Antifungal activity of rezafungin in invasive candidiasis: From bench to bed

Abstract

Invasive mycoses present significant medical challenges, with new populations at risk and the emergence of antifungal resistance. Therapeutic options for antifungal treatment are limited and further complicated by drug–drug interactions, toxicity, and constraints in administration routes. Despite the urgent need for more antifungal drugs, few new antifungal drugs have been introduced over the past decades. Nevertheless, there is hope on the horizon, with new antifungal drugs approved or in late-stage clinical trials.

In this review, we critically examine the antifungal activity of rezafungin, a new-generation echinocandin with distinctive PK/PD properties, including high stability that enables once-weekly dosing, as well as elevated peak concentrations (C_max) and area under the curve (AUC) values, and its therapeutic utility in the treatment of invasive candidiasis. We focus on its mechanism of action, as well as on its broad spectrum of activity against emergent species of Candida and other pathogenic fungi. Additionally, we highlight its utility in the treatment of candidemia and other forms of invasive candidiasis in various medical settings, and the potential future roles of rezafungin and the unmet needs in this field.

Resumen

Las micosis invasoras son un desafío médico importante, con nuevas poblaciones en riesgo y la aparición de resistencia a los fármacos antifúngicos. Las opciones terapéuticas para el tratamiento antifúngico son limitadas y se complican por las interacciones farmacológicas, toxicidad y restricciones en las vías de administración de estos medicamentos. A pesar de la urgente necesidad de nuevos fármacos antifúngicos, se han producido pocas novedades en las últimas décadas. No obstante, hay luz y esperanza en el horizonte, con la aprobación de algunos fármacos antifúngicos o que están en etapas avanzadas de ensayos clínicos.

En esta revisión examinamos la actividad antifúngica de la rezafungina, una equinocandina de nueva generación que presenta características diferenciales en sus propiedades farmacocinéticas, entre las que destacan su elevada estabilidad, que permite una administración semanal, y los altos valores de concentración máxima (Cmax) y área bajo la curva (AUC) alcanzados, y con gran utilidad terapéutica en el tratamiento de la candidiasis invasora. Nos centramos en su mecanismo de acción y farmacocinética, así como en su amplio espectro de actividad antifúngica contra especies emergentes de Candida y otros hongos patógenos. Además, destacamos su utilidad en el tratamiento de la candidemia y de otras formas de candidiasis invasora en diversos entornos médicos, las posibles indicaciones futuras de la rezafungina y las necesidades aun no cubiertas.

Background and perspective

The incidence of invasive mycoses is increasing in association with a growing at-risk population. The estimated annual incidence of invasive mycoses is 6.5 million, with about 3.8 million deaths, of which approximately 2.5 million are directly attributable to the fungal infection.^28,49,74 Fungi are ubiquitous organisms capable of surviving in a vast array of conditions. While the number of different fungi is estimated to be around 12 million, approximately 200 species are considered frequent human pathogens.¹⁴⁴ Furthermore, climate change is favouring the emergence of new resistant fungal species as human pathogens, such as Candidozyma auris (formerly Candida auris) and azole-resistant Aspergillus.^45,47 Moreover, the selective pressure of antifungal prophylaxis, along with advances in molecular testing, may be contributing to the emergence and discovery of formerly less common fungal pathogens that are often resistant to currently available antifungal treatments.^4,82 The spectrum of patients affected by invasive mycoses has expanded significantly in recent years, now including not only individuals with severe immunodeficiencies but also other patient populations. Most invasive mycoses occur in individuals with profound immunodeficiency and are more common in males.³⁰ These infections contribute substantially to increased clinical risk, and are influenced by economic and social determinants, thereby exacerbating existing health disparities.⁶¹ Candidiasis, caused by Candida albicans and increasingly by other Candida species, remains one of the most frequent and significant invasive mycoses. This mycosis is typically endogenous, associated with immunodeficiencies, severe diseases, and serious predisposing factors (e.g., intensive care unit – ICU-stay, use of central venous catheters, broad-spectrum antibiotics, or abdominal surgery). The signs and symptoms of invasive candidiasis are non-specific, with candidaemia being the most common type, potentially leading to disseminated candidiasis affecting multiple organs. Diagnosis is complex, often requiring conventional culture techniques supplemented by non-culture-based assays. Furthermore, the expanding immunodeficient population presents a growing challenge with new risk factors.^{24,60,73,74,88,143}

Patient mortality remains high, with all-cause mortality exceeding 40% among cases. In invasive mycoses caused by antifungal-resistant fungi, mortality increases further and may exceed 80%.^55,115 The attributable mortality from candidaemia and disseminated infections is approximately 30%. The 30-day mortality rate in patients with invasive candidiasis in European hospitals, including ICUs, is 27–42%.^13,44,95 Additionally, the intensive care population may develop new symptoms related to other diseases, such as influenza- and COVID-19-associated mycoses, that pose further threats to the current therapeutic armamentarium.^{47,101,102,117}

The epidemiology of invasive candidiasis is continuously evolving and varies across different geographical regions and patient populations.^{73,74,109,88,110} Advances in healthcare have led to a persistent increase in the immunosuppressed patient population. A significant challenge in the management of patients with invasive candidiasis is determining the optimal timing of antifungal treatment. Early and appropriate initiation of antifungal therapy has been shown to significantly impact patient outcomes; however, treatment is often initiated too late due to, for example, the presence of diagnostic challenges.¹²³ Early antifungal treatment and removal of the central venous catheter are crucial for reducing mortality, with broad-spectrum antifungal prophylaxis decreasing prevalence and improving survival in patients with traditional risk factors.^{24,52,142,143}

Available treatment options for invasive candidiasis include echinocandins (anidulafungin, caspofungin, or micafungin), azoles (fluconazole, itraconazole, isavuconazole, posaconazole, or voriconazole), and polyenes (amphotericin B). Data from direct comparisons and meta-analyses indicate that echinocandins show higher success rates than azoles and similar efficacy to amphotericin B, but with a better safety profile. Global candidiasis guidelines recommend echinocandins as first-line treatments for candidaemia and invasive candidiasis because they have a broad anti-Candida spectrum, are relatively well tolerated, and exhibit limited drug–drug interactions.^22,99 Among these, anidulafungin has demonstrated superior efficacy over fluconazole in treating invasive candidiasis.¹¹² Nevertheless, as classical echinocandins (anidulafungin, caspofungin, micafungin) require daily intravenous infusions, any other echinocandin with an improved pharmacokinetic profile could reduce the frequency of administration.⁵²

Despite established treatments, there is an unmet need for new antifungal drugs and treatment strategies due to increased antifungal resistance and the rising incidence of candidiasis caused by non-C. albicans species, such as C. auris, Nakaseomyces glabratus (formerly Candida glabrata), or Candida parapsilosis.¹⁰ Fluconazole resistance is a concern in these species of Candida, although acquired echinocandin resistance is infrequent. New antifungal drugs and/or novel treatment strategies are also needed to help reduce the risk of nosocomial infections associated with prolonged hospital stays. This may occur when the step-down from daily parenteral echinocandin to oral fluconazole treatment, as recommended by guidelines, is not suitable due to the limited antifungal activity of fluconazole against some species. Antifungal drugs with antibiofilm activity are important in these patient populations with numerous risk factors for biofilm formation (presence of catheters and other biomedical devices), which is a key virulence factor in the pathogenicity of Candida and is associated with increased resistance.²³

Patients requiring long courses of antifungal therapy frequently exhibit drug-related adverse effects, which may be cumulative. The nephrotoxicity, acid-base, and electrolyte disturbances of currently available amphotericin B formulations are well known and obviate attempts for long-term therapy. The triazole class is frequently prescribed to those patients requiring long courses of treatment, but drug–drug interactions are extensive. Fluconazole has poor efficacy in the treatment of several invasive infections and itraconazole absorption is limited by food and gastric pH. Voriconazole is associated with photosensitivity reactions, being rash frequent in addition to hepatotoxicity, and posaconazole may cause drug-induced hypertension, necessitating therapeutic drug monitoring. For these reasons, there is a significant need for new antifungal drugs. However, very few have been introduced as therapeutic tools against mycoses over the last decades. Fortunately, several new antifungal classes are finally in late-stage clinical development.^55,93,94

Rezafungin (Rezzayo – Mundipharma –, formerly SP3025 and CD101 – Cidara Therapeutics–) is considered the first member of a second generation of echinocandins with enhanced pharmacokinetics. Following ReSTORE pivotal phase III clinical study, rezafungin has been approved by the European Medicines Agency (EMA) with input from national authorities, such as the Agencia Española de Medicamentos y de Productos Sanitarios (AEMPS), and Food and Drug Administration (FDA).^5,37,41 Rezafungin exhibits activity against multiple species of Candida, including wild-type, azole-resistant, and some echinocandin-resistant isolates. Rezafungin has distinct pharmacokinetic-pharmacodynamic (PK/PD) properties compared to other echinocandins, such as prolonged half-life and stable molecular structure that allows weekly dosing, high front-loaded therapeutic drug exposures with a high area under the concentration-time curve (AUC), a significant post-antifungal effect, and widespread distribution and penetration at infection sites. High initial plasma concentrations of rezafungin have the potential to rapidly clear Candida from infection sites and may prevent the development of antifungal resistance.²³ Rezafungin has been developed for the treatment of invasive candidiasis, including candidaemia, and its use in the prophylaxis against invasive mycoses in haematopoietic stem cell transplant recipients is currently under investigation. Thus, the recent global guideline for the diagnosis and management of candidiasis published by the European Confederation of Medical Mycology (ECMM) in cooperation with the International Society of Human and Animal Mycology (ISHAM) and the American Society for Microbiology (ASM) recommends echinocandins – including the novel agent rezafungin – as the first-line therapy for candidaemia and for certain forms of invasive candidiasis, such as intra-abdominal candidiasis, due to their broad-spectrum activity and favourable safety profile.²⁴

In this review, we extensively discuss about this promising drug, rezafungin, designed for once-weekly dosing. We also highlight its potential future roles, the clinical trials currently evaluating it, and its spectrum of antifungal activity. We have conducted a literature search including a PubMed search for rezafungin (old and new names), as well as searching the reference lists for additional studies, over the last 10 years. The final section of the manuscript includes three different clinical perspectives: the usefulness of rezafungin in patients from Infectious Diseases, Intensive Care Medicine and Haematology services.

Antifungal activity of rezafungin

Mechanism of action

Rezafungin derives from anidulafungin and has been developed creating structural changes in order to reduce pharmacological and stability problems of first generation echinocandins.¹⁴² Rezafungin, as the other echinocandins, non-competitively inhibits the biosynthesis of a major fungal cell wall carbohydrate, 1,3-β-d-glucan, by binding to the catalytic subunit of 1,3-β-d-glucan synthase enzyme complex, encoded by FKS1 and FKS2 genes (Fig. 1).¹⁰⁸

Fig. 1.

Antifungal mechanisms of action.

Modified from 108.

Pharmacokinetics/Pharmacodynamics (PK/PD)

Rezafungin has been designed to optimise PK/PD properties and reduce hepatotoxicity by reducing its degradation while maintaining the potent antifungal activity and safety profile of the echinocandins.⁹⁷ Rezafungin has a cyclic depsipeptide core and an N-linked acyl lipid side chain, responsible for its antifungal activity. The hemiaminal region of the echinocandin cyclic nucleus has been replaced with a choline aminal ether that reduces the chemical degradation and enhances stability and solubility of rezafungin (Fig. 2). This echinocandin has minimal CYP450 interactions and is chemically stable to biotransformation in liver microsomes or hepatocytes, reducing the risk of hepatotoxicity.^{14,46,97,113,127} Rezafungin mean half-life is of approximately 80 h after the first dose of 400 mg, and 150 h after the second or third doses of 200 mg, indicating linear pharmacokinetics. This property allows a weekly administration of rezafungin, a better tissue penetration and improved safety in comparison with other echinocandins. Mean plasma C_max and AUC have been shown to increase with the dosage. Following single or multiple intravenous doses of rezafungin in healthy adults, rezafungin exposure increases in a dose-dependent manner over the dose range of 50–400 mg. Steady-state is reached after the first 400 mg loading dose, with comparable AUC values maintained with subsequent 200 mg doses. In healthy volunteers who received a single dose of radiolabelled rezafungin, parent rezafungin made up approximately 77% of the total circulating rezafungin exposure, with individual metabolites each making up less than 10% of exposure. Rezafungin is excreted predominantly (approximately 74%) in the faeces, mainly as unchanged drug. The remainder (26%) is excreted in the urine, largely as inactive metabolites. The proportion of the dose recovered in urine as unchanged rezafungin is less than 1% at all dose concentrations, which evinces that renal clearance makes only a minor contribution to overall rezafungin elimination.^37,43,65 Consistent with other approved echinocandins, rezafungin has a low potential for clinically meaningful drug–drug interactions. In healthy subjects, no relevant pharmacokinetic interactions were observed when dosing rezafungin concomitantly with several other drugs concurrently used: Rezafungin had minimal or no effect on drug interactions with cytochrome P450 enzymes (CYP2B6, CYP3A4, CYP1A2, CYP2C8) and transporter proteins (P-gp, OCT-1, OCT-2, MATE1, MATE-2, OATP, and BCRP).^{7,8,14,37,38,40,43,113,116,121}

Fig. 2.

Chemical structure of rezafungin (Wikipedia commons).

Achieving and maintaining therapeutic drug concentrations in the case of antifungal drugs is a significant challenge in special patient populations, such as those with organ dysfunctions (liver or kidney failures) or obesity, severe immunodeficiency or neutropenia, paediatric or elderly patients, among others. Echinocandins exert a concentration-dependent fungicidal activity against Candida cells, reflecting their PK/PD profiles. Consequently, higher drug concentrations than MIC values are necessary for the optimal treatment of invasive candidiasis. This could explain the higher activity of rezafungin. The PK/PD properties of antifungal drugs can be affected by various factors, including the following: organ dysfunction (hepatic impairment/liver failure and renal insufficiency), extracorporeal membrane oxygenation (ECMO), obesity (high body mass index), extreme ages (frailty, changes in body composition, diminished homeostatic control, low tolerance for adverse drug reactions, multiple comorbidities in the elderly, etc.), polypharmacy (use of immunosuppressants and corticosteroids), and critical illness (organ dysfunction and physiological factors such as a hyperdynamic state, third spacing, and hypoalbuminaemia). Dose adjustments may be necessary for some echinocandins in these populations (Table 1). Rezafungin appears to be suitable for these special patient populations without the need for dose modifications.^7,8,23,123

Table 1.

Properties and pharmacokinetic-pharmacodynamic parameters of rezafungin and other echinocandins.^{23,34-37,41,65,93,95,129}

Parameter	Echinocandin
	Rezafungin	Anidulafungin	Caspofungin	Micafungin
Mechanism of action	Inhibition of 1,3-β-d-glucan biosynthesis
Formulation	Powder for reconstitution for intravenous infusion
Dose (mg)	Loading dose: 400Weekly dose: 200	Loading dose: 200Daily dose: 100	Loading dose: 70Daily dose: 50	Daily dose: 100 (50–150)
Cmax (mg/L)	19.2–22.7	7.2	12.1	7.2
Plasma protein binding (%)	96–99	99	97	99
Volume of distribution (L)	39–95	35.2	9.7	25.6
Distribution	Extensive tissue distribution (except brain)
Clearance (L/h)	0.19–0.35	0.87–1.39	0.55–0.78	0.90–1.27
Terminal half-life t1/2 (h)	123–181	24–26	9–11	11–17
Excretion in faeces (%)	38–74	30	35	40
Excretion in urine (%)	14–26 (inactive metabolites) and <1% unchanged rezafungin	1	41	15
Drug interactions	Minimal (negligible interactions with CYP450 isoenzymes)
Need for dose adjustment in special populations^a
Liver failure	No	No	Yes, in moderate failure	Not in severe impairment
Renal failure	No	No	No^b	No^b
Obesity	No	No?	Yes	Yes?
Older patients	No	No	Yes?	No
ICU patients	No	No	No	No

a

Data are limited. Moreover, data on rezafungin in children, patients on extracorporeal membrane oxygenation, patients with severe immunocompromise or neutropenia, HIV-infected individuals, and other special groups are not yet available.

b

Dose escalation may be needed in some patients undergoing continuous veno-venous haemodiafiltration or plasma exchange therapy due to filter adsorption.

Echinocandins are utilised in the outpatient setting (hospital-at-home) for a wide range of clinical conditions, particularly when there is fluconazole resistance or when azole therapy is contraindicated. The once-weekly dosing regimen of rezafungin facilitates its use in the outpatient setting, which is particularly important for many of the special patient populations discussed here (e.g., the elderly and obese patients), both for optimising quality of life and reducing complications related to hospital stays.²¹ Rezafungin may be a suitable option for patients with severe immunodeficiencies without the need for dose adjustment or concerns over drug–drug interactions. However, data concerning the use of rezafungin in this patient population remains scarce. Experience with other echinocandins (though not rezafungin) also suggests that clinical outcomes for neutropenic patients are generally less favourable. Nevertheless, there is insufficient information on rezafungin in children, HIV-infected people, patients on ECMO, among others.^23,24,123

Spectrum of antifungal activity

In vitro susceptibility testing of rezafungin has been conducted in several studies for both wild-type and resistant fungal isolates, using reference methodologies and guidelines from the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST). These studies have shown excellent concordance in testing. The minimum inhibitory concentration 50 and 90 (MIC50 and MIC90) values for rezafungin are comparable to those of anidulafungin, caspofungin, and micafungin.¹⁹

Table 2 shows the activity of rezafungin against the fungi included in the World Health Organization's fungal priority pathogens list.^71,136 Rezafungin is highly active against nine of the twenty pathogenic fungi listed. Notably, rezafungin is highly effective against three of the four critically prioritised fungi: Aspergillus fumigatus, C. albicans, and C. auris. Rezafungin was inactive against three of these twenty fungi, and data are scarce for the remaining eight. However, based on the in vitro activity of other echinocandins, it is likely that rezafungin is not active against them. It is important to underscore that the fungi within the genera Candida and Aspergillus against which rezafungin is active cause more than 90% of invasive mycoses, according to most estimates.²⁸

Table 2.

Activity of rezafungin against the fungi included in the World Health Organization fungal priority pathogens list ¹³⁶.

Priority	Fungal species	Rezafungin activity
Critical priority	Aspergillus fumigatus	Very active
	Candida albicans	Very active
	Candidozyma auris	Very active
	Cryptococcus neoformans	Inactive
High priority	Nakaseomyces glabratus	Very active
	Candida parapsilosis	Very active
	Candida tropicalis	Very active
	Eumycetoma causative agents	Inactive?
	Fusarium	Inactive
	Histoplasma	Inactive?
	Mucorales	Inactive
Medium priority	Pichia kudriavzevii	Very active
	Coccidioides	Inactive?
	Cryptococcus gattii	Inactive
	Lomentospora prolificans	Inactive
	Paracoccidioides	Inactive?
	Pneumocystis jirovecii	Very active
	Scedosporium	Inactive
	Talaromyces marneffei	Inactive

Rezafungin has potent in vitro activity against a broad range of Candida species. Most C. albicans isolates are inhibited by rezafungin concentrations of ≤0.125 mg/L, and most C. auris isolates are inhibited by ≤0.5 mg/L.^12,54,55,141 In most studies, rezafungin MIC90 values were 0.03 mg/L for Pichia kudriavzevii (formerly Candida krusei) and Pichia anomala (formerly Candida pelliculosa), 0.06 mg/L for C. albicans and N. glabratus, 0.125 mg/L for Cyberlindnera fabianii (formerly Candida fabianii), Kluyveromyces marxianus (formerly Candida kefyr), Yarrowia lipolytica (formerly Candida lipolytica), and Candida tropicalis, 0.25 mg/L for Candida dubliniensis and Clavispora lusitaniae (formerly Candida lusitaniae), 0.5 mg/L for C. auris, Candida metapsilosis and Saccharomyces cerevisiae, and 1 mg/L for Candida orthopsilosis and Meyerozyma fermentati (formerly Candida fermentati). C. parapsilosis and Meyerozyma guilliermondii (formerly Candida guilliermondii) exhibited an intrinsic reduced susceptibility phenotype, being inhibited by ≤4 mg/L of rezafungin (Table 3). This observation is similar to those previously reported for other echinocandins, and it is attributed to a polymorphism in the FKS genes of C. parapsilosis, although consistent treatment failures have not been reported.^{11,12,16,18,32,54,55,102,118,131,141,145}

Table 3.

In vitro susceptibility to rezafungin of medically relevant Candida (or formerly Candida) species ^{11,33,16-18,32,82}.

Standard for antimicrobial susceptibility testing	Species of Candida (or formerly within the genus Candida)
	Candida albicans	Candidozyma auris	Nakaseomyces glabratus	Meyerozyma guilliermondii	Pichia kudriavzevii	Candida parapsilosis	Candida tropicalis
Susceptible category – Clinical breakpoints (mg/L)
CLSI	≤0.25	≤0.5	≤0.5	–	≤0.25	–	≤0.25
EUCAST	≤0.08	–	≤0.016	–	≤0.03	≤4	≤0.03
FDA	≤0.125	–	≤0.125	–	–	≤2	≤0.125
MIC ranges (mg/L)
CLSI	≤0.008–1	0.25–0.5	≤0.008–4	0.5–1	≤0.008–1	≤0.008–≥8	≤0.008–2
EUCAST	0.0005–0.004	0.125–0.25	0.04–0.5	0.25–0.5	0.008–0.016	0.25–2	0.002–0.016

CLSI: Clinical and Laboratory Standards Institute; EUCAST: European Committee on Antimicrobial Susceptibility Testing; FDA: Food and Drug Administration; MIC: minimum inhibitory concentration.

Rezafungin exhibits the same or slightly better activity as other echinocandins on Candida isolates harbouring FKS hot spot mutations, which may lead to echinocandin resistance, with higher MICs than those of wild-type isolates without FKS mutations.^{10,11,46,54,76,103,111} The in vitro antifungal spectrum is so similar that Winkler et al.¹⁴⁰ have proposed that other echinocandins, particularly anidulafungin, could be used to predict, by using the CLSI microdilution method and interpretive criteria, the in vitro susceptibility to rezafungin of clinical isolates within the five most common Candida species (some of which have since been reclassified into different genera): C.albicans, N. glabratus, P. kudriavzevii, C. parapsilosis, and C.tropicalis.

There were no MIC increases over time for Candida species, and an extended post-antifungal effect against C. albicans, C. parapsilosis, and N. glabratus has been reported, similar to that of micafungin.¹⁷ The existence of this inhibition of fungal growth after the drug is stopped and its magnitude largely depend on the fungal species and the class of antifungal drug. It should be noted that echinocandins have the longest post-antifungal effect, followed by polyenes and azoles, and this property can have a positive effect on the eradication of fungal burden.⁵⁸

The potential to develop in vitro rezafungin resistance is low for common Candida species. The median frequency of spontaneous mutations conferring reduced susceptibility to rezafungin ranged from 1.4 × 10⁻⁸ to 3.9 × 10⁻⁹. Serial passage results showed that around 20 passages were required to develop resistance.⁷⁹ As previously mentioned, mechanisms of resistance or tolerance to rezafungin are similar to those described for other echinocandins and are primarily associated with mutations in the hot spots of the FKS genes.⁷⁵ However, while higher concentrations of rezafungin, caspofungin and other echinocandins were required to inhibit the in vitro growth of Candida isolates with resistance mechanisms (e.g., FKS1 and FKS2 mutations), dose adjustments of rezafungin were not necessary for achieving mycological cure in patients with resistant isolates in clinical trials.^80,122,124 Moreover, rezafungin is active against early and mature Candida biofilms. Rezafungin concentrations of 0.25–1.0 mg/L reduced biofilm size and prevented the development of mature biofilms.²⁰ In time–kill studies of C. auris, rezafungin concentrations of ≥1 to ≥8 mg/L using RPMI-1640 medium with 50% human serum killed the yeasts in the first 8–12 h at clinically achievable trough concentrations.⁷⁰

Rezafungin has demonstrated efficacy in murine models of neutropenic and other immunocompromised invasive candidiasis, showing potent activity against isolates of C. albicans, including azole-resistant strains, N. glabratus, and C. parapsilosis. In these animal models, rezafungin exhibits concentration-dependent fungicidal activity with similar or better efficacy than other echinocandins, even against azole-resistant isolates of C. dubliniensis, C. tropicalis, and C. auris.^{12,76,77,90,97,148} The AUC/MIC ratio required to achieve efficacy endpoints was lower for rezafungin compared to other echinocandins.^76,77 Rezafungin was more potent than amphotericin B and micafungin against C. auris in immunosuppressed mice, reducing fungal burden in kidneys.⁵³ Rezafungin achieved higher hepatic concentrations compared to micafungin, resulting in improved survival of mice with intra-abdominal candidiasis.¹⁴⁸ Rezafungin also demonstrated benefits in the prophylaxis of immunosuppressed mice models of C. albicans infection. The reduction of Candida colony-forming units was greater with increasing drug concentrations (5, 10, or 20 mg/kg) and when the drug was administered closer to the time of infection (day −1, −3, or −5). Mice that received 20 mg/kg of the drug, which is the human equivalent in mice, had C. albicans cleared.⁸⁹

The activity of rezafungin against Aspergillus is comparable to that of other echinocandins.⁵⁵ Rezafungin minimum effective concentration (MEC) ranges of ≤0.015–0.125 mg/L and ≤0.015–2 mg/L have been reported against A. fumigatus wild–type and azole-resistant species, respectively. Moreover, rezafungin is active with MECs of ≤0.008–0.03 mg/L against Aspergillus flavus, Aspergillus niger, and Aspergillus terreus clinical isolates. Rezafungin also had activity against some cryptic species of Aspergillus, including Aspergillus calidoustus, Aspergillus lentulus, Aspergillus thermomutatus, and Aspergillus udagawae.^103,137,138 In a disseminated infection mouse model of aspergillosis using A. fumigatus, similar survival rates were shown with rezafungin compared to amphotericin B treatment.^89,90 In vivo efficacy has also been demonstrated in a murine model of disseminated aspergillosis with an azole-resistant A. fumigatus isolate harbouring a TR34/L98H CYP51A mutation.^137,138 However, additional studies are warranted to determine whether this preclinical research translates to clinical experience, and if rezafungin has the potential to prevent and treat invasive aspergillosis. Rezafungin, like other echinocandins, is less active or inactive against non-Aspergillus filamentous fungi isolates, such as Fusarium, Lomentospora or Scedosporium, and basidiomycete yeasts, such as, Cryptococcus, Trichosporon, and Rhodotorula, with MICs > 8 mg/L, but in vitro data are lacking for most of the rare and emerging moulds and yeasts.⁴⁶

Echinocandins are not commonly used in pneumocystosis therapy. However, the efficacy of rezafungin has been investigated in the prevention of Pneumocystis murina infection in an immunosuppressed mouse model of Pneumocystis pneumonia. A rezafungin dose of 20 mg/kg was comparable to trimethoprim–sulfamethoxazole in clearing the trophic forms and asci of P.murina. Rezafungin prophylaxis for at least four weeks also prevented the reactivation of pneumocystosis six weeks after discontinuation. Although other echinocandins reduce the number of asci forms, trophic forms do not contain 1,3-β-d-glucan and are not inhibited by anidulafungin, caspofungin, or micafungin, leading to reactivation of infections after discontinuation of prophylaxis. The higher efficacy of rezafungin may be due to its long-acting pharmacokinetics, as Pneumocystis depends on sexual reproduction and the development of asci that contain 1,3-β-d-glucan.^26,55,89

Therapeutical efficacy of rezafungin

EMA has approved the use of rezafungin for the treatment of invasive candidiasis in adults.^5,37 The United States Food and Drug Administration (FDA)^41,121 has also approved its use for treating candidemia and invasive candidiasis in individuals over 18 years old who have no alternative treatment options. These approvals are based on the results obtained from the phase 2 STRIVE (NCT02734862) and phase 3 ReSTORE (NCT03667690) clinical trials, which showed that weekly administration of rezafungin was statistically as active and effective as daily treatment with caspofungin. Moreover, rezafungin was well tolerated and had a safety profile very similar to that of caspofungin.^{56,122,124,128–130}

The STRIVE study was a two-part phase 2 trial of rezafungin, comparing two dosing regimens (400 mg once weekly, and 400 mg in week 1 followed by 200 mg once weekly) with a standard caspofungin therapy (70 mg intravenous once, followed by 50 mg daily). Efficacy was assessed in the modified intention-to-treat population, which included 76, 46, and 61 patients in the high-dose and low-dose rezafungin regimens, and caspofungin treatment groups, respectively. Overall, cure rates at day 14 were comparable between the three treatment arms (60.5% vs. 76.1% vs. 67.2%, respectively). The highest cure rates and lowest mortality were observed with the low-dose rezafungin regimen. Apparent differences between the two rezafungin groups raised the discussion of paradoxical growth with higher concentrations of rezafungin. Both in vitro and animal studies of other echinocandins have reported this phenomenon, where the fungal burden increases at doses above a certain threshold.⁸⁴ However, the differences with rezafungin occurred on day 5, when both treatment arms had received a similar 400 mg dose. Therefore, the hypothesis of a paradoxical growth effect appears to be improbable.¹²⁶

There were no serious or severe adverse events (all were transient and resolved), withdrawals due to adverse events, or deaths. Mild transient infusion reactions such as flushing, nausea, and chest tightness were observed with the third 400 mg dose in the multiple-dose study.¹¹⁶ No clinically meaningful laboratory abnormalities were noted. A randomised, double-blind, phase 1 study was conducted in healthy volunteers to evaluate the cardiac effects of single doses of intravenous rezafungin.³⁸ Unlike anidulafungin, rezafungin did not prolong the QT interval and had no apparent effect on repolarization or QRS duration at doses up to 1400 mg. Echocardiograms showed no change in ejection fraction or other cardiac parameters compared with baseline. Although an increase in PR interval was seen in the 1400 mg dose group, it was considered not clinically relevant. According to the results from the STRIVE study, 400 mg of rezafungin in the first week, followed by 200 mg weekly, was the most effective dosing regimen in the phase 3 randomised double-blind ReSTORE study.¹²⁸

The ReSTORE trial was a multicentre, randomised, double-blind study examining the aforementioned rezafungin dosage 400 mg/200 mg once-weekly protocol to treat candidemia and/or invasive candidiasis. The active comparator was intravenous caspofungin followed by optional fluconazole step-down. The primary endpoints were all-cause mortality at day 30 and global cure at day 14, measured by clinical, radiological, and mycological indices. Patients included in the ReSTORE study had to present one or more systemic symptoms (fever, hypothermia, hypotension, tachycardia, tachypnoea) and a mycological diagnosis of candidemia and/or invasive candidiasis. However, patients with osteomyelitis, endocarditis, meningitis, endophthalmitis, chorioretinitis, central nervous system infections, chronic disseminated candidiasis or urinary tract candidiasis were excluded from the study. The patients were randomly assigned to two different groups. In one group, the patients received intravenous rezafungin in loading dose of 400 mg on day 1, followed by 200 mg on day 8, and then 200 mg once weekly (total of two to four doses). In the other group, the patients received caspofungin in loading dose of 70 mg on day 1, followed by 50 mg once daily for a total treatment duration of two to four weeks. After three days of intravenous treatment, patients could switch to oral treatment (placebo for patients randomised to rezafungin, and fluconazole for those randomised to caspofungin) if certain criteria were met (v.g., fluconazole-susceptible Candida isolate, all symptoms of candidiasis resolved, or a recent blood culture negative for Candida). Randomisation was stratified by diagnosis (candidemia only vs. invasive candidiasis) and by Acute Physiology and Chronic Health Evaluation II (APACHE II) score or absolute neutrophil count (APACHE II ≥ 20 and neutrophils < 500 cells/μL vs. APACHE II < 20 and neutrophils ≥ 500 cells/μL).

The primary efficacy endpoint was the global response, determined based on clinical, mycological, and radiological responses, confirmed by an independent committee on day 14. These responses were also analysed separately as secondary endpoints. Other secondary efficacy endpoints included all-cause mortality on day 30 (the primary endpoint for FDA evaluation) and global response on days 5, 30, end of treatment, and follow-up (days 52–59). When the lower limit of the 95% confidence interval (CI 95%) in relation to the difference in cure rates on day 14 (rezafungin vs. caspofungin) was greater than −20%, non-inferiority was inferred. The modified intention-to-treat population was defined as all subjects with confirmed candidiasis, based on a blood culture or a culture from a normally sterile site obtained four or fewer days before randomisation; those patients received one or more doses of the investigational drug. A total of 199 patients were included in the study, with 100 patients randomised to rezafungin and 99 to caspofungin. Eight patients from the rezafungin group were excluded from the modified intention-to-treat population due to the absence of documented candidiasis (n = 6) and for not having received the treatment (n = 2). With regard to the caspofungin group, five patients were excluded from the study: in four cases there was a lack of documented infection, and in one case the patient did not receive the treatment. Thus, 187 patients were included in the modified intention-to-treat population: 93 in the rezafungin group and 94 in the caspofungin group. No notable differences were observed between the treatment arms in baseline demographic characteristics. The majority of patients (70% vs. 68.7%) had a diagnosis of candidaemia. The APACHE II score was <20 for 84% and 81.8%, and neutrophils were ≥500 cells/μL in 88% and 93.9% of patients, respectively.

The primary endpoint for EMA was the global response confirmed by an independent committee on day 14. The study demonstrated the non-inferiority of rezafungin, with a treatment difference of −1.5% (95% CI: −15.4 to 12.5), with mortality rate values at day 30 of 23.7% and 21.3% in the rezafungin and caspofungin groups, respectively. Non-inferiority of rezafungin in all-cause mortality was demonstrated, with a treatment difference of 2.4% (95% CI: −9.7 to 14.4). The analyses of secondary and exploratory endpoints were consistent with the primary analysis: Mycological eradication rates were similar between treatment groups, and the clinical response rate on day 5 was lower in the rezafungin group than in the caspofungin group (63.4% vs. 74.5%), being similar at subsequent visits (Table 4). Response rates for N. glabratus, C. tropicalis, and C. parapsilosis were higher in the rezafungin group (66.7%, 70%, and 75%) than in the caspofungin group (56%, 58.8%, and 64.7%).⁸⁰

Table 4.

Global response on the efficacy of rezafungin in adults recruited in the ReSTORE study ^5,37,41,128.

Endpoint	Response	Rezafungin (N = 93)	Caspofungin (N = 94)	Difference (95% CI)
At day 5	Clinical cure^a	55 (55.9%)	49 (52.1%)	3.8% (−10.5 to 17.9)
	Clinical cure	59 (63.4%)	70 (74.5%)	−11.0% (−24.0 to 2.3)
	Mycological eradication	64 (68.8%)	58 (61.7%)	7.1% (−6.6 to 20.6)
At day 14	Clinical cure^a	55 (59.1%)	57 (60.6%)	−1.5% (−15.4 to 12.5)
	Clinical cure	62 (66.7%)	63 (67.0%)	−0.4% (−13.8 to 13.1)
	Mycological eradication	63 (67.7%)	62 (66.0%)	1.8% (−11.7 to 15.2)
At day 30	Clinical cure^a	46 (49.5%)	46 (48.9%)	0.5% (−13.7 to 14.7)
	Clinical cure	51 (54.8%)	52 (55.3%)	−0.5% (−14.6 to 13.7)
	Mycological eradication	56 (60.2%)	53 (56.4%)	3.8% (−10.3 to 17.8)
At the end of treatment	Clinical cure^a	56 (60.2%)	59 (62.8%)	−2.6% (−16.4 to 11.4)
	Clinical cure	65 (69.9%)	64 (68.1%)	1.8% (−11.5 to 15.0)
	Mycological eradication	63 (67.7%)	63 (67.0%)	0.7% (−12.7 to 14.1)
At follow-up	Clinical cure^a	42 (45.2%)	39 (41.5%)	3.7% (−10.5 to 17.7)
	Clinical cure	46 (49.5%)	44 (46.8%)	2.7% (−11.6 to 16.8)
	Mycological eradication	48 (51.6%)	49 (52.1%)	−0.5% (−14.7 to 13.7)

a

Evaluated by the Independent Committee. CI: confidence interval.

The subgroup analysis for all-cause mortality showed that, on day 30, mortality rates in patients over 65 years were lower in the rezafungin group (14%) than in the caspofungin group (31.7%). Similar results were seen in patients with candidemia alone and in those with end-organ invasive candidiasis.¹²⁸ An integrated analysis of the STRIVE and ReSTORE trials has further supported the efficacy of rezafungin in invasive candidiasis. The 30-day all-cause mortality was 18.7% for rezafungin and 19.4% for caspofungin (difference −1.5, 95% CI −10.7 to 7.7), in the modified intention-to-treat population analysis (139 patients for rezafungin, 155 for caspofungin). Mycological eradication rates on day 5 in patients with positive blood cultures were 75.5% and 54.9%, respectively (difference 19.2, 95% CI 3.0–35.5). The rezafungin 400 mg/200 mg protocol showed the greatest overall cure, with the highest clinical and mycological cure rates and the lowest rate of all-cause mortality at day 30 across all treatment arms. More patients in the rezafungin group had negative blood cultures at 24 hours than in the caspofungin group (60% vs. 49.1%, respectively), suggesting that rezafungin may be associated with a shorter time to achieve negative blood cultures than caspofungin in patients suffering from invasive candidiasis.¹²⁹ Candidemia cleared more rapidly in the rezafungin-treated patients, possibly reflecting greater fungicidal activity with front-loaded drug exposure, and demonstrated high rates of early treatment efficacy in patients with candidemia. Clinical cure rates were also highest with rezafungin 400 mg/200 mg protocol when differentiating between C. albicans and other Candida species.^122,124

Soriano et al.¹²⁴ conducted a post hoc analysis using pooled data from the STRIVE/ReSTORE trials to determine clinical and mycological cure in the treatment of invasive candidiasis with rezafungin or caspofungin, based on the Candida species causing the infection and the in vitro antifungal susceptibility of the Candida isolates obtained from the patients. This analysis included data from patients with a documented candidiasis within 96 h of randomisation who also received one or more doses of the study drug. A total of 294 patients were included, 139 treated with rezafungin and 155 with caspofungin. Global cure rates at day 14 were similar between both treatment groups in infections caused by C. albicans (61% – rezafungin– vs. 65.2% – caspofungin–) and C. tropicalis (70.4% vs. 63.6%). However, cure rates were higher with rezafungin for invasive candidiasis caused by N. glabratus (71.1% vs. 60%) and C. parapsilosis (78.6% vs. 55.6%). All-cause mortality at day 30 was similar in both treatment arms in infections caused by C. albicans (22% – rezafungin – vs. 18.8% – caspofungin–) and N. glabratus (15.8% vs. 11.4%), but higher in patients with invasive infections by C. tropicalis (18.5% – rezafungin – vs. 31.8% – caspofungin–) and C. parapsilosis (7.1% vs. 29.6%) treated with caspofungin. In cases of invasive candidiasis caused by C. albicans and C. parapsilosis, both rezafungin and caspofungin had similar mycological cure rates at days 5 and 14, although rezafungin achieved higher mycological cure rates in candidiasis caused by N. glabratus and C. tropicalis. Front-loaded dosing may maximise the effect of rezafungin due to its low clearance and long half-life, as well as the concentration-dependent fungicidal activity of echinocandins.¹²²

Of interest, there was no correlation between clinical and microbiological outcomes, and the MICs of the isolates causing the infections in either treatment group. Moreover, only three isolates of the 313 isolates from the 294 pooled patients, tested by the EUCAST method, were non-susceptible to rezafungin: one C. albicans isolate in the caspofungin group, and two N. glabratus isolates, one in the rezafungin group and one in the caspofungin group. Only the N. glabratus isolate from the patient treated with rezafungin had an FKS mutation. However, as the MIC of rezafungin or caspofungin increased, no clear trends were observed in the overall cure rates or mycological eradication rates at day 14 in any patient, regardless of the Candida species.

In the patient infected with a N. glabratus isolate carrying a FKS mutation (FKS2 HS1 F659V) and treated with rezafungin, although global cure was not achieved at day 14, mycological cure was supposed at this time point and the patient was alive at day 30. Data on the effectiveness of rezafungin in patients with FKS mutations conferring echinocandin resistance are limited; however, this patient and two patients with N. glabratus candidiasis under the rezafungin expanded-access programme have been classified as having achieved mycological cure.³ Rezafungin reaches high plasma concentrations early in therapy and may be better positioned to treat invasive candidiasis caused by Candida isolates with higher MICs. This is of particular importance given the shift in the epidemiology of infections caused by N. glabratus, C. parapsilosis, and other Candida species, with a reduced susceptibility to antifungal treatment of the aetiological isolates.⁸⁰

The most relevant safety data regarding the use of rezafungin in adult patients with candidaemia or invasive candidiasis come from the pooled data of the STRIVE and ReSTORE studies. In total, 409 subjects were exposed to at least one dose of rezafungin, with data of 151 patients exposed to the recommended dose available. The median duration of the treatment was 14 days (range: 1–28). In the pooled population, 27.8% of patients randomised to rezafungin and 35.5% of those randomised to caspofungin, switching from intravenous to oral therapy. The rate of adverse events was higher in the rezafungin group than in the caspofungin group (92.1% vs. 83.1%), as were treatment-related adverse events (14.3% vs. 10.8%) and serious adverse events (55% vs. 48.8%). Treatment-related serious adverse events occurred in 2% of the patients in the rezafungin group and 3% in the caspofungin group. The most frequent adverse events were hypokalaemia (14.6% vs. 10.2%), pyrexia (11.9% vs. 6.6%), and diarrhoea (11.3% vs. 10.2%). Adverse events leading to treatment discontinuation occurred in 2% of patients in the rezafungin group and 2.4% in the caspofungin group. The most common serious adverse event was septic shock (6% in both treatment groups). Adverse events of special interest included infusion-related reactions, phototoxicity, tremor, and neuropathy.

Infusion-related reactions may occur with other echinocandins as well. The formulation of rezafungin contains relatively high concentrations of polysorbate, as do the formulations of other echinocandins. Infusion-related reactions may mimic anaphylactic reactions; however, they differ in that they usually resolve spontaneously and do not occur or recur when the drug is administered at slower infusion rates. Distinguishing infusion-related reactions from true anaphylaxis is crucial when using potentially life-saving agents, as an incorrect diagnosis of anaphylaxis may lead to unjustified discontinuation of therapy, whereas treatment can often be safely continued by reducing the infusion rate. The risk of infusion-related reactions can be minimised by adhering to, and not exceeding, the infusion rates recommended in the product information.

The proportion of patients under 65 years of age was higher in the combined rezafungin group (90.8% vs. 77.6%). In patients aged 65 years or older, the incidence of adverse events leading to study discontinuation was 10.9% in the rezafungin group and 27.9% in the caspofungin group. Considering the safety results in special populations, it was concluded that dose adjustment is not necessary in patients aged 65 years or older with hepatic impairment, renal impairment, or based on weight. The number of neutropenic patients included in the pivotal study with rezafungin was small (7.4%). These patients often receive prophylactic antifungal treatment, making their recruitment in clinical trials challenging. However, there are no data suggesting a different response to rezafungin in these patients.²³

Moreover, Honoré et al.⁵⁶ conducted a post hoc, pooled, exploratory analysis of the length-of-stay in hospital and ICU in the STRIVE and ReSTORE studies. They observed that patients treated with rezafungin had a shorter length-of-stay in hospital compared to those treated with caspofungin across all studied parameters, including total length-of-stay in hospital, ICU length-of-stay, and ICU length-of-stay in patients with mechanical ventilation.

Long-term therapy with echinocandins is necessary to treat some infections caused by azole-resistant Candida isolates, such as endocarditis or osteomyelitis, or certain aspergillosis, either in combination with other antifungal drugs or not. Daily intravenous treatment for these patients is associated with a poorer quality of life and a higher risk of health care-associated complications. The ReSTORE study did not include patients with forms of invasive candidiasis that typically require prolonged antifungal treatments, such as candidiasis associated with biofilm development (prosthetic candidiasis) or candidiasis located in organs or tissues in which echinocandins penetration is limited, such as the candidiasis of the central nervous system or the urinary tract.^25,128 However, there is a growing number of anecdotal case reports and small case series describing the successful and long-term use of rezafungin in difficult-to-treat cases of invasive candidiasis, such as natural valve and prosthetic valve endocarditis, osteomyelitis, prosthetic joint infection, or spondylodiscitis,^{4,21,42,50,64,72,92,104,132,134} but more information on its efficacy against these forms of invasive candidiasis, or those caused by uncommon Candida species, would be desirable.

There are no direct comparisons of rezafungin with azoles or polyenes, nor with other echinocandins besides caspofungin. Available meta-analyses suggest rezafungin is as effective as other echinocandins. However, this conclusion is based mainly on the results of the phase 2 STRIVE study of rezafungin, with the results of the phase 3 ReSTORE study not being considered. Therefore, any interpretation should be treated with caution. Unlike the other echinocandins (anidulafungin, caspofungin and micafungin), which are administered by daily intravenous infusion, rezafungin is administered by weekly intravenous infusion. This weekly dosing regimen could offer a significant clinical advantage to these patients. In line with the results of the ReSTORE study, physicians would have considered that 16% of the patients could have been discharged a median of five to six days earlier if they did not require daily antifungal treatment.⁵⁶ In addition to the potential reduction in hospital stay duration and treatment burden, administration of antifungal medication on a weekly basis reduces the total volume of infusion needed, thereby decreasing the risk of intravenous fluid overload, which can be significant in certain patients, such as those with renal impairment or requiring dialysis.^23,56 Unlike other echinocandins, which are also indicated for use in paedriatic populations, the use of rezafungin is limited to adult patients.⁵

Additionally, there are ongoing studies in the recruitment phase regarding the utility of rezafungin.¹⁴² A multicentre, open-label phase 1 study (NCT05534529) aims to evaluate the PK/PD, safety, and tolerability of a single intravenous dose of rezafungin in children receiving antifungal drugs for prophylactic treatment, or to treat confirmed or probable invasive mycoses. However, it should be noted that recruitment to this study has been halted for strategic reasons.

Another phase 2, proof-of-concept, multicentre, open-label, randomised study (NCT05835479) aims to evaluate the efficacy, safety, and tolerability of rezafungin with cotrimoxazole compared to cotrimoxazole monotherapy in adult patients with HIV suffering from Pneumocystis jirovecii pneumonia. A combination approach for the treatment of this fungal infection may be very promising, as the combination of an echinocandin and trimethoprim–sulfamethoxazole not only covers all stages of the Pneumocystis life cycle, but may also allow for dose reduction of trimethoprim–sulfamethoxazole, thereby reducing its associated toxicity and improving tolerability.⁶⁷

Finally, a phase 3 study (NCT04368559, ReSPECT) is comparing the efficacy and safety of intravenous prophylaxis with rezafungin (for preventing invasive mycoses, including P. jirovecii pneumonia) with the standard treatment – fluconazole or posaconazole – for patients who have undergone allogeneic bone marrow transplants. Rezafungin 400 mg/200 mg once weekly is compared with a standard regimen containing daily azole prophylaxis with fluconazole or posaconazole and anti-P. jirovecii prophylaxis with oral trimethoprim–sulfamethoxazole. Fungal-free day 90 survival will be evaluated as the primary outcome.

Infectologist's point of view

The incorporation of rezafungin into the therapeutic toolbox for treating invasive fungal infections is of significant importance to infectious disease specialists for several reasons. Firstly, rezafungin offers an optimal PK/PD profile that facilitates the rapid clearance of candidaemia, a feature inherently associated with improved patient outcomes. In the ReSTORE trial, the overall mortality was approximately 20%, which is notably low for this patient population. A recent European, multicentre, observational case-control study compared mortality in patients with candidaemia to that in matched controls. Matching was based on age, primary underlying disease, ICU versus non-ICU hospitalisation, and major surgery within two weeks prior to the index event. The overall mortality rate among patients with candidaemia was 40%, with an attributable mortality of 18.2%, compared to 22% in the control cohort. These findings suggest that clinical trials, such as ReSTORE, which involve patients with a baseline mortality close to 20%, may have limited power to assess antifungal efficacy in terms of mortality endpoints.

Nevertheless, several studies underscore the prognostic significance of early Candida eradication. A retrospective US study of 827 patients demonstrated that early mycological clearance (≤3 days), defined by the timing of the last positive blood culture, was significantly associated with lower in-hospital (p = 0.005) and 30-day mortality (p = 0.009).⁸¹ Additionally, a subgroup analysis of the ReSTORE trial reported higher rates of mycological eradication by day 5 with rezafungin (77.4%) compared to caspofungin (68.2%), with early clearance correlating with numerically lower mortality.¹²⁹ Early mycological clearance may also play a conceptual role in preventing the development of antifungal resistance, although further data are needed.

Moreover, early eradication of candidiasis has been associated with reduced healthcare costs. The same retrospective study found that early clearance was linked to significantly shorter hospital stays and decreased overall costs (p < 0.001). Candidaemia poses a substantial economic burden, primarily driven by extended hospitalisation and antifungal therapy costs, with per-patient expenditures ranging from $48,487 to $157,574.^51,59,135 Rezafungin has demonstrated other potential cost-saving advantages. Data from the STRIVE and ReSTORE studies indicated that patients treated with rezafungin had shorter hospital and ICU stays compared to those receiving daily echinocandins, resulting in substantial cost savings. These findings support the clinical and economic value of early, aggressive therapy aimed at rapid fungal clearance. Nevertheless, additional country-specific analyses are needed to identify the optimal timeframe for clearance that maximises both clinical and economic outcomes.

Beyond its PK/PD advantages, rezafungin's long half-life and weekly dosing schedule offer clear logistical and clinical benefits. In the Candida III cohort study conducted by the European Confederation of Medical Mycology (ECMM),³¹ which included 621 patients across 64 hospitals in 20 countries, hospitalisation was extended by a median of 14 days in 16% of patients solely to complete parenteral antifungal therapy. An additional 4% required outpatient parenteral antifungal treatment. These data suggest that approximately one in five patients with candidaemia could benefit from the use of rezafungin to expedite hospital discharge.

Apart from ICU or haematologic patients, who will be discussed separately, the majority of “other” patients with candidaemia are elderly, with high comorbidity burdens and extensive use of concomitant medications.⁹¹ The administration of rezafungin on a weekly basis, in conjunction with minimal drug–drug interactions and a favourable safety profile, renders it particularly suitable for this population, including patients with chronic or difficult-to-treat infections. Long-term echinocandin therapy is often required for treating azole-resistant Candida infections – such as endocarditis, osteomyelitis, and some forms of aspergillosis – either as monotherapy or in combination with other agents. Daily intravenous administration in these cases is associated with reduced quality of life and an increased risk of healthcare-associated complications. While the ReSTORE study did not include patients with forms of invasive candidiasis that typically require prolonged antifungal therapy, such as biofilm-associated infections (e.g., prosthetic valve endocarditis) or infections in sites with poor echinocandin penetration (e.g., central nervous system, urinary tract), an increasing number of case reports and small series have documented successful long-term use of rezafungin in such settings. These include natural and prosthetic valve endocarditis, osteomyelitis, prosthetic joint infections, and spondylodiscitis.^{4,21,42,50,64,72,92,104,132,134} However, further evidence is needed to assess its efficacy in these complex clinical scenarios and in infections caused by uncommon Candida species.

Intensivist's point of view

Critically ill patients are at risk of candidaemia and invasive candidiasis, with one-third to half of all candidaemia cases occurring in the ICU. Outcomes remain poor, with an estimated crude mortality rate of 40–55% in the ICU for invasive candidiasis.^48,107 Complicated intra-abdominal infections are the second leading cause of sepsis-related mortality and present a significant challenge for physicians, accounting for nearly 60% of all surgical patients with sepsis. Treatment focuses on effective intra-abdominal source control and appropriate antimicrobial therapy to reduce mortality. However, despite advancements in prevention, diagnosis, and treatment, mortality rates remain high, ranging from 20% to 60%.^13,87

Classical echinocandins are established as first-line treatment for candidaemia¹⁴⁶ and invasive candidiasis, even in the most recent guidelines.²⁴ However, treatment must be individualised due to certain limitations that can influence the efficacy of the treatment and, consequently, the patient's prognosis. These limitations⁶² are mainly related to the speed of action of echinocandins, the avoidance of inappropriate empirical treatment, their activity against non-C.albicans species resistant to azoles, especially C. parapsilosis, N. glabratus, and C. auris, and the PK/PD properties with poor penetration into peritoneal fluid.^87,147 Therefore, new agents and/or dosing strategies are needed for critically ill patients with candidaemia and/or invasive candidiasis.

Rezafungin was developed to treat candidaemia and invasive candidiasis among other indications. Due to its special properties, which overcome the previously described limitations, the new European guidelines²⁴ strongly recommend the use of rezafungin for candidaemia, and do it moderately (at the same level as classical echinocandins) for intra-abdominal candidiasis. In the following paragraphs, we will describe the principal characteristics of rezafungin, focusing on the current limitations of echinocandins.

The timely initiation of appropriate antifungal therapy and its early benefits in reducing the time taken for a blood culture to turn negative

It has been widely studied that timely initiation of appropriate antifungal therapy is essential; delay or inadequate treatment absolutely increases mortality.^48,68,107 Another important factor, less studied until now, is the benefit of reducing the time to negative blood culture. Both phase 2 and phase 3 trials suggested that rezafungin was associated with better time to negative blood culture and day 5 outcomes, including global (ReSTORE) or overall (STRIVE) cure and mycological eradication rates.^128,130 A pooled analysis of STRIVE and ReSTORE also confirmed these findings and suggested a potentially faster time to negative blood culture, especially in patients with a positive blood culture close to randomisation.¹²⁹ This may be particularly relevant in critically ill patients.

A post hoc, pooled, patient-level analysis of STRIVE and ReSTORE investigated the efficacy and safety of rezafungin compared with caspofungin in the subgroup of patients with candidaemia and/or invasive candidiasis who were in the ICU at randomisation. Of 294 patients in STRIVE/ReSTORE, 113 were in the ICU at randomisation (rezafungin n = 46; caspofungin n = 67).⁵⁶ The results provide further support for potential early treatment benefits with the rezafungin dosing regimen, as demonstrated by a reduced time to negative blood culture compared to caspofungin treatment and numerically higher mycological eradication rates on day 5.

The median time to negative blood culture was 18 hours (interquartile range, 12.6–43) in the rezafungin group, as opposed to 38 hours (interquartile range, 15.9–211.3) in the caspofungin group (stratified log-rank p = 0.001; nominal, not adjusted for multiplicity). Interestingly, the approximately 20-h difference in median time to negative blood culture between treatments was greater in the present subgroup of critically ill patients than in the individual trials (3-h treatment difference, approximately).^128,130 Furthermore, the mean time from the first positive blood sample to the initiation of any antifungal treatment for candidaemia/invasive candidiasis, and the time to the first dose of randomised treatment, were both longer for patients who received rezafungin than for those who received caspofungin. This is consistent with the data reported for the ReSTORE trial.¹²⁸ There was a similar trend in favour of rezafungin in the proportions of patients achieving negative blood cultures at 24 and 48 h in the ICU versus the overall population, although treatment differences between the groups were slightly greater in patients in the ICU versus the overall population.¹²⁹

Mycological eradication rates on day 5 and 14 were 78.3% and 71.7% for rezafungin versus 59.7% and 65.7% for caspofungin, respectively. Safety profiles were similar between groups. Overall, the treatment of 17.4% of the patients under rezafungin regimen and that of 29.9% of those under caspofungin was discontinued due to treatment-emergent adverse events. Finally, day 30 all-cause mortality rate was numerically higher, though not statistically significant, with rezafungin, but the mortality attributable to candidaemia/invasive candidiasis appeared to be similar in both treatment groups.

Activity against non-C. albicans species resistant to azoles and echinocandins

The increasing problem of azole and echinocandin resistance, particularly in relation to N. glabratus and C. auris, is a cause for concern.⁶² In the ICU setting, antifungal-resistant isolates spread among patients; outbreaks of azole-resistant C. parapsilosis with similar genotypes and intra-hospital spread of azole-resistant N. glabratus have also been reported.^10,18,33 The lack of activity against these strains is worrying.

As previously mentioned, Soriano et al.¹²⁴ conducted a post hoc analysis using pooled data from the STRIVE/ReSTORE trials to determine clinical and mycological cure in the treatment of invasive candidiasis with rezafungin or caspofungin, based on the Candida species causing the infection and the in vitro antifungal susceptibility of the Candida isolates obtained from patients. Although global cure rates at day 14 were similar between both treatment groups for infections caused by C. albicans (61% – rezafungin – vs. 65.2% – caspofungin–) and C. tropicalis (70.4% vs. 63.6%), it is notable that cure rates were higher with rezafungin for candidiasis caused by N. glabratus (71.1% vs. 60%) and C. parapsilosis (78.6% vs. 55.6%).

Regarding mortality, all-cause mortality at day 30 in candidiasis caused by C. albicans (22% – rezafungin – vs. 18.8% – caspofungin–) and N. glabratus (15.8% vs. 11.4%) was similar in both treatment arms, but higher in those patients treated with caspofungin for invasive infections by C. tropicalis (18.5% – rezafungin – vs. 31.8% – caspofungin–) and C. parapsilosis (7.1% vs. 29.6%). Differences were also observed in mycological cure rates at days 5 and 14, where rezafungin achieved higher mycological cure rates for candidiasis caused by N. glabratus and C. tropicalis. Outcomes by Candida species were not associated with treatment-specific MICs.

Results from the ReSTORE rezafungin phase 3 clinical study also included in vitro results against Candida, but no patients with C. auris infections were included. In a mini-review of in vitro data for Candida species, including C. auris, isolated during clinical trials of three new antifungal drugs, fosmanogepix, ibrexafungerp, and rezafungin, recently published, rezafungin showed a good activity against this fungal species.³² Furthermore, it has been demonstrated that FKS1 mutations raised rezafungin MICs by a smaller amount than anidulafungin and micafungin MICs in C. auris.⁵⁴

The PK/PD properties of echinocandins with poor penetration into peritoneal fluid

Intra-abdominal candidiasis should be addressed separately from candidaemia due to the peculiarity of the required penetration of antifungal drugs into the peritoneal cavity. Intra-abdominal concentrations may be further restricted in critically ill patients where pathophysiological factors alter normal drug distribution. However, echinocandins are recommended as first-line treatment in guidelines.²⁴ Controversial results about caspofungin concentrations in critically ill patients are found in the literature. Echinocandins are highly bound to albumin (>95%), thus, considering that only the unbound fraction is active and passively diffuses to the extravascular space, penetration into the peritoneal fluid is highly compromised. Moreover, the percentage of reduction in echinocandins peritoneal concentrations with respect to serum concentrations was reported to be approximately 33%.^7,8,34–36 Given that drug concentrations in peritoneal fluid are often suboptimal, there is a genuine risk of promoting the emergence of antifungal resistance in Candida isolates responsible for intra-abdominal candidiasis, making intra-abdominal microbiota a reservoir of non-susceptible isolates, as occurs with bacteria. Mean peritoneal concentrations of the three classical echinocandins were reported to be always below the mutant prevention concentrations. For all these reasons, abdominal candidiasis has been pointed out as a hidden reservoir of echinocandin resistance.⁸⁷ The likelihood of achieving effective exposures with current dosing regimens of first-generation echinocandins (anidulafungin, caspofungin, and micafungin) is also unclear, especially regarding resistant Candida species. It is clear that new agents and/or dosing strategies are needed for critically ill patients with candidaemia and/or invasive candidiasis. However, considering the published evidence, these data suggest that higher doses – an approach proposed by some authors – may be required to achieve adequate PK levels in peritoneal fluid. Furthermore, liposomal amphotericin B is advocated as first-line therapy in patients with sepsis/septic shock presenting with candidaemia or endophthalmitis, or with prior exposure to echinocandins and/or fluconazole, or with infections by N. glabratus.⁸⁷ Rezafungin's long half-life (∼5–6 days) and high front-loaded dosing allow for extended-interval dosing and high plasma drug concentrations early in therapy. This is due to the concentration-dependent efficacy of echinocandins for intra-abdominal candidiasis, which overcomes the limitations of previous echinocandins.

To sum up, due to the special characteristics of rezafungin, this new antifungal drug should be considered as first-line therapy in these circumstances over classical echinocandins in the ICU setting.

Haematologist's point of view

Patients with haematological malignancies are at high risk of invasive candidiasis, primarily due to an inadequate immune response. Neutropenia is a major risk factor for the development of invasive candidiasis. Neutropenia may be caused by the underlying haematologic disease itself, may be a side effect of antitumour treatment (cytotoxic drugs or radiotherapy), occur in the context of haematopoietic transplantation (both during the pre-engraftment period and in cases of graft-versus-host disease), or be a side effect of adjuvant medication, including antimicrobial drugs. Neutropenia is also a marker of chemotherapy intensity. Patients with chemotherapy-induced neutropenia associated with infection (known as febrile neutropenia) also exhibit other common risk factors for invasive candidiasis: use of broad-spectrum antibiotics, need for intravascular catheter cannulation for drug administration, development of post-chemotherapy mucositis, need for parenteral nutrition, or admission to critical care units. In this context, patients diagnosed with haematologic malignancies, especially those with acute leukaemia or haematopoietic transplant recipients, have been classically considered at high risk for developing invasive candidiasis.^73,83

The incidence and mortality of invasive candidiasis in patients with haematologic malignancies have decreased in recent years.⁷⁸ The first milestone associated with this change was the universalisation, in the early 1990s, of fluconazole as a prophylactic strategy in patients with high-risk haematologic malignancies, aiming to reduce the previously unacceptable incidence and mortality rates secondary to invasive candidiasis.^85,86,120 Moreover, the approval of new antifungal drugs and their early use in patients with high-risk febrile neutropenia have achieved better infectious control and a decrease in mortality associated with invasive candidiasis.¹²⁵ Despite this, reported mortality rates in invasive candidiasis in haematologic patients vary between 17% and 45%, with significant differences depending on the underlying haematologic disease and whether the patient is a recipient of a haematopoietic stem cell transplant.^44,98,105

Although various studies highlight that invasive candidiasis is currently a less significant problem than in the past, new challenges have emerged in recent years in the context of patients with haematological malignancies. The widespread use of fluconazole and posaconazole for antifungal prophylaxis in high-risk haematologic patients is associated with a decreased incidence of invasive candidiasis due to C. albicans, which has led to an epidemiological shift in the distribution of Candida species causing invasive candidiasis in these patients. It has been described an increase in breakthrough invasive candidiasis and a current predominance of Candida species other than C. albicans, including former Candida species (C. parapsilosis, N. glabratus, P. kudriavzevii), reaching between 66.9% and 90% of the isolates reported in various studies.^105,106 These species are associated with both intrinsic and extrinsic azole resistance. Although this epidemiological change does not seem to negatively impact haematological patients’ mortality, the epidemiological evolution could prompt a re-evaluation of our prophylactic approaches.⁶⁹

Simultaneously, in the last 30 years, the diagnosis and treatment of haematological malignancies have undergone extraordinary progress. The introduction of imatinib for the treatment of chronic myeloid leukaemia in 2001 paved the way for precision medicine, leading to a novel landscape of therapeutic strategies directed against specific targets, aiming to improve patient survival while minimising the toxicity associated with classical chemotherapy drugs.^29,114 Among the different groups of treatments, monoclonal, conjugated and bispecific antibodies, checkpoint inhibitors, small molecule inhibitors and, more recently, CAR-T therapy, stand out.^1,6,96,100 All these drug groups target specific cell lines or signalling pathways and have changed the natural history of various haematological malignancies, such as diffuse large B-cell lymphoma or chronic lymphocytic leukaemia. However, these treatments are associated with different toxicities on the immune system, which, together with their general reservation for patients in a relapsed or refractory status, has led to the emergence of new risk profiles for invasive mycoses in patients with haematologic malignancies.^9,63,119

Rezafungin, a next-generation echinocandin, has potent in vitro activity against most Candida species, including azole-resistant Candida species. Rezafungin combines the high efficacy of echinocandins with a prolonged half-life, a low rate of drug–drug interactions, and a favourable safety profile.⁹⁵ The clinical efficacy of rezafungin in the treatment of candidemia and invasive candidiasis was established in the randomised, controlled Phase 2 STRIVE and Phase 3 ReSTORE trials, which demonstrated that rezafungin is non-inferior to caspofungin for all-cause mortality.^128,130 However, the number of neutropenic patients included in these studies is low, with only 7.4% of the total patients. Additionally, haematological patients are underrepresented in these trials, partly due to the prophylactic antifungal treatment they usually receive, which is an exclusion criterion in most clinical trials. Nevertheless, there are no data suggesting a different response to rezafungin in these patients in terms of efficacy and safety, and no differences in response rate and serious treatment-related adverse effects were detected.

Rezafungin is of particular interest for the treatment of invasive candidiasis in haematological patients. Firstly, rezafungin can be safely administered in patients with renal impairment, as it is non-renally excreted, making it particularly useful in the context of renal insufficiency, a relatively frequent situation in haematological patients secondary to the administration of certain cytotoxic drugs such as cisplatin, methotrexate, melphalan, cyclophosphamide, or as a consequence of different immunosuppressive drugs such as calcineurin inhibitors (cyclosporine or tacrolimus).^93,116 Rezafungin has potential clinical use in the treatment of invasive candidiasis in patients under immunosuppressive treatment after allogeneic transplantation. It can also be safely administered in patients with hepatic insufficiency.⁵⁷ Furthermore, rezafungin does not affect the QTc interval. This makes its use in managing invasive candidiasis in patients receiving chemotherapy treatments that prolong the QT interval, such as arsenic trioxide or tyrosine kinase inhibitors like dasatinib or nilotinib, particularly interesting.^2,38,39,66

Secondly, an important issue with new targeted therapies is drug-to-drug interactions. Rezafungin is neither a substrate, inhibitor, nor inducer of cytochrome P450 enzymes. Additionally, interactions of rezafungin with various drugs, such as venetoclax, have been studied. Co-administration of rezafungin and venetoclax caused a reduction in its AUC or C_max, but did not result in a clinically significant reduction in drug exposure.⁴⁰ Venetoclax, an oral inhibitor of the anti-apoptotic BCL-2 protein, can be used for the treatment of acute myeloid leukaemia in combination with azacitidine as induction therapy in patients ineligible for intensive chemotherapy. This is a high-risk scenario for the development of invasive candidiasis, so the absence of interactions could potentially allow us to treat an infection without antifungal adjustment, thus not compromising our patient's clinical outcomes.

Finally, given the prolonged half-life of rezafungin that allows a weekly administration along with a favourable drug–drug interaction profile, its potential usefulness as antifungal prophylaxis in high-risk haematologic patients is under study. Current guidelines recommend the use of fluconazole or voriconazole/posaconazole as first-line antifungal prophylaxis for allogeneic haematopoietic stem cell transplant recipients. Additionally, prophylactic posaconazole is recommended for patients with graft-versus-host disease.^27,83 Azole prophylaxis decreases the incidence of invasive fungal infection in allogeneic haematopoietic stem cell transplant recipients.^{85,86,120,133,139} However, there are still some concerns, primarily regarding drug-to-drug interactions, toxicity, safety, and tolerability.¹⁵ Echinocandins are not routinely recommended as prophylactic strategies because they must be infused daily, and there is a lack of trials supporting their efficacy in this setting. In this context, the ReSPECT clinical trial, a multicentre, randomised, double-blind, phase 3 clinical trial, will evaluate the efficacy of rezafungin versus the standard of care (fluconazole or posaconazole plus trimethoprim–sulfamethoxazole) as a prophylactic strategy in adults undergoing allogeneic stem cell transplantation. The primary endpoint of fungal-free survival at day 90 aims to assess efficacy during the acute phase of allogeneic haematopoietic stem cell transplantation. This trial will be the first to evaluate the efficacy of an echinocandin in preventing invasive fungal infection in patients undergoing allogeneic haematopoietic stem cell transplantation and will open the door to investigating the potential benefits of antifungal prophylaxis with rezafungin in different settings.

Conclusions

Rezafungin has the potential to complement the existing antifungal toolbox and address many needs in the care of invasive mycoses, improving patient outcomes. The strengths of the echinocandins, combined with the prolonged half-life and robust safety profile of rezafungin, open several possibilities for future use.²³ The greatest promise of rezafungin lies in its prolonged half-life, favourable safety profile, and high initial drug exposure, which can reduce fungal burden and prevent or limit the development of resistance. As outlined in a recent European guideline,²⁴ the primary intended use of rezafungin is the targeted treatment of invasive candidiasis, with a particular emphasis on candidaemia. Furthermore, the weekly intravenous infusion of rezafungin has the potential to facilitate earlier hospital discharge and extended outpatient treatment, which will be of particular benefit to those with candidaemia without focal invasive candidiasis. A key benefit would be the treatment of candidiasis in cases requiring prolonged therapy, such as intra-abdominal candidiasis. For patients requiring daily echinocandin therapy, rezafungin can ease healthcare burdens, and for patients with resistant or refractory infections, the unique pharmacokinetic parameters of rezafungin may offer significant benefits. Patient populations that may benefit include those with abdominal surgery and anastomotic leak. However, the potential benefit of its use in forms of invasive candidiasis, such as endocarditis, endophthalmitis and other fungal sanctuary infections, requires more data as current information is limited to anecdotal cases or small case series.^{3,4,21,42,50,52,64,72,92,104,132,134} The use of rezafungin as a prophylactic agent to prevent invasive candidiasis, aspergillosis, and pneumocystosis, given the activity of rezafungin against Candida, Aspergillus, and Pneumocystis, could surpass current multidrug regimens in onco-haematology patients. Weekly dosing with limited drug–drug interactions may also be attractive for prophylaxis during the early phase of solid organ transplantation (e.g., liver). Furthermore, the theoretical risk of in vivo resistance development (FKS mutant selection) resulting from prolonged drug exposure with subtherapeutic concentrations for at least part of the treatment period is a valid concern. However, there have been no signals thus far, either in vitro or clinical studies, indicating a clinical impact of echinocandin resistance.

The availability of rezafungin with its prolonged antifungal action may represent a significant change in the pharmacological treatment of candidiasis and other invasive mycoses, as it combines the strengths of echinocandins with a prolonged half-life, a low likelihood of drug–drug interactions, and a robust safety profile. These properties may allow for earlier hospital discharge of patients with favourable clinical conditions and enable outpatient treatment when prolonged treatment duration is expected. When ongoing clinical trials are completed, and considering the results of STRIVE and ReSTORE clinical studies, rezafungin may also emerge as an outpatient parenteral antimicrobial therapy for long-term antifungal treatment of candidiasis and potentially other mycoses, such as aspergillosis, as well as for outpatient parenteral antimicrobial prophylaxis.²¹ The authors acknowledge the limitations of this review manuscript as it is not a systematic review or meta-analysis, and there is potential for bias in the selection of data and scientific articles by the authors. Additionally, clinical and microbiological data are continuously expanding and are limited or non-existent in some areas, and patients will be evaluated in future studies.

To conclude, the spectrum of invasive mycoses may change in the coming decades, associated with advances in medicine, the increase in the population at risk of mycoses, and climate changes, which may induce the emergence of new fungal pathogens and antifungal resistance. It is also likely that the treatment of invasive mycoses will evolve. We hope that rezafungin will fill important gaps in the current therapeutic options and contribute to improving patient survival due to its PK/PD advantages, limited drug–drug interactions, and good tolerability. However, further research is required to address the knowledge that will be filled once rezafungin is used in real-world clinical scenarios.

Conflict of interest

GQ has received funding and has participated in Advisory Boards or Speaker services for Astellas Pharma, Evidenze, Gilead, Merck, Sharp & Dohme (MSD), Palau Pharma (Noucor), Pfizer, and Scynexis.

RZ has not specific conflicts of interest related to the current manuscript but declare the following: RZ has received funding and has participated in Advisory Boards and/or Speaker services for Gilead, Pfizer and Mundipharma.

CG has received funding and participated in sponsored talks for Pfizer, MSD, Gilead, Shionogi, AdvanzPharma, bioMérieux, Viatris and Mundipharma; in Advisory Boards for Advanz Pharma and Pfizer; and has received grants from Gilead and Mundipharma.

XMM has no relevant affiliations or financial involvement with any organisation or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Acknowledgments

This study was supported by the Department of Education, Universities, and Research of the Basque Government (GIC21/24 IT1607-22) and by Mundipharma.