Bortezomib-induced peripheral neurotoxicity: A neurophysiological and pathological study in the rat

Abstract

Bortezomib is a new proteasome inhibitor with a high antitumor activity, but also with a potentially severe peripheral neurotoxicity. To establish a preclinical model and to characterize the changes induced on the peripheral nerves, dorsal root ganglia (DRG) and spinal cord, bortezomib was administered to Wistar rats (0.08, 0.15, 0.20, 0.30 mg/kg/day twice [2q7d] or three times [3q7d] weekly for a total of 4 weeks). At baseline, on days 14, 21 and 28 after the beginning the treatment period and during a 4-week follow-up period sensory nerve conduction velocity (SNCV) was determined in the tail of each animal. Sciatic nerve, DRG and spinal cord specimens were processed for light and electron microscope observations and morphometry.

At the maximum tolerated dose bortezomib induced a significant reduction in SNCV, with a complete recovery at the end of the follow-up period.

Sciatic nerve examination and morphometric determinations demonstrated mild to moderate pathological changes, involving predominantly the Schwann cells and myelin, although axonal degeneration was also observed. Bortezomib-induced changes were also observed in DRG and they were represented by satellite cell intracytoplasmatic vacuolization due to mitochondrial and endoplasmic reticulum damage, closely resembling the changes observed in sciatic nerve Schwann cells. Only rarely did the cytoplasm of DRG neurons has a dark appearance and clear vacuoles occurring in the cytoplasm. Spinal cord was morphologically normal.

This model is relevant to the neuropathy induced by bortezomib in the treatment of human malignancies and it could be useful in increasing our knowledge regarding the mechanisms underlying bortezomib neurotoxicity.

Introduction

The ubiquitin-proteasome pathway is the major proteolytic system in the cytosol and nucleus of eukaryotic cells and, in recent years, the proteasome has been identified as a key target for cancer therapy. Proteasome inhibition is an attractive approach to cancer therapy because it can potentially interact with multiple signaling pathways that are critical for tumor cell growth and survival, including cell cycle regulation (King et al., 1996), apoptosis (Orlowski, 1999), cell adhesion (Read et al., 1995), transcription (Desterro et al., 2000) and angiogenesis (Dulic et al., 1994). Moreover, proteasome inhibitors induce apoptotic cell death in cancer cells at doses which are comparatively non-toxic to most normal cells (Voorhees et al., 2003).

Bortezomib is the first of the class of proteasome inhibitors entered into clinical evaluation that have been shown to be cytotoxic to several tumor types (Voorhees et al., 2003). Bortezomib reversibly inhibits proteasome function by targeting the chymotryptic-like site of the proteasome, with little affinity for other proteases such as chymotrypsin and thrombin. Because of the benefit observed in patients with relapsed or refractory myeloma in phase 1 and 2 trials (Jagannath et al., 2004, Orlowski, 2004, Richardson et al., 2003) the drug was fast-tracked by the Food and Drug Administration (FDA), making it available to patients with this kind of advanced myeloma in May 2003 (Dispenzieri, 2005).

The fine mechanism of the toxic action of bortezomib on cancer and non-cancer cells is not completely understood, but one of the major dose-limiting toxicities of bortezomib is peripheral neuropathy (Jagannath et al., 2004, Kondagunta et al., 2004, Richardson et al., 2003, Shah et al., 2004). Despite the clinical relevance of this side effect, the present knowledge of the clinico-pathological features of bortezomib-induced peripheral neurotoxicity is very limited. In fact, no detailed description of the peripheral nerve damage has yet been reported, although in most cases sensory neuropathy occurred in treated patients. No animal models of bortezomib-induced peripheral neurotoxicity have been published so far.

In the present study we used neurophysiological and histopathological methods to investigate the effect of different schedules of bortezomib administration on the nervous system of the rat with the aim of establishing a reliable model of bortezomib neurotoxicity which can then be used to better understand the pathogenesis of the damage and to investigate possible neuroprotective strategies.