Proteasome Inhibitor Associated Neuropathy Is Mechanism Based
Authors: Silverman Lee , Csizmadia Vilmos , Brewer Katherine , Simpson Chris , Alden Carl 
Published: April 2020

Abstract
Background: Peripheral neuropathy is a common side effect of chemotherapeutics, particularly taxanes, platinums, vinca alkaloids, and proteasome inhibitors (PIs). There are currently three chemical scaffolds of PIs in clinical development: leucine boronates, lactacystin analogs, and epoximicin derivatives. Among these, only a leucine boronate, VELCADE, is currently approved for use in humans. Approximately 39% of patients treated with VELCADE develop peripheral neuropathy. Preclinical data suggest that peripheral neuropathy may develop as a consequence of functional effects in the dorsal root ganglia (Silverman et al. 2006; Toxicologic Pathology34:989). Moreover, in vitro data suggests that these effects may be the result of a reorganization of cytoskeletal elements and an accumulation of ubiquitinated proteins in the neuronal cytoplasm (Csizmadia et al. 2008; Neurotoxicology29:232). In vivo and in vitro data suggest that PI-associated neuropathy develops following treatment with PIs of differing chemical scaffolds, consistent with a mechanism based effect. A better understanding of the mechanism underlying PI-associated peripheral neuropathy could potentially lead to improved management strategies for this adverse event while maintaining therapeutic benefit.
Aims:
To investigate if PI-associated neuropathy is mechanism based (a consequence of proteasome inhibition), and If mechanism based, to further understand the mechanism by which proteasome inhibition results in peripheral neuropathy.
Methods: The effects of treating in vitro neuronal cell cultures (PC-12 rat pheochromocytoma cell line treated with nerve growth factor [NGF] in order to induce neuronal differentiation) with leucine boronate, lactacystin analog, or epoximicin PIs, as well as with taxol and cisplatinum, were examined by Western immunoblot and immunohistochemistry specific for ubiquitinated proteins. Additionally, the morphologic effects on the peripheral nervous system of mice treated with leucine boronate or lactacystin analog PIs were examined.
Results: Treatment of PC-12 cells with leucine boronate, lactacystin analog or epoximicin PIs but not with taxol or cisplatinum resulted in perinuclear accumulations of ubiquitinated proteins. In in vivo models, morphologic lesions were observed in the peripheral nervous system of mice treated with both leucine boronate and lactacystin analog PIs. In a time course study with a prototypical PI, dorsal root ganglia was identified as the primary target leading to secondary peripheral nerve degeneration. Routine hematoxylin and eosin histologic analysis showed an accumulation of eosinophilic material within the dorsal root ganglia neurons. Immunohistochemistry showed these accumulations to contain ubiquitinated proteins, consistent with the in vitro observations. The in vivo effects were similar following treatment with either a leucine boronate or a lactacystin analog PI.
Summary: Our in vitro and in vivo studies identified a morphologic effect which among the chemotherapeutics which cause peripheral neuropathy is uniquely associated with PIs. Moreover, these findings were seen regardless of the chemical scaffold.