title: Nonproteasomal targets of the proteasome inhibitors bortezomib and carfilzomib: a link to clinical adverse events.
abstract: OBJECTIVE
  Bortezomib (Velcade), a dipeptide boronate 20S proteasome inhibitor and an approved treatment option for multiple myeloma, is associated with a treatment-emergent, painful peripheral neuropathy (PN) in more than 30% of patients. Carfilzomib, a tetrapeptide epoxyketone proteasome inhibitor, currently in clinical investigation in myeloma, is associated with low rates of PN. We sought to determine whether PN represents a target-mediated adverse drug reaction (ADR).
  
  
  METHODS
  Neurodegenerative effects of proteasome inhibitors were assessed in an in vitro model utilizing a differentiated neuronal cell line. Secondary targets of both inhibitors were identified by a multifaceted approach involving candidate screening, profiling with an activity-based probe, and database mining. Secondary target activity was measured in rats and patients receiving both inhibitors.
  
  
  RESULTS
  Despite equivalent levels of proteasome inhibition, only bortezomib reduced neurite length, suggesting a nonproteasomal mechanism. In cell lysates, bortezomib, but not carfilzomib, significantly inhibited the serine proteases cathepsin G (CatG), cathepsin A, chymase, dipeptidyl peptidase II, and HtrA2/Omi at potencies near or equivalent to that for the proteasome. Inhibition of CatG was detected in splenocytes of rats receiving bortezomib and in peripheral blood mononuclear cells derived from bortezomib-treated patients. Levels of HtrA2/Omi, which is known to be involved in neuronal survival, were upregulated in neuronal cells exposed to both proteasome inhibitors but was inhibited only by bortezomib exposure.
  
  
  CONCLUSIONS
  These data show that bortezomib-induced neurodegeneration in vitro occurs via a proteasome-independent mechanism and that bortezomib inhibits several nonproteasomal targets in vitro and in vivo, which may play a role in its clinical ADR profile.
authors.affiliation: Onyx Pharmaceuticals, Inc South San Francisco, California 94080, USA.
authors.forename: Shirin
authors.initials: S
authors.lastname: Arastu-Kapur
authors.forename: Janet L
authors.initials: JL
authors.lastname: Anderl
authors.forename: Marianne
authors.initials: M
authors.lastname: Kraus
authors.forename: Francesco
authors.initials: F
authors.lastname: Parlati
authors.forename: Kevin D
authors.initials: KD
authors.lastname: Shenk
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Lee
authors.forename: Tony
authors.initials: T
authors.lastname: Muchamuel
authors.forename: Mark K
authors.initials: MK
authors.lastname: Bennett
authors.forename: Christoph
authors.initials: C
authors.lastname: Driessen
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Ball
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Kirk
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000008:administration & dosage / Q000009:adverse effects; D001897:Boronic Acids / Q000008:administration & dosage / Q000009:adverse effects*; D000069286:Bortezomib; D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000008:administration & dosage / Q000009:adverse effects; D016503:Drug Delivery Systems; D064420:Drug-Related Side Effects and Adverse Reactions / Q000209:etiology*; D056945:Hep G2 Cells; D006801:Humans; D008297:Male; D008954:Models, Biological; D009842:Oligopeptides / Q000008:administration & dosage / Q000009:adverse effects*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D061988:Proteasome Inhibitors; D011719:Pyrazines / Q000008:administration & dosage / Q000009:adverse effects*; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D000970:Antineoplastic Agents; D001897:Boronic Acids; D009842:Oligopeptides; D061988:Proteasome Inhibitors; D011719:Pyrazines; D000069286:Bortezomib; C524865:carfilzomib; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex