The Selective Proteasome Inhibitor Carfilzomib Is Well Tolerated in Experimental Animals with Dose Intensive Administration

Carfilzomib (CFZ) is a selective inhibitor of the chymotrypsin-like (CT-L) activity of the proteasome. Phase I trials have demonstrated that doses of carfilzomib from 11 – 20 mg/m² were well tolerated, inhibited blood CT-L activity by ≥80%, and resulted in objective anti-tumor responses in patients with multiple myeloma (MM) and non-Hodgkin’s lymphoma (NHL) (Blood 110:409 & 411). In order to further characterize the safety profile of CFZ, studies in mice, rats and monkeys were conducted to assess the toxicologic consequences of acute and chronic administration. In both rats and monkeys, a single dose of CFZ (equivalent to 24 mg/m²) resulted in > 80% inhibition of CT-L activity in blood and tissues with recovery occurring in most tissues with a t_1/2 of 24 hr. At this dose, monkeys developed a pre-renal azotemia within 24 hr, which was characterized by increases in serum creatinine, blood urea nitrogen (BUN) levels and urine specific gravity. These changes resolved in 72 hr, were not associated with histologic changes in the kidney, and were not exacerbated by consecutive daily administrations, suggesting that CFZ may not have a direct nephrotoxic effect. A single dose in either rats or monkeys induced a transient drop in platelets within 48 hr that resolved within 7 days and neither the level of nor recovery from this thrombocytopenia were affected by consecutive daily administration. This phenomenon has also been noted with bortezomib (BTZ), and is observed clinically with both compounds, suggesting that thrombocytopenia is a hallmark of inhibition of proteasome CT-L activity. However, in contrast to BTZ, which has been reported to induce both lymphopenia and neutropenia in animals, CFZ treatment in both rats and monkeys resulted in acute increases in circulating neutrophils and monocytes which rose within 24 hr of a single dose and returned to baseline in 3 – 4 days. We were unable to detect a significant induction of cytokines or chemokines capable of mediating leukocyte fluxes. However, since the acute phase proteins fibrinogen and C-reactive protein were elevated during the same time period of leukocyte flux, cytokines may play a role in this response. In chronic studies, CFZ was administered on Days 1, 2, 8, 9, 15, and 16 of a 28-day cycle to rats and monkeys for 6 and 9 cycles, respectively. This dosing schedule results in anti-tumor activity in mouse models of MM, NHL and solid tumors and is currently being utilized in Phase 2 studies in MM and solid tumors. In these chronic studies, transient pre-renal azotemia and increases in circulating neutrophils were manifest in monkeys at 24 mg/m² following the first dose of both the first and last cycles of treatment. However, no histologic signs of nephropathy were seen in the kidneys of rats receiving 6 cycles of treatment. In addition, chronic administration of CFZ at doses equivalent to those currently administered in the clinic did not impact on the neurobehavioral function of rats and monkeys. There was no severe degeneration in the dorsal root ganglion or in the sciatic, sural, and peroneal nerves of CFZ treated rats. The data with CFZ are in distinct contrast to the axonal swelling, demyelination and severe neural degeneration that is induced by BTZ in rodents and monkeys. In the clinic, peripheral neuropathy associated with CFZ treatment appears to be less severe and possibly less frequent than what is seen with BTZ. Overall, studies in rodents and monkeys have been largely predictive of the clinical experience to date with CFZ. Together with anti-tumor activity in human tumor xenograft models, the toxicologic data support that CFZ administered at therapeutic doses is well tolerated and prolonged treatment of patients with this agent will be feasible.