Pre-Clinical Activity of the Novel, First-in-Class p97 Inhibitor, CB-5083, in Multiple Myeloma

Hematological malignancies such as multiple myeloma (MM) have an increased reliance on the ubiquitin proteasome system (UPS) presumably as a consequence of their high protein synthetic and secretory burden. Chemical agents that target the proteasome, such as bortezomib and carfilzomib, have been successful in treating multiple myeloma; however patients treated with these drugs ultimately relapse. The AAA-ATPase p97/VCP (p97) facilitates ATP-dependent extraction and degradation of ubiquitinated proteins destined for proteasomal elimination. In addition to ubiquitin-dependent protein degradation, p97 is also closely involved in other aspects of protein homeostasis, including endoplasmic reticulum-associated degradation (ERAD) and autophagy. Pharmacologic inhibition of p97 provides a compelling therapeutic approach for hematological malignancies that rely on tight regulation of protein homeostasis as a component of their survival.

CB-5083 is a novel small molecule inhibitor of p97 ATPase activity with nanomolar enzymatic and cellular potency. Treatment of cancer cells with CB-5083 causes a dramatic increase in poly-ubiquitinated proteins as well as an accumulation of substrates of the UPS and ERAD. CB-5083 causes a profound induction of the unfolded protein response (UPR) with consequent activation of the DR5 death receptor, caspase 8, caspase 3/7 and ultimately cell death. Induction of the UPR occurs to a greater magnitude with CB-5083 when compared to the proteasome inhibitor, bortezomib, suggesting the potential for increased efficacy in cancers with sensitivity to UPR-mediated cell death. In addition, activation of apoptosis and cell death occur more rapidly with CB-5083 than with bortezomib. Sequencing of cell lines made resistant to CB-5083 reveals missense mutations mapping to the D2 ATPase site in p97, supporting on-target association with cytotoxicity.

In an expanded panel of MM cell lines there is no correlation between the cytotoxic sensitivity to CB-5083 and the cytotoxic sensitivity to proteasome inhibitors, suggesting differential mechanisms of cytotoxicity and potential activity of CB-5083 in proteasome inhibitor resistant settings. Compared to myeloma cell lines, CB-5083 has reduced cytotoxic potency in immortalized stromal cell lines and in patient-derived CD138-negative bone marrow mononuclear cells. Furthermore, unlike the reduced potency demonstrated by carfilzomib in the context of MM cell-bone marrow stromal cell (BMSC) interactions, the cyto-reductive potential of CB-5083 is unaffected in co-cultures of MM cells with patient-derived BMSCs or immortalized BMSCs from healthy donors.

In vivo, CB-5083 is orally bioavailable, shows a pharmacodynamic effect in tumor tissue (as measured by poly-ubiquitin accumulation) and demonstrates robust anti-tumor activity across several MM models. CB-5083 treatment of mice bearing subcutaneous xenografts leads to tumor stasis and regression in RPMI8226 and AMO1 MM models, respectively. In advanced models of disseminated, ortho-metastatic disease, intermittent oral administration of CB-5083 demonstrates significant inhibition of myeloma burden and improves survival, with an overall efficacy profile that compares favorably to that of clinically approved proteasome inhibitors. Furthermore, in the Vk*Myc genetically engineered mouse model of MM, treatment with CB-5083 results in a significant reduction in M-spike by 55%. Combination treatment of mice bearing the RPMI8226 subcutaneous xenograft model with CB-5083, dexamethasone and lenalidomide results in tumor regression. Taken together, these data demonstrate that CB-5083 is a potent and selective inhibitor of the p97 ATPase with robust activity in vitro and in vivo in numerous MM models and strongly support clinical evaluation. Based on these observations, a phase 1 dose-escalation trial has recently been initiated and is currently underway in patients with relapsed/refractory multiple myeloma.