Anti-Tumor Phagocytic Cell Activation in Multiple Myeloma By the IAP Antagonist LCL161: Results of a Phase II Clinical Trial

Dexamethasone is a powerful anti-inflammatory, immunosuppressive agent widely used in the treatment of MM with pleiotropic effects on both the tumor, and host immune cells. We report on a completely novel approach to immune modulation using an agent with almost directly opposite effects to glucocorticoids on innate immune cells. LCL161 is an IAP antagonist targeting the cellular inhibitor of apoptosis proteins cIAP1 and -2 (cIAP1/2) that in a phase I study in solid tumors was well tolerated with a cytokine release syndrome as the dose limiting toxicity. We previously reported that loss of cIAP1/2 in MM cells results in the stabilization of NIK and activation of the non-canonical NFkB pathway, a pathway we found frequently activated by a promiscuous array of mutations in multiple myeloma (MM). As expected, we found that LCL161 has no direct anti-tumor activity against MM cells in vitro and ex vivo. However, the non-canonical NFkB pathway is also a key activator of the innate immune response. Remarkably, we found that in vivo LCL161 has dramatic, apoptosis-independent activity associated with marked phagocytosisagainst MM that develops spontaneously in an immunocompetent genetically engineered mouse model of MM (Vk*MYC). Cyclophosphamide (Cy), which has been shown to induce an acute secretory activating phenotype in tumor-associated macrophages, markedly potentiated the anti-tumor effects of LCL161. We show that phagocytes (depleted by liposomal clodronate), but not adaptive immune cells, are required for the anti-tumor effect. Consistently we show that LCL161 activates dendritic cells and macrophages in vitro, and in vivo, and upregulates production of inflammatory cytokines.

A phase 2 clinical trial of LCL161 was conducted in 24 patients with relapsed MM previously exposed to both IMiDs and PIs, but less than 5 prior lines of therapy. Patients received LCL161 1200mg po weekly, with Cy 500mg po weekly added for progressive disease or lack of response. Single agent LCL161 was well-tolerated, with grade 3 or 4 non-hematologic toxicity in four patients, and following the addition of cyclophosphamide in 9 patients. No patients responded to single agent LCL161, and the best response was stable disease in 6/24. In 21 patients, Cy was added after a median of 2 cycles, in whom the best response was stable disease or better in 18/21 (p<0.001 compared to LCL161 alone), and a PR or better (including 1 VGPR and 1 CR) in 5/21 (p<0.001 compared to LCL161 alone). Six months following the initiation of Cy, 35% of patients remain event-free. Two patients had stable disease for more than nine months after stopping treatment. One, relapsing after VDT-PACE, HDM200, RVd x 3 years with t(11;14) and del17p, received only three months of LCL161, the last month in combination with Cy. He then achieved a PR, reconstitution of his uninvolved IgG to normal levels for the first time in more than 3 years and still has not progressed one year after stopping therapy. Similar changes in serum and bone marrow inflammatory cytokines and gene expression were observed in patients following treatment with LCL161 to those observed in the mice. We conclude that LCL161 is able to modulate the tumor microenvironment to exert long-term disease control in combination with Cy, and deserves further study in combination with agents that more directly depend on phagocytic cell activity (e.g., monoclonal antibodies, inhibitors of "Don't eat me" signals).