Abstract IA21: Immunomodulatory therapy of multiple myeloma with IAP antagonists
Authors: Chesi Marta , Fooksman David , Bergsagel Peter Leif 
Published: August 2018

Abstract
Multiple myeloma (MM) is a tumor of terminally differentiated plasma cells that underwent affinity maturation and class switch recombination processes in germinal centers before homing to the bone marrow (BM), where they secrete large amounts of monoclonal immunoglobulins, clinically detected as an M-spike by serum protein electrophoresis (SPEP). Clinical manifestations of MM also include renal impairment, anemia, and bone disease, the latter resulting from abnormal osteoclast activation, which highlights the complex crosstalk between MM cells and their BM microenvironment.
Chromosomal translocations into the immunoglobulin loci are the primary oncogenic events in ~50% of MM cases and are thought to occur as a result of errors during somatic hypermutation or class switch recombination. The other half of MM is characterized by trisomies of odd-numbered chromosomes. Both of these genomic abnormalities are present in the premalignant phase of MM called MGUS, which progresses to MM at a rate of 1% per year. Progression events include mutations of genes in the RAS/MAPK pathways; chromosomal rearrangements of the MYC locus; and mutations, amplifications, or biallelic deletions of genes in the NFkB pathway. Among the latter, the most common are deletions of TRAF3 or cIAP1 and -2 (BIRC2 and -3) and amplification or translocations of NIK (Map3k14), all resulting in NIK stabilization, phosphorylation of IKKα, processing of p100 to p52, NFKB2 nuclear translocation, and activation of the noncanonical NFKB pathway. This can be induced in MM cell lines in vitro by treatment with SMAC mimetics or IAP antagonists (IAPa), which induce cIAP1 auto-ubiquitination and degradation. We postulated that the resulting constitutive activation of the noncanonical NFKB pathway would render MM cells independent on the BM-localized survival signals provided by BAFF and APRIL, leading to extramedullary dissemination. We tested this hypothesis in vivo by exposing Vk*MYC mice with BM-localized MM to the IAP antagonist LCL161 and monitoring disease progression.
In the immunocompetent Vk*MYC transgenic mouse model MYC expression is sporadically induced in germinal center cells by AID-dependent somatic hypermutation, leading to progressive accumulation of monoclonal plasma cells restricted to the BM that can be monitored over time by SPEP and M-spike quantization, as is done clinically in MM patients. Through a very rigorous testing of drugs whose clinical activity in MM has been assessed in clinical trials, we have also validated Vk*MYC mice as a faithful model to predict drug activity in MM, with a positive predictive value of 73% and a negative predictive value of 92%. To our surprise, treatment of Vk*MYC mice with the IAPa LCL161 did not promote MM extramedullary dissemination as we predicted, but rather induced a significant reduction in M-spike levels, indicative of reduction of tumor burden. Based on these preclinical data, we conducted a phase 2 clinical trial of LCL161 in combination with cyclophosphamide in relapsed/refractory MM patients. Results from 25 patients enrolled in the study demonstrated a response in five patients (1CR, 1 VGPR, 2PR, 1MR) and a median PFS of 10 months.
Consistent with in vitro data on MM cell lines, LCL161 treatment did not promote direct apoptosis of Vk*MYC cells but phagocytosis of live cells, detectable by intravital microscopy within four hours after treatment. While LCL161 treatment of BM-derived macrophages (MΦ) in vitro promoted the acquisition of an inflammatory phenotype, with increased expression of IL6, IL12, and CD86 that resembled activation with CD40 agonists, it did not increase their phagocytic ability. In contrast, exposure of MΦ to LCL161 treated tumor cells or to the conditioned medium collected after LCL161 treatment enhanced phagocytosis. We therefore performed gene expression analysis on MM cells collected from Vk*MYC mice and MM patients before and after LCL161 treatment and identified marked upregulation of NFKB target genes, as expected, but also a type-I interferon (IFN) signature. Interestingly, the presence of blocking antibody to the IFN receptor subunit 1 (aIFNAR1) during LCL161 treatment of tumor cells inhibited their phagocytosis in vitro, and pretreatment of Vk*MYC mice with aIFNAR antibody abrogated LCL161 activity in vivo. Pretreatment of Vk*MYC mice with liposomal clodronate, known to deplete phagocytic cells, also abrogated LCL161 activity, indicating that MΦ-dependent phagocytosis, stimulated by the LCL161 induced tumor cell autonomous IFN response, is required for the anti-MM effects of LCL161.
LCL161 has been shown to promote T-cell function in vitro, but in our hand LCL161 retained full anti-MM activity in T-cell deficient mice. However, while ~15% of immunocompetent mice were cured by just two weeks of LCL161 treatment, none of the T-cell deficient mice were, indicating a role for T cells in preventing disease recurrence. Consistently, rechallenge of cured mice with specific MM tumors induced expansion of IFNγ-producing T cells, indicative of reactivation of immunologic memory. We were surprised by the hyporesponsiveness of T cells to LCL161 in vivo and attributed it to the high levels of expression of the coinhibitory molecule PD1 we noted on both CD4 and CD8 cells irrespective of treatment. We also detected high levels of PDL1 on most CD11b+ myeloid cells as well as on a fraction of MM cells, which increased following LCL161 treatment. We therefore sought to combine LCL161 with an antibody against PD1. Remarkably, the combination of LCL161 with aPD1 was curative in all the mice that completed the two-week treatment, and we are in the process of translating this discovery to the clinic by opening a new trial of LCL161 in combination with aPD1.
Citation Format: Marta Chesi, David Fooksman, Peter Leif Bergsagel. Immunomodulatory therapy of multiple myeloma with IAP antagonists [abstract]. In: Proceedings of the Second AACR Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; May 6-9, 2017; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(24_Suppl):Abstract nr IA21.