Identification of Novel Therapeutic Targets in the Clinically Predictive Vk*MYC Mouse Model of Multiple Myeloma
Authors: Chesi Marta , Garbitt Victoria , Bergsagel P. Leif 
Published: October 2019

Abstract
A genetic rearrangement of the MYC locus, resulting in dysregulated expression of MYC, is the most common mutation in human multiple myeloma (MM). The genetically engineered Vk*MYC mouse model is based on dysregulation of MYC, and has been extensively validated as a clinically and biologically faithful model of untreated MM. We previously reported 9 drugs or classes of drugs (DNA alkylators, glucocorticoids, proteasome inhibitors, IMiDs, nab-paclitaxel, histone deacetylase inhibitors, TACI-Ig, perifosine and SNS-032, a CDK2,7,9 inhibitor) with more than 20% partial response (PR) rate in Vk*MYC MM. Among those, the first 5 also have more than 20% PR in patients with MM for a positive predictive value of 56%. Although the HDACi did not show single agent activity in relapsed/refractory MM patients, the PANORAMA phase 3 study of panobinostat in combination with bor+dex has shown superior PFS compared to bor+dex, suggesting the possibility that HDACi may have shown single agent activity if tested in a less advanced clinical setting. In contrast, 11/12 drugs that have less than 20% PR in Vk*MYC MM also have less than 20% PR in patients with MM for a NPV of 92%. Confident that drugs with activity in Vk*MYC mice will likely be effective in the treatment of MM, we have used this model to study novel drugs for prioritization in clinical trials.
Pim kinases are constitutively active serine/threonine kinases identified using retroviral mutagenesis as potent suppressors of MYC induced apoptosis and can be inhibited by the pan-PIM kinase inhibitor LGH447. KPT-276 is a selective inhibitor of nuclear export CRM1/XPO1 inhibitor. Both drugs are active in Vk*MYC MM and are showing early evidence of promising clinical activity, increasing the PPV of the Vk*MYC model of MM to 64% (or 73% if HDACi are considered clinically active).
Also active in the Vk*MYC model were the bromodomain inhibitors GSK I-BET151, CPI-203, CPI-456, OTX-015, which, like JQ1, compete with acetylated histones for the binding to BRD4, inhibiting super-enhancer activity and MYC transcription. The histone methyltransferase enhancer of Zeste homolog 2 (EZH2) induces transcriptional repression through histone H3 lysine 27 trimethylation (H3K27me3) at the promoter of silenced genes. Activating mutations in the SET domain of EZH2 are prevalent in germinal center DLBCL and follicular lymphoma but have not been identified in MM, where the frequent dysregulation of MMSET by t(4;14) translocation or biallelic deletion of UTX are thought to play a tumorigenic role equivalent to EZH2 activation. EZH2 inhibitors have demonstrated activity against lymphoma with EZH2 activating mutations. Interestingly, we found the EZH2 inhibitor CPI-169 active against Vk*MYC MM, identifying EZH2 as a promising new epigenetic target in MM.
EDO-S101 is a novel drug resulting from the fusion of a molecule of bendamustine with a molecule of vorinostat, with the aim of increasing the efficacy of the alkylator through the HDACi-mediated chromatin relaxation that would make DNA more accessible to the damaging effect of bendamustine. It induced a high rate of response in Vk*MYC MM that was sustained for more than 3 months in mice receiving only 2 doses, one week apart. Remarkably EDO-S101 is the only drug we have identified with single agent activity in the very aggressive, multi-drug resistant Vk12653 transplant model of relapsed/refractory MM.
Finally, 2 agents (anti-murine PD-L1 and LCL161) that primarily modulate the immune microenvironment, with little to no direct anti-MM activity, also induced responses. Anti-PD-L1 blocks immunosuppressive signaling from MM cells to PD1 on T cells. LCL161 is an IAP antagonist that leads to constitutive activation of the alternative NFkB pathway with direct stimulation of the innate and adaptive immune systems. Although antibodies to CD38 are among the most exciting new agents for the treatment of MM, they cannot be evaluated in Vk*MYC MM since murine normal or malignant plasma cells do not express CD38. In contrast while SLAMF7 is expressed in Vk*MYC MM, there is no murine equivalent of elotuzumab.
In summary, we report 7 new drugs with single agent activity in Vk*MYC MM, nearly doubling the number of drugs or classes of drugs with promising anti-MM activity. Two (LGH447 and KPT-330) are already beginning to demonstrate clinical efficacy in MM. All of these agents should be prioritized for rapid evaluation in clinical trials of patients with MM.

Disclosures
Bergsagel: Novartis: Research Funding; Constellation Pharmaceutical: Research Funding; OncoEthix: Research Funding; MundiPharma: Research Funding.