The Vk*MYC Mouse Model of Myeloma Identifies Successful Combination Therapies for the Treatment of Bortezomib Refractory Myeloma Patients
Authors: Chesi Marta , Palmer Stephen , Garbitt Victoria , Bergsagel P. Leif 
Published: October 2019

Abstract
Abstract 3015
Several preclinical models are available to assess the efficacy of novel anti-myeloma therapies, each of them with specific utilities. The most commonly utilized are: 1) Human myeloma cell lines (HMCLs) offer the advantage of being easily manipulated and well genetically characterized and therefore are best used to provide in-vitro target validation and to demonstrate specific target inhibition by a drug. On the other hand, they are highly proliferative and do not recapitulate the complexity of the human disease in an endogenous micro-environment. 2) Xenograft studies, in which HMCLs are injected into immunodeficient mice, serve the purpose of demonstrating that in-vivo target inhibition can be achieved under physiological conditions, but again fail to represent the indolence of human MM and its complex interaction with the BM stromal cells. Drug screenings performed on both these models tend inevitably to overestimate the antimyeloma activity of compounds that simply inhibit proliferation, but are ineffective in targeting the bulk of MM tumor. 3) Direct cytotoxic studies on primary patients cells, alone or in co-culture with stroma cell lines, are useful in assessing a pro-apoptotic activity in a way that is completely independent on proliferation. However, once again this in-vitro model does not capture the complete biology of human MM.
The immuno-competent Vk*MYC mouse model of myeloma has already demonstrated high biological fidelity to the human disease, making it an ideal model to study the behavior of myeloma cells in the context of a native microenvironment and immune system. Furthermore, this model offers the advantage of studying both indolent, BM localized, untreated MM (primary MM), and, with the use of transplants into syngeneic hosts, advanced, more proliferative and refractory disease (secondary MM). Using the primary MM model, we have assessed the anti-myeloma activity of 30 known and novel compounds, many of which currently in clinical trials, and found that agents effective in the treatment of patients are active in this model, whereas agents ineffective in the treatment of patients are not. From this study, the most promising class of novel agents in clinical trials are the HDAC inhibitors vorinostat and panobinostat. Based on this promising single agent activity, we sought to model the effects of combination therapy in the secondary myeloma models that we specifically generated to be bortezomib refractory or multidrug resistance. Although transplanted mice treated with full dose single agent bortezomib, vorinostat or panobinostat showed no response and died within three weeks post transplant, recipient mice treated with bortezomib in combination with either vorinostat or panobinostat achieved complete response and are still alive 10 weeks post-transplant.
We conclude that the bortezomib + HDAC inhibitors is an active combination that overcomes in vivo bortezomib resistance. Furthermore, the Vk*MYC model provides an excellent platform for the development of novel combinations aimed to the treatment of refractory MM disease.

Disclosures:
Bergsagel: Amgen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Genentech: Membership on an entity's Board of Directors or advisory committees; Millennium: Speakers Bureau; Novartis: Speakers Bureau.