Preclinical Studies and Phase I Trial of Vactosertib in Combination with Pomalidomide in Relapsed Multiple Myeloma: A Corticosteroid-Free Approach By Targeting TGF-β Signaling Pathway

Multiple Myeloma (MM) is a neoplasm of terminally differentiated plasma cells which proliferate in a permissive bone marrow environment characterized by immunosuppression and osteoclast activation. Although malignant plasma cells do not harbor any mutation in the Transforming Growth Factor-beta (TGF-β) pathway, increased TGF-β secreted by MM cells lead to impaired immune surveillance and promotion of catabolic bone remodeling allowing myeloma progression (Kyrtsonis MC et al. 1998). Here, we conducted preclinical studies in the syngeneic 5T3MM immunocompetent mouse model assessing the efficacy of vactosertib (Vacto), a TGF-β type I receptor antagonist, single agent activity as well as synergistic activity with the third generation immunomodulatory drug, pomalidomide (Pom) and a subsequent corticosteroid-free phase I study to test safety and preliminary efficacy of this combination. (NCT03143985). Methods Preclinical: Mice bearing 5T33MM cells expressing luciferase were treated with Vacto/Pom and the combination for 3 weeks, and evaluated for MM growth by bioluminescence imaging (BLI). Cellular and molecular assays were performed in human RPMI8226 and U266 as well as murine 5T33MM cells via apoptosis, real-time PCR and Western blotting. Peripheral blood monoclonal protein concentration, M-spike, was measured by ELISA. Distal femur trabecular bone structure was assessed by 3D micro-CT. Phase I: pts with relapsed MM with at least two lines of therapies enrolled on a modified Fibonacci 3 + 3 dose escalation design and received escalating dosages of Vacto, 60 mg/d, 120 mg/d and 100 mg BID and 200 mg BID in combination with standard dose of Pom (4mg) without corticosteroids. The study objectives were to assess safety, recommended phase 2 dose, and efficacy of Vacto/Pom in compared to historical control of Pom without corticosteroids (PFS-6: 20% in randomized Phase II study by Richardson et al. Blood. 2014). Vacto tablets, taken daily for 5 days followed by 2 days off, was administered in 28-day cycles, repeated for 6 cycles or until progression of disease or intolerable toxicity. There was no fasting requirement after the first two dose levels. Results:Preclinical: Vacto attenuated the growth and viability of human and murine MM cells by inducing apoptosis and inhibited TGF-β-induced activation of Smad2/3 in MM cells in vitro. In the 5T33MM preclinical model, Vacto inhibited MM progression, as single agent, as measured by peripheral blood monoclonal protein conce