Background: Immunosuppression and osteoclast activation are two hallmarks of the bone marrow environment in Multiple Myeloma (MM). Corticosteroids have been used historically as part of anti-myeloma regimens due to their anti-plasma cell activity, however they potentially could suppress immune system and activate osteoclast further; therefore there is an unmet need for corticosteroid-free approaches in the era of emerging anti-cancer immunotherapy modalities. There is an abundance of Transforming Growth Factor-beta (TGF-β), a crucial cytokine in suppression of immune system as well as catabolic bone remodeling, in the MM microenvironment. Vactosertib (Vacto) is a small molecule TGF-β type I receptor inhibitor that has shown single agent activity against myeloma in the syngeneic 5T33MM murine mouse model. Here, we report the phase Ib trial of Vacto in combination with pomalidomide (Pom) without any corticosteroids (NCT03143985). Methods: pts with relapsed MM with at least two lines of therapies enrolled on a 3 + 3 dose escalation design and received Vacto, 60 mg/d, 120 mg/d, 100 mg BID and 200 mg BID in combination with standard dose of Pom (4mg) without corticosteroids. The primary objectives of the study was to assess safety and recommended phase 2 dose. Vacto tablets, taken once or twice daily for 5 days followed by 2 days without treatment, is administered in 28-day cycles, until progression of disease or intolerable toxicity. Results: 15 pts were enrolled on the study (Table). The most common non-hematologic adverse event (AE) was grade II fatigue and pain in one pt, one episode of grade III renal failure that took less than 7 days to get back to baseline on another patient, sinus bradycardia that reversed to sinus rythem and an Afib that was rate controlled with beta blocerks. No grade IV non-hematologic AE was observed. Three pts had grade III hematologic AE, no grade IV hematologic AE. Three out of 15 pts experienced progression of disease (PFS-6: 80%). Conclusions: The phase Ib data shows safety of this agent in combination with Pom. The efficacy assessment (PFS-6: 80%) is higher than the historical control (PFS-6: 20% in randomized Phase II study by Richardson et al. Blood. 2014) with Pom only (PFS-6: 20%) or Pom with corticosteroids (PFS-6: 40%). Further advancement of this agent in clinical trial pipelines for MM is planned. Clinical trial information: NCT03143985

PR: Partial response, MR: Minimal response, SD: Stable disease, PD: Progressive disease.