8033 Background: MLN9708 is an oral, reversible 20S proteasome inhibitor. The feasibility of combining a proteasome inhibitor with an immunomodulatory drug and a steroid in previously untreated MM has been demonstrated with the RVD regimen. This is the first study of MLN9708 in combination with lenalidomide and dexamethasone (NCT01217957). Here we report the phase (Ph) 1 MTD and preliminary Ph 2 results. Methods: Pts with previously untreated MM, aged ≥18 yrs with measurable disease received oral MLN9708 (phase 1: 1.68–3.95 mg/m2) days 1, 8, and 15, lenalidomide 25 mg days 1–21, and dexamethasone 40 mg days 1, 8, 15, and 22, for up to twelve 28-day cycles. Primary objectives were determination of safety, MTD, and recommended phase 2 dose (RP2D) (Ph 1), and CR+VGPR rate (Ph 2). Results: At data cut-off (Dec 1, 2011), 29 pts had been enrolled (15 Ph 1, 14 Ph 2). Median age was 64 yrs (range 40–82); 69% ISS stage II/III. In Ph 1, the MLN9708 MTD was determined as 2.97 mg/m2 and the RP2D as 2.23 mg/m2; for Ph 2, the RP2D was converted to a 4.0 mg fixed dose based on population PK results. Ph 1 pts have received a median of 6 treatment cycles (range 1–11), 8 received ≥6 cycles; 6 stopped to receive ASCT, 7 are ongoing. Ph 2 pts received a median of 1 (range 1–2), all are ongoing. Grade ≥3 hematologic toxicity was reversible and included anemia (n=2) and thrombocytopenia (n=1). Grade ≥3 nonhematologic toxicity included erythematous rash, syncope, and vomiting (2 pts each). All-grade drug-related peripheral neuropathy was seen in 6 pts (21%), including grade 2 with pain in 2 (both Ph 1 at doses above the MTD). Two pts discontinued due to AE; there were 5 pts who had serious drug-related AE (all Ph 1). Of 19 response-evaluable pts (Ph 1 + Ph 2), all achieved ≥PR, including 5 CR (1 sCR), 4 VGPR, and 10 PR; all remain in response with duration of confirmed response of up to 9.5 months. Of 4 response-evaluable Ph 2 pts, 1 has achieved VGPR and 3 PR to date. Conclusions: Oral MLN9708 plus lenalidomide and dexamethasone appears well tolerated with manageable toxicity. These data show antitumor activity at the RP2D in pts with previously untreated MM, with ≥PR in all pts to date.