Background: Azacitidine (AZA) may overcome drug resistance of relapsed or refractory multiple myeloma (RRMM).Continuous administration should maximize epigenetic effects and safety. Methods: Lenalidomide (len) 25mg (10mg if GFR 30-59 ml/min) d1-21 every 28d and dexamethasone (dex) 40mg weekly (len-dex) were combined with escalating doses of AZA from 30mg/m² SC weekly to 50mg/m²SC twice a week. Dose limiting toxicity (DLT) was assessed during cycle 1. IMWG uniform response criteria (partial response, PR) and adapted EBMT criteria (minor response, MR) were used. Plasma activity of the AZA inactivating enzyme cytidine deaminase (CDA) was measured by HPLC at Zymo Research Corp., CA. Results: Forty patients (pts) with relapsed (n = 3) or refractory (n = 37) MM were enrolled after a median of 4 prior regimens (range 1-10). Their disease was refractory to len (n = 32), bortezomib and / or carfilzomib (n = 32), or both len and proteasome inhibitors (n = 28). The target phase II dose of 50mg/m2 SC twice a wk was reached. One DLT (neutropenic fever without documented infection) occurred in 1 of 6 pts treated with 40mg/m2 AZA twice a week);. Grade 3/ 4 toxicities possibly drug related were seen in 23 pts (58%), neutropenia (13), thrombocytopenia (5), fatigue (3), infection (2), anemia (2), pleural effusion (1), fever (1), and atrial fibrillation (1). Twelve pts achieved > MR, 9 > PR (3 VGPR) yielding 30% clinical benefit response (CBR) and 22.5% response rates. Median time on study was 90 days. Responses lasted between 3 months and 2 years. Five pts remain on study. Plasma CDA activity at screening and during the study (q wk x4, then q 28d) inversely correlated with achieving > MR (p < 0.03, p < 0.01, respectively, Wilcoxon exact test) and varied by median 17%. Pts with screening CDA activity < 1000 mU/mL had 50% CBR, vs. 21% if > 1000mU/mL. Conclusions: AZA was well tolerated up to target 50mg/m² SC twice a week in combination with len-dex . Inverse correlation of CDA activity with response suggests AZA contributed to benefit and supports development of the HPLC assay to select patients for aza nucleoside treatment and guide development of CDA inhibitors. Clinical trial information: NCT01155583