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Background: The selective HDAC6 inhibitor ACY-241, a tablet, is structurally related to ricolinostat (ACY-1215), the first agent in this class in the clinic.Ricolinostat, an oral liquid, demonstrated clinical efficacy in a Phase 2 combination with pomalidomide (Pom) and dexamethasone (Dex) in patients (pts) with relapsed or relapsed-and-refractory multiple myeloma (RRMM) without toxicities greater than those reported with Pom and Dex alone (Raje et al., EHA 2016, S813). Preclinical data demonstrate synergistic activity of ACY-241 with Pom and lenalidomide (Len) in induction of cell cycle arrest and apoptosis in MM cells as well as significant extension of survival in a mouse xenograft model (Niesvizky et al., Blood 2015, 126: 3040). We present updated data on safety and efficacy of the ACY-241/Pom/Dex combination in pts with relapsed or RRMM (ACE-MM-200, NCT02400242). Aims:Determine the safety, tolerability, and preliminary efficacy of ACY-241 monotherapy and combination with Pom and Dex and the recommended dose for further development. Methods:Based on clinical experience with ricolinostat and non-clinical pharmacokinetics (PK) of ACY-241, we designed a first-in-human phase 1a/1b clinical trial of a single-cycle of ACY-241 monotherapy followed by ACY-241 in combination with Pom (4mg) and low-dose Dex in pts with relapsed or RRMM. The starting dose of ACY-241 was chosen to give similar exposure to the therapeutic dose of ricolinostat (160 mg QD). The trial design was chosen to grant pts access to combination therapy with an active regimen while exploring the safety, PK, and pharmacodynamic profile of ACY-241 alone and in combination with Pom/Dex. The PK of Pom and Dex was also assessed. Pts with relapsed or RRMM previously treated with ≥ 2 cycles of Len and a proteasome inhibitor were eligible. Cohorts of 3 pts had ACY-241 PO QD as monotherapy (180, 360 and 480 mg) on days 1-21 of a 28 day cycle. If no DLT was noted in cycle 1 with ACY-241, pts continued to cycle 2 of combo therapy with ACY-241/Pom/Dex. Pharmacodynamic assessments were acetylated tubulin (HDAC6 marker) and acetylated histones (Class 1 HDAC marker) in peripheral blood mononuclear cells. Results: Since June 2015, 40 pts have enrolled (34 safety-evaluable, 6 had no dosing information in the database). Median age was 62 (34-84) years and median number of prior regimens was 3 (1-7). 90% of pts were refractory to last treatment. 83% were refractory to Len and 50% to both bortezomib and Len. 20% of pts had high risk cytogenetics. No monotherapy DLTs were observed at the highest dose explored (480 mg). Common toxicities in the monotherapy safety population (N=15) were all grade 1/2, except 1 pt with grade 3 anemia at the 480 mg dose level. Toxicities included nausea (4 pts, 27%), anemia (3 pts, 20%), dizziness, fatigue, leukopenia and thrombocytopenia (2 pts each, 13%). Doses of 180 mg and 360 mg were explored in combination; one DLT (grade 4 thrombocytopenia) occurred at 360 mg. Common toxicities in the combination therapy safety population (N=33) included neutropenia (13 pts, 40%), fatigue (9 pts, 27%), anemia, leukopenia (6 pts each, 18%), cough, insomnia, rash (4 pts each, 12%), and hyperglycemia (3 pts, 9%). Grade 3/4 toxicities included neutropenia (10 pts, 30%), leukopenia (3 pts, 9%) and anemia (2 pts, 6%). PK results showed a dose-linear increase in exposure with increasing dose, no accumulation and no drug-drug interaction with Pom and Dex. Selective increase in acetylated tubulin was seen at 180 mg with increasing levels of acetylated tubulin and histones at higher doses. Confirmed efficacy data (median follow-up 3.5 months) for combination treatment (N=22, all refractory to last treatment regimen) shows 1 VGPR, 10 PR, 2 MR and 8 SD and 1 PD. Median PFS and duration of response were not reached at time of the data cut. Given the safety profile, PK exposure (Cmax~6 µM) and PD profile, the 360 mg QD dose level was recommended for further clinical exploration of ACY-241 in combination with Pom/Dex. Summary/Conclusion:ACY-241 is well tolerated in combination with Pom/Dex with dose proportional increase in drug exposure. Early response data to combination treatment parallel those observed with ricolinostat/Pom/Dex and compare favorably to historic controls of Pom/Dex. Cohort expansion at 360 mg ACY-241 with Pom/Dex is ongoing to confirm the dose and schedule for a planned pivotal trial of Pom/Dex +/- ACY-241 and to explore selected biomarkers. Disclosures Richardson: Jazz Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees. Nooka:Amgen: Consultancy; Spectrum: Consultancy; Novartis: Consultancy. Raab:Amgen: Consultancy, Research Funding; BMS: Consultancy; Celgene: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Consultancy, Research Funding. Shain:Takeda/Millennium: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novartis: Speakers Bureau; Amgen/Onyx: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Signal Genetics: Research Funding. Matous:Celgene: Consultancy, Speakers Bureau; Takeda Pharmaceuticals International Co.: Speakers Bureau; Seattle Genetics: Research Funding, Speakers Bureau. Agarwal:Celgene: Speakers Bureau; Onyx: Speakers Bureau; Janssen: Speakers Bureau; Amgen: Consultancy; Millennium: Consultancy; AbbVie: Honoraria, Research Funding. Madan:Amgen: Speakers Bureau; Onyx: Speakers Bureau; Takeda: Speakers Bureau; Celgene: Speakers Bureau. Moreau:Novartis: Honoraria; Takeda: Honoraria; Janssen: Honoraria, Speakers Bureau; Celgene: Honoraria; Amgen: Honoraria; Bristol-Myers Squibb: Honoraria. Mateos:Janssen: Honoraria; Celgene: Honoraria; Amgen: Honoraria; Takeda: Honoraria. Facon:Acetylon Pharmaceuticals Inc: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Tamang:Acetylon Pharmaceutical Inc.: Employment. Jones:Acetylon Pharmaceuticals, Inc.: Employment, Equity Ownership. Markelewicz:Acetylon Pharmaceutical Inc.: Employment. Wheeler:Acetylon Pharmaceuticals Inc.: Employment. Trede:Acetylon Pharmaceutials Inc: Employment. Raje:Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees; Merck: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees; AstraZeneca: Research Funding; Eli Lilly: Research Funding. Terpos:Amgen: Consultancy, Honoraria, Other: Travel expenses, Research Funding; Genesis: Consultancy, Honoraria, Other: Travel expenses, Research Funding; Novartis: Honoraria; BMS: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Other: Travel expenses, Research Funding; Takeda: Consultancy, Honoraria; Celgene: Honoraria. Bensinger:Amgen: Honoraria; Celgene: Honoraria; Acetylon Pharmaceuticals Inc.: Honoraria; Amgen: Consultancy; Celgene: Consultancy; Sanofi: Consultancy; Merck: Consultancy; Bristol-Meyers Squibb: Consultancy; Celgene: Speakers Bureau; Takeda: Speakers Bureau; Amgen: Speakers Bureau; Acetylon Pharmaceuticals Inc: Research Funding; Bristol-Meyers Squibb: Research Funding; Celgene: Research Funding; Karyopharm Therapeutics: Research Funding; Merck: Research Funding; Amgen: Research Funding; Sanofi: Research Funding; Takeda: Consultancy.

Background: Ricolinostat, an oral selective HDAC6 inhibitor, is well tolerated as monotherapy (Raje Blood 2012;120:4061). HDAC6 inhibition impairs the aggresome/autophagy pathway, an alternate pathway to proteasome clearance of misfolded proteins (Santo Blood 2012;119:2578-89) providing the rationale for combining ricolinostat with bortezomib (Btz). Pan-HDAC inhibitors vorinostat and panobinostat are active in multiple myeloma (MM) in combo with Btz, but toxicities (thrombocytopenia, fatigue, and GI events) limit dosing exposure. This trial explores activity of ricolinostat in combo with Btz and dexamethasone (Dex) in patients with relapsed or relapsed-and-refractory MM. Methods: Phase 1b was a 3+3 design that explored ricolinostat combined with Btz (1.0-1.3 mg/m2 IV or SQ on days 1, 4, 8, 11) and Dex (20 mg PO on days 1, 2, 4, 5, 8, 9, 11, 12). Patients previously received at least 2 lines of therapy including a proteasome inhibitor and an immunomodulatory agent, had either progressed after or were ineligible for autologous stem cell transplant, and had adequate BM reserve, hepatic function and CrCl ≥30 mL/min. Patients with prior HDAC inhibitor therapy were excluded. Peripheral blood samples were obtained for PK/PD assessment of acetylated tubulin and histones. Results: 50 patients were enrolled to 8 combination dose cohorts to date. Median age was 65, median number of prior regimens was 5 (2-12) and 33 patients were refractory to Btz. The first combination cohort was expanded due to a DLT of asymptomatic increase in amylase. No other DLTs have been observed in the cohort escalations. Common toxicities were predominately low grade (grade 1/2) and included diarrhea (46%), anemia (38%), thrombocytopenia (22%), increased creatinine (18%), fatigue (16%), and nausea (16%). Important grade 3/4 related toxicity included thrombocytopenia (9 patients, 18%) and anemia (6 patients, 12%). Cohorts were expanded sequentially at two dose levels: 160 mg BID and 160 mg QD. Of the 22 patients receiving ricolinostat 160 mg BID common toxicities were diarrhea (68%, grade 1/2), thrombocytopenia (50%, grade 3/4), anemia (45%, grade 2/3), fatigue (36%, grade 1/2), and nausea (36%, grade 1/2). There was a marked increase in the incidence of SAEs related to diarrhea leading to dehydration and hospitalization (27%). Therefore a second cohort expansion at 160 mg QD was initiated. Of the 15 patients receiving ricolinostat at 160 or 240 mg QD common toxicities were diarrhea (33%, grade 1/2), thrombocytopenia (20%, grade 4), anemia (20%, grade 3), increased creatinine (20%, grade 3), hyponatremia (20%, grade 1), and increased ALT (20% grade 3), and peripheral neuropathy (20%, grade 1/2). PK for ricolinostat is similar to that observed in Phase 1a monotherapy, indicating that co-administration of Btz does not significantly impact ricolinostat exposure. Maximal blood levels of ricolinostat were ≥0.5 µM at ≥80 mg correlating with a >2x increase in the HDAC6 PD marker acetylated tubulin. 48 patients were evaluable for response with median follow-up of 3 (1-18) months; ORR (≥PR) was 39% and clinical benefit rate (≥MR) was 44% with 81% SD or better. Of the 22 patients receiving ricolinostat 160 mg BID with median follow-up of 3 (1-18) months, ORR (≥PR) was 32% and clinical benefit rate (≥MR) was 41% with 62% SD or better. Of the 12 patients receiving ricolinostat at 160 or 240 mg QD with median follow-up of 4 (1-8) months, ORR (≥PR) was 33% and clinical benefit rate (≥MR) was 33% with 67% SD or better. Of the 33 Btz refractory patients, 42% had SD or better, and 12% responded including 1 VGPR. Conclusion: Overall, ricolinostat in combination with Btz and Dex was well tolerated. Diarrhea and hematologic toxicity, mostly low grade, was less prevalent with QD than BID dosing and led to fewer hospitalizations and early treatment discontinuation. Response rates were similar between both QD and BID dosing. 160 mg QD is the recommended dose in combination with Btz and Dex. Disclosures Vogl: Constellation Pharmaceuticals: Research Funding; GSK: Research Funding; Millennium Pharmaceuticals: Research Funding; Calithera Biosciences: Research Funding; Celgene Corporation: Consultancy; Acetylon Pharmaceuticals, Inc.: Research Funding. Raje:Acetylon: Research Funding; Amgen: Consultancy; Eli Lilly: Research Funding; Onyx: Consultancy; Celgene Corporation: Consultancy; AstraZeneca: Research Funding; BMS: Consultancy; Takeda: Consultancy; Millenium: Consultancy; Novartis: Consultancy. Jagannath:Celgene: Honoraria; MERCK: Honoraria; Janssen: Honoraria; Novartis Pharmaceuticals Corporation: Honoraria; BMS: Honoraria. Richardson:Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Millennium Takeda: Membership on an entity's Board of Directors or advisory committees; Gentium S.p.A.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Hari:Sanofi: Consultancy; Takeda: Consultancy; Spectrum: Consultancy; Novartis: Consultancy; Janssen: Consultancy; BMS: Consultancy; Celgene: Consultancy. Orlowski:Millennium Pharmaceuticals: Consultancy, Research Funding; Array BioPharma: Consultancy, Research Funding; Onyx Pharmaceuticals: Consultancy, Research Funding; Bristol-Myers Squibb: Consultancy, Research Funding; Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; BioTheryX, Inc.: Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Research Funding; Acetylon: Membership on an entity's Board of Directors or advisory committees; Genentech: Consultancy; Spectrum Pharmaceuticals: Research Funding; Forma Therapeutics: Consultancy. Tamang:Acetylon Pharmaceuticals, Inc.: Employment. Jones:Acetylon Pharmaceuticals, Inc.: Employment, Equity Ownership. Wheeler:Acetylon Pharmaceuticals, Inc: Employment. Markelewicz:Acetylon Pharmaceuticals, Inc: Employment. Lonial:Millennium: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Celgene: Consultancy, Research Funding; Bristol-Myers Squibb: Consultancy, Research Funding; Janssen: Consultancy, Research Funding; Onyx: Consultancy, Research Funding.

Background: Ricolinostat, an oral selective HDAC6 inhibitor, is well tolerated as monotherapy (Raje Blood 2012;120:4061) and demonstrates potent synergistic activity with lenalidomide (Len) and pomalidomide (Pom) in preclinical models (Quayle Blood 2013;122:1952). In an ongoing trial of ricolinostat in combination with Len and dexamethasone (Dex) in patients with relapsed or refractory multiple myeloma (MM), no dose limiting toxicities (DLTs) were observed at up to doses of 160 mg BID, with excellent activity and tolerability (Yee Blood 2014;124:4772). Pan-HDAC inhibitors vorinostat and panobinostat are active in MM in combination with bortezomib (Btz) and Len, but toxicities (thrombocytopenia, fatigue, and GI events) limit dosing exposure. This trial explores activity of ricolinostat in combination with Pom and Dex in a patient population comparable to that in MM-003 (San Miguel Lancet Oncol 2013;14:1055-66). At the time of achieving the primary endpoint of PFS in MM-003, the ORR was 16.6% at 18.1 weeks median follow-up (EMA Pom Celgene Assessment Report EMA/CHMP/427059/2013), and was 31% at 10 months in the same clinical trial (San Miguel Lancet Oncol 2013;14:1055-66). Methods: Phase 1b was a 3+3 design which explored ricolinostat 160 mg QD or BID combined with Pom (4 mg) for 21 days of a 28 day cycle with Dex (40 mg) on days 1, 8, 15 and 22. Patients had measurable disease, adequate BM reserve and hepatic function with CrCl ≥45 mL/min. Refractory was defined as PD on or within 60 days of last therapy. Patients with non-secretory MM, prior Pom or HDAC inhibitor therapy were excluded. Peripheral blood samples were obtained for PK/PD assessment of acetylated tubulin and histones. Patients with serum FLC only disease were excluded from Phase 2. The dose level of 160 mg dose was chosen based on prior experience in which PK plateau was reached at 160 mg of ricolinostat. A sample size of 66 was determined to be adequate to detect an ORR of 44% against a historical rate of 29%. Results: 7 patients were treated in phase 1b: 3 at 160 mg QD and 4 at 160 mg BID. No DLTs were observed; however, all 4 patients had grade 2 diarrhea in the 160 mg BID cohort and grade 3 fatigue and grade 3 or 4 neutropenia attributed to Pom were reported in patients in phase 1b. 1 patient remained on study for 12 cycles before PD; the other 6 Phase 1b patients were withdrawn for PD at 2-4 cycles. Following safety review committee (SRC) review, phase 2 opened at a dose of 160 mg BID, with a predetermined SRC review to occur after 6 patients had completed 1 cycle of therapy. 3 of the 6 patients had clinically relevant diarrhea requiring ricolinostat dose reduction to 160 mg QD in cycle 2 leading to SRC recommendation of QD dosing of ricolinostat. As of April 28, 2015 39 phase 2 patients were evaluable for safety and 28 evaluable for response. Median age was 68 and median number of prior regimens was 4 (2-9) in 3 (2-6) line of therapy. 37 patients were refractory to Len as defined in the entry criteria. Common toxicities were predominantly low grade and included fatigue (40%), diarrhea (38%), neutropenia (36%), anemia (31%), thrombocytopenia (26%), and constipation (21%). Important grade 3/4 related toxicities included neutropenia (5 patients, 13%) and diarrhea (3 patients, 8%). PK of ricolinostat was similar to that observed in combination with Btz and Len. There was no evidence of ricolinostat accumulation or drug-drug interaction with Pom. Median follow-up of 12 (4-32) weeks; ORR (≥PR) was 29% with 3 VGPR and clinical benefit rate (≥MR) was 50% with 68% SD or better for the 28 response evaluable patients. Updated safety and efficacy data will be presented as enrollment continues and data matures including for the 49 patients enrolled in phase 2 to date. Conclusion: These early data with a median follow-up of 12 weeks (compared to historical control of 16.6% ORR at 18.1 weeks for Pom/Dex alone in MM-003) in a rigorously defined patient population suggest that selective HDAC6 inhibition is associated with encouraging tolerability and promising clinical activity. Accrual is continuing in US, Canada, and Europe. Overall, ricolinostat is an active and safe oral agent which combines favorably with Pom and Dex in relapsed-and-refractory MM. Disclosures Raje: Novartis: Consultancy; Takeda: Consultancy; Amgen: Consultancy; Eli Lilly: Research Funding; AstraZeneca: Research Funding; Millenium: Consultancy; Onyx: Consultancy; Acetylon: Research Funding; Celgene Corporation: Consultancy; BMS: Consultancy. Bensinger:Sanofi: Research Funding; Acetylon Pharmaceuticals, Inc: Research Funding; BMS: Research Funding; Novartis: Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Celgene: Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees; Millenium: Research Funding; Onyx: Research Funding; Sanofi: Membership on an entity's Board of Directors or advisory committees. Lonial:Celgene: Consultancy, Research Funding; Janssen: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Bristol-Myers Squibb: Consultancy, Research Funding; Millennium: Consultancy, Research Funding; Onyx: Consultancy, Research Funding. Jagannath:BMS: Honoraria; Novartis Pharmaceuticals Corporation: Honoraria; Janssen: Honoraria; MERCK: Honoraria; Celgene: Honoraria. Valent:Takeda/Millennium: Speakers Bureau; Celgene: Speakers Bureau. Harb:Idera Pharmaceuticals: Research Funding; Astex Pharmaceuticals, Inc.: Research Funding. Sandhu:Janssen: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Celgene: Consultancy, Honoraria. Bahlis:Johnson & Johnson: Research Funding; Johnson & Johnson: Speakers Bureau; Johnson & Johnson: Consultancy; Amgen: Consultancy; Celgene: Consultancy, Honoraria, Research Funding, Speakers Bureau. Reece:Merck: Research Funding; Lundbeck: Honoraria; Amgen: Honoraria; Onyx: Consultancy; Novartis: Honoraria, Research Funding; Bristol-Myers Squibb: Research Funding; Janssen-Cilag: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Millennium Takeda: Research Funding; Otsuka: Research Funding. Terpos:Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel expenses; Takeda: Honoraria; Celgene: Honoraria, Other: Travel expenses; Novartis: Honoraria; Amgen: Honoraria, Other: Travel expenses, Research Funding. Tamang:Acetylon Pharmaceuticals, Inc.: Employment. Jones:Acetylon Pharmaceuticals, Inc.: Employment, Equity Ownership. Wheeler:Acetylon Pharmaceuticals, INC: Employment. Markelewicz:Acetylon Pharmaceuticals, Inc: Employment. Richardson:Celgene: Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Millennium Takeda: Membership on an entity's Board of Directors or advisory committees; Johnson & Johnson: Membership on an entity's Board of Directors or advisory committees.

Background: Ricolinostat, an oral selective HDAC6 inhibitor, is well tolerated as monotherapy (Raje Blood 2012;120:4061) and demonstrates potent synergistic activity with lenalidomide (Len) and pomalidomide (Pom) in preclinical models (Quayle Blood 2013;122:1952). Immunomodulatory drugs (thalidomide, Len, and Pom) trigger anti-tumor activities in multiple myeloma (MM) in part by targeting cereblon, a component of an E3 ubiquitin ligase complex, leading to decreased levels of IZF1/3, c-Myc and IRF4, critical factors in MM proliferation. Preclinical models using ricolinostat in combination with Len was shown to both avoid cereblon downregulation caused by non-selective HDAC inhibitors, and enhance Len mediated downregulation of c-Myc, IRF4 and IZF1/3 to induce synergistic cytotoxicity in MM (Hideshima Blood Cancer J 2015;5:e312). Pan-HDAC inhibitors vorinostat and panobinostat are active in MM in combination with bortezomib (Btz) and Len, but toxicities (thrombocytopenia, fatigue, and GI events) limit dosing exposure. This trial explores activity of ricolinostat in combination with Len and dexamethasone (Dex) in patients with relapsed and relapsed-and refractory MM. Methods: Phase 1b was a 3+3 design which studied ricolinostat combined with Len (15-25 mg QD on Days 1-21 of a 28-day cycle) and Dex (40 mg weekly). In Regimen A, patients were treated with escalating doses of ricolinostat on Days 1-5 and 8-12 of a 28-day cycle with Len and Dex. In Regimen B, ricolinostat was also given on Days 15-19. Doses up to 240 mg QD and 160 mg BID were explored. In Regimen C, patients were treated with 160 mg ricolinostat BID or QD for 21 of 28 days with Len and Dex. Patients had measurable disease, adequate BM reserve, hepatic function, and CrCl ≥50 mL/min. Per the IMWG, refractory was defined as disease that is nonresponsive while on primary or salvage therapy, or progresses on or within 60 days of last therapy. Patients with non-secretory MM or prior HDAC inhibitor therapy were excluded. Peripheral blood samples were obtained for PK/PD assessment of acetylated tubulin and histones. Results: 33 patients have been enrolled to date. Median age was 63 with a median of 3 (2-9) lines of prior therapy. 22 patients were previously treated with Len, and 12 of these patients were refractory to Len as defined in the entry criteria. No MTD was observed at up to 160 mg BID. Common toxicities were predominately low grade and included fatigue (55%), neutropenia (36%), diarrhea (33%), upper respiratory tract infections (30%), and anemia (27%). Important grade 3/4 related toxicities included neutropenia (6 patients, 18%), fatigue (2 patients, 6%), diarrhea (2 patients, 6%), and vomiting (2 patients, 6%). 2 DLTs have been observed: grade 3 syncope in Regimen B at 160 mg BID on Days 1-5, 8-12 and 15-19 and grade 3 muscle cramps in Regimen C at 160 mg BID Days 1-21. The last cohort is being expanded at 160 mg QD Days 1-21 based on prior cohort expansions and other trials of ricolinostat in combination with Btz and Pom where 160 mg ricolinostat QD is better tolerated than BID dosing (Vogl Blood 2014;124:4764). PK for ricolinostat is similar to that observed in Phase 1a monotherapy, indicating that co-administration of Len does not significantly impact ricolinostat exposure. Maximal blood levels of ricolinostat were ≥0.5 µM at ≥80 mg correlating with a >2x increase acetylated tubulin (HDAC6 PD marker) with minimal increase in acetylated histones (class 1 HDAC PD marker). 31 patients were evaluable for response with ORR (≥PR) was 55% with 1 sCR and 7 VGPR and clinical benefit rate (≥MR) was 61% with 71% SD or better. The median follow-up of all 31 patients compared to the 17 responders was 6 (1-33) months and 12 (1-33) months, respectively. 19 patients were previously naïve or responsive (≥MR) to Len with median follow-up of 11 (1-33) months had ORR (≥PR) of 68% and clinical benefit rate (≥MR) of 74%. 12 patients refractory to Len with median follow-up of 5 (3-10) months had ORR (≥PR) of 33% and clinical benefit rate (≥MR) of 42% with 67% SD or better. Conclusion: Ricolinostat is an active and safe oral agent in combination with Len and Dex in relapsed and relapsed-and-refractory MM. No MTD has been identified. A dose of 160 mg QD on Days 1-21 of a 28-day cycle is proposed for phase 2 based on excellent tolerability. The median duration on study for responders was a year, with some patients continuing up to 33 months. Disclosures Bensinger: Sanofi: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Millenium: Research Funding; BMS: Research Funding; Novartis: Research Funding; Sanofi: Research Funding; Onyx: Research Funding; Celgene: Research Funding; Acetylon Pharmaceuticals, Inc: Research Funding. Voorhees:Millennium Pharmaceuticals: Consultancy, Research Funding; Onyx Pharmaceuticals: Research Funding; Celgene: Research Funding; Oncopeptides: Research Funding; Acetylon Pharmaceuticals, Inc.: Research Funding; Janssen: Research Funding; GSK: Research Funding; Oncopeptides: Consultancy; Array BioPharma: Consultancy; Novartis: Consultancy; Celgene: Consultancy; A Takeda Oncology Company: Consultancy, Research Funding; GSK: Consultancy. Berdeja:BMS: Research Funding; Acetylon: Research Funding; Array: Research Funding; Janssen: Research Funding; Curis: Research Funding; Celgene: Research Funding; Abbvie: Research Funding; Novartis: Research Funding; MEI: Research Funding; Takeda: Research Funding; Onyx: Research Funding. Richardson:Gentium S.p.A.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Millennium Takeda: Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding. Tamang:Acetylon Pharmaceuticals, Inc.: Employment. Jones:Acetylon Pharmaceuticals, Inc.: Employment, Equity Ownership. Wheeler:Acetylon Pharmaceuticals, INC: Employment. Markelewicz:Acetylon Pharmaceuticals, Inc: Employment. Raje:AstraZeneca: Research Funding; Acetylon: Research Funding; Onyx: Consultancy; Novartis: Consultancy; Celgene Corporation: Consultancy; Takeda: Consultancy; Millenium: Consultancy; Eli Lilly: Research Funding; Amgen: Consultancy; BMS: Consultancy.