Ricolinostat (ACY-1215), the First Selective HDAC6 Inhibitor, in Combonation with Lenalidomide and Dexamethasone in Patients with Relapsed and Relapsed-and-Refractory Multiple Myeloma: Phase 1b Results (ACE-MM-101 Study)

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.