Translational Focus on Targeting HDAC6 with Ricolinostat Confirms Potent Activity in Preclinical Models of Lymphoma and a Favorable Toxicity Profile in Patients with Relapsed or Refractory Lymphoma

Pan-class HDAC inhibitors are active agents for select subtypes of lymphoma but are often associated with toxicities such as fatigue, diarrhea/constipation, cardiac arrhythmias and myelosuppression. HDAC6 plays a critical role in protein homeostasis by managing misfolded proteins and inducing the unfolded protein response (UPR). Ricolinostat is an isoform selective HDAC6 inhibitor. We evaluated the preclinical effects of ricolinostat and translated the findings into a multicenter Phase 1b study in patients with lymphoma (NCT 02091063).

Cytotoxicity was evaluated in 16 lymphoma cell lines (8 DBCL, 4 MCL, 4 TCL) by Cell TiterGlo and Annexin V. Ricolinostat has activity with IC50 values ranging from 0.9-4.7 uM range, which is in the range observed in clinical PK studies. Ricolinostat displayed marked synergy with bortezomib, PI3K inhibitors, disatinib, alisertib, and ibrutinib with synergy coeficients of 0.24, 0.29, 0.64, 0.07, 0.16 respectively (values<1 imply synergy). In vitro ricolinostat modulates the UPR leading to accumulated misfolded proteins and ultimately apoptosis. Treatment with ricolinostat led to inhibition of the aggresome pathway evidenced by acetylation of a-tubulin and increased poly-ubiquitinated proteins. Upregulation of UPR was demonstrated by acetylation of GRP78 and subsequent dissociation of PERK, increased p-eIF2a, spliced XBP-1 and CHOP. Xenograft studies of the OCI-Ly10 cell line treated with ricolinostat demonstrated significantly prolonged survival compared to control, as well as modulation of the UPR. Evaluation of a collection of primary lymphoma samples by IHC revealed increased HDAC6, GRP78 and XBP-1 expression compared to reactive lymphoid tissue.

In an effort to translate these findings, a multi-center Phase Ib open-label, single agent study designed to study the safety of 2 dose schedules of ricolinostat in patients with relapsed or refractory lymphoid malignancies was initiated. Patients were treated with 160 mg of oral ricolinostat continuously on a 28 day cycle. Patients were accrued sequentially to 2 dose cohorts: 3 patients were enrolled on single daily dosing (QD) schedule, and following safety assessment, patients were enrolled to twice daily dosing (BID). Samples were collected for PK and PD analysis. As of July 25^th 2015, 11 patients were enrolled on the phase Ib study and were evaluated for safety. The median age was 36 yrs (24-86) and 7 (64%) were male. Sixty-four percent had 5 prior treatments (range: 1-15). Histologies included HL (N=3) and NHL (N=8) including DLBCL=3, MZL=2, FL=1, PTCL=1, PTLD=1. Three patients were enrolled in QD dosing and 8 patients in the BID schedule. There were no DLTs at either of the dosing schedules, and the maximum administrable dose was determined on the BID dosing schedule. The median number of cycles completed was 2 and ranged from 1 through 12+ (currently ongoing).

The only grade 3 toxicities reported were anemia (27%), hyponatremia (9%), hypercalcemia (9%) and hypotension (9%), none thought to be attributed to the study drug. Other treatment emergent events included nausea (45%), diarrhea (36%), fatigue (36%) and vomiting (27%) which were all grade 1 and 2. Three patients achieved stable disease, all of whom were treated on the BID schedule (3 of 8, 37.5%), no complete or partial responses have been observed to date. One patient with refractory HL with 14 prior lines of therapy achieved stable disease (SD) and had symptomatic improvement corresponding to an increase of hemoglobin after 9 cycles, as well as resolution of B symptoms. Another patient with PTCL had immediate resolution of B symptoms such as fevers and pruritus after starting QD dosing.

This is the first reported experience with an isoform selective HDAC inhibitor to be studied for the treatment of patients with relapsed or refractory lymphoid malignancies. Preclinical studies support the notion that ricolinostat mediates its effects through the UPR and potently synergizes with complementary agents. Ricolinostat is well tolerated with no DLTs. Thirty-seven percent of patients achieved SD on the BID schedule. PK and PD data will be evaluated for acetylation of alpha-tubulin and modulation of the UPR in an effort to confirm preclinical findings. Efficacy will be evaluated in a phase II study. We believe that ricolinostat in rational combinations will lead to effective treatment for patients with relapsed lymphoid malignancies.