Safety and Efficacy of Abexinostat, a Pan-Histone Deacetylase (HDAC) Inhibitor, in Non-Hodgkin Lymphoma and Chronic Lymphocytic Leukemia: Results of an Ongoing Phase 2 Study

Introduction: Histone deacetylase (HDAC) inhibitors, by blocking HDAC enzymes, can regulate acetylation states of histones and other non-histone proteins. Hyperacetylation of histones in cells can cause transcriptional activation of tumor suppressor genes, as well as genes involved in cell cycle control, cell division, and apoptosis, resulting in antitumor activity. Currently, 3 HDAC inhibitors (HDACi), vorinostat, romidepsin, and belinostat, are approved for the treatment of relapsed or refractory peripheral or cutaneous T-cell lymphoma (T-CL). HDACi in development also show promising results in B-cell malignancies and solid tumors. Abexinostat, an orally available hydroxamate-containing HDACi with good tolerability, differs from approved HDACi due to its unique pharmacokinetic profile and oral dosing schedule, twice daily 4 hours apart, which allows for continuous exposure at concentrations required for efficient tumor cell killing (Mitsiades, et al. Blood. 2003; unpublished data). Abexinostat may, therefore, offer an active and potentially less-toxic treatment option for cancer with a wider therapeutic index than other HDACi in development. Abexinostat showed manageable toxicity and durable responses, including some complete responses (CR), particularly in patients (pts) with relapsed/refractory follicular lymphoma (FL) (Evens ICML 2013; Morschhauser, Invest New Drugs, 2015).

Methods: In this phase 2 trial, pts aged ≥18 years with relapsed/refractory NHL or CLL received oral abexinostat at 80 mg BID for 14 days of a 21-day cycle and continued until progressive disease or unacceptable toxicity. The 80 mg BID dose, which corresponds to the recommended phase 2 dose of 45 mg/m² BID, was identified in phase 1 of the study (Morschhauser, Invest New Drugs, 2015). The primary endpoint was overall response rate (ORR); secondary endpoints included overall survival, progression-free survival, time to tumor progression, duration of response, disease-free survival, and time to treatment failure.

Results: A total of 100 pts (median age, 66.5 years; 52% >65 years; 55% male) were enrolled between Oct 2011 and Jul 2014, including 16 with CLL, 17 with diffuse large B-cell lymphoma (DLBCL), 18 with FL, 16 with mantle cell lymphoma (MCL), 18 with T-CL, and 15 with marginal zone lymphoma (MZL) or other NHL subtypes. The median number of prior regimens across all lymphoma subtypes was 3 (range, 1-11) with a median of 4.5 prior regimens (range, 1-11) for FL pts. All pts received at least one dose of study drug; 55% discontinued due to progressive disease and 25% due to adverse events. Seven pts remain on treatment. Among the 87 pts evaluable for efficacy, ORR was 28% (CR, 5%). Responses by histology are shown in the table. Highest responses were observed in FL, T-CL, and DLBCL with ORRs of 56%, 40%, and 31% and median durations of response of 26.0 weeks (range, 0.1-90.4), 32.1 weeks (range, 6.3-51.3), and 8.1 weeks (range, 3.1-59.0), respectively. Grade ≥3 adverse events (AEs) and any serious AEs (SAEs) were reported in 86% and 46% of pts, respectively. The most frequently reported grade ≥3 treatment-emergent AEs were thrombocytopenia (80%), neutropenia (27%), and anemia (22%). The incidence of any-grade diarrhea was 47% (grade ≥3, 3%). The most commonly reported SAEs included thrombocytopenia (15%), anemia (7%), and pneumonia (6%). The most frequent toxicities that led to discontinuation included hematologic events, such as thrombocytopenia and neutropenia. Gastrointestinal toxicities leading to discontinuation were infrequent with 1 episode of vomiting being reported.

Conclusions: Abexinostat has a manageable toxicity profile in pts with various NHL subtypes that is similar to other HDACi and comparable to other single-agent therapies currently in development. Promising efficacy was observed with abexinostat, especially in FL, T-CL, and DLBCL, with an ORR ≥30% in these subtypes, consistent with the results of an independent study of abexinostat in lymphomas that used a week-on-week-off schedule (Evens ICML 2013). Further investigation of the safety and efficacy of abexinostat in these indications implementing the less dose-intense interval on a week-on-week-off schedule is planned.

Table: Response With Abexinostat by Tumor Type

Tumor type	ORR, % (CR, %)
Overall (N=87)	28% (5%)
FL (n=16)	56% (6%)
T-CL (n=15)	40% (7%)
DLBCL (n=16)	31% (6%)
MCL (n=13)	15% (8%)
MZL/other (n=13)	15% (0%)
CLL (n=14)	0% (0%)