A Phase II Multicenter Study of the Histone Deacetylase Inhibitor (HDACi) Abexinostat (PCI-24781) in Relapsed/Refractory Follicular Lymphoma (FL) and Mantle Cell Lymphoma (MCL)
Authors: Evens Andrew M. , Vose Julie M , Harb Wael , Gordon Leo I. , Langdon Robert , Grant Barbara , Sprague Julian , Plasencia Clara , Sirisawad Mint , Yue Jeanne , Luan Ying , Siek Aaron , Zhou Lei , Balasubramanian Sriram , Bartlett Nancy L. 
Published: October 2019

Abstract
Abstract 55

Background:
Abexinostat (PCI-24781) is a novel oral pan-HDACi that has previously demonstrated potent preclinical activity in lymphoma cell lines and animal models (Evens et al, Clin Ca Res 2009) as well as a tolerable safety profile in phase I solid tumor clinical studies (Undevia et al, ASCO 2008). In a phase I single-agent clinical trial in patients (pts) with multiply relapsed/refractory lymphoma and leukemia, anti-tumor activity was noted in FL and MCL; 4/4 FL pts showed durable clinical benefit with median time on treatment of >8 months; 1 of 2 MCL pts had a complete remission (CR) and remains on treatment for >3 years. Based on these encouraging single-agent abexinostat data, a phase II extension study was completed in relapsed/refractory FL and MCL.

Methods:
The primary objective of this phase II study was objective response rate (ORR, complete [CR] and partial remission [PR]), while reduction in tumor size (CR + PR + stable disease) and safety were also examined. Abexinostat was given orally twice daily at 45mg/m2 on a 4-week cycle for 7 days/week every other week, which is the previously established dose and schedule identified in the phase I study. For pharmacodynamic correlative analyses, tubulin and histone acetylation were measured in peripheral blood mononuclear cells (PBMCs) at pre-dose and 4 hours after the first dose of abexinostat.

Results:
A total of 30 pts were enrolled (n=16 FL and n=14 MCL pts) of which 25 were response-evaluable. The median age was 67 years (36–81) and the median prior therapies were 3 (1–5), 77% of which were rituximab-containing and one-third (33%) had undergone prior autologous stem cell transplant (31% FL, 36% MCL). The ORR in all pts was 48% (12/25). A reduction in tumor size was observed in 86% (12/14) of FL pts, while the ORR in FL was 64% (9/14); the FL intent-to-treat ORR was 56%. With a median follow-up of 10.3 months (1.2–20.9), the progression-free probability was 86% in FL pts. Furthermore, the median duration of response (DOR) in FL pts has not been reached, as 5 responders remain on study. Notably, 4 FL pts were on treatment for over 16 months. Among MCL pts, reduction in tumor size was noted in 27% (3/11), while the ORR was similar at 27% (3/11). The median progression-free survival (PFS) in MCL pts was 4 months (1–12), while the DOR in the 3 responding patients were 2+, 3, and 6+ months. Therapy was overall well tolerated. The most common treatment-related adverse events (AEs) of any grade (> 20% incidence) include: nausea (60%), fatigue (57%), diarrhea (50%), thrombocytopenia (37%), vomiting (30%), anemia (27%), decreased appetite (23%), dysgeusia (20%) and neutropenia (20%). The most common grade 3/4 related AEs (> 5% incidence) were thrombocytopenia (17%), neutropenia (13%), fatigue (13%), and anemia (7%). There were no deaths reported on the study. Pharmacodynamic analyses revealed that a majority of pts had increased tubulin acetylation in PBMCs, however this finding did not correlate with response or toxicity.

Conclusion:
In this phase II study, the novel pan-HDACi, abexinostat, was clinically active and overall well tolerated in relapsed/refractory B-cell lymphoma. The safety profile was consistent with this class of agents with <20% of subjects experiencing grade 3/4 cytopenias (during the course of prolonged treatments). Moreover, there was significant clinical activity noted in FL with an ORR of 64%, which included several durable responses in this multiply-relapsed pt population. Further examination of single-agent abexinostat in FL is warranted.

Disclosures:
Off Label Use: PCI24781 is not FDA approved; it is an investigational agent. Plasencia:Pharmacyclics: Employment, Equity Ownership. Sirisawad:Pharmacyclics: Employment, Equity Ownership. Yue:Pharmacyclics: Employment, Equity Ownership. Luan:Pharmacyclics: Employment, Equity Ownership. Siek:Pharmacyclics: Employment, Equity Ownership. Zhou:Pharmacyclics: Consultancy. Balasubramanian:Pharmacyclics: Employment, Equity Ownership.