An Expanded Treatment Protocol of Panobinostat Plus Bortezomib and Dexamethasone in Patients with Previously Treated Myeloma

Introduction: Panobinostat (PAN) is a potent pan-deacetylase inhibitor that targets multiple myeloma (MM) cells via its effects on epigenetics and protein metabolism. In the PANORAMA 1 phase 3 trial, the combination of PAN, bortezomib (BTZ), and dexamethasone (Dex; PAN-BTZ-Dex) significantly increased progression-free survival compared with placebo plus BTZ and Dex, leading to US Food and Drug Administration approval of the combination for the treatment of patients with MM who have received ≥ 2 prior regimens including BTZ and an immunomodulatory agent. The "PANobinostat EXpansion treatment protocol" (PANEX) study was developed to provide access to PAN and gather additional safety data on the PAN-BTZ-Dex combination until PAN became commercially available.

Methods: PANEX is a multicenter, open-label, expanded treatment protocol study of PAN-BTZ-Dex in adult patients with MM relapsed and/or refractory to ≥ 1 prior line of therapy. During treatment phase (TP) 1 of the study, patients received oral PAN (20 mg; days 1, 3, 5, 8, 10, and 12) plus intravenous (IV) or subcutaneous (SC) BTZ (1.3 mg/m²; days 1, 4, 8, and 11) for eight 21-day cycles. Patients with at least stable disease proceeded to TP2, with maintained PAN and less frequent BTZ dosing (days 1 and 8) for an additional 8 cycles. In both phases, oral Dex (20 mg) was administered on the days of and after BTZ treatment. Reduction to once-weekly BTZ was allowed before TP2 as a dose reduction strategy. Response assessments were completed per modified European Group for Blood and Marrow Transplantation criteria.

Results: A total of 39 patients, with a median age of 70 years (range, 44-88 years), were enrolled in the study. Patients were heavily pretreated, with a median of 3 prior lines of therapy (range, 1-12; 21% ≥ 7 prior therapies), with nearly half of the patients (48.7%) having progressed on their most recent therapy. Most patients received SC BTZ (87.2% [n = 34] vs 12.8% [n = 5] IV BTZ), with a median treatment duration of 9.4 weeks (range, 2-34). A total of 21 patients achieved a partial response, leading to an overall response rate of 53.8%; an additional 17.9% achieved a minimal response, for a clinical benefit rate (≥ minimal response) of 71.8%, with 25.6% having stable disease. Notably, no patients in the current trial had a best response of progressive disease. The most common grade 3/4 hematologic laboratory abnormalities were thrombocytopenia (53.8%), anemia (17.9%), and neutropenia (17.9%). The most common nonhematologic grade 3/4 treatment-emergent adverse events (AEs) were dehydration (28.2%), fatigue (28.2%), diarrhea (17.9%), asthenia (10.3%), and pneumonia (10.3%). Common gastrointestinal treatment-emergent AEs of any grade included diarrhea (48.7%), nausea (25.6%), and vomiting (12.8%), and nearly all cases of dehydration were associated with these gastrointestinal events. Interestingly, in the patients receiving SC BTZ (n = 34), the rate of grade 3/4 diarrhea was 11.8%, approximately half the rate seen with IV BTZ administration in the phase 3 PANORAMA 1 trial (25%). Common serious AEs included dehydration (15.4%), diarrhea (12.8%), and pneumonia (10.3%). AEs led to PAN, BTZ, and Dex dose reductions in 59.0%, 59.0%, and 46.2% of patients, respectively.

Conclusions: Overall, the results from PANEX, albeit in an older population of patients with more advanced MM, support those generated in PANORAMA 1. In the subgroup of patients receiving SC BTZ, the rate of diarrhea, an AE of interest with PAN-BTZ-Dex therapy, appeared to be reduced when compared with PANORAMA 1 data with IV BTZ administration. Further clinical experience with the use of SC BTZ in this combination will help to determine the potential impact of route of BTZ administration on tolerability.