Efficacy of Melflufen, a Peptidase Targeted Therapy, and Dexamethasone in an Ongoing Open-Label Phase 2a Study in Patients with Relapsed and Relapsed-Refractory Multiple Myeloma (RRMM) Including an Initial Report on Progression Free Survival

Background: 

Melflufen is a highly potent anti-angiogenic compound that triggers rapid, robust and irreversible DNA damage and exerts its cytotoxicity through alkylation of DNA.  The lipophilicity of melflufen leads to rapid and extensive distribution into tissues and cells where it binds directly to DNA or is readily metabolized by intracellular peptidases into hydrophilic alkylating metabolites.  With targeted delivery of alkylating metabolites to tumor cells in vitro (such as multiple myeloma that are rich in activating peptidase), melflufen exerts a 20-100 fold higher anti-tumor potency and produces a 20 fold higher intracellular concentration of alkylating moieties compared with melphalan.

Methods:

Melflufen is evaluated in combination with dexamethasone (dex) 40 mg weekly in an ongoing Phase 1/2a study.  RRMM patients with measurable disease and at least 2 prior lines of therapy are eligible (NCT01897714).  Phase 1 established the maximum tolerated dose (MTD) of melflufen to be 40 mg every 3 weeks in combination with low dose dex.  The primary objective of Phase 2a is the overall response rate and safety of the MTD in a total of 55 patients.  Response was investigator assessed at the end of each cycle by IMWG criteria.  Here we present the Phase 2 data as of 14 July 2015 data-cut.

Results:

Thirty-one patients were dosed at the MTD.  The median time from initial diagnosis to first dose of melflufen was 6 years (1-15).  The median number of prior therapies was 4 (2-9).  97% of patients were exposed to immunomodulatory drugs (IMiDs), 90% to proteasome inhibitors (PIs), 77% to melphalan, and 71% had received prior autologous stem cell transplant. 58% were double refractory (IMiDs and PIs) and 42% were triple refractory (IMiDs, PIs and alkylators).  In total, 121 doses of melflufen have been given (1-11 cycles). Median treatment duration was 13 weeks with 9 patients still ongoing.  One patient completed therapy as planned, 15 patients discontinued due to AEs (48%) and 6 due to progression (19%).

Twenty-three patients were evaluable for response (protocol defined as ≥2 doses of melflufen with baseline and follow-up response assessments).  One patient achieved a very good partial response and 10 patients achieved partial response (PR) (1 unconfirmed, still ongoing) for an overall response rate (ORR) of 48%.  Three additional patients achieved minimal response (MR) for a clinical benefit rate (CBR) of 61%.  Time to clinical benefit and response was rapid with 93% of patients achieving ≥ MR after 1-3 cycles and 64% achieving PR after only 1-3 cycles.  Eight patients maintained stable disease and 1 patient had early progressive disease.  Similar ORRs were seen in PI-refractory (43%), IMiD-refractory (40%), alkylator-refractory (62%), double-refractory (38%) and triple-refractory (50%) patients.  The median progression free survival (PFS) is currently at 7.6 months (95% confidence interval: 3.4 - ∞) based on 14 events in 30 patients.

The most frequent adverse events (AE), all grades, occurring in >10% of patients, regardless of relationship to study drug were thrombocytopenia (94%), anemia (84%), neutropenia (61%), leukopenia (42%), pyrexia (36%), asthenia (32%), fatigue and nausea (26%), bone pain (19%), cough, diarrhea, dyspnea, mucosal inflammation and upper respiratory infection (16%) and constipation and epistaxis (13%).  Treatment-related Grade 3 or 4 AEs were reported in 27 patients (87%). Those occurring in >5% of patients were thrombocytopenia (68%), neutropenia (55%), anemia (42%), leukopenia (32%) and febrile neutropenia, fatigue, pyrexia, asthenia and hyperglycemia each occurred in 6% of patients.  Serious AEs occurred in 9 patients (29%), but were only assessed as related to study drug in 5 patients (16%) including 3 febrile neutropenia, 1 fever and 1 pneumonia.  Cycle length has recently been increased to 28 days to improve tolerability with respect to hematologic toxicity.

Conclusion:

Melflufen has promising activity in heavily pretreated RRMM patients where conventional therapies have failed.  The current ORR is 48% and CBR is 61%. Similar results were seen across patient populations regardless of refractory status. The median PFS is encouraging at 7.6 months.  Hematologic toxicity was common, but non-hematologic AEs were infrequent.  Updated results will be presented at the meeting.