Background: MLN9708 is an investigational, oral, potent, reversible, and specific 20S proteasome inhibitor, which immediately hydrolyzes to MLN2238, the biologically active dipeptidyl leucine boronic acid. In preclinical studies, MLN2238 has shown faster proteasome dissociation and greater tissue penetration than bortezomib. Current phase 1 data indicate that oral administration of single-agent MLN9708 was generally well tolerated, with no grade ≥3 peripheral neuropathy, and showed signs of antitumor activity in some patients with relapsed and/or refractory MM. Bortezomib in combination with lenalidomide and dexamethasone demonstrated a 100% partial response or better (≥PR) rate in patients with previously untreated MM, providing the rationale for evaluating oral MLN9708 in place of bortezomib in this combination. Here we report phase 1 data from the first combination study of MLN9708 (ClinicalTrials.gov: NCT01217957).
Methods: Primary phase 1 objectives were to determine the safety, tolerability, and maximum tolerated dose (MTD) of weekly MLN9708 in combination with lenalidomide and dexamethasone; secondary objectives included characterization of the pharmacokinetic (PK)/pharmacodynamic (PD) profile of MLN9708, assessment of PK interaction and response. Adults aged ≥18 years with ECOG performance status of 0 to 2, and adequate renal, hepatic, and hematologic function were eligible. Patients with grade ≥2 peripheral neuropathy or prior/concurrent deep vein thrombosis (DVT)/pulmonary embolism were excluded. Patients received oral MLN9708 weekly on days 1, 8, and 15, lenalidomide 25 mg on days 1–21, and dexamethasone 40 mg on days 1, 8, 15, and 22, for up to twelve 28-day cycles. Patients received thromboprophylaxis with aspirin or low molecular weight heparin. Transplant-eligible patients could undergo stem cell transplant after six cycles. MLN9708 dose escalation, from a starting dose of 1.68 mg/m2, followed a 3+3 scheme based on the occurrence of dose-limiting toxicities (DLTs) in cycle 1. Adverse events (AEs) were evaluated according to NCI-CTCAE v4.0. Response was assessed according to modified EBMT criteria.
Results: At data cut-off (June 29, 2011), 10 patients have been enrolled and treated: three each at 1.68, 2.23, and 2.97 mg/m2 and one at 3.95 mg/m2; median age was 66 years (range 59–77). Patients completed a median of 3 cycles (range 1–6), with three having received 6 cycles; treatment is ongoing in six patients. A DLT of grade 3 fainting was observed in a patient treated at 3.95 mg/m2. The MTD has not yet been reached; the current cohort is receiving 3.95 mg/m2. Drug-related AEs included rash in four patients (two maculopapular, two erythematous); vomiting and fatigue in three patients each; and diarrhea, constipation, and nausea in two patients each. Grade 1 treatment-related peripheral neuropathy was reported in one patient. Serious AEs were seen in four patients: one at 3.95 mg/m2 (DLT of grade 3 fainting), one at 2.23 mg/m2 (grade 3 DVT) unrelated to MLN9708, and two unrelated at 1.68 mg/m2 (grade 3 hypotension; grade 3 gastrointestinal hemorrhage resulting in discontinuation). Two patients required lenalidomide dose reductions to 15 mg due to erythematous rash; no dose reductions were required for MLN9708 or dexamethasone. There were no on-study deaths. Of nine response-evaluable patients, all nine achieved ≥PR, including three very good PR (VGPR) and one complete response. Time to response was rapid; all responders achieved a ≥50% decrease in M-protein in cycle 1, and best response was reached by the end of cycle 4. No patient has progressed to date. One patient with confirmed VGPR discontinued at cycle 6 to undergo stem cell transplant.
Conclusions: MLN9708 administered weekly in combination with lenalidomide and dexamethasone appears to be generally well tolerated in previously untreated MM patients at the MLN9708 dose levels studied, with evidence of antitumor activity in the dose-escalation cohorts. Evaluation continues to determine the MTD of MLN9708 in this combination. Updated results, PK data, assessment of PK interaction, and PD data will be presented.