FDA Approves PEPAXTO® for Relapsed or Refractory Multiple Myeloma

SUMMARY: The FDA on February 26, 2021, granted accelerated approval to PEPAXTO® (Melphalan flufenamide) in combination with Dexamethasone for adult patients with Relapsed or Refractory multiple myeloma, who have received at least four prior lines of therapy and whose disease is refractory to at least one Proteasome Inhibitor, one immunomodulatory agent, and one CD-38 directed monoclonal antibody.

Multiple Myeloma is a clonal disorder of plasma cells in the bone marrow and the American Cancer Society estimates that in the United States, 34,920 new cases will be diagnosed in 2021 and 12,410 patients are expected to die of the disease. Multiple Myeloma (MM) in 2021 remains an incurable disease. Multiple Myeloma is a disease of the elderly, with a median age at diagnosis of 69 years and characterized by intrinsic clonal heterogeneity. Almost all patients eventually will relapse, and patients with a high-risk cytogenetic profile, extramedullary disease or refractory disease have the worst outcomes. The median survival for patients with myeloma is over 10 years. With the introduction of new combinations of antimyeloma agents in earlier lines of therapy, patients with Relapsed/Refractory myeloma often have disease that is refractory to multiple drugs. There is an urgent unmet medical need for agents with novel mechanisms of action that are safe and effective, for patients with aggressive and resistant disease.

PEPAXTO® is a novel, first-in-class peptide-drug conjugate that links a peptide carrier to a cytotoxic agent, resulting in a highly lipophilic compound. The lipophilicity allows PEPAXTO® to readily diffuse across cell membranes and get distributed into cells. Through its passive uptake into cells, the conjugated agent circumvents the development of transporter-associated resistance. The drug compound then leverages aminopeptidases, which are overexpressed in multiple myeloma cells, resulting in the release of the cytotoxic alkylating payload, which irreversibly damages tumor DNA and induces apoptosis.

The HORIZON trial is a pivotal, single-arm, multicenter, Phase II study of PEPAXTO® plus Dexamethasone in heavily pretreated patients with Relapsed or Refractory multiple myeloma. This study included 157 patients with relapsed or refractory disease, of whom 97 patients were triple-class refractory to at least one Immunomodulatory agent, one Proteasome Inhibitor, and a CD38-directed monoclonal antibody, and had received at least four prior lines of therapy. Patients received PEPAXTO® 40 mg IV on day 1 and Dexamethasone 40 mg orally (20 mg for patients 75 years of age or older) on day 1, 8, 15 and 22 of each 28-day cycle, until disease progression or unacceptable toxicity. The Primary end point was Overall Response Rate (Partial Response or better) assessed by the investigator, and Secondary end points included Duration of Response, Progression Free Survival (PFS), Overall Survival (OS), and Safety.

The FDA approval was based on the efficacy in a subgroup of patients (N=97), who were triple-class refractory and had received at least four prior lines of treatment. The Overall Response Rate for the patients within this group of patients was 23.7 % and the Median Duration of Response was 4.2 months. Among these 97 patients, 41% had extramedullary disease (N=40), an aggressive and resistant characteristic associated with poor prognosis. The most common adverse reactions in 20% or more were fatigue, fever, nausea, diarrhea and respiratory tract infection. Most common laboratory abnormalities in 50% or more were cytopenias and increased creatinine.

It was concluded that PEPAXTO® is a novel and innovative therapeutic option for patients with refractory myeloma, and is an important addition to the myeloma treatment armamentarium, in an area of unmet medical need.

Melflufen and Dexamethasone in Heavily Pretreated Relapsed and Refractory Multiple Myeloma. Richardson PG , Oriol A, Larocca A, et al. J Clin Oncol. 2021; 39:757-767