Results from the Phase 1, First-in-Human Study of MEDI2228, a BCMA-Targeted ADC in Patients with Relapsed/Refractory Multiple Myeloma

Background: B-cell maturation antigen (BCMA) is widely expressed on normal plasma cells and malignant plasma cells in multiple myeloma (MM). MEDI2228 is an antibody-drug conjugate (ADC) that targets the extracellular domain of human BCMA with preferential binding to membrane-bound versus circulating soluble BCMA. The ADC is comprised of a fully human antibody to BCMA, site- specifically conjugated to a DNA cross-linking pyrrolobenzodiazepine (PBD) dimer via a protease-cleavable linker. After cell surface binding to BCMA, MEDI2228 is internalized and cleaved in the lysosomal compartment, releasing the active PBD, which cross-links DNA and leads to apoptotic cell death. We report the results from a phase 1, first-in-human, open-label, dose-escalation and expansion trial (NCT03489525) that evaluated the safety, pharmacokinetics (PK), and clinical activity of MEDI2228 in patients (pts) with relapsed/refractory MM (R/R MM).

Methods: Eligible pts were ≥18 years old with confirmed R/R MM as defined by International Myeloma Working Group consensus criteria, had measurable disease, and had an Eastern Cooperative Oncology Group performance status ≤1. Pts had to have progressed after treatment with three classes of standard-of-care anti-myeloma drugs, including proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), and monoclonal antibodies (mAbs). Dose escalation was based on an accelerated titration design, with an mTPI-2 algorithm. During dose escalation, MEDI2228 was administered in sequentially ascending dose levels (0.0125, 0.025, 0.05, 0.1, and 0.2 mg/kg) intravenously every 3 weeks (Q3W). Due to dose-limiting toxicities (DLTs), the dose was de-escalated from 0.2 mg/kg to an intermediate dose level of 0.14 mg/kg. The primary endpoint was safety and tolerability. Secondary endpoints included assessment of preliminary efficacy, PK, and immunogenicity.

Results: As of May 15, 2020, 82 pts received MEDI2228 during dose escalation and expansion. All pts had received prior therapy with an IMiD, PI, and mAb: lenalidomide, 76 (94%); pomalidomide, 68 (84%); bortezomib, 77 (95%); carfilzomib, 64 (79%); and daratumumab, 71 (87%). Pts received between 2–11 lines of prior regimens. At 0.2 mg/kg, 2 pts experienced DLTs (grade 3/4 thrombocytopenia) without bleeding; no DLTs were reported for other dose levels. The maximum tolerated dose was 0.14 mg/kg Q3W. Treatment-related adverse events (TEAEs) occurring in ≥20% of pts in the 0.14 mg/kg cohort were photophobia (53.7%), thrombocytopenia (31.7%), rash (29.3%), increased gamma-glutamyltransferase (24.4%), dry eye (19.5%), and pleural effusion (19.5%). No reports of keratopathy or visual acuity loss were observed in the 0.14 mg/kg cohort. The TEAE profiles at lower-dose levels were similar but with lower incidences compared with the 0.14 mg/kg level. Efficacy was demonstrated at all dose levels (dose, objective response rate [ORR], [n]): 0.0125 mg/kg, 33.3% [1/3]; 0.025 mg/kg, 16.7% [1/6]; 0.05 mg/kg, 33.3% [3/9]; 0.10 mg/kg, 27.8% [5/18]; 0.14 mg/kg, 61.0% [25/41]; 0.2 mg/kg, 40.0%, [2/5]. In the 0.14 mg/kg cohort, ORR was 61.0% (95% confidence interval [CI]: 44.5%, 75.8%); there were 10 (24.4%) very good partial responses (VGPRs), 15 (36.6%) partial responses, and 3 (7.3%) minimal responses. Four of the VGPR pts were immunofixation negative. Most (90.2%) pts in the 0.14 mg/kg cohort had received prior daratumumab. Median duration of response (DOR) in the 0.14 mg/kg cohort was not reached. MEDI2228 exhibited linear PK at doses of ≥0.05 mg/kg Q3W that was minimally impacted by circulating levels of soluble BCMA at baseline. There was no difference in BCMA expression by IHC in the bone marrow of responders compared with nonresponders. Thirty-six pts in the 0.14 mg/kg cohort discontinued treatment due to adverse events (n=24), progressive disease (n=8), patient decision (n=2), investigator decision (n=1), or death (n=1).

Conclusion: MEDI2228 is a BCMA-targeted ADC that demonstrated clinical efficacy at all dose levels explored. MEDI2228 0.14 mg/kg Q3W had a manageable safety profile and an ORR of 61% in a heavily pretreated population with myeloma that is R/R post PI, IMiD, and mAb therapies.