SEA-BCMA, an Investigational Nonfucosylated Monoclonal Antibody: Ongoing Results of a Phase 1 Study in Patients with Relapsed/Refractory Multiple Myeloma (SGNBCMA-001)

Background

Patients (pts) with relapsed/triple-class refractory (R/R; refractory to a proteasome inhibitor, immunomodulatory drug or anti-CD38 antibody) multiple myeloma (MM) have limited treatment options. A need remains for novel combination therapies with manageable toxicity and non-cross-resistant mechanisms of action. B-cell maturation antigen (BCMA) is expressed in most MM pt tumors. SEA-BCMA is an investigational, humanized, nonfucosylated IgG1 monoclonal anti-BCMA antibody. Proposed mechanisms of action of SEA-BCMA include blocking of BCMA-mediated pro-survival and proliferative signaling, mediating antibody-dependent cellular phagocytosis, and displaying enhanced antibody-dependent cellular cytotoxicity. Preclinical data demonstrate promising antitumor activity and versatile combinability. Here, we present preliminary data from the first-in-human Phase 1 clinical trial.

Methods

SGNBCMA-001 (NCT03582033) is an ongoing phase 1, open-label, multicenter, dose-escalation and expansion study to evaluate the safety, tolerability and antitumor activity of SEA-BCMA in adults with R/R MM not previously exposed to any other BCMA-directed therapy. Part A tested monotherapy safety and tolerability with dose escalation (100-1600 mg flat dosing once every 2 weeks [Q2W] by intravenous infusion), and dose expansion at the highest tolerated dose. Parts B and C aim to optimize efficacy by testing intensified dosing (Part B; weekly [Q1W] induction dosing of SEA BCMA for 8 weeks is followed by Q2W maintenance dosing) and dexamethasone (DEX) combination therapy (Part C) in pts who have received ≥3 prior lines of therapy for MM and are triple-class refractory. Responses are assessed per the 2016 International Myeloma Working Group criteria.

Results

As of June 15, 2021, Part A completed enrollment. A total of 55 pts received SEA-BCMA (see Table 1 for pt characteristics).

SEA-BCMA was generally well tolerated, and the maximum tolerated dose was not reached in the 5 dose levels tested in Part A (100, 200, 400, 800, or 1600 mg Q2W; n=2, 2, 2, 7, and 7, respectively). At 800 mg, 1 of 7 pts reported a Grade 3 infusion-related reaction (IRR), which met dose-limiting toxicity (DLT) criteria by lasting >24 hours despite supportive care. This constituted the single DLT observed during dose escalation. Subsequently, dose expansion (n=15) proceeded at 1600 mg Q2W. The most common treatment-emergent adverse events (TEAEs) observed with 1600 mg Q2W (n=22) were fatigue (32%), pyrexia (23%), IRR (23%), and hypertension (23%) for non-hematological events, and anemia (14%) for hematologic events. The most common ≥Grade 3 TEAEs with 1600 mg Q2W were anemia and syncope (9% each). Following a safety monitoring review, premedication with acetaminophen + antihistamine was introduced to mitigate IRRs in expansion cohorts. Parts B and C completed parallel safety run-ins (n=6 each) at the 1600 mg dose with no DLTs observed and with similar tolerability.

Pharmacokinetic (PK) analyses indicate serum SEA-BCMA exposures increased proportionally with increasing dose with a half-life of approximately 10 days. Preliminary analyses suggest the PK profile of SEA-BCMA in combination therapy with DEX (n=5) is comparable to monotherapy and is unaltered with a change in schedule from Q2W to Q1W.

Three of 22 pts who received 1600 mg SEA-BCMA Q2W monotherapy (Part A) achieved a confirmed objective response (OR; OR rate: 14% [95% confidence interval: 2.91, 34.91]; 2 partial responses [PR], 1 very good PR [VGPR]). To date, 3 pts remain on treatment (2 PRs and 1 stable disease). The median duration of treatment was 12 weeks (Figure 1; range 4-88).

In Parts B and C, 2 of 8 (2 PR) and 2 of 12 (1 VGPR, 1 PR) pts reported a confirmed OR; 3 of the 4 responding pts remain on treatment in cycle 7, 8, and 9, respectively.

Conclusions

Preliminary results for SGNBCMA-001 suggest a favorable safety profile and combination potential. SEA-BCMA also showed encouraging duration on treatment and initial antitumor activity in a heavily pre-treated late-line R/R MM pt population. Expansion cohorts testing intensified SEA-BCMA dosing and the addition of DEX are underway to further assess the safety, tolerability, and estimate of SEA-BCMA activity in the context of these treatment approaches. An additional expansion cohort (Part D), testing the combination of SEA-BCMA with pomalidomide and DEX in pts who have received ≥2 prior lines of therapy, is now enrolling.

Figure 1

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Figure 1

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