Results: At data cut-off (March 1, 2023), a total of 310 patients were enrolled in GO39775. 76 patients met the inclusion criteria and were eligible for analysis. Median age was 65 years (range: 43–82), with a median of 6 previous lines of therapy (range: 2–12). 46 patients (60.5%) had IgG isotype, 10 (13.2%) had IgA isotype, and 20 (26.3%) had light chain disease.

Of the 76 patients who met the inclusion criteria, 10 (13.2%) experienced IR (see Table); 5 patients had IgG isotype, 3 had IgA isotype, and 2 had light chain disease. The 10 patients with IR were treated for a median of 323 days (range: 64–364). Of these, 6 patients had IR at a median time of Day 156 (range: 123–260) while on cevostamab, while 4 patients had IR at a median time of Day 242 (range: 133–309) after stopping cevostamab. Of the 6 patients who reconstituted on cevostamab, 1 had IgG isotype, 3 had IgA isotype, and 2 had light chain disease. All 4 patients who reconstituted after stopping cevostamab had IgG isotype. Apart from disease isotype, no other baseline characteristic demonstrated a trend with IR.

Discussion: The majority of patients with a response to cevostamab in the GO39775 study had baseline IP, consistent with previous findings in RRMM. Among eligible responders, 10 patients (13.2%) reconstituted ≥1 polyclonal Ig, highlighting that disease control may allow some patients to reverse IP. Of the patients who had IR, 6 had immune recovery while on cevostamab therapy, which indicates that IR is possible on cevostamab once disease control is achieved. Our findings demonstrate that despite normal plasma cell targeting, IR can occur while receiving a FcRH5xCD3 bispecific antibody. Further analysis on a larger patient pool may provide more insight into the characteristics of patients with RRMM who experience IR on cevostamab therapy.

At data cut-off (18 May 2021), 160 pts had been enrolled (median age: 64 years, range: 33–82 years; male: 58.1%); 21.3% of pts had extramedullary disease. Median number of prior lines of therapy was 6 (range: 2–18). Most pts (85.0%) were triple-class refractory (PI, IMiD, anti-CD38 antibody). 28 pts (17.5%) had received ≥1 prior CAR-T, 13 pts (8.1%) ≥1 prior BsAb, 27 pts (16.9%) ≥1 prior antibody–drug conjugate (ADC), and 54 pts (33.8%) ≥1 prior anti-BCMA targeting agent.

Median follow-up in exposed pts was 6.1 months. Almost all had ≥1 adverse event (Table). The most common was CRS (128/160 pts [80.0%]; Grade [Gr] 1: 42.5%; Gr 2: 36.3%; Gr 3: 1.3%). Immune effector cell-associated neurotoxicity syndrome (ICANS) associated with CRS was observed in 21 pts (13.1%) and in 34/211 (16.1%) CRS events (Gr 1: 8.5%; Gr 2: 6.2%; Gr 3: 1.4%). Most CRS events occurred in C1 (87.2%), arose within 24 hours of cevostamab administration (70.5%), and resolved within 48 hours of onset (83.4%). In the pts with CRS, tocilizumab was used for CRS management in 43.8% and steroids in 25.8% (both agents: 18.0%). In SS dose-escalation (68 pts), 3.6mg was chosen as the most effective C1D1 SS dose for limiting CRS in C1, with no target dose-dependent increase in the rate or severity of CRS observed after the C1D8 administration. Likewise, in DS dose-escalation (30 pts), 0.3/3.6mg was identified as the preferred C1D1/C1D8 DS dose for limiting CRS in C1. Notably, the overall rate of CRS was lower in the pts who received the 0.3/3.6mg/target DS regimen than in those who received the 3.6mg/target SS regimen (77.3% [34/44] vs 88.2% [75/85], respectively). The rate of ICANS associated with CRS was also lower in the 0.3/3.6mg/target DS cohort than in the 3.6mg/target SS cohort (4.5% [2/44] vs 21.2% [18/85], respectively).

At data cut-off, 158/160 pts were efficacy evaluable. In dose-escalation, responses were observed at the 20–198mg target dose levels, and data suggested a target dose-dependent increase in clinical efficacy. Median time to response was 29 days (range: 20–179 days). Two dose-expansion cohorts were opened: ORR was higher at the 160mg dose level (54.5%, 24/44 pts) than at the 90mg dose level (36.7%, 22/60). At target dose levels >90mg, ORRs in pts with prior exposure to CAR-Ts, BsAbs, ADCs, and anti-BCMA targeting agents were 44.4% (4/9 pts), 33.3% (3/9), 50.0% (7/14), and 36.4% (8/22) respectively. Median follow-up among all responders (n=61) was 8.1 months; estimated median duration of response was 15.6 months (95% CI: 6.4, 21.6).

At cut-off (April 13, 2020), 51 pts (median age: 62.0 years [range: 33–80]; high-risk [HR] cytogenetics [1q21, t(4;14), t(14;16), or del(17p)]: 28 pts) had been enrolled into Arm A. Median number of prior lines of therapy was 6 (range: 2–15). Prior treatments included: proteasome inhibitors (PIs), 100% (94.1% refractory); immunomodulatory drugs (IMiDs), 100% (98.0% refractory); anti-CD38 mAbs, 78.4% (92.5% refractory); autologous stem cell transplant, 86.3%. Overall, 66.7% of pts were triple-class refractory (≥1 PI, ≥1 IMiD, and ≥1 anti-CD38 mAb), and 94.1% of pts were refractory to their last therapy.

At cut-off, 46/51 pts were evaluable for efficacy. Responses were observed at the 3.6/20mg dose level and above, in 15/29 pts (51.7%) (Table). Responses included 3 stringent CRs, 3 CRs, 4 VGPRs, and 5 PRs (Table). At the 3.6/20mg dose level and above, responses were observed in pts with HR cytogenetics (9/17), triple-class refractory disease (10/20), and prior exposure to anti-CD38 mAbs (11/22), CAR-Ts (2/3), or ADCs (2/2). Duration of response data are evolving, with 6/15 pts in response for >6 months at cut-off.

Median follow-up for safety was 6.2 months (range: 0.2–26.3 months). Almost all pts (49/51) had ≥1 treatment-related AE. The most common treatment-related AE was CRS (Lee et al. 2014 criteria; 38/51 pts, 74.5%). CRS was Grade (Gr) 1 in 20 pts (39.2%), Gr 2 in 17 pts (33.3%), and Gr 3 in 1 pt (2%) (due to Gr 4 transaminase elevation). CRS was most common in C1 (38 pts) and was uncommon or absent in subsequent cycles (4 pts). Most CRS events (49/58, 84.5%) resolved within 2 days. 18/38 (47.3%) pts with CRS received tocilizumab and/or steroids. Other treatment-related AEs in ≥5 pts were neutropenia and lymphocyte count decreased (6 pts each, 11.8%), aspartate aminotransferase increased and platelet count decreased (5 pts each, 9.8%). Treatment-related Gr 3–4 AEs (20 pts, 39.2%) in ≥3 pts were lymphocyte count decreased (6 pts, 11.8%), neutropenia (5 pts, 9.8%), anemia and platelet count decreased (3 pts each, 5.9%). No treatment-related Gr 5 (fatal) AEs were observed. Treatment-related AEs leading to withdrawal of treatment were uncommon (1 pt, 2.0%). One DLT (Gr 3 pneumonia) was observed in the 3.6/90mg cohort, but the MTD was not reached.