As of May 24, 2022, 101 pts had received MEZI + DEX at the RP2D. Median age was 67 (range 42–85) years, median time since initial diagnosis was 7.4 (1.1–37.0) years, and 20.8% of pts had ISS stage III at study entry. Plasmacytomas were present in 38.6% of pts and 36/101 pts had high-risk cytogenetics (55/101 pts were not evaluable). Median number of prior regimens was 6 (range 3–15). Prior therapies included stem cell transplantation (77.2%) and anti-BCMA therapy (29.7%) (Table 1). All pts were refractory to last myeloma regimen and triple-class refractory (refractory to an IMiD agent [LEN/POM], a PI, and an anti-CD38 mAb), and 39.6% of pts were refractory to LEN, POM, ≥ 2 PIs, and an anti-CD38 mAb. Median follow-up was 5.8 (range 0.5–19.0) months, with a median number of 4 (1–18) cycles received and 23 (22.8%) pts continued on treatment. The main reason for discontinuation was progressive disease (50.5%).

ORR was 39.6%, with 2 (2.0%) stringent complete responses, 3 (3.0%) complete responses, 18 (17.8%) very good partial responses, and 17 (16.8%) partial responses (Table 2). While data are not yet mature, the preliminary median duration of response was 8.3 (95% confidence interval [CI] 5.4–not reached) months and median progression-free survival was 4.6 (95% CI 3.2–6.3) months. ORR was 30.8% in pts with plasmacytomas (N = 39) and 50.0% in pts with prior anti-BCMA therapy (N = 30).

Grade (Gr) 3/4 treatment-emergent adverse events (TEAEs) were reported in 90 (89.1%) pts. Most frequent (≥ 20% pts) hematologic Gr 3/4 TEAEs were neutropenia (74.3%, with 14.9% febrile neutropenia), anemia (32.7%), and thrombocytopenia (25.7%). Gr 3/4 infections were observed in 32.7% of pts; Gr 3/4 pneumonia and COVID-19 were present in 9.9% and 5.0% of pts, respectively. The occurrence of other Gr 3/4 non-hematologic TEAEs was generally low, including gastrointestinal disorders (5.9%), fatigue (4.0%), and rash (1.0%). Seventy-two (71.3%) pts and 29 (28.7%) had MEZI dose interruptions and reductions due to TEAEs, respectively; 6 (5.9%) pts discontinued MEZI due to TEAEs, with 1 pt due to hematological TEAEs.