EX103, a Novel CD20xCD3 Bispecific Antibody, Showed a Favorable Safety and Potent Single-Agent Antitumor Activity in Heavily Pretreated Relapsed/Refractory Patients with B-Cell Non-Hodgkin Lymphoma

Introduction Non-Hodgkin lymphoma (NHL) is one of the most common cancers in the United States, and relapsed or refractory (R/R) B cell NHL had a poor prognosis. In recent years, T cell engaging bispecific antibody (TCB) emerged as a promising therapy. Nevertheless, severe cytokine release syndrome (CRS) remains to be a significant challenge in TCB treatment. To overcome this issue, we designed a new CD20xCD3 bispecific antibody (EX103) with a lower affinity to CD3. In the non-human primate tox study, EX103 induced much lower IL-6 release. Preliminary data from dose escalation part of Phase I clinical study demonstrates that EX103 has an encouraging safety and robust single-agent antitumor activity trend, even at low doses, in heavily pretreated R/R B-NHL patients. Method The dose-escalation part (8 dose cohorts, ranging from 1.2 to 36 mg) of this Phase I, multicenter, single-arm, open-label clinical trial has enrolled R/R CD20+ B-cell non-Hodgkin lymphoma subjects at 4 study sites in China. Objectives include safety, dose finding (RP2D), and antitumor activity. Eligible patients in each cohort received intravenous(iv) step-up doses of EX103 (with 1 priming dose and 2 intermediate doses followed by 1 target dose of EX103) administered in a 28-day dose limiting toxicity (DLT) observation period, and then received a target dose every 2 weeks for up to 12 cycles until disease progression or unacceptable toxicity. Response rates reported are based on the Lugano criteria (2014). Results As of June 2023, 5 cohorts (target dose from 1.2 to 18 mg) of the dose-escalation were completed. Among 13 evaluable patients , 9 of them are aggressive B-NHL (5 Diffuse large B-cell lymphoma (DLBCL), 3 Follicular lymphoma (FL) grade 3B, and 1 Mantle cell lymphoma (MCL)), 2 of them are indolent B-NHL (1 FL grade 1-3A, 1 Marginal zone lymphoma (MZL)), and 2 of them are Chronic lymphocytic leukemia (CLL). No DLT was observed, and the maximum tolerated dose (MTD) was not reached. The most common adverse events (AEs) were CRS, pyrexia, neutropenia, thrombocytopenia. All CRS were grade (Gr) 1-2, no Gr 3 or higher CRS. Most CRS events occurred in first or second treatment cycle, and all CRS related clinical symptoms were resolved within 48 hours. No Immune effector cell-associated neurotoxicity syndrome (ICANS) or other clinically significant neurologic AEs were reported. Among those patients, the median age was 51 years (range 43-66) and median number of prior lines of treatment was 5 (range 2-8). The preliminary Disease Control Rate (DCR) is 84.6% (11/13). Among 9 aggressive NHL patients, the overall response rate (ORR) was 67.7 % (6/9), including 3 DLBCL patients (1 each in 6 or 12 or 18 mg cohorts) and 1 MCL patient (in 3 mg cohort) achieving CR, and 2 grade 3B FL patients achieving PR (1 each in 6 or 18 mg cohorts). Both indolent NHL patients (1 FL (Grade 1-3A) and 1 MZL in 12 mg cohort) achieved PR and the preliminary ORR for indolent NHL was 100.0 % (2/2). Of 2 CLL patients, 1 achieved SD (3mg cohort). It is worth mentioning that two of the patients who failed prior CAR-T treatment had achieved PR after received 6-14 week treatment of EX103. Both patients are still under EX103 treatment. Conclusions Although this study has a limited number of subjects so far and the findings are preliminary, the available dose-escalation data demonstrate that single-agent EX103, even at low doses (3 or 6 mg), exhibits remarkable antitumor activity and favorable safety. Furthermore, EX103 demonstrates promising efficacy in heavily pretreated patients with R/R B-NHL, including some who have previously failed CAR-T treatment. The safety and efficacy of EX103 will be further evaluated in an ongoing clinical study.