Atreca, Inc. reported additional data from ongoing Phase 1b trial, which show that ATRC-101 continues to be well tolerated, and that longer progression free survival was observed in patients with high target expression in this early trial. Patient enrollment based on target expression is ongoing in both the monotherapy and combination cohorts of the study and the company expects to report additional data and go/no-go decisions for Phase 2 development toward the end of 2023. ATRC-101 Update: The Phase 1b trial is a first-in-human, open-label study of ATRC-101 as monotherapy and in combination with pembrolizumab in patients with select solid tumor cancers.

Once every 3-week (Q3W) and once every 2-week (Q2W) schedules have been evaluated at dose levels up to 30 mg/kg with no dose-limiting toxicities observed. Enrollment is ongoing at the 30 mg/kg ATRC-101 dose level in both the Q3W monotherapy and combination arms of the trial, with participants selected based on target expression. As of the data cut-off date of February 17, 2023, a total of 71 participants had been dosed in the trial and evaluated for safety, with 62 participants receiving either the 3, 10 or 30 mg/kg dose including 42 participants treated in the Q3W monotherapy arm, 11 in the Q2W monotherapy arm, and 9 in the combination arm.

Participants enrolled in the study had received a median of five prior lines of treatment. All participants in the combination arm had experienced an unsatisfactory tumor response or progression following prior anti-PD-1 or anti-PD-L1 therapy. ATRC-101 has been generally well-tolerated.

For the 71 participants enrolled, most of the reported adverse events were grade = 2. Only two grade 3 AEs were considered potentially treatment-related, which were one instance each of headache and a small intestinal obstruction. The most common treatment-related AEs were fatigue (n=17, 24%) and pain (n=14, 20%), with no serious adverse events determined to be related to treatment with ATRC-101. The Company continued to observe that high target expression (as defined by an H-score =50) discriminates for anti-tumor activity in the 3, 10 and 30 mg/kg dose cohorts.

Among the 17 participants with a high target expression at screening treated with monotherapy at the higher dose levels, disease control was observed in 59% (10 of 17 patients) including 9 (53%) patients with stable disease (SD) and one (6%) patient with a partial response (PR). With combination patients included, disease control was observed in 58% (n=14 of 24) including 12 (50%) SD, 1 (4%) PR and 1 (4%) CR. By comparison, in participants with a screening H-score < 50 (low), disease control was 25% (6 of 24 patients) with SD as the best overall response for all patients in the monotherapy cohort.

A similar result was observed when including combination patients with 6 of 25 (24%) achieving SD as a best overall response, with none achieving PR or better. Longer progression free survival was observed in patients with high target expression treated in the 3, 10 and 30 mg/kg dose cohorts. A hazard ratio of 0.47 was observed in a Kaplan Meier analysis of monotherapy patients treated at the higher dose levels separated by H-score (n=41), and a hazard ratio of 0.40 was observed in the Kaplan Meier analysis of combination therapy patients separated by H-score (n=49).

The previously reported PR observed in a lung cancer patient and CR observed in a melanoma patient were durable, with a PR extending beyond 300 days and a CR ongoing (418 days as of data cut-off date). Atreca is continuing to enroll subjects in both the Q3W monotherapy and combination therapy cohorts at the 30 mg/kg dose level, with a focus on the recruitment of H-score high participants across tumor types. Additional data from the ongoing study will inform a future decision concerning whether to advance ATRC-101 into Phase 2 studies and in which indications.

Atreca expects to report additional data from the Phase 1b study and provide details on its potential Phase 2 development plans by the end of 2023. Preclinical Pipeline and Discovery Platform: APN-497444 (‘444) is an Atreca-discovered antibody targeting a novel, tumor-specific glycan. ‘444 displays uniform and tumor-selective binding with high target prevalence in colorectal cancer and exhibits compelling pre-clinical anti-tumor activity and initial safety when weaponized as an ADC.

Atreca expects to nominate a clinical candidate from the program in 2023 and is targeting an IND submission in late 2024. APN-346958 (‘958) is a tumor-selective antibody discovered by Atreca recognizing an RNA-binding protein target. ‘958 exhibits compelling pre-clinical anti-tumor activity and initial safety accompanied by robust immune activation and T cell expansion when weaponized as a T cell bispecific antibody.

‘958 was mutually selected as the first joint program combining an Atreca-discovered antibody with Xencor's XmAb® bispecific Fc domain and a cytotoxic T-cell binding domain (CD3), with Atreca leading preclinical and clinical development. Atreca and Xencor expect to name a candidate from the program in 2023 and target an IND submission by early 2025. MAM01/ATRC-501 is a novel, Atreca-discovered antibody licensed to the Gates Medical Research Institute (“Gates MRI”) for the prevention of malaria.

Gates MRI is preparing to file an IND for MAM01/ATRC-501 in 2023. Atreca retains commercial rights in the U.S., Europe and parts of Asia, and potential product development opportunities in those regions include prophylaxis for those traveling to malaria endemic regions. Pipeline expansion has been accelerated by continued investments in the discovery platform, which have enabled Atreca to generate leads against novel targets more efficiently.