Abstract B045: Ataxia telangiectasia- and Rad3-related kinase inhibitor (ATRi) camonsertib in combination with low dose gemcitabine in patients with solid tumors with DNA damage response (DDR) aberrations: Preclinical and Phase 1b results

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Abstract Background: Antitumor activity and good tolerability of camonsertib (cam) monotherapy in patients (pts) with recurrent solid tumors was previously reported. Preclinical data showed that cam-related tumor cell death is potentiated by the replication stress-inducing agent gemcitabine (gem) even at low doses, resulting in synergistic combination activity. We evaluated safety/tolerability and efficacy of cam+gem in this Ph1b trial (TRESR; NCT04497116). Methods: A tumor xenograft (Granta-519; ATM loss of function) was tested with a range of cam+gem doses. Pts >18yr, ECOG 0-1, with advanced solid tumors with deleterious DDR alterations were enrolled. Cam (80-120mg) + de-escalating gem doses (1000-100mg/m2) were given over 21- or 28-day (d) cycles (cam d1-2/3 and d8-9/10 or d15-16/17; gem d1 and d8 or d15). Dose decisions followed BOIN methodology. Endpoints were safety/tolerability, recommended Ph2 dose (RP2D), response rate (RECIST v1.1, CA-125 [GCIG], or PSA [PCWG3]), clinical benefit rate (CBR; response or treatment [Tx] duration ≥16 weeks [w] without progression), ctDNA molecular response rate (MRR; best reduction in mean variant allele frequency ≥50%), and cam PK. Replication stress markers were analyzed in surrogate normal tissue. Results: In a tumor xenograft model, low doses of cam (1/3 of maximum tolerated dose [MTD]) and gem (1/20 of MTD) resulted in tumor regression not observed in single agent arms, with good tolerability. As of May 2023, 54 pts were treated with cam+gem in TRESR (ovarian [n=19], pancreatic [n=8], breast [n=6], other [n=21] tumors); enrollment genes included ATM (n=12), BRCA1 (n=18), BRCA2 (n=16), other (n=8). No drug-drug interactions were noted. RECIST v1.1 or CA-125 responses were observed in 5 pts (40 evaluable): 3 cPR, 1 uPR in pts with ovarian (n=2; 1 ATM, 1 BRCA1), endometrial (gPALB2), and breast (gBRCA1) cancers; 1 CA-125 response in a pt (ovarian/gBRCA1) with SD (on Tx 21w+). Four additional pts (uterine carcinosarcoma, ovarian, liver, lung) with SD were on Tx ≥6 months. CBR was 42.4% (14/33) in evaluable pts and 66.7% (8/12) in pts with gynecological cancers. MRR was 58% (7/12) in evaluable pts. Myelotoxicity, the most frequent Tx-related toxicity, improved with decreasing gem doses. Responses were maintained in the 3 pts with cPR following reductions to ≤200mg/m2 gem; the pts with the uPR and CA-125 responses started Tx at 200 and 100mg/m2 gem, respectively. Preliminary RP2D is 80mg cam (50% of monotherapy RP2D) + 400mg/m2 gem, 28d cycle (1w on/1w off). Related Grade 3+ neutropenia, anemia, and thrombocytopenia occurred in 44%, 11%, and 0% pts, respectively (n=9). Importantly, neutrophil nadir occurred in the 1w off Tx with quick rebound, alleviating the need for dose modifications. Conclusion: The synergy of low doses of cam+gem, demonstrated in preclinical studies, was confirmed in the clinical setting. A safe and tolerable dose and schedule of the combination was identified. The antitumor efficacy supports further development in pts with gynecological cancers; evaluation at RP2D is ongoing. Citation Format: Ezra Rosen, Timothy A Yap, Elisa Fontana, Elizabeth K Lee, Devalingam Mahalingam, Martin Højgaard, Niharika B Mettu, Gregory M Cote, Ruth Plummer, Ian M Silverman, Joseph D Schonhoft, Emeline Bacque, Adrian J Fretland, Gabriela Gomez, Danielle Ulanet, Kezhen Fei, Julia Yang, Maria Koehler, Anne Roulston, Li Li, Michal Zimmermann, Benedito A Carneiro, Stephanie Lheureux. Ataxia telangiectasia- and Rad3-related kinase inhibitor (ATRi) camonsertib in combination with low dose gemcitabine in patients with solid tumors with DNA damage response (DDR) aberrations: Preclinical and Phase 1b results [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B045.