Abstract PO2-04-13: TWT-203: PHASE 1b/2 STUDY OF CFI-402257 AS MONOTHERAPY IN ADVANCED SOLID TUMORS AND IN COMBINATION WITH FULVESTRANT IN PATIENTS WITH ER+/HER2- ADVANCED BREAST CANCER AFTER PROGRESSION ON PRIOR CDK4/6 INHIBITORS AND ENDOCRINE THERAPY

Abstract Background: TTK (Threonine Tyrosine Kinase also known as Monopolar spindle 1), is a dual-specificity serine-threonine kinase critical for anaphase promoting complex inhibition at the spindle assembly checkpoint and is required for chromosome alignment and error correction. TTK inhibition results in premature mitosis exiting with unattached chromosomes potentially leading to aneuploidy and cell death. High TTK tumor levels correlate with worse prognosis and contribute to the survival and proliferation of aneuploid cells. CFI-402257, a potent and selective inhibitor of TTK, inhibits the growth of a variety of human cancer-derived cell lines. A first-in-human phase 1 study of CFI-402257 (NCT02792465) demonstrated a tolerable safety profile when enrolled as a monotherapy in solid tumors, and in combination with fulvestrant in hormone receptor positive, HER2 negative (HR+/HER2-) breast cancer1. The dose for expansion was 168 mg, dose limiting toxicity was dose-dependent neutropenia which was manageable and reversible. Investigator-confirmed partial responses (cPR) were observed in 5 pts (8%) with 32 (50.8%) exhibiting disease control. In the HR+/HER2- breast cancer population previously treated with cyclin dependent kinase 4/6 inhibitors (CDK4/6i) and aromatase inhibitors (N=25), there were 4 cPR’s with a median duration of response of 223 days, with responses emerging after 2 cycles of therapy. Responses were observed with CFI-402257 as a single agent and in combination with fulvestrant. Based on these data, study TWT-203 will focus on advanced solid tumors for dose confirmation then focus on advanced HR+/HER2- breast cancers in combination with an approved endocrine therapy. Methods: In TWT-203 study, safety and clinical activity of CFI-402257 monotherapy will be evaluated in patients (pts) with advanced solid tumors (Part A) or in combination with fulvestrant in pts with HR+/HER2- advanced breast cancer (Part B). Part A will confirm the RP2D using a 3+3 design with a starting dose of 126 mg daily. Part B evaluates CFI-402257 in combination with fulvestrant in pts with HR+/HER2- advanced breast cancer following progression on prior CDK4/6i and endocrine therapy. Efficacy endpoints include overall response rate and disease control rate. Safety endpoints include incidence of treatment emergent adverse events. Exploratory objectives include characterization of protein and molecular alterations relevant to the cell cycle and CFI-402257 response. Results: At data cutoff of 11 May 2023, 14 pts were enrolled. All received monotherapy treatment. Median treatment duration was 2.1 months (range, 0.2-7.3+). Median age was 67 years (57-76). Median number of prior regimens was 3.5 (2-12). Tumor types enrolled were colorectal (n=6, 43%), breast (3, 21%), and endometrial, hepatocellular, leiomyosarcoma, pancreatic, and sarcoma (1 each, 7%). 4 dose levels, from 126 to 252 mg, were studied. Most pts (12, 86%) experienced ≥1 treatment emergent adverse event (TEAE). More than half of pts (8, 57%) experienced ≥1 treatment related TEAE. Most common TEAEs were fatigue (5, 36%) and nausea (4, 29%). 5 pts (36%) experienced TEAEs grade ≥3, most common were fatigue, neutrophil count decrease, white blood cell count decrease (2 pts each, 14.3%). 1 pt (7%) experienced a serious adverse event (hematuria), not related to study therapy. No patients discontinued treatment due to TEAE. All pts who ended treatment were due to disease progression (8, 57%). No grade 5 TEAEs occurred. No dose limiting toxicities were reported. Disease control rate (CR, PR, or SD ≥ 6 weeks from baseline) was 54.5% (6 pts/11): all achieved SD. 3 of the 6 pts remain on study. Conclusion: CFI-402257 is a potent inhibitor of TTK. It is well tolerated with manageable TEAEs, no dose limiting or treatment limiting toxicities, and no treatment related deaths. Dose expansion in the patient population of interest will commence. 1. John Hilton et al. Cancer Res 2023;83(5 Suppl): P6-10-13. Citation Format: Justin Call, Muralidhar Beeram, Alexander I. Spira, Mark Bray, Dih-Yih Chen, Long Kwei, Emily Roberts-Thomson, Trisha Denny, Roger Sidhu, Robert Wesolowski. TWT-203: PHASE 1b/2 STUDY OF CFI-402257 AS MONOTHERAPY IN ADVANCED SOLID TUMORS AND IN COMBINATION WITH FULVESTRANT IN PATIENTS WITH ER+/HER2- ADVANCED BREAST CANCER AFTER PROGRESSION ON PRIOR CDK4/6 INHIBITORS AND ENDOCRINE THERAPY [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-04-13.