Abstract PO3-04-12: SUMIT-BC: Phase 2 randomized study of fulvestrant with or without the cyclin-dependent kinase 7 inhibitor (CDK7i) samuraciclib in advanced hormone receptor positive (HR+) breast cancer after CDK4/6i

Abstract Background: CDK7 has 3 critical roles in cancer: enhanced transcription of oncogenes and upregulation of anti-apoptotic genes, acceleration of the cell cycle through phosphorylation of other CDKs and driving estrogen receptor (ER) resistance to hormonal therapy. Inhibition of CDK7 is therefore a potential novel anti-cancer therapeutic strategy. In an initial single-arm evaluation samuraciclib (CT7001), a once daily (QD) oral CDK7 inhibitor has demonstrated a favourable safety profile and clinical activity in combination with fulvestrant, a selective estrogen receptor degrader (SERD), in patients with HR+/HER2- advanced breast cancer who have previously been treated with a CDK4/6 inhibitor. There was evidence of enhanced benefit in patients with no detectable TP53 mutation from baseline circulating tumor DNA (ctDNA) analysis and in patients without baseline liver metastases [1]. Samuraciclib is associated with low grade gastrointestinal (GI) adverse events such as nausea, vomiting and diarrhea managed with simple prophylaxis and patient counselling. An instant release capsule formulation was used in the initial clinical evaluation of samuraciclib requiring patients to take multiple capsules designed to rapidly release a large amount of material high in the GI tract. In this study a novel single tablet formulation will be administered which may enhance GI tolerability (2). This Phase 2 open-label randomised study will evaluate the efficacy, safety, pharmacokinetics and Quality of Life (QoL) impact of samuraciclib in combination with fulvestrant in comparison to a fulvestrant monotherapy control arm. Consistent with the principles of the current Project OPTIMUS initiative, 2 dose levels of samuraciclib will be evaluated [3]. All patients will undergo baseline ctDNA evaluation of TP53 mutation status to permit a prospective evaluation of its patient selection biomarker potential. Trial Design: Eligible patients will provide written informed consent, be aged 18 years or older, have histologically or cytologically confirmed ER+/HER2- advanced or metastatic breast cancer not amenable to resection or radiotherapy of curative intent, have received an aromatase inhibitor in combination with a CDK4/6 inhibitor in either the adjuvant or advanced setting, be receiving a luteinizing hormone-releasing hormone agonist if pre/perimenopausal and have RECIST v1.1 evaluable disease. Prior SERD, mammalian target of rapamycin inhibitor (mTORi) or chemotherapy for advanced breast cancer are not permitted. All patients will undergo baseline Guardant360 ctDNA analysis to establish their TP53 mutation status among others. 60 patients will be enrolled and will all receive fulvestrant 500mg IM administered on days 1, 15 and 29 and then monthly thereafter. Patients will be randomized 1:1:1 to fulvestrant alone, fulvestrant and samuraciclib 240mg QD or fulvestrant and samuraciclib 360mg QD. Patients will undergo RECIST v1.1 evaluation at baseline every 8-weeks until week 48, followed by every 12-weeks thereafter. The pharmacokinetics of samuraciclib and fulvestrant will be studied though the first 6 months of the study, with highest intensity during month 1. To evaluate Quality of Life (QoL) The Functional Assessment of Cancer Therapy -Breast questionnaire (FACT-B) will be completed. The primary endpoint is clinical benefit response at 24 weeks. Secondary endpoints are tolerability, progression free survival, overall response rate, duration of response, and the pharmacokinetics of fulvestrant and samuraciclib. QoL and correlations between ctDNA detectable TP53 mutations and efficacy/safety finding in this patient population will be reported. The study opened for recruitment in June 2023. References: 1. Coombes et al., SABCS 2021, 2. Bardia et al., JCO 2021, 3. www.fda.gov Citation Format: Glen Clack, Sercan Aksoy, Meritxell Bellet- Ezquerra, Begoña Bermejo, Cagatay Arslan, Maria Castro-Henriques, Irfan Cicin, Mahmut Gumus, Carmen Hinojo-González, Nuri Karadurmus, Denisse Lee Cervantes, Rebeca Lozano Mejorada, Emmanuel de la Mora Jimenez, Fernando Moreno Antón, Sonia Pernas, Timothy Pluard, Carlos Zuloaga Fernandez del Valle, Ozan yazıcı, Stuart McIntosh. SUMIT-BC: Phase 2 randomized study of fulvestrant with or without the cyclin-dependent kinase 7 inhibitor (CDK7i) samuraciclib in advanced hormone receptor positive (HR+) breast cancer after CDK4/6i [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 PO3-04-12.