Olema Pharmaceuticals, Inc. announced interim results from an ongoing Phase 1b/2 clinical study of palazestrant (OP-1250) in combination with CDK4/6 inhibitor ribociclib for the treatment of ER+/HER2- metastatic breast cancer. These results, as of the data cut-off of March 13, 2024, will be presented on May 16, 2024, in a poster session at the 2024 ESMO Breast Cancer Annual Congress in Berlin, Germany (ESMO Breast). The poster, titled ?A Phase 1b/2 Study of Palazestrant (OP-1250) in Combination with Ribociclib in Patients with Estrogen Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative, Advanced and/or Metastatic Breast Cancer?, highlighted that: Across 50 treated patients, the combination of up to 120 mg of palazestrant with the full and approved dose of 600 mg of ribociclib daily was well tolerated, with no new safety signals or enhancement of toxicity and an overall safety profile consistent with the established safety profile of ribociclib plus an endocrine therapy.

Palazestrant did not affect ribociclib drug exposure and ribociclib had no clinically meaningful effect on palazestrant drug exposure. Promising preliminary efficacy was observed to date with a clinical benefit rate (CBR) of 85% across all CBR-eligible patients (11/13), 83% in ESR1-mutant patients (5/6), 86% in ESR1-wild-type patients (6/7), and 83% in prior CDK4/6 inhibitor patients (10/12). Partial responses were observed in five patients through the data cut-off (2 confirmed, 3 unconfirmed) among 23 response-evaluable patients.

Findings from this study support the continued clinical development of palazestrant in combination with ribociclib for the first-line treatment of ER+/HER2- advanced or metastatic breast cancer. Phase 1b/2 Clinical Study Results: Enrollment: As of the data cut-off of March 13, 2024, 50 patients with recurrent, locally advanced or metastatic ER+/HER2- breast cancer with at least four weeks of follow-up were treated with palazestrant (3 patients each at 30 mg once daily and 60 mg once daily, 44 patients at the palazestrant recommended Phase 2 dose (RP2D) of 120 mg once daily) plus ribociclib 600 mg once daily (three weeks followed by one week off treatment). The majority of patients (37 or 74%) were 2nd/3rd line+, with 37 (74%) patients having received prior endocrine therapy for metastatic breast cancer, 35 (70%) patients having received prior CDK4/6 inhibitors (11 or 22% having received two prior lines of CDK4/6 inhibitors), and nine (18%) patients having received chemotherapy for metastatic breast cancer.

Of 48 patients whose circulating tumor DNA (ctDNA) was assessed as of the data cut-off, 27% had activating mutations in ESR1 at baseline. The study is now fully enrolled with 60 patients. Pharmacokinetics: Palazestrant demonstrated favorable pharmacokinetics characterized by high oral bioavailability, dose proportional exposure and a half-life of eight days as a single agent, with steady-state plasma levels showing minimal peak-to-trough variability enabling consistent inhibition of ER for the full dosing interval.

Palazestrant did not affect ribociclib 600 mg drug exposure when compared with published exposure data for single-agent ribociclib. Steady-state trough values showed no clinically significant difference between the combination and single-agent palazestrant. Safety and Tolerability: Treatment with palazestrant up to the RP2D of 120 mg was well tolerated with no dose-limiting toxicities, and the maximum tolerated dose (MTD) was not reached.

The majority of treatment-emergent adverse events (TEAEs) were Grade 1 or 2, and the severity and incidence of adverse events were consistent with the expected safety profile of ribociclib plus endocrine therapy. Ten patients had dose reduction of ribociclib only, due to QTcF prolongation (n=4), neutropenia (n=4), or fatigue (n=2). No patients discontinued palazestrant due to a treatment-related adverse event, and two patients discontinued ribociclib for neutropenia without discontinuation of palazestrant in the 120 mg cohort.

Neutropenia was reversible in all patients and the timing was consistent with ribociclib-related neutropenia. Efficacy: In a maturing dataset, palazestrant showed anti-tumor activity and prolonged disease stabilization in patients both with ESR1 wild-type and ESR1 activating mutations at baseline, and in those previously treated with one or two lines of CDK4/6 inhibitors. Partial responses were observed in five patients (two confirmed, three unconfirmed as of data cut-off) out of 23 response-evaluable patients.

Across patients who were CBR-eligible, the CBR was 85% (11/13) for all patients, 83% (5/6) for patients with ESR1 mutations, 86% (6/7) for patients that were ESR1 wild-type, and for CDK4/6i-pretreated patients the CBR was 83% (10/12). The longest duration on treatment is 44 weeks through the data cut-off, and 66% (33/50) of patients in this data set remain on treatment as of the data cut-off date.