Olema Pharmaceuticals, Inc. announced the first clinical data from the Phase 1 dose-escalation portion of the ongoing Phase 1/2 clinical trial of OP-1250, a complete estrogen receptor (ER) antagonist (CERAN) and a selective ER degrader (SERD) in development for the treatment of metastatic breast cancer and other women?s cancers. These initial data provide strong proof-of-concept for OP-1250 as a once-daily oral monotherapy in women with recurrent, locally advanced or metastatic ER+ /HER2- breast cancer and demonstrate OP-1250?s potential to become a best-in-class endocrine therapy. In the trial, OP-1250 showed highly attractive pharmacokinetics supporting once-daily dosing, favorable tolerability, and clear evidence of anti-tumor activity. Three partial responses (2 confirmed, 1 unconfirmed) and robust target lesion reduction up to 100% were observed in a heavily pretreated patient population. In the recommended Phase 2 dose (RP2D) range of 60-120 mg OP-1250 once daily, the overall response rate (ORR) was 17% and the clinical benefit rate (CBR) was 46%. Interim Results from Phase 1a Dose Escalation of OP-1250 as a Monotherapy: Pharmacokinetics (PK), safety, tolerability, and anti-tumor activity of once-daily OP-1250 monotherapy were evaluated in the open-label, dose-escalation portion of the ongoing Phase 1/2 clinical trial (NCT04505826). As of November 1, 2021, a total of 41 patients with recurrent, locally advanced or metastatic ER+ /HER2- breast cancer were enrolled across 7 dose cohorts (30 mg, 60 mg, 90 mg, 120 mg, 150 mg, 210 mg, and 300 mg once-daily). This was a difficult-to-treat, heavily pretreated population: 95% of patients were previously treated with a cyclin-dependent kinase (CDK) 4/6 inhibitor (9 patients, or 22%, received 2 or more prior CDK4/6 inhibitor regimens), 68% of patients received prior fulvestrant, and 42% received prior chemotherapy in the advanced setting. Overall, patients received a median of 3 prior lines of anti-cancer therapy and 2 prior lines of endocrine therapy in advanced settings. Of 39 patients whose circulating tumor DNA (ctDNA) was assessed, ESR1 mutations were detected in 49% at baseline. Pharmacokinetic analyses demonstrated dose-proportional increases in OP-1250 exposures across all evaluated doses, high oral bioavailability and steady-state plasma levels with minimal peak-to-trough variability. At doses 60 mg and above, OP-1250 achieved exposures exceeding the predicted thresholds for maximal anti-tumor efficacy based on preclinical models. OP-1250 was generally well tolerated, and no dose-limiting toxicities were reported at any of the seven dose levels studied. A maximum tolerated dose was not reached. The majority of reported adverse events were grade 1 or 2 at all dose levels, and the most common treatment-related adverse events as assessed by study investigator were nausea (49%), fatigue (34%), vomiting (22%) and headache (17%). No clinically significant bradycardia, ocular toxicities or diarrhea occurred. A RP2D range of 60 to 120 mg was identified for further evaluation based on pharmacokinetics, favorable tolerability, and initial evidence of anti-tumor efficacy. Three partial responses were observed among 24 efficacy-evaluable patients, including 2 confirmed partial responses and 1 unconfirmed partial response in a patient with robust target lesion reduction of 100%, but whose response remained unconfirmed due to progressive disease with a new lesion appearing at a follow-up visit. All 3 responses occurred in patients with ESR1 mutations and who had previously received CDK4/6 and aromatase inhibitors, and fulvestrant. Four response-eligible patients had target lesion reductions of greater than 30%. Patients were considered efficacy-evaluable for ORR if they had RECIST-measurable disease at baseline and at least one post-baseline tumor assessment or discontinued treatment prior to their first post-baseline assessment, and for CBR if they were enrolled at least 24 weeks prior to the data cut-off date. Across all doses, the ORR was 8% (2/24) and the CBR was 29% (7/24). For the dose levels within the RP2D range, the ORR was 17% (2/12) and the CBR was 46% (6/13). As of the data cut-off date, 32% of patients (13/41) remained on treatment with efficacy data continuing to mature, including both patients with confirmed partial responses.