202P A phase Ib/II study of OP-1250, an oral complete estrogen receptor (ER) antagonist (CERAN) and selective ER degrader (SERD) with palbociclib in patients (pts) with advanced or metastatic ER-positive, HER2-negative breast cancer (MBC)

OP-1250 is small molecule CERAN/SERD that binds to and completely blocks transcriptional activity of wild-type and mutant ER. OP-1250 was well tolerated in a phase I/II monotherapy study (OP-1250-001), and the recommended phase II dose is 120 mg once a day (qd). OP-1250 with palbociclib showed synergistic activity in preclinical models. Here we report updates of pharmacokinetics (PK), drug-drug interactions (DDI), safety and efficacy from a study of OP-1250 with palbociclib (OP-1250-002). Pts with advanced or MBC with progression on or after ≤1 line of endocrine therapy (prior CDK4/6 inhibitors and chemotherapy were allowed) were enrolled into sequential cohorts to receive escalating doses of OP-1250 PO qd with palbociclib 125 mg PO qd for 21 of 28 days, using a 3+3 design, followed by dose expansion. As of January 23, 2023, 20 pts have been treated with palbociclib and OP-1250 doses of 30/60/90/120 mg (n=3/3/3/11). Fourteen received prior CDK4/6 inhibitor; 11 received prior palbociclib. No DLTs occurred. The most common (≥4 pts) treatment emergent adverse events (AEs) were neutropenia, nausea, vomiting, anemia, gastroesophageal reflux, constipation, and thrombocytopenia (all were Grade 1-2, except neutropenia). Grade 3 neutropenia occurred in 11 pts (55%). No Grade 4 AEs occurred. The exposure of OP-1250 (n=18) was consistent with the monotherapy study. Palbociclib exposure at steady state was comparable to published monotherapy data when combined with OP-1250 at all dose levels tested. Anti-tumor activity has been observed including partial responses. OP-1250 did not affect palbociclib PK and no DDIs have been observed with this combination. OP-1250 and palbociclib combination was well tolerated, safety was consistent with individual profiles of each drug as a monotherapy. Tumor responses were observed in this heavily pretreated population. Expanding on our previous report (SABCS 2022), these data provide rationale to continue exploring OP-1250 with the approved dose of palbociclib. Updated data will be presented. (NCT0526610).