PRO-seq analysis of estradiol, OP-1250, fulvestrant and 4OH-tamoxifen treatments at multiple timepoints in breast cancer cells

The estrogen receptor (ER) is a well-established target for the treatment of breast cancer, with the majority of patients presenting as ER-positive (ER+). Endocrine therapy is a mainstay of breast cancer treatment but the development of resistance mutations in response to aromatase inhibitors, poor pharmacokinetic properties of fulvestrant, agonist activity of tamoxifen, and limited benefit for elacestrant leave unmet needs for patients with or without resistance mutations in ESR1, the gene that encodes the ER protein. Here we describe OP-1250, a novel, orally bioavailable complete ER antagonist and selective ER degrader. OP-1250, like fulvestrant, has no agonist activity on the ER and completely blocks estrogen-induced transcriptional activity. In addition, OP-1250 demonstrates favorable biochemical binding affinity, ER degradation, and antiproliferative activity in ER+ breast cancer models that is comparable or superior to other agents of interest. OP-1250 has superior pharmacokinetic properties relative to fulvestrant, including oral bioavailability and brain penetrance, as well as superior performance in wild-type and ESR1-mutant breast cancer xenograft studies. OP-1250 combines well with cyclin-dependent kinase 4 and 6 inhibitors in xenograft studies of ER+ breast cancer models and effectively shrinks intracranially implanted tumors, resulting in prolonged animal survival. With demonstrated preclinical efficacy exceeding fulvestrant in wild-type models, elacestrant in ESR1-mutant models, and tamoxifen in intracranial xenografts, OP-1250 has the potential to benefit patients with ER+ breast cancer. MCF7 or CAMA1 cells were plated in estradiol-depleted charcoal dextran stripped serum-containing media for 24 hours prior to dosing with 316 nM compound or 100 pM 17-beta estradiol and harvested after 15 min, 1 hr, 6 hr or 24 hr treatment. For estradiol pretreated conditions, cells were treated for 24 hours with 17-beta estradiol prior to compound treatment and harvested after 15 min, 1 hr, 6 hr or 24 hr treatment. Harvested samples were processed for precision run-on sequencing.