Background: The G protein-coupled estrogen receptor (GPER) is a broadly expressed G protein-coupled receptor that is tumor suppressive. LNS8801 is an oral, highly selective small molecule agonist of GPER. GPER activation results in c-Myc depletion, inhibition of tumor proliferation, and enhancement of tumor immune recognition. Preclinically, LNS8801 demonstrates potent single-agent and combinatorial anti-cancer activity and can overcome established resistance to standard-of-care anti-cancer therapies including immune checkpoint inhibitors. Methods: The primary objective of this phase 1/1B first-in-human, open-label, multicenter study (NCT04130516) was to determine the safety and tolerability and recommended phase 2 dose (RP2D) of LNS8801 in patients (pts) with locally advanced or metastatic solid tumor malignancies, both as monotherapy and in combination with the anti-PD-1 antibody, pembrolizumab. Dose levels were escalated in a 3+3 fashion and included 10, 40 and 125 mg dosed 3/7, 125 mg daily, and 125 and 250 mg twice daily. Dose limiting toxicity (DLT) was defined via NCI CTCAE v5.0 during the first 21 days of treatment. An increase in prolactin over the initial 12 hrs of dosing was measured to assess systemic GPER signaling. Tumor c-Myc expression was measured as a surrogate of treatment-related biologic response. Radiographic response (RECIST v1.1) was evaluated every 8 weeks until progression. Results: 33 pts (19 M/14 F) with median age 58.8 y and 4 (1-9) prior therapies enrolled. Median duration of treatment was 66 d (1–367+). With monotherapy (n = 28), no DLTs, treatment-related SAEs, or treatment-related study discontinuations were observed up through the maximum administered dose (250 mg bid). Possibly related AEs were grade 1 or 2 and did not correlate with dose level. Exposure was above that predicted for efficacy and t_1/2 was ̃10 hr at all doses. Of 26 evaluable monotherapy pts, 8 (27%) experienced stable disease (SD) for up to a year. All SD pts had a prolactin response. Among tumors expressing GPER, c-Myc depletion was observed in 100% (5/5) of paired pre and on-treatment biopsies. In the combination cohort (n = 5), 2/2 evaluable pts demonstrated net tumor reductions on initial f/up scans, including one RECIST partial response. Based on PK/PD data, 125 mg daily has provisionally been identified as the monotherapy and combination RP2D. Conclusions: LNS8801 is well tolerated and demonstrates signals of anti-tumor activity when administered both as monotherapy and in combination with pembrolizumab. Confirmation of RP2Ds and updated efficacy data will be presented in June. A phase 2A expansion study to evaluate these RP2Ds in clinical settings of high unmet need is now in development. Clinical trial information: NCT04130516