Background Immunotherapy has been revolutionary in its anti-cancer efficacy due to its ability to elicit anti-tumor immune activation and response, rather than non-specific cell death associated with traditional chemotherapies. Specifically, immune checkpoint inhibitors (ICIs) promote T cell activation by blocking the co-inhibitory interplay during antigen presentation, with most FDA-approved ICIs targeting the PD-1/PD-L1 interaction. However, for reasons largely unknown, only 20–40% of patients respond to these treatments. Due to this, there has been a concerted effort to understand the mechanisms of ICI-resistance. Recent studies have uncovered sexual dimorphism in ICI responsiveness, with leukocyte-intrinsic androgen receptor signaling hindering and estrogen receptor signaling promoting anti-tumor efficacy supporting the utility of sex hormone receptor targeting agents as co-therapy. Estrogen action is mediated largely by binding to nuclear receptors: Estrogen Receptor Alpha (ERα) and Estrogen Receptor Beta (ERβ). ERα activation is associated with a host of unwanted side effects including modulation of sex hormone biogenesis and potential stimulation of nascent estrogen-dependent cancers.

Methods The Drug Development Institute at The Ohio State University developed OSU-ERβ-12, a carborane-based ERβ-selective agonist with a similar activity profile but superior preclinical pharmacokinetics to clinically evaluated ERβ-selective agonists. In male C57BL/6J mice with ERα or ERβknocked out, daily dosing of OSU-ERβ-12 at 10/100 mpk in wild-type and ERβ knockout mice identified dose-dependent activation of ERα associated with reduced urogenital tract mass, while ERα knockout mice had no significant urogenital tract mass loss. ERβ selective dose levels of 10 mpk or lower were corroborated using a uterotrophic assay in estrogen-naive female mice.

Results ICI-resistant syngeneic CT26 tumor-bearing mouse studies revealed that treatment with the combination of αPD1 and ERβ-selective dose levels of OSU-ERβ-12 (1 or 10 mpk) resulted in significantly smaller terminal tumor volumes compared to either monotherapy or vehicle controls.

Conclusions Splenocyte immunophenotyping confirmed OSU-ERβ-12 + αPD1 combination treatment significantly increased T cell populations such as CD8⁺PD1⁺ and CD8⁺Tim-3⁺, suggesting adaptive immune activation. Further mechanistic studies are currently underway.