Homologous recombination deficiency in endometrial cancer: shedding light on recent clinical findings

Endometrial cancer is the most common gynecologic malignancy in the United States, with rising incidence and high recurrence rates. Immune checkpoint inhibitors (ICIs) benefit patients with mismatch repair-deficient (dMMR) tumors, but options remain limited for those with mismatch repair-proficient (pMMR) disease. Homologous recombination deficiency (HRD), a genomic instability phenotype, has emerged as a therapeutic target. Poly(adenosine diphosphate-ribose) polymerase inhibitors (PARPis) are being investigated in endometrial cancer, with studies exploring whether HRD predicts response, particularly in combination with ICIs or chemotherapy. This review examines HRD in endometrial cancer, focusing on its molecular basis, clinical implications, and emerging therapeutic strategies. HRD occurs in a sub-set of endometrial cancers, particularly non-endometrioid sub-types, and is linked to genomic instability and platinum sensitivity. The Cancer Genome Atlas (TCGA) molecular classification has improved understanding of HRD prevalence across sub-types. HRD testing remains challenging due to a lack of standardization, with current methods including genomic-scar assays, next-generation sequencing, and functional assays. Clinical trials, such as DUO-E and RUBY-2, suggest that PARPi combined with ICIs or chemotherapy may improve outcomes in pMMR tumors, whereas PARPi monotherapy offers limited benefits. Resistance to PARPi is common, driven by the restoration of homologous recombination repair, replication fork stabilization, and drug efflux. HRD is a promising biomarker and therapeutic target in endometrial cancer. Evidence supports the integration of PARPi for select populations, although further research is needed to refine testing, optimize patient selection, and overcome resistance. Future trials should prioritize predictive biomarkers and novel combinations to maximize the benefits of PARPi in HRD endometrial cancer.