Purpose: Homologous recombination deficiency (HRD) correlates with platinum sensitivity in ovarian cancer patients, which clinically is the most useful predictor of sensitivity to PARPi. To date, there are no reliable diagnostic tools to anticipate response to platinum-based chemotherapy, thus we aimed to develop an ex vivo functional HRD detection test that could predict both platinum-sensitivity and patient eligibility to targeted drug treatments. Experimental Design: We obtained a functional HR score by quantifying homologous recombination (HR) repair after ionizing radiation-induced DNA damage in primary ovarian cancer samples (n=32). Samples clustered in 3 categories: HR-deficient, HR-low and HR-proficient. We analysed the HR score association with platinum sensitivity and treatment response, platinum-free interval (PFI) and overall survival (OS), and compared it with other clinical parameters. In parallel, we performed DNA-sequencing of HR genes to assess if functional HRD can be predicted by currently offered genetic screening. Results: Low HR scores predicted primary platinum sensitivity with high statistical significance (p=0.0103), associated with longer PFI (HR-deficient vs HR-proficient: 531 vs 53 days), and significantly correlated with improved OS (HR score <35 vs >=35, hazard ratio=0.08, p=0.0116). At the genomic level, we identified a few unclear mutations in HR genes and the mutational signature associated with HRD, but, overall, genetic screening failed to predict functional HRD. Conclusions: We developed an ex vivo assay that detects tumor functional HRD and an HR score able to predict platinum sensitivity, which holds the clinically relevant potential to become the routine companion diagnostic in the management of ovarian cancer patients.
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