Combined MEK and PARP inhibition enhances radiation response in rectal cancer

Rectal cancer is most frequently diagnosed at a locally advanced stage and treated by neoadjuvant chemoradiation. Current efforts to improve treatment outcome are focused on intensifying neoadjuvant chemotherapy, which is however associated with higher levels of toxicity. To discover alternative strategies, we established patient-derived rectal cancer organoids (PDOs) that reflect clinical radiosensitivity and used these organoids to screen 1596 drug-radiation combinations. We found that inhibitors of RAS-MAPK signaling, especially MEK inhibitors, strongly enhance radiation response. Mechanistically, MEK inhibitors suppressed radiation-induced activation of RAS-MAPK signaling, and selectively downregulated RAD51, a key component of the homologous recombination DNA repair pathway. Through testing drug-drug-radiation combinations in organoids and cell lines, we identified that a combined PARP and MEK inhibition can further enhance radiosensitivity of colorectal cancers, which was confirmed in mouse xenograft models. Our data support clinical testing of MEK and PARP combination therapy with radiation in locally advanced rectal cancers as an alternative to chemoradiation Microarrays were used to investigate the effect of radiation treatment on gene expression in three patient-derived colorectal cancer organoid lines. We measured total RNA from 3 human patient-derived colorectal cancer organoid lines.