Monotherapy with
PARP inhibitors is effective for the subset of castrate-resistant
prostate cancer (CRPC) with defects in homologous recombination (HR) DNA repair. New treatments are required for the remaining
tumors, and an emerging strategy is to combine
PARP inhibitors with other
therapies that induce DNA damage. Here we tested whether
PARP inhibitors are effective for HR-proficient CRPC, including
androgen receptor (AR)-null
tumors, when used in combination with
CX-5461, a small molecule that inhibits
RNA polymerase I transcription and activates the DNA damage response, and has antitumor activity in early phase I trials. The combination of
CX-5461 and
talazoparib significantly decreased in vivo growth of patient-derived xenografts of HR-proficient CRPC, including AR-positive, AR-null, and
neuroendocrine tumors.
CX-5461 and
talazoparib synergistically inhibited the growth of organoids and cell lines, and significantly increased the levels of DNA damage. Decreased
tumor growth after combination
therapy was maintained for 2 weeks without treatment, significantly increasing host survival. Therefore, combination treatment with
CX-5461 and
talazoparib is effective for HR-proficient
tumors that are not suitable for monotherapy with
PARP inhibitors, including AR-null CRPC. This expands the spectrum of CRPC that is sensitive to PARP inhibition.