Reported genomic alterations in the DNA repair pathway in studies of castrate-resistant prostate cancer (CRPC) patients have prompted interests in targeted therapies related to the DNA damage response (DDR). CHK1 kinase plays a critical role in mediating the DNA damage checkpoint response by contributing to orderly cell cycle progression, regulation of the mitotic spindle-assembly checkpoint response and is essential for homologous recombination repair. The CHK1 kinase (CHK1) inhibitor prexasertib (LY2606368) is currently in clinical development. In this study we investigated the efficacy of prexasertib as single agent in CRPC in vitro cell lines and in vivo tumors. Prexasertib inhibited cell proliferation of prostate cancer cell lines showing the more consistent and potent inhibition in androgen-receptor positive (AR) + models (n=5, IC50 value ranges from 4.3 to 13.1 nM) while AR- cell lines had IC50 values ranging from 6.4 nM to 1000 nM (n=8). A sub-set of cell lines including VCAP, LNCaP, 22RV1 and PC3 underwent additional in vitro studies including cell cycle regulation and programmed cell death induced by prexasertib. A 24-hour treatment with 50 nM prexasertib increased S-phase populations in all cell lines (VCAP, LNCaP, 22RV1 and PC3) and sub-G1 populations in VCAP and 22RV1 cells. Live-cell imaging showed 50 nM prexasertib triggered caspase 3/7 induction by 30, 19, 15 and 10 fold change in VCAP, 22RV1, LNCaP and PC3, respectively when compared with control. Western blot studies characterized the activation of the DDR pathway signaling giving rise to a time- and concentration-dependent DNA damage response leading to induction of pCHK1 (S345), γH2AX, pRPA32(S4/8) and PARP cleavage. Importantly, in all three AR+ cell lines, prexasertib yielded both time- and concentration-dependent inhibition of AR-full length and AR-variant7 expression. In addition, prexasertib demonstrated similar single-agent activity in prostate cancer patient-derived organoid (PDO) models by inhibiting proliferation and increasing apoptosis. Finally, in vivo n=1 studies of six patient-derived xenograft (PDX) models which represent heavily pretreated mCRPC patients yielded single-agent prexasertib efficacy in 4/6 models. Similarly, xenograft models including 22RV1, LNCaP and PC3 also yielded single-agent efficacy compared to vehicle groups. On-going efforts continue to deepen the understanding of the mechanisms underlying the action of prexasertib and potential markers for drug response by using RNA/gene arrays. In conclusion, prexasertib yielded potent single-agent activity in preclinical studies including cancer cell lines, PDO, xenograft and PDX models of castrate-resistant prostate cancer and these data provide rationale of the development of the prexasertib for the treatment of CRPC.

Citation Format: Ann McNulty, Greg Donoho, Jack Dempsey, Adem Abel, Jennifer Stephens, Ricardo Martinez, Damien Gerald, Carole Perruzzi, Marguerita O’Mahony, Christoph Reinhard, Aimee Lin, Wenjuan Wu. The CHK1 kinase inhibitor prexasertib (LY2606368) shows potent single-agent efficacy in in vitro and in vivo models of castrate-resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3513.