Combined CHK1 and PD-L1 blockade as a novel therapeutic strategy against stemness and immunosuppression in ovarian cancer.

[BACKGROUND] Cancer stem cells (CSCs) are considered the 'seeds' of recurrence after chemotherapy, but eliminating CSCs remains notoriously challenging. This study aims to examine whether cell cycle checkpoint kinase 1 (CHK1) blockade can abrogate the stemness of ovarian cancer (OC) cells, making them easier targets of anti-tumor immunity.

[METHODS] Prexasertib was used to block CHK1 in OC cell lines and xenografts, and its cytotoxicity was assessed in vitro and in vivo. In vitro tumor-sphere formation assays and stemness markers were used to evaluate cell stemness. PD-L1 expressions were examined via qRT-PCR, Western blot, flow cytometry, and immunohistochemistry. Prexasertib in combination with anti-PD-L1 antibody Atezolizumab was tested in immune-proficient mice bearing OC xenografts in terms of effects on tumor growth, tumor cell stemness, and tumor infiltrating lymphocytes via tumor volume monitoring, immunohistochemistry, and flow cytometry.

[RESULTS] Prexasertib effectively inhibited CHK1 phosphorylation, exhibited significant anti-tumor effects in vitro and in vivo, accompanied by decreased OC cell stemness. CHK1 was highly expressed in tumor spheres versus tumor cells cultured in 2D system, and Prexasertib treatment suppressed sphere formation and reduced the ALDH cell fraction. Unexpectedly, Prexasertib upregulated PD-L1 expression in tumor cells. In vivo, combining Prexasertib with Atezolizumab led to more remarkable remission of tumors, when compared with Prexasertib or Atezolizumab alone. Meanwhile, the tumor-infiltrating CD8 T cells significantly increased in the combination group, while exhausted T cells decreased; the treatments did not affect CD4 cell infiltration.

[CONCLUSION] Dual targeting of CHK1 and PD-L1 may improve OC treatment by simultaneously suppressing stemness and enhancing anti-tumor immunity.