Small molecule targeting of SERPINB3 as an anticancer strategy in cervical cancer

SERPINB3, also known as squamous cell carcinoma antigen (SCCA) is a cysteine protease inhibitor and prognostic marker associated with poor outcomes in cervical cancer. SERPINB3 blocks lysosomal-mediated cell death by inhibiting lysosomal cysteine proteases such as Cathepsin L and S (CatL, CatS) promoting radioresistance, metastasis, and recurrence. Thus, SERPINB3 is a promising target for translational treatment. We hypothesize that SERPINB3-binding small molecules will sensitize tumor cells to radiation therapy (RT) by disrupting its protease inhibitor activity. Small molecule candidates were identified on a DNA-encoded library screen of 4.4 billion compounds. To test top candidates for cytotoxicity, cervical cancer cell lines CaSki vector control (CTRL) and SERPINB3-overexpressing (B3OE) cells were treated with SERPINB3-targeting small molecules for four hours, followed by increasing doses of RT. Cell death and viability were measured using LDH and CCK8 assays at 48 and 72 hours post-RT, respectively, and significance assessed with ANOVA analyses. Protease activity assays were performed using fluorogenic CatS substrate conversion following preincubation of recombinant SERPINB3 with candidate small molecules. One small molecule enhanced RT-induced cell death in CaSki cells in a concentration-dependent manner. SERPINB3-OE cells displayed even more radiosensitization by this compound compared to CTRL cells. Additionally, no cytotoxic effects were observed in non-malignant END1 cervical epithelial cells. Interestingly, none of the candidate small molecules blocked SERPINB3 function in in vitro protease assays, suggesting an alternative mechanism may be at play. Collectively, these results suggest that small molecule approaches may represent promising therapeutic avenues for targeting SERPINB3 to overcome radioresistance in cervical cancer, and further preclinical development is underway.