γH2AX Expression in Tumors Exposed to Cisplatin and Fractionated Irradiation

Purpose: Is retention of gamma H2AX foci useful as a biomarker for predicting the response of xenograft tumors to cisplatin with X-ray? Is a similar approach feasible using biopsies from patients with cervical cancer? Experimental Design: Mice bearing SiHa, WiDr, or HCT116 xenograft tumors were exposed to cisplatin and/or three daily doses of 2 Gy. Tumors were excised 24 h after treatment and single cells were analyzed for clonogenic fraction and retention of gamma H2AX foci. Tumor biopsies were examined using 47 paraffin-embedded sections from untreated tumors and 24 sections from 8 patients undergoing radiochemotherapy for advanced cancer of the cervix. Results: Residual gamma H2AX measured 24 h after cisplatin injection accurately predicted surviving fraction in SiHa and WiDr xenografts. When a clinically equivalent protocol using cisplatin and fractionated irradiation was employed, the fraction of xenograft cells lacking gamma H2AX ranked survival accurately but underestimated tumor cell kill. Residual gamma H2AX foci were detected in clinical samples; on average, only 25% of tumor nuclei exhibited one or more gamma H2AX foci before treatment and 74% after the start of treatment. Conclusion: gamma H2AX can provide useful information on the response of human tumors to the combination of cisplatin and radiation, but prediction becomes less accurate as more time elapses between treatment and tumor biopsy. Use of residual gamma H2AX as a biomarker for response is feasible when cell survival exceeds similar to 20%, but heterogeneity in endogenous and treatment-induced gamma H2AX must be considered.