Cancer Therapy: Clinical g-H2AX Foci Formation as a Pharmacodynamic Marker of DNA Damage Produced by DNA Cross-Linking Agents: Results from 2 Phase I Clinical Trials of SJG-136 (SG2000)

Purpose: To evaluate g-H2AX foci as a pharmacodynamic marker for DNA damage induced by DNA interstrand cross-linking drugs. Experimental Design: g-H2AX foci formationwas validated preclinically in comparisonwith the Comet assay, and evaluated pharmacodynamically in two phase I studies of different dosing schedules of the novel cross-linking agent SJG-136 (SG2000). Results: The measurement of g-H2AX foci in human fibroblasts and lymphocytes in vitrowas more than 10-fold more sensitive than Comet assay measurement of cross-linking, with peak g-H2AX response 24 hours after the peak of cross-linking. In lymphocytes from a phase I study (every three week schedule), g-H2AX foci were detectable 1 hour following the end of administration, and in all patients, maximum responsewasobserved at 24hours. Significant levels of fociwere still evident at days 8 and15 consistentwith the known persistence of the DNA damage produced by this agent. In two tumor biopsy samples, foci were detected 4 hours postinfusion with levels higher than in lymphocytes. Extensive foci formation was also observed before the third dose in cycle 1 in lymphocytes from a second phase I study (daily 3 schedule). These foci also persisted with a significant level evident before the second cycle (day 21). An increased g-H2AX response was observed during the second cycle consistent with a cumulative pharmacodynamic effect. No clear relationship between foci formation and administered drug dose was observed. Conclusion: This is the first use of g-H2AX as a pharmacodynamic response to a DNA cross-linking agent in a clinical trial setting. Clin Cancer Res; 1–10. 2012 AACR.