cer Therapy : Preclinical elopment of a Validated Immunofluorescence Assay γ H 2 AX as a Pharmacodynamic Marker of R oisomerase I Inhibitor Activity

Download pose: Phosphorylated histone H2AX (γH2AX) serves as a biomarker for formation of DNA doublebreak repair complexes. A quantitative pharmacodynamic immunofluorescence assay for γH2AX eveloped, validated, and tested in human tumor xenograft models with the use of clinically relevant ures. erimental Design: The γH2AX immunofluorescence assay uses a novel data quantitation and imocessing algorithm to determine the extent of nuclear-specific γH2AX staining in tumor needle biand hair follicles collected from mice bearing topotecan-responsive A375 xenografts. After method tion with the topoisomerase I (Top1) inhibitor topotecan, the assay was used to compare pharmaamic properties of three structurally related indenoisoquinoline Top1 inhibitors. ults: γH2AX response to topotecan was quantified over a 60-fold dose range (0.016-1.0 times the e single-dose maximum tolerated dose), and significant pharmacodynamic response was measured mouse equivalent of the 1.5 mg/m clinical dose as well as the lowest dose tested. Responses were a timewindowamenable for biopsy collection in clinical trials. These studies enabled characterization e and time responses for three indenoisoquinolines, resulting in selection of two for clinical evaluation. X response to Top1 inhibitors in hair follicles was also observable above a minimal dose threshold. clusions: Our γH2AX assay is sufficiently accurate and sensitive to quantify γH2AX in tumor samnd will be used in correlative studies of two indenoisoquinolines in a phase I clinical trial at the ples a National Cancer Institute. Data suggest that hair follicles may potentially serve as a surrogate tissue to evaluate tumor γH2AX response to Top1 inhibitors. Clin Cancer Res; 16(22); 5447–57. ©2010 AACR.