Abstract Robust pharmacodynamic (PD) assay results are valuable for informing go/no[[Unable to Display Character: ‐]]go decisions about preclinical and clinical development of new agents and for identifying optimal combinations of targeted agents. The NCI's Division of Cancer Treatment and Diagnosis (DCTD) develops and validates PD assays to obtain accurate information about target engagement in first-in-human clinical trials. The Pharmacodynamic Assay Development and Implementation Section (PADIS) and National Clinical Target Validation Laboratory (NCTVL) were established at the Frederick National Laboratory to create robust PD assays for the research community by validating analytical performance, demonstrating fitness-for[[Unable to Display Character: ‐]]purpose for the clinical assay protocol in preclinical xenograft models, and finalizing companion standard operating procedures (SOPs) for clinical specimen handling and processing. Proven clinical assays are transferred from the NCI to requesting sites in academia and industry through laboratory[[Unable to Display Character: ‐]]based certification and training, centralized access to current SOPs, assistance with assay transfer, and participation in the assay's Quality Assurance Plan. A quantitative immunoassay for intra-tumoral topoisomerase 1 (Top1) levels that allows direct measurement of target engagement is one such assay. Initial evaluation of the Top1 immunoassay using tumor needle biopsies was complicated by high baseline variability in clinical samples and lower than expected protein yields. Top1 levels in baseline biopsies from multiple patients were near the assay's lower limit of quantitation, and therefore did not provide an adequate threshold to quantify decreased Top1 levels following Top1 inhibitor treatment. A more efficient extraction method was developed and validated with xenograft tumors that allowed for larger volumes of lysate to be loaded into the immunoassay without matrix interference. Analysis of second pass biopsies from a subset of the same patients was used to confirm that the new assay procedure increased assay sensitivity by ∼3-fold. Based on this evaluation, the revised set of SOPs is estimated to increase the proportion of clinical biopsies that will be evaluable with the assay to > 80%, compared to approximately 65% with the original procedure. PD assay training on the validated Top1-Immunoassay SOPs is now being offered by NCI's Division of Cancer Treatment and Diagnosis. Funded by NCI Contract No HHSN261200800001E. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A214. Citation Format: Ralph E. Parchment, Katherine Ferry-Galow, Jay Ji, Yiping Zhang, Thomas D. Pfister, Robert J. Kinders, Yvonne A. Evrard, Shivaani Kummar, Joseph E. Tomaszewski, James H. Doroshow. Creating clinical target validation groups via quality assured transfer of robust clinical pharmacodynamic (PD) assays from the NCI: Improved performance of the Top1 immunoassay for analysis of tumor biopsies. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A214.