Prostate specific antigen/solvent interaction analysis (PSA/SIA): review of clinical data, and continued examination of potential PSA isoforms for a new assay for prostate cancer Mark Stovsky MD1, Lee Ponsky MD1, Srinivas Vourganti MD1, Peter Stuhldreher MD1, Mike Siroky MD2, Victor Kipnis PhD3, Olga Fedotoff PhD4, Larissa Mikheeva PhD4, Boris Zaslavsky PhD4, Arnon Chait PhD4,*, J. Stephen Jones MD5 1 Case Western Reserve University School of Medicine − Case Medical Center; 2Veterans Administration Boston Healthcare System; 3Biometry Research Group, National Cancer Institute; 4AnalizaDx; 5Cleveland Clinic, *Presenting author. Introduction: We describe exploratory data obtained using a novel protein structural assay for CaP diagnosis. The assay uses the novel technique called PSA/SIA (PSA/Solvent Interaction Analysis) to detect changes in PSA isoform composition that differentiate benign and malignant disease. We previously reported preliminary data using PSA/SIA on urine samples of 222 pts with excellent performance in biopsy cancer detection (ROC AUC 0.90, Sensitivity 100%, Specificity 80.3%), and also using serum samples in a separate study. We discuss the technology and its scientific and biological underpinnings, and the limitations and opportunities afforded by the original approach. Here we report continued examination of potential PSA isoforms using the same technology with a different SIA assay chemistries. Finally, we offer speculation regarding the origin of the observed structural differences in the PSA molecule and its interactions with other molecules in serum. Methods: 149 serum samples were obtained from multiple clinical sites, collected prior to prostate biopsy. These serum samples were evaluated using PSA-SIA, with biopsy results as gold standard. Separate studies regarding PSA conformation were performed in the presence of selected serum proteins and various analytical techniques. Results: PSA/SIA results are reported using a ratiometric composite structural coefficient, K. Using ROC analysis comparing total serum PSA, %free PSA, and PSA/SIA performed for total PSA and complex PSA (calculated as the difference between total and free PSA concentrations), the corresponding AUC were 0.6, 0.7, 0.77, and 0.83, respectively. Separately, the non-specific interactions of PSA with key serum proteins, including albumin, transferrin, and gamma-globulin were analytically evaluated. Conclusions: PSA/SIA uniquely exploits the structural heterogeneity in PSA resulting in enhanced diagnostic performance of CaP. The assay responds solely to structural changes and/or changes in PSA complex formation, and not to its absolute amount. Superior performance so far has been demonstrated by focusing the assay on complex PSA isoforms in the mixture. Further studies, including further assay optimization, a large scale validation study, and elucidation of the detected underlying structural differences will address these and other issues. References: 1. Stovsky, M. et al., Urology 78 (3), 2011, 601-605. 2. Fedotoff, O., et al., J. Biomolecular Structure and Dynamics 29 (5), 2012, 1051-1064.