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New Dual Monoclonal ELISA for Measuring Plasma Osteopontin as a Biomarker Associated with Survival in Prostate Cancer: Clinical Validation and Comparison of Multiple ELISAs

¹ London Regional Cancer Program, London, Ontario, Canada;² Department of Oncology and⁷ Department of Clinical Epidemiology and Biostatistics, University of Western Ontario, London, Ontario, Canada;³ Department of Pathology, London Health Sciences Centre, London, Ontario, Canada;⁴ Assay Designs, Inc, Ann Arbor, MI;⁵ Division of Molecular Immunology, Institute for Genetic Medicine, Hokkaido University, Sapporo, Japan;⁶ Immuno-Biological Laboratories Co., Ltd., Gunma, Japan.

^aAddress correspondence to this author at: London Regional Cancer Program, 790 Commissioners Road East, London, Ontario N6A 4L6 Canada. Fax 519-685-8646; e-mail ann.chambers{at}Lhsc.on.ca.

Background: A previously developed monoclonal/polyclonal ELISA (Mono/Poly) to detect plasma concentrations of osteopontin (OPN) was shown to provide prognostic information in breast, prostate, and other cancers. Here we describe the clinical validation of a new dual monoclonal (Dual Mono) assay. We compared both assays with 4 assays that recognize defined regions of OPN protein (dual polyclonal systems 5-1, 4-1, 4-3 and polyclonal-monoclonal system 1-3).

Methods: OPN sequences recognized by the monoclonal antibodies that make up the Dual Mono ELISA were identified by Pepscan CLIPS^TM analysis. Using the 6 ELISAs, we measured OPN in plasma from 66 patients with castration-resistant prostate cancer, and we assessed the ability of each assay to predict patient survival.

Results: The assays varied in measured plasma OPN concentrations, with median values ranging from 112 to 1740 µg/L, and ability to predict patient survival. By Cox univariable regression of survival by tertiles of OPN, the Mono/Poly and Dual Mono ELISAs had the highest log-rank ² values. After adjustment for risk factors independently associated with survival in our samples, OPN remained associated with survival only for the Mono/Poly and Dual Mono systems.

Conclusions: OPN plasma values varied significantly depending on the assay used. Only the Mono/Poly and Dual Mono systems were independently associated with survival in a population of men with castration-resistant prostate cancer. The availability of a clinically validated, dual monoclonal–based ELISA will provide consistent reagents for studies of OPN plasma concentrations in cancer and other pathologies.

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