Abstract 404: Development of a RSPO3 CLIA-validated assay as a predictive biomarker for response to anti-RSPO3 antibody treatment in patients with solid tumors
Authors: Zhang Chun , Liu Yuwang , Wang Min , OYoung Gilbert , Kavanagh Joy , McFarlane Cheryl , Cattaruzza Fiore , Yeung Pete , Cain Jennifer , Yen Wan-Ching , Fischer Marcus , Cancilla Belinda , Dobbin Edwina , McCarthy Michelle , Gurney Austin , Faoro Leonardo , Lewicki John , Hoey Tim , Kapoun Ann M. 
Published: July 2016

Abstract
R-Spondin (RSPO) proteins bind to LGR receptors and potentiate Wnt/β-catenin signaling. We have identified a therapeutic anti-RSPO3 antibody targeting the RSPO-LGR pathway. In preclinical studies, RSPO3 gene expression has shown correlation with anti-RSPO3 antibody efficacy in multiple solid tumor types. A qPCR-based RSPO3 assay has been developed as a predictive biomarker for response to the anti-RSPO3 antibody. In addition, RSPO gene fusions may play a role in the activation of Wnt signaling. A gene fusion detection workflow consisting of a RSPO3 CLIA assay, a RSPO3 RUO assay and next generation sequencing (NGS) has also been developed.
We designed 6 qPCR-based assays for the RSPO3 CLIA assay development and 2 assays for the RUO assay. These assays were designed to span exon-exon junctions or target microarray probe set sequences. Amplification sensitivity and specificity were assessed for assay selection. The analytic performance of the candidate RSPO3 CLIA assay and quality control measures were established in a validation study. The validation study included: 1) performance specifications of the RSPO3 assay including analytical sensitivity, linearity, and precision, 2) determination of a reportable range, 3) establishment of a cut-off for the RSPO3 CLIA assay for patient selection, and 4) establishment of quality control procedures. 104 human cancer tissues and 24 independent patient-derived tumor xenografts (PDX) were used in these studies. To evaluate the fusion detection workflow, the RUO assay was performed on samples that tested above the CLIA assay cut-off. The delta Ct difference between the CLIA and RUO assays was calculated to identify potential fusions.
The limit of quantification was established for the RSPO3 CLIA assay. The 95% reference interval was estimated to be (-2.44, 16.02) with 90% confidence interval for the lower bound (-3.45, -2.12) and upper bound (15.26, 16.57). The delta Ct cut-off for the RSPO3 CLIA assay was set based on sensitivity, specificity and prevalence. No statistically significant difference in the total variance across the tested samples was observed. A549 and OV56 were identified to be cell line controls with established acceptable delta Ct limits. Using NGS, RSPO3 fusions were identified in 6 PDX tumors with delta Ct RUO - delta Ct CLIA&gt;7, including a novel fusion. This cut-off was further refined with NGS of 9 clinical samples. Prevalence of the RSPO3 expression and fusions will be presented.
A qPCR based RSPO3 assay was developed and CLIA-validated for use as a potential predictive biomarker for response to anti-RSPO3 therapy. This RSPO3 CLIA assay, together with the fusion detection workflow, will be evaluated in a Phase 1a/b dose escalation study of anti-RSPO3 (OMP-131R10) in advanced solid tumors and in combination with FOLFIRI in metastatic colorectal cancer (NCT02482441).
Citation Format: Chun Zhang, Yuwang Liu, Min Wang, Gilbert OYoung, Joy Kavanagh, Cheryl McFarlane, Fiore Cattaruzza, Pete Yeung, Jennifer Cain, Wan-Ching Yen, Marcus Fischer, Belinda Cancilla, Edwina Dobbin, Michelle McCarthy, Austin Gurney, Leonardo Faoro, John Lewicki, Tim Hoey, Ann M. Kapoun. Development of a RSPO3 CLIA-validated assay as a predictive biomarker for response to anti-RSPO3 antibody treatment in patients with solid tumors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 404.