Multi-Cohort Retrospective Validation of a Predictive Biomarker for Topoisomerase I Inhibitors

ABSTRACT Purpose Camptothecin (CPT) analogues topotecan and irinotecan specifically target topoisomerase I (topoI), and are used to treat colorectal, gastric and pancreatic cancer. Patient response rate for this class of drug varies from 10-30% and there is no predictive biomarker for patient stratification by response. Based on our understanding of CPT drug resistance mechanisms, we developed an IHC-based predictive test (P-topoI-Dx) to stratify the responder versus non-responder patient populations. Patients and mthods The retrospective validation studies included a training set of (n=79) and a validation cohort (n=27) of gastric cancer (GC) patients, and eight cohorts of colorectal cancer (CRC) patient tissue (n=176). Progression-free survival for 6 months was considered positive response to CPT-based therapy. FFPE slides were immunohistochemically stained with anti-phosphospecific topoI-Serine10 (topoI-pS10), quantitated and analyzed statistically. Results We determined a threshold of 35% percent positive staining as offering optimal test characteristics in GC. The GC (n=79) training set demonstrated 76.6% (64-86) sensitivity; 68.8% (41-88) specificity; positive predictive value (PPV) 92.5% (81-98); and negative predictive value (NPV) 42.3% (24-62). The GC validation set (n=27) demonstrated 82.4% (56-95) sensitivity and 70.0% (35-92) specificity. Estimated PPV and NPV were 82.4% (56-95) and 70.0% (35-92) respectively. In the CRC validation set (n=176), the 40% threshold demonstrated 87.5% (78-94) sensitivity; 70.0% (59-79) specificity; PPV 70.7% (61-79) and NPV 87.0 % (77-93). Conclusion The analysis of retrospective data from patients (n=275) provides clinical validity to our P-topoI-Dx IHC test to identify the individuals who are more likely to respond to topoI inhibitors.