P-0099 Two-Thirds of Pancreatic Ductal Adenocarcinomas have low Expression of Hent1 as Determined by the Sp120 Rabbit Monoclonal Antibody Immunohistochemistry Test

Abstract

IntroductionHuman equilibrative nucleoside transporter 1 (hENT1) is the primary membrane channel through which gemcitabine (Gem) enters pancreatic tumor cells, and patients whose tumors have low expression of hENT1 may derive little benefit from Gem therapy. MethodsWe have developed and analytically validated an immunohistochemistry (IHC) in vitro diagnostic (IVD) assay, with a new rabbit monoclonal antibody (SP120) used on the VENTANA BenchMark ULTRA automated slide staining platform, for assessing hENT1 expression in pancreatic ductal adenocarcinomas (PDAC). The distribution of hENT1 expression was initially determined in 392 retrospectively collected pancreatic tumor specimens, and a cut-off was identified that associated with clinical outcome on Gem-based therapy. The distribution of hENT1 expression was then prospectively evaluated in 242 metastasis biopsy specimens from an ongoing pivotal trial of a novel lipid-conjugated Gem (CO-101, which enters tumor cells independently of hENT1), vs. Gem in metastatic PDAC (NCT01124786). ResultsSP120 was shown to be sensitive and specific for membrane expression of hENT1 with variable expression demonstrated across a panel of primary PDAC samples. Using retrospectively collected primary tumor samples (n=201) from a randomized controlled clinical adjuvant trial (RTOG 97-04), we developed and verified a hENT1 scoring algorithm and dichotomizing cut-off for identifying patients least likely to benefit from Gem (hENT1-high vs -low; HR=0.58, p=0.018). We confirmed the predictive value of the hENT1 assay by showing there was no association with clinical outcome in the 5FU-treated control group (hENT1-high vs –low HR=0.92, p=0.7). The distribution of hENT1 expression was similar between these samples and an independent set of 130 PDAC specimens from the AIO-PK0104 study of which 77 were confirmed metastatic biopsies. Furthermore, 100% concordance in hENT1 status (high vs. low) was demonstrated between 16 primary tumors and paired, contemporaneous metastatic lesions. A high level of inter-reader concordance was demonstrated with an overall percent agreement of>90% between three pathologists. Overall, using this cut-off, approximately two thirds of pancreatic tumors displayed low-hENT1 expression across the different data sets. Importantly, in an ongoing clinical trial, this test demonstrated low expression of hENT1 in 65% of metastatic PDAC biopsy specimens, thus confirming the similar distribution of hENT1 expression between primary and metastatic tumors. ConclusionA robust hENT1 IHC IVD has been developed using the SP120 rabbit monoclonal antibody and identifies two thirds of primary and metastatic PDAC cases as having low expression of hENT1. The utility of CO-101 in this low expressing hENT1 group of patients is currently being tested versus gemcitabine in a global pivotal trial.