Preclinical assessment of BT5528 anti-tumor activity in patient-derived xenograft (PDX) models of pancreatic ductal adenocarcinoma (PDAC)

Background: Erythropoietin-producing hepatocellular receptor A2 (EphA2) is a receptor tyrosine kinase critical for cell development; it is highly expressed in a range of solid tumors, and its expression correlates with higher grade, later stage disease, and poor prognosis. There is high unmet need for patients with EphA2-expressing tumors, including pancreatic cancer, which has one of the highest EphA2 expression levels across solid tumors. BT5528 is a Bicycle® Drug Conjugate (BDC®), comprising a highly selective EphA2-targeting bicyclic peptide conjugated to the cytotoxin MMAE via a stable valine-citrulline cleavable linker. BT5528 has low molecular weight (4.4 kDa), enabling rapid and efficient delivery of the BDC® to the tumor and subsequent payload release, with minimal systemic exposure to the conjugate. Here we evaluate EphA2 expression and antitumor activity of BT5528 in murine PDX models of PDAC. Methods: Cohorts of female NOD SCID gamma (NSG) mice were used to generate 16 PDAC PDX models by subcutaneous implantation of patient-derived PDAC tumors into the left abdominal flank. Flash frozen paraffin embedded tumor samples from vehicle treated mice were used to assess EphA2 expression by immunohistochemistry (IHC); Tumor Proportion Score (TPS) was calculated as the number of membrane EphA2+ cells/total number of viable tumor cells x 100. Tumor-bearing mice were treated once weekly for 4 weeks with either vehicle control or BT5528 (3 mg/kg IV). Tumor growth was monitored by caliper measurements and tumor volume was calculated as [(width) 2 x length]/2. Tumor growth inhibition (TGI) was estimated for each treatment group per model using the average response across replicates after 4 weeks.Results: Of 16 PDAC PDX models tested, all 16 displayed some degree of EphA2 membrane staining, with 16/16 considered positive (TPS ≥1%), including models JH029 (20%), Panc163 (30%), and Panc421 (90%). Fourteen of 16 PDAC PDX models were assessed for antitumor activity. Six of 14 models showed high sensitivity to BT5528 (TGI ≥100%), including JH029 (157%), Panc163 (114%), and Panc421 (110%), with most models showing some level of sensitivity to BT5528 (only 3/14 had TGI <50%). Anti-tumor activity of BT5528 was not affected by the extent of desmoplasia, with TGI scores roughly equivalent across mature, intermediate, and immature tumor models. No increase in response was seen with increasing membrane EphA2 TPS. Conclusions: Expression of EphA2 was found in all PDAC PDX models. Most models were sensitive to BT5528 treatment, which was not affected by the degree of desmoplasia or membrane EphA2 TPS. These results may reflect a lack of truly EphA2-negative models in this analysis. These data support the potential for BT5528 to offer a novel option for the treatment of PDAC.