Abstract A004: MEDI3726 (ADCT-401), a novel antibody-drug conjugate targeting PSMA, has potent in vivo antitumor activity in prostate cancer patient-derived xenograft models
Authors: Cho Song , Zammarchi Francesca , Monks Noel R. , Vashisht Kapil , Tammali Ravinder , Schifferli Kevin , Strout Patrick , King Wanda , Dacosta Karma , Fleming Ryan , Williams David G. , Havenith Karin , Hinrichs Mary Jane Masson , Chivers Simon , Dimasi Nazzareno , Howard Phil W. , Hartley John A. , Coats Steve , Herbst Ronald , Berkel Patrick H. van , Tice David A. 
Published: February 2019

Abstract
Prostate specific membrane antigen (PSMA), a type II membrane glycoprotein, is highly expressed in nearly all prostate cancers, with the highest expression in metastatic castration-resistant prostate cancer (mCRPC). The prevalence, increased surface expression in prostate tumors, and constitutive internalization make PSMA an attractive target for an antibody-drug conjugate (ADC) approach in anticancer treatment of patients with mCRPC.
MEDI3726 (previously known as ADCT-401) is an ADC consisting of an engineered version of anti-PSMA antibody J591, site-specifically conjugated with DNA cross-linking pyrrolobenzodiazepine (PBD) dimer SG3249 for targeting prostatic cancer cells. Using prostate cancer cell line models, we have previously shown that MEDI3726 specifically binds to the extracellular domain of PSMA and, once internalized, releases the PBD dimer to cross link DNA and achieve potent in vitro and in vivo cytotoxicity.
Here we investigated the in vivo activity of MEDI3726 in a series of LuCaP prostate cancer patient-derived xenograft (PDX) models. The selected LuCaP models had varying PSMA expression and heterogeneous genetic and phenotypic backgrounds. In agreement with the earlier cell line xenograft data, dose-dependent antitumor activity was observed in PSMA-positive PDX models with durable tumor regressions in models with high PSMA expression. In the PSMA-negative LuCaP 35CR PDX model, MEDI3726 did not have significant antitumor activity, thus highlighting target-mediated in vivo activity. Increased phosphorylation of histone H2AX was observed in xenografts dosed with MEDI3726, confirming DNA damage induced by interstrand cross-linking PBD dimer as the mechanism of antitumor activity of MEDI3726.
In summary, MEDI3726 demonstrated potent and specific in vivo antitumor activity, concurrent with DNA damage, in clinically relevant prostate cancer PDX models. MEDI3726 is being evaluated in phase 1 clinical trial as an anticancer treatment in patients with metastatic castrate-resistant prostate cancer (NCT02991911).
Citation Format: Song Cho, Francesca Zammarchi, Noel R. Monks, Kapil Vashisht, Ravinder Tammali, Kevin Schifferli, Patrick Strout, Wanda King, Karma Dacosta, Ryan Fleming, David G. Williams, Karin Havenith, Mary Jane Masson Hinrichs, Simon Chivers, Nazzareno Dimasi, Phil W. Howard, John A. Hartley, Steve Coats, Ronald Herbst, Patrick H. van Berkel, David A. Tice. MEDI3726 (ADCT-401), a novel antibody-drug conjugate targeting PSMA, has potent in vivo antitumor activity in prostate cancer patient-derived xenograft models [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A004.