Abstract A071: PSMA-specific CARTyrin T-stem cell memory therapy eliminates solid tumor in subcutaneous prostate cancer model
Authors: Smith Jenessa Barbara , Codde Rebecca , Tan Yening , Barnett Barnett E. , Hermanson David , Rengarajan Srinivas , Ostertag Eric M. , Shedlock Devon J. 
Published: February 2019

Prostate-specific membrane antigen isoform 1 (PSMA) is a transmembrane glycoprotein overexpressed in &gt;80% of primary and metastatic prostate cancers. PSMA is involved in folate uptake and confers a proliferative advantage to PSMA-expressing cells. Additionally, PSMA levels increase as cells become androgen-independent, a hallmark of advancing prostate disease. Although PSMA peptide and antibody drug conjugate therapies have shown promise, early first-generation chimeric antigen receptor (CAR) T-cell therapies have lacked clinical efficacy. Here we developed a novel CAR T-cell product (P-PSMA-101) via piggyBac™ transposition of a tri-cistronic transgene encoding a safety switch, a PSMA-specific Centyrin-based CAR (CARTyrin), and a selection gene—features that may improve safety and efficacy compared with previous anti-PSMA CAR T-cell therapies.

We first developed and identified a lead anti-PSMA CARTyrin from over 250 available Centryin binders. We also tested the previously clinically applied anti-PSMA J591 scFv-based CAR for comparability. Initial assessment utilized mRNA delivery of candidate CARTyrins to confirm CAR surface expression and specific T-cell degranulation against PSMA+ prostate tumor cells or PSMA-engineered cells. We then used piggyBac to deliver our tri-cistronic vector system encoding the lead PSMA CARTyrin (P-PSMA-101), J591 scFv CAR, or a BCMA-specific CARTyrin to T cells, resulting in &gt;95% CAR+ T cells after selection and expansion. Importantly, our unique production methodology leads to &gt;60% T-stem cell memory (Tscm) cells, an early memory population that correlates with complete responses in CD19 CAR T-cell clinical trials. In vitro, P-PSMA-101 and J591 CAR T cells specifically proliferated, lysed, and secreted IFN-γ against PSMA+ LNCaP or PSMA-engineered K562s. No evidence of tonic signaling or exhaustion was detected.

P-PSMA-101 demonstrated significantly enhanced antitumor efficacy and survival in comparison to J591 CAR T cell-treated mice (P-PSMA-101 &gt;110 days versus J591 41 days) in a low “stress test” dose of T cells against established subcutaneous LNCaP(fLuc+) solid tumors in NSG mice. The &gt;60% Tscm P-PSMA-101 expanded in vivo and gave rise to differentiated effector CAR+ T cells that were detected in the peripheral blood concomitant with a decrease in tumor burden below detectable caliper and bioluminescent imaging limits. P-PSMA-101 then contracted, yet persisted in the peripheral blood with &gt;70% of T cells exhibiting a Tscm phenotype. On the contrary, J591 CAR T cells did not significantly control tumor burden or expand to appreciable levels in the peripheral blood. In a dose titration study, P-PSMA-101 eliminated established LNCaP tumor in 100% of animals for the duration of the studies (42 days post-treatment), while 2/3 low-dose animals remained tumor-free. Ongoing in vivo studies point towards potent antitumor effects in aggressive bone metastases models of prostate cancer.

P-PSMA-101 is a first-in-class Centyrin-based CAR T-cell therapeutic that exhibits a persistently high frequency of Tscm and mediates durable anti-solid tumor efficacy that surpasses previously established anti-PSMA CAR T-cell therapy in our in vivo models. Future efforts will continue towards clinical application of P-PSMA-101 in patients with metastatic castrate-resistant prostate cancer.

Citation Format: Jenessa Barbara Smith, Rebecca Codde, Yening Tan, Barnett E. Barnett, David Hermanson, Srinivas Rengarajan, Eric M. Ostertag, Devon J. Shedlock. PSMA-specific CARTyrin T-stem cell memory therapy eliminates solid tumor in subcutaneous prostate cancer model [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 A071.