Memory Phenotype in Allogeneic Anti-BCMA CAR-T Cell Therapy (P-BCMA-ALLO1) Correlates with In Vivo Tumor Control
Authors: Tseng Hubert , Zhang Yan , Cranert Stacey A , Richter Maximilian , Marquez Karl , Qiu Jing , Cho Benjamin , Tan Yening , Tong Min , Domingo Christine , Weiss Leslie , Argus Elvira , Sparks Jessica , Ostertag Eric M. , Coronella Julia , Shedlock Devon J 
Published: November 2021

Abstract
The emergence of CAR-T cell therapy has transformed the treatment of the previously refractory/relapsed multiple myeloma (MM). Yet, autologous CAR-T cells suffer from inconsistent manufacturing, long manufacturing timelines, and high cost, which can limit patient accessibility. To address these issues, we engineered a fully allogeneic anti-BCMA CAR-T cell candidate for MM from healthy donors (P-BCMA-ALLO1). Herein, we demonstrate that P-BCMA-ALLO1 maintains a T stem cell memory phenotype (T SCM) through genetic editing, which correlates with antitumor efficacy.
Using Poseida's proprietary non-viral piggyBac® (PB) DNA Delivery System, in combination with the high-fidelity Cas-CLOVER™ (CC) Site-Specific Gene Editing System and a proprietary "booster molecule", we generated P-BCMA-ALLO1 from healthy donor T cells. We used CC to eliminate surface expression of TCR and MHC class I to make fully allogeneic CAR-T cells. In addition to the CAR molecule, PB enables the delivery of a selectable marker allowing the generation of a final cell product that is &gt;95% CAR-positive. The inclusion of the "booster molecule" in the manufacturing process improves the expansion of gene-edited cells without compromising memory phenotype or function. This process can produce up to hundreds of patient doses from a single manufacturing run using one healthy donor, thereby significantly reducing manufacturing cost per dose. We characterized the phenotype and functionality of P-BCMA-ALLO1 using flow cytometry and Nanostring to assess their memory phenotype at both the protein and mRNA levels. Also analyzed was antitumor toxicity and proliferative capacity through multiple rounds of activation using in vitro co-culture assays and serial restimulation, respectively. The relationship of all characterizations with in vivo efficacy was then determined, as defined by control of tumor growth in an immunodeficient RPMI-8226 subcutaneous murine tumor model.
We found that P-BCMA-ALLO1 is comprised of a high frequency of T SCM after editing (Fig. 1), and the maintenance of that memory phenotype correlates with antitumor efficacy. In vivo, these CAR-T cells are potent in controlling tumor growth, comparable to or better than autologous anti-BCMA CAR-T cells. Our analysis revealed that the expression of memory markers at the surface protein level (CD27, CD62L, CD127, CCR7) and mRNA level significantly correlate with in vivo tumor control. Conversely, suboptimal research products with worse in vivo outcomes express a more exhausted gene expression profile. We reveal from our analysis that the most effective P-BCMA-ALLO1 in vivo share similar characteristics: (1) these products were a result of efficient manufacturing, with &gt;90% CAR+ and &gt;99% TCR-; (2) they carry a memory phenotype, with 50-70% T scm and high proliferative capacity after multiple rounds of restimulation; (3) they are &gt;90% viable; and (4) they show strong antitumor efficacy both in vitro and in vivo.
We demonstrate that Tscm percentage in the final product correlates with antitumor activity. P-BCMA-ALLO1 is advancing rapidly towards the clinic (NCT04960579) to positively impact the outcomes of CAR-T therapy for MM patients.
Figure 1: Memory composition of P-BCMA-ALLO1 research products. P-BCMA-ALLO1 consists mostly of stem cell memory (T scm) and central memory (T cm) T cells that are CD62L + as opposed to effector memory (T em) and effector (T eff) T cells.
Figure 1 Figure 1.

Disclosures
Tseng: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Zhang: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Cranert: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Richter: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Marquez: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Qiu: Poseida Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Cho: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tan: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tong: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Domingo: Poseida Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Weiss: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Argus: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Sparks: Poseida Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Ostertag: Poseida: Current Employment, Current equity holder in publicly-traded company. Coronella: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Shedlock: Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company.