Poseida Therapeutics (PSTX) Study Result summary

Study background and rationale

• P-BCMA-ALLO1 is an allogeneic, off-the-shelf, non-viral, TSCM-rich CAR-T therapy for relapsed/refractory multiple myeloma, designed to overcome autologous CAR-T limitations, using proprietary gene editing, a novel single VH binder, and multiple safety features.

• The therapy is manufactured in-house for rapid, on-demand treatment, with a median time from enrollment to infusion of one day and no need for apheresis or bridging therapy.

• The phase I/1b trial enrolled 72 heavily pretreated, high-risk patients, including those with prior BCMA-targeted therapies, extramedullary disease, and high-risk cytogenetics.

• P-BCMA-ALLO1 received FDA RMAT and Orphan Drug designations and is being developed in partnership with Roche.

Study design and patient population

• The open-label, multicenter phase I/1b trial used a 3+3 dose-escalation schema, testing four lymphodepletion regimens (Arms S, A, B, C), with Arm C using Cy 750 mg/m² and Flu 30 mg/m².

• Median age was 67 years; 54% female; 69–70% had high-risk cytogenetics; 26–38% had extramedullary disease; 33% were minorities, including 22% Black or African American.

• Median prior lines of therapy was six; 43% had prior BCMA therapy, 58% prior ASCT, and 29% prior GPRC5D exposure in Arm C.

• All patients had ≥3 prior therapies, including proteasome inhibitor, immunomodulatory drug, and anti-CD38 antibody.

• Arm C included the highest proportion of high-risk, BCMA-exposed, and multi-refractory patients.

Efficacy results

• In Arm C, overall response rate (ORR) was 91%, with 100% ORR in BCMA-naïve and 86% in BCMA-experienced patients.

• 22% of Arm C patients achieved complete or stringent complete response, and 48% achieved very good partial response or higher.

• Arms A and B had ORRs of 42% and 70%, respectively; pooled median duration of response was 232 days (approx. 7.5 months) for patients with >6 months follow-up.

• Median time to response was 16 days, with a median treatment decision-to-response time of ~3.5 weeks.

• Responses included deep, durable remissions in high-risk, multi-refractory patients, with retreatment possible and responses seen after multiple infusions.