Clinical Trials of BCMA-Targeted CAR-T Cells Utilizing a Novel Non-Viral Transposon System

P-BCMA-101 and P-BCMA-ALLO1, autologous and allogeneic BCMA targeting CAR-T cell therapies respectively, are manufactured using a novel transposon-based system called piggyBac (PB). They comprise a high percentage of desirable stem cell memory T-cells associated with efficacy and safety. Single agent P-BCMA-101 data was previously reported showing marked efficacy and minimal toxicity (PRIME study), and exploratory combination cohorts have also been evaluated. P-BCMA-ALLO1 is expected to be first assessed in patients by the time of this presentation. Here we report results for P-BCMA-101 exploratory cohorts and P-BCMA-ALLO1.

PRIME is a Phase 1/2 clinical trial to assess the safety and efficacy of P-BCMA-101 in patients with RRMM (≥ 3 prior lines, including a proteasome inhibitor (PI) and an immunomodulatory agent (IMiD), or double refractory) (NCT03288493). Patients undergo apheresis to harvest T cells and P-BCMA-101 is then manufactured. Patients then receive a 3-day cyclophosphamide (300 mg/m ²/day) / fludarabine (30 mg/m ²/day) lymphodepletion (LDC) regimen. Following 2 days of rest, patients received a single administration of P-BCMA-101 in escalating doses starting at 0.75 x 10 ⁶ CAR-T cells/kg. Similar P-BCMA-101 dose escalation was performed in combination cohorts with rituximab (Rit) or lenalidomide (Len). During the study, use of a nanoplasmid (NP) vector in CAR-T manufacturing was implemented to improve transposition efficiency and cell quality.

As of June 30 ^th, 2021, 90 unique patients have been treated with P-BCMA-101 in 5 dose levels of single agent P-BCMA-101, 3 dose levels of P-BCMA-101 with Rit and 2 dose levels of P-BCMA-101 with Len. The median age is 61 years and 66/37% were M/F. Patients were heavily pre-treated with a median of 6 prior regimens (range 3-18), 100% had received prior PI and IMiD, 74% daratumumab and 63% ASCT. 13 patients were treated in combination with Rit and 9 in combination with Len. Like prior reports of P-BCMA-101, the safety profile compares favorably to other CAR-T. No dose limiting toxicities (DLT) were observed in the single agent or the combination cohorts. The addition of Rit or Len did not increase toxicity. Grade 1-2 CRS was seen in 25% (no G3/4) of patients and ICANS in 7% (2% G3). Tocilizumab was used in 11% of patients. There were no treatment related deaths or unexpected/off-target toxicities. The most common treatment emergent adverse events were cytopenias, infections and constitutional symptoms (≥ G3 neutropenia 74%, thrombocytopenia 30%, anemia 35%), as expected with CAR-T and LDC. The overall response rate (ORR) with the Rit and Len combinations was 73% and 71% respectively. The favorable safety profile allowed outpatient treatment in 23 (25%) patients.

Founded on the P-BCMA-101 data, an allogeneic CAR-T (P-BCMA-ALLO1) using a similar PB construct was created and demonstrated excellent preclinical safety and efficacy leading to the initiation of a Phase 1 clinical trial (NCT04960579). P-BCMA-ALLO1 CAR-T cells are generated from healthy donor T-cells using Cas-CLOVER to knockout Beta-2 microglobulin to prevent host vs graft rejection, TCRβchain for graft vs host disease prevention. Proprietary booster molecules are utilized to increase manufacturing yield and cell quality. The CAR binding domain is a novel anti-BCMA VCAR that produced superior efficacy in tumor models. These constructs contain the same PB transposon transgene cassette, DHFR for gene selection and an iCasp9-based safety switch, successfully assessed in the PRIME study.

The primary objective of the P-BCMA-ALLO1 phase 1 trial is to assess the safety and maximum tolerated dose (MTD) based on DLT in RRMM patients who have received a PI, IMid and CD38 mAb. Secondary objectives will assess the anti-myeloma effect of P-BCMA-ALLO1 and biomarkers. The protocol is designed to treat 40 RRMM patients in a standard 3+3 dose escalation with a P-BCMA-ALLO1 starting dose of 0.75 X 10 ⁶ cells/kg. Patients will receive P-BCMA-ALLO1 following LDC with cyclophosphamide (300 mg/m ²/day) / fludarabine (30 mg/m ²/day X 3 days.

In conclusion, transposon generated autologous CAR-T cells demonstrate excellent clinical activity with a favorable toxicity profile in RRMM, can be safely combined with Len and Rit, and administered in the outpatient setting. Allogeneic CAR-T cells generated from this platform represent a further advance. Current data from both clinical studies will be presented.