Abstract CT070: Solid Tumor Patients Require Higher Cyclophosphamide (cy) Dose Than Multiple Myeloma (MM) Patients to Achieve Adequate Lymphodepletion (LD) Necessary to Enable Allogeneic CAR-T Expansion

Abstract Introduction: Poseida is developing innovative allogeneic T stem cell memory rich CAR-T both for hematologic malignancies (HM) and solid tumors (ST) including P-BCMA-ALLO1 which targets BCMA for MM, and P-MUC1C-ALLO1 targeting MUC1-C for epithelial-derived ST. Optimal LD for allogeneic cell therapies remains to be established. Most research has focused on HM, where patients (pts) have likely undergone HSCT and are, therefore, LD experienced in contrast to ST pts. Consequently, ST pts may require higher doses of LD chemotherapy to achieve LD depth comparable to HM pts. This study sought to compare LD with higher doses of cy across our early phase 1 trials (NCT04960579/NCT05239143) which are enrolling ST and MM pts, respectively. Methods: P-BCMA-ALLO1 and P-MUC1C-ALLO1 are 3+3 phase 1 dose escalation studies enrolling heavily pre-treated pts. LD arm S used cy 300 mg/m2 + fludarabine (flu) 30 mg/m2 × 3 days, followed by CAR-T. Alternative arms tested higher cy doses: P1 (500 mg/m2) and P2 (1,000 mg/m2), both with flu. We assessed LD depth and cellular kinetics (CK) by measuring WBC, serum IL-15, and CAR-T transcripts (qPCR). Results: By Jan 4, 2024, 42 pts received P-BCMA-ALLO1 (22 in S, 13 in P1, 7 in P2), and 30 pts received P-MUC1C-ALLO1 (19 in S, 9 in P1, 2 in P2). ST pts presented with higher baseline WBC compared to MM pts with median counts of 5.7×103 vs. 4.4×103/µL. LD was less pronounced in ST pts with a median WBC nadir of 1.1×103/µL for both S and P1. In MM pts median WBC nadir was 0.6×103 for S vs. 0.2×103 for P1. For P-BCMA-ALLO1, CK at 2×106 cells/kg dose showed a mean Cmax of 130,811 copies (cp)/µg in P2 and 6,191 in P1 compared to 170 in S. Median WBC nadir was significantly (p=0.0002) lower for P2 (0.1×103/µL) compared to S (0.6×103), with corresponding AUC0-13d values of 3.5×103/µL for P2 vs. 10×103 for S (p=0.0013). Peak serum IL-15 in P2 was 66.1 pg/mL, significantly higher than 29.7 in S (p=0.0110). In P1, there was a trend towards improved LD with median WBC nadir for P1 of 0.3×103/µL and AUC0-13d of 6.4×103/µL.P1 LD for P-MUC1C-ALLO1 did not improve CK, with WBC nadir and AUC 0-13d values comparable to S. P-MUC1C-ALLO1 CK at 2×106 cells/kg showed a mean Cmax of 692 cp/µg DNA in S, 23 in P1, and 4,200 in P2. P-MUC1C-ALLO1 pts receiving P2 achieved the deepest LD (WBC nadir: 0.1×103/µL). Peak IL-15 in ST pts was increased and delayed to day 8 in P2, compared to day 0 - 1 in S or P1. Conclusion: Increasing cy dose to 500 or 1,000 mg/m2 improves CK for P-BCMA-ALLO1 pts due to enhanced LD depth. For P-MUC1C-ALLO1 pts, increasing cy to 500 does not improve LD depth or CK compared to 300. This difference suggests the need for higher LD doses in ST pts than MM pts to achieve optimal CK. This differential need for LD chemo for these two studies is unlikely due to cellular characteristics because of similar manufacturing technology with the only difference being the binder. Citation Format: Sabrina Haag, Jeff D. Eskew, Katherine McArthur, Joanne McCaigue, Sepideh Vaziri, Samuel DePrimo, Christopher E. Martin, Catherine Gregovics, Ann Murphy, Hamid Namini, Ellen Christie, Marcela Marinez-Prieto, Rajesh Belani, Stacey Cranert, Julia Coronella, Devon Shedlock. Solid tumor patients require higher cyclophosphamide (cy) dose than multiple myeloma (MM) patients to achieve adequate lymphodepletion (LD) necessary to enable allogeneic CAR-T expansion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT070.