Long Term Follow-up after SCRI-CAR19v1 Reveals Late Recurrences As Well As a Survival Advantage to Consolidation with HCT after CAR T Cell Induced Remission

Background: CD19 chimeric antigen receptor (CAR) T cell therapy has demonstrated robust responses in refractory/relapsed subjects with CD19+ acute lymphoblastic leukemia (ALL). Our Phase 1 clinical trial demonstrated a minimal residual disease (MRD) negative complete remission (CR) rate of 93% at 21 days following SCRI-CAR19v1 (a CD19 specific CAR T cell product) infusion (PMID: 29171004). Though remission is frequently attained, approximately half of patients recur. Controversy exists regarding the benefit of hematopoietic cell transplant (HCT) following CD19 CAR T cell therapy. Although not mandated by the study, our current institutional recommendation for relapsed/refractory patients without a history of allogeneic HCT is to undergo a HCT once in remission following SCRI-CAR19v1. Additionally, we have recommended HCT to those who have a short duration of persistence of SCRI-CAR19v1 in vivo regardless of prior HCT status. We report here the leukemia free survival (LFS) of subjects who proceeded to HCT following remission after SCRI-CAR19v1 infusion.

Methods/Results: We analyzed the first 64 subjects on our Phase 1/2 trial, PLAT-02 (NCT02028455), with follow-up of ≥1 year to evaluate the potential benefit of consolidative HCT. We excluded subjects that did not respond (n=5) or relapsed prior to day 63 (n=9). Thirty-two of the evaluated subjects were treated on the Phase I dose finding portion of the study and 18 were treated on the Phase 2 portion at dose level of 1x10⁶cells/kg. Of the 50 evaluable subjects, 33 had a history of at least one prior HCT, whereas 17 had no history of HCT. Figure A demonstrates the LFS for patients that did and did not undergo HCT following SCRI-CAR19v1.

There is a trend towards improved LFS (p-value 0.057; Figure B) in patients who underwent first HCT following SCRI-CAR19v1. Of the 17 patients without a history of HCT, 3 did not pursue HCT following CAR therapy. Of those 3 subjects, only 1 remains in remission at 28 months. The other 2 subjects relapsed at 6 and 7 months with CD19+ and CD19- disease, respectively. Of the 14 patients that underwent first HCT after SCRI-CAR19v1, 2 relapsed following HCT. One had evidence of MRD by flow at the time of HCT and the other subject relapsed with CD19- disease. The role of second HCT following CAR therapy remains unclear. Of the 33 subjects with a history of HCT, 10 underwent a second HCT following SCRI-CAR19v1 infusion, and 5 are alive and remain in remission. The recurrences included 2 lineage switches and 2 CD19+ (1 was MRD positive by deep sequence prior to HCT), and one transplant related mortality (TRM). Of the 23 subjects that did not undergo a second HCT, 8 remain in remission (p-value not significant (NS); Figure C).

We previously reported that subjects with a short duration of B cell aplasia (BCA) ≤63 days following SCRI-CAR19v1 have an increased risk of relapse. Here we show that subjects with short BCA who attained a CR and did not relapse prior to day 63 demonstrate a clear benefit of consolidative HCT (p-value 0.007; Figure D). Of the 15 subjects with short BCA (regardless of HCT history), 6 did not pursue HCT and all recurred (5 CD19+, 1 CD19-). Of the short BCA subjects that underwent HCT, 1 subject died of TRM (2^ndHCT for this subject) and 2 subjects relapsed following HCT (1 was MRD positive prior to HCT).

All events in the consolidative HCT group have occurred by 20 months following SCRI-CAR19v1. However, we continue to see late relapses following CAR T cell therapy in the group who did not proceed with HCT. Three subjects have relapsed beyond 2 years: 1 with CD19+ disease at 27months, 1 with CD19- disease at 41 months, and 1 patient with lineage switch AML at 38 months. Further analysis is needed to understand the continued long-term risk of relapse following CD19 CAR T cell therapy and the potential role and timing for consolidative HCT in patients with a previous HCT.

Conclusions: We demonstrate a trend towards improved LFS for subjects without a history of HCT who undergo a consolidative HCT following CD19 CAR T cell therapy on PLAT-02. In addition, HCT appears to benefit subjects who attain a CR but are at increased risk of relapse with short-term BCA. Currently, the benefit of second HCT following CD19 CAR T cell therapy is unclear and may be restricted to those that have short functional persistence of SCRI-CAR19v1. Late relapses after SCRI-CAR19v1 have only occurred in those without consolidative HCT, but longer follow up is needed to confirm these findings.

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