Chimeric Antigen Receptor Induced Cytopenia Differs from Chemotherapy Induced Myelosuppression

Background: Cytopenias are a potential complication of chimeric antigen receptor (CAR) therapy. The specific degree, duration, or implication of these cytopenias is not well established, and is further confounded by baseline cytopenias in heavily pre-treated patients with active leukemia. Preclinical models suggest that these cytopenias may be due to a cytokine rich milieu which may contribute to nonspecific suppression of marrow progenitors-independent of the CAR target (Sauter, ASH 2016, abstract 3357); which differs from standard cytotoxic chemotherapy based myelosuppression. We systematically analyzed CAR-related cytopenias on our phase I dose-escalation study of CD22 CAR.

Design: Children and young adults with relapsed/refractory CD22+ acute lymphoblastic leukemia (ALL) treated on our phase I dose escalation anti-CD22 CAR therapy protocol (NCT02315612) were evaluated for time to neutrophil and platelet recovery (defined by absolute neutrophil count (ANC) of > 500 k/mcL x 3 days and achievement of transfusion independent platelet count > 20 k/mcL, respectively). All patients had bone marrow evaluations by morphology and flow cytometry at baseline, prior to lymphodepleting chemotherapy (Fludarabine 25 mg/m² x 3 days and Cyclophosphamide 900 mg/m² x 1 day) and again at day 28 (+/- 4 days) post-CAR. Infections were closely monitored for. All patients were on anti-fungal prophylaxis and were empirically treated for fever during periods of neutropenia.

Results: From December 2014 to July 2017, 30 patients with ALL were treated on study at 3 dose levels. All patients had active bone marrow involvement at baseline with the majority (n=21) having an M2 marrow (>5% blasts) or higher. Twenty-six patients had prior transplants, and 21 patients had prior CD19-directed CAR therapy. Cytokine release syndrome (CRS) was seen in 24/30 (80%), with most patients having grade 1-2 CRS. 18/30 patients (60%) attained a complete remission following CD22 CAR therapy. Baseline hematologic parameters for responders prior to lymphodepletion included grade 2-4 neutropenia and grade 2 and 3 thrombocytopenia, with five patients requiring platelet transfusions (Table 1). Amongst complete responders, the bone marrow at day 28 demonstrated a median cellularity of 28% with trilineage hematopoiesis noted in 16/18 patients. The median CAR T cell % in bone marrow at 1 month was 30%. The median time to ANC nadir was 15 days post CAR infusion (range:10-23 days).17/18 patients had ANC recovery >500 k/mcL, at a median of 22 days post-infusion (range: 15-44 days). One patient had ANC recovery to 260 k/mcL but subsequently developed disease relapse. Eight patients received intermittent GCSF, with clinically relevant responses seen in seven of these patients with a median increase in ANC of 2,180 k/mcL (range: 850-4,100 k/mcL) with limited dosing. The median number of days of GCSF administration to time of ANC recovery was 4, with 3 patients requiring only 1 injection to achieve robust ANC response. Interestingly, only 1/18 patient (6%) had a new onset of infection (clostridium difficile colitis) during the period of CAR associated neutropenia, which was confounded by administration of intravenous antibiotic therapy for fevers that was initiated 24 hours prior to onset of diarrhea. There was no new development of fungal infections or worsening of any pre-existing infections. Platelet recovery to > 20 k/mcL was at a median of 36 days (range: 21-55) post-CAR and was seen in 16 of 18 responders. Two patients who remained platelet transfusion dependent post-CAR included one patient who was later found to have donor-cell monosomy 7 MDS and another patient whose disease relapsed prior to recovery.

Conclusion: In contrast to chemotherapy associated myelosuppression, CAR-therapy associated peripheral blood cytopenias existed despite evidence for trilineage hematopoiesis on the bone marrow. Although the period of CAR associated neutropenia is substantial, with prophylactic antifungals and intermittent growth factor support, this heavily pre-treated population was not at an increased risk of developing serious bacterial or fungal infections over baseline. The rapid response to G-CSF is suggestive of an alternative modality of bone marrow inhibition distinct from chemotherapy induced cytotoxicity. Further evaluation in the context of multiple CAR targets and constructs are needed to further explore this complication.

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