Safe and Efficient Peripheral Blood Stem Cell Collection in Patients with Sickle Cell Disease Using Plerixafor

Background: Hematopoietic stem cell (HSC) gene therapy is potentially curative for sickle cell disease (SCD) provided that a sufficient quantity of long-term engrafting CD34+ HSCs can be safely collected and infused. In patients with SCD, peripheral blood (PB) mobilization with granulocyte colony stimulating factor is contraindicated, and steady-state bone marrow (BM) harvesting is associated with suboptimal HSC quality and yield. Hence, agents that promote safe and effective peripheral HSC mobilization in SCD are required.

Methods: This phase I multicenter study investigated the safety and efficacy of plerixafor mobilization (240 µg/kg) followed by apheresis, processing, and HSC enrichment in 15 adult subjects with SCD (NCT03226691). Hydroxyurea treatment was stopped at least two weeks prior to mobilization and all participants who were not receiving long-term transfusion therapy received prophylactic red blood cell exchange prior to plerixafor administration to achieve a target sickle Hb (HbS) <30%. The primary endpoint was efficient collection of PB HSCs (target 2.0x10⁶ CD34+ cells/kg, minimum 1.5x10⁶) after plerixafor mobilization without serious adverse events (SAEs). Collection products were assessed for total WBC, CD34+, CD3+, and CD19+ composition by flow cytometry. Spearman's correlation test was used to assess the relationship between baseline CD34/µL, total CD34+ cells/kg collected, and total blood volume processed.

Findings: Fifteen participants with SCD (HbSS n=13, HbSC n=1, HbSβ+ n=1) who met inclusion criteria were enrolled at St. Jude Research Children's Hospital (n=3) or NIH (n=12) between July 2017 and February 2019. All participants except one successfully met the minimum target CD34+ cells/kg yield with 2 or fewer mobilization and apheresis procedures, with almost half the participants (n=7) achieving ≥5.0x10⁶ CD34+ cells/kg. The total WBC count increased by an average of 3.2-fold over baseline (range 1.7-5.0) to an average peak WBC count of 26.5x10³/µL (range 14.1-47.4), with counts returning to baseline within 1-2 days. Median total WBC, CD34+, CD19+, and CD3+ cells/kg in the final apheresis product after one (n=12) or two (n=3) collection procedures were 0.9 x10⁹ WBC/kg (range 0.4-1.7), 4.5x10⁶ CD34+ cells/kg (range 0.9-12.0), 3.4x10⁸ CD19+ cells/kg (range 0.5-4.9), and 3.6x10⁸ CD3+ cells/kg (range 1.7-7.3), respectively. There was a strong positive correlation between baseline CD34/µL and total CD34+ cells/kg (rs = 0.7776, 95% CI 0.446-0.9215, p = 0.001). Participants with the lowest pre-apheresis CD34 cell count generally underwent higher blood volume processing (rs = - 0.1443, 95% CI -0.6075 to 0.3921, p = 0.59) in an effort to achieve target yields. However, higher blood volume processing did not translate to higher total CD34+ cells/kg collection yields (rs = - 0.2104, 95% CI -0.6488 to 0.3328, p = 0.43). A median of 97% of positively selected CD34+ cells were CD34^high (range 73.6-99.4%) compared to 1.3% CD34^low (range 0.09-24.4%) phenotype suggesting enrichment for long-term engrafting HSCs. Seven grade III AEs (two non-pain and five pain related) and one grade IV AE (non-pain - hemolysis) occurred and each resolved with symptomatic treatment. Plerixafor mobilization was overall safe and in most cases generated CD34+ cell yields that were sufficient for both genetic modification and back-up allogeneic transplantation (median cell yield 4.2x10⁶ CD34+ cells/kg).

Interpretation: This first multi-institutional phase I study suggests the efficiency and safety of plerixafor mobilization and apheresis collection in 15 adult subjects with SCD. The primary endpoint to obtain a target of 2.0x10⁶ CD34+ cells/kg from the PB was exceeded without SAEs. Importantly, immunophenotyping data suggest that for individuals with SCD, the plerixafor mobilized cell product is more enriched for long-term engrafting HSCs compared to CD34+ cells isolated from BM. Given the risks of general anesthesia and the low quality and yield of CD34+ cells harvested from the BM of individuals with SCD, plerixafor mobilization represents a safe and potentially superior alternative for HSC isolation.