Early Results of the Clinical Collection and Genetic Correction of Fanconi Complement Type A Hematopoietic Stem Cells.

Fanconi anemia (FA) is a genetic syndrome characterized by almost uniform development of aplastic anemia. Current therapies for patients lacking HLA-identical sibling hematopoietic stem cell (HSC) donors have shown high morbidity and mortality in clinical trials. Genetic correction of FA HSC using viral vectors has been demonstrated in animal models. However, harvesting of sufficient CD34+ cells at the time that HSC therapy is clinically indicated is difficult due to the severe bone marrow hypoplasia that accompanies pancytopenia. We have opened two phase I clinical trials that seek to determine if potentially useful numbers of CD34+ cells can be collected early in the course of the disease (collection study) and if these cells, once corrected, can engraft without cytoreduction and demonstrate proliferative advantage in vivo over un-corrected cells (gene transfer study). These studies are being conducted with approval by the FDA/NIH-RAC/Institutional IRB and monitored by an independent DSMB. To date, 4 FA patients have undergone a 20 ml/kg bone marrow harvest (BMH) with an average of 1.3x10⁶ CD34+ cells/kg (range 0.3–2.9x10⁶ CD34+ cells/kg) collected, suggesting that collection of adequate numbers of cells will be challenging, even early in the disease. In the gene transfer study, 3 FA patients with genotype A (FAA) have enrolled, meeting eligibility criteria of FAA, no evidence of malignancy and a minimum of 1x10⁵ viable CD34+ cells/kg for ex vivo culture and gene transfer. BMHs from the 3 patients (2 fresh and one previously cryopreserved) were CD34+ cell selected using the CliniMACS device. Despite collection before significant pancytopenia, an average of only 5x10⁵ CD34+ cells/kg (range 1.5–10x10⁵ CD34+ cells/kg) was purified from these 3 cases representing ~10% of the expected yield from normal individuals. These cells underwent ex vivo gene transfer using cytokine prestimulation in serum-free medium followed 2 exposures to a GALV-MSCV-FANCA vector. Transduction efficiency of the final products determined by real-time PCR analysis of CD34+-derived progenitors averaged 48% (range 40–62%). Equivalent efficiency of correction of mitomycin C hypersensitivity in progenitor cells confirmed this analysis at the functional level. Nucleated cell recovery after ex vivo manipulation was 82–110% of input nucleated cells using freshly harvested bone marrow derived CD34+ cells (N=2). However, despite good CD34+ cell recovery and viability after CD34+ selection, only 6% of input nucleated cells were recovered utilizing CD34+ cells purified from the previously cryopreserved bone marrow and these cells were not re-infused. In the two patients who did receive gene corrected cells, the total cell dose re-infused was 2.5–3.5x10⁵ nucleated cells/kg, reflecting the low number of initial CD34+ cells placed in culture. One patient is now 6 months post re-infusion with no evidence of gene marking observed in her PB or BM. The second patient had detectable FAA vector sequences in her PB early post-infusion (+4 weeks) but had none detected +8 weeks. The data suggest that while gene transfer efficiency in the clinical setting has been significantly improved, collection and expansion (either in vitro or in vivo) of adequate number of HSC may be critical to the success of genetic correction attempts in FA.