Reduced Intensity Conditioned Bone Marrow Transplant with Post-Transplant Cyclophosphamide and Donor-Derived Mesenchymal Stromal Cell Infusions for Recessive Dystrophic Epidermolysis Bullosa

Introduction: Recessive dystrophic epidermolysis bullosa (RDEB) is a severe, life-limiting systemic genodermatosis, characterized by COL7A1 mutation(s) yielding inadequate type VII collagen to maintain the integrity of the cutaneous basement and mucosal membranes. The mainstay of therapy for RDEB is supportive care to minimize the daily morbidity of blistering/scarring, pain/pruritis, systemic inflammation, and high metabolic demand. Leveraging the pluripotency of bone marrow cells, investigations of BMT as a systemic therapy for RDEB showed promise in pre-clinical murine (Tolar J, et al., Blood 2009, 113(5): 1167-74) as well as early phase human clinical trials utilizing fully myeloablative conditioning (Wagner J, et al., NEJM 2010, 363(7): 629-39). However, regimen-rated toxicity limited dampened enthusiasm for the clinical approach.

Methods: In an effort to modulate disease activity with decreased toxicity, we investigated the safety of and preliminary responses to BMT with a reduced-intensity conditioning regimen (rabbit anti-thymocyte globulin 2.5 mg/kg with a methylprednisolone taper, cyclophosphamide 28 mg/kg, fludarabine 150 mg/m², and low dose total body irradiation 300-400 cGy). Graft-versus-host disease (GvHD) prophylaxis included post-transplant cyclophosphamide (PTCy, 50 mg/kg recipient weight/dose on days +3 and 4) and mycophenylate mofetil from day +5 to 35. All except HLA-matched related donor recipients received tacrolimus from day +5 until tapered at day +100. Immunomodulatory donor-derived mesenchymal stromal cells (MSCs, 2 x 10⁶ cells/kg recipient weight/dose) were infused at 60, 100 and 180 days post-BMT. Skin biopsies, medical photography, and dynamic assessments of RDEB disease activity by providers and parents were completed at baseline, day +100, +180 and 1 year post-BMT.

Results: Ten RDEB patients were transplanted at a median age of 9.9 years (range 1.8 to 22.1), with a median follow-up of 16 months (1 year evaluations available for only 4 of 10). Donors were haploidentical related (n=6), HLA-matched related (n=3), and HLA-matched unrelated (n=1). BMT complications included graft failure in 3 patients (2 elected to pursue a 2^nd PTCy BMT), veno-occlusive disease in 2, posterior reversible encephalopathy in 1, and chronic GvHD in 1, the latter deceased. Infectious complications in the first 100 days were limited to 3 viremia, 7 bacteremia, and 0 fungemia episodes. No serious adverse events were observed with MSC infusions. In the 9 ultimately engrafted patients, median donor chimerism at day +180 was 100% in both myeloid and lymphoid fractions of peripheral blood and 27% in skin. Skin biopsies at day +180 show stable (n=7) to improved (n=2) type VII collagen protein expression by immunofluorescence (IF) and gain of anchoring fibril components by transmission electron microscopy in 4 of 9 patients. Early clinical response included a trend toward reduced body surface area of blisters/erosions from a median of 45.5% at baseline to 27.5% at day +100 (p=0.05), with parental measures indicating stable quality of life. Select medical photography and skin biopsy results are shown for Patient 1 [IF: 40x merged dapi (blue) and C7 collagen antibody (red); immunoelectron microscopy with C7 collagen-directed immunostain (black)].

Conclusions: BMT using reduced-intensity conditioning, PTCy and donor-derived MSC infusion for RDEB was largely well tolerated with low rates of GvHD and death from regimen-related toxicity in 1 of 10 patients undergoing 12 total BMTs. Early follow-up suggests this treatment modulates RDEB disease activity but requires longer follow-up for evaluation of efficacy. Importantly, the PTCy BMT platform provides a means of attaining immunotolerance for future donor-derived cellular grafts.

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