Objectives

The primary objective of this study was to determine the feasibility and toxicity of employing fludarabine-based conditioning to reduce transplant-related toxicity while maintaining (or ideally improving) engraftment in allogeneic donor marrow transplantation from matched (and mismatched) unrelated donors in patients with severe aplastic anemia (SAA). Additionally, the study examined the degree of cyclophosphamide (CY) dose reduction achievable with the introduction of fludarabine in the preparative regimen, with the goal of maintaining (or improving) engraftment, reducing major transplant-related toxicity and early deaths, and thereby ultimately improving long-term survival.

Background

Unrelated donor bone marrow transplantation (BMT) is an option for the treatment of patients with severe acquired aplastic anemia who do not have HLA-matched siblings and sustained responses to immunosuppressive therapy. Outcomes after unrelated donor BMT have improved over the past decades, presumably because of closer donor–recipient HLA matching and improved supportive care.

The optimum preparative regimen remains to be established. Ideally, the conditioning regimen should maximize engraftment while minimizing organ toxicity. However, with transplant conditioning at the time of the study, graft rejection and regimen-related toxicity continued to adversely affect patient outcome. Conditioning regimens for unrelated donor BMT in patients with aplastic anemia are typically combinations of cyclophosphamide, anti-thymocyte globulin, and total body irradiation (TBI). Although cyclophosphamide is a key part of the preparative regimen for patients with severe aplastic anemia, the optimum dose remained undefined. In view of the known toxicities linked to high-dose (i.e., 200 mg/kg) cyclophosphamide, a reduction of the cyclophosphamide dose would be desirable.

The study investigated whether the combination of fludarabine, anti-thymocyte globulin, and TBI would enable reduction of the cyclophosphamide dose to less than 200 mg/kg while maintaining engraftment and having a survival similar to or better than that with standard regimens using a cyclophosphamide dose of 200 mg/kg (known to be associated with significant organ toxicity) for unrelated donor transplantation for severe aplastic anemia.

Participants

Between Feb 22, 2006, and Aug 1, 2007, 21 patients were accrued to phase 1 of the study. Phase 2 opened in Nov 5, 2007, and closed to accrual on Dec 2, 2013, with a final enrolment of 96 patients. One patient who withdrew consent prior to transplant was not included in the final analysis.

Patients were eligible for inclusion in the study if they were 65 years old or younger, had adequate organ function, and had an unrelated adult donor HLA matched at the allele-level for HLA A, B, C, and DRB1 or mismatched at a single HLA locus.

Exclusion criteria were Karnofsky performance status of less than 60, clonal cytogenetic abnormalities, Fanconi’s anemia, or other marrow failure syndromes. Bone marrow was the only stem cell source allowed because of the poor outcome reported in patients with severe acquired aplastic anemia transplanted with peripheral blood cells.

Design

The study was a prospective phase I/II dose optimization study. All patients were given a fixed dose of anti-thymocyte globulin (either thymoglobulin: 3 mg/kg IV daily x 3 or ATGAM 30 mg/kg IV daily x 3, on Days –4 to –2), Fludarabine (30 mg/m2 IV daily x 4, on Days – 5 to –2), and TBI (200 cGy from a linear accelerator at ≤ 20 cGy/min on Day –1).

The phase I portion of the trial tested each of four dose levels of cyclophosphamide (0 mg/kg, 50 mg/kg, 100 mg/kg, and 150 mg/kg) for adequate safety and graft retention. 6 patients were allocated to each of the doses, except for the 0 mg/kg dose which was closed after accrual of three patients due to secondary graft failure. The phase II portion of the trial refined the dose selection and allocated an additional 68 patients to the optimal dose, as assigned by the Bayesian approach. The starting cyclophosphamide dose was 150 mg/kg, and was de-escalated depending on engraftment and toxicity. Eight patients were accrued to the cyclophosphamide dose of 150 mg/kg, which was then closed because of excess toxicity. Patients were subsequently enrolled to the 100 mg/kg dose in cohorts of six. While enrollment at this dose paused until all patients in a cohort had their day 100 assessments, additional patients were assigned to cyclophosphamide 50 mg/kg. The last 20 patients were preferentially assigned to cyclophosphamide 50 mg/kg to balance accrual to the two doses.

The primary endpoint was the selection of the optimum cyclophosphamide dose based on assessments of graft failure (primary or secondary), toxicity, and early death during 100 days of follow-up after the transplant.

Conclusions

The study identified cyclophosphamide 50 mg/kg as the most desirable dose in combination with TBI 2 Gy, fludarabine 120 mg/m², and anti-thymocyte globulin for engraftment and early survival for unrelated donor transplantation in patients with severe acquired aplastic anemia. A dose of 100 mg/kg, although slightly less desirable, also provided promising early results. These cyclophosphamide doses should be assessed further, ideally in the context of a randomized trial.

Anderlini P, Wu J, Gersten I, et al. Cyclophosphamide conditioning in patients with severe aplastic anaemia given unrelated marrow transplantation: a phase 1-2 dose de-escalation study. Lancet Haematol. 2015;2(9):e367–e375. doi:10.1016/S2352-3026(15)00147-7

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