Fludarabine-based Conditioning for Severe Aplastic Anemia (BMT CTN 0301)

BACKGROUND:

Aplastic anemia (AA) remains a life-threatening illness. Treatment options include supportive care (transfusions, growth factors, etc.), immunosuppression therapy and stem cell transplantation. Only the latter two have favorably impacted the natural history of the disease. The prognosis of AA patients, particularly severe aplastic anemia (SAA), as defined by Camitta et al., who fail to respond to immunosuppressive therapy (IS) or who relapse after an initial response to IS is poor. Although many of these patients can be supported in the short term with growth factors, transfusions and possibly rechallenged successfully with IS, the cumulative morbidity and mortality from infection, hemorrhage or transfusion-related complications is substantial.

While allogeneic bone marrow transplantation is potentially curative in AA, no more than 25% of patients have a human leukocyte antigen (HLA)-identical sibling donor. Cyclophosphamide (CY)-antithymocyte globulin (ATG) has been recommended as the preparative regimen of choice in sibling donor transplants. Results of bone marrow transplantation from alternative donors, such as matched unrelated donors and mismatched related donors in AA patients who have failed IS, have largely been unsatisfactory. The cyclophosphamide-ATG conditioning regimen has proved inadequate in ensuring engraftment in allogeneic transplants from matched, unrelated donors for AA. This was the major reason why total body radiation (TBI) has been added to the conditioning regimen.

Graft failure is a very serious and frequently life-threatening or fatal event following matched unrelated donor (MUD) allografts in aplastic anemia. It is an immunologically mediated event. Risk factors for graft failure include the use of HLA nonidentical or unrelated donors, a poor marrow nucleated cell dose as well as prolonged transfusional support prior to BMT (which increases the probability of patient sensitization to multiple antigens). While some patients may achieve autologous hematopoietic recovery, prolonged pancytopenia is common and infection-related morbidity and mortality are very substantial. Reconditioning for a second allograft from the same or a different donor is frequently not successful. While the addition of TBI and intensive pre-transplant conditioning has led to a sizable improvement in engraftment rates, this has come with a price, particularly in adult patients. Transplant-related toxicity has been a major and frequent problem. Radiation-induced pulmonary toxicity in particular has been common, usually in the form of diffuse alveolar damage or diffuse interstitial pneumonitis. In addition, Graft Versus Host Disease (GVHD)-related morbidity and mortality in these patients have also been substantial.

DESIGN NARRATIVE:

The study is a prospective Phase I/II dose optimization study. All patients are given a fixed dose of ATG (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/m^2 IV daily x 4, on Days - 5 to -2), and TBI (200 cGy (centigray) from a linear accelerator at less than 20 cGy/min on Day -1). The starting CY dose will be 150 mg/kg (50 mg/kg intravenously daily, Days -4 to -2), and will be de-escalated depending on engraftment and toxicity. The Phase I portion of the trial (maximum of 24-27 patients) tests each of four dose levels of CY for adequate safety and graft retention. The Phase II portion of the trial refines the dose selection and allocates an additional 70 patients to the optimal dose, at which two-year post-transplant survival will be assessed. The combined enrollment in Phase I and II will total 94 patients.

The study is a prospective single-arm Phase I/II dose-selection and evaluation study. The study will seek the optimal dose level of CY based on assessments of graft failure, toxicity and early death during 100 days of follow-up post-transplant. A brief synopsis is given below.

Phase I - Test Each Dose for Adequate Safety and Graft Retention

1. Proceed from the highest dose (150 mg/kg CY) to the lowest dose (0 mg/kg CY), treating a minimum of six patients at each dose.
2. Evaluate the 100-Day outcomes for toxicity, death and graft failure on each patient enrolled at the current dose, or until stopping criteria are met.
3. If there are three or more graft failures at the current dose, the current dose and all lower doses are closed to further enrollment.
4. If there are five or more severe regimen-related toxicities and/or early deaths at the current dose, the current dose is closed to further enrollment, and the next lower dose is tested.
5. Dose de-escalation ceases once all four doses are tested or closed to further enrollment.

Phase II - Refine Dose Selection and Allocate Patients to the Optimal Dose

1. Treat each newly enrolled patient at the most desirable of the dose levels remaining open to enrollment. This can involve de-escalation, escalation, or no change in dose.
2. As each patient completes the observation period, evaluate the 100-Day outcomes for graft failure, toxicity and/or early death for this patient, or until stopping criteria are met.
3. If there are excess (according to the criteria in Table 5.8) graft failures, that patient's dose and all lower doses are closed to further enrollment.
4. If there are excess (according to the criteria in Table 5.8) toxicities and/or early deaths, that patient's dose is closed to further enrollment.
5. Re-evaluate the desirability of the current dose level based on the 100-Day outcomes for toxicity and/or early death and graft failure.
6. Repeat steps 1-5 until 54 patients are enrolled in Phase II, or all dose levels are closed to further enrollment.

Dosage Levels for CY:

3 Days (Day -4, -3, -2): Dose of 50 mg/kg/day; total dose of 150 mg/kg; dose level 3

2 Days (Day -3, -2): Dose of 50 mg/kg/day; total dose of 100 mg/kg; dose level 2

1 Day (Day -2): Dose of 50 mg/kg/day; total dose of 50 mg/kg; dose level 1

0 Days (None): No dose; no total dose; dose level 0

There may be wait periods between enrollment of successive patients and/or cohorts for endpoint assessment. Under these circumstances, the final decision about waiting versus treating the patient off study will be made at the local transplant center.