We investigated a novel graft-versus-host disease (GVHD) prophylactic regimen, sirolimus and mycophenolate mofetil (MMF), in patients with advanced hematologic malignancies receiving myeloablative hematopoietic cell transplantation (HCT) from matched related donors (MRD). The sirolimus and MMF combination was chosen based on intriguing evidence of sirolimus, and less dramatically MMF, conserving the inhibition of T cell proliferation compared to cyclosporine (CSP) in an AGVHD murine model as well as multiple reports supporting sirolimus’ preservative or augmentative effects on the T regulatory cell population (CD4+CD25+, FoxP3+) in animals and humans in comparison to cyclosporine (CSP) and a theoretical reduction in the incidence and severity of AGVHD. In addition, this regimen may cause less severe toxicities compared with standard methotrexate and/or calcineurin inhibitor (CI)-containing GVHD prophylactic regimens. Sirolimus was begun as an oral medication on day -3 with a 12 mg loading dose followed by 4 mgs daily, dose adjusted to maintain serum trough levels of 3–12 ng/ml. MMF was begun on D0 at 15 mg/kg IV every 12 hours. Mycophenolic acid (MPA) pharmacokinetics were conducted on days 2 and 21 post-HCT. Sirolimus and MMF were scheduled to be tapered simultaneously beginning 100 days post-HCT. FoxP3+ cell populations in the peripheral blood (PB) were analyzed before and after HCT in the patients receiving sirolimus and MMF GVHD prophylaxis as well as in 15 control patients with similar disease categories and preparative regimens receiving CI-containing GVHD prophylaxis. Based on pre-study stopping rules of expected grade II–IV AGVHD incidence, the trial was closed to accrual after enrollment of 11 patients. The median age of the 11 patients was 51 years (26–59). The diagnoses of the study patients included myelodysplasia (5), acute myelogenous leukemia (3) and nonhodgkin lymphoma (3). Seven patients received busulfan, etoposide and cyclophosphamide (CY); three patients received carmustine, etoposide and CY and one patient received total body irradiation, etoposide and CY as the preparative regimens. Sirolimus was discontinued in 4 patients due to concern for toxicity-related events including severe sinusoidal obstructive syndrome (1), altered mental status (1), portal vein thrombosis (1) and risk of poor wound healing after abdominal exploratory surgery (1) 9 days, 9 days, 61 days and 39 days post-HCT, respectively. Six of 11 patients developed grade II-IV AGVHD a median of 15.5 days post-HCT (range 11–25 days), 3 of the 6 with grade IV AGVHD. Two of the 3 patients with grade IV AGVHD required discontinuation of sirolimus 9 days post-HCT due to toxicity. Five of the 6 AGVHD patients had skin-only and one had liver, gut and skin involvement. All patients responded to AGVHD therapy and there were no AGVHD-related deaths. One patient died 54 days post-HCT due to complications of colonic ulceration believed related to oral MMF therapy. Two patients died of relapsed disease at 103 and 304 days post-HCT. At a median follow-up of 416 days (328–652 days), 8 of 11 patients were alive without disease, 5 with active chronic GVHD. The median MPA area under the curve (AUC) on day 2 was 12.8 mcg*hr/mL (6.4–17.4) and day 21 was 17.9 mcg*hr/mL (8–26.6). We measured a statistically significant increase in the percentage of CD4+FoxP3+ cells per total CD4+ cells in the sirolimus/MMF study group compared to the CI historical control group with a mean of the individual patient means over the first 100 days post-HCT of 16% versus 6%, respectively (p.0005). Despite the increased percentage of CD4+FoxP3+ cells per total CD4+ cells identified in the PB of the sirolimus/MMF cohort, the incidence of grade II-IV AGVHD was similar to the CI cohort (6 of 11 versus 7 of 15, respectively). However, all but one patient had skin-only AGVHD and there were no GVHD-related deaths in the sirolimus/MMF group. All patients requiring early withdrawal of sirolimus had received the busulfan-containing preparative regimen with two of these patients developing severe AGVHD. In a subsequent GVHD prophylaxis trial including sirolimus and MMF, the busulfan-containing preparative regimen will be eliminated to optimize sirolimus and MMF administration with the continued goal of reducing the incidence and severity of AGVHD.

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