Evaluation of Posttransplant Methotrexate to Facilitate Engraftment in the Canine Major Histocompatibility Complex-Haploidentical Nonmyeloablative Transplant Model

Background. Posttransplant cyclophosphamide has been shown to control graft-versus-host disease and facilitate engraftment in the major histocompatibility complex-haploidentical transplant setting. Here, we hypothesized that methotrexate (MTX) could be used in a similar fashion. In patients with genetic diseases, the use of MTX rather than an alkylating agent such as cyclophosphamide would be preferable due to its reduced risk of promoting secondary malignancies. Method. Using our standard conditioning regimen consisting of a specific anti-CD44 mAb (S5) and 200 cGy total body irradiation followed by postgrafting immunosuppression with cyclosporine and mycophenolate mofetil as a control group, we compared outcomes with experimental animals receiving the same regimen with the addition of a single, large dose of posttransplant MTX on day +3 (50–400 mg/m2). Results. Adding MTX at all dose levels did not abrogate initial engraftment and controlled graft-versus-host disease in most cases. Dogs receiving MTX at the first dose level (50 mg/m2) improved time to rejection compared with controls (P=0.03) but did not decrease overall rates of rejection (P=0.56). However, increasing the dose of MTX beyond 50 mg/m2 seemed to have detrimental effects in both average (P=0.04) and peak (P=0.002) donor chimerism. Increasing the dose of MTX also promoted more profound lymphopenia. Finally, delaying cyclosporine and mycophenolate mofetil until after MTX administration did not seem to significantly improve engraftment kinetics. Conclusion. Adding high-dose MTX seemed to benefit the duration of donor chimerism at the lowest dose studied, but there was no benefit when escalating MTX doses to toxicity.