Chronotherapy of sublethal doses of methotrexate on rats allogeneic heart grafts

A potential dosing-time dependent efficacy and toxicity of methotrexate (MTX) was tested using a rat heterotopic heart transplantation (HHT) model. Chronopharmacokinetics of this agent was also studied in rats. Inbred rat strains of DA (RT1^a) and PVG/c (RT1^c) were used as a donor and recipient, respectively. Adult female PVG/c rats with DA heart grafts were intraperitoneally administered with a sublethal dose (0.25mg/kg) of MTX at 10:00 or 22:00 daily from day 0 to day 20 after HHT operation (n=8 in each group). Changes in the recipient body weight and hematological parameters were examined and survival of allograft hearts and the recipients were observed. MTX concentrations in plasma of rats after 0.25 mg/kg MTX injection were measured by an immunoassay. The recipient's lethality of a high dose MTX administration at 10:00 or 22:00 was 38% or 25%, respectively. Permanent graft survivals of the former or the latter were obtained 5 out of 8 or 6 out of 8 cases, but no significant difference. However, dosing-time dependent toxicity such as body weight and hematological data was statistically different, showing those of the 10:00 group seven than the 22:00 group. Area under the curves (AUC) of MTX concentrations at these two different times was also significantly different; the 10:00 group showed a higher concentration curve. In conclusion, chronotherapy of MTX on organ transplantation might reduce toxicity with potent immunosuppressive effect.