Effect of liposomal methylprednisolone on heart allograft survival and immune function in rats

Liposomal methylprednisolone (L-MPL) applied in monotherapy prolonged cardiac allograft survival in rats in comparison with the same dosage regimen of drug in solution (Solu-Medrol). The most efficacious treatment consisted of a 2-mg/kg i.v. dose of L-MPL twice a week (group III), producing survival up to 30 days, followed by a 4-mg/kg/week dose of L-MPL (group IV) and a single 2-mg/kg dose of L-MPL (group II). Survival in animals receiving Solu- Medrol as a 2-mg/kg dose twice a week (group V) did not differ from untreated animals. Only daily 4-mg/kg doses of methylprednisolone (MPL) in solution (group VI) were as effective as group III. The concentrations of MPL in liver and spleen were detectable for 26 days after the last dose of L-MPL, showing tissue selective sequestration of drug. Treatment at these low doses did not suppress endogenous corticosterone determined 24 hr or later in plasma. The administration of steroid caused significant immunosuppression in most animals as measured by inhibition of splenocyte blastogenesis induced with phytohemagglutinin. Cellular immunity data did not differ significantly between groups, but alterations occurred at day 14 to 15 after surgery: CD3, CD4 and ratio CD4:CD8 subsets of cells showed minimum values; CD8, CD4CD8, CD25 and white blood cell counts were at maximum at this time. Slight but significant differences between Immunoglobulin M suppression in group II compared to group I or V were found, whereas Immunoglobulin G values were unchanged. The transplantation and treatment with steroid decreased the total body weight of animals but increased weights of internal organs, particularly spleen, similarly for all groups. L-MPL offers promise in reducing doses of MPL needed for prevention of acute graft rejection which may moderate the adverse effects of this steroid.