Dextran-methylprednisolone succinate as a prodrug of methylprednisolone: in vitro immunosuppressive effects on rat blood and spleen lymphocytes

The in vitro immunosuppressive activity of a conjugate of methylprednisolone (MP) with dextran 70 kDa (DEX-MPS) was tested using the lymphocyte proliferation assay after stimulation of lymphocytes with concanavalin A (Con-A). Blood and spleen lymphocytes, isolated from drug-free male Sprague–Dawley rats, were used in the assay. First, the optimum concentration of Con-A for stimulation of lymphocytes was determined. The inhibition of the lymphocyte proliferation was then tested in the presence of 0.25, 0.5, 1.0, 2.5, 5.0, 10, 20, and 50 nM concentrations (MP equivalent) of DEX-MPS or free MP. The maximum stimulation of lymphocytes with Con-A was observed at mitogen concentrations of 2.5 and 10 μg/ml for the spleen and blood lymphocytes, respectively. For free MP, sigmoidal relationships were observed between the effect (% inhibition of lymphocyte proliferation) and the logarithm of MP concentration. Additionally, the maximum inhibitory effect (Imax) and MP concentration producing half of Imax (IC50) were, respectively, 98% and 1.38 nM for the blood and 86% and 3.1 nM for the spleen lymphocytes. For MP conjugated to dextran, the response–log concentration curves were substantially shifted to the right with IC50 values of 40 and 52 nM for the blood and spleen lymphocytes, respectively. It is concluded that compared with free MP, the steroid attached to dextran has minimal immunosuppressive activity. Therefore, to be effective in vivo, DEX-MPS should release MP in the body.