Synergism between sirolimus and 1,25-dihydroxyvitamin D3 in vitro and in vivo.

The active form of vitamin D, 1 alpha, 25-(OH)2D3, displays immunomodulatory effects in vitro and in vivo at pharmacological levels. We evaluated the dose-effect relationship of 1,25(OH)2D3 and sirolimus (rapamycin, RAP) in vitro, on the inhibition of PHA-stimulated PBMC proliferation, by using the median effect analysis. Pharmacological concentrations of 1,25(OH)2D3 (between 10(-9) and 3 x 10(-6) M) interacted synergistically with RAP (combination index value of 0.01 for 50% suppression of PBMC proliferation). In vivo, the effect of 1,25(OH)2D3 and RAP combinations on the evolution of experimental allergic encephalomyelitis in SJL mice was analyzed. 1,25(OH)2D3, given i.p., in monotherapy, at a dose of 2 micrograms/kg every two days, from day -3 until day +19 after disease induction, or RAP, injected daily at a dose of 0.3 mg/kg for the same period, decreased EAE incidence (paralysis in 70 and 55% of the animals, respectively, versus 98% in the placebo treated group, p < 0.001). The combination treatment using the two drugs in these subtherapeutical doses provided near-total clinical (8% paralysis, p < 0.001 compared to monotherapy with 1,25(OH)2D3 or RAP) and histological protection, comparable to that obtained with RAP in monotherapy at a threefold higher dose (1 mg/kg/d). When the two drugs were given using an alternate day administration schedule (RAP at 0.6 mg/kg and 1,25(OH)2D3 at 2 micrograms/kg, each given on alternate days from day -3 to 19), near total protection was again obtained (13% paralysis, p < 0.001 versus control). These in vitro and in vivo data support the existence of synergistic interactions between 1,25(OH)2D3 and RAP. Considering the narrow therapeutic windows of both RAP and vitamin D-related compounds in autoimmune disease models, combinations of these drugs could find clinical application in reducing their individual therapeutically efficient doses to non-toxic levels.