The in vivo immunosuppressive properties of a novel, marine-derived compound, discodermolide, are reported here. Discodermolide was effective in suppressing the graft-versus-host splenomegaly response of BALB/c→tCB6F1 (BALB/c × C57BL/6J)F1 grafted mice at 5.0, 2.5, and 1.25 mg/kg, when administered as daily, i.p. injections, for 7 days. Mice treated with 5.0 and 2.5 mg/kg demonstrated a high degree of suppression (219 and 150%, respectively); however, these dosages were associated with some degree of morbidity (2/5 and 4/5 survivors for 5.0 and 2.5 mg/kg, respectively). Mice that were treated with 1.25, 0.625, and 0.313 mg/kg remained healthy after a 7-day regimen, and continued to demonstrate suppression of splenomegaly (106%, 72%, and 76% suppression, respectively). Splenocytes obtained from discodermolide-treated, allogeneic grafted mice were suppressed in their ability to respond in vitro to optimal mitogenic concentrations of concan-avalin A, and natural-killer-cell activity directed against YAC-1 tumor cells, compared with vehicletreated, allogeneic grafted control mice. Lower dosages (2.5 and 1.25 mg/kg) of discodermolide, however, did not affect the subsequent ability of splenocytes obtained from these mice to produce IL-2 following in vitro stimulation with Con A. This was observed to be in contrast to the immunosuppressive activity observed with cyclosporine treatment of mice (150 mg/kg) for the ex vivo suppression of splenocyte production of IL-2. Treatment of normal, nongrafted mice with similar high dosages of discodermolide (5.0 mg/kg) for 4 days did not affect the primary antibody response of mice immunized with sheep red blood cells as measured by hemagglutination activity of their serum. These results suggest that dis-codermolide's in vivo immunosuppressive action ap
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