Laboratory studies on the photostability of fumagillin, the active ingredient of Fumidil B

Fumagillin (as the dicyclohexylammonium salt) has been found to be extremely unstable when solutions in 50% ethanol are exposed to sunlight in small vials, undergoing a series of degradations with half-lives in the range of seconds to minutes. Similar results were obtained with photolyses of Fumidil B in either 50% ethanol or sugar syrup. Decomposition also occurred in fluorescent room light. Exposure of fumagillin to sunlight for three days caused almost complete disappearance of UV absorption of fumagillin and all its immediate photoproducts. The reactions involved in the photolyses are apparently reversible Z:E rearrangements in the unsaturated ester portion of the molecule, and do not involve the pharmacologically active moiety. Samples of fumagillin in syrup, irradiated for 0, 0.5, 5, 30, or 360 minutes were all effective in protecting caged bees from nosema disease. While long exposures to sunlight probably should be avoided, brief exposure causes no obvious loss of activity.