Fig. 7. Degradation and inactivation of GW280430A. This occurs by two novel processes: pH-sensitive chemical hydrolysis (reaction I, lower pathway ) and adduction of cysteine, replacing chlorine and saturating the fumarate double bond to yield a new five-membered heterocyclic ring (reaction II, upper pathway ). The products of both reactions are presumably inactive. Reaction II (cysteine adduction) is most likely the rate-limiting step with respect to the kinetics of GW280430A (see text).