BINDING OF THE EPOXIDE CRYPTOPHYCIN ANALOG, LY355703 TO ALBUMIN AND ITS EFFECT ON IRT VITRO ANTIPROLIFERATIVE ACTIVITY

Cryptophycin, isolated from the cyanobacterium Nostoc, is a cytotoxic dioxadiazacyclohexadecenetetrone which causes rapid depletion of micro tubules in intact cells. In the present report, the effect of protein binding of a new synthetic cryptophycin analog, LY355703 (cryptophycin 52), is discussed. In handling the compound, it was found to bind ext ensively to surfaces, and a high degree of plasma protein binding was also observed (about 99% in human plasma). Similarly, while LY355703 d isplays potent antiproliferative activity against several human tumor cell lines in vitro (IC(50)s ranging from 12 to 40 pM), the addition o f human or bovine serum albumin (BSA) to CCRF-CEM cells adapted to ser um-free (UltraCHO) medium markedly reduced its antiproliferative activ ity. For example, the IC(50)s for LY355703 in BSA at 0, 4 and 40 mg/ml were 2, 19 and 34 pM, respectively. In comparison, the IC50, only inc reased 2-fold (4210-8530 pM) for taxol over the same BSA concentration range. When log phase CCRF-CEM cells were exposed to 1 mu M [H-3]LY35 5703, there was a rapid accumulation of drug, so that LY355703 reached steady state within 10 min. The rate of LY355703 uptake in log-phase CCRF-CEM human leukemia cells was a linear function of concentration o ver a wide range (0.25-50 mu M), although the cytotoxicity IC50 was 19 pRI. Drug accumulation was not inhibited by sodium azide. Although cr yptophycin was observed to bind extensively to albumin, binding did no t markedly modulate cryptophycin uptake by CCRF-CEM cells. Overall, th ese results demonstrate that attention must be given to the binding pr operties of LY355703 and similar cryptophycins while handling these co mpounds, and that binding to albumin (and probably other cellular comp onents as well) is a significant factor for interpretation of results both in vitro and in vivo.