Uptake and metabolism of BuCast: a glycoprotein processing inhibitor and a potential anti-HIV drug.

We have previously shown (Sunkara et al., 1989; Taylor et al., 1991) that 6-o-butanoyl castanospermine (BuCast) was a 30-50-fold better inhibitor of HIV syncytia formation than castanospermine (Cast). Radiolabeled Cast and BuCast were used to study the uptake and metabolism of these compounds in cultured cells and in mice. BuCast was preferentially taken up by cells compared to Cast. Approximately 30-50-fold higher radioactivity was found in cells treated with BuCast compared to cells treated with Cast during the initial 4-6 h of labeling. HPLC analysis showed that once BuCast was taken up by cells, it was rapidly converted to Cast. Mice given oral doses of BuCast had 5-10-fold higher levels of Cast in the plasma and tissues as compared to Cast treated mice. However, when the compounds were given i.v., the levels of plasma and tissue radioactivity obtained from Cast of BuCast were equivalent suggesting rapid conversion of BuCast to Cast in the blood. In mice orally treated with BuCast, HPLC analysis suggested that only Cast was found in the plasma and tissues. With multiple dosing of mice, additive results were obtained, suggesting that multiple doses may be used to obtain higher concentrations of the compound in the target cells. These data suggest that the lipophilic properties of butanoyl side chain on the Cast ring makes BuCast significantly better absorbed, and this may help to alleviate some of the gut toxicity associated with Cast treatment.