Assay development for determination of DZ2002, a new reversible SAHH inhibitor, and its acid metabolite DZA in blood and application to rat pharmacokinetic study

Methyl(S)-4-(6-amino-9 H-purin-9-yl)-2-hydroxybutanoate(DZ2002) is a potent reversible inhibitor of S-adenosyl-L-homocysteine hydrolase(SAHH). Due to its ester structure, DZ2002 is rapidly hydrolyzed in rat blood to 4-(6-amino-9 H-purin-9-yl)-2-hydroxybutyric acid(DZA) during and after blood sampling from rats; this hampers accurate determination of the circulating DZ2002 and its acid metabolite DZA in rats. To this end, a method for determining the blood concentrations of DZ2002 and DZA in rats was developed by using methanol to immediately deactivate blood carboxylesterases during sampling. The newly developed bioanalytical assay possessed favorable accuracy and precision with lower limit of quantification of 31 nM for DZ2002 and DZA. This validated assay was applied to a rat pharmacokinetic study of DZ2002. After oral administration, DZ2002 was found to be extensively converted into DZA. The level of systemic exposure to DZ2002 was significantly lower than that of DZA. The apparent oral bioavailability of DZ2002 was 90%–159%. The mean terminal half-lives of DZ2002 and DZA were 0.3–0.9 and 1.3–5.1 h, respectively. The sample preparation method illustrated here may be adopted for determination of other circulating ester drugs and their acid metabolites in rodents.

Copper ions inactivate S-ade-nosylhomocysteine hydrolase

S-adenosylhomocysteine (AdoHcy) hydrolase is an enzyme that regulates biomethylation and some other physiological processes. Recombinant AdoHcy hydrolase was overexpressed in E. coli JM109 and purified with ion exchange and gel filtration chromatographies. The effects of copper ions (Cu2+) on the activity of AdoHcy hydrolase were investigated and the results showed that Cu2+ inhibited the enzyme’s activity by a concentration and time-dependent process. The inhibition constant (Ki) and the apparent rate constant (Kapp) were calculated to be (14±4) nmol·L-1 and (1.08±0.15) min-1, respectively. The existence of the natural substrate Ado could to some extent prevent Cu2+ from inactivating the enzyme, suggesting that copper ions possibly could compete with the natural substrate on enzyme’s substrate binding site. Further studies on the mechanism of inhibition are being carried out.