Bioactivation to free radicals and cytotoxicity of 1,1-dichloro-1-fluoroethane (HCFC-141b)

Dual negative precursor ion scan approach for rapid detection of glutathione conjugates using liquid chromatography/tandem mass spectrometry

Screening for conjugates formed by the tripeptide glutathione with new chemical entities is an essential step during the drug discovery process, as the formation of these conjugates serves as an indicator for the presence of reactive electrophilic intermediates. To increase the selectivity and throughput of this analysis, various mass spectral scan types have evolved over time. Historically, samples were analyzed under positive ionization conditions for the neutral loss of m/z 129 (loss of the pyroglutamic acid moiety from glutathione); however, more recently, negative precursor ion scanning for the loss of m/z 272 (deprotonated gamma-glutamyl-dehydroalanyl-glycine from glutathione) has emerged as a more selective tool. Further increasing the selectivity, we report on an extension of this methodology by incorporating a simultaneous dual negative precursor ion scan for two commonly observed ion fragments from glutathione conjugates, m/z 272 and 254 (the dehydrated form of m/z 272). This negative dual precursor ion scan methodology was first validated using substrates known to undergo metabolic bioactivation (diclofenac, carbamazepine, and 3-methyl indole) and has then been applied to the routine analysis of proprietary compounds undergoing active lead optimization. In comparison to alternative scan methodologies, the increased selectivity offered by this simultaneous dual precursor method results in a reduction in the generation of false positive results as well as reduced data interpretation time.

Effect of oxygen on the dehalogenation of 1,2-Dibromo-3-chloropropane by cytochrome P450cam (CYP101)

Cytochromes P450 are known to exhibit diverse catalytic functions against a large number of hydrocarbon substrates. The determinants of specific activity(ies) that operate on specific substrates have not been widely explored. Earlier, we showed that dehalogenation of 1,2-dibromo-3-chloropropane (DBCP) by P450cam (CYP101) monooxygenase exhibits oxygen- and substrate-dependent product distributions and reaction rates (1). Bromochloroacetone was the major conversion product when incubation media were saturated with oxygen, whereas allyl chloride was the sole product accounting for virtually all of the DBCP converted in the absence of oxygen. In an effort to develop a quantitative understanding of the effect of oxygen on product distribution and reaction rate, we have identified first generation products and measured reaction rates at four oxygen levels ranging from 0.01% to 100% saturation. In addition to bromochloroacetone and allyl chloride, a number of bromochloropropene isomers were identified in the presence of oxygen and are thought to be formed by an elimination mechanism. These products accounted for greater than 97 mol % of the reacted DBCP, which was run to high conversion (60-100 mol % DBCP converted). These measurements suggest that P450cam acts on the DBCP substrate through hydroxylation to produce 1-bromo-3-chloroacetone, through reduction to produce allyl chloride, and through elimination to produce bromochloropropene, with oxygen concentration determining the extent of each activity. A global data fitting kinetic model that describes the time-varying concentrations of substrate and products was developed to quantify the controlling level of oxygen on these multiple activities. The parameters of the model were compared with independent measurements and data from the literature.

Bioactivation and toxicity in vitro of HCFC-123 and HCFC-141b: role of cytochrome P450

The bioactivation and cytotoxicity in vitro of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1,1-dichloro-1-fluoroethane (HCFC-141b), two replacements for some ozone-depleting chlorofluorocarbons (CFC), were investigated in rat liver microsomes and isolated rat hepatocytes. Both compounds were activated by cytochrome P450 to reactive metabolites, as indicated by: (i) the depletion of exogenous and cellular glutathione, (ii) the increased LDH release from hepatocytes, (iii) the loss of microsomal P450 content and activities, and (iv) the formation of free radical species observed in the presence of the two compounds. Moreover, the formation of two stable metabolites and an increased production of conjugated dienes, a marker of lipid peroxidation, were observed for both HCFC-123 and HCFC-141b. The biotransformation of both compounds by pyridine- and phenobarbital-induced rat liver microsomes and the inhibition of LDH release by 4-methylpyrazole and troleandomycin indicate that P450 2E1, 2B and, possibly, also 3A are the isoforms involved in the bioactivation and toxicity of HCFC-123 and HCFC-141b in the rat.