An early understanding of key metabolites of drugs is crucial in drug discovery and development. As a result, several in vitro models typically derived from liver are frequently used to study drug metabolism. It is presumed that these in vitro systems provide an accurate view of the potential in vivo metabolites and metabolic pathways. However, no formal analysis has been conducted to validate their use. The goal of the present study was to conduct a comprehensive analysis to assess if the three commonly used in vitro systems, pooled human liver microsomes, liver S-9 fraction, and hepatocytes, adequately predict in vivo metabolic profiles for drugs. The second objective was to compare the overall capabilities of these three systems to generate in vivo metabolic profiles. Twenty-seven compounds in the Pfizer database and 21 additional commercially available compounds of diverse structure and routes of metabolism for which the human ADME data was available were analyzed in this study to assess the performance of the in vitro systems. The results suggested that all three systems reliably predicted human excretory and circulating metabolite profiles. Furthermore, the success in predicting primary metabolites and metabolic pathways was high (>70%), but the predictability of secondary metabolites was less reliable in the three systems. Thus, the analysis provides sufficient confidence in using in vitro systems to reliably produce primary in vivo human metabolites and supports their application in early discovery to identify metabolic spots for optimization of metabolic liabilities anticipated in humans in vivo. However, the in vitro systems cannot solely mitigate the risk of disproportionate circulating metabolites in humans and may need to be supplemented with metabolic profiling of plasma samples from first-in-human studies or early human radiolabeled studies.
ABSTRACT:Sunitinib is an oral multitargeted tyrosine kinase inhibitor approved for the treatment of advanced renal cell carcinoma, imatinibrefractory gastrointestinal stromal tumor, and advanced pancreatic neuroendocrine tumors. The current studies were conducted to characterize the pharmacokinetics, distribution, and metabolism of sunitinib after intravenous and/or oral administrations of
CYP3cide (PF-4981517; 1-methyl-3-[1-methyl-5-(4-methylphenyl)-1H-pyrazol-4-yl]-4-[(3S)-3-piperidin-1-ylpyrrolidin-1-yl]-1H-pyrazolo[3,4-d]pyrimidine) is a potent, efficient, and specific time-dependent inactivator of human CYP3A4. When investigating its inhibitory properties, an extreme metabolic inactivation efficiency (k inact /K I ) of 3300 to 3800 ml ⅐ min ؊1 ⅐ mol ؊1 was observed using human liver microsomes from donors of nonfunctioning CYP3A5 (CYP3A5 *3/*3). This observed efficiency equated to an apparent K I between 420 and 480 nM with a maximal inactivation rate (k inact ) equal to 1.6 min ؊1 . Similar results were achieved with testosterone, another CYP3A substrate, and other sources of the CYP3A4 enzyme. To further illustrate the abilities of CYP3cide, its partition ratio of inactivation was determined with recombinant CYP3A4. These studies produced a partition ratio approaching unity, thus underscoring the inactivation capacity of CYP3cide. WhenCYP3cide was tested at a concentration and preincubation time to completely inhibit CYP3A4 in a library of genotyped polymorphic CYP3A5 microsomes, the correlation of the remaining midazolam 1-hydroxylase activity to CYP3A5 abundance was significant (R 2 value equal to 0.51, p value of <0.0001). The work presented here supports these findings by fully characterizing the inhibitory properties and exploring CYP3cide's mechanism of action. To aid the researcher, multiple commercially available sources of CYP3cide were established, and a protocol was developed to quantitatively determine CYP3A4 contribution to the metabolism of an investigational compound. Through the establishment of this protocol and the evidence provided here, we believe that CYP3cide is a very useful tool for understanding the relative roles of CYP3A4 versus CYP3A5 and the impact of CYP3A5 genetic polymorphism on a compound's pharmacokinetics.
An efficient synthesis of imidazoles with isotope labeling at different positions of the five-membered ring was developed. We carried out a detailed mechanistic study of the photosensitized oxidation of isotope-labeled imidazole derivatives. A new product, CO(2), was observed in the photooxidation of 2-H,N1-H imidazoles, but not in 2-substitituted imidazoles. The carbon of CO(2) derives from the 2C of imidazole. As shown by 18O experiments, both oxygen atoms of CO(2) originate mainly from one molecule of oxygen. Transient intermediates were detected by low-temperature NMR in the photosensitized oxidation of the isotope-labeled imidazoles. Quantitative analysis of the 13C NMR at different temperatures and times correlates the formation of one intermediate with the loss of another, thus allowing the complete decomposition pathway of the transient intermediates to be established. Singlet oxygen reacts with 4,5-diphenylimidazole via a [4 + 2] cycloaddition to form a 2,5-endoperoxide, which, upon warming, decomposes to a hydroperoxide. The hydroperoxide in one pathway loses water to form an imidazolone 7, which is hydrolyzed to a hydroxyimidazol-2-one 11. In another pathway, the hydroperoxide rearranges to diol 8. The diol rearranges to a carbamate 9 by opening and reclosing the five-membered ring. 9 decomposes to CO(2) and benzil diimine. A labile NH in the imidazole is crucial for the decomposition of the initially formed endoperoxide, otherwise the endoperoxide decomposes to regenerate starting material. Many similarities exist between the photooxidations of imidazole and guanosine in organic solvent, suggesting that the two reactions share a similar reaction mechanism with singlet oxygen.
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