The Experts below are selected from a list of 315 Experts worldwide ranked by ideXlab platform
Xinxin Ding - One of the best experts on this subject based on the ideXlab platform.
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Abstract 5465: Role of CYP2A13 in NNK Bioactivation and lung tumorigenesis:In vivostudies using a CYP2A13-humanized mouse model
Carcinogenesis, 2012Co-Authors: Vandana Megaraj, Xin Zhou, Fang Xie, Zhihua Liu, Jaime D'agostino, Weizhu Yang, Xinxin DingAbstract:The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent lung procarcinogen. Previously, we have demonstrated that NNK-induced lung tumorigenesis depends on target tissue Bioactivation by pulmonary P450 enzymes (Weng et al., 2007, Cancer Res 67:7825-7832). However, the specific P450 enzyme(s) responsible for NNK Bioactivation in the lung have not been identified. The aim of the present study was to test the hypotheses that 1) mouse CYP2A5 plays an essential role in NNK Bioactivation and tumorigenesis in mouse lung, and 2) transgenic expression of human CYP2A13, known to be selectively expressed in the respiratory tract and be the most efficient enzyme for NNK Bioactivation in vitro, can increase the rates of NNK Bioactivation and incidence of tumorigenesis in the mouse lung. Kinetic parameters of microsomal NNK Bioactivation in vitro, tissue levels of O 6 -methylguanine (O 6 -mG) DNA adducts formed in vivo, and incidence of NNK-induced lung tumors were determined for wild-type, Cyp2a5-null, and CYP2A13-humanized (CYP2A13-transgenic/Cyp2a5-null) mice. The results indicated that, in both liver and lung microsomes, the loss of CYP2A5 resulted in significant increases in the Km value for the formation of 4-oxo-4-(3-pyridyl)-butanal (OPB), which represents the reactive intermediate that can lead to the formation of O 6 -mG DNA adducts; but, the gain of CYP2A13 led to recovery of the activity in the lung, but not in the liver. The levels of O 6 -mG, the DNA adduct highly correlated with lung tumorigenesis, were significantly higher in the lungs of the wild-type mice and CYP2A13-humanized mice, than in the Cyp2a5-null mice. Furthermore, lung tumor bioassays, performed for the three mouse strains on A/J background, and using NNK at various doses (30 -200 mg/kg; single i.p. dose), revealed that lung tumor multiplicity determined at 4 months post-treatment was significantly greater in wild-type and CYP2A13-humanized mice than in Cyp2a5-null mice, for all NNK doses tested. The difference in lung tumor multiplicity between CYP2A13-humanized mice and Cyp2a5-null mice was greater at low (30 mg/kg) NNK dose (∼70%) than at high (200 mg/kg) NNK dose (∼30%). In control experiments, rates of systemic clearance of NNK and its major circulating metabolite, NNAL, were confirmed to be not different among the three mouse strains. These results indicated that CYP2A13 is a low-Km enzyme in catalyzing NNK Bioactivation in vivo, and they provide strong support for the idea that genetic polymorphisms of CYP2A13 can influence the risks of tobacco-induced lung tumorigenesis in humans. Moreover, the CYP2A13-humanized mouse model will be valuable for testing new, CYP2A13-targeted chemopreventive agents for lung carcinogenesis. (Supported in part by NIH grant CA092596) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5465. doi:1538-7445.AM2012-5465
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role of cyp2a5 in the Bioactivation of the lung carcinogen 4 methylnitrosamino 1 3 pyridyl 1 butanone in mice
Journal of Pharmacology and Experimental Therapeutics, 2012Co-Authors: Xin Zhou, Jaime Dagostino, Fang Xie, Xinxin DingAbstract:The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent lung carcinogen. Previously, we have demonstrated that NNK-induced lung tumorigenesis in mice depends on target-tissue Bioactivation by pulmonary cytochrome P450 (P450) enzymes. The present study was designed to test the hypothesis that mouse CYP2A5 plays an essential role in NNK Bioactivation in mouse lung. The role of CYP2A5 in NNK Bioactivation was studied both in vitro and in vivo, by comparing the kinetic parameters of microsomal NNK metabolism and tissue levels of O6-methylguanine (O6-mG) (the DNA adduct highly correlated with lung tumorigenesis) between wild-type (WT) and Cyp2a5-null mice. In both liver and lung microsomes, the loss of CYP2A5 resulted in significant increases in the apparent Km values for the formation of 4-oxo-4-(3-pyridyl)butanone, which represents the reactive intermediate that produces O6-mG in vivo. The loss of CYP2A5 did not change circulating levels of NNK or 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol in mice treated intraperitoneally with NNK at either 20 or 100 mg/kg. However, the levels of lung O6-mG were significantly lower in Cyp2a5-null than in WT mice; the extent of the reduction was greater at the 20 mg/kg dose (∼40%) than at the 100 mg/kg dose (∼20%). These results indicate that CYP2A5 is the low-Km enzyme for NNK Bioactivation in mouse lung. It is noteworthy that the remaining NNK Bioactivation activities in the Cyp2a5-null mice could be inhibited by 8-methoxypsoralen, a P450 inhibitor used previously to demonstrate the role of CYP2A5 in NNK-induced lung tumorigenesis. Thus, P450 enzymes other than CYP2A5 probably also contribute to NNK-induced lung tumorigenesis in mice.
Harihara M Mehendale - One of the best experts on this subject based on the ideXlab platform.
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toxicokinetics and toxicity of thioacetamide sulfoxide a metabolite of thioacetamide
Toxicology, 2007Co-Authors: Jaya Chilakapati, Midhun C Korrapati, Ronald A Hill, Alan Warbritton, John R Latendresse, Harihara M MehendaleAbstract:Abstract Thioacetamide (TA) is bioactivated by CYP2E1 to TA sulfoxide (TASO), and to the highly reactive sulfdioxide (TASO 2 ), which initiates hepatic necrosis by covalent binding. Previously, we have established that TA exhibits saturation toxicokinetics over a 12-fold dose range, which explains the lack of dose–response for Bioactivation-based liver injury. In vivo and in vitro studies indicated that the second step (TASO → TASO 2 ) of TA Bioactivation is less efficient than the first one (TA → TASO). The objective of the present study was to specifically test the saturation of the second step of TA Bioactivation by directly administering TASO, which obviates the contribution from first step, i.e. TA → TASO. Male SD rats were injected with low (50 mg/kg, ip), medium (100 mg/kg) and high (LD 70 , 200 mg/kg) doses of TASO. Bioactivation-mediated liver injury that occurs in the initial time points (6 and 12 h), estimated by plasma ALT, AST and liver histopathology over a time course, was not dose-proportional. Escalation of liver injury thereafter was dose dependent: low dose injury subsided; medium dose injury escalated upto 36 h before declining; high dose injury escalated from 24 h leading to 70% mortality. TASO was quantified in plasma by HPLC at various time points after administration of the three doses. With increasing dose (i.e., from 50 to 200 mg/kg), area under the curve (AUC) and C max increased more than dose proportionately, indicating that TASO Bioactivation exhibits saturable kinetics. Toxicokinetics and initiation of liver injury of TASO are similar to that of TA, although TASO-initiated injury occurs at lower doses. These findings indicate that Bioactivation of TASO to its reactive metabolite is saturable in the rat as suggested by previous studies with TA.
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saturation toxicokinetics of thioacetamide role in initiation of liver injury
Drug Metabolism and Disposition, 2005Co-Authors: Jaya Chilakapati, Midhun C Korrapati, Ronald A Hill, K Shankar, Harihara M MehendaleAbstract:Thioacetamide (TA), a potent centrilobular hepatotoxicant, undergoes a two-step Bioactivation mediated by microsomal CYP2E1 to TA sulfoxide (TASO), and further to TA-S,S-dioxide (TASO2), a reactive metabolite that initiates cellular necrosis. Our earlier studies showed that Bioactivation-mediated liver injury of TA is not dose-proportional. The objective of this study was to examine whether increasing doses of TA lead to enzyme saturation, thereby resulting in lack of dose-response for injury: Bioactivation of TA --> TASO --> TASO2 may follow zero-order kinetics. A 12-fold dose range of TA (50, 300, and 600 mg/kg i.p.) was injected into male Sprague-Dawley rats. TA and TASO were quantified in plasma, liver, and urine by high-performance liquid chromatography. With increasing doses, the apparent elimination half-lives of TA and TASO increased linearly, indicating that TA Bioactivation exhibits saturation kinetics. Increasing TA dose resulted in greater-than-proportional increases in plasma TA and TASO levels. The TASO/TA ratio was inversely proportional to the dose of TA. Covalent binding of 14C-TA-derived radiolabel to liver macromolecules showed a less-than-dose-proportionate increase with a 12-fold higher dose. Less than dose-proportional covalent binding was confirmed in liver microsomal incubations with 14C-TA. Three-fold higher excretion of TASO was seen in urine at the highest dose (600 mg/kg) compared with the lowest dose (50 mg TA/kg). Incubation of TA with rat liver microsomes and purified baculovirus-expressed rat and human CYP2E1 Supersomes, over a concentration range of 0.01 to 10 mM, revealed saturation of TA conversion to TASO at and above 0.05 mM TA concentration, comparable to in vivo plasma and liver levels achieved upon administration of higher doses. Calculated K(m) values for TA (0.1 mM) and TASO (0.6 mM) suggest that the second step of TA Bioactivation is 6-fold less efficient. Collectively, the findings indicate saturation of CYP2E1 at the first (TA to TASO) and second (TASO to TASO2) steps of TA Bioactivation.
Gary M Williams - One of the best experts on this subject based on the ideXlab platform.
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n nitrosodiethylamine mechanistic data and risk assessment Bioactivation dna adduct formation mutagenicity and tumor initiation
Pharmacology & Therapeutics, 1996Co-Authors: Lynne Verna, John Whysner, Gary M WilliamsAbstract:Abstract N-Nitrosodiethylamine (NDEA) is DNA reactive after Bioactivation and produces tumors in every animal species tested. Bioactivation is effected by several P450 isozymes including CYP2E1, which is ethanol inducible. Tumor formation in rat liver was proportional to O 4 -ethyldeoxythymidine formation in DNA, which was generally proportional to NDEA dose. At low doses in the 0.033–1.1 ppm range, the doseresponse for esophageal tumor formation was sublinear, possibly due to DNA repair. Although no epidemiological studies have specifically evaluated NDEA, sufficient exposure levels would be expected to cause cancer in humans.
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n nitrosodiethylamine mechanistic data and risk assessment Bioactivation dna adduct formation mutagenicity and tumor initiation
Pharmacology & Therapeutics, 1996Co-Authors: Lynne Verna, John Whysner, Gary M WilliamsAbstract:Abstract N-Nitrosodiethylamine (NDEA) is DNA reactive after Bioactivation and produces tumors in every animal species tested. Bioactivation is effected by several P450 isozymes including CYP2E1, which is ethanol inducible. Tumor formation in rat liver was proportional to O 4 -ethyldeoxythymidine formation in DNA, which was generally proportional to NDEA dose. At low doses in the 0.033–1.1 ppm range, the doseresponse for esophageal tumor formation was sublinear, possibly due to DNA repair. Although no epidemiological studies have specifically evaluated NDEA, sufficient exposure levels would be expected to cause cancer in humans.
Stephen S. Hecht - One of the best experts on this subject based on the ideXlab platform.
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Recent Studies on Mechanisms of Bioactivation and Detoxification of 4-(Methylnitrosamino)-1-(3-Pyridyl)-1-Butanone (NNK), A Tobacco-Specific Lung Carcinogen
Critical reviews in toxicology, 1996Co-Authors: Stephen S. HechtAbstract:This article reviews recent advances in the biochemistry and molecular biology of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific pulmonary carcinogen believed to be involved in the induction of lung cancer in smokers. Several aspects of NNK Bioactivation are addressed, including identification of its metabolites in laboratory animals and humans, cytochrome P450 enzyme involvement in its metabolic activation, DNA and protein adduct formation, biological significance of the major DNA adducts formed, and mutations in oncogenes from tumors induced by NNK. Collectively, the presently available data provide a reasonably clear picture of NNK Bioactivation in rodents, although there are still important gaps in our mechanistic understanding of NNK-induced tumorigenesis. The studies in rodents and primates have facilitated development of methods to assess NNK Bioactivation in humans, which will be applicable to studies of lung cancer susceptibility and prevention.
Grover P. Miller - One of the best experts on this subject based on the ideXlab platform.
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Meloxicam methyl group determines enzyme specificity for thiazole Bioactivation compared to sudoxicam
Toxicology letters, 2020Co-Authors: Dustyn A. Barnette, Mary A. Schleiff, Noah R. Flynn, S. Joshua Swamidass, Arghya Datta, Grover P. MillerAbstract:Abstract Meloxicam is a thiazole-containing NSAID that was approved for marketing with favorable clinical outcomes despite being structurally similar to the hepatotoxic sudoxicam. Introduction of a single methyl group on the thiazole results in an overall lower toxic risk, yet the group’s impact on P450 isozyme Bioactivation is unclear. Through analytical methods, we used inhibitor phenotyping and recombinant P450s to identify contributing P450s, and then measured steady-state kinetics for Bioactivation of sudoxicam and meloxicam by the recombinant P450s to determine relative efficiencies. Experiments showed that CYP2C8, 2C19, and 3A4 catalyze sudoxicam Bioactivation, and CYP1A2 catalyzes meloxicam Bioactivation, indicating that the methyl group not only impacts enzyme affinity for the drugs, but also alters which isozymes catalyze the metabolic pathways. Scaling of relative P450 efficiencies based on average liver concentration revealed that CYP2C8 dominates the sudoxicam Bioactivation pathway and CYP2C9 dominates meloxicam detoxification. Dominant P450s were applied for an informatics assessment of electronic health records to identify potential correlations between meloxicam drug-drug interactions and drug-induced liver injury. Overall, our findings provide a cautionary tale on assumed impacts of even simple structural modifications on drug Bioactivation while also revealing specific targets for clinical investigations of predictive factors that determine meloxicam-induced idiosyncratic liver injury.
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Significance of Multiple Bioactivation Pathways for Meclofenamate as Revealed through Modeling and Reaction Kinetics.
Drug metabolism and disposition: the biological fate of chemicals, 2020Co-Authors: Mary A. Schleiff, Noah R. Flynn, Sasin Payakachat, Benjamin Mark Schleiff, Anna O. Pinson, Dennis W. Province, S. Joshua Swamidass, Gunnar Boysen, Grover P. MillerAbstract:Meclofenamate is a nonsteroidal anti-inflammatory drug used in the treatment of mild-to-moderate pain yet poses a rare risk of hepatotoxicity through an unknown mechanism. Nonsteroidal anti-inflammatory drug (NSAID) Bioactivation is a common molecular initiating event for hepatotoxicity. Thus, we hypothesized a similar mechanism for meclofenamate and leveraged computational and experimental approaches to identify and characterize its Bioactivation. Analyses employing our XenoNet model indicated possible pathways to meclofenamate Bioactivation into 19 reactive metabolites subsequently trapped into glutathione adducts. We describe the first reported Bioactivation kinetics for meclofenamate and relative importance of those pathways using human liver microsomes. The findings validated only four of the many Bioactivation pathways predicted by modeling. For experimental studies, dansyl glutathione was a critical trap for reactive quinone metabolites and provided a way to characterize adduct structures by mass spectrometry and quantitate yields during reactions. Of the four quinone adducts, we were able to characterize structures for three of them. Based on kinetics, the most efficient Bioactivation pathway led to the monohydroxy para-quinone-imine followed by the dechloro-ortho-quinone-imine. Two very inefficient pathways led to the dihydroxy ortho-quinone and a likely multiply adducted quinone. When taken together, Bioactivation pathways for meclofenamate accounted for approximately 13% of total metabolism. In sum, XenoNet facilitated prediction of reactive metabolite structures, whereas quantitative experimental studies provided a tractable approach to validate actual Bioactivation pathways for meclofenamate. Our results provide a foundation for assessing reactive metabolite load more accurately for future comparative studies with other NSAIDs and drugs in general. Significance Statement Meclofenamate Bioactivation may initiate hepatotoxicity, yet common risk assessment approaches are often cumbersome and inefficient and yield qualitative insights that do not scale relative Bioactivation risks. We developed and applied innovative computational modeling and quantitative kinetics to identify and validate meclofenamate Bioactivation pathways and relevance as a function of time and concentration. This strategy yielded novel insights on meclofenamate Bioactivation and provides a tractable approach to more accurately and efficiently assess other drug Bioactivations and correlate risks to toxicological outcomes.
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Dual mechanisms suppress meloxicam Bioactivation relative to sudoxicam
Toxicology, 2020Co-Authors: Dustyn A. Barnette, Mary A. Schleiff, Noah R. Flynn, S. Joshua Swamidass, Laura R. Osborn, Matthew K. Matlock, Grover P. MillerAbstract:Thiazoles are biologically active aromatic heterocyclic rings occurring frequently in natural products and drugs. These molecules undergo typically harmless elimination; however, a hepatotoxic response can occur due to multistep Bioactivation of the thiazole to generate a reactive thioamide. A basis for those differences in outcomes remains unknown. A textbook example is the high hepatotoxicity observed for sudoxicam in contrast to the relative safe use and marketability of meloxicam, which differs in structure from sudoxicam by the addition of a single methyl group. Both drugs undergo Bioactivation, but meloxicam exhibits an additional detoxification pathway due to hydroxylation of the methyl group. We hypothesized that thiazole Bioactivation efficiency is similar between sudoxicam and meloxicam due to the methyl group being a weak electron donator, and thus, the relevance of Bioactivation depends on the competing detoxification pathway. For a rapid analysis, we modeled epoxidation of sudoxicam derivatives to investigate the impact of substituents on thiazole Bioactivation. As expected, electron donating groups increased the likelihood for epoxidation with a minimal effect for the methyl group, but model predictions did not extrapolate well among all types of substituents. Through analytical methods, we measured steady-state kinetics for metabolic Bioactivation of sudoxicam and meloxicam by human liver microsomes. Sudoxicam Bioactivation was 6-fold more efficient than that for meloxicam, yet meloxicam showed a 6-fold higher efficiency of detoxification than Bioactivation. Overall, sudoxicam Bioactivation was 15-fold more likely than meloxicam considering all metabolic clearance pathways. Kinetic differences likely arise from different enzymes catalyzing respective metabolic pathways based on phenotyping studies. Rather than simply providing an alternative detoxification pathway, the meloxicam methyl group suppressed the Bioactivation reaction. These findings indicate the impact of thiazole substituents on Bioactivation is more complex than previously thought and likely contributes to the unpredictability of their toxic potential.