Desacetyl-diltiazem displays severalfold higher affinity to CYP2D6 compared with CYP3A4

HiMolformer: Integrating graph and sequence representations for predicting liver microsome stability with SMILES

In the initial stages of drug discovery or pre-clinical studies, understanding the metabolic stability of new molecules is crucial. Recently, research on pre-trained deep learning for molecular property prediction has been actively progressing, with various models being made open-source. However, most of these models rely on either 2D graph or 1D sequence for training, and the representation varies depending on the data format used. Consequently, combining multiple representations can broaden the scope of learning and may potentially be a manageable and most effective method to enhance performance. Therefore, we propose a novel hybrid model for predicting metabolic stability, which integrates representations from both graph-based and sequence-based models pre-trained for molecular features. This approach utilizes the combined strengths of 2D topological and 1D sequential information of molecules. HiMol, a graph-based graph neural network (GNN) model, and Molformer, a sequence-based Transformer model, were selected for integration, thus we named it HiMolformer. HiMolformer demonstrated superior performance compared to other models. We also focus on regression task for prediction with a empirical dataset from Korea Chemical Bank (KCB), comprising 3,498 molecules with mouse liver microsome (MLM) and human liver microsome (HLM) data obtained from actual metabolic reaction experiments. To the best of our knowledge, it is the first attempt to develop MLM and HLM prediction models using regression with a single SMILES input. The source code of this model is available at https://github.com/YUNSEOKWOO/HiMolformer.

A novel human stem cell-based biomarker assay for in vitro assessment of developmental toxicity

BACKGROUND

Testing for developmental toxicity according to the current regulatory guidelines requires large numbers of animals, making these tests very resource intensive, time-consuming, and ethically debatable. Over the past decades, several alternative in vitro assays have been developed, but these often suffered from low predictability and the inability to provide a mechanistic understanding of developmental toxicity.

METHODS

To identify embryotoxic compounds, we developed a human induced pluripotent stem cells (hiPSCs)-based biomarker assay. The assay is based on the differentiation of hiPSCs into functional cardiomyocytes and hepatocytes. Proper stem cell differentiation is investigated by morphological profiling and assessment of time-dependent expression patterns of cell-specific biomarkers. In this system, a decrease in the expression of the biomarker genes and morphology disruption of the differentiated cells following compound treatment indicated teratogenicity.

RESULTS

The hiPSCs-based biomarker assay was validated with 21 well-established in vivo animal teratogenic and non-teratogenic compounds during cardiomyocyte and hepatocyte differentiation. The in vivo teratogenic compounds (e.g., thalidomide and valproic acid) markedly disrupted morphology, functionality, and the expression pattern of the biomarker genes in either one or both cell types. Non-teratogenic chemicals generally had no effect on the morphology of differentiated cells, nor on the expression of the biomarker genes. Compared to the in vivo classification, the assay achieved high accuracy (91%), sensitivity (91%), and specificity (90%).

CONCLUSION

The assay, which we named ReproTracker®, is a state-of-the-art in vitro method that can identify the teratogenicity potential of new pharmaceuticals and chemicals and signify the outcome of in vivo test systems.

Pharmacomicrobiomics: Exploiting the Drug-Microbiota Interactions in Antihypertensive Treatment

Hypertension is a leading risk factor for cardiovascular diseases and can reduce life expectancy. Owing to the widespread use of antihypertensive drugs, patients with hypertension have improved blood pressure control over the past few decades. However, for a considerable part of the population, these drugs still cannot significantly improve their symptoms. In order to explore the reasons behind, pharmacomicrobiomics provide unique insights into the drug treatment of hypertension by investigating the effect of bidirectional interaction between gut microbiota and antihypertensive drugs. This review discusses the relationship between antihypertensive drugs and the gut microbiome, including changes in drug pharmacokinetics and gut microbiota composition. In addition, we highlight how our current knowledge of antihypertensive drug-microbiota interactions to develop gut microbiota-based personalized ways for disease management, including antihypertensive response biomarker, microbial-targeted therapies, probiotics therapy. Ultimately, a better understanding of the impact of pharmacomicrobiomics in the treatment of hypertension will provide important information for guiding rational clinical use and individualized use.

Development of template systems for ligand interactions of CYP3A5 and CYP3A7 and their distinctions from CYP3A4 template

Starting from established CYP3A4 Template (DMPK. 2019, and 2020), CYP3A5 and CYP3A7 Templates have been constructed to be reliable tools for verification of their distinct catalytic properties. A distinct occupancy was observed on CYP3A4-selective ligands, but not on the non-selective ligands, in simulation experiments. These ligands often invade into Bay-1 region during the migration from Entrance to Site of oxidation in simulation experiments. These results offered an idea of the distinct localization of Bay-1 residue on CYP3A5 Template, in which the Bay-1 residue stayed closely to Template border. The idea also accounted for the higher oxidation rates of CYP3A5, than of CYP3A4, of noscapine and schisantherin E through their enhanced sitting-stabilization. Typical CYP3A7 substrates such as zonisamide and retinoic acids took their placements without occupying a left side region of Template for their metabolisms. In turn, the occupancies of the left-side region were inevitably observed among poor ligands of CYP3A7. Altered extent of IJK-Interaction or localization of a specific residue at the left-side would thus explain distinct catalytic properties of CYP3A7 on Template. These data suggest the alteration of each one of Template region, from CYP3A4 Template, led to the distinct catalytic properties of CYP3A5 and CYP3A7 forms.

Topological Analysis of Electron Density in Large Biomolecular Systems

BACKGROUND

A great step toward describing the structure of the molecular electron was made in the era of quantum chemical methods. Methods play a very important role in the prediction of molecular properties and in the description of the reactivity of compounds, which cannot be overestimated. There are many works, books, and articles on quantum methods, their applications, and comparisons. At the same time, quantum methods of a high level of theory, which give the most accurate results, are time-consuming, which makes them almost impossible to describe large complex molecular systems, such as macromolecules, enzymes, supramolecular compounds, crystal fragments, and so on.

OBJECTIVES

To propose an approach that allows real-time estimation of electron density in large systems, such as macromolecules, nanosystems, proteins.

METHODS

AlteQ approach was applied to the tolopogical analysis of electron density for "substrate - cytochrome" complexes. The approach is based on the use of Slater's type atomic contributions. Parameters of the atomic contributions were found using high resolution X-ray diffraction data for organic and inorganic molecules. Relationships of the parameters with atomic number, ionization potentials and electronegativities were determined. The sufficient quality of the molecular electron structure representation was shown under comparison of AlteQ predicted and observed electron densities. AlteQ algorithm was applied for evaluation of electron structure of "CYP3A4 - substrate" complexes modeled using BiS/MC restricted docking procedure. Topological analysis (similar to Atoms In Molecules (AIM) theory suggested by Richard F.W. Bader) of the AlteQ molecular electron density was carried out for each complex. The determination of (3,-1) bond, (3,+1) ring, (3,+3) cage critical points of electron density in the intermolecular "CYP3A4 - substrate" space was performed.

RESULTS

Different characteristics such as electron density, Laplacian eigen values, etc. at the critical points were computed. Relationship of pKM (KM is Michaelis constant) with the maximal value of the second Laplacian eigen value of electron density at the critical points and energy of complex formation computed using MM3 force field was determined.

CONCLUSION

It was shown that significant number of (3,-1) bond critical points are located in the intermolecular space between the enzyme site and groups of substrate atoms eliminating during metabolism processes.

Mapping human microbiome drug metabolism by gut bacteria and their genes

Individuals vary widely in their responses to medicinal drugs, which can be dangerous and expensive owing to treatment delays and adverse effects. Although increasing evidence implicates the gut microbiome in this variability, the molecular mechanisms involved remain largely unknown. Here we show, by measuring the ability of 76 human gut bacteria from diverse clades to metabolize 271 orally administered drugs, that many drugs are chemically modified by microorganisms. We combined high-throughput genetic analyses with mass spectrometry to systematically identify microbial gene products that metabolize drugs. These microbiome-encoded enzymes can directly and substantially affect intestinal and systemic drug metabolism in mice, and can explain the drug-metabolizing activities of human gut bacteria and communities on the basis of their genomic contents. These causal links between the gene content and metabolic activities of the microbiota connect interpersonal variability in microbiomes to interpersonal differences in drug metabolism, which has implications for medical therapy and drug development across multiple disease indications.

Use of Human Plasma Samples to Identify Circulating Drug Metabolites that Inhibit Cytochrome P450 Enzymes

Drug interactions elicited through inhibition of cytochrome P450 (P450) enzymes are important in pharmacotherapy. Recently, greater attention has been focused on not only parent drugs inhibiting P450 enzymes but also on possible inhibition of these enzymes by circulating metabolites. In this report, an ex vivo method whereby the potential for circulating metabolites to be inhibitors of P450 enzymes is described. To test this method, seven drugs and their known plasma metabolites were added to control human plasma at concentrations previously reported to occur in humans after administration of the parent drug. A volume of plasma for each drug based on the known inhibitory potency and time-averaged concentration of the parent drug was extracted and fractionated by high-pressure liquid chromatography-mass spectrometry, and the fractions were tested for inhibition of six human P450 enzyme activities (CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A4). Observation of inhibition in fractions that correspond to the retention times of metabolites indicates that the metabolite has the potential to contribute to P450 inhibition in vivo. Using this approach, norfluoxetine, hydroxyitraconazole, desmethyldiltiazem, desacetyldiltiazem, desethylamiodarone, hydroxybupropion, erythro-dihydrobupropion, and threo-dihydrobupropion were identified as circulating metabolites that inhibit P450 activities at a similar or greater extent as the parent drug. A decision tree is presented outlining how this method can be used to determine when a deeper investigation of the P450 inhibition properties of a drug metabolite is warranted.

Effect of the human therapeutic drug diltiazem on the haematological parameters, histology and selected enzymatic activities of rainbow trout Oncorhynchus mykiss

Diltiazem is a pharmaceutical belonging to a group of calcium channel blockers (CCB) that is widely used in the treatment of angina pectoris and hypertension. The objective of the present study was to assess the effect of diltiazem on rainbow trout (Oncorhynchus mykiss). Juvenile trout were exposed for 21 and 42 days to three nominal concentrations of diltiazem: 0.03 μg L(-1) (environmentally relevant concentration), 3 μg L(-1), and 30 μg L(-1) (sub-lethal concentrations). The number of mature neutrophilic granulocytes was significantly increased by 450 and 400% in fish exposed to 3 μg L(-1) and 30 μg L(-1) diltiazem compared to the control, respectively. Antioxidant enzyme activity was affected in liver and gills of fish exposed to all tested concentrations of diltiazem but the changes were mostly transient and not concentration dependent. Creatine kinase activity was markedly increased (ranging from 520 to 845%) at all tested diltiazem concentrations at the end of the exposure indicating muscle and/or kidney damage. The highest concentration was associated with histological changes in heart, liver, and kidney. These alterations can be attributed to the effects of diltiazem on the cardiovascular system, similar to those observed in the human body, as well as to its metabolism. At the environmentally relevant concentration, diltiazem was found to induce some alterations in the blood, gills, and liver of fish, indicating its potential for adverse effects on non-target organisms in the aquatic environment.

Investigation of diltiazem metabolism in fish using a hybrid quadrupole/orbital trap mass spectrometer

RATIONALE

Diltiazem, a calcium channel blocker drug, is widespread in the environment because of its incomplete elimination during water treatment. It can cause negative effects on aquatic organisms; thus, a rapid and sensitive liquid chromatography/mass spectrometry (LC/MS) method to detect its presence was developed. Our approach is based on accurate mass measurements using a hybrid quadrupole-orbital trap mass spectrometer that was used to measure diltiazem and its metabolites in fish tissue.

METHODS

Blood plasma, muscle, liver, and kidney tissues of rainbow trout (Oncorhynchus mykiss), exposed for 42 days to 30 μg L(-1) diltiazem, were used for the method development. No metabolite standards were required to identify the diltiazem biotransformation products in the fish tissue.

RESULTS

Overall, 17 phase I diltiazem metabolites (including isomeric forms) were detected and tentatively identified using the MassFrontier spectral interpretation software. A semi-quantitative approach was used for organ-dependent comparison of the metabolite concentrations.

CONCLUSIONS

These data increase our understanding about diltiazem and its metabolites in aquatic organisms, such as fish. These encompass desmethylation, desacetylation and hydroxylation as well as their combinations. This study represents the first report of the complex diltiazem phase I metabolic pathways in fish.

Bioconcentration, metabolism and half-life time of the human therapeutic drug diltiazem in rainbow trout Oncorhynchus mykiss

Diltiazem is a human therapeutic drug and a member of the group of calcium channel blockers having widespread use in the treatment of angina pectoris and hypertension. The objective of the present study was to assess the bioconcentration, metabolism, and half-life time of diltiazem in rainbow trout Oncorhynchus mykiss. Juvenile trout were exposed for 21 and 42 days to three nominal concentrations of diltiazem: 0.03 µg L(-1) (environmentally relevant concentration), 3 µg L(-1), and 30 µg L(-1) (sub-lethal concentrations). The bioconcentration factor (BCF) of diltiazem was relatively low (0.5-194) in analysed tissues, following the order kidney > liver > muscle > blood plasma. The half-life of diltiazem in liver, kidney, and muscle was 1.5 h, 6.2 h, and 49 h, respectively. The rate of metabolism for diltiazem in liver, kidney, muscle, and blood plasma was estimated to be 85 ± 9%, 64 ± 14%, 46 ± 6%, and 41 ± 8%, respectively. Eight diltiazem metabolites were detected. The presence of desmethyl diltiazem (M1), desacetyl diltiazem (M2), and desacetyl desmethyl diltiazem (M3) suggests that rainbow trout metabolize diltiazem mainly via desmethylation and desacetylation, similar to mammals. In addition, diltiazem undergoes hydroxylation in fish. At environmentally relevant concentrations, diltiazem and its metabolites were identified in liver and kidney, indicating the potential for uptake and metabolism in non-target organisms in the aquatic environment.

Bioavailability enhancers of herbal origin: an overview

Recently, the use of herbal medicines has been increased all over the world due to their therapeutic effects and fewer adverse effects as compared to the modern medicines. However, many herbal drugs and herbal extracts despite of their impressive in-vitro findings demonstrates less or negligible in-vivo activity due to their poor lipid solubility or improper molecular size, resulting in poor absorption and hence poor bioavailability. Nowadays with the advancement in the technology, novel drug delivery systems open the door towards the development of enhancing bioavailability of herbal drug delivery systems. For last one decade many novel carriers such as liposomes, microspheres, nanoparticles, transferosomes, ethosomes, lipid based systems etc. have been reported for successful modified delivery of various herbal drugs. Many herbal compounds including quercetin, genistein, naringin, sinomenine, piperine, glycyrrhizin and nitrile glycoside have demonstrated capability to enhance the bioavailability. The objective of this review is to summarize various available novel drug delivery technologies which have been developed for delivery of drugs (herbal), and to achieve better therapeutic response. An attempt has also been made to compile a profile on bioavailability enhancers of herbal origin with the mechanism of action (wherever reported) and studies on improvement in drug bioavailability, exhibited particularly by natural compounds.

Discovery and early development of AVI-7537 and AVI-7288 for the treatment of Ebola virus and Marburg virus infections

There are no currently approved treatments for filovirus infections. In this study we report the discovery process which led to the development of antisense Phosphorodiamidate Morpholino Oligomers (PMOs) AVI-6002 (composed of AVI-7357 and AVI-7539) and AVI-6003 (composed of AVI-7287 and AVI-7288) targeting Ebola virus and Marburg virus respectively. The discovery process involved identification of optimal transcript binding sites for PMO based RNA-therapeutics followed by screening for effective viral gene target in mouse and guinea pig models utilizing adapted viral isolates. An evolution of chemical modifications were tested, beginning with simple Phosphorodiamidate Morpholino Oligomers (PMO) transitioning to cell penetrating peptide conjugated PMOs (PPMO) and ending with PMOplus containing a limited number of positively charged linkages in the PMO structure. The initial lead compounds were combinations of two agents targeting separate genes. In the final analysis, a single agent for treatment of each virus was selected, AVI-7537 targeting the VP24 gene of Ebola virus and AVI-7288 targeting NP of Marburg virus, and are now progressing into late stage clinical development as the optimal therapeutic candidates.

Effect of hesperidin on the oral pharmacokinetics of diltiazem and its main metabolite, desacetyldiltiazem, in rats

Objectives

This study was to investigate the effect of hesperidin, an antioxidant, on the bioavailability and pharmacokinetics of diltiazem and its active major metabolite, desacetyldiltiazem, in rats.

Methods

A single dose of diltiazem was administered orally (15 mg/kg) in the presence or absence of hesperidin (1, 5 or 15 mg/kg), which was administered 30 min before diltiazem.

Key findings

Compared with the control group (given diltiazem alone), hesperidin (5 or 15 mg/kg) significantly altered the pharmacokinetic parameters of diltiazem, except for 1 mg/kg hesperidin. The area under the plasma concentration-time curve from time 0 h to infinity (AUC0-∞) was significantly (5 mg/kg, P < 0.05; 15 mg/kg, P < 0.01) increased by 48.9–65.3% and the peak plasma concentration (Cmax) was significantly (P < 0.05) increased by 46.7–62.4% in the presence of hesperidin (5 or 15 mg/kg). Consequently, the absolute bioavailability (F) of diltiazem with hesperidin was significantly (5 mg/kg, P < 0.05; 15 mg/kg, P < 0.01) higher than that in the control group. Hesperidin (5 or 15 mg/kg) significantly (P < 0.05) increased the AUC0-∞ and 15 mg/kg of hesperidin significantly (P < 0.05) increased the Cmax of desacetyldiltiazem. However, the metabolite-parent ratio (MR) of desacetyldiltiazem was not significantly changed in the presence of hesperidin.

Conclusions

Hesperidin significantly enhanced the oral bioavailability of diltiazem in rats. It might be considered that hesperidin increased the intestinal absorption and reduced the first-pass metabolism of diltiazem in the intestine and in the liver via an inhibition of cytochrome P450 3A or P-glycoprotein.

New insights into the structural characteristics and functional relevance of the human cytochrome P450 2D6 enzyme

To date, the crystal structures of at least 12 human CYPs (1A2, 2A6, 2A13, 2C8, 2C9, 2D6, 2E1, 2R1, 3A4, 7A1, 8A1, and 46A1) have been determined. CYP2D6 accounts for only a small percentage of all hepatic CYPs (< 2%), but it metabolizes approximately 25% of clinically used drugs with significant polymorphisms. CYP2D6 also metabolizes procarcinogens and neurotoxins, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, and indolealkylamines. Moreover, the enzyme utilizes hydroxytryptamines and neurosteroids as endogenous substrates. Typical CYP2D6 substrates are usually lipophilic bases with an aromatic ring and a nitrogen atom, which can be protonated at physiological pH. Substrate binding is generally followed by oxidation (5-7 A) from the proposed nitrogen-Asp301 interaction. A number of homology models have been constructed to explore the structural features of CYP2D6, while antibody studies also provide useful structural information. Site-directed mutagenesis studies have demonstrated that Glu216, Asp301, Phe120, Phe481, and Phe483 play important roles in determining the binding of ligands to CYP2D6. The structure of human CYP2D6 has been recently determined and shows the characteristic CYP fold observed for other members of the CYP superfamily. The lengths and orientations of the individual secondary structural elements in the CYP2D6 structure are similar to those seen in other human CYP2 members, such as CYP2C9 and 2C8. The 2D6 structure has a well-defined active-site cavity located above the heme group with a volume of approximately 540 A(3), which is larger than equivalent cavities in CYP2A6 (260 A(3)), 1A2 (375 A(3)), and 2E1 (190 A(3)), but smaller than those in CYP3A4 (1385 A(3)) and 2C8 (1438 A(3)). Further studies are required to delineate the molecular mechanisms involved in CYP2D6 ligand interactions and their implications for drug development and clinical practice.

Technique for energy decomposition in the study of "receptor-ligand" complexes

A new methodology to describe the interactions in "receptor-ligand" complexes is presented. The methodology is based on a combination of the 3D/4D QSAR BiS/MC and CoCon algorithms. The first algorithm performs the restricted docking of compounds to receptor pockets. The second determines the relationships between the bioactivity and the parameters of interactions in the "receptor-ligand" complexes, including a new formalism for estimating hydrogen bond energies.

Proof of a 1-(3-chlorophenyl)piperazine (mCPP) intake—Use as adulterant of cocaine resulting in drug–drug interactions?

Since 2005, increasing numbers of seizures of the designer drug of abuse 1-(3-chlorophenyl)piperazine (mCPP) have been reported. This paper describes the unequivocal proof of a mCPP intake. Differentiation from the intake of its precursor drugs trazodone and nefazodone was performed by a systematic toxicological analysis (STA) procedure using full-scan GC-MS after acid hydrolysis, liquid-liquid extraction and microwave-assisted acetylation. The found mCPP/hydroxy-mCPP ratio indicated altered metabolism of this cytochrome (CYP) 2D6 catalyzed reaction compared to published studies using the same procedure. Although this might be ascribed to a poor metabolizer (PM) phenotype, genotyping revealed no PM genotype but indications for an intermediate metabolizer genotype. However, a PM phenotype could also be caused by drug-drug interactions with CYP2D6 inhibitors or substrates such as the co-consumed cocaine and diltiazem and/or diltiazem metabolites, respectively. In conclusion, the presented data indicate a possible relevance of CYP2D6 polymorphism and/or drug interactions to mCPP toxicokinetics, which is important for risk assessment of this new designer drug of abuse, in particular if it is used as adulterant of CYP2D6 substrates such as cocaine.

Enhanced diltiazem bioavailability after oral administration of diltiazem with quercetin to rabbits

The aim of this study was to investigate the effect of quercetin on the bioavailability of diltiazem after administering diltiazem (15 mg/kg) orally to rabbits either co-administered or pretreated with quercetin (2, 10, 20 mg/kg). The plasma concentrations of diltiazem in the rabbits pretreated with quercetin were increased significantly (p<0.05, at 2 mg/kg; p<0.01, at 10 and 20 mg/kg) compared with the control, but the plasma concentrations of diltiazem co-administered with quercetin were not significant. The areas under the plasma concentration-time curve (AUC) and the peak concentrations (Cmax) of the diltiazem in the rabbits pretreated with quercetin were significantly higher (p<0.05, at 2 mg/kg; p<0.01, at 10 and 20 mg/kg) than the control. The absolute bioavailability (AB%) of diltiazem in the rabbits pretreated with quercetin was significantly (p<0.05 at 2 mg/kg, p<0.01 at 10 and 20 mg/kg) higher (9.10-12.81%) than the control (4.64%). AUC, AB% and Cmax of diltiazem co-administered with quercetin were higher than the control, but these were not significant. The bioavailibility of diltiazem in the rabbits pretreated with quercetin is increased significantly compared with the control, but not in the rabbits co-administered with quercetin. The increased bioavailability of diltiazem in the rabbits pretreated with quercetin might have been resulted result from the quercetin, which inhibits the efflux pump P-glycoprotein and the first-pass metabolizing enzyme CYP 3A4.

Naphthalene-induced respiratory tract toxicity: metabolic mechanisms of toxicity

The lung, which is in intimate contact with the external environment, is exposed to a number of toxicants both by virtue of its large surface area and because it receives 100% of the cardiac output. Lung diseases are a major disease entity in the U.S. population ranking third in terms of morbidity and mortality. Despite the importance of these diseases, key issues remain to be resolved regarding the interactions of chemicals with lung tissue and the factors that are critical determinants of chemical-induced lung injury. The importance of cytochrome P450 monooxygenase dependent metabolism in chemical-induced lung injury in animal models was established over 25 years ago with the furan, 4-ipomeanol. Since then, the significance of biotransformation and the reasons for the high degree of pulmonary selectivity for a myriad of different chemicals has been well documented, mainly in rodent models. However, with many of these chemicals there are substantial differences in the susceptibility of rats vs. mice. Even within the same species, varied levels of the respiratory tract respond differently. Thus, key pieces of data are still missing when evaluating the applicability of data generated in rodents to primates, and as a result of this, there are substantial uncertainties within the regulatory community with regards to assessing the risks to humans for exposure to some of these chemicals. For example, all of the available data suggest that the levels of cytochrome P450 monooxygenases in rodent lungs are 10-100 times greater than those measured in the lungs of nonhuman primates or in man. At first glance, this suggests that a significant margin of safety exists when evaluating the applicability of rodent studies in the human, but the issues are more complex. The intent of this review is to outline some of the work conducted on the site and species selective toxicity and metabolism of the volatile lung toxic aromatic hydrocarbon, naphthalene. We argue that a complete understanding of the cellular and biochemical mechanisms by which this and other lung toxic compounds generate their effects in rodent models with subsequent measurement of these cellular and biochemical events in primate and human tissues in vitro will provide a far better basis for judging whether the results of studies done in rodent models are applicable to humans.

Kinetic interactions of dopamine and dobutamine with human catechol-O-methyltransferase and monoamine oxidase in vitro

Dopamine and dobutamine are often infused together into acutely ill patients requiring temporary support of cardiac and renal function, but whether these catecholamines affect the metabolic clearance of each other is not established. We determined the kinetics of dopamine and dobutamine as substrates and inhibitors of each other, i.e., apparent V(max), K(m), and K(i), with crude preparations of human blood mononuclear cell catechol-O-methyltransferase (COMT) and platelet monoamine oxidase (MAO) at pH 7.4 and 37 degrees C. Values of V(max) for dopamine and dobutamine as substrates for COMT were 0.45 and 0.59 nmol of 3-O-methyl product formed per milligram of protein per minute, whereas those for K(m) were 0.44 and 0.05 mM, respectively. Dopamine and dobutamine were competitive inhibitors of each other in this reaction. The K(i) for dopamine as an inhibitor of dobutamine methylation was 1.5 mM, whereas that for dobutamine as an inhibitor of dopamine methylation was 0.015 mM. Dopamine but not dobutamine was a substrate for MAO. The V(max) for dihydroxyphenylacetaldehyde formation from dopamine was 0.29 nmol/mg protein/min and the K(m) for dopamine was 0.38 mM. Dobutamine was a noncompetitive inhibitor of dopamine oxidation in this reaction (K(i) congruent with 1.19 mM). The high apparent K(m) and K(i) values derived for dopamine and dobutamine when tested with these two human enzymes in vitro suggest that these catecholamines do not interfere with the metabolism of each other when both are infused together at therapeutic concentrations.