In vitro tests for predicting drug-drug interactions: the need for validated procedures

Screening for clinically relevant drug-drug interactions between direct oral anticoagulants and antineoplastic agents: a pharmacovigilance approach

BACKGROUND

Use of direct oral anticoagulants (DOACs) in patients with cancer remains suboptimal due to the concern regarding potential drug-drug interactions (DDIs) with antineoplastic treatments. However, the clinical relevance of these DDIs is unknown.

METHODS

We conducted a pharmacovigilance study of adverse event (AE) reports from the US Food and Drug Administration Adverse Event Reporting System from 1/1/2004 to 12/31/2021. AE reports containing DOACs and antineoplastic agents with CYP3A4/P-gp inhibitory or inducing activity suggested by published pharmacokinetic studies were included (n = 36,066). The outcomes of interest were bleeding or stroke, identified by MedDRA dictionary version 25.0. We used disproportionality analyses (DPA), logistic regression models (LR), and Multi-item Gamma-Poisson Shrinker (MGPS) (Empirical Bayes Geometric Means (EBGM) and 90% credible intervals (90% CIs)) algorithms to identify the safety signal of DDIs.

RESULTS

The highest bleeding reporting rates for each drug class were the combination of DOACs with neratinib (39.08%, n = 34), tamoxifen (21.22%, n = 104), irinotecan (20.54%, n = 83), and cyclosporine (19.17%, n = 227). The highest rate of stroke was found for prednisolone (2.43%, n = 113). In the primary analysis, no signal of DDIs by the antineoplastic therapeutic class was detected by MGPS, DPA, and LR approaches. By individual antineoplastic drug, DOACs-neratinib was the only signal detected [EBGM (EB05-EB95) = 2.71 (2.03-3.54)].

CONCLUSION

No signal of DDIs between DOACs and antineoplastic agents was detected, except for DOAC-neratinib. Most DDIs between DOACs and antineoplastic agents may not be clinically relevant. The DDIs between DOACs and neratinib should be further examined in future research.

Molecular Modeling Identification of Key Secondary Metabolites from Xylopia aethiopica as Promising Therapeutics Targeting Essential Measles Viral Proteins

This study computationally screened three key compounds (vanillin (VAN), oxophoebine (OPB), and dihydrochalcone (DHC)) derived from Xylopia aethiopica (Guinea pepper), a medicinal plant with known antiviral activity, against key druggable measles virus (MV) proteins (fusion protein (FUP), haemagglutinin protein (HMG), and phosphoprotein (PSP)). Each molecular species was subjected to a 100 ns molecular dynamics (MD) simulation following docking, and a range of postdynamic parameters including free binding energy and pharmacokinetic properties were determined. The docking scores of the resulting OPB-FUP (-5.4 kcal/mol), OPB-HMG (-8.1 kcal/mol), and OPB-PSP (-8.0 kcal/mol) complexes were consistent with their respective binding energy values (-25.37, -28.74, and -40.68 kcal/mol), and higher than that of the reference standard, ribavirin (RBV) in each case. Furthermore, all the investigated compounds were thermodynamically compact and stable, especially HMG of MV, and this observation could be attributed to the resulting intermolecular interactions in each system. Overall, OPB may possess inhibitory properties against MV glycoproteins (FUP and HMG) and PSP that play important roles in the replication of MV and measles pathogenesis. While OPB could serve as a scaffold for the development of novel MV fusion and entry inhibitors, further in vitro and in vivo evaluation is highly recommended.

Molecular Docking, Validation, Dynamics Simulations, and Pharmacokinetic Prediction of Phytochemicals Isolated From Croton dichogamus Against the HIV-1 Reverse Transcriptase

The human immunodeficiency virus (HIV) infection and the associated acquired immune deficiency syndrome (AIDS) remain global challenges even after decades of successful treatment, with eastern and southern Africa still bearing the highest burden of disease. Following a thorough computational study, we report top 10 phytochemicals isolated from Croton dichogamus as potent reverse transcriptase inhibitors. The pentacyclic triterpenoid, aleuritolic acid (L12) has displayed best docking pose with binding energy of -8.48 kcal/mol and Ki of 0.61 μM making it superior in binding efficiency when compared to all docked compounds including the FDA-approved drugs. Other phytochemicals such as crotoxide A, crothalimene A, crotodichogamoin B and crotonolide E have also displayed strong binding energies. These compounds could further be investigated as potential antiretroviral medication.

Repurposing drugs and identification of inhibitors of integral proteins (spike protein and main protease) of SARS-CoV-2

The outbreak of Coronavirus infection (COVID-19) has prompted the World Health Organisation (WHO) to declare the outbreak, a Public Health Emergency of International concern. As part of the efforts to discover lead compounds for clinical use, 53 molecules were screened using molecular docking and dynamic simulations (MDS) techniques to identify potential inhibitors of SARS-CoV-2 spike protein (COVID-19 Sgp) and main protease (COVID-19 Mpro) or both. Lopinavir (LPV), nelfinavir (NEF), hydroxychloroquine (HCQ), remdesivir (RDV) and an irreversible inhibitor of SARS-CoV (N3) were used as standard drugs for COVID-19 Mpro, while zafirlukast (ZFK) and cefoperazone (CSP)) as standard drugs for COVID-19 Sgp. After 100 ns of MDS, with reference to standard drugs (N3, -52.463 Kcal/mol, NEF, -51.618 Kcal/mol, RDV, -48.780 Kcal/mol, LPV, -46.788 Kcal/mol, DRV, -33.655 Kcal/mol and HCQ, -21.065 Kcal/mol), five molecules, HCR, GRN, C3G, EGCG, and K7G were predicted to be promising inhibitors of COVID-19 Mpro with binding energies of -53.877 kcal/mol, -50.653 Kcal/mol, -48.600 kcal/mol, -47.798 kcal/mol and -46.902 kcal/mol, respectively. These lead molecules were then docked at receptor-binding domain (RBD) of COVID-19 Sgp to examine their inhibitory effects. C3G, GRN and K7G exhibited higher binding energies of -42.310 kcal/mol, -32.210 kcal/mol, -26.922 kcal/mol than the recorded values for the reference drugs (CSP, -35.509 kcal/mol, ZFK, -24.242 kcal/mol), respectively. The results of the binding energy and structural analyses from this study revealed that C3G, GRN and K7G could serve as potential dual inhibitors of COVID-19 Sgp and COVID-19 Mpro, while HCR and EGCG would be inhibitors of COVID-19 Mpro.Communicated by Ramaswamy H. Sarma.

Chemical Group Profiling, In Vitro and In Silico Evaluation of Aristolochia ringens on α-Amylase and α-Glucosidase Activity

Diabetes mellitus (DM) has become a global scourge, and there is a continuous search for novel compounds as viable alternatives to synthetic drugs which are often accompanied by severe adverse effects. Aristolochia ringens is among the scientifically implicated botanicals effective in the management of several degenerative diseases including DM. The current study evaluated the inhibitory mechanism(s) of root extract of A. ringens on α-amylase and α-glucosidase in vitro and in silico, while its constituents were characterized using liquid chromatography-mass spectrometric technique. The extract had concentration-dependent inhibitory effect on the study enzymes, and the inhibition compared well with that of standard drug (acarbose) with respective IC₅₀ values of 0.67 mg/mL (α-amylase) and 0.57 mg/mL (α-glucosidase) compared with that of the extract (0.63 and 0.54 mg/mL). The extract competitively and uncompetitively inhibited α-amylase and α-glucosidase, respectively. Of the identified compounds, dianoside G (−12.4, −12.5 kcal/mol) and trilobine (−10.0, −10.0 kcal/mol) had significant interactions with α-amylase and α-glucosidase, respectively, while magnoflorine and asiatic acid also interacted keenly with both enzymes, with quercetin 3-O-glucuronide and strictosidine showing better affinity towards α-glucosidase. These observations are suggestive of involvement of these compounds as probable ligands contributing to antidiabetic potential of the extract. While studies are underway to demystify the yet to be identified compounds in the extract, the data presented have lent scientific credence to the acclaimed in vivo antidiabetic potential of the extract and suggested it as a viable source of oral hypoglycaemic agent.

An insight on the nature of biochemical interactions between glycyrrhizin, myricetin and CYP3A4 isoform

Drug interaction studies are imperative to gain insights into the beneficial or harmful effects of therapeutic and dietary agents. This study investigated the mechanism of modulatory roles of glycyrrhizin (GLH) and myricetin (MYC) on the human CYP3A4 isoform using in silico and in vitro methods. While MYC had concentration-dependent inhibitory effect on CYP3A4 (IC₅₀ : 10.5 ± 0.55 μM) with characteristic K_m and V_max values of 1.13 μM and 1.54 nM/min, respectively, GLH exhibited no inhibitory effect on CYP3A4 activity in vitro. These observations are consistent with the results of in silico evaluations where the effect of MYC compared well with that of ketoconazole (a known CYP3A4 inhibitor) against CYP3A4. Overall, the established interactions between the study compounds and CYP3A4 could potentiate clinically vital drug-drug interactions and has lent credence to the mechanism of modulatory effect of MYC and GLH on CYP3A4 that could guide their safe use as therapeutic agents. PRACTICAL IMPLICATIONS: Myricetin (MYR) and glycyrrhizin (GLH) occur freely in commonly ingested foods and their supplements are recommended for the treatment of several debilitating diseases such as diabetes, cancer, and cardiovascular complications. This study provided an insight on the possible interactions that could be established between these compounds (MYR and GLH) and CYP3A4 when ingested and metabolized by the liver. The results suggested possibilities of potential clinical drug-drug interactions and advocates for their cautious use within the therapeutic dose in food supplements or medications to avoid probable liver damage.

Antibacterial properties and carbonic anhydrase inhibition profiles of azido sulfonyl carbamate derivatives

Aim: The aim of this study was to identify inhibition of carbonic anhydrase I and II (CA I and II) isozymes by azido sulfonyl carbamates through both in vitro and in silico approaches and also to determine the drug-likeness properties and antibacterial activities of azido sulfonyl carbamates. Methods & Results: In vitro inhibition and molecular docking studies of azido sulfonyl carbamate derivatives (1-4) on isozymes were performed. Except for derivative 4, all derivatives inhibited human CA I and II. Almost all compounds had antibacterial effects. The docking results showed that compound 3 had the best results, with binding energy of -8.20 kcal/mol for human CA I and -8.24 kcal/mol for human CA II. Conclusion: Molecule 4 inhibited only CA I. Its usage as a potential chemotherapeutic agent specific to the CA I isozyme may be considered.

Innovation in bioanalytical strategies and in vitro drug-drug interaction study approaches in drug discovery

Failure to evaluate actual toxicities of investigational molecules in drug discovery is majorly due to inadequate evaluation of their pharmacokinetics. Limitation of conventional drug metabolism profiling procedure demands advancement of existing approaches. Various techniques such as 3D cell culture system, bio microfluidic OoC model, sandwich culture model is in pipeline to be employed at their full potential in drug discovery phase. Although they outweigh the conventional techniques in various aspects, a more detailed exploration of applicability in terms of automation and high throughput analysis is required. This review extensively discusses various ongoing innovations in bioanalytical techniques. The review also proposed various scientific strategies to be adopted for prior assessment of interaction possibilities in translational drug discovery research.

The pharmacokinetic properties of HIV-1 protease inhibitors: A computational perspective on herbal phytochemicals

Acquired Immune Deficiency Syndrome is the most severe phase of Human Immunodeficiency Virus (HIV) infection. Recent studies have seen an effort to isolate phytochemicals from plants to repress HIV, but less studies have focused on the effects of these phytochemicals on the activities of enzymes/transporters involved in the metabolism of these drugs, which is one of the aims of this study and, to examine the antiviral activity of these compounds against HIV-1 protease enzyme using computational tools. Centre of Awareness-Food Supplement (COA®-FS) herbal medicine, has been said to have potential anti-HIV features. SWISSTARGETPREDICTION and SWISSADME servers were used for determination of the enzymes/transporters involved in the metabolism of these protease inhibitor drugs, (PIs) (Atazanavir, Lopinavir, Darunavir, Saquinavir) and the effects of the selected phytochemicals on the enzymes/transporters involved in the metabolism of these PIs. Using Computational tools, potential structural inhibitory activities of these phytochemicals were explored. Two sub-families of Cytochrome P450 enzymes (CYP3A4 and CYP2C19) and Permeability glycoprotein (P-gp) were predicted to be involved in metabolism of the PIs. Six phytochemicals (Geranin, Apigenin, Fisetin, Luteolin, Phthalic acid and Gallic acid) were predicted to be inhibitors of CYP3A4 and, may slowdown elimination of PIs thereby maintain optimal PIs concentrations. Free binding energy analysis for antiviral activities identified four phytochemicals with favourable binding landscapes with HIV-1 protease enzyme. Epigallocatechin gallate and Kaempferol-7-glucoside exhibited pronounced structural evidence as potential HIV-1 protease enzyme inhibitors. This study acts as a steppingstone toward the use of natural products against diseases that are plagued with adverse drug-interactions.

Herb-drug interactions: a novel algorithm-assisted information system for pharmacokinetic drug interactions with herbal supplements in cancer treatment

PURPOSE

To develop a system to estimate the risk of herb-drug interactions that includes the available evidence from clinical and laboratory studies, transparently delineates the algorithm for the risk estimation, could be used in practice settings and allows for adaptation and update.

METHODS

We systematically searched Drugbank, Transformer, Drug Information Handbook, European and German Pharmacopoeia and MEDLINE for studies on herb-drug interactions of five common medicinal plants (coneflower, ginseng, milk thistle, mistletoe and St. John's wort). A diverse set of data were independently extracted by two researchers and subsequently analysed by a newly developed algorithm. Results are displayed in the form of interaction risk categories. The development of the algorithm was guided by an expert panel consensus process.

RESULTS

From 882 publications retrieved by the search, 154 studies were eligible and provided 529 data sets on herbal interactions. The developed algorithm prioritises results from clinical trials over case reports over in vitro investigations and considers type of study, consistency of study results and study outcome for clinical trials as well as identification, permeability, bioavailability, and interaction potency of an identified herbal perpetrator for in vitro investigations. Risk categories were assigned to and dynamically visualised in a colour-coded matrix format.

CONCLUSIONS

The novel algorithm allows to transparently generate and dynamically display herb-drug interaction risks based on the available evidence from clinical and laboratory pharmacologic studies. It provides health professionals with readily available and easy updatable information about the risk of pharmacokinetic interactions between herbs and oncologic drugs.

Pharmacokinetic interactions study between carvedilol and some antidepressants in rat liver microsomes - a comparative study

Background and aims

Cardiovascular diseases and depressive disorders are some of the most frequent diseases. The probability of concomitant prescription of antihypertensive and antidepressive medication is increasing. The aim of this study was to investigate the enzyme inhibition by bupropion, sertraline and fluvoxamine on the metabolism of carvedilol using rat pooled liver microsomes and to assess the importance of these interactions from the pharmacokinetic mechanism point of view.

Methods

Two substrate concentrations (0.5 and 1 μM) and four inhibitor concentrations (0, 0.1, 0.75 and 1.5 μM) were used for each tested inhibitor.

Results

The results of the in vitro experiments showed a significant decrease of the metabolic rate of carvedilol to 4′-hydroxyphenyl carvedilol, for all tested inhibitors, when the inhibitor was added to the incubation mixture containing the substrate. Moreover, an increase of the area under the concentration-time curve for carvedilol was observed after incubation with each tested inhibitor compared with the control state (no inhibitor). The most potent inhibitor was sertraline, followed by fluvoxamine and bupropion.

Conclusion

The co-administration of tested antidepressants led to a significant alteration of carvedilol’s metabolism in vitro. CYP2D6 inhibition is the main pharmacokinetic mechanism that can explain these drug-drug interactions, with possible clinical implications.

Metabolic interactions of environmental toxicants in humans

A description of the interactions between environmental toxicants following simultaneous exposure or exposure in close temporal sequence is presented. At the metabolic level, such interactions may be based on induction, inhibition, or activation of xenobiotic-metabolizing enzymes. Cytotoxicity may also play a role, particularly in affecting induction of xenobiotic-metabolizing enzymes. The effects of interactions manifested at the level of the expression of toxic endpoints may result from interactions at the metabolic level or may have other causes. New approaches to genome-wide effects (e.g., microarray studies) are also discussed.

A sandwiched microarray platform for benchtop cell-based high throughput screening

The emergence of combinatorial chemistries and the increased discovery of natural compounds have led to the production of expansive libraries of drug candidates and vast numbers of compounds with potentially interesting biological activities. Despite broad interest in high throughput screening (HTS) across varied fields of biological research, there has not been an increase in accessible HTS technologies. Here, we present a simple microarray sandwich system suitable for screening chemical libraries in cell-based assays at the benchtop. The microarray platform delivers chemical compounds to isolated cell cultures by 'sandwiching' chemical-laden arrayed posts with cell-seeded microwells. In this way, an array of sealed cell-based assays was generated without cross-contamination between neighbouring assays. After chemical exposure, cell viability was analyzed by fluorescence detection of cell viability assays on a per microwell basis using a standard microarray scanner. We demonstrate the efficacy of the system by generating four hits from toxicology screens towards MCF-7 human breast cancer cells. Three of the hits were identified in a combinatorial screen of a library of natural compounds in combination with verapamil, a P-glycoprotein inhibitor. A fourth hit, 9-methoxy-camptothecin, was identified by screening the natural compound library in the absence of verapamil. The method developed here miniaturizes existing HTS systems and enables the screening of a wide array of individual or combinatorial libraries in a reproducible and scalable manner. We anticipate broad application of such a system as it is amenable to combinatorial drug screening in a simple, robust and portable platform.

Preclinical testing on insects predicts human haematotoxic potentials

The substitution of insects for laboratory animals in toxicity testing is likely to become a reality in the framework of prescreening. Haematotoxicological studies of newly developed chemicals, such as food components, drugs, etc. performed on insects can offer advantages in, for example, environmental toxicology. Reliable routine predictions should produce an increase in our knowledge of haemocyte physiology. Although the differences between human physiology and morphology and those of insects are great, the basic functions of insect haemocytes and mammalian leukocytes appear not to have changed during evolution. The use of insects in haematotoxicity assays represents a preclinical testing strategy which will lower costs, accelerate screening and offer ethical benefits.

Development and full validation of six inhibition assays for five major cytochrome P450 enzymes in human liver microsomes using an automated 96-well microplate incubation format and LC–MS/MS analysis

Substrate inhibition assays for five of the major CYP enzymes (phenacetin for CYP1A2, diclofenac for CYP2C9, (S)-mephenytoin for CYP2C19, dextromethorphan for CYP2D6 and midazolam and testosterone for CYP3A4) in human liver microsomes were developed. Fully automated incubations were conducted in a 96-well format under optimized enzyme kinetic conditions. Metabolites of probe substrates were analyzed with rapid LC-MS/MS methods. The assays were fully validated following the procedure for validating bioanalytical methods recommended by regulatory agencies. Quality control samples and a positive control CYP inhibitor were included in each assay. The IC(50) values determined for typical CYP inhibitors were reproducible and consistent with those reported in the literature. The high quality and throughput of these assays make them ideally suited for providing information for decision making in late drug discovery and early development and for providing labeling input for new drug registrations.

Reactive oxygen species in in vitro pesticide-induced neuronal cell (SH-SY5Y) cytotoxicity: role of NFkappaB and caspase-3

Oxidative stress has been implicated in pesticide-induced neurotoxicity, based on its role in the cascade of biochemical changes that lead to dopaminergic neuronal cell death. We have, therefore, examined the role of oxidative stress caused by the pesticides endosulfan and zineb in human neuroblastoma cells (SH-SY5Y) in culture. Upon treatment with 50-200 microM concentrations of either of these pesticides, SH-SY5Y cells generated both superoxide anion and hydrogen peroxide in a dose-and time-dependent manner. Mixtures of the pesticides significantly enhanced the production of these reactive oxygen species compared to individual pesticide exposures. Pesticide treatment decreased superoxide dismutase, glutathione peroxidase, and catalase activities in SH-SY5Y cells. Additionally, these pesticides induced lipid peroxide (thiobarbituric acid reactive products) formation in these cells. While both pesticides individually (at 100 microM) increased caspase-3 activity, cells exposed to a mixture of the pesticides exhibited significantly low levels of this enzyme, probably due to excessive necrotic cell death. Furthermore, exposure to these pesticides increased nuclear NFkappaB activity. Taken together, these findings suggest that the cytotoxicity of endosulfan and zineb, both individually and in mixtures may, at least in part, be associated with the generation of reactive oxygen species with concomitant increased expression of NFkappaB.

Validated method for rapid inhibition screening of six cytochrome P450 enzymes by liquid chromatography-tandem mass spectrometry

In vitro drug interaction data can be used in guiding clinical interaction studies, or, the design of new candidates. To make such a claim, it must be assured that the in vitro data obtained is confident. To meet this need, a rapid liquid chromatography-tandem mass spectrometry (LC/MS/MS) method has been validated and employed for routine screening of new chemical entities for inhibition of six major human cytochrome P450 (CYP) isoforms using cDNA-expressed CYPs. Probe substrates were used near the Michaelis-Menten constant (K(m)) concentration values for CYP1A2 (phenacetin), CYP2C9 (tolbutamide), CYP2C19 (S-mephenytoin), CYP2D6 (dextromethorphan) and CYP3A4 (midazolam and dextromethorphan). The major metabolites of CYP-specific probe substrates were quantified. The LC/MS/MS method was found to be accurate and precise within the linear range of 1.0-2000 ng/ml for each analyte in enzyme incubation mixture. The lower limit of quantification (LLOQ) was 1.0 ng/ml. The limit of detection (LOD) for the tested analytes was 0.48 ng/ml or better based on signal-to-noise ratio >3. The inhibition potential of the six CYP isoforms has been evaluated using their known selective inhibitors. The 50% inhibitory concentrations (IC(50) values) measured by this method demonstrated high precision and are consistent with the literature values.

Exposure to mixtures of endosulfan and zineb induces apoptotic and necrotic cell death in SH-SY5Y neuroblastoma cells,in vitro

A number of epidemiological studies have demonstrated a strong association between the incidence of Parkinson's disease and pesticide exposure. Earlier it was demonstrated that exposure to the pesticides endosulfan and zineb, alone and in combination, caused neurodegeneration in vivo. It was hypothesized that these pesticides cause neurotoxicity, in part, by enhancing apoptotic cell death. SH-SY5Y human neuroblastoma cells, which retain a catecholaminergic phenotype, were exposed to endosulfan, zineb or a combination of these chemicals, in vitro. For mixture studies, concentrations of pesticides (100 microM each) were chosen based on LC(25) (lethal concentration) that would result in minimum cell death. Exposure to a mixture of pesticides exhibited significantly (P < or = 0.05) higher toxicity than each one alone. Both pesticides were found to cause apoptotic cell death that was concentration (50-400 microM) dependent. A flow cytometric (7-aminoactinomycin D) assay was used to distinguish live, early apoptotic and late apoptotic/necrotic populations. Exposure to mixtures of the pesticides enhanced both early apoptosis and late apoptosis/necrosis compared with either chemical alone. Visual evaluation using a DNA ladder assay and a fluorescence Annexin V/PI assay confirmed the contribution of both apoptotic and necrotic processes. These findings suggest that the cytotoxicity of endosulfan and zineb, both individually and in mixtures, is associated with the occurrence of early and late apoptotic/necrotic processes in SH-SY5Y human neuroblastoma cells and support the contention that pesticide-induced neuronal cell death leading to neurodegenerative disease may, at least in part, be associated with early and late apoptosis of dopaminergic neurons.

Pesticides induced oxidative stress in thymocytes

The role of oxidative stress in immune cell toxicity caused by the pesticides lindane, malathion and permethrin was investigated in thymic cells from C57BL/6 mice. Thymocytes treated with any of these pesticides (concentrations ranging between 50-150 microM) were found to generate both superoxide ((*)O(2) (-)) and H(2)O(2). The production of (*)O(2) (-) was detected with hydroethidine-ethidium bromide assay. H(2)O(2) production was monitored with a flow cytometric fluorescent (DCFH-DA) assay. All three pesticides stimulated (*)O(2) (-) release after 5 min exposure. Lindane and permethrin, but not malathion, continued to have significant (p < or = 0.05) effects on (*)O(2) (-) generation following 15 min of exposure. The lindane + malathion mixture was found to cause more-than-additive increase in (*)O(2) (-) production compared to individual pesticide treatments (at both 5 and 15 min). However, the effect of the lindane + permethrin mixture was not significantly different than individual components of this mixture. The effects of these pesticides on levels of antioxidant enzymes were also investigated, and only mixtures were found to have significant (p < or = 0.05) effects. Thus, lindane + malathion and lindane + permethrin mixtures increased total superoxide dismutase (SOD) specific activity, had no effect on catalase levels and inhibited GSH-peroxidase and GSH-reductase specific activities. Although the results of these studies do not explain the mechanism of action of these pesticides on the generation of (*)O(2) (-) and H(2)O(2), it is worthy of note that mixtures of these chemicals have oxidative responses greater than those of single chemicals.

In vitroscreening of drug metabolism during drug development: can we trust the predictions?

Due to the importance of drug metabolism for drug action, side effects, interactions and interindividual differences, in vitro assays of drug metabolism are widely employed during drug development. Validation of the in vitro systems is still in a rather elementary stage, but some tentative conclusions about their performance for predicting various important in vivo characteristics of drugs can be attempted. So far, prediction of drug-drug interactions on the basis of in vitro screens has been advanced to a relatively reliable level. Prediction of other important characteristics, such as metabolic stability and consequent half-life and dosage schedule, or induction potential, is less reliable. Although metabolite profiles and participating enzymes are predicted quite accurately with advanced analytical techniques, an important problem here is the lack of reliable methods to detect reactive metabolites or products, which may be of significance regarding toxicity.

Prediction of Drug Metabolism and Interactions on the Basis of in vitro Investigations

Drug metabolism profoundly affects drug action, because almost all drugs are metabolised in the body and thus their concentrations and elimination rates are dependent on metabolic activity. Drug metabolism contributes substantially to interindividual differences in drug response and is also often involved in drug interactions, resulting in either therapeutic failure or adverse effects. Knowledge about the metabolism of a new chemical entity and its affinity to drug-metabolising enzymes helps in the drug development process by providing important information for the selection of a lead compound from among a number of substances pharmacologically equally effective in their therapeutic response. In drug development protocols, metabolism characteristics should be assessed very early during the development process. This has been made possible by the advances made especially in analytical capabilities and in in vitro technologies that are employed to predict in vivo metabolite profile, pharmacokinetic parameters and drug-drug interaction potential.

VALIDATED ASSAYS FOR HUMAN CYTOCHROME P450 ACTIVITIES

The measurement of the effect of new chemical entities on human cytochrome P450 marker activities using in vitro experimentation represents an important experimental approach in drug development. In vitro drug interaction data can be used in guiding the design of clinical drug interaction studies, or, when no effect is observed in vitro, the data can be used in place of an in vivo study to claim that no interaction will occur in vivo. To make such a claim, it must be assured that the in vitro experiments are performed with absolute confidence in the methods used and data obtained. To meet this need, 12 semiautomated assays for human P450 marker substrate activities have been developed and validated using approaches described in the GLP (good laboratory practices) as per the code of U.S. Federal Regulations. The assays that were validated are: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), bupropion hydroxylase (CYP2B6), amodiaquine N-deethylase (CYP2C8), diclofenac 4'-hydroxylase and tolbutamide methylhydroxylase (CYP2C9), (S)-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1), felodipine dehydrogenase, testosterone 6 beta-hydroxylase, and midazolam 1'-hydroxylase (CYP3A4 and CYP3A5). High-pressure liquid chromatography-tandem mass spectrometry, using stable isotope-labeled internal standards, has been applied as the analytical method. This analytical approach, through its high sensitivity and selectivity, has permitted the use of very low incubation concentrations of microsomal protein (0.01-0.2 mg/ml). Analytical assay accuracy and precision values were excellent. Enzyme kinetic and inhibition parameters obtained using these methods demonstrated high precision and were within the range of values previously reported in the scientific literature. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of cytochrome P450 activities.

Gastrointestinal-tract models and techniques for use in safety pharmacology

The gastrointestinal tract (GI tract), which extracts nutrients, electrolytes, minerals, and water, is prone to injury as a result of oral drug administration. Clinical assessment of the GI tract is often limited to measurements of transit time and observations of vomiting or diarrhoea, despite the existence of methods and techniques capable of assessing specific changes in GI function at the membrane, cell, and whole animal levels. Membrane studies, record the uptake of solutes, and electrolyte transport, assessing the affects of compounds on transepithelial GI transport and flux. Such methods lend themselves to permeability, immunohistochemistry, morphology, and molecular biology techniques. Isolated cells from the GI tract or cultured cell lines provide knowledge of regulation and function at a cellular level, whilst motility patterns, taken in vivo or from biopsies, provide information at a more integrated level. In anesthetised animals, ligated segments of the intestine can be infused with test compounds, providing information about absorptive and secretory processes important for the treatment of diarrhoea. Computer simulations and modelling are used to predict the disposition of a chemical and its metabolite and can, to some extent, replace animal testing, thereby reducing development costs. Indeed, software programs can be used to simulate the dissolution, absorption, distribution, metabolism, and excretion (ADME) properties of potential drugs in the human GI tract. Finally, advances in the field of imaging, combined with endoscopy, have resulted in a wireless capsule, allowing the inspection of the GI tract anatomy and pathology without surgical intervention. It is concluded that the field of safety pharmacology could rapidly, cheaply, and routinely incorporate membrane, isolated tissue, and endoscopy techniques for GI tract testing of drugs.