Zidovudine azido-reductase in human liver microsomes: activation by ethacrynic acid, dipyridamole, and indomethacin and inhibition by human immunodeficiency virus protease inhibitors

Selective Azidooxygenation of Alkenes Enabled by Photo-induced Radical Transfer Using Aryl-λ3-azidoiodane Species

The photolytic radical-induced vicinal azidooxygenation of synthetically important and diverse functionalized substrates including unactivated alkenes is reported. The photoredox-catalyst/additive-free protocol enables intermolecular oxyazidation by simultaneously incorporating two new functionalities; C-O and C-N across the C=C double bond in a selective manner. Mechanistic investigations reveal the intermediacy of the azidyl radical facilitated via the photolysis of λ³-azidoiodane species and cascade proceeding to generate an active carbon-centered radical. The late-stage transformations of azido- and oxy-moieties were amply highlighted by assembling high-value drug analogs and bioactive skeletons.

The material-enabled oxygen control in thiol-ene microfluidic channels and its feasibility for subcellular drug metabolism assays under hypoxia in vitro

Tissue oxygen levels are known to be critical to regulation of many cellular processes, including the hepatic metabolism of therapeutic drugs, but its impact is often ignored in in vitro assays. In this study, the material-induced oxygen scavenging property of off-stoichiometric thiol-enes (OSTE) was exploited to create physiologically relevant oxygen concentrations in microfluidic immobilized enzyme reactors (IMERs) incorporating human liver microsomes. This could facilitate rapid screening of, for instance, toxic drug metabolites possibly produced in hypoxic conditions typical for many liver injuries. The mechanism of OSTE-induced oxygen scavenging was examined in depth to enable precise adjustment of the on-chip oxygen concentration with the help of microfluidic flow. The oxygen scavenging rate of OSTE was shown to depend on the type and the amount of the thiol monomer used in the bulk composition, and the surface-to-volume ratio of the chip design, but not on the physical or mechanical properties of the bulk. Our data suggest that oxygen scavenging takes place at the polymer-liquid interface, likely via oxidative reactions of the excess thiol monomers released from the bulk with molecular oxygen. Based on the kinetic constants governing the oxygen scavenging rate in OSTE microchannels, a microfluidic device comprising monolithically integrated oxygen depletion and IMER units was designed and its performance validated with the help of oxygen-dependent metabolism of an antiretroviral drug, zidovudine, which yields a cytotoxic metabolite under hypoxic conditions.

Rapid, user-friendly, and inexpensive detection of azidothymidine

Strict adherence to highly active antiretroviral therapy (HAART) is very important to improve the quality of life for HIV-positive patients to reduce new infections and determine treatment success. Azidothymidine (AZT) is an antiretroviral drug commonly used in HAART treatment. In this research, an "add, mix, and measure" assay was developed to detect AZT within minutes. Three different probes designed to release fluorophores when samples containing AZT are added were synthesized and characterized. The limit of detection to AZT in simulated urine samples was determined to be 4 μM in 5 min for one of the probes. This simple and rapid point-of-care test could potentially be used by clinicians and health care workers to monitor the presence of AZT in low resource settings.

Late Stage Azidation of Complex Molecules

Selective functionalization of complex scaffolds is a promising approach to alter the pharmacological profiles of natural products and their derivatives. We report the site-selective azidation of benzylic and aliphatic C-H bonds in complex molecules catalyzed by the combination of Fe(OAc)₂ and a PyBox ligand. The same system also catalyzes the trifluoromethyl azidation of olefins to form derivatives of natural products containing both fluorine atoms and azides. In general, both reactions tolerate a wide range of functional groups and occur with predictable regioselectivity. Azides obtained by functionalization of C-H and C=C bonds were converted to the corresponding amines, amides, and triazoles, thus providing a wide variety of nitrogen-containing complex molecules.

Colorimetric Detection of Azidothymidine Using an Alkyne-Modified Dextran Substrate

Monitoring the adherence of patients taking highly active antiretroviral therapy (HAART) is a key step in treating an HIV infection, especially in resource-limited settings in the developing world. For most regimens, when patients are not at least 95% adherent to their drug schedule, there is a loss of effectiveness in treatment resulting in increases in health care costs, increases in the rate of transmission, and reduction of positive patient outcomes. Currently, subjective methods such as pill counting, electronic drug monitoring, and patient self-reporting are the only ways clinicians can track adherence and intervene in cases of noncompliance. We address this issue by developing a dipstick-based point-of-care azide-alkyne click chemistry assay with colorimetric read-out that directly tests for the presence of one common HAART drug in patient urine. An alkyne-modified dextran was synthesized and characterized by NMR and then used to colorimetrically report the presence of azidothymidine, an azide-containing HAART drug, in urine samples. The assay is specific to azide-containing molecules that are not naturally present in the urine and is sensitive to physiologically relevant urine concentrations as low as 750 uM. This point-of-care device is a strong alternative in resource-limited settings over current direct measurement techniques that are expensive and require trained users such as HPLC.

An azide-modified nucleoside for metabolic labeling of DNA

Metabolic incorporation of azido nucleoside analogues into living cells can enable sensitive detection of DNA replication through copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and strain-promoted azide-alkyne cycloaddition (SPAAC) "click" reactions. One major limitation to this approach is the poor chemical stability of nucleoside derivatives containing an aryl azide group. For example, 5-azido-2'-deoxyuridine (AdU) exhibits a 4 h half-life in water, and it gives little or no detectable labeling of cellular DNA. In contrast, the benzylic azide 5-(azidomethyl)-2'-deoxyuridine (AmdU) is stable in solution at 37 °C, and it gives robust labeling of cellular DNA upon addition of fluorescent alkyne derivatives. In addition to providing the first examples of metabolic incorporation into and imaging of azide groups in cellular DNA, these results highlight the general importance of assessing azide group stability in bioorthogonal chemical reporter strategies.

In vitro metabolism and identification of human enzymes involved in the metabolism of methylnaltrexone

Methylnaltrexone (MNTX) is a peripherally acting mu-opioid receptor antagonist and is currently indicated for the treatment of opioid-induced constipation in patients with advanced illness who are receiving palliative care, when response to laxative therapy has not been sufficient. Sulfation to MNTX-3-sulfate (M2) and carbonyl reduction to methyl-6alpha-naltrexol (M4) and methyl-6beta-naltrexol (M5) are the primary metabolic pathways for MNTX in humans. The objectives of this study were to investigate MNTX in vitro metabolism in human and nonclinical species and to identify the human enzymes involved in MNTX metabolism. Of the five commercially available sulfotransferases investigated, only SULT2A1 and SULT1E1 catalyzed M2 formation. Formation of M4 and M5 was catalyzed by NADPH-dependent hepatic cytosolic enzymes, which were identified using selective chemical inhibitors (10 and 100 microM) for aldo-keto reductase (AKR) isoforms, short-chain dehydrogenase/reductase including carbonyl reductase, alcohol dehydrogenase, and quinone oxidoreductase. The results were then compared with the effects of the same inhibitors on 6beta-naltrexol formation from naltrexone, a structural analog of MNTX, which is catalyzed mainly by AKR1C4. The AKR1C inhibitor phenolphthalein inhibited MNTX and naltrexone reduction up to 98%. 5beta-Cholanic acid 3alpha,7alpha-diol, the AKR1C2 inhibitor, and medroxyprogesterone acetate, an inhibitor of AKR1C1, AKR1C2, and AKR1C4, inhibited MNTX reduction up to 67%. Other inhibitors were less potent. In conclusion, the carbonyl reduction of MNTX to M4 and M5 in hepatic cytosol was consistent with previous in vivo observations. AKR1C4 appeared to play a major role in the carbonyl reduction of MNTX, although multiple enzymes in the AKR1C subfamily may be involved. Human SULT2A1 and SULT1E1 were involved in MNTX sulfation.

Effect of potential amine prodrugs of selective neuronal nitric oxide synthase inhibitors on blood-brain barrier penetration

Several prodrug approaches were taken to mask amino groups in two potent and selective neuronal nitric oxide synthase (nNOS) inhibitors containing either a primary or secondary amino group to lower the charge and improve blood-brain barrier (BBB) penetration. The primary amine was masked as an azide and the secondary amine as an amide or carbamate. The azide was not reduced to the amine under a variety of in vitro and ex vivo conditions. Despite the decrease in charge of the amino group as an amide and as carbamates, BBB penetration did not increase. It appears that the uses of azides as prodrugs for primary amines or amides and carbamates as prodrugs for secondary amines are not universally effective for CNS applications.

Toward refinement of the colony-forming unit-granulocyte/macrophage clonogenic assay: Inclusion of a metabolic system

This work represents a first attempt to refine the colony-forming unit-granulocyte/macrophage (CFU-GM) clonogenic assay by incorporating liver microsomes and co-factors as a metabolic system into the in vitro test system in response to an ECVAM recommendation. From the comparison of results obtained with the CFU-GM clonogenic assay currently used and with the new experimental protocol, different toxicity on granulocyte/macrophage precursors was demonstrated, when drugs with a known metabolism in vivo were tested.

Drug-Metabolizing Ability of Molybdenum Hydroxylases

Molybdenum hydroxylases, which include aldehyde oxidase and xanthine oxidoreductase, are involved in the metabolism of some medicines in humans. They exhibit oxidase activity towards various heterocyclic compounds and aldehydes. The liver cytosol of various mammals also exhibits a significant reductase activity toward nitro, sulfoxide, N-oxide and other moieties, catalyzed by aldehyde oxidase. There is considerable variability of aldehyde oxidase activity in liver cytosol of mammals: humans show the highest activity, rats and mice show low activity, and dogs have no detectable activity. On the other hand, xanthine oxidoreductase activity is present widely among species. Interindividual variation of aldehyde oxidase activity is present in humans. Drug-drug interactions associated with aldehyde oxidase and xanthine oxidoreductase are of potential clinical significance. Drug metabolizing ability of molybdenum hydroxylases and the variation of the activity are described in this review.

Antiretrovirals, Part II: Focus on Non-Protease Inhibitor Antiretrovirals (NRTIs, NNRTIs, and Fusion Inhibitors)

The second in a series reviewing the HIV/AIDS antiretroviral drugs. This review summarizes the non-protease inhibitor antiretrovirals: nucleoside and nucleotide analogue reverse transcriptase inhibitors (NRTIs), the nonnucleoside reverse transcriptase inhibitors (NNRTIs), and cell membrane fusion inhibitors. In an overview format for primary care physicians and psychiatrists, this review presents the mechanism of action, side effects, toxicities, and drug interactions of these agents.

Induction of zidovudine glucuronidation and amination pathways by rifampicin in HIV-infected patients

AIMS

The objective of the study was to determine the effect of multiple doses of rifampicin on the steady-state pharmacokinetics of zidovudine and its 5'-glucuronosyl (GZDV) and 3'-amino (AMT) metabolites.

METHODS

Eight asymptomatic HIV-infected patients (seven male, one female) participated in this three-period longitudinal study. Each patient received zidovudine (200 mg every 8 h) for 14 days (period 1), followed by rifampicin (600 mg every 24 h) with zidovudine for 14 days (period 2), and then zidovudine alone for a further 14 days (period 3). Blood and urine samples were collected over 6 h on the last day of each period for measurements of zidovudine and GZDV by h.p.l.c.-u.v. and AMT by h.p.l.c.-m.s-m.s.

RESULTS

Compared with zidovudine-alone values in period 1, 14 days of coadministration with rifampicin significantly increased zidovudine oral clearance (89%) and formation clearances to GZDV (100%) and AMT (82%). Correspondingly, there were decreases in maximum plasma concentration (43%), AUC (47%) and urine recovery (37%) of zidovudine. GZDV/zidovudine and AMT/zidovudine AUC ratios increased by 99% and 36%, respectively, despite a significant 29% decrease in AMT AUC. After stopping rifampicin for 14 days, values of these pharmacokinetic parameters returned to within 26% of baseline. Over the three periods AMT plasma levels were <18 ng ml-1 (n=6) and <40 ng ml-1 (n=2), and molar AMT/zidovudine AUC ratios ranged from 1.7% to 4.5%.

CONCLUSIONS

Rifampicin induced zidovudine glucuronidation and amination pathways resulting in decreased plasma and urine exposures to zidovudine. AMT plasma exposure decreased because induction was more pronounced for the major GZDV metabolite. The magnitude of the residual inductive effect was minimal at 14 days after stopping rifampicin.