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Opioids vary not only in their receptor specificity order 5 mg warfarin free shipping, but also in their efficacy at dif- ferent types of receptors buy warfarin with mastercard. Some agents act as agonists on one type of receptor and antago- nists or partial agonists at another 2 mg warfarin, producing a very complicated pharmacological pic- ture. Most opioids are pure agonists, pentazocine and nalorphine are partial agonists, and naloxone and naltrexone act as antagonists (Table 1). They either close a voltage-gated Ca2+ channel on presynaptic nerve terminals and thereby reduce trans- mitter release, or they hyperpolarize and thus inhibit postsynaptic neurons by opening K+ channels (2–4). The presynaptic action (depressed transmitter release) has been demonstrated for release of a large number of neurotransmitters, including acetylcho- line, norepinephrine, glutamate, serotonin, and substance P (2). All three major receptors are present in high concentrations in the dorsal horn of the spinal cord. Receptors are present both on spinal cord pain transmission neurons and on the primary afferents that relay the pain message to them. Opioid agonists inhibit the release of excitatory transmitters from these primary afferents, and they directly inhibit the dorsal horn pain transmission neuron. This spinal action has been exploited clinically by direct application of opioid agonists to the spinal cord, which provides a regional analgesic effect while minimizing the unwanted respiratory depression, nausea and vomiting, and sedation that may occur from the supraspinal actions of systematically administered drugs (2). Different combinations of opioid receptors are found in supra- spinal regions implicated in pain transmission and modulation. Of particular impor- tance are opioid-binding sites in pain-modulating descending pathways, including the rostral ventral medulla, the locus ceruleus, and the midbrain periaqueductal gray area. At these sites as at others, opioids directly inhibit neurons, yet neurons that send pro- cesses to the spinal cord and inhibit pain transmission neurons are activated by the drugs. In addition, part of the pain-relieving action of exogenous opioids involves the release of endogenous opioid peptides (2). Clinical use of opioid analgesics consists primarily in balancing the analgesia against adverse side effects. Their depressive effect on neuronal activity, increase in pain threshold, and sedation is often accompanied by euphoria. Introduction: The Size of the Problem Man has used drugs for recreational purposes as long as history itself. In the last 30 yr the number of people 128 Moallem, Balali-Mood, and Balali-Mood Table 3 Pharmacodynamic Properties of Opioids Central Nervous System Effects Suppression of pain; analgesia Drowsiness and decreased mental alertness; sedation Respiratory function depression (at the same dose that produce analgesia) Euphoria Psychotomimetic effects (nightmares, hallucinations) Suppression of cough; codeine is used primarily as antitussive Miosis, mediated by parasympathetic pathways Nausea and vomiting, by activating the brain stem chemoreceptor trigger zone Antimuscarinic effects by meperedine Peripheral Effects Increased intracranial pressure Hypotension, if cardiovascular system is stressed Bradycardia Decreased peristalsis; constipation Decreased gastric acid secretion Inhibition of fluid and electrolyte accumulation in intestinal lumen Increased tone of intestinal smooth muscle Increased tone of sphincter of Oddi; increased biliary pressure Increased tone of detrusor muscle and vesical sphincter Decreased uterine tone Stimulation of the release of antidiuretic, prolactine, and somatotropine hormones Inhibition of luteinizing hormone release Skin flushing and warming; sweating; itching Immune system modulation using recreational drugs, particularly opioids, appears to have increased. By 1997, 25% of the population reported using illicit drugs at some point in their lives and 10% within the last year. In 1999, there were 179,000 treatment admissions for primary- injection drug abuse and 34,000 admissions for secondary-injection drug abuse in the United States. Opiates accounted for 83% of substance abuse treatment admissions for injection drug abuse, followed by methamphetamine/amphetamines (11%) and cocaine (5%). Injection drug admissions of young people aged 15–25 yr old increased between 1992 and 1999. Injection drug users tended to use drugs for many years before enter- ing the substance abuse treatment system. Heroin treatment admission rates between 1993 and 1999 increased by 200% or more in 6 states and by 100–199% in another 11 states. The West and Northeast had the highest heroin treatment admission rates between 1993 and 1999 (Website of National Institute of Drug Abuse, 2001). Australian mortality data for 1992 indicate that approximately 401 male deaths and 161 female deaths occurred as a result of opiate use. This represents some 15,429 and 6261 person-years of life lost to age 70, for males and females, respectively (6). Thus, methadone would appear to be 19 times more toxic than heroin, simi- lar to previous findings in New York. Yet methadone is a manufactured pharmaceutical product, whereas heroin is usually adulterated from the street (7). Although methadone has been used as a maintenance therapy for opiate addicts, several reports on the fatal methadone overdose have been published (8,9). This powerful and potentially addictive pain- killer used by millions of Americans is causing rapid hearing loss and even deafness (10). In another report, sublingual buprenorphine caused 20 fatalities in France over a 6-mo period in five urban areas. Buprenorphine and its metabolites were found in post- mortem fluids and viscera (11). Pathophysiology of Opiate Use The physiologic effects of opioids are actually the result of interaction between the individual agent and multiple receptors. Morphine-like drugs produce analgesia, drowsiness, changes in mood, and mental clouding. A significant feature of the analgesia is that it occurs without loss of consciousness, although drowsiness commonly occurs.
This interaction led to 15 deaths in Japan from 5-fluorouracil toxicity due to elevated exposure to the drug buy 5 mg warfarin with amex. This basic approach was also used to predict the inhibitory effect of verapamil enantiomers and their major metabolites discount 2mg warfarin fast delivery, norverapamil and N-desalkyl verapamil (D617) generic warfarin 5 mg line, on midazolam clearance. The model predicted the nonlinear disposition of paroxetine with an accumu- lation ratio that is fivefold higher than the expected accumulation if linear kinetics were assumed; this was in excellent agreement with the observed five- to sixfold greater accumulation. These predictions are in reasonable agreement with observed data but the incorporation of in vitro microsomal binding was essential for good predictive accuracy. Quantitative prediction of irreversible inhibition at the level of the gut wall remains challenging because of the added uncertainty in the effective inhibitor concentration at this site. However, a model that successfully predicted the grapefruit juice–felodipine interaction at the gut wall has been described and used to recommend consumption strategies for mini- mizing the severity of the interaction, although prospective evaluations of the predictions were not described (125). An attractive strategy for predicting the clinical significance of irreversible inhibition is to use human hepatocytes wherein the ‘‘natural’’ turnover of enzymes might be preserved and in vivo cellular concentrations of inhibitors and metabolites would be achieved. This property could affect a drug’s own metabolism or the metabolism of coadministered drugs, which could lead to serious drug interactions. Even though in vitro Ki values have been determined for a number of drugs and have been used to predict an in vivo interaction, the effect of mechanism-based inhibitors can be observed at in vivo concentrations below these Ki values. A theoretical basis and application have been presented that applies in vitro estimates of mechanism-based inhibitors to accurately predict in vivo drug interactions. In vitro forcasting of drugs which may interfere with the biotransformation of midazolam. Thiophene derivatives as new mechanism- based inhibitors of cytochromes P-450: inactivation of yeast-expressed human liver cytochrome450 2C9 by tienilic acid. Oxidation of tienilic acid by human yeast- expressed cytochromes P-450 2C8, 2C9, 2C18 and 2C19: evidence that this drug is a mechanism-based inhibitor specific for cytochrome P-450 2C9. Electrospray ionization mass spectrometric analysis of intact cytochrome P450: identification of tienilic acid adducts to P450 2C9. Inhibition of cyclosporine and tetrahydrocan nabinol metabolism by cannabidiol in mouse and human microsomes. Characterization of cytochrome P450 3A inactivation by cannabidiol: possible involvement of cannabidiol-hydroxyquinone as a P450 inacti- vator. Suicide inactivation of rat liver cytochrome P-450 by chloramphenicol in vivo and in vitro. Identification of the heme adduct and an active site peptide modified during mechanism-based inactivation of rat liver cytochrome P450 2B1 by secobarbital. Mechanism-based inactivation of human liver cytochrome P450 2A6 by 8-methoxypsoralen. Oxidative metabolism of spironolactone: evi- dence for the involvement of electrophilic thiosteroid species in drug-mediated destruction of rat hepatic cytochrome P450. Inhibition of mixed-function oxidations by substrates forming reduced cytochrome P-450 metabolic-intermediate complexes. Direct characterization of the selectivity of furafylline as an inhibitor of human cytochromes P450 1A1 and 1A2. Characterization of the enzymatic and non- enzymatic peroxidative degradation of iron porphyrins and cytochrome P-450 heme. Particular ability of cytochromes P450 3A to form inhibitory P450-iron-metabolite complexes upon metabolic oxidation of aminodrugs. Evaluation of atypical cytochrome P450 kinetics with two-substrate models: evidence that multiple substrates can simultaneously bind to cytochrome P450 active sites. An in vitro model for predicting in vivo inhibition of cytochrome P450 3A4 by metabolic intermediate complex formation. Differences in the inhibition of cytochromes P450 3A4 and 3A5 by metabolite-inhibitor complex-forming drugs. Cytochrome P-450 complex formation by dirithromycin and other macrolides in rat and human livers. An evaluation of potential mechanism- based inactivation of human drug metabolizing cytochromes P450 by monoamine oxidase inhibitors, including isoniazid. Diltiazem inhibition of cytochrome P-450 3A activity is due to metabolite intermediate complex formation. Prediction of cytochrome P450 3A inhibition by verapamil enantiomers and their metabolites. Mechanism-based inactivation of cytochrome P450s 1A2 and 3A4 by dihydralazine in human liver microsomes. Inactivation of cytochrome P450 3A4 by berga- mottin, a component of grapefruit juice. The licorice root derived isoflavan glabridin inhibits the activities of human cytochrome P450S 3A4, 2B6, and 2C9. Mechanism-based inactivation of cytochrome P450 3A4 by 17 alpha-ethynylestradiol: evidence for heme destruction and covalent binding to protein. Inhibition of oral contraceptive steroid-metabolizing enzymes by steroids and drugs.
Phenocopies of poor metabolizers of omeprazole caused by liver disease and drug treatment cheap warfarin 2mg overnight delivery. Metabolic disposition of proguanil in extensive and poor metabolisers of S-mephenytoin 4 -hydroxylation recruited from0 an Indonesian population buy warfarin 2mg on-line. Inter-subject variability in the metabolism of proguanil to the active metabolite cycloguanil in man discount warfarin 2mg amex. The pharmaco-kinetics and activation of proguanil in man: consequences of variability in drug metabolism. Proguanil metabolism is determined by the mephenytoin oxidation polymorphism in Vietnamese living in Denmark. Evidence for the polymorphic oxida- tion of debrisoquine and proguanil in a New Zealand Maori population. Evidence for the polymorphic oxidation of debrisoquine and proguanil in a Khmer (Cambodian) population. Chloro-guanide metabolism in relation to the efficacy in malaria prophylaxis and the S-mephenytoin oxidation in Tanzanians. Pharmacokinetic evaluation of proguanil: a probe phenotyping drug for the mephenytoin hydroxylase polymorphism. Proguanil metabolism in relation to S-mephenytoin oxidation in a Turkish population. Relation between chlor- oguanide bioactivation to cycloguanil and the genetically determined metabolism of mephenytoin in humans. Inherited amplification of an active gene in the cytochrome P450 2D-locus as a cause of ultrarapid metabolism of debrisoquine. Metoprolol and debrisoquin metabolism in Nigerians: lack of evidence for polymorphic oxidation. Lower prevalence of the debrisoquin oxidative poor metabolizer phenotype in American black versus white subjects. Molecular basis for rational mega- prescribing in ultrarapid hydroxylators of debrisoquine. Cytochrome P450 2D6 variants in a Caucasian population: allele frequencies and phenotypic consequences. Oxidation phenotype—a major deter- minant of metoprololol metabolism and response. Metoprolol metabolism and debrisoquine oxidation polymorphism—population and family studies. Metoprolol oxidation polymorphism in a Korean population: comparison with native Japanese and Chinese populations. Differential stereoselective of metoprolol in extensive and poor debrisoquin metabolisers. Utility of a one-point (3-hour postdose) plasma metabolic ratio as a phenotyping test using metoprolol in two East Asian pop- ulations. Polymorphic dextromethorphan metabolism: co-segregation of oxidative O-demethylation with debrisoquin hydroxylation. Dextromethorphan O-demethylation in liver microsomes as a prototype reaction to monitor cytochrome P-450 db 1 activity. Concordance of P450 2D6 (debrisoquine hydroxylase) phenotype and genotype: inability of dextromethorphan metabolic ratio to dis- criminate reliably heterozygous and homozygous extensive metabolizers. Polymorphism of dextro- methorphan metabolism: relationships between phenotype, genotype and response to the administration of encainide in humans. Prediction of phenotype for dextro- methorphan O-demethylation by using polymerase chain reaction in healthy vol- unteers. Antidepressant drug interactions and the cytochrome P450 system—the role of cytochrome P4502D6. Pharmacokinetic drug interactions of new antidepressants: a review of the effects of the metabolism of other drugs. Correlations among the metabolic ratios of three test probes (metoprolol, debrisoquine and sparteine) for genetically determined oxidation polymorphism in a Japanese population. Dissociation of co-regulatory control of debrisoquin/phenformin and sparteine oxidation in Ghanaians. Evidence for a dissociation in the control of sparteine, debrisoquine and metoprolol metabolism in Nigerians. Metoprolol alpha-hydroxylation polymorphism in the San Bushmen of Southern Africa. Polymorphism of the 4-hydroxylation of debrisoquine in the San Bushmen of Southern Africa. Dextromethorphan metabolism in Jordanians: dissociation of dextromethorphan O-demethylation from debrisoquine 4-hydroxylation. Cytochrome P450 2E1 genotype and chlorzox- azone metabolisminhealthy and alcoholic Caucasian subjects. Both cytochromes P450 2E1 and 1A1 are involved in the metabolism of chlorzoxazone.