Propoxyphene and norpropoxyphene plasma concentrations after oral propoxyphene in cirrhotic patients with and without surgically constructed portacaval shunt

Prediction and validation of enzyme and transporter off-targets for metformin

Metformin, an established first-line treatment for patients with type 2 diabetes, has been associated with gastrointestinal (GI) adverse effects that limit its use. Histamine and serotonin have potent effects on the GI tract. The effects of metformin on histamine and serotonin uptake were evaluated in cell lines overexpressing several amine transporters (OCT1, OCT3 and SERT). Metformin inhibited histamine and serotonin uptake by OCT1, OCT3 and SERT in a dose-dependent manner, with OCT1-mediated amine uptake being most potently inhibited (IC50 = 1.5 mM). A chemoinformatics-based method known as Similarity Ensemble Approach predicted diamine oxidase (DAO) as an additional intestinal target of metformin, with an E-value of 7.4 × 10(-5). Inhibition of DAO was experimentally validated using a spectrophotometric assay with putrescine as the substrate. The Ki of metformin for DAO was measured to be 8.6 ± 3.1 mM. In this study, we found that metformin inhibited intestinal amine transporters and DAO at concentrations that may be achieved in the intestine after therapeutic doses. Further studies are warranted to determine the relevance of these interactions to the adverse effects of metformin on the gastrointestinal tract.

Pharmacokinetics of opioids in liver disease

The liver is the major site of biotransformation for most opioids. Thus, the disposition of these drugs may be affected in patients with liver insufficiency. The major metabolic pathway for most opioids is oxidation. The exceptions are morphine and buprenorphine, which primarily undergo glucuronidation, and remifentanil, which is cleared by ester hydrolysis. Oxidation of opioids is reduced in patients with hepatic cirrhosis, resulting in decreased drug clearance [for pethidine (meperidine), dextropropoxyphene, pentazocine, tramadol and alfentanil] and/or increased oral bioavailability caused by a reduced first-pass metabolism (for pethidine, dextropropoxyphene, pentazocine and dihydrocodeine). Although glucuronidation is thought to be less affected in liver cirrhosis, and clearance of morphine was found to be decreased and oral bioavailability increased. The consequence of reduced drug metabolism is the risk of accumulation in the body, especially with repeated administration. Lower doses or longer administration intervals should be used to remedy this risk. Special risks are known for pethidine, with the potential for the accumulation of norpethidine, a metabolite that can cause seizures, and for dextropropoxyphene, for which several cases of hepatotoxicity have been reported. On the other hand, the analgesic activity of codeine and tilidine depends on transformation into the active metabolites, morphine and nortilidine, respectively. If metabolism is decreased in patients with chronic liver disease, the analgesic action of these drugs may be compromised. Finally, the disposition of a few opioids, such as fentanyl, sufentanil and remifentanil, appears to be unaffected in liver disease.

[Conventional techniques for analgesia: opioids and non-opioids. Indications, adverse effects and monitoring]

Morphine dosage must be carefully adapted in patients with renal failure or severe liver failure. The i.v. route is used for morphine titration in the post anaesthesia care unit (PACU), or for analgesia in children. Systematic (not on demand) intramuscular or subcutaneous morphine must be administered at intervals not longer than 4 hours. Dosage is best determined after i.v. titration in the PACU. Codeine, administered orally, is metabolised into morphine. Codeine has almost no effect in 7% of Caucasians and at least 15% of Asians. Nalbuphine, which has a sedative effect and a short half-life, is mainly used in children. Paracetamol (acetaminophen) is used orally or rectally, most often in combination with codeine. Paracetamol dosage is 60-90 mg.kg-1.d-1, including a 20 mg (orally), or 40 mg (rectally) loading dose. Its therapeutic ratio is low, with a potential hepatic toxicity. Dosage must be lowered in alcoholics or in patients under isoniazide therapy. Non-steroidal anti-inflammatory drugs are powerful antinociceptive agents. Their use must be restricted to the first 5 postoperative days. Their major contraindications are kidney failure, risk of gastrointestinal bleeding, coagulation disorders, allergy. They also have a marked morphine sparing effect and reduce therefore the respiratory depression induced by morphine.

[Drug therapy for tumor pain I. Properties of non-opioids and opioids.]

Analgesic pharmacotherapy represents one of the major approaches to the treatment of cancer pain, since it is used in almost every patient. A thorough evaluation of the physical and mental status of the patient and of the pain is as necessary as a sound understanding of the pharmacokinetic and pharmacodynamic characteristics of the analgesics selected. The World Health Organization (WHO) has issued a basic 3 stage progression for the treatment of cancer pain, the "WHO Analgesic Ladder". Assignment to the stages depends mainly on the intensity of the pain rather than on its specific aetiology. Mild to moderate pain is treated with non-opioid drugs; moderate to severe pain, with a combination of a "weak" opioid and a non-opioid; and "strong" opioids should be used in combination with a non-opioid in the case of severe pain. Adjuvant drugs can be added if specifically indicated. Nonopioid analgesics include non-acidic compounds, e. g. paracetamol and metamizole, and acidic non-opioids, e. g. acetylsalicylic acid and newer non-steroidal anti-inflammatory drugs (NSAID). In contrast to most of the opioid analgesics, they have a ceiling effect for analgesia. Addiction and tolerance are extremely rare concerns. Opioids can be subgrouped into "weak" (e. g., codeine, dextropropoxyphene) and "strong" opioids (e. g., morphine) and also into drugs interacting with different opioid-receptor subtypes. Whereas pure agonists (e. g., morphine) produce increasingly intense analgesia with increasing dose, partial agonists and agonist-antagonists have a ceiling effect for analgesia and therefore have only a minor role in the treatment of chronic pain in cancer patients. Adverse effects occur in most patients in a dose-dependent manner. The most common of these is constipation; nausea, vomiting and sedation occur mostly at the start and can usually be treated effectively. The appropriate dosage, route of administration and dosage scheme of analgesics needs to be worked out for each individual patient in intensive work with the patient and a close follow-up, for years if necessary. Some analgesics may not be available in some countries, or only in specific preparations.

D-propoxyphene and norpropoxyphene kinetics after the oral administration of D-propoxyphene: a new approach to liver function?

In an attempt to design a liver function test which takes into account both portal-systemic shunting and hepatocellular dysfunction, we investigated a group of patients with cirrhosis with or without surgical porta-caval shunt for d-propoxyphene and its major metabolite, norpropoxyphene kinetics. A small dose of d-propoxyphene (0.7 mg/kg body weight) was given orally to seven normal subjects, 15 patients with cirrhosis and seven patients with cirrhosis and surgical portacaval shunt. D-propoxyphene and norpropoxyphene areas under the plasma concentration-time from 0 to 4-h (AUC) were determined by the trapezoidal method. As d-propoxyphene is a high extraction drug and since the production of norpropoxyphene should reflect the amount of d-propoxyphene available to the hepatocytes, we tested the hypothesis that norpropoxyphene/d-propoxyphene AUC ratios should reflect both the degree of portal-systemic shunting and the severity of hepatocyte dysfunction. Norpropoxyphene/d-propoxyphene AUC ratios were significantly lower in patients with cirrhosis (mean +/- S.D.: 0.92 +/- 0.59) than in controls (2.51 +/- 0.45) and also significantly lower in patients with cirrhosis and a surgical shunt (0.53 +/- 0.23) than in patients with cirrhosis but without surgical shunt (1.10 +/- 0.63). Moreover, there was an overall statistically significant correlation between norpropoxyphene/d-propoxyphene AUC ratios and branched to aromatic amino acids ratios (rs = 0.91) and fasting venous NH4 (rs = -0.63). On the other hand, there was only a weak correlation between norpropoxyphene/d-propoxyphene AUC ratios and the 14C-aminopyrine breath test (rs = 0.43). These data suggest that the norpropoxyphene/d-propoxyphene AUC ratio reflects both shunting and reduced hepatocellular function.(ABSTRACT TRUNCATED AT 250 WORDS)

Cancer pain management. Current strategy

Pain is among the most prevalent symptoms experienced by cancer patients. A strategy for the management of cancer pain is now widely accepted, and when well implemented, is usually effective. Unfortunately, many oncologists are ill-prepared for the task of pain assessment and management, and the outcomes achieved in clinical practice are often suboptimal. The various elements in the pain management strategy are described. Patient assessment, the use of primary therapies and systemically administered nonopioid and opioid analgesics are pivotal to the strategy. Practical aspects of opioid pharmacotherapy encompass drug selection and dosing considerations including selection of an appropriate route of administration, dose titration, and the management of side effects. Specific approaches are described for the treatment of patients for whom an acceptable balance between relief and side effects of opioids is not achieved. These comprise noninvasive interventions, including the use of adjuvant analgesics, psychological therapies, and physiatric techniques, and invasive interventions, such as the use of intraspinal opioids, neural blockade, and neuroablative techniques. Finally, the use of sedation in the treatment of patients with pain that is refractory to other interventions is addressed. The skilled application of this strategy can provide adequate relief to the vast majority of patients, most of whom will respond to systemic pharmacotherapy alone. Patients with refractory pain should see specialists in pain management or palliative medicine who can address these difficult problems.

Individual variation in first-pass metabolism

Individual variation in pharmacokinetics has long been recognised. This variability is extremely pronounced in drugs that undergo extensive first-pass metabolism. Drug concentrations obtained from individuals given the same dose could range several-fold, even in young healthy volunteers. In addition to the liver, which is the major organ for drug and xenobiotic metabolism, the gut and the lung can contribute significantly to variability in first-pass metabolism. Unfortunately, the contributions of the latter 2 organs are difficult to quantify because conventional in vivo methods for quantifying first-pass metabolism are not sufficiently specific. Drugs that are mainly eliminated by phase II metabolism (e.g. estrogens and progestogens, morphine, etc.) undergo significant first-pass gut metabolism. This is because the gut is rich in conjugating enzymes. The role of the lung in first-pass metabolism is not clear, although it is quite avid in binding basic drugs such as lidocaine (lignocaine), propranolol, etc. Factors such as age, gender, disease states, enzyme induction and inhibition, genetic polymorphism and food effects have been implicated in causing variability in pharmacokinetics of drugs that undergo extensive first-pass metabolism. Of various factors considered, age and gender make the least evident contributions, whereas genetic polymorphism, enzymatic changes due to induction or inhibition, and the effects of food are major contributors to the variability in first-pass metabolism. These factors can easily cause several-fold variations. Polymorphic disposition of imipramine and propafenone, an increase in verapamil first-pass metabolism by rifampicin (rifampin), and the effects of food on propranolol, metoprolol and propafenone, are typical examples. Unfortunately, the contributions of these factors towards variability are unpredictable and tend to be drug-dependent. A change in steady-state clearance of a drug can sometimes be exacerbated when first-pass metabolism and systemic clearance of a drug are simultaneously altered. Therefore, an understanding of the source of variability is the key to the optimisation of therapy.

Clinical pharmacokinetics in patients with liver disease

From considerations of hepatic physiology and pathology coupled with pharmacokinetic principles, it appears that altered drug elimination in liver disease may result from the following mechanisms: reduction in absolute cell mass, in cellular enzyme content and/or activity, in portal vein perfusion due to extrahepatic/intrahepatic shunting, or of portal perfusion of hepatocyte mass due to decreased portal flow or sinusoidal perfusion; increase in arterial perfusion relative to portal perfusion; preferential perfusion of the sinusoidal midzone and terminal zones by arterioles; potential for direct mixing of arterial blood within the space of Disse; reduced exchange across the endothelial lining; and impaired diffusion within the space of Disse. In general, oxidative drug metabolism is impaired in liver disease and the degree of impairment of oxidisation differs between drugs but correlates best with the degree of sinusoidal capillarisation, i.e. the degree of access of the drug from the sinusoid to the hepatocyte. Drug conjugation appears to be relatively unaffected by liver disease, whereas elimination by biliary excretion correlates best with the degree of intrahepatic shunting and not with sinusoidal capillarisation. As the latter should impair hepatocyte access of all compounds similarly, a potentially important mechanism could be impaired access of oxygen to hepatocytes as oxidative metabolism is much more sensitive to oxygen supply than are conjugation or biliary excretion. This suggests a potentially important therapeutic role for agents which increase the hepatic oxygen supply. Useful adjunctive strategies may also derive from the oxygen limitation hypothesis. Anaemia should be targeted as a critically important variable, as should oxygen-carrying capacity, i.e. modification of the smoking habit. Additionally, enzyme inducers such as barbiturates may be used if overriding hypoxic constraints are removed by oxygen supplementation. Agents likely to seriously compromise arterial perfusion of the hepatic vascular bed should be avoided, e.g. those causing postural hypotension or vasospasm. Vasodilators can be used to actively promote arterial perfusion. While the effect of liver disease on drug handling is highly variable and difficult to predict, there are well recognised principles for modifying dosage. These include halving the dose of drugs given systemically (or of low clearance drugs given orally) and a 50 to 90% reduction in the dose of drugs with a high hepatic clearance given orally. Changes in the pharmacodynamic effects of drugs (either alone or in addition to pharmacokinetic changes) can also be profound, and awareness of this possibility should be increased.

Management of cancer pain. Pharmacology and principles of management

The pain of the vast majority of patients with cancer can be controlled with the use of analgesic drugs. Pharmacokinetic and clinical studies have provided the basis for guidelines for the pharmacologic management of cancer pain. The fundamental concept underlying these guidelines is individualization of therapy, which has as its goal the maximization of pain relief and the minimization of adverse drug effects. This article presents these guidelines and discusses the pharmacologic properties and adverse effects of analgesic drugs commonly used in the treatment of cancer pain. Also reviewed are methods for avoiding and treating the adverse effects of analgesics.

Dextropropoxyphene overdose. Epidemiology, clinical presentation and management

This paper comprehensively reviews the worldwide situation regarding acute overdosage of dextropropoxyphene (propoxyphene). The changing epidemiology of this type of poisoning over the last 20 years is described with discussion of concurrent trends and, in particular, the effects of different preventive measures adopted in various countries. The clinical pharmacology of dextropropoxyphene relevant to the clinical toxic effects resulting from acute overdosage is described, and the management is detailed. In particular, the importance of early diagnosis and treatment is stressed in view of the potentially lethal complications that may suddenly occur with this poisoning. Recommendations for the correct use of the specific narcotic antagonist, naloxone, are made, together with other intensive supportive measures. As dextropropoxyphene is frequently taken together with other toxic agents, the concomitant effects of alcohol and sedative drugs are described and the treatment of paracetamol (acetaminophen) in combination with dextropropoxyphene is emphasised. The most effective preventive measures for the future are suggested, but caution is advised regarding the prescription for 'at risk' patients of alternative analgesics, which may be no safer in overdosage.

Effect of severe alcoholic liver disease on the disposition of methadone in maintenance patients

We studied methadone disposition in 11 maintenance patients with alcoholic liver disease of such severity that liver biopsy was contraindicated. Nine methadone-maintained patients with recent alcohol abuse but minimal or no evidence of liver disease served as controls. Most kinetic indices, including the apparent oral clearance and area under the concentration-time curves, were similar in patients and controls. Although the apparent terminal half-life of methadone was longer (p = 0.04) in the patients with liver disease, the peak plasma methadone level was lower (p = 0.03). None of the patients had signs or symptoms of methadone overdosage or abstinence at the time of study. Six patients and one control had flattened plasma methadone concentration-time curves. We hypothesize that, in severe liver disease, damage to hepatic drug-metabolizing systems is offset by damage to the capacity of the liver to store and release unchanged methadone. The usual methadone maintenance dose may be continued in stable patients with severe alcoholic cirrhosis.

Human pharmacokinetics of analgesics and methods for their determination in biological fluids

The main pharmacokinetic data of analgesics--biological half-lives, apparent volumes of distribution, total body clearances--obtained in humans, and their clinical relevance are summarized. Special emphasis has been given to the analytical methods used for the quantitative determination of these drugs in biological fluids.

First-pass elimination. Basic concepts and clinical consequences

First-pass elimination takes place when a drug is metabolised between its site of administration and the site of sampling for measurement of drug concentration. Clinically, first-pass metabolism is important when the fraction of the dose administered that escapes metabolism is small and variable. The liver is usually assumed to be the major site of first-pass metabolism of a drug administered orally, but other potential sites are the gastrointestinal tract, blood, vascular endothelium, lungs, and the arm from which venous samples are taken. Bioavailability, defined as the ratio of the areas under the blood concentration-time curves, after extra- and intravascular drug administration (corrected for dosage if necessary), is often used as a measure of the extent of first-pass metabolism. When several sites of first-pass metabolism are in series, the bioavailability is the product of the fractions of drug entering the tissue that escape loss at each site. The extent of first-pass metabolism in the liver and intestinal wall depends on a number of physiological factors. The major factors are enzyme activity, plasma protein and blood cell binding, and gastrointestinal motility. Models that describe the dependence of bioavailability on changes in these physiological variables have been developed for drugs subject to first-pass metabolism only in the liver. Two that have been applied widely are the 'well-stirred' and 'parallel tube' models. Discrimination between the 2 models may be performed under linear conditions in which all pharmacokinetic parameters are independent of concentration and time. The predictions of the models are similar when bioavailability is large but differ dramatically when bioavailability is small. The 'parallel tube' model always predicts a much greater change in bioavailability than the 'well-stirred' model for a given change in drug-metabolising enzyme activity, blood flow, or fraction of drug unbound. Many clinically important drugs undergo considerable first-pass metabolism after an oral dose. Drugs in this category include alprenolol, amitriptyline, dihydroergotamine, 5-fluorouracil, hydralazine, isoprenaline (isoproterenol), lignocaine (lidocaine), lorcainide, pethidine (meperidine), mercaptopurine, metoprolol, morphine, neostigmine, nifedipine, pentazocine and propranolol. One major therapeutic implication of extensive first-pass metabolism is that much larger oral doses than intravenous doses are required to achieve equivalent plasma concentrations. For some drugs, extensive first-pass metabolism precludes their use as oral agents (e. g. lignocaine, naloxone and glyceryl trinitrate).(ABSTRACT TRUNCATED AT 400 WORDS)