Influence of age and smoking on drug kinetics in man: studies using model compounds

Liver diseases and aging: friends or foes?

The liver is the only internal human organ capable of natural regeneration of lost tissue, as little as 25% of a liver can regenerate into a whole liver. The process of aging predisposes to hepatic functional and structural impairment and metabolic risk. Therefore, understanding how aging could affect the molecular pathology of liver diseases is particularly important, and few studies to date have tackled this complex process. The most common liver disease, affecting one-third of the overall population, is nonalcoholic fatty liver disease (NAFLD), characterized by an intrahepatic accumulation of lipids. NAFLD can evolve into nonalcoholic steatohepatitis (NASH) in the presence of oxidative stress and inflammation. NASH is a serious risk factor for disabling and deadly liver diseases such as cirrhosis and hepatocellular carcinoma (HCC). Old age seems to favor NAFLD, NASH, and ultimately HCC, in agreement with the inflamm-aging theory, according to which aging accrues inflammation. However, the incidence of HCC drops significantly in the very elderly (individuals aged more than 70) and the relationship between the progression of NAFLD/NASH/HCC and very old age is obscure. In this review, we discuss the literature and we argue that there might be an age window in which the liver becomes resistant to the development of injury; this needs to be studied to understand fully the interaction between age and liver diseases from a therapeutic perspective.

The drug-consuming patient and his drugs. I. The patient

Patient characteristics and pre-hospital drug consumption have been studied in 506 consecutive patients acutely admitted to a department of medicine. Women were older (median 69 years) than men (median 63 years) and weighed less (mean 60 vs. 74 kg). Serum creatinine values were above the normal limits in 110 patients (22%). Forty-eight per cent of men and 26% of women were smokers. Heavy alcohol intake was found in 12% of men and 4% of women. It is concluded that a typical patient group shows much greater variations in age, weight, renal function, smoking and drinking habits--factors all known to influence the results of drug treatment--than is generally taken into account when routine drugs are prescribed.

Implication of aging on anesthetic drugs

PURPOSE OF REVIEW

The elderly population is increasing in number each year, and more patients are presenting for anesthesia and surgery. One of the key areas for improving the care of the elderly is a better understanding of the influence of aging on drug pharmacokinetics and dynamics.

RECENT FINDINGS

We now know more about the effects of risk factors on the occurrence of postoperative complications, and strategies to improve outcome after anesthesia and surgery. Two such strategies include the role of perioperative beta-adrenoceptor blockade in obtunding cardiovascular responses and myocardial ischaemia, and the provision of effective perioperative analgesia. Both topics have featured in key publications during the past year. Cognitive dysfunction following surgery occurs in about 10% of elderly patients; possible etiologies include a decline in central nervous system cholinergic function. One major disease of the elderly is Parkinson's disease, which offers challenges to the anesthesiologist both with regard to alterations of physiology and in choice of anesthetic drugs and techniques.

SUMMARY

The effects of comorbidity and intercurrent medications may alter the normal anesthetic practice of the clinician's care of the elderly patient. Further studies in these key areas may lead to improved outcomes.

Smoking delays the response to treatment in episcleritis and scleritis

PURPOSE

To evaluate the influence of smoking on comorbidity, treatment, visual and general outcome in patients with scleritis.

METHODS

The smoking habits of 103 patients with a diagnosis of episcleritis or scleritis were evaluated. These patients were treated by one ruling protocol at the Leiden University Medical Center between 1997 and 2000. Medical records of each patient were evaluated in detail. Data on possible factors concerning smoking were collected by postal questionnaire.

RESULTS

Of all 103 patients diagnosed with either episcleritis or scleritis, 41 (39.8%) were smoking during treatment of the scleral inflammation. In total, 19 patients (18.4%) had a smoking history while 43 (41.7%) patients have never smoked. The response to any of the given medications could be delayed by at least 4 weeks in many smoking patients (odds ratio (OR) 5.4 [95% confidence interval 1.9-15.5]), particularly those with posterior scleritis. Smoking patients above the age of 48 years were even more likely to respond belatedly to any given therapy (OR 6.6 [2.1-20.7]). However, having a smoking history did not delay the response. Furthermore, smoking did not worsen the visual prognosis and was not associated with additional recurrences or ocular complications after successful treatment.

CONCLUSIONS

Although scleritis patients who smoked during treatment eventually responded, there was frequently over a month's delay before the medication became effective when compared to nonsmokers. This was irrespective of the type of disease or given therapy. As a consequence, smokers required more intensive therapy than those who did not smoke.

Issues in the perioperative management of the elderly patient with cardiovascular disease

The elderly patient may show normal physiological changes of the cardiovascular and respiratory systems that accompany aging, as well as features of intrinsic cardiac disease. The latter include: a past history of myocardial infarction or ischaemic heart disease; history of congestive cardiac failure; angina; arterial hypertension (BP >140/90mm Hg); and conduction disorders. A key aspect to the safe and effective anaesthetic management of the elderly patient with cardiac disease is a careful preoperative assessment and optimisation of pre-existing drug therapies. All cardiac medications should be continued up to and including the morning of surgery with the exception of anticoagulation involving warfarin, and perhaps large doses of angiotensin converting enzyme inhibitors and angiotensin II receptor antagonists in patients with hypertension or heart failure. Anaesthetic techniques used in these patients should avoid episodes of excessive hypotension after induction of anaesthesia or large blood loss, or the combination of hypertension and tachycardia after noxious stimulation. The latter physiological disturbances are pivotal for the development of myocardial ischaemia. Both premedication (if used) and anaesthesia should avoid excessive sedation and respiratory depression. The choice of anaesthetic technique may vary between: a balanced technique involving an opiate and a volatile agent; an intravenous technique utilising infusions of propofol; or regional anaesthesia with or without additional sedation. There are no good data to suggest any one technique is better than the rest. The occurrence of ischaemia in the perioperative period may precede the postoperative development of significant cardiac morbidity and mortality (including myocardial infarction or unstable angina, congestive cardiac failure, cerebrovascular accidents, and severe arrhythmias). A number of strategies have been examined to reduce these adverse outcomes. The effect of acute beta-adrenoceptor blockade in treatment-naive patients is associated with reduction in the haemodynamic response to noxious stimuli and decreased ECG evidence of myocardial ischaemia, as well as a reduction in the number of cardiac adverse events. Other drugs (calcium channel antagonists, alpha(2)-agonists and adenosine modulators) have a less predictable influence on both myocardial ischaemia and hard cardiac outcomes. There is inadequate evidence at present to define the optimal time course for acute beta-blockade, or the groups of patients in whom preoperative beta-blockade should be initiated in the absence of contraindications. Nevertheless, addition of beta-blockers to the preoperative regimen should be considered in patients with evidence of or at risk for coronary disease undergoing major surgery. There is also evidence that long-term beta-adrenoceptor or calcium channel blockade or nitrate therapy for the high-risk cardiac patient offers little protection against silent myocardial ischaemia, nonfatal infarction, cardiac failure and cardiac death.

Child and adolescent psychopharmacology. Important developmental issues

This article begins with an overview of developmental variations in pharmacokinetics and pharmacodynamics and then discusses issues related to decision making before beginning treatment with medication, including diagnosis, impairment, and consent. Subsequently, specific disorders, such as attention deficit hyperactivity disorder, mood disorders, and obsessive-compulsive disorder, are briefly discussed, with focus on issues relevant to a developmental approach to pediatric psychopharmacology.

In vivo age-related changes in hepatic drug-oxidizing capacity in humans

Very few studies have been carried out looking at how the effects of drugs and their toxicity in humans change during their lifespan (developing and ageing). The purpose of this study is to review the literature on the changes in probe-drug metabolism, classified by cytochrome P450 (P450 or CYP) at five stages in life: neonates < 4 weeks, infants < 12 months, children < 19 years, young/mature adults 20-64 years, and elderly adults > 65 years. The main probe drugs include caffeine and theophylline, whose metabolism is catalysed by CYP1A2, tolbutamide, phenytoin and ibuprofen, catalysed by CYP2C9, amitriptyline and nortriptyline, catalysed by CYP2C19, acetaminophen, catalysed by CYP2E1 and lidocaine, midazolam and terfenadine, catalysed by 3A3/4. From the published in vivo studies two different patterns of drug metabolism can be identified: (i) activity is low immediately after birth, increases, then peaks at the young/mature adult level and, finally, decreases in old age (drugs catalysed by CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A3/4) and (ii) activity increases rapidly after birth to reach a level equivalent to that in the young/mature adult, then gradually decreases and finally decreasing faster in old age (drugs catalysed by CYP2E1). Further study of the changes in P450 with age is warranted to help prevent adverse reactions and to guide us in tailoring therapy better for the individual patient.

Accuracy of the one-sample method for determination of antipyrine clearance in elderly subjects

The purpose of this study was to evaluate the validity of the one-sample abbreviated method for determination of the pharmacokinetic parameters of antipyrine in the elderly. Antipyrine pharmacokinetics were studied in 15 elderly women (mean age 86 years). Antipyrine (1 g) was administered orally and pharmacokinetic parameters were determined by the one-sample (24 h) and multiple-sample (3, 6, 9, 12 and 24 h) methods. Mean antipyrine clearance for the one-sample study (19.72 +/- 1.51) was almost identical to that obtained with the multiple-sample approach (20.73 +/- 1.57), and the two methods were very well correlated (r = 0.989). Relative standard deviations between individual clearances values for multiple-sample vs. one-sample studies averaged 1.6%. Values of elimination half-life were likewise very similar for the abbreviated (17.41 +/- 1.21) and complete (17.99 +/- 1.09) methods, with a significant correlation (r = 0.857). Although values were underestimated by 10% in the one-sample approach, no difference in the volume of distribution with the multiple-sample study was observed. When the unbiased volume of distribution value was determined from the total elimination curve against time, the influence of biased volume of distribution resulted in a 5.1% deviation in antipyrine clearance in the one sample method. The findings indicate that antipyrine pharmacokinetic parameters can be estimated with reasonable precision and accuracy in the elderly using a simplified one-sample procedure.

Influence of nutrition on liver oxidative metabolism

The liver plays a major role in the disposition of the majority of drugs. This is due to the presence of several drug-metabolizing enzyme systems, including a group of membrane-bound mixed-function oxidative enzymes, mainly the cytochrome P450 system. Hepatic oxidative capacity can be assessed by changes in antipyrine metabolism. Different drugs and other factors may induce or inhibit the cytochrome P450-dependent system. This effect is important in terms of the efficacy or toxicity of drugs that are substrates for the system. Microsomal oxidation in animals fed with protein-deficient diets is depressed. The mixed-function oxidase activity recovers after a hyperproteic diet or the addition of lipids. Similar findings have been reported in patients with protein-calorie malnutrition, although results in the elderly are conflicting. Different studies have revealed that microsomal oxidation is impaired by total parenteral nutrition and that this effect is absent when changing the caloric source from carbohydrates to a conventional amino acid solution or after lipid addition, especially when administered as medium-chain/long-chain triglyceride mixtures. Peripheral parenteral nutrition appears to increase antipyrine clearance.

Cigarette smoking and clinically significant drug interactions

OBJECTIVE

To review clinically significant drug interactions associated with cigarette smoking.

DATA SOURCES

Data from scientific literature were identified by using a MEDLINE search. Data were extracted, evaluated, and summarized for this review.

STUDY SELECTION

Findings and experiences were selected from clinical, epidemiologic, and pharmacokinetic studies; review articles; case studies; abstracts; letters to the editor, and proceedings.

DATA EXTRACTION

Data from human studies published in English were evaluated. Only interactions deemed clinically significant are included in this review. Conclusions derived from review articles on the subject of smoking and drug interactions also were used.

DATA SYNTHESIS

Cigarette smoking can affect drug therapy via pharmacokinetic and pharmacodynamic mechanisms. Pharmacokinetic drug interactions are presented for theophylline, tacrine, insulin, flecainide, propoxyphene, propranolol, diazepam, and chlordiazepoxide. Pharmacodynamic interactions are described for antihypertensive and antianginal agents, antilipidemics, oral contraceptives, and histamine2-receptor antagonists.

CONCLUSIONS

Cigarette smoking can reduce the efficacy of certain drugs or make drug therapy more unpredictable. Pharmacokinetic interactions may cause smokers to require a larger dosage of certain drugs through an increase in plasma clearance, a decrease in absorption, an induction of cytochrome P450 enzymes, or a combination of these factors. Pharmacodynamic interactions may increase the risk of adverse events in smokers with cardiovascular or peptic ulcer disease, and in women who smoke and use oral contraceptives. Healthcare professionals should pay special attention to patients with these profiles and should try to prevent cigarette smoking or encourage patients to discontinue this addictive habit.

Effects of aging on antipyrine clearance: predictive factors of metabolizing capacity

The purpose of this study was to identify variables that can account for the decline of antipyrine clearance (CLAP) in elderly adults and that may help predict a reduction in metabolizing capacity. For comparison, ClAP was determined in 177 elderly (mean age 82 years) and 25 young (mean age 29 years) volunteers. Antipyrine (1 g) was administered orally and ClAP was determined by the one-sample saliva method. Mean ClAP was reduced by 38% and antipyrine half-life increased by 64% in old subjects. Multiple regression analysis of ClAP revealed an independent value for age, serum aspartate transaminase (AST), and height in the elderly. The independent variables collectively accounted for 27% of the variance explained. Age, high serum AST, use of diuretics, and no consumption of drugs known to stimulate oxidative metabolism were selected by multivariate analysis (logistic model) as independent predictors of a low metabolizing capacity. The findings indicate that factors other than age may contribute to impaired hepatic oxidative metabolism in the elderly.

Assessment of antipyrine kinetics from saliva or plasma: influence of age

The purpose of this study was to investigate whether antipyrine estimation in saliva provides valid information on plasma antipyrine clearance (APCl) and can be useful as an index of changes in drug metabolism with age. Antipyrine kinetics was studied in 93 elderly (mean age 82 years) and 23 young (mean age 29 years) volunteers. Plasma antipyrine half-life (APt1/2) increased and plasma APCl declined with age. No significant difference between plasma- and saliva-derived parameters was found in either young or old subjects. However, the saliva/plasma ACCl ratio tended to increase with age. A highly significant correlation between saliva and plasma APCl or APt1/2 was found in young subjects. Values were less closely related in the elderly and the slope of the saliva/plasma APCl relationship was significantly different in both groups of subjects. Residual variance was higher in the regressions corresponding to the elderly. The findings in the study indicate that the relationship between saliva and plasma kinetics in young subjects becomes less reproducible with age.

Influence of age on the disposition and renal handling of enprofylline in rats

The effects of ageing on the pharmacokinetics, renal handling and protein binding of enprofylline were investigated in 6-, 13- and 18-month-old male Fischer 344 rats. Concentrations of enprofylline in plasma and urine were determined by HPLC, and pharmacokinetic parameters were estimated by model-independent methods. No significant differences in the volume of distribution, systemic clearance of enprofylline or urinary recovery of unchanged enprofylline (> 85%) were observed among any of the groups of rats. The dissociation constant and free fatty acid concentration in plasma increased with age. Age-dependent decreases in the systemic clearance for unbound drug were observed, and the volume of distribution for unbound drug tended to decrease with age. The ratio of systemic clearance for unbound drug to the glomerular filtration rate (GFR) decreased with ageing. Ageing was associated with decreases in the apparent maximum capacity of transport (Vmax)(223.33, 160.24 and 142.98 micrograms min-1 kg-1 for 6-, 13- and 18-month-old rats, respectively) and in the tubular secretory intrinsic clearance (Vmax/Km) of enprofylline (75.45, 51.03 and 44.13 mL min-1 kg-1, respectively), while a slight change in the Michaelis-Menten constant (Km) was observed. These results indicate that the mechanism responsible for age-related changes in the disposition and renal handling of enprofylline may be responsible for a decrease in the ability of the tubular anion transport system.

Extent and variability of the first-pass elimination of adinazolam mesylate in healthy male volunteers

The pharmacokinetics of adinazolam and N-desmethyladinazolam (NDMAD) were studied in 14 healthy male volunteers who received 15 mg adinazolam mesylate orally as a solution and 5 mg adinazolam mesylate intravenously in a crossover design. Two weeks prior to the crossover study, each subject received 5 mg/kg indocyanine green (ICG) as an intravenous bolus injection to estimate liver blood flow. The absolute bioavailability (F), calculated as the dose-corrected ratio of oral to iv adinazolam area under the curve (AUC) values, was found to be 39%. NDMAD AUC values were similar following oral and iv administration, and adinazolam mean absorption time was approximately 0.77 hr. Thus, adinazolam is completely and rapidly absorbed after oral administration in man; the incomplete bioavailability is due to first-pass metabolism. Mean liver blood flow, adinazolam systemic clearance, blood/plasma ratio, and extraction ratio were 1189 ml/min, 498 ml/min, 0.70, and 0.57, respectively. The extraction ratio agrees with that calculated as 1-F (0.62), suggesting that the liver is primarily responsible for first-pass metabolism of adinazolam. The unbound fraction of adinazolam in plasma was 0.31 (range, 0.25-0.36); adinazolam free intrinsic clearance (a reflection of metabolic capacity) was 4285 ml/min (range, 2168-6312 ml/min). These results suggest that the majority of the variability in adinazolam plasma concentrations following oral administration is due to the variability in the metabolic capacity of the liver for adinazolam, rather than variability in plasma protein binding.

Quantifying hepatic function in the presence of liver disease with phenazone (antipyrine) and its metabolites

The disposition of phenazone (antipyrine), a low extraction compound with low protein binding, is known to be altered in the presence of various types of hepatic dysfunction. As such, its pharmacokinetics may be useful in the objective characterisation of altered liver function. Understanding the known effects of various liver disease states upon the disposition of this probe may provide insight into future applications. This article provides a review of background information about normal plasma phenazone pharmacokinetics, urinary metabolite disposition and tabulations of reported total body clearances of the drug in the presence of cirrhosis, fatty liver, hepatitis and cholestasis in humans. An estimate is made of the sensitivity and specificity of phenazone testing for the verification of the presence of cirrhosis based on this compiled literature.

Hepatic drug metabolism and aging

Although there is considerable variation in the effect of age on drug biotransformation, the metabolism of many drugs is impaired in the elderly. Age-related physiological changes, such as a reduction in liver mass, hepatic metabolising enzyme activity, liver blood flow and alterations in plasma drug binding may account for the decreased elimination of some metabolised drugs in the elderly. It is difficult, however, to separate an effect of aging from a background of marked variation in the rate of metabolism due to factors such as individual metabolic phenotype, environmental influences, concomitant disease states and drug intake. The prevailing data suggest that initial doses of metabolised drugs should be reduced in older patients and then modified according to the clinical response. In most studies the elderly appear as responsive as young individuals to the effects of compounds which induce or inhibit the activity of cytochrome P450 isozymes. Concurrent use of other agents, which induce or inhibit drug metabolism, mandates dose adjustment as in younger patients. Many questions remain unanswered. For instance, limitations of in vitro studies prevent any firm conclusion about changes in hepatic drug metabolising enzyme activity in the elderly. With aging, some pathways of drug metabolism may be selectively affected, but this has not been adequately scrutinised. The possibility that metabolism of stereoisomers may be altered in the elderly has not been adequately tested. The effect of aging on the distribution of polymorphic drug metabolism phenotypes is still not established, despite potential implications for disease susceptibility and survival advantage.

The effect of smoking on theophylline kinetics in healthy and asthmatic elderly males

The effect of smoking on theophylline kinetics was investigated in four groups of males over the age of 65 yr. The first group consisted of 14 healthy non-smoking males, the second group consisted of 8 healthy smoking males. The first and the second groups were administered a single oral dose of 300 mg tablet of sustained-release anhydrous theophylline product and serial blood samples were collected over 24 hr. The third group consisted of 22 clinically stable asthmatic non-smoking males, the fourth group consisted of 9 clinically stable asthmatic smoking males. The third and the fourth groups, were administered multiple oral doses equivalent to 300 mg of anhydrous theophylline from sustained-release Oxtriphylline every 12 hr for 72 hr; and blood samples were collected every two hours between 72-82 hrs. Serum theophylline concentrations were determined by HPLC. There were no significant differences between the smokers and non-smokers in observed maximum concentration (Cmax), its time (tmax), the area under the time-concentration curve (AUCo-t), and total body clearance rate (Cl). Aging appears to offset the known smoking inducing effect on the hepatic microsomal enzymes.

Medication and the elderly. Issues of prescription and use

Use of medication has become an increasingly important issue in the health and health care of older populations. The demographic changes resulting in greater numbers of individuals 65 and over, accompanied by a higher prevalence of chronic disease, focus attention on prescription and over-the-counter drugs, the major strategy in treatment regimens for these conditions. In addition, the numbers of new and powerful drugs being developed increase daily. This article reviews and evaluates existing information on the prevalence of medication use and adverse drug reactions. It examines the role of physicians and patients in occurrence and management of side effects of drugs and in patterns of prescription and use. Following this, it identifies areas of research necessary to address the gaps in the current state of knowledge in these areas and discusses the need for strong and substantial information about this essential therapeutic strategy in health care for older people.

A comparative pharmacokinetic study of conventional propranolol and long acting preparation of propranolol in patients with cirrhosis and normal controls

1 Six male patients with alcoholic cirrhosis and seven normal control subjects were each given 80 mg twice daily of conventional propranolol for 1 week and 160 mg once daily of a long acting preparation (LA) of propranolol for 1 week. 2 Plasma propranolol levels were measured at regular intervals on the first and seventh days of both weeks and also following an acute intravenous infusion of 10 mg propranolol on a separate occasion. 3 After the single intravenous dose the elimination half-life tended to be prolonged in the cirrhotic group (median 7.15 h) compared with controls (median 2.92 h) (P = 0.055). 4 After multiple oral dosing with 80 mg twice daily of conventional propranolol the steady-state plasma concentration (Css), area under the curve (AUC tau), peak concentration (Cmax) and trough concentration (Cmin) were significantly higher in cirrhotic patients and the peak: trough ratio (Cmax/Cmin) was significantly lower than controls. 5 After multiple oral dosing with 160 mg LA once daily Cmin was significantly higher than Cmax/min significantly lower in cirrhotic patients; Css, AUC and Cmax were higher than controls but not statistically different. 6 Within both subject groups the bioavailability of 80 mg twice daily of conventional propranolol tended to be greater than 160 mg LA once daily. Cmax was significantly higher in both groups and Css higher in the cirrhotic group with conventional propranolol. 7 In the cirrhotic group the mean reduction in supine heart rate in the steady state was 31.8% with conventional 80 mg twice daily propranolol and 23.75% with 160 mg LA once daily.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors affecting the pharmacokinetics of nifedipine

The plasma pharmacokinetics of nifedipine and the formation of its metabolites have been studied in volunteers under conditions which would affect the activity of the cytochrome P-450 system. The pharmacokinetics of a 10-mg capsule of nifedipine were not significantly different between smokers and non-smokers of similar age. After pretreatment with cimetidine, which inhibits the activity of cytochrome P-450, the peak plasma concentration and area under the plasma-time concentration curve for nifedipine were increased by a mean 84%. In contrast, pre-treatment with ranitidine which has little effect on cytochrome P-450, did not significantly alter nifedipine pharmacokinetics. Smoking does not contribute significantly to the variability in nifedipine pharmacokinetics. However, the interaction between nifedipine and cimetidine, but not ranitidine, may be of clinical importance.

Drug interactions in surgical patients

Drug interactions, defined as when the administration of a single substance (drug, nutrient, or tobacco) modifies the response to a drug, occur relatively frequently in surgical patients and may result in increased morbidity and lengthened hospital stay. Drug interactions also account for some instances of drug ineffectiveness or exaggerated pharmacologic response. There are many types of drug interactions. However, most of them are related to altered drug pharmacokinetic properties, where there are alterations in drug absorption, distribution, metabolism, or elimination; or altered drug pharmacodynamic actions, where two agents may have synergistic, additive, or antagonistic pharmacologic effects. The term, drug interaction, usually refers to pairs of drug substances administered concurrently, but more than two agents may be involved. When patients are taking a large number of different medications, there may be multiple drug interactions with additive or antagonistic effects, the overall effects of which are difficult to predict. There are hundreds of reported drug interactions, and some may be of important clinical consequence. In surgical patients, the majority of drug interactions involve histamine-2 blockers (particularly cimetidine), digoxin, warfarin, or a variety of agents that may be administered during anesthesia. Recognition of the potential for adverse drug interactions is of primary importance in minimizing their effects. Usually, potentially interacting drugs may be administered concurrently as long as appropriate patient or laboratory assessments are performed. For some agents, such as digoxin or theophylline, serum drug concentrations may aid in the avoidance of adverse drug interactions.

Determinants of drug disposition in man

This article reviews some of the important determinants of variation in drug disposition such as age, gender, body weight, diet, environmental influences, drug - protein interactions, compliance, drug - drug interactions, endogenous substances, disease states, circadian variation and genetics.

Cirrhosis of the liver markedly impairs the elimination of mexiletine

To study the effects of cirrhosis of the liver on the pharmacokinetics of mexiletine a single i.v. dose of 200 mg was administered to six cirrhotic patients and to six healthy controls. The distribution of mexiletine in both study groups was similar, as indicated by similar values of V1 and Vss, but it tended to occur more slowly in the cirrhotics. The plasma protein binding of mexiletine was unchanged in the patients with cirrhosis. The elimination of mexiletine was markedly retarded in the cirrhotics, as indicated by its lower total clearance (2.31 vs. 8.27 ml/kg/h,) lower total elimination rate constant (0.059 vs 0.353 h-1), and longer elimination half-life (28.7 vs 9.9 h). The antipyrine half-life was 38.3 h in the patients and 14.7 h in the controls. One healthy volunteer had a Morgagni-Stokes-Adams type of syncopal attack 5 min after administration of mexiletine due to disturbance of AV conduction induced by the drug. Thus, on a pharmacokinetic basis the loading dose of mexiletine need not be modified in cirrhotic patients, whereas the maintenance dosage should be reduced to one fourth - one third of the usual dose.

The influence of age and smoking on the elimination of disopyramide

The influences of smoking and age on the elimination kinetics of disopyramide were studied in 27 subjects. Total elimination clearance of disopyramide was measured after an infusion to steady state. The total elimination clearance was significantly (P less than 0.05) decreased in elderly non-smoking patients compared with young non-smoking subjects (1.54 +/- 0.33 vs 2.12 +/- 0.67 ml kg-1 min-1) (mean +/- s.d.). Smoking more than 20 cigarettes per day significantly (P less than 0.05) increased total elimination clearance in elderly (2.02 +/- 0.35 vs 1.54 +/- 0.33 ml kg-1 min-1), while no significant induction by tobacco was observed in young healthy persons. Serum concentrations of alpha 1-acid glycoprotein, the major binding protein of disopyramide, were significantly higher (P less than 0.001) in the elderly patients. However, the volume of distribution (V) was significantly (P less than 0.001) greater in the elderly patients (2.44 +/- 0.64 vs 1.16 +/- 0.15 1 kg-1). Steady-state serum concentrations of the free drug were significantly (P less than 0.01) lower in the young volunteers (0.75 +/- 0.13 micrograms ml-1) than in the elderly (0.90 +/- 0.10 micrograms ml-1). The half-life of disopyramide was significantly shorter (P less than 0.01) in the young volunteers than in the elderly patients. No difference was observed in the relationship between the serum concentration of disopyramide and its main dealkylated metabolite in the groups studied. The results indicate that it might be advisable to reduce the dosage of disopyramide by approximately 30% in elderly non-smokers compared with young subjects.

No effect of smoking on sulfamethazine acetylation

Seven healthy subjects who smoked at least one pack of cigarettes/d were acetylator-phenotyped with sulfamethazine (SMZ) while they were actively smoking, and again at least one month after they had completely stopped. There were no significant differences of the acetylation profiles in the smoking vs. nonsmoking state, as measured by %ASMZ in serum six hours post-SMZ dosing or by %ASMZ in urine five to six hours post-SMZ dosing. We conclude that smoking does not influence SMZ acetylation and probably does not affect drug acetylation in general.

Age-and sex-related differences in rat liver microsomal enzymes and their inducibility by toluene

In the liver microsomes of toluene-treated and control Sprague-Dawley rats (n = 148, males and females aged 13-93 days), the contents of cytochrome P-450 and b5 and the activities of NADPH-cytochrome c reductase and four monooxygenases were studied. In male control rats, cytochrome contents and NADPH-cytochrome c reductase, aminopyrine N-demethylase and aniline hydroxylase activities increased to the age of one month, and after a slight decrease in cytochrome concentrations, the average adult level was reached by the age of two months. Aniline hydroxylase and 7-ethoxycoumarine O-deethylase activities decreased to about half at the same age period. In control female rats, the activities of aminopyrine N-demethylase and aniline hydroxylase decreased after the age of one month, and they remained at a considerably lower level in adult females than in males. The sex-dependence of other enzymes was negligible. Toluene induction was already well developed in the youngest age group of both sexes; in most cases the induced enzyme levels in young rats were as high or higher than in adults. In adult female rats, toluene induction of all enzymes was weaker than in males. In male rats, the toluene-induced level of aniline hydroxylase and 7-ethoxycoumarine O-deethylase showed deep minima at the age of 43-53 days, at the puberty of rats.

Effects of tobacco smoking and oral contraceptive use on theophylline disposition

The independent as well as interactive effects of chronic (greater than 6 months) oral contraceptive (OC) use and cigarette smoking on single-dose (4 mg/kg) theophylline disposition were assessed in 49 young, healthy women. Significant elevations (40%) in theophylline plasma clearance were found in women who smoked. OC use resulted in decreases in clearance of a similar magnitude (28%). These factors do not appear to interact with respect to theophylline disposition. The combination of main effects tended to cancel one another (clearance of 49.1 ml h-1 kg-1 ideal body weight for OC non-user, non-smoker, vs 49.7 ml h-1 kg-1 for OC user-smoker). Single dose exposure to OC in non-users did not significantly alter theophylline pharmacokinetics for the group as a whole. However, in the subgroup of smoking subjects, significant decreases in clearance were evident (P less than 0.05). Analogous results were found for half-life. Volume of distribution was slightly diminished in smokers, but was unaffected in OC users. Areas under the serum concentration-time (AUC) profiles of norgestrel and ethinyloestradiol were examined in 27 women as indices of OC exposure. The smallest values of theophylline clearance were found in the subjects with largest AUC of both OC steroids. Appropriate statistical analyses of data which are influenced by multiple factors are discussed. Special concern is needed when the factor partitioning process yields subgroups of unequal sizes.

Pharmacokinetics of brotizolam in the elderly

Disposition of brotizolam in patients aged 71-93 years was compared with that of healthy young subjects aged 21-26 years. The mean elimination half-life of brotizolam was about twice as long in the elderly as in the young subjects: 9.3 (4.0-19.5) h and 4.8 (3.1-6.3) h respectively. Increase in elimination half-life was attributable to a decrease in hepatic clearance, i.e. 40 (20-58) ml/min in the elderly and 109 (77-156) ml/min in the young. Volume of distribution and protein binding were the same with mean values of 0.56 (0.45-0.72) l/kg and 9.0 (6.8-11.9) % in the elderly and 0.63 (0.40-0.77) l/kg and 8.4 (7.5-9.4) % in the young. Absorption rate of brotizolam was relatively slow in the elderly with a mean peak time of 1.7 h compared with 1.1 h in the young. Mean bioavailability was almost 70% for both groups. Normalized for body weight and dose (0.25 mg) mean peak concentrations were 247 (137-395) ng ml-1 kg in the young and 343 (251-446) ng ml-1 kg in the elderly. It is unlikely that substantial drug accumulation will occur if elderly patients ingest 0.25 mg brotizolam nightly.

Cimetidine as an inhibitor of drug metabolism: therapeutic implications and review of the literature

Cimetidine has been reported to decrease the biotransformation of drugs metabolized by the MFOE system. Additionally, cimetidine decreases liver blood flow and increases the bioavailability of drugs with high hepatic extraction ratios. Patients receiving cimetidine in conjunction with drugs known to interact with cimetidine in conjunction with drugs known to interact with cimetidine are at risk of experiencing toxicity. When appropriate, reducing the dosage of these agents or switching to an alternative drug will minimize the incidence of side effects. Clinicians should be suspicious if patients experience exaggerated drug effects when cimetidine therapy is begun.

Pharmacokinetics and first-pass metabolism of levomepromazine in the rat

The time course of the blood concentrations of levomepromazine and its two major non-polar metabolites in man were studied in the rat after single oral and intraarterial doses of levomepromazine hydrochloride. The blood levels of N-monodesmethyl levomepromazine were on average 179% of the levomepromazine levels after oral doses, but only 15% of the levomepromazine levels after intraarterial doses. The blood levels of levomepromazine sulfoxide, relative to the levomepromazine levels, were also generally higher after oral doses than after parenteral doses, on average 65% and 25%, respectively. Large interindividual variations were observed in the blood levels of levomepromazine and in the systemic availability of the drug after oral doses. The distribution phase lasted for about 8 hrs, the mean apparent volume of distribution was 16.6 l/kg, and the total body clearance was on average 12.3 ml/min. It is concluded that the rat provides a suitable model for the kinetics of levomepromazine in man, and that the sulfoxide and N-monodesmethyl metabolites of the drug are mainly formed by first-pass metabolism after oral doses in the rat, either in the liver or in the gut.

[On the pharmacokinetics of spironolactone in the elderly]

The pharmacokinetics of canrenone were compared in 10 elderly (77.2 years) patients and 10 young (20.1 years) female persons after multiple oral dosing of 100 mg Spironolactone during steady-state. The concentrations were determined using both a specific HPLC-assay and a nonspecific fluorometric assay. Maximum as well as mean concentrations of canrenone in serum of the elderly subjects were approximately twice as high as those in the young. This was the consequence of an impaired capacity for elimination of spironolactone in the elderly subject. In addition the ratio of the other fluorigenic metabolites and of canrenone were higher in the elderly. Thus also shifts in the metabolic pathways of spironolactone occur with progressing age.

Substrate specificity of age-related changes in the inducibility of hepatic microsomal monooxygenases in middle-aged rats

Hepatic microsomal monooxygenase induction was investigated in young-adult and middle-aged male Fischer 344 rats. Monooxygenase components and drug metabolism activities were determined in liver microsomes prepared from rats treated with phenobarbital (PB, beta-naphthoflavone (BNF) or methyltestosterone (MT) and compared with values from untreated rats. PB and BNF effects on cytochrome P-450 concentration and cytochrome c reductase activity were similar in young-adult and middle-aged animals. However, the extent of cytochrome P-450 induction by MT was less in the older animals. The age-related changes in induction of drug metabolism activities differed with different substrates for the monooxygenase system. In contrast to the inducibility of benzphetamine N-demethylation and aniline hydroxylation, which was diminished in the older rats, the inducibility of nitroanisole O-demethylation was enhanced. The results imply that qualitative changes in the microsomal enzyme system occurred as the animals progress from young to middle adulthood.