Archives of Gerontology and Geriatrics 32 (2001) 255– 263 www.elsevier.com/locate/archger The aging liver: consequences for drug treatment in old age J. Zeeh Geriatrische Fachklinik Georgenhaus, D-98617 Meiningen, Germany Received 29 November 2000; received in revised form 25 February 2001; accepted 26 February 2001 Abstract Numerous age-related changes in hepatic structure and function have been described although liver function seems to be quite well maintained in old age. Few consistent and reproducible observations and a lack of correlation between structural and functional data characterize the present state of our knowledge. However, a decline in liver volume and blood flow has been shown in older subjects, which is the physiologic basis of reduced hepatic drug clearance in this age group. In contrast to renal clearance, no reliable method exists to estimate hepatic drug clearance. The contribution of age to altered drug clearance in the elderly is difficult to assess as drug interactions, numbers and types of drugs taken at a time, underlying disease and increased interindividual variability are superimposed to the aging process. Therefore, after decades of research into the matter, the old and well known aphorism ‘start lower — go slower’ is valid more than ever in the field of geriatric prescribing. Not only renally excreted drugs but also substances metabolized and excreted by the liver should be used in a starting dose which is 30 – 40% smaller than the average dose used in middle aged adults. © 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Aging of liver; Drug metabolization in old liver; Hepatic detoxification 1. Introduction Due to age-associated changes and the sequelae of previous diseases, the elderly, i.e., persons older than 75 years, represent a distinct population. They differ from younger adults in terms of pharmacokinetics and pharmacodynamics (Greenblatt et E-mail address: georgenhaus@t-online.de (J. Zeeh). 0167-4943/01/$ - see front matter © 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 1 6 7 - 4 9 4 3 ( 0 1 ) 0 0 0 9 0 - 5 256 J. Zeeh / Arch. Gerontol. Geriatr. 32 (2001) 255–263 al., 1986; Platt, 1986; Montamat et al., 1989) and these differences can be mainly attributed to decreased liver and renal function, reduced cardiac output, impaired pulmonary function and to reductions in body weight, muscle mass and changes in body composition (Wynne et al., 1989; Durnas et al., 1990; Lindeman, 1992). Furthermore, the possibility of drug interactions has to be taken into account as elderly patients frequently receive extensive drug treatment for coexistent illnesses. The incidence of many diseases increases with age and multimorbidity occurs most often in the elderly and oldest old. Therefore, prescribing for those frail elderly subjects is a major challenge for the medical practitioner. On the one hand, the frail elderly may require, and may potentially benefit from several medications. On the other hand, such polypharmacotherapy may also be harmful, as untoward drug reactions are recognized causes of morbidity in old age. Hurwitz (1969) in a pioneering survey of more than 1200 elderly patients admitted to an acute care hospital, found the incidence of adverse drug reactions more than tripled beyond the age of 70 years. This tendency may even be more pronounced in frail nursing home patients. It has been shown that 11.6% of all patients beyond the age of 65 years admitted to community hospitals had been admitted because of drug-induced conditions (Colt and Shapiro, 1989). In part, this is due to the fact that with declining renal and hepatic function and with a smaller body mass, ‘usual’ adult doses may represent relative overdoses for many elderly. For renally excreted drugs this may be overcome by meticulous assessment of actual renal function, whereas at present no routine tests of hepatic drug metabolizing capacity are available. Therefore, therapeutic drug monitoring may contribute to avoid dose related drug toxicity. Whereas anaphylactic and allergic drug reactions seem to occur independently from age, the frequency of drug– drug interactions and adverse drug reactions due to unexpectedly high blood concentrations of the drug are increasing in elderly patients. Therefore, the elderly not only have an increase in adverse drug reactions associated with abnormally high blood levels of many drugs, but also have more adverse effects at therapeutic blood levels. Several mechanisms may account for that (Table 1). For most side effects encountered in the elderly, remarkably little data is available to support the commonly held view that the incidence of adverse drug reactions increases with patient age as an independent risk factor (Nolan and O’Malley, 1988). After taking into account the doses administered and the duration of therapy, a positive correlation between increasing patient age and the occurrence of side effects may no longer be observed (Gurwitz and Avorn, 1991). This underscores the importance of careful and sensible prescribing for the elderly. However, comorbidity and limited reserve capacities of many organ systems often make the elderly more vulnerable for untoward side effects of drugs (Table 2). 2. Morphological changes of the aging liver Macroscopically the liver is described as undergoing brown atrophy with old age. In autopsy studies, aging was found to be associated with a 24% reduction in liver J. Zeeh / Arch. Gerontol. Geriatr. 32 (2001) 255–263 257 Table 1 Age-related changes and potentially resulting problems in the pharmacotherapy of elderly persons and their estimated relative importance Confounding factors Relative importance Remarks and examples Compliance problems in old patients taking many drugs Very important Errors sharply increase when more than four drugs are being prescribed at a time Altered drug absorption from the aging gastrointestinal tract Altered protein binding due to altered plasma protein pattern Altered volume of distribution due to changing body composition and body mass Impaired liver function Impaired renal excretion Unimportant Altered end organ susceptibility to drug effects Failure of compensatory mechanisms Very important Possibly important Important Important Very important Very important Increases glomerular excretion and tissue drug concentration Diazepam has a markedly prolonged half-life in the elderly due to a increased volume of distribution Glomerular filtration decreases with age The risk of falls is increased in subjects taking sedative drugs Arthritis and impaired postural control may aggravate the risk of falls after a variety of psychotropic drugs weight in males and a 18% reduction in females. This trend has been confirmed with different techniques (Table 3) and in general, the reduction of liver size is noted to be in the order of 25– 35% (Le Couteur and McLean, 1998). Apart from this decline in liver volume, there are few if any age-associated changes in liver structure correlating with perturbations in hepatic function. Physiological and morphological studies suggest that, compared to other organs, the liver seems to age fairly well. Table 2 Characteristic side effects of drugs frequently used in the elderly Target organ system, effect Offended drugs Reference Metabolic system, hypokalaemia Genitourinary system, urinary retention Gastrointestinal system, ulcer bleed Cardiovascular system, postural hypotension Nervous system, confusion Diuretics Levy and Lye (1987) Anticholinergics, benzodiazepines Iber et al. (1994) Nonsteroidal anti-inflammatory drugs Iber et al. (1994) Diuretics, antihypertensive drugs Sabanathan et al. (1987) Anticholinergics, psychotropic drugs, H2-blockers Psychotropic drugs Iber et al. (1994) Locomotor system, falls and fractures Ray et al. (2000), Ray et al., (1987) J. Zeeh / Arch. Gerontol. Geriatr. 32 (2001) 255–263 258 Table 3 Liver volume and age (Wynne et al., 1989; Zeeh and Platt, 1990; Iber et al., 1994; Le Couteur and McLean, 1998) Technique n Autopsy 1582 Ultrasound Ultrasound Ultrasound Ultrasound Age range (years) 26 50 65 32 20–80 25–80 B50–B80 24–91 B55–\70 Decline relative (%) Decline absolute (ml) Year of publication 24% (male), 18% (female) 24% 32% 37% 20 1933 – 1303 990 – 1474 934 1446 1157 1978 1988 1989 1994 Routine liver function tests do not show age-associated changes. Perhaps the most compelling evidence for the maintenance of hepatic function into advanced age comes from clinical practice, wherein livers from elderly subjects, including one donor aged 86 years, have been transplanted successfully (Schmucker, 1998). Fig. 1. Liver perfusion as determined by extrarenal sorbitol clearance (ER-S-Cl; Zeeh et al., 1988) in 86 subjects aged 40 –95 years without liver disease (own data, black columns denote mean values, topped grey columns indicate +SD). J. Zeeh / Arch. Gerontol. Geriatr. 32 (2001) 255–263 259 3. Hepatic blood flow Old age is unambiguously associated with a reduction in hepatic blood flow of about 35 –40% (Fig. 1). This has been documented using a variety of technical methods including dye dilution and indicator clearance, indicator distribution and Doppler ultrasound (Wynne et al., 1989; Woodhouse and Wynne, 1992; Zoli et al., 1999). This decrease is probably due to a diminished splanchnic blood flow which reduced input of blood into the portal vein. Bile flow and bile salt formation are reduced by about 50% reflecting, at least in part, impairment of energy dependent and microtubule-dependent transport processes (Le Couteur and McLean, 1998). 4. Principles of hepatic drug clearance By examining the characteristics and behaviour of some drugs and their metabolites it is possible to infer certain characteristics about the aging liver. Phase I of hepatic drug metabolism leads to structural alteration of the metabolized drug by oxidation, reduction and hydrolysis and depends on the amount and activity of the specific microsomal enzymes. Thereafter, phase II metabolism leads to conjugation with chemical ligands such as glucuronide, sulphate, acetate or glutathione, again dependent upon the activity of specific cytosolic enzymes. Hepatic clearance (Clhep) is defined by the equation Clhep =Q × E (1) where Q is the hepatic blood flow and E is the hepatic extraction fraction, the fractional removal of the given drug by the liver. Some substances eliminated by the liver have an extraction rate that approximates unity. This type of metabolism is called flow limited, because hepatic clearance will be almost equal to hepatic blood flow. Using this principle, the clearance of highly extracted like indocyanine green and sorbitol have been used to estimate hepatic blood flow (Zeeh et al., 1988). On the other hand, the clearance of drugs with a low extraction fraction is not influenced by blood flow. The metabolism of these drugs is influenced by intrinsic clearance (a term that describes total enzyme activity and liver mass) and/or protein binding and is termed capacity-limited (Table 4). The clearances of those compounds, such as antipyrine and galactose, have been used to describe liver size. The relationship between hepatic clearance (Clhep) and parameters such as hepatic flow (Q), intrinsic clearance (Clint) and the unbound fraction ( fu) has been described by various models. The simplest model is the venous equilibrium model, which assumes complete mixing of substrates within the liver and can be summarized as follows: Clhep = Q× fu ×Clint . Q +( fu ×Clint) (2) Phase I and phase II are terms used to describe the major enzymatic pathways in the liver, metabolizing drugs and other xenobiotics. Both pathways tend to increase 260 J. Zeeh / Arch. Gerontol. Geriatr. 32 (2001) 255–263 Table 4 Examples of high- and low-extraction drugs (Durnas et al., 1990; Zeeh and Platt, 1993) High-extraction drugs Low-extraction drugs Amitryptiline Bromocriptine Clomethiazole Diltiazem Dothiepin Ergotamine Isosorbitdinitrate Labetalol Metoprolol Nifedipine Propranolol Triamterene Verapamil Antipyrine Carbamazepine Chlordiazepoxide Caffeine Diazepam Flunitrazepam Indomethazine Oxazepam Phenprocoumone Phenytoin Prednisolone Theophylline the water solubility of compounds and facilitate renal excretion. Phase I reactions alter the structure of a compound by oxidation, reduction, hydrolysis and demethylation and are performed mostly by the cytochrome-P (CYP) system of enzymes within the endoplasmic reticulum. Phase II reactions involve the addition of polar chemical groups such as glucuronide, sulphate, glycine, glutathione and acetate. These reactions occur mainly in the cytosol. Recent studies indicate that the activity of phase I enzymes is more dependent on the delivery of oxygen than that of phase II enzymes. Many drugs, including some tricyclic antidepressants and benzodiazepines, are metabolized extensively by both pathways prior to excretion by the urine or bile. Drug elimination can also be influenced by many other factors such as drug absorption, extra-hepatic metabolism, tissue distribution, protein binding and renal excretion. Therefore, in order to assess the effects of age on the liver, it is preferable to examine drugs that are not influenced by renal excretion or other routes of elimination and where absorption is not affected by age. It should be noted that these traditional approaches to the understanding of hepatic drug metabolism make the assumptions that there are no limitations on the supply of oxygen and/or other cofactors and that other substrates including drugs have unimpeded access to enzymes. It also assumes that no changes in physicochemical parameters such as intracellular pH and osmolality will impact on enzyme function. 5. Potential limitations of drug studies on aging The aging process is quite different from person to person, so that among a group of 70-year-old persons, there are a few who appear much younger, those who appear that age and some who appear older. This heterogeneity of an aging cohort of normal individuals is also evident in physiological terms, which affects the metabolism, excretion and toxicity of drugs (Iber et al., 1994). Furthermore, J. Zeeh / Arch. Gerontol. Geriatr. 32 (2001) 255–263 261 subclinical diseases in apparently normal persons, the parallel intake of drugs both over-the-counter and prescribed ones, smoking habits, nutritional differences and other factors may be superimposed to the aging process. In patients, the effects of disease and of comedications need to be considered which makes things even more complex. At all ages there is wide genetic variability or polymorphism in the rates of drug clearance, which is commonly 2–5-fold and occasionally up to 30-fold and clearly persists even in old age. Examples are the acetylator status and differences within the family of cytochrome P-450 isoenzymes. Furthermore, differences in the clearance or hepatic metabolism of one stereoisomer compared with the other have been found for metoprolol, verapamil, salbutamol and felodipine. All these factors provide ‘noise’ and contribute to the wide variability in drug clearance studies performed to detect the effects of aging. 6. Findings, considerations and hypotheses In an attempt to reduce the incidence of adverse drug reactions, much research into the effect of age on drug metabolism has been done. Despite significant efforts, the effect of age on hepatic drug metabolism continues to be a controversial issue. Well known changes in the clearance of drugs that undergo hepatic metabolism may be attributed to age associated alterations in hepatic enzyme activity and, more importantly, to reduced liver size and hepatic blood flow. The ability of the aging liver to metabolize drugs does not decline in a similar way and extent for all pharmacological agents. Several changes in hepatic function and structure have been noted in the elderly, and many of them seem to be of limited relevance. Two of them, however, are of major importance. These are an absolute and relative to body-weight decrease in liver size and mass and a reduced regional blood flow to the organ (Hammerlein et al., 1998). The most frequent ¨ changes involve the mixed-function oxidase system of phase I oxidation and reduction, with little or no change in the processes of phase II conjugation. However, there have also been reports where the activities and amounts of liver microsomal monooxygenases remained unaltered with increasing age (Schmucker et al., 1990). The reasons for these discrepancies between clinically observed decline in drug elimination and unchanged metabolic activity in vitro are probably attributable to interindividual variability, and to the difficulty of getting representative liver samples from healthy elderly subjects. A review of age-related change in drug clearance established that patterns of change are not simply explained in terms of hepatic blood flow, hepatic mass and protein binding changes. In particular, the maintained clearance of drugs subject to conjugation processes while oxygen-dependent metabolism declines and all in-vitro tests of enzyme function have been normal, requires further explanations. By analogy with our understanding of drug metabolism in liver disease, it may be assumed, that a hepatocyte diffusion barrier to oxygen develops with increasing age. This provides a plausible explanation for the paradox and has some experimental support (Le Couteur and McLean, 1998). 262 J. Zeeh / Arch. Gerontol. Geriatr. 32 (2001) 255–263 7. Consequences for clinical practice More than ten years after our previous review of the topic (Zeeh and Platt, 1990) and after an other decade of research into the consequences of aging on drug treatment of the elderly patient, the old and well known aphorism ‘start lower — go slower’ is valid more than ever. 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