Pitfalls of pharmacokinetic dosage guidelines in renal insufficiency

Renal function, bisphenol A, and alkylphenols: results from the National Health and Nutrition Examination Survey (NHANES 2003-2006)

BACKGROUND

Urinary excretion of bisphenol A (BPA) and alkylphenols (APs) was used as a biomarker in most previous studies, but no study has investigated whether urinary excretion of these environmental phenols differed by renal function.

OBJECTIVE

We estimated the association between renal function and urinary excretion of BPA and APs.

METHODS

Analyses were conducted using data from the National Health and Nutrition Examination Survey (NHANES) 2003-2006. Renal function was measured as estimated glomerular filtration rate (eGFR) calculated by the Modification of Diet in Renal Disease (MDRD) Study equation and by the newly developed Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. Regression models were used to calculate geometric means of urinary BPA and APs excretion by eGFR category (≥ 90, 60-90, < 60 mL/min/m(2)) after adjusting for potential confounding factors.

RESULTS

When we used the MDRD Study equation, participants without known renal disease (n=2,573), 58.2% (n=1,499) had mildly decreased renal function or undiagnosed chronic kidney disease. The adjusted geometric means for urinary BPA excretion decreased with decreasing levels of eGFR (p for trend=0.04). The associations appeared primarily in females (p for trend=0.03). Urinary triclosan excretion decreased with decreasing levels of eGFR (p for trend < 0.01) for both males and females, and the association primarily appeared in participants < 65 years of age. The association between BPA and eGFR was nonsignificant when we used the CKD-EPI equation.

CONCLUSIONS

Urinary excretion of triclosan, and possibly BPA, decreased with decreasing renal function. The associations might differ by age or sex. Further studies are necessary to replicate our results and understand the mechanism.

ADME-tox issues for the elderly

BACKGROUND

The elderly, defined as people aged >65 years, are a heterogeneous population. With increasing age there is increased prevalence of disease for which medication may be indicated and increased drug toxicity.

OBJECTIVE/METHODS

This review of the published scientific and clinical data on absorption, distribution, metabolism, excretion and toxicity (ADME-tox) in old age informs prescribers and drug developers of age-related factors that determine drug dose, safety and efficacy.

RESULTS/CONCLUSION

Increased inter-individual variability is a major feature of toxicology in old age. Changes in ADME-tox associated with normal ageing are currently better described than changes in the frail aged. Reduced hepatic and renal clearance are the most significant toxicokinetic changes seen in normal ageing. Drug toxicity in old age is influenced by changes in the quantity, affinity and responsiveness of drug targets, physiologic reserve and response to injury. Further investigation of the effects of medications on cognitive and physical functions in older adults is required.

Geriatric pharmacology

When faced with the geriatric dog or cat, the practitioner should consider the following: 1. Avoid using any drugs at all unless there are definite therapeutic indications. If the patient has some degree of renal insufficiency, try to select drugs that are hepatically metabolized and excreted in bile rather than eliminated by the kidneys (eg, doxycycline, tolfenamic acid). If hepatic insufficiency is present, select drugs that do not undergo metabolism before renal excretion (eg, penicillins, cephalosporins). 2. If therapeutic drug monitoring is available, tailor the drug dosage regimen to that specific patient (eg, phenobarbital, digoxin, amino-glycosides). 3. If therapeutic drug monitoring is unavailable, determine if there are clinically proven adjusted dosage regimens for specific drugs. The package insert on human pharmaceutics often gives guidelines for adjusting dosages in geriatric patients. 4. If the drug has not been sufficiently studied to have dosage adjustment recommendations, determine if there is sufficient information about its kinetics to estimate the proper drug dose in a geriatric patient. Some general guidelines for commonly used drugs in geriatric veterinary patients are provided in Table 1. In general, if the Vd changes in your patient, change the dose. If the elimination half-life changes, change the dosing interval. 5. Carefully monitor treated patients for signs of efficacy and toxicity.

Drug dosing in chronic kidney disease

Patients with chronic kidney disease (CKD) are at high risk for adverse drug reactions and drug-drug interactions. Drug dosing in these patients often proves to be a difficult task. Renal dysfunction-induced changes in human pathophysiology regularly results may alter medication pharmacodynamics and handling. Several pharmacokinetic parameters are adversely affected by CKD, secondary to a reduced oral absorption and glomerular filtration; altered tubular secretion; and reabsorption and changes in intestinal, hepatic, and renal metabolism. In general, drug dosing can be accomplished by multiple methods; however, the most common recommendations are often to reduce the dose or expand the dosing interval, or use both methods simultaneously. Some medications need to be avoided all together in CKD either because of lack of efficacy or increased risk of toxicity. Nevertheless, specific recommendations are available for dosing of certain medications and are an important resource, because most are based on clinical or pharmacokinetic trials.

Drug therapy in the elderly

Drug dosage in the elderly requires an understanding of the age-dependent changes in drug disposition and sensitivity. The most important pharmacokinetic alteration is a decline in renal function, the elderly should therefore be treated as renally insufficient patients. Metabolic clearance is primarily reduced with drugs that display high hepatic extraction, whereas the metabolism of drugs with low hepatic extraction usually is not diminished. The reduction of metabolic clearance is especially pronounced in malnourished or frail patients. The water content of the aging body decreases, the fat content rises. Hence the distribution volume of hydrophilic drugs may be reduced in the elderly, resulting in increased plasma concentrations. In contrast, the distribution volume of liphophilic drugs is increased, their plasma concentrations may decrease. Intestinal absorption of most drugs is not altered in the elderly. Aside of these pharmacokinetic changes, one of the characteristics of old age is a progressive decline in counterregulatory (homeostatic) mechanisms. Therefore, drug effects are attenuated less, the responses are usually stronger than in younger subjects, the rate and intensity of adverse effects are higher. Examples of drug actions augmented is this manner are postural hypotension with agents that lower blood pressure, dehydration and electrolyte disturbances in response to diuretics, bleeding complications with oral anticoagulants, hypoglycemia with antidiabetics, and gastrointestinal irritation with non-steroidal anti-inflammatory drugs. The brain is an especially sensitive drug target in old age. Psychotropic drugs, anticonvulsants, and centrally acting antihypertensives may impede intellectual function and motor coordination. Hence drugs should be used restrictively in geriatric patients.

Hepatic drug metabolism and transport in patients with kidney disease

The disposition of many drugs is altered in patients with acute (AKD) and chronic kidney disease (CKD). A decline in renal clearance of several drugs has been correlated significantly with residual renal function (ie, creatinine clearance) of subjects. Reductions in nonrenal clearance of some compounds also have been reported and associated with clearance of markers of oxidative and/or conjugative metabolism or P-glycoprotein-mediated transport. Although initial accounts of reduced hepatic microsomal cytochrome P-450 (CYP) content and activity in animal models of AKD and CKD were published almost 25 years ago, it is only in the last decade that technical advances in molecular biology and clinical pharmacology have enabled researchers to begin to characterize the phenotypic expression of individual enzymes and, importantly, distinguish the molecular and/or genetic basis for these changes. The selective modulation of hepatic CYP enzyme activity observed in kidney disease is caused, at least in part, by differentially altered expression of several CYP isoforms. This review summarizes data available through June 2003 regarding the effect of AKD and CKD on drug metabolism. Knowledge of the impact and nature of these alterations associated with kidney disease may facilitate the individualization of medication management in this patient population.

When drug therapy gets old: pharmacokinetics and pharmacodynamics in the elderly

The age-related changes in the functions and composition of the human body require adjustments of drug selection and dosage for old individuals. Drug excretion via the kidneys declines with age, the elderly should therefore be treated as renally insufficient patients. The metabolic clearance is primarily reduced with drugs that display high hepatic extraction ('blood flow-limited metabolism'), whereas the metabolism of drugs with low hepatic extraction ('capacity-limited metabolism') usually is not diminished. Reduction of metabolic drug elimination is more pronounced in malnourished or frail subjects. The water content of the aging body decreases, the fat content rises, hence the distribution volume of hydrophilic compounds is reduced in the elderly, whereas that of lipophilic drugs is increased. Intestinal absorption of most drugs is not altered in the elderly. Aside of these pharmacokinetic changes, one of the characteristics of old age is a progressive decline in counterregulatory (homeostatic) mechanisms. Therefore drug effects are mitigated less, the reactions are usually stronger than in younger subjects, the rate and intensity of adverse effects are higher. Examples of drug effects augmented is this manner are postural hypotension with agents that lower blood pressure, dehydration, hypovolemia, and electrolyte disturbances in response to diuretics, bleeding complications with oral anticoagulants, hypoglycemia with antidiabetics, and gastrointestinal irritation with non-steroidal anti-inflammatory drugs. The brain is an especially sensitive drug target in old age. Psychotropic drugs but also anticonvulsants and centrally acting antihypertensives may impede intellectual functions and motor coordination. The antimuscarinic effects of some antidepressants and neuroleptic drugs may be responsible for agitation, confusion, and delirium in elderly. Hence drugs should be used very restrictively in geriatric patients. If drug therapy is absolutely necessary, the dosage should be titrated to a clearly defined clinical or biochemical therapeutic goal starting from a low initial dose.

Physiological modelling of renal drug clearance

A physiological model of renal drug clearance is presented with the aim of establishing a basis for adjusting drug dosing regimens in renal insufficiency. In agreement with the morphology of blood supply to the nephron, the model assumes serial arrangement of the processes involved in drug excretion. Fractional extraction by filtration in the glomeruli is defined in terms of the product of the unbound fraction of the drug, the filtration fraction being responsible for the limited extraction efficiency of this process. For a description of the limitations of the tubular secretory process by plasma flow through peritubular capillaries, the parallel tube model is utilized. The assumption of direct proportionality between the transport maximum of the secretory process and filtrate flow in the tubules permits a quantitative comparison of the intrinsic tubular secretion clearance and the effectiveness of the filtration process. Provided that the secretory mechanism is highly effective, renal clearance becomes dependent only on kidney plasma flow and the fraction of drug not reabsorbed in the tubules. Tubular reabsorption results only in a proportional decrease in renal clearance. The model predicts proportionality of renal drug clearance to GFR, which as a rule is used for dosage adjustment of drugs in renal insufficiency, only for compounds exclusively excreted by filtration. Compounds also excreted by tubular secretion in general exhibit a curvilinear relationship. The curvature is less pronounced as an increasing fraction of the drug is protein bound in blood. Therefore, for dosage adjustment of drugs secreted in the tubules and highly bound in blood, proportionality between renal clearance and GFR can serve as a reasonable approximation.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of indomethacin in the elderly

In a cross-sectional study the pharmacokinetics of indomethacin were studied in old and young adults without manifest organ failure. Total clearance of indomethacin after a single oral dose of 50mg was 0.8 ml/min/kg in elderly individuals (mean 79.5 +/- 1.3 years) compared with 1.4 ml/min/kg in younger individuals (mean 36.9 +/- 3.0 years). The apparent elimination rate constant averaged 0.23 h-1 in the aged and 0.32 h-1 in the young people. Oral bioavailability was close to 1 in the young but 0.77 in the elderly. The apparent volume of distribution was similar in each group. Based on these results it is suggested that the maintenance dose of indomethacin be reduced by 25% in the elderly.