Hepatic drug metabolism and aging

Utilization of Lower-Dose Cyclobenzaprine in the Older Inpatient

Background In older inpatients, anticholinergic medications can increase the risk of complications that may increase length of stay (LOS). Cyclobenzaprine is an anticholinergic medication associated with mental status changes, falls, and injuries in older patients. Objective The purpose of this study is to determine whether use of a lower cyclobenzaprine dose (5 mg) compared with higher dosing (10 mg) will affect LOS, 30-day readmission rates, and need for injectable psychotropic agents in inpatients 65 years of age and older. Methods This was a retrospective cohort analysis comparing outcomes in patients 65 years of age and older who received either a 5 mg or 10 mg cyclobenzaprine dose during their inpatient admission over a 2.5-year period. The primary outcome was hospital LOS, adjusted using multivariate linear regression. Secondary outcomes included 30-day readmission rate adjusted using logistic regression and use of injectable antipsychotics or benzodiazepines. A sub-analysis evaluated the impact of the institution's implementation of a geriatric prescribing context (GEM-CON) on cyclobenzaprine dose selection. Results The adjusted LOS was 32.7% longer (95% CI 25.9%-39.9%) for patients exposed to higher-dose cyclobenzaprine. Use of injectable antipsychotics or benzodiazepines was also significantly greater in the higher-dose group (P < 0.001; P = 0.025). Cyclobenzaprine dose was not significantly associated with readmission on multivariate analysis (OR = 0.93, 95% CI 0.45-1.93). After GEM-CON implementation, there was a significant increase in use of the recommended lower cyclobenzaprine dose (P < 0.001). Conclusion Use of lower cyclobenzaprine dosing in older inpatients is associated with reduced hospital LOS and need for injectable antipsychotics and benzodiazepines.

Comparison of Hepatic Metabolite Profiles between Infant and Adult Male Mice Using 1H-NMR-Based Untargeted Metabolomics

Although age-related characteristics of hepatic metabolism are reported, those in infants are not fully understood. In the present study, we performed untargeted metabolomic profiling of the livers of infant (3-week-old) and adult (9-week-old) male ICR mice using ¹H-NMR spectroscopy and compared 35 abundant hepatic metabolite concentrations between the two groups. The liver/body weight ratio did not differ between the two groups; however, serum glucose, blood urea nitrogen, total cholesterol, and triglyceride concentrations were lower in infants than in adults. Hepatic carbohydrate metabolites (glucose, maltose, and mannose) were higher, whereas amino acids (glutamine, leucine, methionine, phenylalanine, tyrosine, and valine) were lower in infant mice than in adult mice. The concentrations of ascorbate, betaine, sarcosine, and ethanolamine were higher, whereas those of taurine, inosine, and O-phosphocholine were lower in infant mice than in adult mice. The differences in liver metabolites between the two groups could be due to differences in their developmental stages and dietary sources (breast milk for infants and laboratory chow for adults). The above results provide insights into the hepatic metabolism in infants; however, the exact implications of the findings require further investigation.

Alternation of EEG Characteristics During Transcutaneous Acupoint Electrical Stimulation-Induced Sedation

Recent studies have shown that applying acupuncture during general anesthesia can reduce the dosage of anesthetics. Hence, it is speculated that acupuncture may have a sedative effect. However, existing studies employed acupuncture in combination with anesthetics, which makes determine acupuncture's role in producing sedation difficult. In this work, we investigated the sedative effect of acupuncture by using transcutaneous acupoint electrical stimulation (TAES) at bilateral Zusanli (ST36), Shenmen (HT7) and Sanyinjiao (SP6). Using a cross-over design, 2 separate sessions, that are, the resting and TAES sessions, were conducted for each subject. The sedative effect was quantified by using the bispectral index (BIS). The difference in brain activities between resting and TAES sessions was investigated by analyzing the simultaneously recorded EEG signals. Our results showed that a statistically significant difference in BIS values existed between resting and TAES sessions, which suggested that TAES alone was capable of inducing observable sedation. Using power spectrum analysis, we showed that TAES-induced sedation was accompanied by a reduction in alpha band power and an increment in delta band power. Permutation entropy was lower during the TAES session, which suggested that TAES reduced the complexity of the EEG signal. Moreover, a significant reduction in the global strength of brain functional connections was observed during TAES. These findings suggest that TAES alone can induce observable sedative effects, and this sedation effect is accompanied by changes in brain activities that have shown to be correlated with consciousness.

Ontogeny of Drug-Metabolizing Enzymes

Almost 50% of prescription drugs lack age-appropriate dosing guidelines and therefore are used "off-label." Only ~10% drugs prescribed to neonates and infants have been studied for safety or efficacy. Immaturity of drug metabolism in children is often associated with drug toxicity. This chapter summarizes data on the ontogeny of major human metabolizing enzymes involved in oxidation, reduction, hydrolysis, and conjugation of drugs. The ontogeny data of individual drug-metabolizing enzymes are important for accurate prediction of drug pharmacokinetics and toxicity in children. This information is critical for designing clinical studies to appropriately test pharmacological hypotheses and develop safer pediatric drugs, and to replace the long-standing practice of body weight- or surface area-normalized drug dosing. The application of ontogeny data in physiologically based pharmacokinetic model and regulatory submission are discussed.

Serum estradiol level according to dose and formulation of oral estrogens in postmenopausal women

This study was performed to evaluate serum estradiol level in postmenopausal women using oral menopausal hormone therapy (MHT) with different doses and formulations of estrogens. A total of 344 postmenopausal women who received oral MHT was included in this cross-sectional study. Serum estradiol level was compared according to formulation (estradiol hemihydrate [EH] or valerate [EV], conjugated estrogen [CE]) and dose (estradiol 1 or 2 mg, CE 0.45 or 0.625 mg) of the estrogens. Mean age and years since menopause were 56.9 and 7.9 years, respectively. Mean duration of MHT was 27.4 months. Since serum estradiol levels were not significantly different at either dose, EH and EV at the same dose were combined for comparisons: estradiol 1 mg and 2 mg. The serum estradiol level with estradiol 2 mg (107.6 pg/mL) was significantly higher by 60% than with estradiol 1 mg (65.8 pg/mL) or CE 0.45 mg (60.1 pg/mL), and it was also significantly higher than with CE 0.625 mg (76.8 pg/mL). Our findings suggest that serum estradiol level is not directly proportional to estrogen dose. In terms of serum concentration, CE 0.45 mg is equivalent to estradiol 1 mg.

Clinical pharmacology of the dual orexin receptor antagonist ACT-541468 in elderly subjects: Exploration of pharmacokinetics, pharmacodynamics and tolerability following single-dose morning and repeated-dose evening administration

BACKGROUND

The dual orexin receptor antagonist ACT-541468 showed sedative pharmacodynamic effects during initial clinical testing in adult subjects. The present study explored pharmacokinetics, pharmacodynamics and tolerability in healthy elderly subjects.

METHODS

Double-blind, placebo-controlled, randomised, single-ascending dose study in 24 male/female elderly (65-80 years, 5, 15 and 25 mg in the morning, 6/2 active/placebo per group). Additionally, 10 subjects (8/2 active/placebo) received 25 mg for 7 days in the evening. Pharmacokinetics, pharmacodynamics (saccadic peak velocity, adaptive tracking, body sway, visual analogue scales according to Bowdle and Bond and Lader, Karolinska Sleepiness Scale) and tolerability were assessed. In particular, pharmacodynamics results are to be interpreted exploratorily.

RESULTS

Absorption was quick with a median time to maximum concentration of ∼ 1.0 h. The mean elimination half-life was 8.5-9.8 h, the area under the curve and the maximum plasma concentration increased proportionally with dose. Following repeated evening administration of 25 mg, minimal accumulation was observed. There were no pharmacodynamic effects at 5 mg. At 15 mg, saccadic peak velocity (degree/s; SD) was reduced (69; 38), while other variables showed no effects. At 25 mg, effects on all objective pharmacodynamic parameters were observed. At 8-12 h post-dose, there were no differences to placebo and no next-day effects on pharmacodynamic variables after evening administration. Elderly subjects reported fewer adverse events compared to adults in previous studies.

CONCLUSION

ACT-541468 in elderly subjects was well tolerated and pharmacokinetics and pharmacodynamics are compatible with a drug for the treatment of insomnia. Clinicaltrials.gov: NCT02571855.

Age-associated changes of cytochrome P450 and related phase-2 gene/proteins in livers of rats

Cytochrome P450s (CYPs) are phase-I metabolic enzymes playing important roles in drug metabolism, dietary chemicals and endogenous molecules. Age is a key factor influencing P450s expression. Thus, age-related changes of CYP 1–4 families and bile acid homeostasis-related CYPs, the corresponding nuclear receptors and a few phase-II genes were examined. Livers from male Sprague-Dawley rats at fetus (−2 d), neonates (1, 7, and 14 d), weanling (21 d), puberty (28 and 35 d), adulthood (60 and 180 d), and aging (540 and 800 d) were collected and subjected to qPCR analysis. Liver proteins from 14, 28, 60, 180, 540 and 800 days of age were also extracted for selected protein analysis by western blot. In general, there were three patterns of their expression: Some of the drug-metabolizing enzymes and related nuclear receptors were low in fetal and neonatal stage, increased with liver maturation and decreased quickly at aging (AhR, Cyp1a1, Cyp2b1, Cyp2b2, Cyp3a1, Cyp3a2, Ugt1a2); the majority of P450s (Cyp1a2, Cyp2c6, Cyp2c11, Cyp2d2, Cyp2e1, CAR, PXR, FXR, Cyp7a1, Cyp7b1. Cyp8b1, Cyp27a1, Ugt1a1, Sult1a1, Sult1a2) maintained relatively high levels throughout the adulthood, and decreased at 800 days of age; and some had an early peak between 7 and 14 days (CAR, PXR, PPARα, Cyp4a1, Ugt1a2). The protein expression of CYP1A2, CYP2B1, CYP2E1, CYP3A1, CYP4A1, and CYP7A1 corresponded the trend of mRNA changes. In summary, this study characterized three expression patterns of 16 CYPs, five nuclear receptors, and four phase-II genes during development and aging in rat liver, adding to our understanding of age-related CYP expression changes and age-related disorders.

Mitochondrial form, function and signalling in aging

Aging is often accompanied by a decline in mitochondrial mass and function in different tissues. Additionally, cell resistance to stress is frequently found to be prevented by higher mitochondrial respiratory capacity. These correlations strongly suggest mitochondria are key players in aging and senescence, acting by regulating energy homeostasis, redox balance and signalling pathways central in these processes. However, mitochondria display a wide array of functions and signalling properties, and the roles of these different characteristics are still widely unexplored. Furthermore, differences in mitochondrial properties and responses between tissues and cell types, and how these affect whole body metabolism are also still poorly understood. This review uncovers aspects of mitochondrial biology that have an impact upon aging in model organisms and selected mammalian cells and tissues.

Alcohol-Related Diagnoses in Hospital Admissions for All Causes Among Middle-Aged and Older Adults: Trends and Cohort Differences From 1993 to 2010

OBJECTIVE

This aim of this study was to characterize trends in alcohol-related hospital admissions among middle-aged and older adults from 1993 to 2010 in relation to age, gender, race, and cohort membership.

METHOD

This study utilized repeated cross-sectional data from the Nationwide Inpatient Sample. Using alcohol-related classified admissions, yearly rates and longitudinal trends of alcohol-related inpatient hospitalizations based on age, period, birth cohort, gender, and race were estimated.

RESULTS

Among those aged 45 and older, admissions rose from an estimated 610,634 to more than 1,134,876, and rates of any alcohol-related diagnosis also increased from 1993 to 2010. Rates for men were consistently higher than women, and rates for Blacks were higher than Whites. Age was associated with decreasing rates, but post-World War II cohorts displayed higher rates over time.

DISCUSSION

Rates of alcohol-related admissions are increasing among adults above age 45, which may be a function of cohort effects. Training the health care workforce is crucial to respond to this trend.

Irinotecan, a key chemotherapeutic drug for metastatic colorectal cancer

Irinotecan hydrochloride is a camptothecin derivative that exerts antitumor activity against a variety of tumors. SN-38 produced in the body by carboxylesterase is the active metabolite of irinotecan. After irinotecan was introduced for the treatment of metastatic colorectal cancer (CRC) at the end of the last century, survival has improved dramatically. Irinotecan is now combined with 5-fluorouracil, oxaliplatin and several molecularly-targeted anticancer drugs, resulting in the extension of overall survival to longer than 30 mo. Severe, occasionally life-threatening toxicity occurs sporadically, even in patients in relatively good condition who have a low risk of chemotherapy-induced toxicity, often causing the failure of irinotecan-based chemotherapy. Clinical pharmacological studies have revealed that such severe toxicity is related to exposure to SN-38 and genetic polymorphisms in UDP-glucuronosyltransferase 1A1 gene. The large inter- and intra-patient variability in systemic exposure to SN-38 is determined not only by genetic factors but also by physiological and environmental factors. This review first summarizes the roles of irinotecan in chemotherapy for metastatic CRC and then discusses the optimal dosing of irinotecan based on the aforementioned factors affecting systemic exposure to SN-38, with the ultimate goal of achieving personalized irinotecan-based chemotherapy.

Gender differences in stressful life events, social support, perceived stress, and alcohol use among older adults: results from a National Survey

Stressful life events, perceived stress, and social support relationships with consumption, at-risk drinking, and alcohol use disorder (AUD) were studied in a population-based sample of current drinkers age 60+ in the National Epidemiologic Survey of Alcohol and Related Conditions (Wave 2; 2004-2005; n = 4,360). Stressful life events were associated with AUD among men and women, and crime victimization among men only. However, greater perceived stress was associated with lower consumption among women and greater odds of AUD in men, highlighting differences in the relationship between stress and alcohol use by gender that may be the result of the stress alcohol link.

Effect of caloric restriction on hepatic sinusoidal system and stellate cells in mice

Aging associated changes in liver include reduced hepatic blood flow, increased number of stellate cells, and collagen deposits in perisinusoidal space. We tested the possibility of mitigating these changes with caloric restriction. Two-month-old mice were subjected to 30 percent caloric restriction for 12 months and then examined for the effect of caloric restriction on the sinusoidal network, collagen deposition, and the number of stellate cells. Using intravital fluorescence microscopy, assessments were made on sinusoidal diameter, density, volumetric flow, perfusion index, and autofluorescence of vitamin A that was primarily stored with lipid droplets in stellate cells. A significant effect was observed in the vitamin A autofluorescence of stellate cells; stellate cell associated fluorescence was diminished in terms of number and size of fluorescent spots. Caloric restriction reduced collagen deposits in liver sections and lowered the gene expression of α1-(I) collagen but not α-smooth muscle actin. No differences were detected in sinusoidal dimension measurements. Our results showed that caloric restriction was effective in ameliorating the increase in stellate cells and the mild fibrosis in old mice. However, caloric restriction had no impact on stellate cell activity level as indicated by the unaffected α-smooth muscle actin expression.

Optimization of cancer chemotherapy on the basis of pharmacokinetics and pharmacodynamics: from patients enrolled in clinical trials to those in the 'real world'

  Cytotoxic anticancer drugs are the most challenging therapeutic agents among all medicines with relatively narrow efficacy profiles. Therefore, medical oncologists have to practically manage the risk of severe toxic effects to optimize treatment outcomes. Dose and treatment-schedule recommendations for cytotoxic anticancer agents are determined on the basis of clinical trials. Patients enrolled in clinical trials are those likely to receive the drug in clinical practice, excluding those with conditions such as organ dysfunction, obesity, advanced age, or comorbidity. On the other hand, the 'real world' includes large numbers of such patients who do not meet the eligibility criteria of clinical trials. However, there is a paucity of data from sufficiently powered pharmacokinetic and pharmacodynamic studies to support dosage recommendations in such patients. Consequently, dose levels and treatment schedules for chemotherapy in these subjects are somewhat arbitrary and not evidence-based. Pharmacokinetic and pharmacodynamic studies of patients in the 'real world' are needed to address this issue. In this review article, we describe general aspects of clinical pharmacology in cancer patients enrolled in clinical trials and those in the 'real world,' and introduce recent findings regarding the pharmacokinetic and pharmacodynamic properties of irinotecan and S-1 in 'real world' cancer patients.

CYP2D6 genotype affects age-related decline in flecainide clearance: a population pharmacokinetic analysis

OBJECTIVE

To investigate the association between age-related decline in flecainide clearance and CYP2D6 genotype, we conducted a population pharmacokinetic analysis of flecainide using routine therapeutic drug monitoring data.

METHODS

Population pharmacokinetic analysis was performed on retrospective data from 163 genotyped patients treated with oral flecainide for supraventricular tachyarrhythmias. The CYP2D6 genotype was categorized as CYP2D6 homozygous extensive metabolizers (hom-EMs; n=57), heterozygous extensive metabolizers (het-EMs; n=79), and intermediate metabolizers and poor metabolizers (IMs/PMs; n=27).

RESULTS

Population pharmacokinetic analysis revealed that estimated glomerular filtration rate, body weight, female sex, and aging were important factors for estimating flecainide clearance. The metabolic clearance was decreased age dependently in a curvilinear fashion, where the lower clearance was observed in greater than 60 years for het-EMs and greater than 55 years for IMs/PMs. The reduction in metabolic clearance in elderly (70 years) patients compared with middle-aged (52 years) patients was different among the CYP2D6 genotype groups: 22.1 and 49.5% in CYP2D6 het-EMs and IMs/PMs, respectively, and no change in hom-EMs. A 11.4% reduction in estimated glomerular filtration rate in elderly patients compared with middle-aged patients corresponded to 6.1% decline in flecainide clearance. Overall, the age-related decline in flecainide clearance was 6.1% in hom-EMs, 16.3% in het-EMs, and 28.9% in IMs/PMs groups.

CONCLUSION

This study suggests that CYP2D6 genotype is a determinant factor of age-related decline in flecainide clearance.

Predicted metabolic drug clearance with increasing adult age

AIM

To determine the effect of increasing adult age on predicted metabolic drug clearance.

METHOD

Predicted metabolic drug clearances (CLPT ) were determined using in vitro-in vivo extrapolation coupled with physiological-based pharmacokinetic modelling and simulation (IVIVE-PBPK) in Simcyp®. Simulations were conducted using CYP-selective 'probe' drugs with subjects in 5 year age groups (20-25 to 90-95 years). CLPT values were compared with human pharmacokinetic data stratified according to age (young = 20-40 years and elderly = 65-85 years) and gender. Age-related changes in the physiological parameters used for IVIVE of CLPT were described.

RESULTS

Predicted metabolic drug clearances decreased with increasing adult age to approximately 65-70 years: caffeine from 1.5 to 1.0 ml min(-1)  kg(-1) (a 33% decrease), S-warfarin from 0.100 to 0.064 ml min(-1)  kg(-1) (36%), S-mephenytoin from 4.1 to 2.5 ml min(-1)  kg(-1) (39%), desipramine from 10.6 to 7.3 ml min(-1)  kg(-1) (31%) and midazolam from 5.4 to 3.9 ml min(-1)  kg(-1) (27%). Except for S-mephenytoin, predictions were within 3.5-fold of clearances from clinical studies when stratified by age and gender. A trend towards higher CLPT was observed in females, but this was only statistically significant in larger virtual trials. Physiological parameters that determine CLPT decreased with increasing adult age: mean microsomal protein g(-1) of liver, liver weight, hepatic blood flow and human serum albumin concentration.

CONCLUSION

Decreased metabolic clearance in the elderly was predicted by Simcyp® and was generally consistent with limited clinical data for four out of five drugs studied and the broader literature for drugs metabolized by CYP enzymes. IVIVE-PBPK may be increasingly useful in predicting metabolic drug clearance in the elderly.

Aging and Clinical Pharmacology: Implications for Antidepressants

The elderly frequently have changes in pharmacokinetics, sensitivity to medications, homeostatic reserve (ability to tolerate physiological challenges), exposure to multiple medications, and adherence. All of these age-associated factors can potentially influence total exposure to medication, adverse effects, and subsequent treatment outcome. Most clinical trials are performed with healthy, younger adults. Extrapolating the results of these trials to the elderly may be inappropriate, particularly for the antidepressant treatment of depression. The authors review these age-associated differences and discuss their implications for antidepressant use in older adults.

Adult age and ex vivo protein binding of lorazepam, oxazepam and temazepam in healthy subjects

AIM

To see if adult age correlates with ex vivo protein binding of lorazepam, oxazepam and temazepam in healthy subjects.

METHODS

Sixty healthy drug free subjects were recruited in the age groups 18-39, 40-64 and ≥65 years. Plasma albumin concentrations were determined. Ex vivo unbound fractions (f(u)) were assessed by spiking samples and measuring the free and total concentrations.

RESULTS

No correlation of age with f(u) was seen. The study was powered to demonstrate a change in f(u) of ≥7-10%. A decline in plasma albumin concentration of ~0.03 g l(-1) year(-1) was seen with increasing age (P= 0.032) and was associated with increased f(u) of lorazepam (P= 0.009) and oxazepam (P= 0.014).

CONCLUSIONS

There was no association of adult age with ex vivo f(u) of lorazepam, oxazepam or temazepam in healthy subjects.

Pharmacokinetic considerations for digoxin in older people

OBJECTIVE

This review aims to arm readers with a deep understanding of pharmacokinetics of digoxin.

DATA SOURCES

Pharmacology and pharmacokinetic references texts, and peer reviewed medical journal manuscripts indexed on Medline included based on currency, accuracy and appropriateness.

RESULTS

Physiologic changes and disease associated with aging have an impact on pharmacokinetics and pharmacodynamics of medications. Altered drug response and increased adverse reactions are common amongst the elderly. The narrow therapeutic index of digoxin and pharmacokinetic changes associated with aging increases the risk of toxicity. In the aging population, a number of factors combine to increase the risk, severity and likelihood of hospitalisation or death due to adverse drug effects: changes to absorption, distribution, metabolism and excretion,increased susceptibility to drug sensitivity,co-existing pathology,polypharmacy.

CONCLUSION

A thorough understanding of digoxin pharmacokinetics in the older person is essential for improved therapeutic outcomes, improved compliance, reduced morbidity and improved quality of life.

Influence of age and obesity on serum estradiol, estrone, and sex hormone binding globulin concentrations following oral estrogen administration in postmenopausal women

OBJECTIVE

Hormone therapy (HT) increases the risk of venous thrombosis and stroke. Risk of venous thrombosis and stroke is higher in older, overweight, and obese women using HT. However, the impact of age and obesity on estrogen concentrations among HT users is not well defined.

METHOD

We measured serum levels of estrone, total and free estradiol, and SHBG in 180 postmenopausal women participating in the Estrogen in the Prevention of Atherosclerosis Trial (EPAT), 91 receiving estradiol therapy (ET) and 89 taking placebo, every 6 months over 2 yr. Mean on-trial levels of estrogens and SHBG were compared across age, body mass index (BMI), and waist to hip ratio categories among ET users and placebo separately.

RESULTS

Among the ET users, total (P = 0.01) and free estradiol (P = 0.002) were significantly directly associated with BMI adjusted for age. SHBG was inversely related to waist to hip ratio adjusted for age (P = 0.005). Age was not associated with any of the estrogen or SHBG concentrations in ET or placebo groups. BMI was positively associated with estrone concentrations among older but not younger ET users (P for interaction = 0.03).

CONCLUSION

Overweight and obese women using ET attain greater concentrations of estrogen compared to women with normal BMI, whereas ET users with abdominal obesity attain lower SHBG levels. Obese older women using ET have the highest concentration of estrone. It may be useful to consider age and obesity when prescribing HT to minimize the risk of venous thrombosis or stroke in postmenopausal women. Further research regarding relationships among circulating hormone levels and risk for these conditions is required to substantiate this conclusion.

Effect of aging on glucuronidation of valproic acid in human liver microsomes and the role of UDP-glucuronosyltransferase UGT1A4, UGT1A8, and UGT1A10

Valproic acid (VPA) is a widely used anticonvulsant that is also approved for mood disorders, bipolar depression, and migraine. In vivo, valproate is metabolized oxidatively by cytochromes P450 and beta-oxidation, as well as conjugatively via glucuronidation. The acyl glucuronide conjugate (valproate-glucuronide or VPAG) is the major urinary metabolite (30-50% of the dose). It has been hypothesized that glucuronidation of antiepileptic drugs is spared over age, despite a known decrease in liver mass. The formation rates of VPAG in a bank of elderly (65 years onward) human liver microsomes (HLMs) were measured by liquid chromatography/tandem mass spectrometry and compared with those in a younger (2-56 years) HLM bank. In vitro kinetic studies with recombinant UDP-glucuronosyltransferases (UGTs) were completed. A 5- to 8-fold variation for the formation of VPAG was observed within the microsomal bank obtained from elderly and younger donors. VPAG formation ranged from 6.0 to 53.4 nmol/min/mg protein at 1 mM substrate concentration (n=36). The average velocities at 0.25, 0.5, and 1 mM VPA were 7.0, 13.4, and 25.4 nmol/min/mg protein, respectively, in the elderly HLM bank. Rates of VPAG formation were not significantly different in the HLM bank obtained from younger subjects. Intrinsic clearances (V(max)/K(m)) for several cloned, expressed UGTs were determined. UGT1A4, UGT1A8, and UGT1A10 also were found to catalyze the formation of VPAG in vitro. This is the first reported activity of these UGTs toward VPA glucuronidation. UGT2B7 had the highest intrinsic clearance, whereas UGT1A1 demonstrated no activity. In conclusion, our investigation revealed no differences in VPAG formation in younger versus elderly HMLs and revealed three other UGTs that form VPAG in vitro.

Pharmacokinetic-Pharmacodynamic Crisis in the Elderly

Advances in medical technology have led to improved survival after catastrophic illnesses. Many of the survivors require ongoing care including tracheostomy, mechanical ventilation, tube feedings, and indwelling venous catheters. Repeated hospitalizations may be necessary to treat infectious complications resulting from resistant organisms requiring intravenous antibiotic therapy. Because prolonged intravenous access may be difficult or even impossible in these patients, alternative means of therapy are necessary. Linezolid is the first of a new class of antimicrobial agents known as the oxazolidinones with activity against gram-positive bacteria similar to that of vancomycin and yet its oral bioavailability allows for enteral administration. We present our retrospective experience with oral linezolid in a cohort of pediatric intensive care unit patients. Primary infectious disease issues included endocarditis, tracheitis, pneumonia, or central line sepsis resulting from Staphylococcus epidermidis, methicillin-resistant Staphylococcus aureus, and Enterococcus. Treatment was initiated with vancomycin and changed to enteral linezolid (10 mg/kg every 12 hours). The duration of therapy with linezolid varied from 7 days to 6 weeks. All of the patients were discharged home to complete their course of enteral linezolid. No complications related to linezolid therapy were noted, and all of the patients completed their prescribed course of therapy without the need for rehospitalization. Our preliminary experience suggests that oral linezolid offers an effective alternative to intravenous vancomycin for the treatment of infections resulting from gram-positive bacteria and avoids the need for prolonged vascular access.

Risks of combined alcohol/medication use in older adults

BACKGROUND

Many older adults (ie, those aged >65 years) drink alcohol and use medications that may be harmful when consumed together.

OBJECTIVE

This article reviews the literature on alcohol and medication interactions, with a focus on older adults.

METHODS

Relevant articles were identified through a search of MEDLINE and International Pharmaceutical Abstracts (1966-August 2006) for English-language articles. The following medical subject headings and key words were used: alcohol medication interactions, diseases worsened by alcohol use, and alcohol metabolism, absorption, and distribution. Additional articles were identified by a manual search of the reference lists of the identified articles, review articles, textbooks, and personal reference sources.

RESULTS

Many older adults drink alcohol and take medications that may interact negatively with alcohol. Some of these interactions are due to age-related changes in the absorption, distribution, and metabolism of alcohol an medications. Others are due to disulfiram-like reactions observed with some medications, exacerbation of therapeutic effects and adverse effects of medications when combined with alcohol, and alcohol's interference with the effectiveness of some medications.

CONCLUSIONS

Older adults who drink alcohol and who take medications are at risk for a variety of adverse consequences depending on the amount of alcohol and the type of medications consumed. It is important for clinicians to know how much alcohol their older patients are drinking to be able to effectively assess their risks and to counsel them about the safe use of alcohol and medications. Similarly, it is important for older adults to understand the potential risks of their combined alcohol and medication use to avoid the myriad of problems possible with unsafe use of these substances..

Lymphoma in Older Patients

One half of patients newly diagnosed with lymphoma are older than 60 years and a significant proportion of them older than 80 years. Older patients treated for lymphoma may not tolerate the high-dose therapies used in younger patients, usually because of the presence of concomitant diseases. Diffuse large B-cell lymphoma represents more than 60% of all lymphomas seen in older patients. Clinical presentation and prognostic parameters are identical to those described in young patients. However, response rate is usually lower in elderly patients compared with young patients, even if the patients are treated with the cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) regimen. Therefore, event-free and overall survival rates are shorter in elderly patients, even if disease-free survival rates are not really shorter than in young patients. Rituximab added to the CHOP regimen has recently been shown to dramatically improve the survival of these older patients without increasing the toxicity of the treatment. Patients older than 80 years may also be treated with rituximab plus CHOP, except for those having severe organ failure secondary to other diseases. Very few of these older patients may benefit from a salvage treatment after relapse.

Effects of repeated green tea catechin administration on human cytochrome P450 activity

PURPOSE

Preclinical studies suggested that green tea or green tea catechins can modulate the activities of drug-metabolizing enzymes. We conducted this clinical study to determine the effect of repeated green tea catechin administration on human cytochrome P450 (CYP) enzyme activities.

METHODS

Forty-two healthy volunteers underwent a 4-week washout period by refraining from tea or tea-related products. At the end of the washout period, study participants received a cocktail of CYP metabolic probe drugs, including caffeine, dextromethorphan, losartan, and buspirone for assessing the activity of CYP1A2, CYP2D6, CYP2C9, and CYP3A4, respectively. Blood and urine samples before and 8 h after probe drug administration were collected to determine parent drug and metabolite concentrations for measurements of baseline CYP enzyme activities. Following the baseline evaluation, study participants underwent 4 weeks of green tea catechin intervention at a dose that contains 800 mg epigallocatechin gallate (EGCG) daily. The green tea catechin product was taken on an empty stomach to optimize the p.o. bioavailability of EGCG. The EGCG dose given in this study exceeded the amounts provided by average green tea consumption. Upon completion of the green tea catechin intervention, the postintervention CYP enzyme activities were evaluated as described above.

RESULTS

There are large between-subject variations in CYP enzyme activities in healthy individuals. Four weeks of green tea catechin intervention did not alter the phenotypic indices of CYP1A2, CYP12D6, and CYP12C9, but resulted in a 20% increase (P = 0.01) in the area under the plasma buspirone concentration-time profile, suggesting a small reduction in CYP3A4 activity.

CONCLUSIONS

We conclude that repeated green tea catechin administration is not likely to result in clinically significant effects on the disposition of drugs metabolized by CYP enzymes.

Human variability in hepatic and renal elimination: implications for risk assessment

Hepatic metabolism and renal excretion constitute the main routes of xenobiotic elimination in humans. Improving human risk assessment for threshold contaminants requires the incorporation of quantitative data related to their elimination (toxicokinetics) and potential toxic effects (toxicodynamics). This type of data provides a scientific basis to replace the standard uncertainty factor (UF = 10) allowing for the consideration of human variability in toxicokinetics and toxicodynamics. This review focuses on recent research efforts aiming to incorporate human variability in hepatic and renal elimination (toxicokinetics) into the risk assessment process. A therapeutic drug database was developed to quantify pathway-related variability in human phase I and phase II hepatic metabolism as well as renal excretion in subgroups of the population (healthy adults, neonates and the elderly), using data on compounds cleared primarily through each route (> 60% dose). For each subgroup of the population and elimination route, pathway-related UFs were then derived to cover 95-99% of each subgroup. Overall, the default toxicokinetic UFs would not cover neonates, the elderly for most elimination routes and any subgroup of the population for compounds metabolized via polymorphic isozymes (such as CYP2C19 and CYP2D6). These pathway-related UFs allow the incorporation of in vivo metabolism and toxicokinetic data in the risk assessment process and provide a flexible intermediate option between the default UF and chemical-specific adjustment factors (CSAFs) derived from physiologically based pharmacokinetic models. Implications of human variability in hepatic metabolism and renal excretion for chemical risk assessment are discussed.

The influence of old age and enzyme inducing comedication on the pharmacokinetics of valproic acid at steady-state: A case-matched evaluation based on therapeutic drug monitoring data

PURPOSE

To evaluate the influence of aging on the pharmacokinetics of valproic acid (VPA) at steady-state and on the susceptibility of VPA metabolism to enzyme induction by antiepileptic comedication.

METHODS

The database of the therapeutic drug monitoring service of a large neurological hospital was searched to identify patients aged > or = 65 years stabilized on VPA therapy. Apparent VPA oral clearance (CL/F) calculated for each elderly patient was compared with that determined in an equal number of VPA-treated controls aged 20-50 years and matched for gender, body weight and antiepileptic drug (AED) comedication.

RESULTS

A total of 71 elderly patients aged 70.0+/-4.4 years, including 20 receiving enzyme inducing AEDs, was included in the main evaluation. In the absence of enzyme inducing comedication, VPA CL/F in the elderly was similar to that found in non-elderly controls (9.7+/-4.6 versus 10.2+/-4.6mlh(-1)kg(-1)). Elderly patients on enzyme inducing comedication, on the other hand, had lower CL/F values than enzyme induced younger controls (11.7+/-5.4 versus 16.0+/-6.3mlh(-1)kg(-1), p<0.05). Since VPA CL/F is known to increase with increasing dosage, a lower VPA dosage in elderly patients comedicated with enzyme inducers compared with controls may have contributed to differences in CL/F between the two groups.

CONCLUSIONS

In the absence of enzyme inducing comedication, VPA clearance in the elderly was comparable to that observed in controls. VPA clearance in elderly patients receiving enzyme inducing AEDs was lower than in controls, the difference being probably due to an influence of age as well as to the fact that mean VPA dosage was lower in these patients than in controls. Since our measurements of clearance were based on total serum VPA concentrations and VPA binding to plasma proteins is known to be reduced in old age, it is likely that the clearance of unbound, pharmacologically active, VPA was decreased to an important extent in the elderly, presumably as a result of a decline in drug metabolizing capacity.

Modulation of cytochrome P450 activity: implications for cancer therapy

Although metabolism mediated by cytochrome P450 isoenzymes is known to play a major part in the biotransformation of anticancer agents in vivo, few clinical studies have investigated activity of cytochrome P450s and therapeutic outcome in people with cancer. Variability between individuals in the pharmacokinetics of cancer chemotherapy has important consequences in terms of therapeutic efficacy and safety. We discuss here the effect of drug metabolism mediated by cytochrome P450 on therapeutic outcome. As examples, the biotransformation pathways of cyclophosphamide, ifosfamide, tamoxifen, docetaxel, paclitaxel, and irinotecan are discussed. Since most anticancer agents are transformed by enzymes, better knowledge of their metabolic pathways could help improve treatment outcome and safety. Furthermore, a more complete understanding of the metabolism of anticancer agents through phenotyping and genotyping approaches will facilitate the prediction of interactions between drugs. More clinical evidence is needed on the metabolic transformation and drug interactions with these agents to improve cancer therapeutics.

Systems-ADME/Tox: resources and network approaches

The increasing cost of drug development is partially due to our failure to identify undesirable compounds at an early enough stage of development. The application of higher throughput screening methods have resulted in the generation of very large datasets from cells in vitro or from in vivo experiments following the treatment with drugs or known toxins. In recent years the development of systems biology, databases and pathway software has enabled the analysis of the high-throughput data in the context of the whole cell. One of the latest technology paradigms to be applied alongside the existing in vitro and computational models for absorption, distribution, metabolism, excretion and toxicology (ADME/Tox) involves the integration of complex multidimensional datasets, termed toxicogenomics. The goal is to provide a more complete understanding of the effects a molecule might have on the entire biological system. However, due to the sheer complexity of this data it may be necessary to apply one or more different types of computational approaches that have as yet not been fully utilized in this field. The present review describes the data generated currently and introduces computational approaches as a component of ADME/Tox. These methods include network algorithms and manually curated databases of interactions that have been separately classified under systems biology methods. The integration of these disparate tools will result in systems-ADME/Tox and it is important to understand exactly what data resources and technologies are available and applicable. Examples of networks derived with important drug transporters and drug metabolizing enzymes are provided to demonstrate the network technologies.

Pharmacokinetics in older persons

BACKGROUND

Physiologic changes and disease-related alterations in organ function occur with aging. These changes can affect drug pharmacokinetics in older persons.

OBJECTIVE

This article reviews age-related changes in pharmacokinetics and their clinical relevance.

METHODS

A PubMed search was conducted using the terms elderly and pharmacokinetics. Other reviews were also included for literature searching. The review includes literature in particular from 1990 through April 2004. Some articles from before 1990 were included to help illustrate principles of age-related pharmacokinetics.

RESULTS

There are minor changes in drug absorption with aging. The effect of aging on small-bowel transporter systems is not yet fully established. Bioavailability of highly extracted drugs often is increased with age. Transdermal absorption may be delayed, especially in the case of water-soluble compounds. Fat-soluble drugs may distribute more widely and water-soluble drugs less extensively in older persons. Hepatic drug metabolism shows wide interindividual variation, and in many cases, there is an age-related decline in elimination of metabolized drugs, particularly those eliminated by the cytochrome enzyme system. Any decrement in cytochrome enzyme metabolism appears nonselective. Synthetic conjugation metabolism is less affected by age. Pseudocapillarization of the sinusoidal endothelium in the liver, restricting oxygen diffusion, and the decline in liver size and liver blood flow may influence age-related changes in rate of hepatic metabolism. Frailty, physiological stress, and illness are important predictors of drug metabolism in older individuals. Inhibition of drug metabolism is not altered with aging, but induction is reduced in a minority of studies. Renal drug elimination typically declines with age, commensurate with the fall in creatinine clearance. Renal tubular organic acid transport may decline with age, while the function of the organic base transporter is preserved but may be less responsive to stimulation.

CONCLUSION

Changes in pharmacokinetics occur due to age-related physiologic perturbations. These changes contribute to altered dose requirements in older persons, particularly in the case of drugs eliminated by the kidney. Interindividual variation, disease, frailty, and stress may overshadow age-related changes.

The refinement of uncertainty/safety factors in risk assessment by the incorporation of data on toxicokinetic variability in humans

The derivation of safe levels of exposure in humans for compounds that are assumed to cause threshold toxicity has relied on the application of a 100-fold uncertainty factor to a measure for the threshold, such as the no observed adverse effect level (NOAEL) or the benchmark dose (BMD). This 100-fold safety factor consists of the product of two 10-fold factors allowing for human variability and interspecies differences. The International Programme on Chemical Safety has suggested the subdivision of these 10-fold factors to allow for variability in toxicokinetics and toxicodynamics. This subdivision allows the replacement of the default uncertainty factors with a chemical-specific adjustment factor (CSAF) when suitable data are available. This short review describes potential options to refine safety factors used in risk assessment, with particular emphasis on pathway-related uncertainty factors associated with variability in kinetics. These pathway-related factors were derived from a database that quantified interspecies differences and human variability in phase I metabolism, phase II metabolism, and renal excretion. This approach allows metabolism and pharmacokinetic data in healthy adults and subgroups of the population to be incorporated in the risk-assessment process and constitutes an intermediate approach between simple default factors and chemical-specific adjustment factors.

Human variability in xenobiotic metabolism and pathway-related uncertainty factors for chemical risk assessment: a review

This review provides an account of recent developments arising from a database that defined human variability in phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferases, glucuronidation, glycine conjugation, sulphation) and renal excretion. This database was used to derive pathway-related uncertainty factors for chemical risk assessment that allow for human variability in toxicokinetics. Probe substrates for each pathway of elimination were selected on the basis that oral absorption was >95% and that the metabolic route was the primary route of elimination of the compound (60-100% of a dose). Intravenous data were used for compounds for which absorption was variable. Human variability in kinetics was quantified for each compound from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups of the population using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve (AUC)) and acute exposure (Cmax) (data not presented here). The pathway-related uncertainty factors were calculated to cover 95%, 97.5% and 99% of the population of healthy adults and of each subgroup. Pathway-related uncertainty factors allow metabolism data to be incorporated into the derivation of health-based guidance values. They constitute an intermediate approach between the general kinetic default factors (3.16) and a chemical-specific adjustment factor. Applications of pathway-related uncertainty factors for chemical risk assessment and future refinements of the approach are discussed. A knowledge-based framework to predict human variability in kinetics for xenobiotics showing a threshold dose below which toxic effects are not observed, is proposed to move away from default assumptions.

Impact of inter-individual differences in drug metabolism and pharmacokinetics on safety evaluation

Safety evaluation aims to assess the dose-response relationship to determine a dose/level of exposure for food contaminants below which no deleterious effect is measurable that is 'without appreciable health risk' when consumed daily over a lifetime. These safe levels, such as the acceptable daily intake (ADI) have been derived from animal studies using surrogates for the threshold such as the no-observed-adverse-effect-level (NOAEL). The extrapolation from the NOAEL to the human safe intake uses a 100-fold uncertainty factor, defined as the product of two 10-fold factors allowing for human variability and interspecies differences. The 10-fold factor for human variability has been further subdivided into two factors of 10(0.5) (3.16) to cover toxicokinetics and toxicodynamics and this subdivsion allows for the replacement of an uncertainty factor with a chemical-specific adjustment factor (CSAF) when compound-specific data are available. Recently, an analysis of human variability in pharmacokinetics for phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferase, glucuronidation, glycine conjugation, sulphation) and renal excretion was used to derive pathway-related uncertainty factors in subgroups of the human population (healthy adults, effects of ethnicity and age). Overall, the pathway-related uncertainty factors (99th centile) were above the toxicokinetic uncertainty factor for healthy adults exposed to xenobiotics handled by polymorphic metabolic pathways (and assuming the parent compound was the proximate toxicant) such as CYP2D6 poor metabolizers (26), CYP2C19 poor metabolizers (52) and NAT-2 slow acetylators (5.2). Neonates were the most susceptible subgroup of the population for pathways with available data [CYP1A2 and glucuronidation (12), CYP3A4 (14), glycine conjugation (28)]. Data for polymorphic pathways were not available in neonates but uncertainty factors of up to 45 and 9 would allow for the variability observed in children for CYP2D6 and CYP2C19 metabolism, respectively. This review presents an overview on the history of uncertainty factors, the main conclusions drawn from the analysis of inter-individual differences in metabolism and pharmacokinetics, the development of pathway-related uncertainty factors and their use in chemical risk assessment.

Aging biology and geriatric clinical pharmacology

Population aging evokes doomsday economic and sociological prognostication, despite a minority of older people suffering significant dependency and the potential for advances in therapeutics of age-related disease and primary aging. Biological aging processes are linked mechanistically to altered drug handling, altered physiological reserve, and pharmacodynamic responses. Parenteral loading doses need only be adjusted for body weight as volumes of distribution are little changed, whereas oral loading doses in some cases may require reduction to account for age-related increases in bioavailability. Age-related reduction of hepatic blood flow and hepatocyte mass and primary aging changes in hepatic sinusoidal endothelium with effects on drug transfer and oxygen delivery reduce hepatic drug clearance. Primary renal aging is evident, although renal clearance reduction in older people is predominantly disease-related and is poorly estimated by standard methods. The geriatric dosing axiom, "start low and go slow" is based on pharmacokinetic considerations and concern for adverse drug reactions, not from clinical trial data. In the absence of generalizable dosage guidelines, individualization via effect titration is required. Altered pharmacodynamics are well documented in the cardiovascular system, with changes in the autonomic system, autacoid receptors, drug receptors, and endothelial function to modify baseline cardiovascular tone and responses to stimuli such as postural change and feeding. Adverse drug reactions and polypharmacy represent major linkages to avoidable morbidity and mortality. This, combined with a deficient therapeutic evidence base, suggests that extrapolation of risk-benefit ratios from younger adults to geriatric populations is not necessarily valid. Even so, therapeutic advances generally may convert healthy longevity from an asset of fortunate individuals into a general social benefit.

Quantitative structure-pharmacokinetic relationship modelling: apparent volume of distribution

The purpose of this study was to develop a quantitative structure-activity relationship (QSAR) for the prediction of the apparent volume of distribution (Vd) in man for a heterogeneous series of drugs. The relationship of many computed, and some experimental, structural descriptors with Vd, and the Vd corrected for protein binding (unbound Vd), was investigated. Models were constructed using stepwise regression analysis for all the 70 drugs in the dataset, as well as for acidic drugs and basic drugs separately. The predictive power of the models was assessed using half the chemicals as a test set, and revealed that the models for Vd yielded lower prediction errors than those constructed for the unbound Vd (mean fold error of 2.01 for Vd compared with 2.28 for unbound Vd). Moreover, the separation of the compounds into acids and bases did not reduce the prediction error significantly.

Effects of Malnutrition on Cytochrome P450 1A2 Activity in Elderly Patients

Little is known of the influence of nutritional status on cytochrome P450 (CYP) 1A2 activity in elderly patients. Thirty elderly institutionalised patients with malnutrition (group A, aged 88 +/- 5 years) and 24 without (group B, aged 81 +/- 9 years) were included. Malnutrition was defined as weight loss of >10% over the previous 6 months and/or a body mass index (BMI) <21 kg/m2 and albuminaemia < or = 32 g/L. CYP1A2 activity was evaluated by the plasma paraxanthine/caffeine (PAX/CAF) metabolic ratio. The plasma PAX/CAF metabolic ratio was similar in both groups regardless of nutritional status (0.34 +/- 0.13 [A] versus 0.30 +/- 0.11 [B]; p = 0.11). The CYP1A2 metabolic ratio was not correlated to either BMI, serum albumin or renal clearance. CYPI A2 activity, as measured by the plasma PAX/CAF ratio, was not influenced by nutritional status in elderly patients.

Same Old Story? Do We Need to Modify Our Supportive Care Treatment of Elderly Cancer Patients? Focus on Antiemetics

The incidence of cancer is highest among individuals > or =65 years of age. Physiological changes associated with aging, such as cognitive decline, renal and hepatic dysfunction, can often complicate treatment options, and the elderly represent a particular challenge to the oncologist because of the high incidence of comorbidity and polypharmacy. Effective supportive care is of particular importance in elderly cancer patients as they may recover less satisfactorily if adverse events are poorly controlled. Nevertheless, evidence suggests that supportive care agents, for example, antiemetics, are underutilised in this patient population. Chemotherapy and radiotherapy regimens are frequently associated with nausea and vomiting--symptoms that can have deleterious effects on vulnerable patients if not adequately managed. The serotonin 5-HT3-receptor antagonists represent a class of antiemetics that are currently regarded as the gold standard treatment for chemotherapy- and radiotherapy-induced nausea and vomiting. They are recommended as first-line treatment for patients at moderate-to-high risk of emesis. However, antiemetic guidelines do not differentiate between these agents and, more importantly, do not contain specific recommendations for the elderly. Pharmacological differences exist between the commonly available 5-HT3-receptor antagonists (dolasetron, granisetron, ondansetron, tropisetron and palonosetron), namely receptor sensitivity, duration of action, metabolism and tolerability profile. Of particular concern with prescriptions to elderly cancer patients is the convenience of once-daily administration, the low potential for drug-drug interactions and cardiovascular adverse effect profile. Moreover, the addition of the newly approved neurokinin NK1-receptor antagonist aprepitant to the choice of antiemetic regimen may complicate therapy and exacerbate the drug-drug interaction risk in elderly patients. Therefore, the use of antiemetics that are well tolerated and with the lowest risk of drug-drug interactions is imperative in this patient population and will enable even those patients with several comorbidities and a high level of polypharmacy to receive effective antiemetic therapy.

Drug interactions with angiotensin receptor blockers: a comparison with other antihypertensives

The ever-increasing introduction of new therapeutic agents means that the potential for drug interactions is likely to escalate. Numerous different classes of drugs are currently used to treat hypertension. The angiotensin receptor blockers offer one of the newest approaches to the management of patients with high blood pressure. Compared with other classes of antihypertensive agents, the angiotensin receptor blockers appear overall to have a low potential for drug interactions, but variations within the class have been detected. Losartan and irbesartan have a greater affinity for cytochrome p450 (CYP) isoenzymes and, thus, are more likely to be implicated in drug interactions. There is pharmacokinetic evidence to suggest that such interactions could have a clinical impact. Candesartan cilexetil, valsartan and eprosartan have variable but generally modest affinity and telmisartan has no affinity for any of the CYP isoenzymes. In vitro studies and pharmacokinetic/pharmacodynamic evaluation can provide evidence for some interactions, but only a relatively small number of drug combinations are usually studied in this way. The absence of any pharmacokinetic evidence of drug interaction, however, should not lead to complacency. Patients should be made aware of possible interactions, especially involving the concurrent use of over-the-counter products, and it may be prudent for all patients receiving antihypertensive treatment to be monitored for possible drug interactions at their regular check-ups. The physician can help by prescribing agents with a low potential for interaction, such as angiotensin receptor blockers.

Principles of drug therapy for the elderly patient

Physicians will treat larger numbers of elderly patients as the US population ages. Being treated simultaneously for more than 1 condition with multiple prescription drugs is only 1 reason why elderly patients are at greater risk of experiencing adverse drug reactions. The need for physicians to minimize the incidence of these reactions has become incumbent on both physicians and administrators. We review the underlying reasons why the elderly population is at risk of adverse drug reactions and summarize the principles of drug-drug interaction, metabolism, and distribution, which can help elderly patients receive proper pharmacological treatment.

Pharmacology of cytotoxic agents: a helpful tool for building dose adjustment guidelines in the elderly

Aging is associated with multidimensional changes, including alterations in physiological functions, co-morbidities and poly-medications. These changes may lead to modifications in the absorption, distribution, metabolism and excretion of drugs. The lack of a scientific basis for optimal drug dosing in the elderly is a major problem. The development and validation of guidelines are therefore essential to improve treatment administration and monitoring in elderly patients. Even though it has been widely demonstrated that standard therapies used in adults may be of great benefit in the elderly, there may be a higher incidence of toxicity. This could be avoided by using dosage individualization based on a sound knowledge of the physiological factors implicated in the pharmacokinetic (PK) characteristics of the drugs administered and in their observed pharmacodynamic (PD) effects in each patient. The so-called "population modeling" approach renders such studies feasible by allowing the analysis of PK-PD relationships from sparse observational data.

Pharmacokinetic properties of zolpidem in elderly and young adults: possible modulation by testosterone in men

AIMS

The influence of ageing on the pharmacokinetics of zolpidem, an extensively prescribed hypnotic medication, was evaluated in healthy human volunteers.

METHODS

A series of 16 elderly (age: 61-85 years) and 24 young (age: 22-42 years) volunteers received single 5 mg oral doses of zolpidem tartrate. Serum zolpidem concentrations were determined by HPLC with fluorescence detection in samples drawn during 8 h after dosage. The effect of testosterone on zolpidem biotransformation was evaluated in vitro using human liver microsomes. Possible induction of CYP3A protein expression and function was studied in cultured human hepatocytes.

RESULTS

Among men, apparent oral clearance of zolpidem was decreased in elderly compared to young subjects (3.8 vs 11.0 ml min-1 kg-1, P < 0.01), Cmax was increased (93 vs 40 ng ml-1, P < 0.01), and half-life increased (2.7 vs 1.5 h, P < 0.03). Among women, zolpidem oral clearance was decreased in the elderly (3.0 vs 5.8 ml min-1 kg-1, P < 0.02), Cmax increased (108 vs 60 ng ml-1, P < 0.001), with no difference in t1/2 (2.3 vs 2.4 h). Among male subjects, free serum testosterone concentrations were lower in the elderly (10.5 vs 19.0 pg ml-1, P < 0.01), and were significantly correlated with zolpidem clearance (r2 = 0.46, P < 0.001). Multiple regression analysis indicated a greater relative contribution of serum testosterone than age to the oral clearance of zolpidem among men. In human liver microsomes, co-incubation of zolpidem (10 micro m) with varying concentrations of testosterone produced activation of biotransformation of zolpidem to its principal hydroxylated metabolite. Maximum activation was achieved at equimolar concentrations of testosterone (10 micro m). However, testosterone did not induce immunoactive CYP3A4 expression or catalytic function in cultured human hepatocytes.

CONCLUSIONS

The increased Cmax and lower oral clearance of zolpidem in the elderly are consistent with recommendations of lower clinical doses of zolpidem in the elderly. Our clinical and in vitro data both suggest that reduced free serum testosterone may have a modulatory role in age-dependent changes in zolpidem pharmacokinetics in men.

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.

Drug treatment of epilepsy in elderly people: focus on valproic Acid

Despite old age being the commonest time of life to develop epilepsy, relatively little is known about the condition in later years. Antiepileptic drugs (AEDs) are the mainstay of treatment and valproic acid (VPA) has been prescribed for older patients with seizures for over 35 years. VPA is available in a variety of formulations. The drug is generally rapidly absorbed, although there are no data on the extent of oral absorption in the elderly. The volume of distribution (Vd) and elimination half-life have been compared in older and younger patients. One study reported no change in either parameter between elderly and younger patients (Vd: 0.16 vs 0.14 L/kg; elimination half-life: 15.3 vs 13.0h), the other found an increase in both for older patients (Vd: 0.19 vs 0.13 L/kg; elimination half-life 14.9 vs 7.2h). Total VPA clearance is similar in young and elderly subjects. The drug does not induce the metabolism of hepatic enzymes, but can act as a metabolic inhibitor, raising plasma concentrations of lamotrigine, phenobarbital (phenobarbitone), carbamazepine-10-11-epoxide, lorazepam, nimodipine and zidovudine. Concomitant use of VPA may also lead to an elevation in phenytoin, diazepam, warfarin, amitriptyline and chlorpromazine concentrations. A number of enzyme-inducing AEDs such as phenytoin, phenobarbital, primidone and carbamazepine can increase the clearance of VPA. Plasma concentrations of VPA may also rise when the drug is administered with felbamate, stiripentol, aspirin (acetylsalicylic acid), naproxen, phenylbutazone, isoniazid, fluoxetine and chlorpromazine. The majority of elderly patients present with partial and/or secondary generalised seizures, although a few have long-standing primary generalised seizures. Results from meta-analyses and randomised studies of patients comparing VPA with other AED monotherapies suggest that the drug is as effective as carbamazepine, phenytoin and phenobarbital in treating these seizure types. Although some of these studies recruited older patients, there have been no randomised double-blind trials examining the efficacy of VPA with other AEDs in an exclusively elderly cohort. There is no direct correlation between efficacy and plasma VPA concentrations. The majority of older patients require lower doses of AEDs than younger adults. Higher VPA doses may be needed in patients taking drugs which induce hepatic microsomal enzymes. Once-daily dosing of the controlled-release preparation can help to improve compliance and may render some frail elderly people seizure free. There is a perception that the elderly are generally more susceptible to the adverse effects of AEDs than younger adults, although there are few data to validate this claim. Dose-dependent and idiosyncratic reactions may be more frequent. Common adverse effects of VPA include gastrointestinal symptoms and tremor. Slow-dose escalation and controlled-release preparations may minimise these. In summary, VPA is a long established AED. Its broad spectrum of action and dosing schedule are favourable properties for its use in older people. To accurately establish the place of this and other AEDs in treating elderly patients with epilepsy, well designed clinical trials are urgently required in this vulnerable population.

Clinical pharmacokinetic profile of modafinil

Modafinil is a unique wake-promoting agent for oral administration. Its pharmacological properties are distinct from those of other CNS agents, and it selectively targets neuronal pathways in the sleep/wake centres of the brain. After single or multiple oral doses, modafinil is readily absorbed, reaching maximum plasma concentrations at 2-4 hours after administration and pharmacokinetic steady state within 2-4 days. Its pharmacokinetics are dose-independent between 200 and 600 mg/day. The elimination half-life is approximately 12-15 hours, which is largely reflective of the pharmacokinetics of the longer-lived l-enantiomer. Modafinil is primarily eliminated via metabolism, mainly in the liver, with subsequent excretion in the urine. Less than 10% of the dose is excreted as unchanged drug. Metabolism is largely via amide hydrolysis, with lesser contributions from cytochrome P450 (CYP)-mediated oxidative pathways. In patients who are renally or hepatically compromised, the elimination processes can be slowed, and in a similar manner (although to a lesser extent), elimination in the elderly may be reduced due to normal effects of aging. Because modafinil is administered concomitantly with other medications, the potential for metabolic drug-drug interactions has been examined both in vitro and in vivo. In vitro, modafinil was observed to produce a reversible inhibition of CYP2C19 in human liver microsomes. It also caused a small, but concentration-dependent, induction of CYP1A2, CYP2B6 and CYP3A4 activities and suppression of CYP2C9 activity in primary cultures of human hepatocytes. Clinical studies have been conducted to examine the potential for interactions with methylphenidate, dexamfetamine, warfarin, ethinylestradiol and triazolam. The only substantive interactions observed were with ethinylestradiol and triazolam, apparently through induction of CYP3A4, primarily in the gastrointestinal system. Overall, the results of the interaction studies suggest that modafinil has potential to affect the pharmacokinetics of drugs that are metabolised by certain CYP enzymes. Compounds that induce or inhibit CYP activity are unlikely to have major effects on the pharmacokinetics of modafinil. In summary, the results show that modafinil is a moderately long-lived drug that is well absorbed and extensively metabolised.