Liver Diseases

Pharmacokinetic Considerations Relating to Tacrolimus Dosing in the Elderly

Relaxation of the upper age limits for solid organ transplantation coupled with improvements in post-transplant survival have resulted in greater numbers of elderly patients receiving immunosuppressant drugs such as tacrolimus. Tacrolimus is a potent agent with a narrow therapeutic window and large inter- and intraindividual pharmacokinetic variability. Numerous physiological changes occur with aging that could potentially affect the pharmacokinetics of tacrolimus and, hence, patient dosage requirements. Tacrolimus is primarily metabolised by cytochrome P450 (CYP) 3A enzymes in the gut wall and liver. It is also a substrate for P-glycoprotein, which counter-transports diffused tacrolimus out of intestinal cells and back into the gut lumen. Age-associated alterations in CYP 3A and P-glycoprotein expression and/or activity, along with liver mass and body composition changes, would be expected to affect the pharmacokinetics of tacrolimus in the elderly. However, interindividual variation in these processes may mask any changes caused by aging. More investigation is needed into the impact aging has on CYP and P-glycoprotein activity and expression. No single-dose, intense blood-sampling study has specifically compared the pharmacokinetics of tacrolimus across different patient age groups. However, five population pharmacokinetic studies, one in kidney, one in bone marrow and three in liver transplant recipients, have investigated age as a co-variate. None found a significant influence for age on tacrolimus bioavailability, volume of distribution or clearance. The number of elderly patients included in each study, however, was not documented and may have been only small. It is likely that inter- and intraindividual pharmacokinetic variability associated with tacrolimus increase in elderly populations. In addition to pharmacokinetic differences, donor organ viability, multiple co-morbidity, polypharmacy and immunological changes need to be considered when using tacrolimus in the elderly. Aging is associated with decreased immunoresponsiveness, a slower body repair process and increased drug adverse effects. Elderly liver and kidney transplant recipients are more likely to develop new-onset diabetes mellitus than younger patients. Elderly transplant recipients exhibit higher mortality from infectious and cardiovascular causes than younger patients but may be less likely to develop acute rejection. Elderly kidney recipients have a higher potential for chronic allograft nephropathy, and a single rejection episode can be more devastating. There is a paucity of information on optimal tacrolimus dosage and target trough concentration in the elderly. The therapeutic window for tacrolimus concentrations may be narrower. Further integrated pharmacokinetic-pharmacodynamic studies of tacrolimus are required. It would appear reasonable, based on current knowledge, to commence tacrolimus at similar doses as those used in younger patients. Maintenance dose requirements over the longer term may be lower in the elderly, but the increased variability in kinetics and the variety of factors that impact on dosage suggest that patient care needs to be based around more frequent monitoring in this age group.

Extrahepatic conditions and hepatic encephalopathy in elderly patients

PURPOSE

Extrahepatic conditions can cause, exacerbate, or mimic hepatic encephalopathy in any patient with advanced liver disease, particularly in older persons. The aim of this study was to characterize the clinical features and frequency of extrahepatic conditions and the effect of therapeutic interventions upon the encephalopathy.

DESIGN

Survey.

SETTING

Inner city community hospital.

METHODS

Retrospective chart review of 294 elderly patients (age 65-97) with liver disease and suspected hepatic encephalopathy, during a 15-year period, that included 188 men and 106 women.

RESULTS

Extrahepatic conditions were found in 64 patients (22%); 29 (10%) patients had > 1 extrahepatic condition. Category and frequency of the extrahepatic conditions found in these 64 patients were as follows: urinary tract infection, 21 (33%); cellulitis/infected pressure ulcers, 16 (25%); pneumonia, 16 (25%); septicemia (with positive blood cultures), 10 (16%); silent myocardial infarction, 10 (16%); drug toxicity (nonsteroidal anti-inflammatory drugs, sedatives, hypnotics, antidiabetics), 6 (9%); meningitis, 6 (9%); head injury, 5 (8%); stroke, 5 (8%); and subdural hematoma, 5 (8%).

CONCLUSION

A significant proportion of elderly patients with liver disease and presumptive diagnosis of hepatic encephalopathy may have extrahepatic condition(s), and the treatment of the latter may improve clinical outcome of such patients. A high index of suspicion, low threshold of diagnostic measures, and prompt treatment of any associated extrahepatic condition are essential to prevent significant morbidity and mortality of these patients.

Liver function and phase I drug metabolism in the elderly: a paradox

Aging is accompanied by marked changes in the physiology of many organs, as well as in their constituent cells. These nonpathological alterations in structure and/or function may affect normal physiological processes in the elderly (individuals > 65 years), for example drug disposition. The liver plays a major role in drug clearance and aging has been reported to diminish this hepatic capacity, particularly the clearance of drugs that undergo mandatory oxidation by the microsomal cytochrome P450-dependent mono-oxygenase systems. Liver volume and blood flow decline with age in humans and, no doubt, this contributes to the diminished clearance of drugs that exhibit first-pass kinetic profiles. Changes in liver morphology with aging that have been described in rodents are limited to the hepatocytes, for example accumulation of dense bodies and loss of smooth surfaced endoplasmic reticulum. There is no evidence that the increase in intracellular lipofuscin adversely affects hepatocyte functions. A number of studies have documented significant age-related declines in the amounts, specific activities and rates of induction of liver microsomal mono-oxygenases in inbred male rats. On the basis of a variety of clinical tests, most liver functions in humans appear to be well preserved. The most remarkable characteristic of liver function in the elderly is the increase in interindividual variability, a feature that may obscure age-related differences. Most in vitro studies using nonhuman primate or human liver tissue did not detect age-related deficiencies in cytochrome P450-dependent microsomal mono-oxygenases. On the other hand, there have been recent reports of age-related, but not gender-related, declines in the in vitro activities of several human liver mono-oxygenases, for example the cytochrome P450 isoform CYP3A. Nevertheless, reduced liver volume and blood flow in the elderly permit the reconciliation of: the in vivo clinical pharmacokinetic data indicative of reduced hepatic drug clearance; and the absence of significant age-related declines in the amounts or in vitro activities of liver microsomal mono-oxygenases.