The 'apparent clearance' of free phenytoin in elderly vs. younger adults

Medicines optimisation in older people: Taking age and sex into account

There are a number of complex and seemingly ignored issues around prescribing safely and effectively for older people, particularly for very old women. These issues include polypharmacy, possible compliance issues and communication barriers between patient, specialists and general practitioners (GPs). There are specific pharmacokinetic (PK) and pharmacodynamic (PD) parameters that change in older age generally, and in women more specifically, which if ignored are likely to cause symptoms and to impair quality of life when drug dosage is unchanged. These changed PK and PD parameters are not all-or-nothing processes, but a continuum across age, sex and comorbidity. Very old people also have less 'reserve' when drugs are used in 'standard' doses, are more likely to have multiple concurrent therapies, and the risk of adverse effects of drugs in this group is very high. Doctors need to consider these issues when providing therapy for this group, or when trying to unravel the complex prescribing cascade here. This review outlines general principles to consider when prescribing for older people, focusing on age- and sex-related changes in both PK and PD processes.

The effect of combination therapy on the plasma concentrations of traditional antiepileptics: a retrospective study

The present study aimed at determining the differences in plasma concentrations of traditional antiepileptics such as phenytoin, carbamazepine, and valproic acid in patients receiving monotherapy and combination therapy. In addition, the effect of gender and age on plasma drug concentration was evaluated in these patients. For this purpose, plasma trough concentrations obtained during routine therapeutic monitoring of these drugs were assessed retrospectively. The average plasma concentrations reached the apparent therapeutic ranges, except for the average plasma concentration of phenytoin, which was below the therapeutic range in patients who received only phenytoin or in combination with the other agents. Phenytoin when combined with carbamazepine or valproic acid significantly decreased the average plasma concentrations of these drugs to subtherapeutic concentrations. The results showed that plasma carbamazepine concentrations were higher in men than in women, whereas plasma concentrations of valproic acid and phenytoin were higher in women than in men. The difference in this regard between men and women was found to be statistically significant for phenytoin. The difference between the average plasma concentrations of carbamazepine, phenytoin, and valproic acid among age groups was not significant. In conclusion, our study measured the average plasma antiepileptic drug concentrations in patients with epilepsy who were receiving monotherapy and combination therapy and were routinely monitored, and has thus shown the importance of drug monitoring in the evaluation of the effectiveness of these drugs.

Understanding the time course of pharmacological effect: a PKPD approach

The key concepts that underpin the choice of drug and dosing regimen are an understanding of the drugs' effectiveness, the potential for adverse effects, and the expected time course over which both desired and adverse effects are likely to occur. Research in clinical pharmacology should therefore address three fundamental questions: (1) What is the magnitude of drug effects (beneficial or adverse) from a given dose? (2) How quickly will any given effects occur? (3) How long will these effects last? Under steady-state conditions, only the magnitude of drug effects can be examined. This requires researchers to consider non-steady-state conditions, which require more complex models and an understanding of the mechanisms that drive the time course of drug effect. The aim of this review is to provide a conceptual framework for understanding the time course of drug effects using pharmacokinetic-pharmacodynamic models. Key examples will illustrate how this can inform the optimal use of drugs in the clinic.

Drug metabolism in older people--a key consideration in achieving optimal outcomes with medicines

Hepatic clearance plays a key role in determining the systemic exposure of drugs and metabolites, which in turn has a major effect on variability in the beneficial and adverse effects of medicines. Aging results in a number of significant changes in the human liver including reductions in liver blood flow, size, drug-metabolizing enzyme content, and pseudocapillarization. Drug metabolism is also influenced by comorbid disease, frailty, concomitant medicines, and (epi)genetics. These changes have the potential to alter the hepatic clearance of drugs but need to be interpreted in the context of the pharmacokinetic (and pharmacodynamic) characteristics of the drug of interest. There is growing evidence that the age-related changes in the liver not only result in a decrease in the hepatic clearance of unbound drug but also influence variability in response to medicines in older people.