Factors affecting free (unbound) lignocaine concentration in suspected acute myocardial infarction

Effects of aging, renal dysfunction, left ventricular systolic impairment, and weight on steady state pharmacokinetics of perhexiline

MATERIALS AND METHODS

Two hundred patients at steady-state on long-term perhexiline were identified retrospectively. The ratio of maintenance dose to steady-state plasma concentration (dose:[Px]) was correlated with the following putative determinants via simple and multiple linear regression analyses: age, weight, left ventricular ejection fraction (LVEF), and creatinine clearance (CrCl, Cockroft-Gault formula). A Mann-Whitney U test was performed to determine if severe left ventricular systolic impairment affected maintenance dose.

RESULTS

Advanced age, left ventricular systolic impairment, and renal impairment were frequently encountered. Using simple linear regression, age was a negative correlate of dose:[P] (R = 0.23, P = 0.001), whereas weight (R = 0.27, P = 0.0001) and CrCl (R = 0.30, P < 0.0001) were positive correlates. Mann-Whitney U analysis showed no difference between dose: [Px] among patients with LVEF of less than 30% versus 30% or greater. Advancing age was strongly associated with decreasing weight (R = -0.45, P < 0.00001) and calculated CrCl varied directly with weight, as expected (R = 0.66, P < 0.0001). Stepwise multiple linear regression using age, LVEF, CrCl, and weight as potential predictors of dose:[P] yielded only weight as a significant determinant.

DISCUSSION

Perhexiline has become a "last-line" agent for refractory angina as a result of complex pharmacokinetics and potential toxicity. Use has increased predictably in the aged and infirm who have exhausted standard medical and surgical therapeutic options. Beyond genotype, the effect of patient characteristics on maintenance dose has not been explored in detail. In this study, dose requirement declined with age in a frail and wasting population as a result of weight-related pharmacokinetic factors. LVEF had no apparent effect on maintenance dose and should not be considered a contraindication to use.

CONCLUSION

A weight-adjusted starting dose may facilitate the safe and effective prescription of perhexiline and is calculated by 50 + 2 × weight (kg) mg/d, rounded to the closest 50 mg/day.

Nonlinear pharmacokinetics: clinical Implications

Nonlinear pharmacokinetics (in other words, time or dose dependences in pharmacokinetic parameters) can arise from factors associated with absorption, first-pass metabolism, binding, excretion and biotransformation. Nonlinearities in absorption and bioavailability can cause increases in drug concentrations that are disproportionately high or low relative to the change in dose. One of the more important sources of nonlinearity is the partial saturation of presystemic metabolism exhibited by such drugs as verapamil, propranolol and hydralazine. In such cases, circulating drug concentrations are sensitive not only to dose size but also to rate of absorption: slower absorption may decrease the overall systemic availability. The binding of drugs to plasma constituents, blood cells and extravascular tissue may exhibit concentration dependence. This can cause pharmacokinetic parameters based on total blood or serum drug concentrations to be concentration-dependent. Often, in these cases, parameters based on free drug concentration appear linear. An important consideration in regard to concentration-dependent serum binding is the difficulty in relating total concentration to a usual therapeutic range if free concentration is a better indicator of drug effect. Measurement of free concentration is needed in these cases, particularly if the intersubject variability in binding is high. An example of this behaviour is valproic acid. Partial saturation of elimination pathways can result in the well known behaviour typical of Michaelis-Menten pharmacokinetics. Small changes in dosing rate can make much larger differences in steady-state concentration. The time to achieve a given fraction of steady-state becomes longer as the dosing rate approaches the maximum elimination rate. Alcohol and phenytoin are examples of drugs that exhibit such behaviour. The sensitivity of steady-state concentration and cumulation rate to changes in dosing rate are both influenced by the magnitude of parallel first-order elimination pathways: even a first-order pathway that is only 1 to 2% of maximum clearance (which occurs at very low concentration) can be an important determinant of steady-state concentration and cumulation rate when concentrations are high. Theophylline and salicylate have significant parallel first-order elimination pathways as well as saturable pathways. Autoinduction causes an increase in clearance with long term administration. In some cases, dosage adjustment must be made to compensate for the increase, and the possibility that the degree of induction may be dose- or concentration-dependent must be kept in mind. Carbamazepine is a major example of autoinduction. It is fortunate that only a few of the many hundreds of drugs in use exhibit nonlinear behaviour that leads to clinical implications.(ABSTRACT TRUNCATED AT 400 WORDS)