A simple and accurate midpoint-back-extrapolation method for pharmacokinetic individualization of aminoglycoside therapy

Intravenous verapamil kinetics in rats: marked arteriovenous concentration difference and comparison with humans

The pharmacokinetics of verapamil, a calcium channel blocker, were studied in male Sprague-Dawley rats following i.v. administration at a dose of 1 mg kg-1. Both arterial and venous blood were collected and the plasma drug concentrations were determined by reversed-phase high-performance liquid chromatography. Verapamil was distributed to the extravascular tissues very rapidly as indicated by the large Vdss (2.99 +/- 0.57 l kg-1) and Vd beta (5.08 +/- 0.54 l kg-1). The apparent terminal plasma T1/2, MRTiv, and CLp were 1.59 +/- 0.46, 1.26 +/- 0.12 h, and 40.4 +/- 9.73 ml min-1 kg-1, respectively. Marked arterial/venous differences were found with a considerable influence on the MRT and Vdss, and the terminal phase venous levels were higher than arterial levels by 103, 69, and 90%, respectively, for the three rats studied. The distribution of verapamil between plasma and erythrocytes occurred very rapidly and was identical in vitro and in vivo. The average blood to plasma and plasma to blood cell concentration ratios were 0.85 and 1.47, respectively. In contrast to propranolol, blood data rather than plasma data should be used to predict the hepatic extraction ratio of verapamil (0.87). The plasma protein binding of verapamil in humans (90%) and rats (95%) were quite similar and constant over the wide concentration range studied. A comparison of some pharmacokinetic parameters between rats and humans is presented and the potential shortcomings of using T1/2 or CLp and the advantage of using CLu (unbound plasma clearance) in interspecies scaling is also discussed.

An updated comparison of drug dosing methods. Part III: Aminoglycoside antibiotics

Aminoglycoside antibiotics continue to be useful for the treatment of Gram-negative infections. Available dosing methods include predictive algorithms and nomograms, pharmacokinetics-based dosing methods, and methods that incorporate Bayesian forecasting. The individualised Sawchuk-Zaske and Bayesian methods have been extensively evaluated since the previous review in the Journal. Both methods continue to be rapid and accurate means of individualising dosage requirements for patients with diverse pharmacokinetic profiles. The predictive performance of the Bayesian method can be further enhanced when population-based parameters reflect the patient population being monitored. There are now several cost-effectiveness studies that demonstrate that pharmacokinetic dosing services for aminoglycosides result in cost savings, better therapeutic concentrations, fewer toxic serum concentrations, and shorter mean durations of hospital stay and aminoglycoside therapy. Further studies are needed for cost-effectiveness and comparison of various dosing methods in paediatric and neonatal patients.

Clinical pharmacokinetic considerations in the elderly. An update

There are numerous studies of drug handling in the elderly, but it is difficult to assess the significance of changes seen in vitro, or after single-dose administration, because they are often compensated by other mechanisms at steady-state. However, a knowledge of these studies is important as the results alert the investigator to possible treatment problems. The high incidence of adverse drug reaction in the elderly population leaves no doubt that improvements in therapy are needed. Research has been directed at seeking patterns of abnormality in the elderly on which to base recommendations for alterations in dosage regimens. The major shortcoming of this approach has been the failure to distinguish between the effect of chronological age on drug pharmacokinetics, and drug kinetics in elderly people with multiple pathology. The latter concern appreciates the variety of factors involved and the importance of treating each patient as an individual: presentation of mean data is confusing and misleading. The objective of drug treatment in any age group, but particularly in the elderly, is to administer the smallest possible dose which gives adequate therapeutic benefit throughout the entire dosage interval with the minimum of side effects. For most drugs the safe starting dose in the elderly is one-third to half that recommended in the young. Vigilance for potential side effects with plasma concentration monitoring, if available, should help keep toxicity to a minimum. When other medications are added or changed, the possibility of interaction should be anticipated. Methods for individualisation of dosage regimens and the use of sustained-release formulations in the elderly are discussed. Dosage alteration in the elderly in terms of reduced dose frequency, rather than dose size, may help improve compliance. A knowledge of the pharmacokinetics of a drug helps determine which approach will be most beneficial.

Performance of two predictive methods for calculation of the key pharmacokinetic parameters for gentamicin dosage individualization

Performance of two methods for determination of the apparent volume of distribution (Vd) and the elimination half-life (t1/2) for gentamicin was evaluated in 20 non-obese acutely ill patients. The patients had varying degrees of renal function. Initial creatinine clearance ranged from 22.7-103.1 ml/min, with a mean value (+/- SEM) of 63.26 (+/- 5.74) ml/min, and serum creatinine concentration was 1.37 (+/- 0.13) mg/dl, with a range of 0.70-3.0 mg/dl. The total daily dose of gentamicin ranged from 1.85-4.71 mg/kg. Two different methods were used for Vd calculation: the Hull-Sarubbi method and Chiou midpoint-back-extrapolation method. For t1/2 estimation, the Hull-Sarubbi and Cutler methods were used. These values were compared with the values obtained by the Sawchuk-Zaske method. Mean predicted error (ME), mean absolute error (MAE), and root mean squared error (RMSE) were calculated for each method. Prediction bias and precision were compared statistically between each method by calculating the 95% confidence intervals of the delta MA and delta MAE, respectively. The MAEs revealed that the precision of Vd predictions were within 1.04 litre and 0.62 litre for the Hull-Sarubbi and Chiou methods, respectively. For the elimination half-life, none of the methods performance exhibited substantial bias. The Hull-Sarubbi method, however, was less biased and more precise than the Cutler method.