An updated comparison of drug dosing methods. Part II: Theophylline

Pharmacokinetic Dosing Algorithms

Pharmacokinetic algorithms for dosing range from practical nomograms and formulas to conventional or Bayesian, weighted, least-squares regression methods that adapt parameter-values to concentration data. Choice of an algorithm depends on the degree of risk from undertreatment and overtreatment, the availability of measurable, pharmacodynamic endpoints to guide dosage selection during treatment, and the level of effort needed to ensure safe and effective therapy. The concept of pharmacokinetic-pharmacodynamic dosing is introduced, and characteristic pharmacokinetic dosing algorithms are presented along with their limitations.

Estimation of pharmacokinetic parameters based on the patient-adjusted population data

Population pharmacokinetic data, adjusted for patient characteristics, are recommended for the design of initial dosage regimens of some therapeutically monitored drugs in patients for whom patient-specific data are not available. However, despite widespread use by clinicians such as pharmacists, a clear understanding of the principles of population pharmacokinetics, including data collection and analysis and its limitations, is often lacking. This article describes the 2 main methods of obtaining population kinetic data, namely the two-stage method and nonlinear mixed effect model, and their applications to the pharmacokinetic-based design of dosage regimens. Additionally, some numerical examples are provided to assist the reader in understanding the material. The author uses these tools in a pharmacokinetics course taught to PharmD students.

Methods for clinical monitoring of cyclosporin in transplant patients

Cyclosporin was introduced into clinical practice in the early 1980s and has since been shown to prolong survival for transplant recipients. Because cyclosporin is a narrow therapeutic index drug and there are significant consequences associated with 'subtherapeutic' and 'supratherapeutic' concentrations, cyclosporin therapy is monitored as part of routine patient follow-up. However, the optimal method for the therapeutic drug monitoring of cyclosporin has yet to be defined. Currently, the most common method involves monitoring pre-dose trough concentrations, but this method is less than ideal. Other methods of monitoring cyclosporin therapy include monitoring the area under the concentration-time curve, limited sampling strategies, monitoring of single concentrations other than troughs and pharmacodynamic monitoring. Bayesian forecasting has been used successfully in clinical practice with other drugs with narrow therapeutic indices. However, few studies are available regarding Bayesian forecasting and cyclosporin. Existing studies are preliminary in nature and involve the old Sandimmun formulation rather than the Neoral formulation. Although these methods show promise, they have not gained widespread acceptance. This is because of their impracticality and the lack of prospective studies comparing other monitoring methods with trough concentration monitoring. Further comparative studies evaluating the impact of the specific monitoring method on definite patient outcomes are warranted.

Controlled-release hydrophilic tablets for individualized theophylline therapy

Directly compressible controlled-release (CR) theophylline tablet formulations with a non-zero-order drug release were prepared using various grades of Methocels. These tablet formulations were employed in the individualization of therapy with the aid of a pharmacokinetic simulation model developed with STELLA II computer software. In vitro drug release data were used to simulate plasma concentration-time (C,t) profiles based on a wide range of previously reported patient pharmacokinetic parameters (clearances of 2-5 L/hr and apparent volumes of distribution of 20-50 L). The simulations indicated that formulations containing low-viscosity Methocels (E4, K4, and K4CR) were suitable for individualizing theophylline therapy. Average steady-state concentrations were well within the therapeutic range of 10-20 micrograms/ml. High-viscosity polymers such as E10CR, K15, and K15CR yielded subtherapeutic concentrations and were deemed unsuitable. Thus, a pharmacokinetic simulation program capable of predicting in vivo C,t profiles (even though theophylline release occurred by a non-zero order) may be useful for individualizing theophylline therapy that involves CR formulations.

Abbott PKS system: a new version for applied pharmacokinetics including Bayesian estimation

Abbott Laboratories has developed a new software package (Abbottbase pharmacokinetic system or PKS package) that employs the principles of pharmacokinetics to assist clinical pharmacologists and clinicians in designing dosage regimens. This software, which runs on IBM PC compatibles, allows Bayesian estimation of individual pharmacokinetic parameters. The aim of the present study was to validate this new system in routine clinical practice for amikacin (40 intensive care unit patients) and theophylline (20 patients). By using the program one or more times during the treatment (50 cases for amikacin and 46 cases for theophylline), dosing recommendations were obtained in real time for all patients. The predictive performance (bias and precision) was assessed by comparing predicted drug concentrations with those measured 24-48 h after dosage recommendation. In the case of amikacin, precision and bias were computed separately for peak and trough levels. In all cases, no statistically significant bias was observed. Finally, our results demonstrate the accuracy of the program in predicting drug levels for amikacin and theophylline. Consequently, the PKS package is reliable for the choice of optimal dosage regimen for amikacin and theophylline.

Pharmacokinetic optimisation of asthma treatment

Asthma is generally managed with bronchodilator therapy and/or anti-inflammatory drugs. Guidelines now advocate selection of drugs and pharmaceutical formulations (long-acting vs short-acting, inhaled vs systemic) on the basis of disease severity. Theophylline has a narrow therapeutic margin. Clearance is highly variable and plasma concentrations should be monitored to avoid the occurrence of plasma concentration-related adverse effects. The rate of absorption of theophylline differs depending on the sustained release formulation administered. Some products do not provide sufficient plasma drug concentrations for therapeutic efficacy over a 12-hour period, particularly in patients with high clearance rates (e.g. children and patients who smoke). Administration of drugs via inhalation offers several advantages over systemic routes of administration (e.g. adverse effects are decreased). Inhalation is now advocated as first-line therapy. Aerosol medications available for the treatment of asthma are beta 2-agonist (including the newer long-acting agents such as salmeterol), corticosteroids, anticholinergic drugs, sodium cromoglycate (cromolyn sodium) and nedocromil. To reach the airways, aerosolised particles should be 1 to 5 microns in diameter. Particles of this size can be produced by nebuliser for continuous administration or by metered-dose inhaler and drug powder inhaler for unit dose medication. For efficient use of the metered-dose inhaler, slow inhalation and actuation must be coordinated. However, efficacy and convenience can be improved when spacer devices are used. Furthermore, spacer devices lessen the oropharyngeal adverse effects of inhaled corticosteroids. Dry powder inhalers are more easily used by children and elderly patients than metered-dose inhalers. Regardless of the device used, a maximum of 10% of the inhaled dose reaches the airways. The rest of the dose is swallowed and absorbed through the gastrointestinal tract. Most inhaled drugs have low oral bioavailability, either because of a high first-pass metabolism (beta 2-agonists and glucocorticoids) or because of lack of absorption (sodium cromoglycate). Sulphation of beta 2-agonists occurs in the wall of the gastrointestinal tract and extensive metabolism of inhaled corticosteroids occurs in the liver. Low bioavailability of the swallowed fraction contributes to reduced adverse effects. The pharmacokinetic properties of an inhaled drug are of interest. The fraction of the dose absorbed through the lung has the same disposition characteristics as an intravenous dose, and the swallowed fraction has the same disposition as an orally administered dose. However, for many drugs, pharmacokinetic data after inhalation are limited and cannot be used as a criteria for selection of therapy.(ABSTRACT TRUNCATED AT 400 WORDS)

Prediction of acetaminophen concentrations in overdose patients using a Bayesian pharmacokinetic model

A pharmacokinetic program using population-based parameter estimates and a Bayesian forecasting model was retrospectively evaluated for predicting acetaminophen serum concentrations in overdose patients. Dynamic disposition factors known to affect acetaminophen disposition (emesis, activated charcoal, N-acetylcysteine, etc.) were included in the program. Twenty six patients who reported an acetaminophen ingestion of at least 70 mg/kg within 24 h of presentation to the hospital and had at least one measured acetaminophen concentration were included. Prediction of initial acetaminophen concentrations using only population-based parameter estimates resulted in a percent mean error (%ME) and percent mean absolute error (%MAE) of 9.3 and 42.2, respectively. Using only the initial concentration as feedback, the Bayesian forecasting model accurately predicted the second acetaminophen concentration (%ME = 4.0, %MAE = 23.6). The Bayesian model also accurately predicted all concentrations within 8 h of the ingestion (%ME = 10.6, %MAE = 24.0). The prediction of concentrations between 2 to 4 h and 4 to 4.5 h after ingestion with only population-based parameter estimates resulted in %ME of 17.0 and 13.2, respectively, and %MAE of 36.5 and 35.1, respectively. Our data suggests that acetaminophen serum concentrations occurring within the first 4.5 h after ingestion can be reliably predicted by the set of population-based parameter estimates evaluated. Once a single acetaminophen concentration is available, the Bayesian forecasting model can accurately predict subsequent concentrations within the first 8 h after an acetaminophen ingestion.

Impact of therapeutic drug monitoring of intravenous theophylline regimens on serum theophylline concentrations in the medical intensive care unit

OBJECTIVE

The primary objective of this study was to determine if a clinical pharmacist ensuring appropriate use and performing careful pharmacokinetic analysis of serum theophylline concentrations (STCs) can result in optimal theophylline regimens for patients in a medical intensive care unit (ICU).

DESIGN

In phase 1 of the study, housestaff physicians determined the theophylline dosing regimen. A clinical pharmacist prospectively followed these initial patients, but did not intervene. Patients in phase 2 received intravenous theophylline, with all dosing regimens being based on prospective therapeutic drug monitoring (TDM) by a clinical pharmacist.

PARTICIPANTS

The first phase enrolled 11 consecutive patients admitted into the medical ICU and requiring intravenous theophylline. This was followed by a second phase of 14 medical ICU patients whose intravenous theophylline regimen was determined by a clinical pharmacist performing pharmacokinetic analysis.

MAIN OUTCOME MEASURES

Questions were organized into the following sections: (1) number of STC blood samples drawn, (2) number of emergency STCs performed, (3) number of concentrations per infusion-rate changes, (4) number of STC results in the pharmacist-generated regimens that were within the physicians' predetermined acceptable range.

RESULTS

In phase 1, 27 theophylline rate changes occurred, with a mean +/- SD of 1.01 +/- 0.3 STCs per day. In phase 2, 44 theophylline infusion rates were changed, with a mean +/- SD of 0.62 +/- 0.3 STCs per day. The TDM group had a significant reduction (p less than 0.001) in inappropriate concentrations (15 vs. 40 percent) as set by predetermined criteria, and fewer emergency theophylline concentration requests (2 vs. 14). Serum theophylline concentrations ordered by the physician were achieved in 85 percent of the pharmacist-generated regimens.

CONCLUSIONS

Pharmacokinetic analysis when performing TDM in the determination of intravenous theophylline regimens can result in optimal therapy for patients, while significantly reducing the number of STCs required.