Incorporation of first-order uptake rate constants from simple mammillary models into blood-flow limited physiological pharmacokinetic models via extraction efficiencies

Use of Pharmacokinetic Data Under the FDA's Redbook II Guidelines for Direct Food Additives

Experience with food additive petitions submited after publication of the Food and Drug Administration's Redbook I (U. S. FDA, 1982) guide lines indicated a number of areas in which improvements were needed, and advances in toxicol-ogy testing during the last decade required additional rev is ions. In March 1993, the FDA's Center for Food Safety and Applied Nutrition (CFSAN) distributed copies of a draft of Redbook II for public comment. Since that time, revisions have been made based on comments received on the initial draft. This article describes the rationale for Redbook II guidance on the design of pharm acoki-netic studies and discusses some common problems the FDA has encountered in reviewing pharmacokine tic data submitted as part of food additive petitions. Points emphasized are that (1) pharmaco kinetic information is needed for the interpretation of toxicity studies and is most use ful when conducted before major toxicity studies, (2) the use of whole-body autoradiography is encouraged as a means to select tissues of interest, and as a substitute for dissection and tis-sue sampling, (3) kinetic and mechanistic studies conducted with blood compo-nents, tissue slices, hepatocytes, and othercell types in vitro ofien provide more useful information on the fate of chemicals in specific tissues than information extracted from whole-animal studies. The intention of th e new guide lines for pharmaco kinetic studies is to increase the information content of data gathered and to encourage the use of pharmaco kinetic models and results in the selection of doses for subchronic, chronic, and developmental toxicity studies.

Whole-body physiologically based pharmacokinetic models

This review summarizes the most recent developments in and applications of physiologically based pharmacokinetic (PBPK) modeling methodology originating from both the pharmaceutical and environmental toxicology areas. It focuses on works published in the last 5 years, although older seminal papers have also been referenced. After a brief introduction to the field and several essential definitions, the main body of the text is structured to follow the major steps of a typical PBPK modeling exercise. Various applications of the methodology are briefly described. The major future trends and perspectives are outlined. The main conclusion from the review of the available literature is that PBPK modeling, despite its obvious potential and recent incremental developments, has not taken the place it deserves, especially in pharmaceutical and drug development sciences.

Whole body pharmacokinetic models

The aim of the current review is to summarise the present status of physiologically based pharmacokinetic (PBPK) modelling and its applications in drug research, and thus serve as a reference point to people interested in the methodology. The review is structured into three major sections. The first discusses the existing methodologies and techniques of PBPK model development. The second describes some of the most interesting PBPK model implementations published. The final section is devoted to a discussion of the current limitations and the possible future developments of the PBPK modelling approach. The current review is focused on papers dealing with the pharmacokinetics and/or toxicokinetics of medicinal compounds; references discussing PBPK models of environmental compounds are mentioned only if they represent considerable methodological developments or reveal interesting interpretations and/or applications.The major conclusion of the review is that, despite its significant potential, PBPK modelling has not seen the development and implementation it deserves, especially in the drug discovery, research and development processes. The main reason for this is that the successful development and implementation of a PBPK model is seen to require the investment of significant experience, effort, time and resources. Yet, a substantial body of PBPK-related research has been accumulated that can facilitate the PBPK modelling and implementation process. What is probably lagging behind is the expertise component, where the demand for appropriately qualified staff far outreaches availability.

A fractal approach to heterogeneous drug distribution: calcium pharmacokinetics

PURPOSE

To point out the importance of heterogeneity in drug distribution processes and develop a noncompartmental approach for the description of the distribution of drug in the body.

METHODS

A dichotomous branching network of vessels for the arterial tree connected to a similar venous network was used to describe the heterogeneity of blood flow in the successive generations of the networks. The relevant kinetics of drug distribution in the well perfused and the deep tissues was considered to take place under well stirred (homogeneous) and understirred (heterogeneous) conditions, respectively.

RESULTS

A "homogeneous model" with classical kinetics (which is mathematically equivalent with the one-compartment model) was developed for these drugs which are confined to well perfused ("well stirred") spaces. A "heterogeneous model" was proposed for the drugs reaching understirred spaces using a decreasing with time rate coefficient (fractal kinetics) to model the diffusion of drug under heterogeneous conditions. The analysis of the model equations revealed that the homogeneous model can be considered as a special case of the heterogeneous model. Concentration-time plots of multiexponential type were generated using the heterogeneous model equation. The empirically used power functions of time for the analysis of calcium clearance curves, were found to be similar to the equation adhering to the heterogeneous model. Fittings comparable to multiexponential models were obtained when the heterogeneous model equation with only one adjustable parameter was applied to six sets of long period calcium data.

CONCLUSIONS

The heterogeneous processes of drug distribution in the body can obey the principles of fractal kinetics. Calcium clearance curves were analysed with the heterogeneous model. The validity of multicompartmental models which are based on the concept of homogeneity to describe drug distribution should be reconsidered.