Early human studies are normally conducted in healthy subjects to assess acute safety and, if applicable, to monitor biochemical and pharmacological responses. These studies also provide pharmacokinetic, metabolic, and biopharmaceutical data, all of which are essential to characterize the drug's profile in humans-ultimately in the target patient population. Physiological conditions, such as body weight and composition, hepatic and renal function, and cardiovascular function vary within and among patients and are often different from healthy sub-jects. Individuals also differ in their genetic profile, die-tary habits, and in the case of patients, in the degree of lishing concentration-response relationships the use of plasma data, obtained from a peripheral vein, may hin-der the ability to establish the relationship because of temporal differences, and even differences at equilib-rium, between active site and plasma. Here PBPK, with its ability to predict the kinetics of drugs in tissues as well as plasma, has application.

1. Users

PBPK software development is very expensive and the user group is relatively small, at least at the moment. It appears that the community of PBPK technology users is neatly split between "experts" (ie, those who actually have done PBPK modeling using any software tool that they had at their disposal) and "novices" (ie, those about to use PBPK models for the first time). This view is probably simplistic and does not represent the reality exactly. It is nevertheless useful for a discussion of pre-sent needs in software. In general, "experts" have little need for, and do not see the relevance of customer-built, specific, or user-friendly software, as they are usually experts also in general modeling methodology (eg, ordi-nary differential equations, probability theory, etc). In contrast, novices need user-friendly tools that encourage them to learn PBPK modeling and to appreciate its limitations. Given this split, there is no single preferred software available that meets all needs.
Because of their ability to formally incorporate prior knowledge and sources of variability, Bayesian methods and the application of nonlinear mixed effects modeling are essential in the development of mechanism-based PK/PD models. Often information on different drugs and/or information on the same drug but obtained under different conditions needs to be simultaneously analyzed to derive the in vivo stimulus-response relationship and to obtain estimates of physiological rate constants of the dynamic system. Furthermore, the incorporation of in-formation from different sources (ie, in vitro bio-assays) may be required.

2. Environmental Risk Assessment Using PBPK

There is a close relationship between environmental risk assessment and risk assessment of therapeutic agents. The main differences are that in environmental risk as-sessment, chemicals not intended for human therapeutic or nutritional consumption must be studied but cannot be administered to human volunteers, chemical toxicity is the main objective of observation, and the principal aim is to inform risk assessment and reduction procedures. On occasion, human data are available from accidental exposure to such chemicals.

3. Software for Whole Body PBPK Modeling

Software for whole body PBPK, as with other PK tools, should be able to perform both simulation and parameter optimization. It appears that there is an inverse relation-ship for existing software between user-friendliness and flexibility. Appendix 2 provides a listing of available software for PBPK modeling, including in silico predic-tion. It ranges from low-level programming languages, generic tools developed primarily for engineering pur-poses, to tools for biological or PK/PD modeling. The problem of in silico prediction has been briefly men-tioned in the Early Candidate Selection section. Absorp-tion, distribution, and metabolism may be simulated us-ing a variety of software. However, currently there does not seem to be specific software for the simulation of excretion, and the current practice at least for renal ex-cretion is to scale renal clearance across mammalian species using allometric relationships, which seems to work adequately for this particular process.
The process of assessing the health risks associated with human exposure to toxic environmental chemicals inevi-tably relies on several assumptions, estimates, and ra-tionalizations. Some of the greatest challenges result from the necessity to extrapolate from the conditions in the studies providing evidence of the toxicity of the chemical to the anticipated conditions of exposure in the environment or workplace. For risk assessments based on animal data, the most obvious extrapolation that must be performed is from the tested animal species to hu-mans. However, others are also generally required: from high dose to low dose, from one exposure route to an-other, and from one exposure time frame to another. PBPK modeling provides a powerful method for in-creasing the reliability of these extrapolations and is one that is being increasingly accepted by environmental protection agencies.

4. Standardization

The presently available software is very diverse in both quality and scope of application. A future goal could be standardization; this would ensure that whoever uses PBPK technology does so in a way that is reasonably uniform with that done by others in the field. This is cer-tainly not the case today, raising serious problems of model verification. That said, it is doubtful that this goal of standardization will ever be truly reached in view of Model Verification and Documentation
As in any form of modeling, verification is an important issue in PBPK. Verification should be considered as a multidimensional approach that reflects current theories and experimental data relating to the particular system of interest, together with model purpose, formulation, and identification. A similar problem arises for model docu-mentation, in particular for publication in the scientific literature. Because of page-length limitations in most journals, model documentation is often reduced to such an extent that it cannot be fully analyzed by the review-ers and used for replication studies by other scientists. Solutions to this problem should be found, for example, by making this information available to the reviewer and later for the interested readers through a Web site.

5. Quality of Input Data

The quality of data derived from databases has been briefly mentioned in the Physiological and Anatomic Databases section. The same concerns may be raised for the molecular properties used for in silico prediction or in vitro data used for estimation of parameters characterizing absorption, distribution, and metabolism. Of particular concern is that in vitro data, although usually are quite reliable from a qualitative point of view, are much less reliable from a quantitative viewpoint. Another limiting factor is the variability in some assay system components. A good example is illustrated by the use of human hepatocytes: their availability seems to be inversely correlated with the hepatocyte "quality" (including the amount of information available on the donor). On the other hand, animal hepatocytes are not always optimal, in particular when a given species does not express enzymes important for metabolism in human beings.