The difference between PK and DK has been described (Wikipedia) as
"Pharmacokinetics may be simply defined as what the body does to the drug, as opposed to pharmacodynamics which may be defined as what the drug does to the body"
I nice description of mixed models in PK/DK is provided here: http://www4.stat.ncsu.edu/~davidian/webinar.pdf
The reason ADMB is useful for such models are:
- The flexibility of ADMB allows numerical ODE schemes to be implemented.
- Robust and fast parameter estimation.
- Mixed mode functionality