Pharmacokinetics: Physiological Model


PHYSIOLOGIC MODEL
o   Also called as Blood flow rate –limited models and perfusion rate limited models.
o   Physiological models describe the drug disposition in terms of realistic physiological parameters such as blood flow and tissue partition co-efficient.
o   The number of compartments to be included in the model depends upon the disposition characteristics of drug.
o   Organs such as bone that have no drug penetration are excluded.
o   Most physiologic models are based on the assumption that the drug movement within a body region is much more rapid than its rate of delivery to that region by the perfusion blood and, therefore the model is said to be perfusion rate limited. The assumption is however applicable to the highly membrane permeable drugs i.e. low molecular weight, poorly ionized and highly lipophilic drugs.
o   For, highly polar, ionized and charged drugs, the model is referred to as membrane permeation rate limited.


ADVANTAGES OF PHYSIOLOGIC MODELING:
·         Mathematical treatment is straightforward.
·         Data fitting is not required; drug concentration in various body regions can be depicted on the basis of organ or tissue volume, perfusion rate and experimentally determined tissue to plasma partition coefficient.
·         The influence of altered physiology or pathology on drug disposition can be easily predicted from changes in various pharmacokinetic parameters since the parameters correspond to actual physiologic and anatomic measures.
·         Correlation of data in several animal species is possible and with some drugs, can be extrapolated to humans.
The only disadvantage of these comprehensive models is obtaining experimental data, which is very exhaustive

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