**3.4 Permeability assessment at the preformulation stage**

After dissolution, when a drug is present in the physiological fluids, such as in gastric juices, the small intestine or in plasma, it must have to penetrate cells and tissues to reach the intended site for action. Drug penetration is mediated by various transport mechanisms both passive and active. The drug must diffuse through aqueous pores in tissues or partition with the lipid components of cells to cause passive diffusion, however, active diffusion requires energy. The early drug development process; involves the in vitro models to forecast drug permeation because they offered a simple, repeatable way of monitoring drug absorption rate and mechanism with a favourable cost-benefit ratio. Some of the drug permeability assessment models are listed here.

**In-vitro permeability assessment models:**

