n the small intestine near the absorption site. PH-dependent materials, which are insoluble in the acidic medium of the stomach, but dissolve in the intestinal fluid, are called enteric materials [94, 95].

These materials have been used to avoid the degradation of labile drugs caused by the acidic medium or gastric enzymes, to lessen irritation of the gastric mucosa, and to deliver drugs selectively to the site of absorption [94, 95]. Enteric coating materials are polymers, which have acid groups. In the acidic medium of the stomach the acid groups are nonionized, and the coating material is insoluble. Fast dissolution and drug release take place in the upper intestine as a function of pH change in the environment. The polymer acid groups are ionized at higher pH and the material dissolves [95].

Cellulose acetate phthalate (CAP) was the first synthetic polymer described in 1937, which gained soon high popularity as a gastric resistant polymer.
Later polyvinyl acetate phthalate (PVAP) and hydroxypropyl methylcellulose phthalate (HPMCP) were preferred, because of their lower permeability in the gastric fluid and improved stability against hydrolysis. Today the methacrylate copolymers Eudragit® L and S are two of the most widely used polymers for this purpose.

The drug release from the pH-sensitive nanoparticles follows certain mechanisms which include:
1- Drug burst releases when the nanoparticle carriers dissolve at specific pH conditions:
They usually exhibited burst release profiles because of the dissolution characters of the carriers; drug release from conventional nanoparticles was mainly by diffusion. For pH- sensitive