Drug absorption gastric mucosa

After the tablet disintegrates in the stomach the drug molecules are dispersed in gastric juice with or without partially digested food, and normally only a small proportion will penetrate the gastric mucosa and enter the blood circulation - partly because the stomach presents only a small surface area for absorption. Drugs are absorbed across mucosal surfaces but there are factors which determine how much is absorbed and at what rate in any particular site. The first set of factors is to do with the dmg molecule itself. 

Drugs in this class are well absorbed but their bioavailability may be limited either by first-pass metabolism or by P-glycoprotein-mediated efflux from the intestinal mucosa. In vitro-in vivo correlations of dissolution rate with the rate of drug absorption are expected if dissociation is slower than gastric emptying rate. If dissociation is sufficiently rapid, gastric emptying will limit absorption rate. 

It is obvious that drugs such as nifedipine and both isosorbide-5-mononitrate and isosorbide dinitrate, which have non-specific, wide absorption sites and so are well absorbed along the entire GI tract, may not be suitable candidates for GRDDS. Also, drugs that are irritant to the gastric mucosa " and those undergoing significant first-pass metabolism may have some limitations. Relevant examples of the latter type are nifedipine, propranolol, levodopa, diltiazem, metopro-lol and 5-fluorouracil. 

Since oral administration is the most common and useful, there are many opportunities in developing oral dosage forms. The oral route faces several obstacles to drug absorption, such as the first pass effect, individual variabihty of bioavailability, and drug-induced local irritation to gastric and intestinal mucosa. According to the US Food and Drug Administration (FDA) Biopharmaceutics Classification System... 

The insolubility of acetylsalicylic acid is a contributing factor to its irritant action on the gastric mucosa it is hydrolysed in aqueous solution. It has, however, been incorporated in a suppository base with macrogols 1540 and 6000 which increase its solubility and appear to improve the absorption of the drug [198]. 

---