Many drugs exhibit absorption windows in particular portion of gastrointestinal tract which can limit the bioavailability of orally administered compounds and can be a major obstacle to the development of controlled release formulations for such regional specific drugs. Various approaches to increase the residence of drug formulations at or above the absorption window to enhance the bioavailability of such drugs are presented in this review. A good understanding of gastrointestinal transit in humans and the effect of factors such as food can be helpful in the design of rational drug delivery systems that will have clinical advantage. Different approaches presently being explored includes (i) Modification of intestinal transit time (ii) bioadhesive microspheres that have a slow intestinal transit; and (iii) the gastroretentive dosage system, which is further divided into several approaches like floating , swelling, expanding and magnetic systems. These approaches are promisingly found to improve bioavailability of different narrow absorption window drugs compared to conventional dosage forms.
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