In addition, the absorption of drugs can be limited by their poor chemical and biological stability, as well as by physiological barriers, including pH, efflux transporters, and metabolic enzymes. Furthermore, orally administered drugs can be targeted to particular regions within the gastrointestinal (GI) tract for localized treatment of pathological conditions such as stomach and colorectal cancers, infections, inflammations, bowel diseases, gastro-duodenal ulcers, and gastroesophageal reflux disorders ( Figure 1).ĭespite these advantages, the development of oral formulations presents several challenges, which are mainly attributed to the physicochemical properties of drugs, including poor water solubility and membrane permeability. The compliance of patients to oral formulations is generally higher than that to other parenteral routes such as intravenous, subcutaneous, and intramuscular injections, as well as to inhalation for asthma medications ( Ingersoll and Cohen, 2008). Around 84% of the best-selling pharmaceutical products are orally administered and are currently valued at $35 billion, with an annual growth rate of 10% ( Prasad et al., 2017). Current estimates indicate that oral formulations represent about 90% of the global market share of all pharmaceutical formulations intended for human use. Around 60% of established small-molecule drug products available commercially are administered via the oral route. Oral medication is the most common form of drug administration because of advantages such as convenience of drug administration via the oral route, patient preference, cost-effectiveness, and ease of large-scale manufacturing of oral dosage forms. The challenges and development aspects of pediatric formulations will also be addressed. To this end, this review will discuss the physiological, and pharmaceutical barriers influencing drug bioavailability for the oral route of administration, as well as the conventional and novel drug delivery strategies. Different pharmaceutical technologies and drug delivery systems including nanocarriers, micelles, cyclodextrins and lipid-based carriers have been explored to enhance oral drug absorption.
Attempts to overcome these factors have focused on understanding the physicochemical, biochemical, metabolic and biological barriers which limit the overall drug bioavailability. Various factors govern oral drug absorption including drug solubility, mucosal permeability, and stability in the gastrointestinal tract environment. It is the most preferred route, due to its advantages, such as non-invasiveness, patient compliance and convenience of drug administration. The oral route is the most common route for drug administration. 3Department of Pharmacognosy, College of Pharmacy, King Saud University, Riyadh, Saudi Arabia.2Nanobiotechnology Unit, Department of Pharmaceutics, College of Pharmacy, King Saud University, Riyadh, Saudi Arabia.
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