Oral route bioavailability factors

Any decrease in the total area under the curve for intramuscular or oral versus intravenous administration would represent a decrease in bioavailability based on that route of administration (16). Common factors that influence bioavailability include ... 

Thyroxine is absorbed best in the duodenum and ileum absorption is modified by intraluminal factors such as food, drugs, gastric acidity, and intestinal flora. Oral bioavailability of current preparations of L-thyroxine averages 80% (Table 38-1). In contrast, T3 is almost completely absorbed (95%). T4 and T3 absorption appears not to be affected by mild hypothyroidism but may be impaired in severe myxedema with ileus. These factors are important in switching from oral to parenteral therapy. For parenteral use, the intravenous route is preferred for both hormones. 

Absorption The process by which a dmg molecule moves from the site of administration into the systemic circulation (bloodstream). When a dmg is administered intravenously (IV), the dmg is 100% absorbed (bioavailability is 100%). However, when a drag is administered via other routes [such as orally (by mouth, PO, per os), subcutaneously (under the skin), intradermal (into the skin)], its absorption (bioavailabihty) is influenced by many factors, including the rate of dissolution, metabolism before absorption and the ability to cross the gastrointestinal tract (Martinez and Amidon, 2002). Therefore, bioavailability, as detailed in Chapter 2, is one of the essential tools in... 

Most peptides and proteins are currently formulated as parenteral formulations because of their poor oral bioavailability. Nevertheless, oral delivery of peptides and proteins would be the preferred route of administration if bioavailability issues could be overcome, as it offers the advantages of convenient, pain-free administration. Although various factors such as permeability, chemical and metabolic stability and gastrointestinal transit time can affect the rate and extent of absorption of orally administered peptides and proteins, molecular size is generally considered the ultimate obstacle [36]. 

Oral administration is the most popular route due to ease of ingestion, pain avoidance, versatility, (to accommodate various types of dmg candidates), and, most importantly, patient compliance [119]. In addition, solid oral delivery systems do not require sterile conditions and are, therefore, less expensive to manufacture. Orally delivered pharmacologically active compounds must have favorable absorption and clearance properties, and satisfactory metabolic stability to provide adequate systemic exposure to elicit a pharmacodynamic response. If the compounds possess reasonable physicochemical properties have low to intermediate clearance and reasonable absorption, adequate oral bioavailabdity may be achieved [120]. Indeed, oral bioavailability, defined as the rate and extent to which the active dmg is absorbed from a pharmaceutical form and becomes available at the site of dmg action [121], is influenced by several factors including solubility, permeability, intestinal and liver metabolism, rapid biliary and other efflux pump-mediated excretion, and conditions in the gastrointestinal milieu [122,123]. Thus, both absorption and elimination processes determine the oral bioavailability F of a given dmg. Accordingly, F can be estimated as... 

Bioavailability is the percentage of a dose the reaches the bloodstream. One hundred percent bioavailability occurs when the medication is injected into a vein compared with between 20% and 40% for a medication administered orally. Medication particles that do not reach the bloodstream are either misdirected or destroyed during the absorption process. For example, some particles of a medication that is administered orally are destroyed by hydrochloric acid in the stomach. The dose of a medication reflects the bioavailability of the medication based on the route that it is administered to the patient. For example, a medication that is administered orally may have a dose four times higher than if the medication is administered intravenously. Factors that alter bioavailability are ... 

Bio availability (symbol F) is a measure of the extent to which a drug reaches the bloodstream and is available at its site of action. The bioavailability of a drug administered by intra-venous (i.v.) injection is defined as 1 (since the entire dose is available in the systemic circulation). Problems start to appear, however, if the drug is administered by a non-parenteral route (e.g. oral, rectal, topical). In these cases, the bioavailability of a drug is often considerably less than 1 due to a number of factors, such as poor absorption from the gut (in the case of an oral medicine), extensive binding to circulating plasma proteins, or rapid first pass metabolism in the liver. 

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