Overview Of Factors Affecting Oral Drug Bioavailability

bioavailability of drugs

The oral absorption and bioavailability of a particular drug from a specific pharmaceutical dosage form is complicated. Yet, the significant steps involved in the process of oral drug absorption are four different parameters – drug dissolution, drug solubility, drug permeability, and pre-systemic metabolism. Many factors may adversely affect the processes mentioned above, thereby finally affecting the rate and extent of a drug’s oral bioavailability. Such factors may be categorized into three different categories -. The first category represents physicochemical properties, the second one represents physiological factors, and the third one represents dosage form factors. These three factors affecting the bioavailability and bioequivalence of the pharmaceutical drug product are as follows –


Physicochemical Factors Contributing to Bioavailability – 

Solubility – 

The critical parameter for establishing the oral bioavailability of a pharmaceutical substance is figuring its active compound dissolution. Solubility is defined as the extent to which an active ingredient is removed from its solid base with the aid of a desirable solvent.

Permeability – 

The pharmaceutical drug’s permeability should be assessed by evaluating specific drug characteristics such as hydrophilicity, lipophilicity, molecular flexibility, molecular size, etc. Both solute volume and hydrogen bond potential contribute to the permeability of the active ingredient within the biological cells’ microenvironment.

Physiological Properties Contributing to Bioavailability – 

How successful your orally administered medication turns out is determined by the gastro-intestinal processing of your active drug and its delivery system. Breakdown of the drug components reflecting their delivery to the intestine, absorption via lumen, intestinal metabolism, active extrusion, first-pass hepatic extraction plays a vital role in the measurement of the orally administered pharmaceutical entity.

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Presence of the bile salts – 

The presence of the sufficient quantity of the bile salts improvises the oral bioavailability of the poorly water-soluble components, thereby enhancing both their dissolution and solubility rates. Both the dissolution rate and the interfacial energy barrier are inversely proportional to each other.

The correlation between gastric emptying and intestinal transit time – 

Gastric emptying and their GI transmit time, both contribute to the onset and evaluation of the extent of absorption of the orally administered drugs. Gastric emptying plays a crucial role in establishing a plasma concentration profile. At the same time, the intestinal transmit significantly influences the process of absorption.

Liver metabolism – 

As human hepatic clearance contributes primarily to studying the oral drug absorption process, the liver is one of the target organs involved in the process bioavailability of drugs metabolism. Only a small portion or fraction of the drug entering your liver gets metabolized while undergoing transitioning through your liver.

Further, the orally administered pharmaceutical drugs may also undergo an extensive metabolism within your gut lumen in the presence of the digestive enzymes or gut microflora.

Food Effect – 

Evaluating the effect of food on the orally administered drug is a complex process. The physiological changes triggered by the food intake may slow down the gastric emptying procedure, thereby increasing its pH.

Thus, tracing the bioavailability and bioequivalence of an orally administered pharmaceutical product is a complex process and varies with the interdependence of the physicochemical properties and the physiological conditions of the concerned individual.

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