Absorption of Drugs

Absorption of Drugs

A drug injected intravascularly (intravenously and/or intra-arterially) directly enters the systemic circulation and exerts its pharmacological effects. However, majority of drugs are administered extravascularly, generally orally. If intended to act systemically, such drugs can exert their pharmacological actions only when they come into blood circulation from their site of application, and for this, absorption is an important prerequisite step.

Drug absorption is defined as the process of movement of unchanged drug from the site of administration to systemic circulation. Following absorption, the effectiveness of a drug can only be assessed by its concentration at the site of action. However, it is difficult to measure the drug concentration at such a site. Instead, the concentration can be measured more accurately in plasma. There always exist a correlation between the plasma concentration of a drug and the therapeutic response and thus, absorption can also be defined as the process of movement of unchanged drug from the site of administration to the site of measurement i.e. plasma. This definition takes into account the loss of drug that occurs after oral administration due to presystemic metabolism or first-pass effect.

Fig. 2.1. Plots showing significance of rate and extent of absorption in drug therapy.

Not only the magnitude of drug that comes into the systemic circulation but also the rate at which it is absorbed is important. This is clear from Fig. 2.1.

A drug that is completely but slowly absorbed may fail to show therapeutic response as the plasma concentration for desired effect is never achieved. On the contrary, a rapidly absorbed drug attains the therapeutic level easily to elicit pharmacological effect. Thus, both the rate and the extent of drug absorption are important. Such an absorption pattern has several advantages:

1. Lesser susceptibility of the drug for degradation or interaction due to rapid absorption.

2. Higher blood levels and rapid onset of action.

3. More uniform, greater and reproducible therapeutic response.

Drugs that have to enter the systemic circulation to exert their effect can be administered by three major routes:

1. The Enteral Route: includes peroral i.e. gastrointestinal, sublingual/buccal and rectal routes. The GI route is the most common for administration of majority of drugs.

2. The Parenteral Route: includes all routes of administration through or under one or more layers of skin. While no absorption is required when the drug is administered i.v., it is necessary for extravascular parenteral routes like the subcutaneous and the intramuscular routes.

3. The Topical Route: includes skin, eyes or other specific membranes. The intranasal, inhalation, intravaginal and transdermal routes may be considered enteral or topical according to different definitions.

Table 2.1 compares the bioavailability/absorption pattern and advantages and disadvantages of drugs administered by common routes.

TABLE 2.1.

Bioavailability/absorption of drug from common routes of drug administration