Absorption: The First Stage of Pharmacokinetics
Updated: Sep 16, 2022
Absorption is the first stage of pharmacokinetics and alludes to the process of drugs entering the blood circulation from a site of administration. The rate and extent of absorption rely on the route of administration, the formulation and chemical properties of the drug, and physiologic variables that can affect the site of absorption. Moreso to direct intravenous administration, the amount and speed of drug absorption are associated with the route of drug administration, physical and chemical properties, and absorption environment.
The Four Stages of Pharmacokinetics
Factors of Absorption
After administration, the rate and extent of the drug will not be entirely absorbed into the bloodstream. The amount absorbed into the circulatory system is dubbed bioavailability, conveyed as a percentage of the amount provided.
Bioavailability and rate of absorption rely on how efficiently the drug can diffuse from its site of administration. Diffusion is in reference to when the substance is spreading out like going from a location of high concentration to low concentration. Drugs able to pass through membranes can diffuse quicker than ones that cannot.
A drug’s ability to permeate a membrane is dependent on certain factors of a drug. Drugs that are larger molecules, ionized chemicals, and hydrophilic substances have difficulty passing through membranes. This is due to the phospholipid bilayer that forms cell membranes consisting of hydrophilic heads and uncharged tails that oppose hydrophilic and ionized molecules.
Diffusion that occurs on its own is passive diffusion, but another type is active transport mechanisms. These mechanisms like transport proteins and ion channels utilize energy but can transfer larger molecules against concentration gradients. Active transport allows for larger molecules to move through membranes and gets absorbed.
The route of administration, or the journey that a drug takes in the body, is another factor that the rate of absorption and bioavailability are also dependent on. Certain journeys of a drug are more efficient than others. The route of administration is usually affected by the drug’s pharmaceutics, like how dosage forms of a drug can decide how it is used.
Routes of Administration
Major Routes of Administration
Intravenous administration of a drug does not require absorption since the drug is transferred from the site of administration directly to the bloodstream. For instances of intravenous administration, the whole dose of the drug can navigate to the sites of drug action. Other routes of administration yield less ability to navigate as a result of incomplete absorption. When this happens, less of the drug travels through the bloodstream to the target site of the drug.
Extravascular delivery routes, specifically oral dosing, are pertinent and prominent mechanisms of drug administration. Dissimilar to intravenous administration, where the drug is injected precisely into the plasma, pharmacokinetic models after extravascular drug administration need to consider systemic drug absorption from the site of administration (I.e., the gut, lung, etc. plasma).
Extravascular drug delivery is made more difficult by different factors at the absorption site, alongside potential drug degradation, metabolism, and significant inter-and intrapatient distinctions in the rate and extent of absorption. Absorption and metabolic factors are differentiated via pharmacokinetic methods. The deviations in systemic drug absorption can be reduced via biopharmaceutical design of the dosage forms to offer expected and dependable drug therapeutics. The greatest advantage of intravenous administration relative to extravascular drug absorption is that the extent and rate of systemic drug additions are attentively measured.
Overall, the goal of drug administration is to obtain a drug concentration that is therapeutic in the bloodstream. There are different routes of administration like:
1. Enteral routes pass through the gastrointestinal tract and tend to have enteral routes that have low bioavailability and slow rates of absorption.
Sublingual(buccal)- Drugs provided beneath the tongue or confined to the cheek pouch and absorbed from these regions into the circulation nearby.
Oral- Common drug route, where the passage of the drug goes from the gut into the blood is determined by physiochemical and biological variables alongside dosage forms.
Rectal- Suppositories are administered for circumstances where the oral route is challenging.
2. Parenteral routes do not pass through the gastrointestinal tract, commonly involving a form of injections, however, non-injection routes are available.
Intravenous injection-utilized in accelerated clinical responses, allowing for relatively accurate drug concentrations into the circulation, since bioavailability is not a factor.
Intra-arterial injection- utilized in significant circumstances, like anticancer drugs, to distribute a high drug concentration to a specific target tissue.
Intrathecal injection- the blood-brain barrier prevents the passage of many drugs to cerebrospinal fluid. In certain situations, mainly life-threatening, antibiotics, antifungals, and anticancer drugs are injected by lumbar puncture and injection in subarachnoid space.
Intramuscular injection- drugs may be injected into the buttocks, thigh, and arm. Due to variables in vascularity rates of absorption differ.
Subcutaneous injection- Certain drugs, like insulin, are commonly delivered subcutaneously. Drug absorption is slower subcutaneously than intramuscularly as a result of poorer vascularity. Absorption can be assisted by heat, massage, or vasodilators. It can also be slowed by coadministration of vasoconstrictors, usually used to prolong local anesthetics.
Inhalation- Volatile anesthetics, alongside drugs that affect pulmonary function, and delivered as aerosols. The large alveolar area and blood supply lead to quick absorption into the circulatory system.
Eye- for appropriate provincial effects.
Intravaginal- for contraception or infections.
Intranasal- for the comfort of relative symptoms.
Skin- topical drug delivery for skin diseases reduces systemic exposure. Despite this, systemic absorption is possible and changes with the range, site, state of skin, and drug.
Drug patches- a consistent amount of the drug arrives in the bloodstream.
Drug Administration Routes Summary
Absorption in pharmacokinetics is simply the movement of a drug from the site of administration to the bloodstream, comprehension of how the drug is absorbed in the circulatory system is a pertinent aspect of pharmacokinetics.
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