Definition/Introduction

Pharmacokinetics describes the study of drug absorption, distribution, metabolism, excretion, and how the body affects the drug. The application of pharmacokinetic methods to ensure patients are treated safely and effectively is known as clinical pharmacokinetics. The introduction of pharmacokinetics as a discipline has facilitated the development of rational drug therapy, the understanding of drug action and metabolism, the understanding of the concentration-effect relationship, and the establishment of dosage regimens.[1] 

The most fundamental principle in pharmacokinetics theory is drug absorption, defined as the transportation of the unmetabolized drug from the administration site to the body circulation system.[2] Several mechanisms of drug absorption have been identified, including passive diffusion, carrier-mediated membrane transport such as active and facilitated diffusion, and other nonspecific drug transporters, such as P-glycoprotein. Different factors can affect drug absorption; these factors can be classified as drug-specific and patient-specific. The percentage of drug absorption varies among different routes of administration, such as oral, subcutaneous (SQ), transdermal, intravenous (IV), and intramuscular (IM). Since the oral route is the primary route of administration, the primary emphasis of this article will be on gastrointestinal (GI) drug absorption. The bioavailability of a drug product is known as the rate and extent of its absorption. Understanding the drug absorption process and affecting factors is essential in achieving better bioavailability and therapeutic effects.[3] This article will discuss the different mechanisms of drug absorption, the affecting factors, and their correlation to bioavailability.

Issues of Concern

Regardless of the absorption site, the drug must cross the cell membrane to reach the systemic circulation. This can occur through passive (simple) diffusion or carrier-mediated membrane transporters. 

The most common mechanism of absorption for drugs is passive diffusion. This process can be explained through the Fick law of diffusion, in which the drug molecule moves according to the concentration gradient from a higher drug concentration to a lower concentration until equilibrium is reached.[4] Passive diffusion can occur in an aqueous or lipid environment. Aqueous diffusion occurs in the aqueous compartment of the body, such as interstitial space, or through aqueous pores in the endothelium of blood vessels. Drugs bound to albumin or large plasma proteins cannot permeate most aqueous pores. On the other hand, lipid diffusion occurs through the lipid compartment of the body. Therefore, it is considered the most critical factor for drug permeability due to the greater number of lipid barriers that separate the body's compartments. The lipid-aqueous partition coefficient of the drug can be used to determine how rapidly the drug moves between lipid and aqueous mediums.

Another mechanism of absorption is via carrier-mediated membrane transporters. Numerous specialized carrier-mediated membrane transport systems are in the body to transport ions and nutrients, particularly in the intestine. Such systems include active and facilitated diffusion. Active diffusion is an energy-consuming system essential for GI absorption and renal and biliary excretion of many drugs. This process facilitates the absorption of some lipid-insoluble drugs, which mimic natural physiological metabolites such as 5-fluorouracil from the GI tract. In contrast to passive diffusion, active diffusion enables the movement of drugs from regions with low drug concentrations to regions with higher drug concentrations.

With active diffusion, the carrier binds to form a complex with the drug. This complex facilitates the transportation of the drug across the membrane and then disassociates on the other side. The carrier molecule may be highly specific to the drug molecule. Drugs sharing similar structures can compete for the carrier in absorption sites. Since only a small number of carrier molecules are available, the binding sites on the carrier may become saturated if the drug concentration is very high, after which the dose increases do not affect the drug concentration. While some transporters facilitate absorption, transporters such as P-glycoprotein (P-gp) can effectively impede drug absorption. P-gp (MDR1) is an energy-dependent efflux transporter that facilitates the secretion of molecules back into the intestinal lumen, thereby restricting overall absorption.[5] Facilitated diffusion is another transporter system that plays a minor role in drug absorption. It is similar to the active diffusion system in that both are saturable and exhibit drug selectivity and competition kinetics. However, the main differences are that facilitated diffusion does not require energy and, unlike active transport, does not enable the movement against a concentration gradient. An example of a facilitated diffusion system is the organic cation transporter 1 (OCT1), which facilitates the movement of drugs like metformin.[6]

Drug-specific factors that affect drug absorption include drugs' physicochemical and pharmaceutical variables. One example of the physicochemical variables is the drug solubility and the effect of pH and pK_a, where most drugs act as weak acids or bases in solutions in both ionized and non-ionized forms. The ionized drugs are hydrophilic and cannot cross the cell membrane. In contrast, the non-ionized drugs appear to be lipophilic and can penetrate the cell membrane easily by simple diffusion. The distribution of weak electrolytes across membranes would result from the pH gradient across the membrane and the drug's pK_a. Weakly acidic drugs are easily absorbed in an acidic environment such as the stomach. Meanwhile, weakly basic drugs are not absorbed until they reach the higher pH medium in the small intestine.[7][8] 

Other physicochemical variables such as particle size and surface area, dissolution rate, amorphism, polymorphism characteristics, and nature of the dosage form will also affect systemic drug absorption.[9] The dissolution rate is the amount of the solid substance that turns into a solution per time at standard pH conditions, solvent composition, and temperature, with a constant surface area. For example, cisapride is a gastroprokinetic agent with low aqueous solubility. However, it has good oral bioavailability due to its rapid dissolution rate in GI fluids. Particle size is inversely related to the dissolution rate. Thus, reducing particle size increases surface area and a higher dissolution rate. Micronizing the drug particles increases the dissolution rate and solubility. For example, digoxin is found to have 100% bioavailability in the micronized tablet. Furthermore, the internal structure of the drug can be either in a crystalline or amorphous form.

Polymorph is a term in which the solid substance has more than one crystalline form. The polymorphs can vary in their physical properties, such as solubility, hardness, and melting point. For example, chloramphenicol palmitate has three polymorphic forms: A, B, and C. Among these, form B is found to have the highest absorption and bioavailability. Pharmaceutical variables include the presence of different excipients (inactive ingredients), which may increase or decrease the absorption rate depending on the added ingredient. There are several dosage forms in which the drug can be administered. Each dosage form has a different absorption rate depending on many factors, including the dosage form's nature and the administration site. Generally, for orally administered dosage forms, solutions have a higher absorption rate. Other pharmaceutical variables include drug expiration and storage conditions.[7]

Patient-specific factors affecting drug absorption (physiological variables) include age, gastric emptying time, intestinal transit time, disease status, blood flow at the absorption site, presystemic metabolism, and GI content. With increased age, many physiological changes occur, which may lead to decreased drug absorption. Critically ill patients may have reduced blood flow to the GI tract, resulting in reduced drug absorption. Generally, intestinal absorption is more critical for most drugs than any other site in the GI tract due to the increased surface area of the intestinal mucosa. The duodenal mucosa has the quickest drug absorption because of such anatomical characteristics as villi and microvilli, which provide a large surface area. However, these villi are much less abundant in other parts of the GI tract. Drugs may be absorbed from the GI tract at a different rate. Before orally administered drugs reach the circulation, they can be metabolized within the gut wall or the liver. This is known as first-pass metabolism, which will decrease the amount of active drug absorbed. Food content appears to affect the absorption rate of many orally administered drugs. For example, the absorption rate of levodopa is decreased when administered with protein-containing food, and the absorption of albendazole is enhanced with lipid-containing food.[10]

Clinical Significance

The benefit and toxicity of a drug are determined by its concentration in the plasma. Bioavailability is essential in maintaining drug plasma concentration within the therapeutic range. Since the bioavailability of a drug is directly dependent on the rate and extent of drug absorption at the site of administration, factors affecting drug absorption, including the route of administration, directly affect the bioavailability of that drug. For a drug with optimal physicochemical absorption properties under normal physiological conditions, the rate and extent of drug absorption are directly affected by the route by which the drug is administered.

Generally, the order of bioavailability among different routes of administration ranked highest to lowest is parenteral, rectal, oral, and topical, respectively.[2] Drugs administered intravenously (IV) achieve 100% bioavailability because they can directly reach the systemic circulation without the absorption process. IV drugs are usually administered when a rapid onset of response is required, such as in emergency cases. Other situations where IV drugs are required include unconscious patients, those who have a non-functional GI tract, when there is an unavailability of an oral dosage form to avoid hepatic first-pass metabolism, or when there is a need for tissue penetration that is not achievable by oral therapy.[11] 

Although the bioavailability of orally administered drugs is complex and variable depending on the factors affecting the absorption process, it is more convenient for many patients. It is the most common route of administration used for most drugs.[12] Bioavailability and pharmacokinetic studies aim to identify a suitable dosage regimen for a new drug candidate to ensure its therapeutic effectiveness and safety, to identify new formulations of the existing drugs, compare the bioavailability for a drug with different dosage forms or the same dosage form of another manufacturer, and to control the quality of a drug product during the early stages of marketing by the determination of physicochemical and physiological factors on absorption.[13]

Absolute bioavailability is a test in which the bioavailability of an orally administered drug is compared to its bioavailability following IV administration. Bioavailability helps identify the absorption characteristics of a drug administered orally. Another type of bioavailability test is called relative bioavailability, in which the bioavailability of an orally administered drug is compared with that of an oral standard of the same drug. It is also known as comparative bioavailability. In contrast to absolute bioavailability, it identifies a drug's absorption characteristics of different formulations. Bioequivalence studies are conducted to differentiate between two drug products having the same active ingredients. It is helpful in the comparison between brand and generic formulations.[14] 

Drug absorption and bioavailability are essential aspects of pharmacokinetics. They influence drug effectiveness and safety. They can also affect the onset, intensity, and sometimes the duration of action. Many factors can affect the absorption and bioavailability of drugs; some are drug-specific, while others are patient-specific. Bioavailability and pharmacokinetic studies aim to optimize drug use by maintaining the drug concentration within the therapeutic range.

Nursing, Allied Health, and Interprofessional Team Interventions

Medication management requires an interprofessional team of healthcare professionals, including nurses, laboratory technologists, pharmacists, and physicians. Without proper medication management, morbidity and mortality from various health conditions can be high. An adequate understanding of drug absorption can facilitate interprofessional collaborations to improve patient health outcomes.