Pharmacokinetics: How Your Body Processes Drugs

Absorption: Getting the Drug Into the Blood

Absorption is the movement of a drug from its site of administration into the systemic circulation. The route of administration fundamentally shapes this process. Intravenous (IV) injection bypasses absorption entirely by delivering the drug directly into the bloodstream, achieving 100% bioavailability. Every other route requires the drug to cross at least one biological membrane before reaching the blood.

For orally administered drugs, the most common route, absorption primarily occurs in the small intestine. Despite the stomach's acidic environment favoring the absorption of weakly acidic drugs, the small intestine's enormous surface area (roughly 250 square meters due to the folds, villi, and microvilli of the intestinal lining) makes it the dominant absorption site for most oral drugs. The drug must dissolve in the gastrointestinal fluid, cross the lipid bilayer of the intestinal epithelial cells, and enter the capillaries draining into the portal vein.

Several factors determine how well a drug is absorbed orally. Lipophilicity matters because cell membranes are composed of lipid bilayers, so lipophilic (fat-soluble) molecules cross more easily than hydrophilic (water-soluble) ones. Ionization state matters because most drugs are weak acids or weak bases that exist in equilibrium between ionized and unionized forms. Only the unionized form is lipophilic enough to cross membranes passively. The Henderson-Hasselbalch equation predicts the ratio of ionized to unionized drug at any given pH. Molecular size also plays a role: Christopher Lipinski's Rule of Five, developed at Pfizer, predicts that poor absorption is more likely when a molecule has a molecular weight above 500 daltons, more than 5 hydrogen bond donors, more than 10 hydrogen bond acceptors, or a calculated log P (a measure of lipophilicity) greater than 5.

Other routes of administration each have distinctive absorption characteristics. Sublingual administration (under the tongue) allows drugs to enter the blood through the thin oral mucosa, bypassing first-pass metabolism. Nitroglycerin for angina is given this way because first-pass metabolism would destroy most of the dose. Transdermal patches deliver drugs slowly through the skin for sustained systemic effects, as with nicotine patches or fentanyl patches. Inhalation delivers drugs to the lungs, whose massive surface area and thin alveolar membranes allow rapid absorption, ideal for drugs targeting the respiratory system (albuterol for asthma) or requiring fast systemic onset (inhaled anesthetics). Intramuscular and subcutaneous injections deposit the drug in tissue, from which it is absorbed into nearby capillaries at a rate determined by local blood flow and the drug's formulation.

Bioavailability and First-Pass Metabolism

Bioavailability (often abbreviated as F) is the fraction of an administered dose that reaches the systemic circulation in active form. For an IV dose, bioavailability is by definition 100% (F = 1). For any other route, bioavailability is typically less than 100% because of incomplete absorption, chemical degradation in the gut, and, for oral drugs, first-pass metabolism in the liver.

First-pass metabolism (also called the first-pass effect or presystemic metabolism) occurs because blood from the gastrointestinal tract drains into the portal vein, which delivers it to the liver before it reaches the systemic circulation. The liver's rich supply of metabolic enzymes can transform a significant portion of the drug into inactive metabolites before it ever reaches its target. Morphine has an oral bioavailability of only about 25% because roughly 75% is metabolized during first pass. Propranolol's oral bioavailability is only about 25 to 35% for the same reason. Some drugs have such extensive first-pass metabolism that oral administration is impractical, necessitating alternative routes.

Bioavailability is a critical parameter in drug development. When a generic version of a brand-name drug is approved, the manufacturer must demonstrate bioequivalence: the generic must produce the same plasma drug concentration profile (same peak concentration and same total exposure) as the original. Two formulations are considered bioequivalent if the 90% confidence interval for the ratio of their key pharmacokinetic parameters falls within 80 to 125%. This standard ensures that patients can switch between brand and generic versions without clinically meaningful differences in drug exposure.

Distribution: Where Drugs Go in the Body

Once a drug enters the systemic circulation, it distributes throughout the body. Distribution is not uniform: a drug's physicochemical properties, blood flow to different organs, and the ability to cross membrane barriers all influence where it accumulates.

Highly perfused organs like the liver, kidneys, heart, and brain receive drugs quickly because they receive a large fraction of cardiac output. Muscle and skin receive drugs more slowly, and poorly perfused tissues like fat and bone receive them slowest of all. However, lipophilic drugs may eventually accumulate in adipose tissue because of its high lipid content, creating a drug reservoir that slowly releases drug back into the bloodstream even after dosing stops. This is why highly lipophilic drugs like thiopental (an anesthetic) and THC (the active component of cannabis) can have effects that persist long after administration.

Plasma protein binding significantly affects distribution. Many drugs bind reversibly to plasma proteins, primarily albumin (which binds acidic drugs) and alpha-1-acid glycoprotein (which binds basic drugs). Only the unbound (free) fraction of drug is pharmacologically active, because only free drug can cross membranes, bind to receptors, and undergo metabolism and excretion. Warfarin, for example, is approximately 99% protein-bound, meaning that only about 1% of the total plasma warfarin is free and active. Small changes in protein binding, caused by liver disease (which reduces albumin production) or displacement by another highly bound drug, can produce large changes in the free drug concentration and potentially cause toxicity.

The volume of distribution (Vd) is a theoretical pharmacokinetic parameter that relates the total amount of drug in the body to the plasma concentration. It is calculated as Vd = total drug in body / plasma concentration. A small Vd (close to the plasma volume of about 3 liters) suggests the drug remains mostly in the blood. A Vd larger than total body water (about 42 liters in a 70 kg person) indicates the drug is extensively distributed into tissues. Chloroquine, an antimalarial, has a Vd of over 13,000 liters, meaning it is overwhelmingly concentrated in tissues rather than blood.

The blood-brain barrier (BBB) is one of the most important distribution barriers in pharmacology. Formed by tight junctions between brain capillary endothelial cells, the BBB restricts the passage of hydrophilic and large molecules from the blood into the brain. Only small, lipophilic molecules or those with specific transport mechanisms can cross. This barrier protects the brain from potentially harmful substances but also makes treating brain diseases challenging: many potentially effective drugs simply cannot reach their target. Strategies to overcome the BBB include designing prodrugs that exploit active transport mechanisms, using nanoparticle delivery systems, and, in extreme cases, direct intracerebral injection.

Metabolism: Transforming Drugs for Elimination

Drug metabolism, also called biotransformation, converts drugs into more water-soluble forms that can be excreted by the kidneys. The liver is the primary metabolic organ, though metabolism also occurs in the gut wall, kidneys, lungs, and even the blood itself. Metabolism typically reduces or eliminates a drug's pharmacological activity, but not always: some drugs are administered as inactive prodrugs that require metabolic activation to produce their therapeutic effect.

Phase I reactions modify the drug molecule by introducing or exposing a functional group through oxidation, reduction, or hydrolysis. The cytochrome P450 (CYP) enzyme superfamily dominates Phase I metabolism. These enzymes are embedded in the smooth endoplasmic reticulum of hepatocytes and use molecular oxygen to oxidize substrates. CYP3A4 alone is responsible for the metabolism of roughly half of all clinically used drugs. Other important CYP isoforms include CYP2D6 (responsible for metabolizing many cardiovascular and psychotropic drugs), CYP2C9 (warfarin, phenytoin), CYP2C19 (omeprazole, clopidogrel), CYP1A2 (caffeine, theophylline), and CYP2E1 (ethanol, acetaminophen).

Phase II reactions attach a polar conjugating group to the drug or its Phase I metabolite, dramatically increasing water solubility. The most common Phase II reaction is glucuronidation, catalyzed by UDP-glucuronosyltransferase (UGT) enzymes. Other Phase II pathways include sulfation, acetylation, methylation, and glutathione conjugation. The resulting conjugates are generally pharmacologically inactive, highly water-soluble, and readily excreted by the kidneys or in bile.

CYP enzyme activity varies substantially between individuals due to genetic polymorphisms, environmental factors, and drug interactions. Enzyme induction occurs when exposure to certain substances (rifampin, carbamazepine, chronic alcohol use, smoking) increases the expression of CYP enzymes, accelerating the metabolism of co-administered drugs. Enzyme inhibition occurs when a substance (ketoconazole, erythromycin, grapefruit juice, cimetidine) blocks a CYP enzyme, slowing metabolism and raising plasma levels of co-administered drugs. These interactions can have serious clinical consequences, particularly for drugs with narrow therapeutic indices.

Excretion: Removing Drugs from the Body

Excretion is the irreversible removal of a drug or its metabolites from the body. The kidneys are the most important excretory organs for drugs, though some substances are excreted in bile (and eliminated in feces), exhaled through the lungs, or secreted in sweat or breast milk.

Renal excretion involves three mechanisms. Glomerular filtration passively filters unbound drug from the blood through the glomerular capillaries into the nephron tubule. Only free drug (not protein-bound) is filtered. Active tubular secretion uses carrier-mediated transport in the proximal tubule to pump drug from the blood into the tubular fluid, a process that can handle both bound and unbound drug because the binding equilibrium shifts as free drug is removed. Tubular reabsorption occurs in the distal tubule, where lipophilic, unionized drug passively diffuses back from the tubular fluid into the blood. Manipulating urine pH can change the ionization state of drugs and alter their reabsorption: alkalinizing the urine with sodium bicarbonate traps weakly acidic drugs (like aspirin) in their ionized form in the tubular fluid, promoting their excretion. This principle is used clinically to treat certain drug overdoses.

Biliary excretion is important for large, polar molecules and drug-glucuronide conjugates. These substances are actively transported from hepatocytes into bile, which drains into the small intestine. Some glucuronide conjugates are hydrolyzed by bacterial enzymes in the gut, regenerating the active drug, which can then be reabsorbed (enterohepatic circulation). This recycling prolongs the drug's presence in the body and can make elimination slower than kidney excretion alone would predict.

Key Pharmacokinetic Parameters

Half-life (t1/2) is the time required for the plasma concentration of a drug to decrease by 50%. After approximately five half-lives, over 97% of the drug has been eliminated, and the drug is considered effectively cleared. Half-life also determines how long it takes to reach steady state during repeated dosing: steady state is reached after approximately five half-lives of consistent dosing, at which point the rate of drug input equals the rate of drug elimination.

Clearance (CL) is the volume of plasma from which drug is completely removed per unit time. It reflects the body's overall efficiency at eliminating a drug and is determined primarily by hepatic metabolism and renal excretion. Total body clearance is the sum of hepatic clearance, renal clearance, and any other elimination routes. Clearance, together with bioavailability, determines the maintenance dose needed to achieve a desired average steady-state concentration.

Area under the curve (AUC) represents the total drug exposure over time. Plotted on a graph of plasma concentration versus time, the AUC is the integral of the concentration-time curve from zero to infinity. AUC is directly proportional to the total amount of drug that reaches the systemic circulation and is the primary measure used in bioequivalence studies.

These parameters are not merely academic. They directly inform clinical dosing decisions. A drug with a long half-life can be given once daily. A drug with a short half-life requires multiple daily doses or a sustained-release formulation. Patients with kidney disease need dose reductions for renally cleared drugs. Patients with liver disease need dose adjustments for hepatically metabolized drugs. Understanding pharmacokinetics turns drug prescribing from guesswork into quantitative science.