# Lecture 01: Pharmacokinetics
## Unit 2.12: Pharmacology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the processes of drug absorption and bioavailability
2. Explain drug distribution and plasma protein binding
3. Describe drug metabolism and the cytochrome P450 system
4. Explain drug elimination and clearance
5. Describe pharmacokinetic parameters and calculations
6. Explain special population considerations in pharmacokinetics
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## Lecture Outline
### I. Overview of Pharmacokinetics
Pharmacokinetics represents one of the two fundamental pillars of pharmacology, describing what the body does to a drug once it enters the system. This discipline stands in complementary relationship to pharmacodynamics, which conversely examines what the drug does to the body. The acronym ADME serves as the foundational framework for understanding pharmacokinetics, representing the four sequential processes of Absorption, Distribution, Metabolism, and Excretion that every drug undergoes. Understanding these processes is essential for predicting drug behavior, optimizing therapeutic regimens, and minimizing adverse effects in clinical practice.
The four ADME processes occur in a coordinated sequence that determines the ultimate fate of any administered medication. Absorption refers to the entry of a drug from its site of administration into the systemic circulation, which varies significantly based on the route of administration. Distribution describes the movement of drug molecules from the bloodstream into various tissues and body compartments. Metabolism involves the biochemical transformation of drug molecules, primarily in the liver, while excretion encompasses the elimination of drugs and their metabolites from the body.
Several key pharmacokinetic parameters quantify drug behavior within the body and guide clinical dosing decisions. Bioavailability (F) represents the fraction of an administered dose that reaches the systemic circulation in unchanged form. Volume of distribution (Vd) is an apparent volume that describes the relationship between total drug in the body and plasma concentration. Clearance (CL) measures the volume of plasma from which drug is completely removed per unit time, while half-life (t1/2) indicates the time required for plasma concentration to decrease by fifty percent.
Clinical application of pharmacokinetic principles guides rational drug dosing strategies in patient care. Loading doses are calculated to rapidly achieve therapeutic plasma concentrations, particularly important for drugs with long half-lives. Maintenance doses sustain therapeutic levels at steady state by replacing the amount of drug eliminated during each dosing interval. Dosing intervals are typically determined based on the drug's half-life to maintain concentrations within the therapeutic window. Drug interactions that alter ADME processes can significantly impact therapeutic outcomes and must be carefully considered in polypharmacy situations.
Panel A: Circular diagram showing the four ADME processes with arrows indicating drug flow from oral administration through absorption in the GI tract, distribution to tissues, metabolism in the liver, and excretion via kidneys. Panel B: Plasma concentration-time curve showing the absorption phase rising to Cmax, followed by the distribution and elimination phases, with therapeutic range indicated by horizontal dashed lines. Panel C: Mathematical formulas displayed for bioavailability (F = AUC oral/AUC IV), volume of distribution (Vd = Dose/C0), clearance (CL = Rate of elimination/C), and half-life (t1/2 = 0.693 x Vd/CL). Panel D: Clinical dosing diagram comparing loading dose administration achieving immediate therapeutic levels versus gradual accumulation with maintenance doses alone over multiple half-lives.
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### II. Drug Absorption
Drug absorption occurs through several distinct mechanisms that determine how medications cross biological membranes to reach the systemic circulation. Passive diffusion represents the most common absorption mechanism, driven by concentration gradients and favoring lipophilic drug molecules that can readily traverse the lipid bilayer of cell membranes. Facilitated diffusion employs carrier proteins to transport drugs across membranes without requiring energy expenditure. Active transport mechanisms can move drugs against their concentration gradients but require ATP hydrolysis, while pinocytosis enables absorption of large molecules through vesicular engulfment.
Multiple factors influence the rate and extent of drug absorption from any administration site. Lipophilicity enhances absorption as more lipid-soluble drugs traverse biological membranes more easily. Molecular size affects absorption inversely, with smaller molecules absorbing more readily than larger compounds. The ionization state of a drug significantly impacts absorption, as non-ionized forms cross membranes more efficiently according to the pH-partition hypothesis. Surface area at the absorption site plays a critical role, with the gastrointestinal tract providing approximately 200 square meters for absorption. Blood flow to the absorption site determines how quickly absorbed drug is carried away, maintaining concentration gradients.
Various routes of administration offer distinct pharmacokinetic profiles suited to different clinical situations. Intravenous administration provides 100% bioavailability with immediate onset, serving as the reference standard for bioavailability comparisons. Intramuscular and subcutaneous injections achieve 75-100% bioavailability with onset within minutes, suitable for medications requiring parenteral delivery. Oral administration offers convenience but variable bioavailability and slower onset of 30-90 minutes due to absorption and first-pass effects. Sublingual and buccal routes bypass first-pass metabolism, providing rapid onset for drugs like nitroglycerin. Inhalation achieves rapid systemic delivery for volatile anesthetics and respiratory medications, while transdermal administration provides sustained release over hours to days.
First-pass metabolism represents a critical phenomenon that reduces oral bioavailability of many drugs before they reach systemic circulation. This process occurs as orally administered drugs pass through the gut wall and liver via the portal circulation before entering the systemic bloodstream. Enzymes in the intestinal epithelium and hepatocytes metabolize a fraction of the absorbed drug during this first passage. Drugs with extensive first-pass metabolism include morphine, propranolol, and nitroglycerin, which have dramatically lower oral bioavailability compared to parenteral administration. Alternative routes such as intravenous, sublingual, transdermal, and rectal administration can circumvent first-pass metabolism when higher bioavailability is clinically necessary.
Panel A: Cellular membrane diagram illustrating four absorption mechanisms - passive diffusion showing lipophilic drug crossing the lipid bilayer, facilitated diffusion with carrier protein, active transport with ATP expenditure, and pinocytosis engulfing large molecules. Panel B: Comparative chart of administration routes showing IV needle entering vein (100% F, immediate), oral tablet in GI tract (variable F, 30-90 min), sublingual tablet dissolving under tongue (rapid onset), and transdermal patch on skin (sustained release). Panel C: Portal circulation diagram showing oral drug absorption through intestinal villi, portal vein carrying drug to liver, first-pass metabolism occurring in hepatocytes with CYP450 enzymes, and remaining drug entering systemic circulation via hepatic vein. Panel D: Bar graph comparing bioavailability of high first-pass drugs by route, showing morphine, propranolol, and nitroglycerin with dramatically higher bioavailability via IV versus oral administration.
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### III. Bioavailability
Bioavailability quantifies the fraction of an administered drug dose that reaches the systemic circulation in pharmacologically active form and represents a fundamental pharmacokinetic parameter. Absolute bioavailability is calculated by comparing the area under the plasma concentration-time curve (AUC) following non-intravenous administration to the AUC following intravenous administration of the same dose. The formula F = AUC(oral) / AUC(IV) yields a value between 0 and 1, with intravenous administration defined as having 100% bioavailability. Relative bioavailability compares two different formulations or routes of the same drug, useful for assessing generic formulations against brand-name products.
Numerous factors influence the bioavailability of orally administered medications and must be considered in clinical prescribing. Drug formulation significantly affects bioavailability, with liquid preparations generally absorbing faster than tablets, and extended-release formulations designed for sustained absorption. First-pass metabolism in the gut wall and liver reduces bioavailability, sometimes dramatically, as with drugs like morphine. Drug interactions can alter bioavailability through effects on gastric pH, motility, or metabolic enzymes. Gastrointestinal conditions including pH changes, altered motility, and presence of food can enhance or diminish absorption. Efflux transporters, particularly P-glycoprotein, actively pump drugs back into the intestinal lumen, reducing the amount reaching systemic circulation.
Bioequivalence testing ensures that generic medications perform equivalently to their brand-name counterparts and represents a regulatory requirement for generic approval. Two formulations are considered bioequivalent when they demonstrate the same rate and extent of absorption under similar experimental conditions. Generic drug manufacturers must conduct bioequivalence studies demonstrating that their product falls within acceptable limits compared to the reference product. Key metrics assessed include AUC (extent of absorption), Cmax (peak concentration), and Tmax (time to peak concentration). Drugs with narrow therapeutic indices require more stringent bioequivalence criteria due to the clinical significance of even small variations in drug exposure.
Understanding bioavailability has direct clinical implications for drug dosing and patient care decisions. Drugs with low oral bioavailability require substantially higher oral doses compared to parenteral administration to achieve equivalent plasma concentrations. When high first-pass metabolism limits oral bioavailability, alternative routes such as sublingual, transdermal, or rectal may provide improved drug delivery. Food effects on bioavailability dictate whether medications should be taken with meals, on an empty stomach, or without regard to food. P-glycoprotein inhibitors can increase bioavailability of substrate drugs, requiring dose adjustments to prevent toxicity from increased systemic exposure.
Panel A: Graph overlaying two plasma concentration-time curves comparing oral and IV administration of the same drug, with AUC shaded under each curve and the bioavailability formula F = AUC(oral)/AUC(IV) displayed. Panel B: Diagram showing factors affecting bioavailability including formulation type (tablet vs liquid vs extended-release), gut wall metabolism, hepatic first-pass effect, and P-glycoprotein efflux pumping drug back into intestinal lumen. Panel C: Bioequivalence study plot showing 90% confidence intervals for AUC ratio and Cmax ratio of test generic versus reference brand product, with acceptance limits of 0.80-1.25 marked. Panel D: Clinical decision flowchart for low bioavailability drugs showing options of increased oral dose, alternative administration route (sublingual, transdermal, IV), or P-gp inhibitor co-administration with associated monitoring requirements.
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### IV. Drug Distribution
Volume of distribution (Vd) represents a theoretical pharmacokinetic parameter that relates the total amount of drug in the body to its plasma concentration, providing insight into the extent of tissue distribution. Calculated using the formula Vd = Dose / C0, where C0 is the initial plasma concentration, Vd does not correspond to any actual physiological space but rather indicates how extensively a drug distributes beyond the plasma compartment. A low Vd of approximately 3-5 liters suggests the drug remains primarily in the plasma volume, characteristic of highly protein-bound or very hydrophilic drugs. Medium Vd values of 15-20 liters indicate distribution into extracellular fluid, while high Vd exceeding 40 liters or even exceeding total body water suggests extensive tissue binding and distribution.
Multiple physicochemical and physiological factors determine how drugs distribute throughout the body after reaching systemic circulation. Lipophilicity strongly influences distribution, with highly lipid-soluble drugs readily crossing cell membranes to accumulate in tissues, resulting in high volumes of distribution. Plasma protein binding restricts distribution by keeping drug molecules in the vascular compartment, as only unbound drug can cross capillary membranes. Tissue binding to intracellular proteins, nucleic acids, or fat stores increases apparent Vd by sequestering drug outside the plasma. Regional blood flow determines the rate of drug delivery to various tissues, with highly perfused organs receiving drug more rapidly. Physiological barriers including the blood-brain barrier and placenta restrict distribution of many drugs to these protected compartments.
Plasma protein binding plays a crucial role in drug distribution, activity, and elimination by determining the fraction of drug available for pharmacological action. Albumin, the most abundant plasma protein, primarily binds acidic drugs such as warfarin, phenytoin, and salicylates with high affinity. Alpha-1-acid glycoprotein preferentially binds basic drugs including lidocaine, propranolol, and many antidepressants. Only the free, unbound fraction of drug can cross membranes, interact with receptors, undergo metabolism, and be excreted. Displacement interactions occur when two highly protein-bound drugs compete for binding sites, potentially causing transient increases in free drug concentration and enhanced pharmacological effects.
The blood-brain barrier presents a specialized challenge for drugs intended to act on the central nervous system or those where CNS penetration causes unwanted effects. This barrier consists of capillary endothelial cells connected by tight junctions, surrounded by astrocyte foot processes, creating a highly restrictive interface between blood and brain tissue. Only small, lipophilic, non-ionized molecules can readily cross the blood-brain barrier through passive diffusion. P-glycoprotein efflux transporters in brain capillary endothelium actively pump many drugs back into the bloodstream, limiting CNS penetration. Alternative delivery strategies including intrathecal injection, intranasal administration, or development of prodrugs that cross the barrier and are activated in the CNS can circumvent this barrier when central nervous system delivery is required.
Panel A: Three-compartment diagram showing plasma (3-5L), extracellular fluid (15-20L), and total body water with tissue binding (40+ L), with example drugs listed for each volume of distribution category. Panel B: Illustration of factors affecting distribution including lipophilicity gradient across cell membrane, plasma protein binding in capillary, tissue protein binding in cells, and variable blood flow to different organs. Panel C: Molecular diagram of albumin binding acidic drugs (warfarin, phenytoin) and alpha-1-acid glycoprotein binding basic drugs (lidocaine, propranolol), showing equilibrium between bound and free drug forms. Panel D: Blood-brain barrier cross-section showing tight junction between endothelial cells, astrocyte foot processes, P-glycoprotein pumping drugs back into blood, and small lipophilic molecule successfully crossing into brain parenchyma.
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### V. Drug Metabolism
Drug metabolism serves the essential purpose of converting lipophilic drug molecules into more polar, hydrophilic metabolites that can be efficiently excreted from the body through renal and biliary routes. The liver serves as the primary site of drug metabolism due to its abundant expression of metabolic enzymes, large size, and receipt of the entire portal blood supply from the gastrointestinal tract. Metabolic reactions are classified into Phase I functionalization reactions and Phase II conjugation reactions, which often occur sequentially. While metabolism typically results in pharmacological inactivation, some drugs undergo metabolic activation to form active metabolites, and in some cases, toxic reactive intermediates may be generated.
Phase I metabolic reactions introduce or expose functional groups on drug molecules, preparing them for subsequent conjugation reactions or directly enhancing their polarity for excretion. Oxidation reactions, catalyzed primarily by the cytochrome P450 enzyme superfamily, represent the most common Phase I transformation and include hydroxylation, N-dealkylation, and epoxidation. Reduction reactions convert ketones to alcohols or reduce nitro groups and are catalyzed by various reductases. Hydrolysis reactions cleave ester and amide bonds through the action of esterases and amidases, converting prodrugs to active forms or inactivating drugs with these susceptible linkages. The functional groups introduced or revealed by Phase I reactions serve as handles for Phase II conjugation.
The cytochrome P450 enzyme system comprises a superfamily of heme-containing monooxygenases that collectively metabolize the majority of clinically used drugs. CYP3A4 represents the most abundant isoform in both liver and intestine, responsible for metabolizing approximately 50% of drugs including statins, macrolide antibiotics, calcium channel blockers, and HIV protease inhibitors. CYP2D6 metabolizes many psychotropic medications, beta-blockers, and opioids including codeine conversion to morphine, with significant genetic polymorphism affecting activity. CYP2C9 catalyzes metabolism of warfarin, phenytoin, and many non-steroidal anti-inflammatory drugs, while CYP2C19 metabolizes proton pump inhibitors and activates the antiplatelet prodrug clopidogrel. CYP1A2 metabolizes caffeine, theophylline, and some antipsychotics, with activity induced by cigarette smoking and inhibited by fluoroquinolones.
Phase II conjugation reactions attach endogenous polar molecules to drugs or their Phase I metabolites, substantially increasing water solubility and facilitating excretion. Glucuronidation, catalyzed by UDP-glucuronosyltransferases (UGTs), represents the most common Phase II reaction, attaching glucuronic acid to hydroxyl, carboxyl, amino, or sulfhydryl groups. Sulfation by sulfotransferases conjugates sulfate groups to hydroxyl and amino functions, occurring at lower drug concentrations before switching to glucuronidation at higher concentrations. Acetylation by N-acetyltransferases (NAT1 and NAT2) conjugates acetyl groups to aromatic amines and hydrazines, with NAT2 showing significant genetic polymorphism. Glutathione conjugation by glutathione S-transferases serves primarily as a detoxification pathway for reactive electrophilic intermediates that might otherwise cause cellular damage.
Panel A: Liver-centric diagram showing drug entering via portal vein, undergoing Phase I metabolism in hepatocytes with CYP450 enzymes on endoplasmic reticulum, followed by Phase II conjugation, and metabolites exiting via hepatic vein or bile. Panel B: Chemical reaction scheme showing Phase I oxidation adding hydroxyl group to aromatic ring, reduction of ketone to alcohol, and hydrolysis of ester bond, with enzyme names labeled. Panel C: Pie chart showing relative contribution of CYP450 isoforms to drug metabolism (CYP3A4 50%, CYP2D6 25%, CYP2C9 10%, CYP2C19 5%, CYP1A2 5%, others 5%) with example substrate drugs listed for each. Panel D: Phase II conjugation reactions illustrated showing glucuronidation attaching glucuronic acid via UGT, sulfation adding sulfate group, acetylation with acetyl-CoA, and glutathione conjugation with GSH.
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### VI. CYP450 Interactions
Enzyme induction increases the synthesis of cytochrome P450 enzymes, accelerating the metabolism of substrate drugs and potentially reducing their therapeutic efficacy. Rifampin stands as the most potent known inducer, capable of increasing the activity of multiple CYP isoforms including 3A4, 2C9, 2C19, and 1A2 by 10-fold or more within days of initiation. Anticonvulsants including carbamazepine, phenytoin, and phenobarbital induce CYP3A4 and CYP2C isoforms, causing significant interactions with oral contraceptives, immunosuppressants, and many other medications. The herbal supplement St. John's wort induces CYP3A4 and P-glycoprotein, resulting in clinically significant reductions in exposure to HIV protease inhibitors, cyclosporine, and warfarin. The clinical consequence of induction is increased drug metabolism leading to decreased plasma concentrations and potentially therapeutic failure, requiring dose increases or alternative therapy selection.
Enzyme inhibition decreases the activity of CYP450 enzymes, slowing metabolism of substrate drugs and increasing their plasma concentrations with potential for toxicity. Grapefruit juice inhibits intestinal CYP3A4 through irreversible mechanism-based inhibition by furanocoumarins, increasing bioavailability of substrates like simvastatin, cyclosporine, and felodipine. Azole antifungals, particularly ketoconazole and itraconazole, potently inhibit CYP3A4, causing dangerous interactions with many commonly prescribed medications. Macrolide antibiotics, especially erythromycin and clarithromycin, inhibit CYP3A4 through mechanism-based inactivation, while azithromycin is notably free of this interaction. Fluoxetine and paroxetine strongly inhibit CYP2D6, reducing conversion of codeine to morphine and metabolism of many other psychotropics. The H2-receptor antagonist cimetidine inhibits multiple CYP isoforms, while ranitidine and famotidine have minimal interaction potential.
Prodrugs present unique considerations for CYP450 interactions because they require metabolic activation to exert their pharmacological effects. Codeine relies on CYP2D6-mediated conversion to morphine for analgesic activity, rendering it ineffective in poor metabolizers and potentially dangerous in ultra-rapid metabolizers who generate excessive morphine. Clopidogrel requires CYP2C19-mediated oxidation to its active thiol metabolite, with poor metabolizers experiencing reduced antiplatelet effect and increased cardiovascular event risk. Tamoxifen undergoes CYP2D6-mediated conversion to the more potent antiestrogen endoxifen, with CYP2D6 inhibitors potentially reducing its efficacy in breast cancer treatment. Cyclophosphamide requires activation by multiple CYP isoforms, with individual variation in metabolism affecting both efficacy and toxicity of this alkylating agent.
Genetic polymorphisms in CYP450 enzymes create substantial interindividual variability in drug metabolism, giving rise to distinct metabolizer phenotypes with clinical implications. Poor metabolizers possess two non-functional alleles and exhibit markedly decreased enzyme activity, predisposing to drug accumulation and toxicity with substrates or therapeutic failure with prodrugs. Intermediate metabolizers carry one reduced-function allele, displaying moderately decreased metabolic capacity. Extensive metabolizers possess two normal-function alleles and exhibit typical enzyme activity, representing the most common phenotype for most CYP isoforms. Ultra-rapid metabolizers carry gene duplications or gain-of-function variants, showing increased enzyme activity that may cause therapeutic failure with substrates or toxicity with prodrugs. Pharmacogenomic testing for CYP2D6, CYP2C19, and CYP2C9 is increasingly used to guide dosing of high-risk medications.
Panel A: Diagram showing enzyme induction mechanism with rifampin binding nuclear receptor (PXR), increasing CYP3A4 gene transcription, producing more enzyme protein, and resulting in decreased substrate drug concentrations. Panel B: Inhibition diagram showing ketoconazole binding directly to CYP3A4 active site, blocking substrate access, resulting in increased substrate drug concentrations and potential toxicity. Panel C: Prodrug activation pathway showing codeine converted by CYP2D6 to morphine, clopidogrel converted by CYP2C19 to active thiol metabolite, and tamoxifen converted by CYP2D6 to endoxifen, with bars indicating effect of poor metabolizer status on each. Panel D: Population distribution curves showing metabolizer phenotypes from poor (PM) to ultra-rapid (UM) for CYP2D6, with percentage of Caucasian population in each category and clinical implications listed.
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### VII. Drug Excretion
Renal excretion represents the primary elimination pathway for most drugs and their metabolites, involving three distinct processes that occur in different segments of the nephron. Glomerular filtration allows passage of free (unbound) drug molecules with molecular weight below approximately 20,000 daltons through the fenestrated glomerular capillaries into Bowman's capsule. Tubular secretion in the proximal tubule involves active transport of drugs from peritubular blood into the tubular lumen via organic anion transporters (OATs) for acidic drugs and organic cation transporters (OCTs) for basic drugs. Tubular reabsorption occurs throughout the nephron as lipophilic drugs in the filtrate passively diffuse back across tubular epithelium into peritubular capillaries. Urinary pH manipulation through acidification or alkalinization can enhance excretion of weak acids or bases by trapping them in ionized form within the tubular lumen.
Renal impairment necessitates dose adjustment for drugs eliminated primarily by the kidneys to prevent accumulation and toxicity. Glomerular filtration rate (GFR) serves as the primary measure of renal function, with equations incorporating serum creatinine, age, sex, and race to estimate GFR. Patients with mild renal impairment (GFR greater than 60 mL/min) typically require no dose adjustment for most medications. Moderate impairment (GFR 30-60 mL/min) often requires dose reduction or extended dosing intervals, particularly for renally eliminated drugs with narrow therapeutic indices. Severe impairment (GFR less than 30 mL/min) necessitates significant dose reductions, and dialysis patients may require supplemental doses after hemodialysis sessions for drugs that are efficiently removed.
Hepatic excretion occurs via the biliary system and represents an important elimination pathway for large, polar molecules and drugs undergoing extensive conjugation. Active transporters in the canalicular membrane of hepatocytes, including P-glycoprotein, multidrug resistance-associated proteins (MRPs), and bile salt export pump (BSEP), secrete drugs and metabolites into bile. Enterohepatic circulation occurs when drugs excreted in bile are reabsorbed from the intestine, prolonging their duration of action and creating secondary peaks in plasma concentration-time profiles. Drugs undergoing significant enterohepatic circulation include estrogens, digoxin, and morphine glucuronide, with clinical implications for drug interactions that disrupt this cycle. Antibiotics that alter gut flora can reduce bacterial enzymes that deconjugate drug glucuronides, decreasing enterohepatic recirculation and lowering drug exposure.
Alternative excretion routes contribute minimally to overall drug elimination but have important clinical considerations in specific circumstances. Pulmonary excretion provides the primary elimination route for volatile anesthetics and contributes to alcohol elimination, with exhalation rate dependent on alveolar ventilation and blood-air partition coefficient. Excretion into sweat and saliva generally represents minor elimination pathways but can be used for therapeutic drug monitoring through non-invasive sampling. Excretion into breast milk raises important considerations for nursing mothers, as lipophilic, weakly basic drugs with low protein binding tend to accumulate in milk and may affect the nursing infant. Understanding these alternative routes helps predict drug behavior and guide clinical decision-making in special populations.
Panel A: Nephron diagram showing glomerular filtration of free drug through fenestrated capillary, proximal tubule secretion via OAT and OCT transporters, and passive reabsorption of lipophilic drug in distal tubule and collecting duct. Panel B: Dose adjustment chart showing GFR categories (>60, 30-60, <30 mL/min, dialysis) with corresponding adjustment strategies and example drugs requiring modification at each level. Panel C: Enterohepatic circulation diagram showing hepatocyte secreting drug-glucuronide into bile, storage in gallbladder, release into duodenum, bacterial deconjugation, reabsorption of free drug in ileum, and return to liver via portal vein. Panel D: Alternative excretion routes illustrated including alveolar excretion of volatile anesthetics, sweat gland secretion, salivary gland excretion, and mammary gland secretion into breast milk with infant exposure considerations.
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### VIII. Pharmacokinetic Parameters
Half-life (t1/2) represents the time required for plasma drug concentration to decrease by 50% and serves as a fundamental parameter for determining dosing frequency and predicting drug accumulation. The formula t1/2 = 0.693 x Vd / CL mathematically relates half-life to volume of distribution and clearance, demonstrating that half-life increases with greater distribution and decreases with higher clearance. Approximately 4-5 half-lives are required to reach steady-state concentrations during repeated dosing, at which point the amount of drug administered equals the amount eliminated. Similarly, after discontinuing a drug, 4-5 half-lives are needed for essentially complete elimination from the body. Clinically, drugs with short half-lives require more frequent dosing or extended-release formulations, while drugs with very long half-lives may be dosed once daily or even less frequently.
Clearance (CL) quantifies the volume of plasma from which drug is completely removed per unit time and serves as the primary parameter determining steady-state drug concentrations. Total body clearance equals the sum of clearances by all eliminating organs, typically dominated by renal clearance (CLrenal) and hepatic clearance (CLhepatic). Drugs with high hepatic extraction ratios (greater than 0.7) have clearance limited by hepatic blood flow, making them sensitive to changes in cardiac output or liver blood flow. Drugs with low hepatic extraction ratios (less than 0.3) have clearance limited by intrinsic hepatic enzyme activity and protein binding, making them more sensitive to enzyme induction or inhibition. Clearance can be calculated from plasma concentration data using the formula CL = Rate of elimination / C, where C is the plasma concentration.
Steady state represents the pharmacokinetic condition achieved during repeated dosing when the rate of drug input equals the rate of drug elimination, resulting in consistent average plasma concentrations. Time to reach steady state depends solely on the drug's half-life, requiring approximately 4-5 half-lives regardless of dose or dosing frequency. The steady-state concentration (Css) is determined by the relationship between dosing rate and clearance, with Css = Dosing rate / CL for continuous infusions. Loading doses can bypass the slow approach to steady state by immediately achieving target concentrations, calculated as Loading dose = Vd x Ctarget. This strategy is particularly valuable for drugs with long half-lives where waiting for steady state would unacceptably delay therapeutic effect.
Clinical dosing calculations apply pharmacokinetic parameters to design rational drug regimens that achieve and maintain therapeutic concentrations. The loading dose formula LD = (Vd x Ctarget) / F incorporates volume of distribution, target concentration, and bioavailability to determine the initial dose needed to rapidly achieve therapeutic levels. Maintenance dose is calculated as MD = (CL x Css x tau) / F, where tau represents the dosing interval, ensuring that drug eliminated between doses is replaced. Dosing interval selection considers the drug's half-life and therapeutic window, with intervals typically ranging from one to three half-lives depending on the acceptable degree of concentration fluctuation. These calculations must be adjusted for individual patient factors including renal function, hepatic function, body composition, and concurrent medications affecting drug disposition.
Panel A: Semi-logarithmic plasma concentration-time plot demonstrating half-life as the time for concentration to decrease by 50%, with the formula t1/2 = 0.693 x Vd/CL and arrows marking successive half-life intervals. Panel B: Clearance diagram showing total body clearance as sum of hepatic and renal components, with high extraction drug clearance limited by blood flow and low extraction drug clearance limited by enzyme activity. Panel C: Steady-state accumulation curve showing drug concentration increasing over 5 half-lives during repeated dosing until rate in equals rate out, with loading dose superimposed achieving immediate therapeutic level. Panel D: Clinical dosing calculation worksheet showing formulas for loading dose (LD = Vd x Ctarget/F) and maintenance dose (MD = CL x Css x tau/F) with example calculations for a drug with specified parameters.
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### IX. Special Populations
Pediatric patients exhibit significant pharmacokinetic differences from adults that necessitate careful dose adjustment and age-appropriate formulations. Gastric pH is higher in neonates due to reduced acid secretion, affecting absorption of pH-dependent drugs and potentially increasing bioavailability of acid-labile medications. Body composition changes dramatically during development, with neonates having higher total body water (70-80% versus 55-60% in adults) and lower fat content, affecting distribution of hydrophilic and lipophilic drugs respectively. Hepatic metabolic capacity is immature at birth, with CYP450 isoforms developing at different rates; CYP3A7 predominates in fetal liver while CYP3A4 activity increases over the first months of life. Renal function, including glomerular filtration and tubular secretion, is reduced in neonates and approaches adult values by approximately one year of age. Pediatric dosing typically uses weight-based calculations (mg/kg) with age-specific adjustments to account for these developmental pharmacokinetic differences.
Geriatric patients experience age-related physiological changes that alter drug pharmacokinetics and increase susceptibility to adverse effects. Absorption is relatively preserved in healthy aging, though reduced gastric acid production and altered gastrointestinal motility may affect specific drugs. Distribution is altered by decreased lean body mass, increased body fat, and reduced total body water, increasing the volume of distribution for lipophilic drugs and decreasing it for hydrophilic compounds. Hepatic metabolism declines due to reduced liver blood flow (approximately 40% decrease by age 65), decreased liver mass, and variably reduced CYP450 activity, particularly affecting high-extraction drugs. Renal function progressively declines with aging, with GFR decreasing approximately 1 mL/min per year after age 40, necessitating dose reduction for renally eliminated drugs. The clinical principle of "start low, go slow" guides prescribing in elderly patients to minimize adverse effects while achieving therapeutic goals.
Pregnancy induces substantial physiological adaptations that affect pharmacokinetics throughout gestation and must be considered when prescribing for pregnant women. Absorption may be altered by decreased gastric acid secretion, prolonged gastrointestinal transit time, and pregnancy-related nausea and vomiting. Distribution changes include increased total body water, expanded plasma volume, increased body fat, and decreased plasma protein concentrations, all contributing to increased volumes of distribution. Hepatic metabolism shows variable changes in CYP450 activity, with CYP3A4 and CYP2D6 activity increased while CYP1A2 and CYP2C19 activity is decreased during pregnancy. Renal blood flow and glomerular filtration rate increase by 50% during pregnancy, accelerating elimination of renally cleared drugs. Placental transfer exposes the fetus to medications, with lipophilic, un-ionized, low molecular weight compounds crossing most readily, necessitating careful consideration of fetal safety.
Obesity significantly affects drug pharmacokinetics and presents challenges for appropriate dose selection in this increasingly common patient population. Lipophilic drugs exhibit increased volumes of distribution in obese patients due to extensive distribution into adipose tissue, potentially requiring larger loading doses. Hydrophilic drugs distribute primarily into lean body mass, and dosing based on total body weight may lead to overdosing; ideal body weight or adjusted body weight is often more appropriate. Hepatic blood flow and metabolic capacity may be enhanced in obesity, though non-alcoholic fatty liver disease, common in obese patients, may impair metabolism. Renal function is often increased in obesity due to hyperfiltration, though this may transition to chronic kidney disease over time. Dosing strategies in obesity are drug-specific and may utilize total body weight, ideal body weight, adjusted body weight, or lean body mass depending on the drug's physicochemical properties and elimination pathways.
Panel A: Developmental timeline showing maturation of pharmacokinetic processes in pediatric patients including gastric pH normalization, body composition changes (water and fat percentages), CYP450 isoform development, and renal function maturation from birth through childhood. Panel B: Geriatric physiology diagram illustrating age-related changes affecting pharmacokinetics: decreased lean mass and increased fat, reduced liver blood flow and mass, decreased GFR, and altered plasma proteins. Panel C: Pregnancy pharmacokinetic changes showing expanded plasma volume, increased GFR, variable CYP450 changes, and placental drug transfer with fetal circulation. Panel D: Obesity dosing considerations comparing lipophilic drug distribution into expanded adipose tissue versus hydrophilic drug distribution limited to lean mass, with dosing weight recommendations for each drug type.
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### X. Therapeutic Drug Monitoring
Therapeutic drug monitoring (TDM) involves measuring drug concentrations in biological fluids, typically plasma or serum, to optimize dosing and ensure safe, effective therapy. TDM is most valuable for drugs with narrow therapeutic indices where small changes in concentration can result in therapeutic failure or toxicity. Variable pharmacokinetics due to genetic polymorphisms, disease states, drug interactions, or patient non-compliance also justify monitoring to ensure adequate drug exposure. Drugs with serious, concentration-related toxicities benefit from monitoring to detect accumulation before clinical toxicity manifests. TDM can also assess patient adherence when therapeutic failure occurs despite appropriate dosing.
Several drug classes routinely require therapeutic drug monitoring due to their pharmacokinetic and safety profiles. Aminoglycoside antibiotics (gentamicin, tobramycin, amikacin) require monitoring of both peak levels to ensure efficacy and trough levels to prevent nephrotoxicity and ototoxicity. Vancomycin trough concentrations guide dosing to achieve target levels of 10-20 mcg/mL for most infections while minimizing nephrotoxicity risk. Anticonvulsants including phenytoin (therapeutic range 10-20 mcg/mL), carbamazepine, and valproic acid require monitoring due to variable metabolism and potential for toxicity. Lithium monitoring maintains levels within the narrow therapeutic range of 0.6-1.2 mEq/L, with higher levels causing potentially fatal toxicity. Cardiac glycosides, particularly digoxin (therapeutic range 0.8-2.0 ng/mL, often 0.5-1.0 ng/mL for heart failure), require monitoring due to their narrow therapeutic index and serious dysrhythmogenic potential.
Proper timing of sample collection is essential for meaningful interpretation of drug concentration data. Trough levels, collected immediately before the next scheduled dose, reflect the minimum concentration during the dosing interval and are most commonly used for monitoring. Peak levels, collected after drug distribution is complete, assess maximum concentrations and are important for concentration-dependent antibiotics. Sampling should occur after steady state is reached, typically requiring 4-5 half-lives of consistent dosing to ensure concentrations reflect the current regimen. For drugs with significant protein binding such as phenytoin, free drug levels may be more clinically relevant, particularly in patients with hypoalbuminemia or renal failure.
Dose adjustment based on TDM results follows systematic approaches to achieve target concentrations safely and efficiently. The measured concentration is compared to the established therapeutic range, considering the clinical context, indication, and patient response. For drugs with linear pharmacokinetics, proportional dose adjustments can be calculated based on the ratio of target to measured concentration. Non-linear drugs like phenytoin require more careful adjustment using specific equations or iterative approaches. After dose adjustment, repeat monitoring should occur after a new steady state is achieved to confirm appropriate response. Clinical correlation remains essential, as some patients may respond well at concentrations outside traditional therapeutic ranges, and drug levels should always be interpreted in the context of clinical efficacy and tolerability.
Panel A: Decision algorithm for therapeutic drug monitoring showing criteria for TDM candidacy including narrow therapeutic index, variable pharmacokinetics, serious toxicity risk, and compliance concerns, with example drugs meeting each criterion. Panel B: Reference table displaying drugs requiring routine TDM with their therapeutic ranges, sampling times, and common clinical indications (vancomycin, gentamicin, phenytoin, lithium, digoxin). Panel C: Sampling timeline diagram showing correct timing for trough sample (immediately before next dose), peak sample (after distribution phase), and steady-state requirement (after 4-5 half-lives), with plasma concentration curve illustrating each point. Panel D: Dose adjustment flowchart showing process from measured concentration through comparison to target, calculation of new dose (proportional for linear kinetics, specialized equations for non-linear), implementation, and verification at new steady state.
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## Summary
- Pharmacokinetics encompasses ADME processes: Absorption, Distribution, Metabolism, and Excretion, describing what the body does to drugs
- Bioavailability quantifies the fraction of drug reaching systemic circulation, significantly affected by first-pass metabolism in the gut wall and liver
- Volume of distribution relates total body drug to plasma concentration, with low Vd indicating plasma retention and high Vd reflecting extensive tissue distribution
- Plasma protein binding to albumin and alpha-1-acid glycoprotein restricts distribution and activity, as only free drug is pharmacologically active
- Drug metabolism occurs primarily in the liver through Phase I reactions (CYP450 oxidation) and Phase II conjugation reactions (glucuronidation, sulfation, acetylation)
- CYP3A4 metabolizes approximately 50% of drugs and is subject to numerous clinically significant inductions and inhibitions
- Renal excretion involves glomerular filtration, tubular secretion, and tubular reabsorption, requiring dose adjustment based on GFR
- Half-life determines dosing interval, with 4-5 half-lives required to reach steady state or achieve complete elimination
- Special populations including pediatric, geriatric, pregnant, and obese patients require individualized pharmacokinetic considerations
- Therapeutic drug monitoring optimizes therapy for narrow therapeutic index drugs including aminoglycosides, vancomycin, phenytoin, lithium, and digoxin
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## Key Terms
| Term | Definition |
|------|------------|
| Bioavailability | Fraction of administered drug that reaches the systemic circulation in unchanged, active form |
| First-pass effect | Pre-systemic metabolism of drug in the gut wall and liver before reaching systemic circulation |
| Volume of distribution | Theoretical volume into which a drug distributes, relating total body drug to plasma concentration |
| Half-life | Time required for plasma drug concentration to decrease by 50% |
| Clearance | Volume of plasma completely cleared of drug per unit time by all elimination pathways |
| Steady state | Pharmacokinetic equilibrium where rate of drug administration equals rate of elimination |
| CYP450 | Cytochrome P450 enzyme superfamily responsible for Phase I oxidative drug metabolism |
| Therapeutic drug monitoring | Clinical practice of measuring drug concentrations to guide individualized dosing |
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