10.1 Pharmacokinetics, ADME & Receptor Dynamics

10.1 Pharmacokinetics, ADME & Receptor Dynamics

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Introduction to Pharmacokinetics vs. Pharmacodynamics

Pharmacology is divided into two core conceptual disciplines:

1. Pharmacokinetics (PK): What the body does to the drug. PK governs the time course of drug concentration in plasma and tissues through the processes of Absorption, Distribution, Metabolism, and Elimination (ADME).
2. Pharmacodynamics (PD): What the drug does to the body. PD investigates receptor interactions, biochemical signal transduction cascades, graded and quantal dose-response relationships, and the magnitude of physiologic or toxicologic effects.

$\text{Dose Administered} \xrightarrow{\textbf{Pharmacokinetics}} \text{Plasma Concentration} \xrightarrow{\textbf{Pharmacodynamics}} \text{Clinical Effect / Toxicity}$

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ADME: Absorption & Bioavailability

Absorption Mechanisms

For a pharmacologic agent to exert a systemic effect, it must traverse biological lipid bilayer membranes to enter the vascular compartment. Drug absorption occurs primarily through:

• Passive Transcellular Diffusion: Driven by concentration gradients; favored by non-ionized, low-molecular-weight, highly lipophilic molecules.
• Facilitated Diffusion & Active Transport: Mediated by membrane solute carrier (SLC) transporters or ATP-binding cassette (ABC) efflux pumps (e.g., P-glycoprotein / MDR1).
• Paracellular Transport: Aqueous filtration through intercellular junctions.

Bioavailability ($F$)

Bioavailability ($F$) is defined as the fraction or percentage of an administered drug dose that reaches the systemic circulation in an active, unchanged molecular state:

$F = \frac{\text{AUC}_{\text{oral}}}{\text{AUC}_{\text{IV}}} \times 100\%$

Where $\text{AUC}$ represents the Area Under the Plasma Concentration-Time Curve.

• Intravenous (IV) Administration: Bypasses absorption barriers and first-pass hepatic extraction entirely, yielding $F = 1.0$ ($100\%$).
• Oral (PO) Administration: Typically exhibits $F < 1.0$ due to incomplete gastrointestinal mucosal dissolution, gastric acid degradation, bacterial enzymatic metabolism, P-glycoprotein extrusion, and first-pass hepatic metabolism.

First-Pass Hepatic Elimination

Drugs absorbed through the gastrointestinal tract (stomach, small intestine, and proximal colon) enter the mesenteric venous drainage and travel via the hepatic portal vein directly to the liver before accessing systemic circulation. Extensive hepatic extraction or enzymatic biotransformation during this primary transit markedly reduces systemic bioavailability. For example, lidocaine undergoes approximately 70% to 90% first-pass hepatic metabolism, rendering oral administration clinically ineffective and necessitating parenteral or topical delivery.

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Henderson-Hasselbalch Equation, Ion Trapping & Local Anesthetic Failure

The Physicochemical Basis of Ionization

The majority of therapeutic drugs are either weak acids or weak bases. Biological membranes consist of hydrophobic phospholipid bilayers that are permeable to uncharged, lipid-soluble molecules, but act as impermeable barriers to charged, water-soluble ionized species.

• Weak Acids ($HA \rightleftharpoons H^+ + A^-$):
  • Non-ionized (protonated) form: $HA$ (uncharged, lipophilic, membrane-permeable).
  • Ionized (unprotonated) form: $A^-$ (negatively charged, hydrophilic, membrane-impermeable).
• Weak Bases ($BH^+ \rightleftharpoons H^+ + B$):
  • Ionized (protonated) form: $BH^+$ (positively charged, hydrophilic, membrane-impermeable).
  • Non-ionized (unprotonated) form: $B$ (uncharged, lipophilic, membrane-permeable).

The mathematical relationship governing the ratio of non-ionized to ionized species is defined by the Henderson-Hasselbalch equation:

$\text{pH} - \text{p}K_a = \log\left(\frac{[\text{unprotonated}]}{[\text{protonated}]}\right)$

$\text{For Weak Acids:} \quad \text{pH} - \text{p}K_a = \log\left(\frac{[A^-]}{[HA]}\right)$

$\text{For Weak Bases:} \quad \text{pH} - \text{p}K_a = \log\left(\frac{[B]}{[BH^+]}\right)$

Ion Trapping in Renal Tubular Clearance

When a drug crosses a biological membrane into a physiological compartment with a different pH, the fraction that ionizes cannot diffuse back across the lipid barrier, becoming "trapped" in that compartment.

High-Yield Board Pearl: Local Anesthetics in Acidic Infected Tissue

Local anesthetics (e.g., lidocaine, bupivacaine, mepivacaine) are chemically weak bases with dissociation constants ($\text{p}K_a$) ranging between $7.8$ and $9.0$:

$BH^+ \text{ (Charged, active intracellular blocker)} \rightleftharpoons B \text{ (Uncharged base, membrane-permeable)} + H^+ \text{ (Proton)}$

In healthy subcutaneous connective tissue with a physiological extracellular pH of $7.4$, a sufficient fraction of the anesthetic exists in the uncharged free base form ($B$) to penetrate the hydrophobic lipid epineurium and axonal membrane of sensory peripheral nerves. Once inside the axoplasm (physiological pH $7.2$), the uncharged base $B$ re-equilibrates into the charged protonated form $BH^+$, which binds with high affinity to the intracellular channel pore of voltage-gated sodium channels ($Na_V$) to halt depolarization and conduct nociceptive blockade.

Volume of Distribution & Plasma Protein Binding

Apparent Volume of Distribution ($V_d$)

The Volume of Distribution ($V_d$) is a theoretical pharmacokinetic parameter that equates the total amount of drug present in the entire body to the concentration of drug measured in the vascular plasma compartment:

$V_d = \frac{\text{Total Amount of Drug in Body}}{\text{Plasma Drug Concentration } (C_0)} = \frac{\text{Dose (IV)}}{C_0}$

$V_d$ does not reflect an actual physical anatomical volume; rather, it reflects the degree to which a drug partitions into peripheral tissues versus remaining confined within circulating plasma:

Plasma Protein Binding

Drugs in systemic circulation exist in a dynamic equilibrium between a protein-bound fraction and an unbound (free) fraction:

• Albumin: Primary binding protein for acidic and neutral drugs (e.g., Warfarin, Phenytoin, Diazepam, NSAIDs, Ceftriaxone).
• Alpha-1-Acid Glycoprotein (AAG): Acute-phase reactant synthesizing primarily in the liver; binds basic drugs (e.g., Lidocaine, Bupivacaine, Propranolol, Quinidine).

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Biotransformation: Phase I vs. Phase II Metabolism

The ultimate goal of hepatic biotransformation is to convert lipophilic compounds into polar, water-soluble, hydrophilic metabolites that can be excreted by the kidneys or biliary tract.

Phase I Reactions: Functionalization

Phase I reactions introduce or unmask a polar functional group ($-OH, -NH_2, -SH, -COOH$) on the parent substrate. These reactions are catalyzed predominantly by the Cytochrome P450 (CYP450) monooxygenase enzyme superfamily localized within the smooth endoplasmic reticulum of hepatocytes.

• Reaction Types: Oxidation (CYP-mediated), Reduction, and Hydrolysis.
• Metabolic Outcome: Yields active, inactive, or toxic metabolites. In the case of prodrugs (e.g., codeine, clopidogrel, enalapril), Phase I bioactivation is required to generate the therapeutically active compound.
• Geriatric Decline: Phase I metabolism declines significantly in elderly individuals due to decreased hepatic blood flow and hepatic microsomal mass, prolonging drug half-lives.

Cytochrome P450 Inducers vs. Inhibitors

Modulation of hepatic CYP450 isozymes (chiefly CYP3A4, CYP2C9, CYP2C19, CYP2D6, CYP1A2) accounts for major drug-drug interactions encountered on licensing examinations:

Phase II Reactions: Conjugation

Phase II reactions conjugate the parent drug or Phase I intermediate with an endogenous hydrophilic substrate, creating large, highly polar, biologically inactive molecules ready for excretion.

• Reaction Types:
  • Glucuronidation: Most common Phase II pathway; catalyzed by UDP-glucuronosyltransferases (UGT). Deficient in neonates (causing chloramphenicol "Gray Baby Syndrome").
  • Sulfation: Catalyzed by sulfotransferases.
  • Acetylation: Catalyzed by N-acetyltransferase (NAT2).
  • Glutathione Conjugation: Neutralizes toxic electrophilic intermediates, notably NAPQI generated by acetaminophen.
• Geriatric Preservation: Phase II conjugation reactions (mnemonic: "Geriatrics have more GAS" — Glucuronidation, Acetylation, Sulfation) remain relatively preserved in elderly patients.
• Pharmacogenetic Variation in Acetylation: Individuals are genetically categorized as fast acetylators or slow acetylators based on NAT2 polymorphisms. Slow acetylators possess reduced hepatic NAT2 activity, predisposing them to elevated drug concentrations and severe adverse reactions, including Drug-Induced Lupus Erythematosus (DILE) from Hydralazine, Procainamide, or Isoniazid (INH), as well as INH-induced peripheral neuropathy.

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Quantitative Pharmacokinetics: Dosing & Elimination Kinetics

                    First-Order vs. Zero-Order Elimination
     First-Order Elimination                 Zero-Order Elimination
   [Plasma Conc vs. Time: Curve]           [Plasma Conc vs. Time: Linear]
   Concentration                           Concentration
   │ *                                     │ *
   │   *                                   │   *
   │     *                                 │     *
   │       *                               │       *
   │         *                             │         *
   └───────────── Time                     └───────────── Time
   - Constant fraction (%)/time            - Constant amount/time
   - Rate proportional to conc             - Rate independent of conc (saturable)
   - Half-life is constant                 - Half-life decreases as conc falls
   - Most drugs at normal doses            - Phenytoin, Ethanol, high-dose Aspirin

First-Order vs. Zero-Order Elimination Kinetics

Core Pharmacokinetic Equations

$\textbf{Clearance (CL):} \quad CL = \frac{\text{Rate of Elimination}}{C_p} = \frac{0.693 \times V_d}{t_{1/2}}$

$\textbf{Half-Life } (t_{1/2}): \quad t_{1/2} = \frac{0.693 \times V_d}{CL}$

$\textbf{Steady-State Concentration } (C_{ss}): \quad C_{ss} = \frac{F \times \text{Dose}}{\text{CL} \times \tau}$

Where $\tau$ is the dosing interval.

Attainment of Steady State

When a drug is administered via continuous IV infusion or regular intermittent maintenance dosing, steady state ($C_{ss}$) is reached when the rate of drug administration equals the rate of drug elimination. The time required to attain steady state depends exclusively on the drug's elimination half-life ($t_{1/2}$), completely independent of the dose, dosing interval, or route of administration:

• $1\text{ half-life} = 50\% \text{ of } C_{ss}$
• $2\text{ half-lives} = 75\% \text{ of } C_{ss}$
• $3\text{ half-lives} = 87.5\% \text{ of } C_{ss}$
• $4\text{ half-lives} = 93.75\% \text{ of } C_{ss}$
• $4 - 5\text{ half-lives} = 95 - 97\% \text{ of } C_{ss}$ (Clinical Steady State attained)

Similarly, complete drug elimination (washout) following drug discontinuation requires $4$ to $5$ half-lives.

Loading Dose vs. Maintenance Dose Calculations

$\textbf{Loading Dose (LD):} \quad \text{LD} = \frac{C_{ss} \times V_d}{F}$

$\textbf{Maintenance Dose (MD):} \quad \text{MD} = \frac{C_{ss} \times CL \times \tau}{F}$

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Pharmacodynamics: Dose-Response Relationships & Antagonism

Graded Dose-Response Curves: Potency vs. Efficacy

• Potency: Refers to the concentration or dose of a drug required to produce $50\%$ of its maximal effect, represented by the $EC_{50}$ (effective concentration 50). On a semi-logarithmic dose-response curve, a leftward shift indicates greater potency (less drug is needed to elicit the response).
• Efficacy: Refers to the maximal biological response ($E_{\max}$) achievable by a drug at saturating concentrations. On a dose-response plot, efficacy is represented by the height (plateau) of the curve. Clinically, efficacy is vastly more important than potency.

Competitive vs. Non-Competitive Antagonists

Therapeutic Index & Therapeutic Drug Monitoring

The Therapeutic Index (TI) is a quantitative metric of drug safety, defined as the ratio between toxic dose and effective dose:

$\text{Therapeutic Index (TI)} = \frac{TD_{50}}{ED_{50}} \quad \left(\text{or } \frac{LD_{50}}{ED_{50}} \text{ in preclinical animal models}\right)$

• Wide Therapeutic Index: Large safety margin; plasma drug concentration monitoring is unnecessary (e.g., penicillin, beta-blockers).
• Narrow Therapeutic Index (NTI): Small difference between therapeutic efficacy and lethal toxicity; requires therapeutic drug monitoring (TDM) of trough/peak levels to prevent catastrophe.
• Classic Narrow TI Drugs: Warfarin, Digoxin, Lithium, Theophylline, Vancomycin, Aminoglycosides (Gentamicin/Tobramycin), Phenytoin, and Carbamazepine.