PHS 110 Fall 2026 - Lecture 2 Study Guide

Core Principles of Pharmacokinetics and Pharmacodynamics

• Pharmacodynamics (PD):

  • Defined as the study of the biochemical and physiological effects of drugs on the body and the molecular mechanisms of their actions.
  • Focuses fundamentally on what a drug does to the body.
  • Encompasses drug-receptor interactions, signal transduction pathways, dose-response relationships, therapeutic windows, and the distinction between primary therapeutic effects and unwanted adverse drug reactions (side effects).

• Pharmacokinetics (PK):

  • Defined as the study of the movement of drugs into, through, and out of the body over time.
  • Focuses fundamentally on what the body does to the drug.
  • Comprises four primary quantitative phases: Absorption, Distribution, Metabolism, and Excretion (commonly abbreviated as the ADME framework).
  • Governs parameters including rate of onset, peak plasma concentration ($C_{\max}$), time to reach peak concentration ($T_{\max}$), half-life ($t_{1/2}$), clearance ($CL$), and volume of distribution ($V_d$).

Clinical Pharmacology Case Studies: Warfarin and Thalidomide

• Warfarin:

  • Therapeutic Class and Clinical Use: An oral anticoagulant agent widely prescribed for the prevention and treatment of thromboembolic events, such as deep vein thrombosis (DVT), pulmonary embolism (PE), and stroke prevention in patients with atrial fibrillation or mechanical heart valves.
  • Mechanism of Action: Inhibits the enzyme vitamin K epoxide reductase complex 1 ($\text{VKORC1}$), thereby depleting the reduced form of vitamin K ($\text{vitamin KH}_2$) required for the post-translational carboxylation of glutamate residues on vitamin K-dependent clotting factors ($\text{Factor II}$, $\text{Factor VII}$, $\text{Factor IX}$, and $\text{Factor X}$), as well as the endogenous anticoagulant proteins C and S.
  • Narrow Therapeutic Index (NTI): Warfarin possesses a remarkably narrow margin between safe therapeutic efficacy and fatal toxicity. Underdosing increases the risk of stroke and thrombosis, whereas overdosing precipitates severe internal hemorrhage.
  • Clinical Monitoring: Requires frequent and rigorous laboratory monitoring of blood coagulation parameters using the Prothrombin Time (PT), standardized internationally as the International Normalized Ratio (INR). The target therapeutic INR for most indications ranges between $2.0$ and $3.0$.
  • Metabolism and Interactions: Heavily metabolized by hepatic cytochrome P450 enzymes (particularly $\text{CYP2C9}$). Highly susceptible to food-drug interactions (e.g., changes in dietary intake of green leafy vegetables rich in vitamin K) and drug-drug interactions (e.g., drugs that induce or inhibit $\text{CYP2C9}$, such as amiodarone, fluconazole, or rifampin), requiring constant dose adjustments.

• Thalidomide:

  • Historical Context: Marketed widely in Europe and other regions during the late 1950s and early 1960s as a safe, non-barbiturate sedative and an effective treatment for morning sickness (hyperemesis gravidarum) in pregnant women.
  • Teratogenicity: Proven to be severely teratogenic when ingested during early pregnancy (specifically during embryonic limb-bud development between days 20 and 36 post-conception). It resulted in thousands of infants born worldwide with severe congenital malformations, most notably phocomelia (a condition characterized by the severe shortening, stunting, or total absence of long limbs, with hands or feet attached directly or nearly directly to the torso), along with ocular, auditory, and cardiac defects.
  • Stereochemistry Considerations: Thalidomide exists as a chiral molecule with two enantiomers: the $(R)$-enantiomer confers the desired sedative and hypnotic properties, whereas the $(S)$-enantiomer is predominantly responsible for the teratogenic and anti-angiogenic actions. Separation of enantiomers is clinically ineffective because thalidomide undergoes rapid, spontaneous in vivo racemization (interconversion between the $(R)$ and $(S)$ forms at physiological pH).
  • Regulatory Impact: The catastrophic outcome of thalidomide use led to radical changes in pharmaceutical legislation, particularly in the United States, prompting the passage of the Kefauver-Harris Drug Amendments of 1962. This legislation required pharmaceutical manufacturers to provide scientific proof of both drug safety and substantial efficacy through rigorous, well-controlled clinical trials prior to market approval.
  • Modern Clinical Indications: Reintroduced into clinical medicine under strict risk evaluation and mitigation systems (such as the STEPS or REMS programs) for targeted conditions where its immunomodulatory and anti-angiogenic properties are clinically valuable, such as the treatment of newly diagnosed and refractory multiple myeloma and erythema nodosum leprosum (ENL).

Characteristics of an Ideal Drug

• Effectiveness (Efficacy): The drug elicits the specific, intended pharmacological response for which it is administered. Efficacy is the single most critical characteristic of any drug.
• Safety: The drug cannot produce harmful effects even at very high doses and across prolonged periods of administration. In clinical reality, no drug is completely safe; all drugs carry the potential to produce adverse effects.
• Selectivity: The drug elicits only the specific response for which it is given, binding solely to its target receptor without interacting with unintended cellular targets or producing side effects.
• Reversible Action: The biological effects of the drug must be reversible within an appropriate therapeutic time frame (e.g., general anesthetics or oral contraceptives must wear off completely).
• Predictability: The patient's response to a given dose can be anticipated reliably without extreme idiosyncratic variations across diverse populations.
• Ease of Administration: The route of administration and dosage schedule should be simple and convenient (e.g., once-daily oral dosing), enhancing patient compliance and decreasing dosing errors.
• Freedom from Drug-Drug Interactions: The compound should not augment or inhibit the pharmacological activity, metabolism, or clearance of concurrent medications.
• Low Cost: The medication should be economical to synthesize, formulate, distribute, and purchase, ensuring broad accessibility.
• Chemical Stability: The drug formulation should retain its potency, stability, and sterility across extended shelf-lives without rapid degradation under standard environmental conditions.
• Simple Generic Name: The chemical entity should possess a clear, easily pronounceable, and memorable non-proprietary name to avoid prescribing and dispensing confusion.

Molecular Mechanisms: Lock and Key Model of Drug-Receptor Interactions

• Foundational Concept of Receptors:

  • A receptor is a macromolecular cellular component (typically a specialized protein located on the plasma membrane, inside the cytoplasm, or within the nucleus) that binds specific endogenous ligands (e.g., hormones, neurotransmitters) or exogenous drug molecules to initiate a cellular response.
  • The drug acts as a ligand that seeks out and binds to the complementary binding site on the receptor macromolecule.

• The Lock and Key Hypothesis:

  • Formulated by Emil Fischer, this classical model postulates that the molecular structure of the drug (the key) possesses a defined, rigid three-dimensional stereochemical shape that fits precisely into a preformed, complementary binding pocket on the receptor (the lock).
  • Proper geometric conformation, spatial orientation, charge distribution, and functional group alignment are strictly required for binding to occur.
  • Binding initiates an intracellular signal cascade or conformational change that either activates or completely blocks the biological function of that receptor.

• Chemical Forces Mediating Drug-Receptor Binding:

  • Electrostatic Interactions: Ionic attractions between oppositely charged functional groups (e.g., basic amine and acidic carboxylate), hydrogen bonding, and dipole-dipole attractions. These are the most common initial forces directing the drug into its binding pocket.
  • Van der Waals Forces: Weak, transient attractions between electron clouds of closely aligned neutral atoms; significant primarily when a high degree of complementary surface fit exists.
  • Hydrophobic Interactions: Driving forces governed by entropy that cause nonpolar drug moieties to associate within nonpolar hydrophobic clefts of the receptor, displacing ordered water molecules.
  • Covalent Bonds: Irreversible, high-energy chemical bonds formed between reactive drug functional groups and amino acid side chains on the receptor (e.g., aspirin acetylating cyclooxygenase). Most drug-receptor interactions rely on non-covalent, reversible forces.

• Functional Drug Classifications at Receptors:

  • Agonists: Molecules that bind to the receptor, fit the active site, and induce a conformational change that activates the receptor, mimicking the effect of endogenous physiological ligands (possess both affinity and high intrinsic activity).
  • Antagonists: Molecules that bind to the receptor active site with high affinity but lack intrinsic activity. They do not activate the receptor; instead, they prevent endogenous agonists or other drugs from binding, thereby blocking downstream signaling.

Routes of Drug Administration

• Enteral Routes (Involving the Gastrointestinal Tract):

  • Oral (PO - Per Os): Swallowed into the gastrointestinal system; absorbed predominantly via the small intestine.
  • Sublingual and Buccal: Placed under the tongue (sublingual) or between the gum and cheek (buccal); absorbed directly across the oral mucosa into the systemic venous circulation.
  • Rectal (PR - Per Rectum): Inserted into the rectum as suppositories or enemas; partially avoids portal circulation ($\sim 50\%$\ bypass) and provides a viable route during vomiting, unconsciousness, or pediatric distress.

• Parenteral Routes (Injectable / Non-GI Routes):

  • Intravenous (IV): Injected directly into a systemic vein; eliminates the absorption phase entirely, yielding instant, complete systemic bioavailability ($100\%$).
  • Intramuscular (IM): Injected into deep, vascularized skeletal muscle tissue (e.g., deltoid, gluteus medius); offers rapid or sustained depot release depending on the aqueous or oil-based vehicle.
  • Subcutaneous (SC / SubQ): Injected into the adipose tissue beneath the dermis; allows slow, sustained, and predictable absorption (e.g., insulin, heparin).
  • Intradermal (ID): Injected into the upper layers of the skin dermis; primarily utilized for diagnostic testing (e.g., tuberculin PPD test) or specific vaccinations.

• Alternative and Topical Routes:

  • Inhalation: Aerosols, powders, or gases delivered into the bronchial tree and alveolar spaces; provides rapid local pulmonary delivery or rapid systemic absorption due to large alveolar surface area and high capillary blood flow.
  • Transdermal: Applied via adhesive skin patches formulated for continuous, systemic absorption across the stratum corneum into the dermal microvasculature (e.g., nicotine, fentanyl, nitroglycerine patches).
  • Topical: Applied directly to mucous membranes, eyes, ears, or skin surfaces for strictly local therapeutic effects, minimizing systemic exposure.
  • Intrathecal / Epidural: Introduced directly into the subarachnoid space or spinal canal to bypass the blood-brain barrier for central nervous system targeted therapies or regional anesthesia.

Oral Drug Administration: Advantages and Disadvantages

• Advantages of the Oral Route:

  • Convenience and Ease: Highly accessible and universally accepted by patients; allows self-administration without specialized medical equipment, sterile needles, or trained clinical personnel.
  • Cost-Effectiveness: Oral dosage forms (tablets, capsules, suspensions) are significantly less expensive to manufacture, package, store, and transport than sterile, pyrogen-free parenteral formulations.
  • Safety and Reversibility: Absorption is gradual rather than instantaneous, providing a window of opportunity to manage acute toxicity or accidental overdose through gastric decontamination (e.g., administration of activated charcoal, gastric lavage, or induced emesis).
  • Non-Invasive: Avoids pain, needle phobia, local tissue damage, and catheter-associated venous thrombophlebitis or systemic bloodstream infections.

• Disadvantages of the Oral Route:

  • Variable Absorption: Extent and rate of drug absorption depend heavily on erratic physiological variables, including gastric emptying time, intestinal motility, regional GI pH, surface area, and the presence or absence of food.
  • First-Pass Hepatic Elimination: Absorbed molecules must travel through the portal venous system directly to the liver, where extensive pre-systemic metabolism can inactivate substantial amounts of the parent compound before reaching target tissues.
  • Gastrointestinal Irritation: Many active pharmaceutical ingredients directly irritate the gastric mucosal lining, provoking nausea, vomiting, dyspepsia, or ulceration (e.g., nonsteroidal anti-inflammatory drugs - NSAIDs).
  • Chemical and Enzymatic Inactivation: Susceptible to degradation by gastric hydrochloric acid ($\text{pH} \approx 1.5\text{ to }2.0$), proteolytic digestive enzymes (pepsin, trypsin), or resident microflora enzymes, making the oral delivery of peptide and protein drugs (e.g., insulin) non-viable without specialized formulation coatings.
  • Patient Compliance and Condition Barriers: Ineffective for patients who are actively vomiting, completely unconscious, uncooperative, dysphagic, or experiencing acute medical emergencies requiring immediate onset of drug action.

The ADME Framework in Pharmacokinetics

• A – Absorption:

  • The movement of a drug from its site of administration across biological biological membranes into the bloodstream or systemic circulation.
  • Governed by passive diffusion, facilitated diffusion, active transport, and endocytosis.
  • Key determinants include drug lipophilicity, ionization status ($pK_a$ versus environmental $\text{pH}$), molecular size, formulation dissolution rate, and local blood flow.

• D – Distribution:

  • The reversible movement and dispersion of a drug between the systemic vascular space and the various extravascular fluids, tissues, and intracellular compartments of the body.
  • Influenced by physiological tissue perfusion rates, capillary membrane permeability (e.g., tight junctions of the blood-brain barrier), plasma protein binding (primarily to albumin for acidic drugs and $\alpha_1$-acid glycoprotein for basic drugs), tissue-specific binding affinities, and overall lipid solubility.

• M – Metabolism (Biotransformation):

  • The enzymatic modification of a parent drug into more polar, water-soluble metabolites to facilitate excretion from the body.
  • Occurs primarily in the liver, though secondary metabolism occurs in kidneys, lungs, intestines, and plasma.
  • Phase I Reactions: Functionalization processes (oxidation, reduction, hydrolysis) typically catalyzed by the Cytochrome P450 ($\text{CYP450}$) enzyme superfamily, which introduce or unmask a polar functional group ($-\text{OH}$, $-\text{NH}_2$, $-\text{SH}$).
  • Phase II Reactions: Conjugation processes (glucuronidation, sulfation, glutathione conjugation, acetylation) where an endogenous polar molecule is covalently attached to the functional group, yielding highly hydrophilic, inactive compounds readily excreted.

• E – Excretion (Elimination):

  • The permanent physical removal of the unchanged drug or its biotransformed metabolites from the body.
  • The primary organ of excretion is the kidney (via glomerular filtration, active tubular secretion, and passive tubular reabsorption into the urine).
  • Biliary and fecal excretion represents another major pathway (via hepatic drug transporters pumping substances into bile, entering the intestine).
  • Minor excretion pathways include pulmonary exhalation (volatile gases), sweat, saliva, and breast milk.

Bioavailability and Quantitative Pharmacokinetics

• Bioavailability ($F$) Defined:

  • The fraction or percentage of an administered drug dose that successfully reaches the systemic circulation in an unchanged, chemically active form.
  • For any drug administered intravenously (IV), bioavailability is by definition complete: $F = 1.0$ or $100\%$.
  • For extravascular routes (especially oral administration), $F$ is commonly less than $1.0$ ($< 100\%$) due to incomplete intestinal absorption and extensive first-pass hepatic biotransformation.

• Mathematical Formula:

  • Bioavailability is calculated by comparing the Area Under the plasma concentration-time Curve ($\text{AUC}$) following oral dosing to the $\text{AUC}$ following an equal intravenous dose:         $F = \frac{\text{AUC}_{\text{oral}}}{\text{AUC}_{\text{IV}}} \times \frac{\text{Dose}_{\text{IV}}}{\text{Dose}_{\text{oral}}} \times 100\%$
  • When the administered doses for both routes are identical ($\text{Dose}_{\text{oral}} = \text{Dose}_{\text{IV}}$), the formula simplifies to:         $F = \frac{\text{AUC}_{\text{oral}}}{\text{AUC}_{\text{IV}}} \times 100\%$

• Quantitative Example:

  • Scenario: A patient is administered a $100\,\text{mg}$ dose of a drug intravenously, generating an Area Under the Curve of $\text{AUC}_{\text{IV}} = 50\,\text{mg}\cdot\text{h/L}$. Subsequently, the same patient is administered a $100\,\text{mg}$ oral dose of the same drug, generating an Area Under the Curve of $\text{AUC}_{\text{oral}} = 35\,\text{mg}\cdot\text{h/L}$.
  • Calculation:$F = \frac{35\,\text{mg}\cdot\text{h/L}}{50\,\text{mg}\cdot\text{h/L}} \times \frac{100\,\text{mg}}{100\,\text{mg}} = 0.70$
  • Result: The absolute bioavailability ($F$) of the oral drug formulation is $0.70$, or $70\%$. This demonstrates that $30\%$ of the ingested dose was eliminated through incomplete intestinal absorption or hepatic first-pass metabolism before entering systemic arterial circulation.

First-Pass Hepatic Metabolism and Nitroglycerine

• Mechanism of First-Pass (Pre-systemic) Hepatic Metabolism:

  • When a drug is absorbed through the stomach and intestines, it enters the mesenteric venules.
  • These venules drain directly into the hepatic portal vein, which channels the entire blood volume carrying the absorbed drug directly into the hepatic sinusoids of the liver prior to reaching the inferior vena cava and systemic arterial circulation.
  • If the liver contains high concentrations of active metabolic enzymes (e.g., $\text{CYP450}$ enzymes) with high affinity for the drug, a massive fraction of the dose is enzymatically transformed and cleared during this initial passage, drastically reducing systemic bioavailability.

• Nitroglycerine Case Study:

  • Therapeutic Action: A potent direct vasodilator used for the immediate relief and prophylaxis of acute angina pectoris attacks (ischemic cardiac chest pain).
  • Oral Ingestion Limitation: When swallowed as an oral tablet, nitroglycerine is subjected to near-total pre-systemic hepatic extraction, with first-pass hepatic metabolism exceeding $90\%$ to $96\%$. Consequently, swallowed nitroglycerine delivers an unpredictable and therapeutically ineffective concentration to coronary blood vessels.
  • Sublingual Delivery Solution: Formulated as a small, rapidly dissolving tablet placed directly under the tongue (sublingually).
  • Anatomical Vascular Bypass: The sublingual mucosal tissue contains a rich venous plexus that drains directly into the internal jugular veins and superior vena cava, entering systemic circulation directly while completely bypassing the mesenteric vessels, the hepatic portal system, and liver first-pass metabolism.
  • Clinical Consequence: Delivers rapid, robust plasma concentrations within $1$ to $3\,\text{minutes}$, terminating acute myocardial ischemia rapidly.

Drug Development, Patent Protection, and Regulatory Standards

• Brand-Name Versus Generic Drugs:

  • Brand-Name (Proprietary / Trade) Drug: The commercial product developed, researched, patented, and brought to market by an innovator pharmaceutical company under a registered trademarked name (e.g., Lipitor, Coumadin).
  • Generic (Non-Proprietary) Drug: A pharmaceutical equivalent developed to match a brand-name drug in active pharmaceutical ingredient (API), dosage form, strength, route of administration, quality, performance characteristics, and therapeutic indication, marketed after the innovator's patent and exclusivity periods expire (e.g., atorvastatin, warfarin).

• Patent Protection of Pharmaceuticals:

  • Duration and Purpose: In the United States, patents are granted by the U.S. Patent and Trademark Office (USPTO) for a standard legal period of $20\,\text{years}$ from the initial filing date.
  • Effective Commercial Life: Because patent applications are filed early during preclinical testing (prior to clinical trials), an innovator firm typically expends $8$ to $12\,\text{years}$ of the patent life executing clinical investigations and obtaining regulatory clearance, leaving an effective market exclusivity period of approximately $7$ to $12\,\text{years}$.
  • Economics: Market exclusivity permits the innovator company to charge premium prices to recoup extensive research and development (R&D) investments (often exceeding hundreds of millions or billions of dollars) and generate profits. Upon patent expiration, competing generic manufacturers enter the market, inducing price reductions of up to $80\%$ to $85\%$ through open competition.

• Bioequivalence:

  • Scientific Definition: Two pharmaceutical products containing identical active moieties are considered bioequivalent when they display no statistically significant difference in the rate and extent to which the active ingredient becomes available at the intended site of drug action when administered at the same molar dose under similar experimental conditions.
  • Regulatory Parameters: Evaluated by testing the generic and brand formulations in pharmacokinetic crossover human studies and comparing two primary metrics:
    • Peak plasma concentration ($C_{\max}$), indicating rate of absorption.
    • Area under the plasma concentration-time curve ($\text{AUC}$), indicating total extent of absorption.
  • FDA Standard: The $90\%$ confidence intervals for the ratio of the geometric mean values (Generic/Innovator) of both $C_{\max}$ and $\text{AUC}$ must fall entirely within the bioequivalence bio-acceptance interval of $80.00\%$ to $125.00\%$ ($0.80\text{ to }1.25$).

Essential Pharmacological and Regulatory Abbreviations

• GI:

  • Stands for: Gastrointestinal.
  • Definition: Refers collectively to the continuous organ tract spanning from the mouth to the anus, including the esophagus, stomach, small intestine (duodenum, jejunum, ileum), and large intestine (colon, rectum), which functions in the ingestion, digestion, absorption of nutrients and drugs, and elimination of wastes.

• IND:

  • Stands for: Investigational New Drug (or Investigational New Drug Application).
  • Definition: An official regulatory filing submitted by a pharmaceutical sponsor to the United States Food and Drug Administration (FDA) following successful preclinical testing (laboratory and animal models). It requests clinical authorization to administer an unapproved experimental compound or dynamic biological agent to human subjects across Phase 1, Phase 2, and Phase 3 clinical trials.

• NDA:

  • Stands for: New Drug Application.
  • Definition: The formal, comprehensive vehicle through which a pharmaceutical sponsor formally proposes that the FDA approve a new pharmaceutical entity for commercial sale and clinical distribution in the United States. The NDA compiles complete preclinical and human clinical trial data (Phases 1–3), drug chemistry, manufacturing processes, pharmacology, toxicology, and proposed product labeling demonstrating that the drug is both safe and effective for its stated indications.

• ANDA:

  • Stands for: Abbreviated New Drug Application.
  • Definition: An application submitted to the FDA for the review and commercial approval of a generic drug product. It is termed "abbreviated" because sponsors are not required to reproduce expensive, time-consuming preclinical safety data or Phase 1–3 clinical safety and efficacy trials. Instead, the generic applicant must scientifically demonstrate bioequivalence and manufacturing quality comparable to the reference listed brand-name drug.