Principles of Pharmacology — Page-by-Page Notes (Pages 3-38)
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Maggot therapy: historical and contemporary relevance
Maggot therapy can be used to clean wounds and prevent infection.
Historical observations by Baer linked maggots in wounds with reduced fever, less necrosis, and better tissue healing in severe infections.
Maggots ingested necrotic tissue while leaving healthy tissue intact when sterilized.
Popular in the 1930s–1940s until penicillin became a simpler alternative.
Modern interest in maggot therapy for wounds infected with antibiotic-resistant bacteria.
In the EU, Japan, and Canada maggots are regarded as medicinal drugs; in 2005 the FDA approved maggots as a medical device.
The healing mechanism is not fully understood, but maggot secretions appear to suppress the immune system, reduce inflammation, may enhance cell growth, and can increase wound oxygen concentration. Pharmacology aims to identify and isolate the active components.
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William S. Baer and maggot therapy
Baer, an orthopedic surgeon at Johns Hopkins, observed battlefield wound healing advantages with maggots.
Maggots are thought to ingest dead tissue, thereby cleansing wounds and enabling new tissue formation; sterilized maggots prevent secondary infections.
Maggot therapy declined after penicillin but re-emerges for antibiotic-resistant wounds.
Current understanding: maggot secretions can suppress immune responses, reduce inflammation, and potentially promote tissue regeneration and oxygenation.
Pharmacology context
The re-emergence of maggot therapy reflects a broader trend in pharmacology to isolate active components for therapeutic use.
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Pharmacology: scope and historical context
Pharmacology is the scientific study of drug actions and effects on living organisms.
Early pharmacology focused on naturally occurring substances; plants have a long therapeutic history (Egypt, India; Ebers Papyrus lists >700 remedies including castor oil and opium).
Traditional medicine systems (Chinese herbal remedies) are extensive and continue today.
World Health Organization estimates up to 80% of people in developing countries rely on herbs or plant-derived medicines.
In 1999, herbal medicines and natural product-derived drugs represented about half of the top 20 drugs in the US.
Despite popularity, herbal remedies can be dangerous; Web Box 1.1 discusses benefits and dangers.
Pharmacology subdivisions and drug action
Neuropharmacology: drug-induced changes in nervous system function.
Psychopharmacology: drug-induced changes in mood, thinking, and behavior.
Neuropsychopharmacology: aims to identify chemicals that alter nervous system function to change behavior; also uses chemical agents as probes into neurobiology.
Drug action concept: the molecular changes produced when a drug binds to a target site or receptor.
Site of action vs site of effect: actions at one site can produce effects elsewhere (e.g., atropine dilates pupils via iris muscles, whereas morphine’s primary effects occur in the brain).
Drugs have multiple effects: therapeutic effects and side effects; there are no inherently good or bad drugs—context of use and delivery determine safety and ethics.
Distinction between drug effects that are specific (drug–receptor interactions) and nonspecific effects that depend on individual factors (docs, mood, environment).
Placebo and nonspecific effects
Placebo: pharmacologically inert; can produce therapeutic-like effects and side effects; depends on mind–body interaction, ritual, and conditioning.
Classic ulcer placebo study shows higher relief when a physician provides the placebo with assurance vs. administration by a nurse (Levine, 1973).
Mechanisms include Pavlovian conditioning (sensory cues associated with medication) and conscious expectations of relief.
Placebo effects are relevant to personalized medicine and can be exploited to improve outcomes; some use placebos in ethically appropriate ways.
Nocebo effect: negative expectations can increase anxiety and pain via brain processes; warnings about side effects can elevate reported adverse effects.
Placebos are central to clinical trials, especially in double-blind designs, to separate true drug effects from expectation effects.
Active placebo: produces some side effects to maintain blinding when a true placebo would be obviously inert.
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Placebo, nocebo, and research design details
Double-blind experiments: neither participants nor researchers know who receives the active drug; reduces bias.
Ethical considerations in using placebos when effective treatments exist; sometimes older therapies are used as comparators.
Placebo effects can occur in animals, suggesting environmental cues contribute to outcomes.
Therapeutic potential of placebo: mind–body interactions can augment treatment outcomes.
Naming drugs (Box 1.1 intro in pharmacology in action)
Drug names follow multiple schemes: chemical name, generic (nonproprietary) name, brand (trademarked) name, slang or street names.
Example: diazepam has chemical name 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one and generic name diazepam; brand name Valium.
Street names for drugs (e.g., marijuana and cocaine) are compiled by agencies like NIDA; names vary by time and region.
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Drug naming and common terms (Box 1.1, continued)
Box 1.1 elaborates on how drugs are named and why there can be confusion: chemical names are unwieldy; generic names are official; brand names are manufacturer-specific; slang names vary across locales.
NIDA maintains lists of street names for common drugs, illustrating the diversity of nomenclature in illicit use.
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Pharmacokinetic factors and drug action (overview)
Drug effect depends on: how much drug reaches the target and how quickly it does so (bioavailability and pharmacokinetics).
Five key pharmacokinetic factors determining bioavailability (Figure 1.2):
1) Routes of administration – how and where drug enters the body.
2) Absorption and distribution – passage through membranes into blood and distribution to tissues.
3) Binding – binding to receptors or depot binding to plasma proteins; some drugs store in bone/fat.
4) Inactivation (metabolism) – liver and other tissues chemically modify drugs.
5) Excretion – metabolites or unchanged drug eliminated via kidneys or bile.The total amount of drug in the body reflects a balance of absorption and inactivation; both influence intensity and duration of effects.
Routes of administration and absorption dynamics
Enteral methods (GI tract): generally slower onset and more variable plasma levels; include oral (PO) and rectal (suppositories).
Parenteral methods: do not use the GI tract; include injection (IV, IM, IP, SC), inhalation, and topical.
Oral absorption (PO): requires dissolution in stomach fluids and passage through gastric wall; must resist stomach acid/enzymes.
First-pass metabolism: orally administered drugs may undergo significant hepatic metabolism before reaching systemic circulation, reducing bioavailability (sometimes >90% loss).
Rectal administration can partially bypass first-pass metabolism depending on placement, but absorption is irregular.
IV administration provides the fastest, most precise drug delivery but carries overdose and anaphylaxis risks and bypasses absorption barriers.
IM injection offers slower, steadier absorption; absorption rate can be modulated by co-injected vasoconstrictors and depots (e.g., oil suspensions like Depo-Provera).
IP (intraperitoneal) injections are common in animals, rapid but less predictable than IV.
SC (subcutaneous) injections are slow and steady; absorption can be influenced by local blood flow; non-aqueous suspensions or implants slow release.
Inhalation delivers drugs rapidly due to large lung surface area and proximity to blood, with very rapid CNS effects (e.g., nicotine, THC, crack cocaine).
Topical and patch delivery provide local or systemic effects; transdermal patches avoid first-pass metabolism but have limited permeation; technologies like microneedles and iontophoresis expand skin permeability and allow self-administration.
Sublingual administration bypasses gastric digestion and first-pass metabolism, delivering rapid systemic effects.
Intranasal delivery can provide local relief (nasal decongestion) or systemic effects and can bypass the blood–brain barrier in some cases; intranasal oxytocin studies in autistic adults are cited as Web Box 1.2.
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More on routes and absorption specifics
Oral and rectal routes are enteral; all others are parenteral.
Stomach-emptying rate, food intake (especially fatty meals), activity level, and other factors influence absorption and onset.
Some drugs undergo extensive first-pass metabolism; high first-pass effects may necessitate higher oral doses or alternative administration routes.
Inhalation provides rapid CNS effects due to pulmonary absorption and swift transport to the brain via arterial circulation.
Snorting (intranasal cocaine) achieves rapid CNS effects but can cause nasal septum perforation due to vasoconstriction and irritation from contaminants.
Accidental dermal absorption of toxins (lead, organophosphates, carbon tetrachloride) demonstrates skin as a potential route for toxic exposure.
Transdermal delivery with patches offers controlled, sustained delivery and avoids first-pass metabolism; however, skin permeability is a limitation.
Emerging skin delivery methods (ultrasound-enhanced permeation, iontophoresis, microneedles) improve feasibility for larger molecules and vaccines.
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Advanced administration methods and CNS-targeted delivery
Epidural injection bypasses the blood–brain barrier to deliver spinal anesthetics directly into cerebrospinal fluid around the spinal cord.
Intracranial (intracerebral) or intracerebroventricular injections allow precise targeting of brain tissue or CSF-filled ventricles for research or therapeutic purposes; used in animals and research contexts.
Infusion pumps can deliver antibiotics or hormones directly into cerebral ventricles; sensor-regulated pumps can respond to physiological cues (e.g., insulin delivery by an implantable device).
Risks of implantable pumps include infection and clogging, but benefits include stable drug delivery and potential for pulsatile administration that mimics biological rhythms.
Gene therapy uses DNA to modulate expression of specific proteins to treat diseases; major challenges include delivering DNA to target cells and achieving controlled expression.
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Continued routes and CNS-targeting delivery
Gene therapy challenges include delivery vectors, controlling expression, and avoiding immune responses; combining with specialized delivery systems (e.g., intracerebral infusion) can be explored in Chapter 4.
The CNS presents barriers to drug delivery; bypass methods (epidural, intracranial, CSF delivery) are important for certain therapies.
The blood–brain barrier selectively restricts many molecules from entering the brain; lipid-soluble agents cross more readily than hydrophilic ones.
Intranasal and other noninvasive routes retain promise for CNS delivery while reducing systemic exposure.
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Continued pharmacokinetic and delivery science
Blood–brain barrier considerations and alternative delivery strategies are central to CNS pharmacology.
The chapter also notes environment and technology advances that enable new delivery systems (e.g., microneedles, iontophoresis, ultrasound).
Epidural and intracranial routes illustrate how clinicians can target the CNS directly when necessary.
Gene therapy uses DNA to adjust protein expression; one challenge is achieving targeted, durable expression with manageable safety profiles.
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Ionization and absorption: pH effects
Ionization state affects membrane permeability and thus absorption; non-ionized forms cross lipid membranes more readily than ionized forms.
The ionization state of drugs depends on the pH of the environment and the drug’s acid/base properties.
Example: aspirin in stomach acid (pH ≈ 2.0) remains largely non-ionized and cross membranes readily; in the more basic intestinal environment, aspirin dissociates more, becomes ionized, and absorption slows.
The figure (Figure 1.6) illustrates how ionization influences passage through membranes and distribution within the body.
Body composition and distribution
At baseline, distribution is influenced by body composition; adipose tissue vs. water content varies across individuals and sexes.
Women generally have a higher body fat percentage relative to water than men, impacting drug distribution and concentration at target sites.
In children, smaller total body water results in higher drug concentration for a given dose.
Across routes, distribution is affected by tissue perfusion; highly vascularized organs (heart, brain, kidneys, liver) accumulate drugs quickly; redistribution occurs as equilibrium is established.
Depot binding (to plasma proteins or fat) sequesters drugs and reduces immediate availability at target sites.
A drug’s half-life and clearance contribute to duration of action via first-order vs zero-order kinetics (section on pharmacokinetics elaborates these concepts).
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Therapeutic drug monitoring and interspecies dose extrapolation (Box 1.2; Pharmacology in Action)
Interspecies dose extrapolation
When translating drug doses from one species to another, size alone is insufficient; multiple factors influence dosing across species.
Example: Tusko the elephant case demonstrates perils of dose scaling based solely on weight; LSD dose scaled from humans led to fatal overdose in an elephant.
Important factors for cross-species dosing:
Depot binding differences (protein binding varies by species and binding site numbers)
Metabolic differences (CYP450 enzyme family variations; species-specific metabolism)
Conjugation pathways (glucuronidation: humans readily glucuronidate; cats lack glucuronidation; rats are rapid acetylators; dogs are poor acetylators; humans are intermediate)
Differences in CNS neurotransmitter systems and receptor distribution/affinity across species
Implication: dose conversions require understanding of pharmacokinetics and pharmacodynamics beyond simple weight-based scaling; receptor characteristics and species-specific metabolism must guide extrapolation.
Box 1.2 (Pharmacology in Action): Interspecies dose extrapolation details and examples; practical guidance for veterinary and translational research.
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Section summary and clinical pharmacology context
Key takeaways from pharmacokinetics and pharmacodynamics concepts summarized:
Placebo is inert but can produce real therapeutic and side effects; placebo responses depend on ritual, conditioning, and expectations.
The two main determinants of drug effect are: (i) how much drug reaches target sites; (ii) how quickly it reaches those sites.
Bioavailability is determined by absorption and inactivation; five factors govern bioavailability: routes of administration, absorption/distribution, depot binding, inactivation, and excretion.
Route of administration affects onset, blood levels, duration, convenience, safety, and special uses; enteral vs parenteral distinction.
Oral and rectal routes are enteral; all others are parenteral.
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Pharmacodynamics: drug–receptor interactions (intro)
Core idea: pharmacodynamics deals with how drugs affect cells and tissues via molecular targets (enzymes, transporters, receptors, and other proteins).
Receptors: large proteins located on the cell surface or inside cells; ligands (drugs, neurotransmitters) bind receptors with selectivity.
Two major receptor types (briefly introduced):
Extracellular (cell-surface) receptors that relay signals to intracellular effectors (ions channels or G proteins).
Intracellular receptors (cytoplasmic or nuclear) that alter gene expression after ligand binding.
Ligands, agonists, antagonists, and efficacy concepts
An agonist activates receptors to produce a biological response; an antagonist binds but does not elicit a response, blocking receptor activation.
Efficacy reflects the maximal effect a ligand can produce; affinity reflects how tightly a ligand binds.
Receptor binding and dynamic regulation
Binding is transient; ligands dissociate and can re-bind.
Binding induces conformational changes that trigger intracellular signaling cascades.
Receptors can undergo up-regulation (increase in receptor numbers) or down-regulation (decrease) in response to chronic stimulation or blockade.
Long-term receptor regulation occurs over weeks; second messenger-mediated changes can be rapid.
Receptor subtypes and selective targeting
Receptors have subtypes with distinct distributions; drugs aim for selective receptor subtype binding to maximize therapeutic effects and minimize side effects.
Example: xanthines (caffeine, theophylline, theobromine) have differing affinities for xanthine receptor subtypes, leading to different clinical profiles.
Dose–response concept at receptor level
Dose–response curves plot mean response vs. dose, usually on a semi-log scale, showing an S-shaped curve.
ED50: dose producing 50% of the maximum effect; ED100: dose achieving full maximal effect.
Potency: relates to the dose required to achieve a given effect (lower ED50 = higher potency); efficacy: maximal achievable effect, equal across different drugs if they share the same mechanism.
Example: hydromorphone, morphine, and codeine have parallel dose–response curves (same mechanism) but different potencies (different ED50s).
Aspirin demonstrates a different mechanism and efficacy compared to opiates, reflected in a different curve shape.
Equations and notations to note
ED50: dose giving 50% of maximal effect.
ED100: dose giving 100% of maximal effect (full receptor occupancy assumption).
Potency vs. efficacy concepts discussed with dose–response curves.
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Drug categories (Box 1.3, pharmacology in action)
Primary categories based on neurochemical and behavioral effects:
CNS stimulants: increase brain activity, arousal (e.g., amphetamine, cocaine, methylphenidate); caffeine, theophylline, and theobromine are also stimulants with varying potency.
CNS depressants: depress CNS function, causing relaxation, sedation, sedation-related side effects; include barbiturates, benzodiazepines, and ethanol; marijuana can have depressant or hallucinogenic effects depending on dose.
Analgesics: reduce pain perception; include narcotics/opiates (morphine, codeine, heroin, fentanyl) and non-narcotic analgesics (aspirin, acetaminophen, ibuprofen).
Hallucinogens (psychedelics): distort perception and mood; include mescaline, psilocybin, LSD, MDMA; PCP/ketamine can be categorized variably due to dissociative/hallucinogenic properties.
Psychotherapeutic drugs: antipsychotics (haloperidol, chlorpromazine), antidepressants (sertraline, fluoxetine, duloxetine), mood stabilizers (lithium, valproate, carbamazepine).
Receptors and signaling (overview)
Ligands (drugs, neurotransmitters) bind receptors; binding triggers conformational changes and downstream signaling.
Receptors may mediate rapid ion flux or longer-term gene regulation depending on their coupling (ion channel vs G protein vs intracellular receptor).
Two main receptor types (revisited with figures)
Extracellular receptors (cell-surface) initiate intracellular signaling cascades.
Intracellular receptors (cytoplasmic/nuclear) alter gene expression and downstream protein synthesis.
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Receptor structure and signaling (continued)
Key receptor features:
Specific molecular fit (lock-and-key) governs affinity; high affinity enhances binding likelihood.
Efficacy varies: full agonists, partial agonists, antagonists, and inverse agonists create a spectrum of biological responses.
Some ligands bind but do not produce a response (neutral antagonists) and can block active ligands.
Agonist/antagonist continuum and receptor activation
Figure illustrates a continuum from full agonist to full antagonist/inverse agonist with intermediate partial agonists and partial inverse agonists.
Efficacy influenced by receptor occupancy and ligand-induced receptor conformation.
Receptor life cycle and regulatory changes
Receptors undergo regulation over time (up-/down-regulation), altering sensitivity and response to drugs.
Chronic drug exposure can cause receptor down-regulation; antagonist exposure can cause up-regulation; this contributes to tolerance and withdrawal phenomena.
Receptor subtype selectivity and drug design
Targeting receptor subtypes can improve selectivity and therapeutic outcomes while minimizing side effects.
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Receptors, efficacy, and occupancy (continued)
Conceptual framework for receptor interactions:
Agonists: activate receptors to elicit a response; efficacy depends on receptor conformation and signaling efficacy.
Antagonists: block receptor activation; may be competitive (displaced by sufficient agonist) or noncompetitive (binds elsewhere or disrupts signaling).
Examples of receptor antagonism
Competitive antagonism: naloxone reduces morphine analgesia by occupying opioid receptors; increasing morphine doses can overcome the blockage (dose–response curves shift right but max effect remains).
Noncompetitive antagonism: reduces maximum effect and alters curve shape; not overcome by simply increasing agonist dose.
Pharmacodynamic interactions: multiple forms
Physiological antagonism: two drugs act on different receptors but produce opposing physiological effects (e.g., one increases heart rate, another decreases it).
Additive effects: combined effects equal the sum of individual effects.
Potentiation: combined effects exceed the sum of individual effects, potentially via pharmacokinetic interactions or depot binding changes.
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Chronic drug use and biobehavioral effects (overview)
Chronic use leads to tolerance and/or sensitization; most often, response diminishes with repeated exposure, but sometimes certain effects intensify (as with amphetamine).
Drug-taking history significantly shapes drug action outcomes; context and environment modulate pharmacokinetics and pharmacodynamics.
Tolerance overview and definitions
Tolerance: diminished response to a drug after repeated exposure; larger doses may be required for the same effect.
Cross-tolerance: tolerance to one drug can diminish the effect of a second drug due to shared mechanisms.
Tolerance can be complex and uneven across drug effects; some effects show rapid tolerance while others persist.
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Tolerance characteristics and mechanisms (Table 1.8 summary inferred)
General features:
Reversibility: tolerance wanes if drug use stops.
Dependence on dose, frequency, and environment.
Some drugs show rapid tolerance (e.g., LSD); others require weeks (barbiturates); some show little to no tolerance (antipsychotics).
Tolerance mechanisms can be metabolic (increased drug metabolism), pharmacodynamic (neural adaptations), or behavioral (learning and conditioning).
Types of tolerance (Table 1.9 and notes): metabolic, pharmacodynamic, behavioral; some drugs show multiple forms.
Classical conditioning and behavioral tolerance
Conditioning links cues (paraphernalia, environment) with drug effects; cues can eventually trigger compensatory responses opposite to the drug’s effects.
An example: morphine injections in the same environment can produce a conditioned hypo/hyperthermic response, indicating tolerance via learned cues.
Different environments (same drug, different setting) can lead to reduced tolerance in new contexts, which may contribute to overdose risks when users change settings.
Experimental evidence (Figure 1.22, 1.23)
Classical conditioning of drug-related cues demonstrates the learned associations driving tolerance.
Morphine tolerance experiments show that context-specific cues can modulate the degree of tolerance in rats.
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Drug antagonism and interaction shapes (continuation of Figure 1.20 discussion)
Antagonism and interaction patterns
Competitive antagonism (Naloxone vs Morphine): shifting the dose–response curve to the right; higher agonist doses can overcome the antagonist.
Noncompetitive antagonism: typically alters the curve’s shape and cannot be overcome by higher agonist doses; max efficacy is not reached.
Additional interaction patterns
Physiological antagonism: two drugs interact across distinct physiological systems to reduce each other’s effects.
Additive: sum of individual drug effects.
Potentiation: combined effects exceed the sum of individual effects, often involving pharmacokinetic interactions or altered metabolism.
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Tolerance and behavioral adaptations (continued)
Chronic use and learning effects
Repeated exposure can trigger both pharmacological and behavioral adaptations, including learning processes, that influence tolerance development and withdrawal.
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Tolerance details and summary of forms
Summary of Table 1.9 (types of tolerance across several drugs):
Barbiturates: metabolic, pharmacodynamic, and behavioral tolerance present.
Alcohol: metabolic, pharmacodynamic, and behavioral tolerance present.
Morphine: metabolic, pharmacodynamic, and behavioral tolerance present.
Amphetamine: pharmacodynamic and behavioral tolerance (no metabolic tolerance).
Cocaine: pharmacodynamic and behavioral tolerance; no metabolic tolerance.
Caffeine: metabolic and possible behavioral tolerance; evidence mixed for pharmacodynamic tolerance.
Nicotine: pharmacodynamic and possible behavioral tolerance.
LSD: pharmacodynamic tolerance; limited or no behavioral tolerance.
Conditioning and the development of craving and withdrawal
Classical conditioning with cues can contribute to withdrawal experiences and craving during abstinence.
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Conditioning, environment, and tolerance (continued)
Morphine tolerance experiments (Figure 1.23) illustrate how context can influence the degree of tolerance observed.
Rats treated in the same environment show more pronounced tolerance compared to those treated in a different environment, due to environmental cue associations.
Implications for human use and overdose risk
Changing drug-taking environments or routines can alter the expression of tolerance, potentially increasing risk of overdose when a familiar dose is taken in a novel context.
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Closing notes on tolerance studies and the role of learning
The interplay of pharmacokinetic and pharmacodynamic tolerance with learned associations helps explain complex withdrawal and craving phenomena.
Understanding these mechanisms aids in designing safer pharmacotherapies and educating patients about the risks tied to chronic drug use.
Key formulas and concepts to remember
Therapeutic index:
Safety factor:
Five half-lives to steady state:
ED50, ED100, and concept of receptor occupancy for maximal effect on a dose–response curve (S-shaped curve)
Ionization and absorption: nonionized forms cross membranes more readily; ionization state depends on pH and drug pKa; example dynamics around pH 2.0 (stomach) vs higher pH (intestine).
Receptor dynamics: up-/down-regulation; competitive vs noncompetitive antagonism; partial agonists and inverse agonists; agonist–antagonist interactions.
Dose–response relationships across analgesics and non-opioids illustrate differing receptor mechanisms and maximum efficacy.