FOUNDATIONS OF PHARMACOLOGY

Pharmacodynamics

Drug Targets and Mechanisms

• Receptors are common drug targets. Important targets include ion-channel receptors, G protein-coupled receptors, transmembrane receptors, intracellular receptors that regulate gene expression, and enzymes. Some drugs act through nonreceptor mechanisms (Peterson & Randazzo, 2022; Woo & Robinson, 2019).
• Four major receptor families mediate most drug-receptor interactions. Ligand-gated ion channels open or close in response to binding, producing rapid changes in ion flow across the membrane. Transmembranous receptors span the membrane and signal through an intracellular enzymatic domain, such as tyrosine-kinase activity, or an associated second-messenger system. G protein-coupled receptors activate an intracellular G protein that triggers a second-messenger cascade, producing a slower but often amplified response. Intracellular receptors are located in the cytoplasm or nucleus and bind lipid-soluble drugs that cross the cell membrane directly, altering gene transcription and producing effects with a slower onset and longer duration (Peterson & Randazzo, 2022).
• Affinity describes how strongly a drug binds to a receptor. Drug-receptor binding is generally reversible, selective, and graded (Ernstmeyer & Christman, 2023; Peterson & Randazzo, 2022).
• An agonist binds to a receptor and produces an effect. An antagonist binds without activating the receptor and limits the effect of an agonist (Ernstmeyer & Christman, 2023; Peterson & Randazzo, 2022).
• Intrinsic activity is the ability of a receptor-bound drug to activate the receptor. A full agonist can produce the system’s maximal response, while a partial agonist has lower intrinsic activity and produces a lower maximal response even when receptors are occupied. In the presence of a full agonist, a partial agonist can reduce the overall response by competing for the same receptors (Peterson & Randazzo, 2022).
• Selectivity is a drug’s preferential action at one receptor or target relative to others. Selectivity is concentration dependent and does not eliminate unintended effects, especially as exposure increases (Ernstmeyer & Christman, 2023; Peterson & Randazzo, 2022).

Dose-Response Relationships

• A graded dose-response curve describes the magnitude of response in an individual or system as the dose or concentration increases. It helps compare potency and maximal efficacy (Woo & Robinson, 2019).
• A quantal dose-response curve describes the proportion of a population that experiences a defined outcome at each dose. The outcome is categorical, such as seizure prevention, adequate anesthesia, or a specified toxicity (Woo & Robinson, 2019).
• Below the minimum effective concentration, the intended effect may be absent or inadequate. Above the minimum toxic concentration, harmful effects become increasingly likely (Ernstmeyer & Christman, 2023).
• The therapeutic window is the concentration range in which benefit is expected without unacceptable toxicity. The therapeutic index compares doses associated with therapeutic and toxic effects and provides a relative measure of safety (Ernstmeyer & Christman, 2023).

Potency, Efficacy, and Clinical Value

• Greater potency shifts a dose-response curve to the left because a lower dose produces the same specified effect. Potency alone does not establish better outcomes, greater safety, or superior value.
• Greater efficacy raises the maximum effect that can be achieved. When the required clinical effect exceeds one drug’s ceiling, a more efficacious option may be needed.
• A drug with lower potency may still be preferable if it has stronger evidence, a wider therapeutic window, fewer interactions, easier monitoring, better tolerability, or lower cost.
• Clinical benefit depends on whether the mechanism and achievable response match the patient’s therapeutic objective.

Applied Example: Opioid Receptor Pharmacology

Morphine and hydromorphone produce comparable analgesic efficacy, but hydromorphone’s greater affinity for the mu-opioid receptor means a smaller concentration achieves that same effect (Peterson & Randazzo, 2022).

Naloxone is a mu-opioid antagonist with an affinity for the receptor higher than that of most opioid agonists. Given to a patient on opioid therapy, it competes for and displaces the agonist from the receptor, reversing analgesic and respiratory-depressant effects without producing agonist activity itself. Following intravenous administration, reversal may begin within one to two minutes. Onset varies by route, which should be considered when evaluating response and the need for repeat dosing. This illustrates competitive antagonism directly (Peterson & Randazzo, 2022).

Buprenorphine is a partial mu-opioid receptor agonist with unusually high receptor affinity but lower intrinsic activity than a full agonist. Its partial agonism produces a ceiling effect on respiratory depression relative to full opioid agonists, but clinically significant or fatal respiratory depression can still occur, particularly when buprenorphine is combined with alcohol, benzodiazepines, or other central nervous system depressants. Its high receptor affinity also allows buprenorphine to displace a full agonist such as heroin or methadone. When buprenorphine is initiated before sufficient spontaneous withdrawal has developed after the last full-agonist dose, this displacement can precipitate opioid withdrawal (Substance Abuse and Mental Health Services Administration [SAMHSA], 2021).

Reading a dose-response curve clinically: once increasing a drug’s dose no longer produces additional benefit, the patient may have reached that drug’s efficacy ceiling, and further increases may add toxicity without improving the intended response. The clinician should reassess the diagnosis, treatment target, adherence, drug exposure, timing, interactions, and toxicity before deciding whether to continue the regimen, switch therapies, or add a medication with a different mechanism.

Therapeutic and Adverse Effects

• The intended effect results from sufficient activity at the therapeutic target. Adverse effects may arise from the same target in another tissue, excessive target activity, activity at another target, toxic metabolites, or immune-mediated reactions.
• A side effect is a known secondary effect that may be tolerable, beneficial, or problematic. An adverse drug reaction is a harmful and unintended response associated with use of the medication at usual doses.
• A narrow therapeutic window leaves limited separation between effective and harmful exposure. These medications often require conservative dose changes, closer clinical monitoring, and sometimes serum concentration monitoring (Ernstmeyer & Christman, 2023).
• The clinical response reflects the intended drug action, the patient’s receptor response, and the ability of physiologic systems to compensate for the medication’s effects.

Patient-Specific Variation

• Age can alter receptor number, signaling, homeostatic reserve, and sensitivity. Older adults may experience a greater clinical response at a concentration tolerated by a younger adult.
• Disease can change the drug target or the physiologic system that compensates for the drug’s effect. Heart failure, autonomic dysfunction, electrolyte abnormalities, endocrine disease, and neurologic disease can alter response.
• Genetics may change a receptor, enzyme, transporter, immune response, or signaling pathway. A pharmacogenomic effect may therefore alter pharmacodynamics even when drug concentration is unchanged.
• Tolerance can reduce response after repeated exposure and may result in a need for a larger dose to produce the same effect (Ernstmeyer & Christman, 2023).
• Adherence, expectations, concurrent substances, diet, smoking, alcohol use, and environmental exposures can modify the observed response or make it difficult to determine whether a drug is effective.

Clinical Application and Monitoring

• Define a measurable therapeutic objective before starting treatment. The outcome may be symptom relief, a physiologic target, prevention of an event, replacement of a deficiency, or disease modification.
• Identify what will show benefit, what will show harm, and when each should be assessed. The monitoring interval should reflect expected onset, peak effect, duration, disease urgency, and the time required for the outcome to change.
• Titrate to the patient’s response rather than to dose alone when the medication permits titration. Use the lowest dose that reliably meets the therapeutic objective with acceptable adverse effects.
• When response is inadequate, assess diagnosis, adherence, technique, timing, interactions, pharmacokinetics, and the achievable efficacy of the medication before increasing the dose.

Common Interpretation Errors

• Using potency as a synonym for efficacy.
• Assuming that receptor selectivity eliminates off-target effects at higher doses.
• Attributing treatment failure to pharmacodynamics before evaluating adherence, absorption, metabolism, and clearance.
• Treating every adverse effect as an allergy or every predictable side effect as a reason to stop therapy.
• Monitoring a laboratory value without defining how the result will change treatment.

Related YourDNP Resources

• Pharmacokinetics
• Pharmacogenomics
• Rational Drug Selection
• Quiz: Foundations of Pharmacology

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References

Ernstmeyer, K., & Christman, E. (Eds.). (2023). Pharmacokinetics and pharmacodynamics. In Nursing pharmacology (2nd ed.). Chippewa Valley Technical College. https://www.ncbi.nlm.nih.gov/books/NBK595006/

Peterson, A. M., & Randazzo, A. M. (2022). Pharmacokinetic basis of therapeutics and pharmacodynamic principles. In V. P. Arcangelo, A. M. Peterson, V. F. Wilbur, & T. M. Kang (Eds.), Pharmacotherapeutics for advanced practice: A practical approach (5th ed.). Wolters Kluwer.

Substance Abuse and Mental Health Services Administration. (2021). Medications for opioid use disorder: Treatment improvement protocol (TIP) series 63 (Publication No. PEP21-02-01-002). U.S. Department of Health and Human Services. https://store.samhsa.gov/sites/default/files/SAMHSA_Digital_Download/PEP21-02-01-002.pdf

Woo, T. M., & Robinson, M. V. (2019). Review of basic principles of pharmacology [PowerPoint slides]. F. A. Davis.