Lesson
From Dose to Exposure to Response
Pharmacokinetics turns a dose and a clock into a concentration-time profile. Pharmacodynamics asks what that exposure does. Keeping those questions separate makes every later calculation and treatment decision clearer.
- PK versus PD
- ADME
- Route
- Formulation
- Exposure
- Response
Route, formulation, dissolution, permeability, and first pass.
Perfusion, barriers, transporters, ionization, and binding.
Enzymes, transporters, kidney, liver, and other routes.
Separate exposure from effect
Pharmacokinetics describes absorption, distribution, metabolism, and excretion over time. Pharmacodynamics connects concentration or exposure with receptor activity, biomarkers, benefit, and toxicity. Dose is an input, concentration is an exposure measure, and response is an effect.
Trace the complete path
A dose may be released from a formulation, dissolve, cross membranes, undergo intestinal or hepatic first-pass loss, distribute into tissues, bind proteins, form metabolites, and leave through kidney, bile, lung, or other routes. One patient can have changes in several steps at once.
Avoid universal formulation rankings
Intravenous administration places drug directly into the circulation, but the relative onset of sublingual, orally disintegrating, immediate-release, delayed-release, and extended-release products is product specific. An orally disintegrating tablet may be swallowed after disintegration and may not produce faster systemic absorption.
Think in concentration-time curves
A single concentration is one point on a curve. The curve's shape reflects the rate of input, distribution, and elimination. Interpretation requires the dose, route, formulation, clock time, sample time, and clinical response.
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Lesson
Measure Systemic Input
Absorption is the movement of drug from the administration site toward systemic circulation. Bioavailability describes the rate and extent of systemic availability, integrating formulation performance, permeability, transport, and presystemic loss.
- Dissolution
- Permeability
- First pass
- AUC
- Cmax and Tmax
- Absolute and relative F
Systemic exposure over the sampled concentration-time curve.
Correct the comparison when doses are unequal.
Multiply the AUC ratio by the inverse dose ratio.
Release before crossing
A solid oral dose generally disintegrates and dissolves before drug can permeate. Particle size, crystal form, excipients, coating, food, gastric emptying, intestinal surface area, solubility, permeability, and transporters can all affect input. Rapid disintegration does not guarantee complete absorption.
Account for first-pass loss
Drug absorbed from much of the gastrointestinal tract enters portal blood and may be metabolized in the intestinal wall or liver before reaching systemic circulation. Low oral bioavailability can therefore reflect poor absorption, presystemic extraction, or both.
Read exposure measures together
AUC summarizes systemic exposure across a specified time interval. Cmax is the observed peak concentration, and Tmax is the time of that peak. Similar AUC does not require similar Cmax, Tmax, onset, or fluctuation.
Normalize the comparison
For linear kinetics, absolute bioavailability is calculated as F equals AUC from the extravascular route divided by IV AUC, multiplied by the IV dose divided by the extravascular dose. Relative bioavailability uses another nonintravenous product as the reference. Formula: F = (AUCev / AUCiv) x (Doseiv / Doseev).
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Lesson
Interpret Distribution and Binding
Distribution connects the amount in the body with the concentration measured in plasma. Perfusion, barriers, transporters, ionization, tissue affinity, and protein binding determine where drug can go and how total concentration relates to active unbound exposure.
- Perfusion
- Permeability
- Ionization
- Protein binding
- Unbound concentration
- Apparent Vd
Albumin and other proteins alter the measured fractions.
Perfusion, permeability, ionization, and transport shape movement.
A proportional volume, not an anatomic container.
Cross the relevant barrier
Small, lipophilic, unionized molecules may cross membranes readily, but molecular size, blood flow, capillary structure, the blood-brain barrier, inflammation, and uptake or efflux transporters can dominate tissue access. Distribution is organ and drug specific.
Separate total from unbound
Total plasma concentration includes protein-bound and unbound drug. Unbound drug can more readily distribute, bind many targets, undergo metabolism, and be filtered. A lower albumin concentration can lower total concentration while increasing the unbound fraction.
Use correction estimates cautiously
Albumin correction equations can provide a rough estimate for selected drugs, but their accuracy varies by patient and clinical setting. When altered binding or clinical discordance matters, a directly measured unbound phenytoin concentration is preferable to automatic dose escalation from a corrected total value.
Treat Vd as a proportional volume
Apparent volume of distribution equals amount of drug in the body divided by plasma concentration. Vd can exceed anatomic body volume because it represents partitioning, not a container. Formula: Vd = amount in body / concentration.
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Lesson
Map Enzymes, Metabolites, and Transporters
Biotransformation can end activity, preserve activity, activate a prodrug, or create toxicity. Drug transporters can govern entry, tissue exposure, and elimination alongside enzymes.
- Phase I
- Phase II
- CYP
- UGT
- Active metabolites
- Transporters
Oxidation, reduction, or hydrolysis can activate or inactivate.
Conjugation can occur directly and does not guarantee inactivity.
Uptake and efflux alter absorption, tissue access, and clearance.
Do not force a two-step sequence
Phase I commonly includes oxidation, reduction, and hydrolysis. Phase II commonly includes glucuronidation, sulfation, acetylation, methylation, and other conjugations. A drug can undergo either phase first, skip one phase, or form active or toxic products.
Follow parent and metabolite
The parent concentration may decline while an active metabolite sustains efficacy or toxicity. Organ dysfunction, enzyme inhibition, or transporter changes can alter the parent and metabolite in different directions.
Distinguish inhibition from induction
Inhibition can reduce pathway activity soon after sufficient inhibitor exposure. Induction generally develops as enzyme or transporter expression increases and may persist after the inducer is stopped. The magnitude depends on how much of the victim drug relies on that pathway.
Include transporters
P-glycoprotein and other uptake or efflux systems influence absorption, barrier penetration, hepatic uptake, tubular secretion, and biliary elimination. The globally harmonized ICH M12 framework now guides contemporary CYP, UGT, and transporter interaction assessment.
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Lesson
Convert Concentration Into Elimination
Clearance describes the efficiency of irreversible drug removal. It connects a measured concentration with the amount eliminated per unit time and can combine renal, hepatic, and other pathways.
- Elimination rate
- Total clearance
- Renal clearance
- Hepatic clearance
- AUC
- Urine pH boundary
Function, binding, flow, pH, and transporters matter.
Blood flow, unbound fraction, enzymes, and transporters interact.
Independent organ clearances can contribute to the whole.
Keep the units visible
Clearance equals elimination rate divided by concentration and is expressed as volume per unit time. For linear IV exposure, clearance can also be estimated as dose divided by AUC. For an extravascular dose, Cl = F x dose / AUC when assumptions are satisfied.
Resolve renal pathways
Kidney elimination can involve glomerular filtration, active secretion, and tubular reabsorption. Protein binding, kidney function, transporters, urine flow, and molecular ionization influence the result. Use the kidney function method specified in current evidence or labeling.
Resolve hepatic pathways
Hepatic clearance can depend on blood flow, unbound fraction, and intrinsic enzyme or transporter capacity. High-extraction and low-extraction drugs respond differently to changes in perfusion, binding, or metabolic capacity.
Set a safe urine pH boundary
Urinary alkalinization can enhance elimination in selected poisonings, including significant salicylate toxicity, but it requires protocolized bicarbonate therapy and close monitoring. Routine urinary acidification is not recommended because metabolic acidosis, rhabdomyolysis, and other harms can occur.
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Lesson
Recognize Linear and Saturable Elimination
Most therapeutic concentration-time profiles can be approximated with first-order elimination over a relevant range, but capacity-limited systems can become nonlinear. When saturation matters, dose and concentration no longer remain proportional.
- First order
- Zero order
- Michaelis-Menten
- Vmax
- Km
- Phenytoin
A constant fraction is removed during each equal interval.
Elimination approaches a ceiling as concentration rises.
Small dose changes can produce large concentration changes.
Identify first-order behavior
A constant fraction of the current amount is removed during each equal time interval. Concentration declines exponentially, clearance is approximately constant, and a consistent half-life can describe the observed range.
Identify capacity limitation
When an elimination pathway is saturated, an approximately constant amount may be removed per unit time. Apparent clearance falls as concentration rises, and the concept of a single fixed half-life becomes unreliable.
Read the Michaelis-Menten curve
Elimination rate = Vmax x concentration divided by Km plus concentration. At concentrations well below Km, behavior is closer to first order. As concentration rises toward and above Km, the rate approaches Vmax.
Respect nonlinear phenytoin
Phenytoin metabolism is saturable. Small incremental dose changes can produce substantial concentration increases, especially at higher exposure. Confirm formulation, adherence, sampling, symptoms, interactions, and unbound exposure when relevant.
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Lesson
Calculate the Elimination Slope
The elimination rate constant connects clearance and volume with the log-linear decline of concentration. Correct sampling and model assumptions matter as much as the arithmetic.
- ke
- Cl divided by Vd
- Exponential decay
- Natural logarithm
- Two-level estimate
- Sampling interval
The elimination rate constant has units of inverse time.
Use post-distribution levels under stable linear conditions.
Actual sample times define the log-linear slope.
Derive ke from disposition
For a one-compartment first-order model, ke = Cl / Vd. A clearance of 5 L/h and Vd of 50 L gives ke = 0.10 h^-1. The inverse-time unit represents the fraction eliminated per unit time.
Predict a later concentration
For stable first-order elimination, C2 = C1 x e^(-ke x t). With C1 10 mg/L, ke 0.22 h^-1, and t 8 hours, C2 is about 1.72 mg/L.
Estimate ke from two levels
If two post-distribution concentrations are in the same elimination phase, ke = ln(C1 / C2) / elapsed time. Use actual collection times and confirm that no dose or major physiologic change occurred between them.
Reject invalid samples
A sample collected during infusion or early distribution should not be treated as an elimination-phase peak. Scheduled time labels do not repair an incorrectly timed specimen. Document the timing error and obtain or model a valid sample.
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Lesson
Connect Half-Life, Washout, and Steady State
Half-life summarizes the time required for concentration or amount to fall by half under a defined model. The same time constant governs exponential washout and the approach toward steady state during repeated input.
- Half-life
- 0.693
- Washout
- Accumulation
- Steady state
- Changing physiology
Also 0.693 times Vd divided by clearance.
Linear exposure approaches steady state over successive half-lives.
About 3 percent remains after five linear half-lives.
Calculate half-life
For first-order one-compartment behavior, t1/2 = 0.693 / ke = 0.693 x Vd / Cl. Greater Vd lengthens half-life when clearance is unchanged. Greater clearance shortens it when Vd is unchanged.
Track washout
After one half-life, 50 percent remains; after two, 25 percent; after three, 12.5 percent; after four, 6.25 percent; and after five, about 3 percent. Clinical effect may last longer or shorter than plasma washout.
Track accumulation
With repeated input and linear stable kinetics, the fraction of steady state reached is approximately 50, 75, 87.5, 93.8, and 96.9 percent after one through five half-lives. Dose size changes the concentration target, not the time constant.
Know when the shortcut fails
A loading dose can move concentration near a target immediately but does not shorten half-life. Nonlinear kinetics, multicompartment distribution, active metabolites, and changing clearance require more detailed interpretation.
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Lesson
Design Loading and Maintenance Inputs
A loading dose and a maintenance regimen solve different problems. Loading supplies an amount needed to approach a target concentration. Maintenance replaces the amount eliminated over time.
- Target concentration
- Loading dose
- Maintenance rate
- Bioavailability
- Dose interval
- Peak-trough fluctuation
Approach a target concentration without changing half-life.
Replace average elimination over time.
Control average exposure and peak-trough fluctuation.
Load from volume
Loading dose = target concentration x Vd / F. An oral drug with target 1.5 mg/L, Vd 500 L, and F 0.6 has a calculated loading dose of 1,250 mg before product-specific rounding and safety limits.
Maintain from clearance
Average maintenance dosing rate = target steady-state concentration x clearance / F. Clearance, not Vd, determines how quickly the body must be resupplied under linear conditions.
Use interval to shape fluctuation
A single average dosing rate can be delivered as different doses and intervals. Shorter intervals or prolonged input generally reduce peak-trough fluctuation. Longer intervals generally increase fluctuation and may improve convenience.
Do not confuse speed with steady state
A loading dose can rapidly approach a concentration target when delay is undesirable. It does not change elimination, and an excessive loading dose cannot be repaired by the idea that maintenance will eventually stabilize.
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Lesson
Turn a Drug Level Into a Decision
Therapeutic drug monitoring is a clinical reasoning process, not a lookup table. A measured concentration becomes useful when it answers a defined question and is interpreted with the curve, the patient, and the treatment goal.
- TDM indication
- Peak and trough
- Post-distribution
- Steady state
- Therapeutic range
- Dose decision
Efficacy, toxicity, adherence, interaction, or changing clearance.
Dose history, draw time, distribution, and steady state.
Integrate concentration with response, safety, and patient goals.
Choose drugs and questions carefully
TDM is most useful when exposure varies substantially, the therapeutic margin is narrow, concentration relates to efficacy or toxicity, and the result can change care. Useful questions include toxicity, nonresponse, adherence, interactions, organ change, or individualized target attainment.
Place the sample on the curve
A trough is drawn immediately before a scheduled dose. A peak or post-distribution sample is drug and infusion specific. Actual administration and collection times must be recorded. A mislabeled or mistimed sample should not be forced into a target range.
Interpret steady state honestly
A pre-steady-state sample can still answer some questions, especially toxicity or model-based dosing, but it should not be interpreted as a steady-state trough. Dose changes, missed doses, dialysis, acute organ changes, and new interactions can invalidate the prior curve.
Treat the patient, not the interval
A published therapeutic range is a population reference. Some controlled patients need no change outside it, while others have toxicity within it. Integrate response, toxicity, indication, unbound exposure where relevant, and the risk of changing therapy.
Quick check
Module test
Check the connections.
Each attempt draws 10 questions from the complete 140 question bank.
Each attempt draws a fresh set and rearranges the answer choices.
References
Current clinical foundation.
Lecture material was synthesized with the following contemporary guidance. Verify local policy and current guidance before applying clinical information.
- FDA. Bioavailability Studies Submitted in NDAs or INDs: General Considerations. April 2022.
- FDA. Population Pharmacokinetics. February 2022.
- FDA and ICH. M12 Drug Interaction Studies. August 2024.
- FDA. Drug Development and Drug Interactions: Substrates, Inhibitors, and Inducers.
- DailyMed. Phenytoin sodium extended-release capsule prescribing information.
- AACT and EAPCCT. Position Paper on Urine Alkalinization. Clinical Toxicology. 2004.