Endpoint explanations

The figure follows a drug from its dosage form, through absorption, into the systemic circulation, and shows how that exposure is quantified. On the plasma concentration–time curve, the peak concentration (Cmax) and the time at which it occurs (Tmax) index the rate of absorption, while the area under the curve (AUC) measures the extent of exposure — the total amount of active moiety that actually reaches the circulation.

Panel 2 defines absolute bioavailability. Because an intravenous dose enters the bloodstream completely, it is taken as the 100% reference, and any non-intravenous route (oral, subcutaneous, …) is compared against it after correcting for dose — so F is the dose-normalised ratio of the two AUCs. A value below 100% reflects drug that never reaches the circulation intact, through incomplete absorption across the gut wall and/or presystemic (first-pass) loss in the gut and liver.

Panel 3 contrasts this with relative bioavailability, which compares two non-intravenous formulations or routes with each other — for example a new tablet against a reference oral solution — rather than against IV. The same dose-corrected AUC ratio is used, so the result expresses how the test formulation performs against the chosen reference, not against the theoretical maximum.

The figure starts from the definition: AUC is the area enclosed beneath the plasma (or serum/whole-blood) concentration–time curve, written formally as the integral of concentration over time, ∫ C(t) dt. Geometrically it sums every moment of exposure, so it captures the total amount of drug the body is exposed to after a dose — not just the peak.

Panel 2 distinguishes the two forms in which AUC is reported. AUC0–t is measured directly from time zero to the last sample with a quantifiable concentration (tlast), typically by trapezoidal integration of the observed points. Because sampling stops while drug is still present, the remaining tail from tlast out to infinity is estimated from the terminal decline and added on: AUC0–∞ = AUC0–t + extrapolated tail. The dashed segment in the figure is exactly this extrapolated portion.

Panel 3 explains why AUC matters. Reported in units of concentration × time (e.g. ng·h/mL), it is the standard index of total systemic exposure, and together with the peak concentration Cmax it forms the pair of parameters on which bioavailability and bioequivalence assessments rest.

The figure reads Cmax straight off the concentration–time profile: it is the single highest drug concentration reached after dosing, marked at the apex of the curve, with the time at which it occurs being tmax. Because exposure is sampled at discrete time points, Cmax is the largest of those measured concentrations — an observed value, not a number interpolated or fitted between samples.

Panel 2 makes the determination explicit: Cmax = max[C(ti)] over the sampled times ti. The practical consequence, shown by the sampling dots, is that adequate sampling around the expected peak is needed — if the schedule is too sparse near the maximum, the observed Cmax (and tmax) can under-capture the true peak.

Panel 3 explains why it is reported. Cmax indexes peak systemic exposure and, as a hybrid parameter, reflects both the rate and the extent of absorption. Reported in concentration units (e.g. ng/mL), it is paired with AUC (total exposure) and tmax (timing) and, together with AUC, anchors bioavailability and bioequivalence assessment.

The figure locates Tmax on the concentration–time profile: it is the time after dosing at which the highest observed concentration (Cmax) is reached, read straight off the x-axis at the curve's apex. Because concentrations are only measured at discrete sampling times, Tmax is taken as the sampling time ti at which the observed Cmax occurs (Tmax = ti where C(ti) = Cmax) — an observed value, not one interpolated between samples. A schedule that is too sparse near the peak therefore blurs Tmax, which is why adequate sampling around the expected peak is needed.

Tmax serves as a simple index of the rate of absorption: an earlier Tmax generally signals that the drug appears in the systemic circulation faster, while a later Tmax reflects slower absorption (or, for modified-release products, deliberately delayed release).

Tmax is reported in units of time (hours or minutes) and is presented alongside Cmax and AUC. Unlike those two, however, it is generally not a formal bioequivalence acceptance criterion — regulators (e.g. EMA) do not require a statistical evaluation of Tmax, and compare it descriptively only when the rate of onset is clinically important.

Volume of distribution is an apparent pharmacokinetic volume relating the amount of drug in the body to a measured plasma or blood concentration. It is a proportionality term, not an anatomical space, and can exceed total body water when tissue partitioning or binding lowers the circulating concentration.

The reported parameter must be identified. Vc describes the central compartment, Vss the steady-state distribution volume, and Vz the volume associated with the terminal phase. After extravascular dosing, Vz/F is an apparent volume that also depends on the unknown bioavailable fraction F.

A larger value is generally consistent with greater distribution away from the sampled circulation, but comparisons require the same parameter definition, species, route, tested molecular or product form, unit normalization, and pharmacokinetic analysis.

The figure builds protein binding up from the reversible reaction at its core: a drug molecule and a plasma protein associate and dissociate continuously (drug + protein ⇌ protein–drug complex), so at any instant the drug exists as a bound pool and a free pool in dynamic equilibrium. The two complementary ways of quantifying this are shown as formulas — percent bound (100 × bound/total) and the fraction unbound, fu = Cunbound/Ctotal = free/total.

The unbound pool is generally the fraction directly available for passive distribution and target binding, while the bound pool remains in reversible equilibrium and can replenish free drug. This relationship is dynamic rather than a permanent separation into active and inactive drug.

High percent binding corresponds to a low fraction unbound when both are defined on the same basis. The pharmacokinetic consequence cannot be inferred from binding alone because clearance, distribution, transporter activity, target kinetics, species, matrix, concentration, and assay method can all affect unbound exposure.

The left panel fixes the anatomy: the barrier itself is the brain capillary endothelium, whose cells are sealed by tight junctions, so a compound moving from blood/plasma into the brain interstitial fluid must cross those cells rather than slip between them. The central message — echoed in the definition — is that this crossing is not captured by one universal number; different assays quantify different aspects of brain entry and exposure.

The middle panel lays out the reported parameters and what each measures. logBB is the base-10 logarithm of the total brain-to-plasma concentration ratio, log10(Cbrain,total/Cplasma,total), so it reflects total, binding-dependent drug. Kp,uu,brain is the unbound brain-to-plasma partition coefficient (Cu,brain/Cu,plasma). Papp or Pe describes in vitro permeability, %ID/g describes in vivo uptake per gram of tissue, and Kin or PS describes unidirectional influx or permeability-surface area behaviour.

The right panel is the interpretation key: the parameters are placed by whether they are measured in vitro or in vivo and whether they capture total or unbound drug. The essential cautions follow — these metrics are distinct and must not be used interchangeably; total-drug metrics such as logBB do not equal free-drug exposure; Kp,uu,brain best reflects pharmacologically relevant unbound brain exposure; and one should always state the exact parameter, its units, and the assay context, because “BBB penetration” is not a single number.

Cell-monolayer assays such as Caco-2 or MDCK commonly report apparent permeability, Papp, from the transport rate normalized to membrane area and donor concentration. Papp is reported separately for apical-to-basolateral and basolateral-to-apical transport.

PAMPA is a cell-free artificial-membrane assay and commonly reports effective permeability, Pe. It measures passive donor-to-acceptor permeation and does not provide a biological apical/basolateral transporter pair.

An efflux ratio, Papp(B→A)/Papp(A→B), is meaningful only when both directions come from a matched cell-monolayer experiment. Papp and Pe values depend on the barrier model, membrane composition, direction, pH, donor and acceptor conditions, and calculation method, so they should not be pooled as interchangeable measurements.

Serum or plasma stability describes loss of intact peptide during incubation in a defined biological matrix. Results are commonly reported as percent intact at a stated time or as an apparent degradation half-life estimated from measurements over time.

A typical experiment incubates peptide in serum or plasma, removes aliquots at specified times, quenches the reaction, and quantifies the intact analyte by a stated analytical method. The fitted model and treatment of the initial value determine how an apparent half-life is obtained.

The result measures stability under the recorded in vitro conditions. Species, serum versus plasma, anticoagulant, matrix dilution, temperature, peptide concentration, sampling schedule, quench procedure, and analytical method can change the value. It can inform metabolic-liability assessment but does not by itself predict in vivo clearance or pharmacokinetic half-life.

The figure defines protease stability as resistance to cleavage by one or more defined, purified proteases (e.g. trypsin, chymotrypsin, pepsin, pronase, carboxy-/aminopeptidases) under controlled buffer, pH, temperature, and enzyme-concentration conditions. The intact peptide is converted by those proteases into cleaved fragments, and what is measured is the percentage of intact peptide surviving over time and/or an apparent degradation half-life (t1/2).

The middle panel shows the workflow: the peptide is combined with the protease solution, incubated under defined conditions (commonly ~37 °C), aliquots are quenched at several time points to stop the reaction, and the remaining intact peptide is quantified by LC–MS or RP–HPLC. Plotting percent intact against time gives a decay curve whose 50% crossing is the apparent half-life.

Unlike whole serum or plasma, a purified-protease assay tests susceptibility to a specified enzyme preparation. Cleavage is protease-specific, and the result can vary with enzyme source and activity, enzyme-to-substrate ratio, buffer and pH, temperature, concentrations, sampling time, quench, analytical method, and kinetic model. Values should be compared only when these conditions and the reported result form are aligned.

Clearance is a proportionality parameter relating elimination rate to concentration and is expressed as volume per time. The label may refer to total systemic, renal, hepatic, biliary, metabolic, or another source-defined clearance, so the clearance type must accompany the value.

For linear pharmacokinetics after intravenous dosing, total clearance can be estimated as DoseIV/AUC. After oral or other extravascular dosing, Dose/AUC gives apparent clearance, CL/F, which also depends on bioavailability. Organ-specific and in vitro clearance measures have different definitions and cannot be substituted for systemic CL.

Under linear steady-state conditions, average concentration is inversely related to systemic clearance for a fixed dosing rate. Cross-study comparisons require the same clearance definition, species, route, analyte or molecular form, dose range, units, body-weight or other normalization, and pharmacokinetic method.

Half-life is the time associated with a 50% decrease in a stated amount, concentration, or intact-analyte signal. Equal successive halving times imply a first-order log-linear process and should not be assumed when the fitted model or phase is unspecified.

An in vitro degradation half-life is estimated in a defined matrix or protease assay. An in vivo pharmacokinetic half-life refers to a specified disposition phase. For a linear terminal phase, t1/2,z = ln(2)/λz and can be written as 0.693 × Vz/CL when Vz and CL are defined consistently.

ADMETatlas contains in vivo, in vitro, and records whose setting is not specified. These categories are not interchangeable. Interpretation therefore requires the recorded setting, phase or endpoint definition, species, matrix, route, dose, sampling design, and estimator rather than an assumption based on the endpoint name alone.

This endpoint preserves source-reported concentration metrics used to describe a cytotoxic, inhibitory, effective, or lethal response. Labels include IC50, CC50, LC50, EC50, and LD50, and the source definition determines the biological effect represented by each value.

Some values are estimated from concentration-response data, while others are reported as thresholds or summary values without a recoverable fitting procedure. IC50, CC50, LC50, EC50, and LD50 must not be relabelled as one another. Interpretation requires the test system, source-defined effect, assay readout, exposure conditions when available, units, and calculation method.

This endpoint records a percentage observation at a stated test concentration. The source may report viability, cytotoxicity, inhibition, cell death, or another assay-defined percentage, and the readout name must be retained.

Viability and cytotoxicity are exact complements only when the source defines them on the same population, control, and normalization scale. Different assays can measure metabolic activity, membrane integrity, proliferation, or death and need not yield 100% minus one another. A single observation cannot be converted into an IC50 or CC50.

This endpoint preserves source-reported concentration metrics for haemolytic effects, including HC50, LC50, IC50, EC50, MHC, and LD50. HC50 is commonly estimated from a concentration-response series as the concentration associated with 50% haemolysis.

MHC is a source-defined minimum concentration meeting a stated haemolysis criterion and is not inherently a curve-fitted 50% value. Other metric labels likewise retain their source definitions. Comparisons require the same erythrocyte source, effect definition, assay and normalization conditions, exposure conditions when reported, units, and calculation method.

This endpoint is the percent-observation form of hemolysis — the bottom-right quadrant of the figure. The peptide is tested at a single stated concentration: RBCs are incubated with it, the sample is centrifuged, and the hemoglobin released into the supernatant is read by absorbance (≈540 nm).

Percent haemolysis is often normalized to negative and full-lysis controls, but the controls, wavelength, background correction, denominator, and calculation formula are source-specific. A percentage observation is interpretable only with the tested concentration, erythrocyte source, and assay definition, and cannot be converted to HC50 or MHC from one point.

The figure frames lipophilicity as how a compound distributes between two immiscible phases — classically n-octanol (organic) and water (aqueous) — with higher values meaning greater hydrophobicity (a stronger preference for octanol). Two related but distinct coefficients are reported, and the figure separates them deliberately.

logP (Panel 2) is the IUPAC partition coefficient: the log10 ratio of a single neutral species between octanol and water, logP = log10([neutral]octanol / [neutral]water). Because it is defined for the neutral form only, it is pH-independent and reflects the compound's intrinsic lipophilicity.

logD (Panel 3) is the distribution coefficient: the log10 ratio of all species — neutral plus ionized — at a stated pH, logD(pH) = log10([all species]octanol / [all species]water). Since ionized forms partition preferentially into water, logD is pH-dependent and the measurement pH must always be reported (e.g. logD7.4). For non-ionizable compounds logD ≈ logP, but for ionizable compounds the two can differ substantially — and partition systems (octanol/water, alkane/water, RP-HPLC retention) measure distinct phenomena and are never merged.