A plasma concentration alone does not reveal what fraction of an oral dose reached systemic circulation. Estimating absolute bioavailability requires a reference exposure and attention to the amount administered by each route.
A historical evacetrapib study by Cannady and colleagues illustrates the method. The investigators combined an oral dose with a much smaller intravenous dose labeled using a stable carbon isotope, allowing the laboratory to distinguish the two sources of drug in plasma.
Two routes studied at the same time
Eight healthy adults, seven men and one woman, completed the open-label study. Each received a 130 mg oral evacetrapib tablet while an intravenous infusion delivered approximately 175 micrograms of carbon-13-labeled evacetrapib over four hours. Samples were collected through 168 hours.
The labeled tracer was measured separately from unlabeled evacetrapib using high-performance liquid chromatography with tandem mass spectrometry. The carbon-13 label was stable rather than radioactive.
Simultaneous administration reduced the problem of comparing clearance on different study days. The interpretation still depended on the labeled and unlabeled drug having comparable disposition under the study conditions.
Why raw concentrations would give the wrong comparison
The oral and intravenous doses differed greatly. Their unadjusted concentrations or exposure areas therefore could not be compared as though equal amounts had been administered.
The researchers divided each area under the concentration-time curve by the corresponding dose. They then compared dose-normalized oral exposure with dose-normalized intravenous exposure. The intravenous route supplied the reference for drug entering systemic circulation.
Using exposure extrapolated to infinity, the estimated absolute bioavailability was 44.8%, with a 90% confidence interval of 42.2% to 47.6%. The estimate based on exposure through the last measurable concentration was similar, at 44.3%.
Bioavailability is not the same as absorption alone
The result describes systemic availability under the tested conditions. It does not, by itself, separate incomplete gastrointestinal absorption from loss before the drug reaches systemic circulation. Additional evidence is needed to distinguish those contributions.
It also does not mean that an oral dose was 44.8% effective, or that the result is a therapeutic target. Bioavailability describes drug disposition, not the magnitude of clinical benefit.
The delivered dose required verification
Preliminary work showed that evacetrapib could bind to infusion equipment. The researchers conditioned the infusion system and measured solution exiting the line before and after administration to determine the delivered tracer dose. Nominal preparation concentration alone would have been insufficient.
This detail matters because the intravenous dose appears in the denominator of the exposure comparison. A sophisticated analytical assay cannot compensate for an incorrectly characterized delivered dose.
Limits of the result
This was a small, single-dose study of a particular formulation in healthy adults. It does not establish bioavailability for other formulations, patients with organ impairment or an unknown individual’s dosing circumstances. Its value here is methodological: a valid bioavailability estimate requires the route, delivered dose, exposure interval and comparison assumptions to remain explicit.
Source: Cannady EA and colleagues. Absolute bioavailability of evacetrapib in healthy subjects determined by simultaneous administration of oral evacetrapib and intravenous carbon-13-labeled evacetrapib as a tracer. Journal of Labelled Compounds and Radiopharmaceuticals. 2016;59:238-244. PubMed record.
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