A change in a probe drug’s peak concentration is not automatically evidence that an enzyme was inhibited or induced. A historical 2015 lisdexamfetamine interaction study examined several probes and showed why peak, exposure area and administration route must remain separate.
What was administered?
Thirty healthy adults entered a randomized, open-label, two-period crossover study; 29 completed it. Each participant received a four-drug Cooperstown cocktail alone and with a single oral 70-mg dose of lisdexamfetamine dimesylate, separated by a seven-day washout.
Caffeine 200 mg, dextromethorphan 30 mg and omeprazole 40 mg were given orally. Midazolam 0.025 mg/kg was given intravenously. Blood sampling continued for 72 hours. The probes were selected to examine CYP1A2, CYP2D6, CYP2C19 and CYP3A-related disposition under these conditions.
What happened to omeprazole?
Its geometric least-squares mean peak plasma concentration was 677.9 ng/mL without lisdexamfetamine and 466.9 ng/mL with it. In contrast, area under the concentration–time curve extrapolated to infinity was 1,428 versus 1,499 ng·h/mL. These are geometric model estimates, distinct from the arithmetic means also tabulated in the paper.
A lower peak with a similar exposure area can reflect a change in the shape or timing of the concentration curve. It does not necessarily mean less drug reached the systemic circulation overall. Nor does the peak alone distinguish altered absorption from altered metabolism.
Did every endpoint meet the same comparison criterion?
No. The authors assessed geometric mean ratios and 90% confidence intervals using a 0.80–1.25 bioequivalence range. Omeprazole and dextromethorphan peak endpoints were exceptions to the otherwise generally similar exposure comparisons. Omitting those exceptions would overstate the uniformity of the results.
The authors interpreted the exposure-area findings as showing no alteration of CYP1A2, CYP2D6 or CYP3A activity under the tested conditions. For omeprazole and CYP2C19, they left an enzyme-versus-absorption explanation open. A research summary should preserve that distinction rather than announce that every CYP pathway was definitively unaffected.
Why is the intravenous route important?
The midazolam probe bypassed oral absorption. Its findings cannot be treated as an experiment on every process affecting an orally administered CYP3A substrate. The route is part of the experimental question, not a detail to remove when generalizing the result.
Similarly, this study examined how lisdexamfetamine affected the probes. It was not a direct test of how a strong inhibitor of a metabolic pathway would affect amphetamine exposure. Reversing the direction of an interaction question requires additional evidence.
What does the experiment leave unresolved?
The participants were healthy, doses were given once and the sample was small. Chronic coadministration, different probe doses, disease states and complex prescribing histories were not reproduced. The absence of serious or severe treatment-emergent events in this short experiment cannot establish long-term safety.
For toxicological interpretation, identify the administered prodrug salt, the measured probe and metabolite, route, plasma endpoint and statistical comparison. The study supplies a constrained pharmacokinetic result. It does not establish equivalent clinical effects, rule out every interaction or convert a concentration into an impairment measure.
Source
Ermer J, Corcoran M, Martin P. Lisdexamfetamine dimesylate effects on cytochrome P450 substrate pharmacokinetics in healthy adults. Drugs in R&D. 2015;15(2):175–185. DOI: 10.1007/s40268-015-0090-z. Full article and complete title; bibliographic record.