Model-based exposure-response analysis to quantify age related differences in the response to scopolamine in healthy subjects

AimSubjects with increasing age are more sensitive to the effects of the anti-muscarinic agent scopolamine, which is used (among other indications) to induce temporary cognitive dysfunction in early phase drug studies with cognition enhancing compounds. The enhanced sensitivity has always been attributed to incipient cholinergic neuronal dysfunction, as a part of the normal aging process. The aim of the study was to correlate age-dependent pharmacodynamic neuro-physiologic effects of scopolamine after correcting for differences in individual exposure. MethodsWe applied a pharmacokinetic and pharmacodynamic modelling approach to describe individual exposure and neurocognitive effects of intravenous scopolamine administration in healthy subjects. ResultsA two-compartment linear kinetics model best described the plasma concentrations of scopolamine. The estimated scopolamine population mean apparent central and peripheral volume of distribution was 2.66 1.050l and 62.10 +/- 10.100l, respectively and the clearance was 1.09 +/- 0.096lmin(-1). Age was not related to a decrease of performance in the tests following scopolamine administration in older subjects. Only the saccadic peak velocity showed a positive correlation between age and sensitivity to scopolamine. Age was, however, correlated at baseline with an estimated slower reaction time while performing the cognitive tests and to higher global and frontal frequency bands measured with the surface EEG. ConclusionsMost of the differences in response to scopolamine administration between young and older subjects could be explained by pharmacokinetic differences (lower clearance) and not to an enhanced sensitivity when corrected for exposure levels.