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Abstract
1. The pharmacokinetics of loprazolam and its principal pharmacologically active metabolite, the piperazine N-oxide, were compared in young subjects (aged 21–25 years) and elderly patients (aged 63–86 years) following single oral evening doses (0.5 mg and 1 mg).
2. Plasma loprazolam was assayed by a specific h.p.l.c./g.c. method. The N-oxide metabolite was assayed by gas chromography.
3. Mean times to peak plasma concentration of loprazolam did not differ significantly between young and elderly subjects and ranged from 1.6–2.7 h. There was, however, a longer mean time to peak concentration of the N-oxide metabolite in the elderly but this was only statistically different after the 0.5 mg dose (4.5 mg young, 6.4 h elderly).
4. Mean peak plasma concentrations of loprazolam did not differ significantly between young and elderly nor did plasma concentrations of the N-oxide metabolite.
5. Although the mean elimination half-life of loprazolam was not statistically signiticantly different between young and elderly subjects (range 10.9–16.0 h) there was a trend towards somewhat longer half-lives in the elderly. Furthermore, there was a small but significant increase in the half-life of the N-oxide metabolite in the elderly after the 1 mg dose from 11.7 h to 16.7 h.
6. The areas under the plasma concentration time curves for both loprazolam and its N-oxide were greater in the elderly being some 50–68% (mean 132.0 and 111.5 ng/ml h) above those found in young subjects (mean 89.8 and 66.0 ng/ml h).
7. Despite these changes the maximum accumulation ratio predicted from these single dose studies would be 1.16 in the young and 1.40 h in the elderly, assuming once nightly dosing.