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Vol. 289, Issue 2, 1067-1074, May 1999
-OH-Midazolam
Leiden/Amsterdam Center for Drug Research, Division of
Pharmacology, Leiden, The Netherlands (B.T., M.F.O., V.M.M.H., M.D.);
and
Stanford University School of Medicine, Department of
Anesthesia, Stanford, California (J.W.M.)
The pharmacodynamic interaction between midazolam and its active
metabolite
-OH-midazolam was investigated to evaluate whether estimates of relevant pharmacodynamic parameters are possible after
administration of a mixture of the two. Rats were administered 10 mg/kg
of midazolam, 15 mg/kg of
-OH-midazolam, or a combination of 3.6 mg/kg of midazolam and 35 mg/kg of
-OH-midazolam. Increase in the
11.5- to 30-Hz frequency band of the electroencephalogram was
used as the pharmacodynamic endpoint. The pharmacodynamics of midazolam
and
-OH-midazolam after combined administration were first analyzed
according to an empirical and a competitive interaction model to
evaluate each model's capability in retrieving the pharmacodynamic
estimates of both compounds. Both models failed to accurately estimate
the true pharmacodynamic estimates of midazolam and
-OH-midazolam.
The pharmacodynamic interaction was subsequently analyzed according to
a new mechanism-based model. This approach is based on classical
receptor theory and allows estimation of the in vivo estimated receptor
affinity and intrinsic in vivo drug efficacy. The relationship
between stimulus and effect is characterized by a monotonically
increasing function f, which is assumed to be identical
for midazolam and
-OH-midazolam. The pharmacodynamic interaction is
characterized by the classical equation for the competition between two
substrates for a common receptor site. This mechanism-based interaction
model was able to estimate the pharmacodynamic parameters of both
midazolam and
-OH-midazolam with high accuracy. It is concluded that
pharmacodynamic parameters of single drugs can be estimated after a
combined administration when a mechanistically valid interaction model
is applied.
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