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Vol. 27, Issue 10, 1220-1223, October 1999
Division of Biopharmaceutics and Pharmacokinetics, Paclitaxel pharmacokinetics are nonlinear with saturable metabolism
and saturable distribution to the tissues. The saturable distribution
has in previous pharmacokinetic modeling been described as a saturable
transport process, whereas the present study was undertaken to
investigate alternative explanations. Using a sparse sampling scheme
(on average 3.3 samples per profile), 101 plasma concentration-time
profiles in 22 female patients with metastatic cancer of the breast or
ovary were monitored. It was found that the observed data could be
equally well described by saturable tissue binding as well as by
capacity-limited tissue transport. The data were better described by a
model where equilibrium was achieved with drug in the central rather
than in the peripheral compartment. Models where the binding was
assumed to be an instantaneous or a noninstantaneous process were
tried, but the data did not allow resolution between these two
possibilities. The value at which the saturable transport was
half-maximal was 0.55 µM. The Kd values of
the binding models were 0.06 to 0.12 µM. These are close to the
values reported as a threshold for drug toxicity of paclitaxel,
suggesting a possible connection between the binding sites involved in
the pharmacokinetics and the mechanism responsible for the
toxicity. For all models, a saturable elimination of paclitaxel was included using the Michaelis-Menten model.
Km for the elimination ranged in the
different models from 2.5 to 5.6 µM.
Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics
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