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Extensive and saturable accumulation of paclitaxel by the human platelet.

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Department of Cell and Molecular Pharmacology and Experimental Therapeutics, Medical University of South Carolina, Charleston 29425, USA.


Little is known about the cellular distribution of paclitaxel in humans. In the present study we examined the distribution of [3H]-paclitaxel in human blood. When 1 microM paclitaxel was incubated with fresh blood at 37 degrees C, the platelet/plasma concentration ratio was 240 +/- 17 (mean +/- SEM), whereas the red blood cell (RBC)/plasma concentration ratio was only 0.59 +/- 0.05. In kinetics experiments using platelet-rich plasma, we observed that the platelet accumulation of paclitaxel was highly temperature- and concentration-dependent. Scatchard analysis of the 37 degrees C uptake data demonstrated a dissociation constant (Kapp) of 0.80 +/- 0.10 microM and a maximal binding capacity of 672 +/- 102 pmol/10(9) platelets. It is proposed that the platelet accumulation of paclitaxel reflects binding to microtubules and may serve as a useful model for binding to less accessible cellular sites.

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