Evidence that the antitumor agent hydroxyurea enters mammalian cells by a diffusion mechanism

Very little information is available about the process responsible for the uptake of the antitumor agent hydroxyurea by mammalian cells. Therefore we have investigated the transport of hydroxy[14C]-urea into Chinese hamster ovary cells. Using a convenient and reproducible 2 min. assay we found that hydroxyurea was taken up in a linear nonsaturable fashion between 0.01 mM and 100 mM drug. The Km for hydroxyurea uptake was essentially zero and the Vmax appeared to be infinite, suggesting a diffusion mechanism. The observation that intracellular drug concentrations were consistently less than medium concentrations indicated that uptake was not an active process. Experiments performed with the metabolic inhibitor sodium azide, and investigations at different assay temperatures also gave results consistent with a mechanism of drug diffusion. In total, the results obtained in this study are in agreement with the proposal that hydroxyurea enters the cell primarily by a process of diffusion. These observations provide a more complete understanding of the mode of action of this widely used drug.