The principal advantage of photodynamic therapy is a selective eradication of neoplastic tissues due to a high dye uptake by the tumor. A considerable excess of photosensitizer (PS) concentration in a tumor in comparison to different types of normal tissue has been demonstrated in vivo and in vitro. There are different suppositions about the mechanism of this excess. We suggest one of the likely mechanisms of a penetration of hydrophobic PS through a cell plasma membrane. Accordingly, the developed mathematical model has yielded a time dependence and steady state values of PS concentration in cell membrane and cytoplasm based on a hypothetical mechanism of PS penetration into the cell and taking into account the values of extra- and intracellular pH. A distribution of different PS ionic species was obtained as a function of external pH. Computer analysis of the model has allowed us to suggest that a selective PS accumulation in a neoplasm in vivo may be determined by the greater lipid fraction inside malignant cells relative to normal ones and low extra- and intracellular pH in cancerous tissues.
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