Photosensitizer accumulation in spontaneous multidrug resistant cells: a comparative study with Rhodamine 123, Rose Bengal acetate and Photofrin.

The influence both of overexpression of multidrug transporter proteins and of phenotype changes occurring in cells developing spontaneous resistance on the accumulation of photosensitizer molecules was studied on two tumor-derived cell lines (B16, A2780) expressing the MDR-1 phenotype. Rhodamine 123, Rose Bengal acetate (a fluorogenic substrate that is restored to the native active molecule by specific enzyme activity inside cells) and Photofrin were considered. The two resistant variants accumulate Rhodamine 123 to a lesser extent than the respective wild types. Treatment with verapamil markedly enhances Rhodamine 123 accumulation in resistant cells, blocking the drug's extrusion. The amount of Rose Bengal is larger in resistant cells than in wild type cells. Verapamil does not affect drug accumulation, although it significantly impairs the efflux process. The results are explained by the enhancement of both membrane traffic and esterase activity resulting in intracellular Rose Bengal production that counterbalances the increased ability in the outward transport of resistant cells. Photofrin is accumulated to a lower degree in resistant than in wild type cells. Verapamil does not alter the drug accumulation, although the release process is somewhat affected. Different intracellular turnovers of Photofrin take place in the cell variants, and the release of the monomeric fluorescent fractions is greater in resistant than in wild type cells.