Intracellular distribution and intracellular dynamics of a spin-labeled analogue of doxorubicin fluorescence and EPR spectroscopy.

Fluorescence spectroscopy revealed a dramatic difference between the intracellular distributions of doxorubicin (DOX) and its paramagnetic analogue, ruboxyl (Rb). Modification of the anthracyclin structure by introduction of a paramagnetic label at position 14 in the DOX molecule resulted in reduced DNA binding and enhanced partitioning into phospholipid membranes, as evidenced by the fluorescence-quenching experiments. Higher partitioning into cell membranes resulted in stronger intracellular fluorescence of Rb. In addition, a paramagnetic label on the Rb molecule provided a unique opportunity for an EPR investigation of the drug's intracellular distribution and dynamics. The observed changes in the EPR spectra of drug-containing cells during their life cycle suggested either a progressive release of the intercalated drug, cell membrane fluidization, or both.