Flow cytometric analysis and confocal imaging of anticancer alkylaminoanthraquinones and their n-oxides in intact human cells using 647-nm krypton laser excitation

Flow cytometry and laser-scanning confocal fluorescence microscopy have been used in the study of the pharmacodynamics, in single intact cells, of two novel alkylaminoanthraquinones (AQ4 and AQ6), structurally based upon the mid-red excitable but very weakly fluorescent anticancer agent mitoxantrone, together with their respective N-oxide derivatives (AQ4NO and AQ6NO). The drug design rationale was that N-oxide modifications generates prodrug forms suitable for selective bioreductive-activation in hypoxic tumor cells. DNA-binding ranked in the order of mitoxantrone > AQ6 > AQ4 > AQ6NO >> AQ4NO. Using both cytometric methods a similar ranking was found for whole cell and nuclear location in human transformed fibroblasts. However, AQ6 showed enhanced nuclear uptake compared with mitoxantrone, in keeping with its greater capacity to inhibit DNA synthesis. Partial charge neutralisation by N-oxide derivatization resulted in loss of DNA synthesis inhibition but retention of the ability to accumulate in the cytosol, an important property for prodrug development. We conclude that both flow cytometry and confocal imaging revealed biologically significant differences between analogues for subcellular distribution and retention properties. The study demonstrates the potential for these complementary 647-nm krypton laser line-based fluorometric methods to provide relevant structure-activity information in anthraquinone drug-design programmes.