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Acquired resistance to anthracyclines is characterised by a lower sensitivity to these agents, associated with impaired accumulation of drug. We have examined the ability of aclacinomycin A (ACM) associated with doxorubicin (DOX), to increase intranuclear DOX concentrations and, consequently, to enhance cytotoxic effects against drug resistant cells in vitro. A recently developed microspectrofluorometric technique is used to measure intranuclear DOX concentrations in sensitive and DOX-resistant K562 cells treated with DOX and ACM. Fluorescence emission spectra are collected from a microvolume of single living cell nuclei. From both DOX and ACM model fluorescence spectra (free, DNA-bound and metabolites), the intranuclear spectral profile is analysed according to the amount of each component. This quantitative analysis determines intranuclear DOX concentrations with an error of 10%. Non-cytotoxic doses of ACM, in combination with DOX, increase cytotoxic activity of DOX against K562 resistant cells. When DOX-resistant cells are exposed simultaneously to ACM and DOX, significant increases in intranuclear DOX concentrations are found compared with the case of exposure to DOX alone. The measure of the intranuclear retention of DOX shows that ACM partly blocks the DOX efflux in resistant cell nuclei, resulting in enhanced accumulation of DOX. These data lead us to conclude that ACM-DOX association partly reverses the DOX resistance at clinically achievable concentrations.

Intranuclear drug concentration in cells treated with doxorubicin (DXR) or with 4'-deoxy-4'-iododoxorubicin (IDX) was measured by means of a quantitative microspectrofluorometric technique recently developed by us. Resolution of free and bound drug contributions in fluorescence emission spectra, as collected from a microvolume of single living cell nuclei, provided concentration data with about 10% indetermination. Uptake of DXR and IDX into the nucleus of K562 cells and DXR-resistant K562/DXR cells could then be studied with a sensitive, nondestructive technique. Growth inhibitory concentrations of K562 and K562/DXR cells, when measured with respect to drug content in the medium, differed by a factor of 25 in the case of DXR and by a factor of three in the case of IDX. By contrast, intranuclear drug concentrations measured at corresponding growth inhibitory concentrations are found to be nearly constant, i.e., independent of cellular-resistant phenotype and of anthracycline structure. This result supports an identical mechanism of action for the two drugs, most probably targeted to the nucleus, and ascribes to intracellular transport the different potency of the two drugs in the two cell lines.

Doxorubicin-DNA association has been studied by quantitative microspectrofluorometry. Fluorescence emission spectra from a microvolume of single living cell nuclei treated with doxorubicin have been analyzed in terms of difference in spectral shape and fluorescence yield between free and DNA-bound drug. Contribution of each spectral component to the total signal was calculated by least-squares linear regression. With this method of analysis, total drug concentration has been determined with an error of less than 10%. Moreover, the uptake into the nucleus has been studied in a non destructive way, avoiding use of 14C-labelled drug. Kinetic studies of drug accumulation into the nuclei were conducted on sensitive and resistant cells.