Direct relation of DNA lesions in multidrug-resistant human myeloma cells to intracellular doxorubicin concentration.

Using a human myeloma cell line selected for resistance to doxorubicin (8226/DOX), which expresses the multidrug resistance phenotype, we studied the effects of drug accumulation on DNA damage and cytotoxicity in multidrug-resistant cells. The resistant 8226 subline showed a decrease in [14C]doxorubicin accumulation as compared to the sensitive (8226/S) subline at all time points investigated. DNA alkaline elution techniques were used to examine the formation of single and double strand breaks and DNA-protein cross-links following exposure to doxorubicin in both sensitive and resistant sublines. When 8226/S and 8226/DOX40 cells were exposed to the same extracellular concentration of doxorubicin there was a difference in the quantity of DNA lesions occurring, with the 8226/DOX40 line exhibiting less DNA damage. However, when the extracellular concentration of drug was adjusted to yield equivalent intracellular concentrations these differences were removed and both lines exhibited the same degree of damage for all three DNA lesions. The same DNA lesions were also studied in isolated nuclei from sensitive and resistant cells. Such a model removes any confounding factors due to drug accumulation such as altered cytosolic distribution and/or metabolism of drug. We observed no difference in the formation of single or double strand breaks, or DNA-protein cross-links when the nuclei were exposed to the same concentration of doxorubicin. Results from colony-forming assays have shown that when resistant and sensitive 8226 cells were exposed to high concentrations of doxorubicin, there was a good correlation between DNA damage, drug accumulation, and cytotoxicity. This relationship did not hold for lower concentrations of doxorubicin, for which there was a lack of correlation between drug accumulation and cytotoxicity. Such findings may possibly be due to a prolonged retention of drug by the sensitive cell line as compared to the resistant cells. Further studies are under way to examine this possibility.