The anthrapyrazoles have entered clinical trials and show significant activity against
breast cancer. However, these drugs are cardiotoxic and ineffective in multidrug-resistant (MDR)
tumor cells. We have reported previously on the synthesis and antitumor characteristics of the 9-aza-anthrapyrazoles and their lack of
cardiotoxicity; unfortunately, the leading candidates are cross-resistant in MDR-expressing cells. The results also indicated that the side arm structures of 9-aza-anthrapyrazole play a critical role in determining the drug resistance in MDR-expressing cells-only compounds that have a tertiary
amine on both side arms are not cross-resistant. To further elucidate the biochemical and pharmacological impact of the side arm structures, one of the 9-aza-anthrapyrazole compounds,
BBR 3422 [2-(2-aminoethyl)-5-(2-methylaminoethyl)indazolo[4,3-g,h]
isoquinoline-6(2H)-one], was selected to be photolabeled with N-hydroxysuccinimidyl-4-azidosalicylic
acid (
NHS-ASA). In comparison to the parental compound, the photolabeled
BBR 3422 was not as cytotoxic or
DNA active, but it competed better than the parental compound against
azidopine on
P-glycoprotein labeling. In addition, confocal microscopic studies showed that
BBR 3422 was clustered mainly in the cell nucleus, but its photolabeled analogue was located in the cytoplasm of the human
breast cancer cell line MCF-7. Only a trace amount of both compounds was detected in the
doxorubicin-derived resistant cell line MCF-7/ADR. The treatment of MCF-7/ADR cells with
verapamil increased the intracellular amounts of both compounds.