Tallimustine lesions in cellular DNA are AT sequence-specific but not region-specific

Tallimustine (FCE 24517) is an AT-specific alkylating antitumor derivative of distamycin. This study examined levels of tallimustine lesions in intracellular DNA, their sequence- and region-specificity, and the long- range distribution of the drug binding motif. Tallimustine adducts in DNA converted to strand breaks by heating allowed the quantitation of drug lesions. In bulk DNA of intact human leukemia CEM cells, tallimustine formed 0.15 ± 0.04 and 0.64 ± 0.18 lesions/kbp at 5 and 50 μM, respectively. These lesions represent monoadducts as no interstrand cross-links or DNA- protein cross-links were detected. Tallimustine adducts in intracellularly treated DNA showed a general preference for sequences with T-tracts, suggesting a propensity for intrinsically bent motifs. Major drug-adducted sites identified by repetitive primer extension, included 5'-TTTTGPu-3' and 5'-TTTTGC-3' motif. Despite the high specificity at the nucleotide level, tallimustine did not differentiate among bulk DNA and three discrete AT-rich regions of genomic DNA examined by quantitative PCR stop assay with lesion frequencies ranging from 0.23 to 0.39 lesions/kbp at 25 μM drug. In comparisons of lesion frequencies and cytotoxicity, tallimustine adducts are ~50 times more lethal than relatively nonsequence specific cisplatin adducts but are > 100 times less lethal than lesions by an unrelated AT-specific drug, bizelesin. However, the 5'TTTTGPu-3' motifs targeted by tallimustine are relatively infrequent and scattered throughout the genome. In contrast, the motifs 5'-T(A/T)₄A-3' motifs targeted by bizelesin, while also infrequent, cluster in defined AT-rich islands. The lack of region- specificity may be the reason tallimustine adducts, despite high AT- specificity at the nucleotide level, are less lethal than region-specific bizelesin adducts.