Intercalation binding of 4-butylaminopyrimido [4 ',5 ': 4,5]selenolo (2,3-b)quinoline to DNA: Relationship with in vitro cytotoxicity

The interaction between double-stranded (ds) DNA and the 4-butylamino pyrimido[4',5':4,5]selenolo(2,3-b)quinoline (BPSQ) has been studied with spectral luminescence methods. Binding constant values determined by absorption and fluorescence titration indicated a binding constant of 4.24 x 10(4) and 5.9 x 10(4) M-1, respectively. The interaction was markedly suppressed by increasing the salt concentration. BPSQ exhibited a strong specificity for the guanine-cytosine (GC) sequence in total DNA at an ionic strength of 0.01 (5.54 x 10(6) M-1). BPSQ increased the viscosity of sonicated rod-like DNA fragments, producing a calculated increment in length of 2.4Angstrom per bound drug molecule. A circular dichroism experiment showed that BPSQ might interact with DNA according to two possible binding modes depending on its structure and concentration. The first mode concerned intercalation of BPSQ with its long axis perpendicular to the long axis of the DNA helix. The same drug was able to bind to external sites, second mode, once the intercalation sites were saturated at high concentration. The results from cytotoxicity assays indicate that BPSQ was found to be toxic to all cell lines tested with IC50 values ranging from 2.8 to greater than or equal to20muM. It was concluded that the binding of BPSQ to DNA occurs by a mechanism of intercalation, which probably accounts for its reported antitumor activity.