INTERACTION OF ANTICANCEROUS DRUGS WITH SPECIFIC OLIGONUCLEOTIDES

Abstract

Intra-cellular DNA represents the target for a wide variety of clinically important antibiotics and synthetic compounds that exhibit antitumour activities. It acts as a 'flexible receptor' capable of expanding its minor groove to accommodate bulky ligand dimers or distorting the sugar-phosphate backbone to allow architecturally complex intercalators to thread chains of sugar residues between the DNA base pairs. These diverse set of antibiotics binding to DNA in a variety of ways have a common ability to act as inhibitors of DNA transcription and replication. As a result these antibiotics are of great interest as potential anticancer agents. An understanding of structure and details of dynamics of drug-DNA interactions at molecular level is important in rationale designing of new families of antitumour agents. Adriamycin, an anthracycline antibiotic is isolated from Streptomyces peucetius. It is active against a number of solid tumours and has a wide spectrum of antitumour activity. The present study has been undertaken to study the conformational features of adriamycin in DO by using ID and 2D NMR techniques. The three-dimensional conformation of adriamycin specified by spin-spin coupling constants and interproton distances has been compared with X-ray crystal structure of daunomycin (the X-ray crystal structure of adriamycin is not available in literature). The conformation of deoxyhexanucleotide d-(CGATCG) has been investigated in DO. Geometry of deoxyribose sugar of each residue is obtained on the basis of double-quantum filter COSY cross peak patterns, spin-spin coupling constants and sums of couplings obtained from 1D/2D NMR spectra. This is supplemented with interproton distances obtained from NOESY spectra recorded at various mixing times (x ). The intra-residue base-sugar proton NOEs are used to derive information about glycosidic bond rotation ix) • The complex of adriamycin with d-(CGATCG) is prepared by titrating the hexamer with drug and monitoring the same by ID NMR at every step. The 2:1 adriamycin to d-(CGATCG) concentration ratio complex finally obtained, is studied by ID NMR and 2D NOESY in detail. The changes in chemical shifts of various protons in 2:1 complex are attributed to stacking interactions and alteration in relative base-base overlap. The data of 2D NOESY spectra is used to assign all proton NMR signals unambiguously. The sugar puckering, helix sense, glycosidic bond rotation, etc. are estimated in complexed form. Several intramolecular NOE connectivities, within adriamycin and DNA molecule, reflect the changes in conformation of DNA and drug on complexation. The intermolecular NOEs between ring D protons of adriamycin and base protons of 5'-3' d-CpG step give direct evidence of intercalation of adriamycin chromophore between d-CpG base pairs. Intermolecular NOEs between daunosamine sugar and A3.T4 base pair indicate the proximity of daunosamine sugar to the third base pair in the hexamer sequence. The results are discussed with a view to understand implications of binding of the drug adriamycin to DNA.