Sabarubicin Hydrochloride

We studied the binding of two anthracycline drugs, Doxorubicin and Sabarubicin, to a model telomeric sequence 5'-d[GGG(TTAGGG)(3)]-3' (21-mer), assuming the basket G-quadruplex (G4) conformation in Na(+)-rich aqueous solution. We used an approach that combines spectroscopic and microcalorimetric techniques to obtain information about ground and excited state properties of the most stable complexes. Both drugs bind to the 21-mer in basket conformation and complexes of 1:1 and 2:1 drug : 21-mer stoichiometry coexist in solution. Binding constants were determined from fluorescence and isothermal titration calorimetry experiments. For both drugs association is driven by enthalpy and disfavoured by entropy in the case of two sequential binding events to different sites. The drug fluorescence is completely quenched in the 1:1 complex, most likely by electron transfer from the guanine system to the anthraquinone moiety, while part of the emission survives in the 2:1 complex. Circular dichroism (CD) of the individual complexes is dominated by the G-quadruplex signal in the UV and by the anthracycline signal in the near-UV and Vis region. The experimental CD spectra combined with conformational calculations at MM level and quantum mechanical calculation of the rotational strength of the electronic transitions afforded information on the binding geometries.

Among the disaccharide derivatives of the antitumor anthracycline doxorubicin, sabarubicin (Men10755) is more active and less cytotoxic than doxorubicin. It showed a strong in vivo antitumor activity in all preclinical models examined, in conjunction with a better tolerability, and is now in phase II clinical trials. The interaction of sabarubicin and Men10749 (a similar disaccharide with a different configuration at C-4' of the proximal sugar) with the hexanucleotides d(CGTACG)(2) and d(CGATCG)(2) was studied by a combined use of 2D-(1)H and (31)P NMR techniques. Both (1)H and (31)P chemical shifts of imino protons and phosphates allowed to established the intercalation sites between the CG base pairs, as it occurs for other anthracyclines of the series. The dissociation rate constants (k(off)) of the slow step of the intercalation process were measured for Men10755 and Men10749, by NMR NOE-exchange experiments. The increase of k(off) , with respect of doxorubicin, showed that the intercalation process is significantly faster for both drugs, leading to an average residence time for sabarubicin into d(CGTACG)(2) sixfold shorter than for doxorubicin. This could give account of both higher cytoplasmic/nuclear ratio and lower cellular uptake of sabarubicin in comparison with doxorubicin and accordingly of the lower cytotoxicity of these disaccharide analogues. A relevant number of NOE interactions allowed the structure of the complexes in solution to be derived through restrained MD calculations. NMR-DOSY experiments were performed with several drug/oligonucleotide mixtures in order to determine the structure and the dimension of the aggregates.