Quaternarized pdppz: synthesis, DNA-binding and biological studies of a novel dppz derivative that causes cellular death upon light irradiationw

The development of novel low molecular weight cancer therapeutics is a highly topical area of research. Such molecules should ideally possess good water solubility, possess rapid cellular uptake, undergo selective localisation and be able to induce programmed cell death of the cancer cells. In recent work, we have demonstrated that pyrrolo-1,5-benzoxazepine, sulfur-substituted a-alkyl phenethyl-amine, naphthalimide, and polyamide structures show potential as anticancer agents in a variety of cancer cell lines, as well as in solid tumours. In other related work, we have developed new polypyridyl ligands possessing various organic fluorophores, either directly attached, or conjugated via short spacers to the Ru(II)polypyridyl unit, for use as DNA targeting ligands and imaging agents. Herein, we present the synthesis of 1, a novel quaternarized polypyridyl molecule formed from 2, pyrazino[2,3-h]dipyrido[3,2-a:20,30-c]phenazine, or pdppz. This new species is based on combining, in a single structure, two well known polypyridyl ligands, dipyrido[3,2-a:20,30-c]phenazine (dppz) and 1,4,5,8-tetraazaphenanthrene (TAP), that are commonly used as DNA binding ligands in Ru(II) polypyridyl complexes, and has not, to the best of our knowledge, been synthesised before. The quaternarisation of polypyridyl ligands has recently been shown by Thomas et al. to be an effective way of increasing the DNA binding affinity of such organic structures. Hence, we embarked on investigating the ability of 1, to bind to DNA, as due to its extended aromatic surface, in concert with its cationic character, we foresaw that 1 would be an ideal candidate as a DNA intercalator.Moreover, its cationic nature also furnishes 1 with good aqueous solubility and should facilitate its cellular uptake in various cancer cell lines, where its ability to initiate apoptosis could potentially be controlled by light irradiation. The synthetic pathway of 1 and 2 is shown in Scheme 1. The synthesis of 2 was achieved by condensation of 5,6diaminoquinoxaline 4 with 1,10-phenanthroline-5,6-dione 3 by reflux in EtOH yielding 2 as a grey solid in 95% yield. Subsequent reflux in dibromoethane over 48 h and precipitation from methanol with diethyl ether yielded 1 as a pure beige solid in 78% yield. Synthesised as its chloride salt, 1 was watersoluble, and its photophysical properties were investigated in 10 mM phosphate-buffered aqueous solutions at pH 7.4. The characteristic absorption spectrum together with the excitation and emission spectra of 1 are shown in Fig. 1. The absorption spectrum of 1 shows absorbance maxima at ca. 285 nm, 310 nm, 370 nm, 390 nm along with a broad shoulder centred at 460 nm. The band at 285 nm (e= 31400 cm 1 M ) is characteristic of p–p* transitions of the phen moiety while the intense band at 310 nm (e=44 100 cm 1 M ) is attributed to p–p* transitions with less intense bands at 370 nm and 390 nm (e = 6900 cm 1 M ) attributed to n–p* transitions within the phenazine part of the ligand. These transitions are typical of the related compound dppz. Upon excitation into the intense band at 310 nm 1 gave intense emission with lmax at 515 nm (FF = 0.053). Similarly, the excitation of 1 (lem = 515 nm) yielded a spectrum structurally identical to the absorption spectrum. Scheme 1 Synthesis of 2 (free ligand) and corresponding dicationic organic derivative 1. (i) EtOH, reflux; (ii) dibromoethane, reflux.