DNA binding and biological activity of some platinum(II) intercalating compounds containing methyl-substituted 1,10-phenanthrolines

This study documents the first detailed investigation into the relationship between molecular structure and biological activity of platinum(II) complexes containing methylated derivatives of 1,10-phenanthroline (phen). A series of square planar platinum(II) compounds incorporating methylated derivatives of phen, 4-methyl-1,10-phenanthroline (4-Mephen), 5-methyl-1,10-phenanthroline (5-Mephen), 4,7-dimethyl-1,10-phenanthroline (4,7-Me₂phen), 5,6-dimethyl-1,10-phenanthroline (5,6-Me₂phen) and 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-Me₄phen) were synthesised and the relationship between their structure and biological activity investigated. The biological activity of these compounds was quantified using the in vitro cytotoxicity assay against the L1210 Murine leukaemia cell line. Large variation in cytotoxicities with different methylation was observed. The 5- and 5,6-methylated derivatives of phen displayed a greater biological activity, with IC₅₀ values of 2.8 ± 0.8 µM and 1.5 ± 0.3 µM respectively, compared with the phen compound, with an IC₅₀ value of 9.7 ± 0.3 µM, while all the others were inactive with IC₅₀ values over 50 µM. Binding constants were determined using circular dichroism spectroscopy (CD) and induced circular dichroism (ICD). ICD was used to highlight any differences in the spectra. Viscometry studies and linear dichroism (LD) experiments indicate that the platinum(II) complexes intercalate although for [Pt(en)(4-Mephen)]Cl₂ and [Pt(en)(4,7-Me₂phen)]Cl₂ this mode of binding appears to be concentration dependent. The binding of the platinum(II) complexes to the oligonucleotide d(GTCGAC)₂ was studied using two-dimensional ¹H NMR spectroscopy. The addition of each metal complex to the hexamer d(GTCGAC)₂ produced upfield shifts of the metal complex resonances, characteristic of intercalation. Through the observation of NOE cross-peaks, two-dimensional NMR studies provided insight into the site and groove preferences of these compounds when binding to DNA.