Replacement of a thiourea-S with an amidine-NH donor group in a platinum-acridine antitumor compound reduces the metal's reactivity with cysteine sulfur.

The reactivity of two DNA-targeted platinum-acridine conjugates with cysteine sulfur was studied. The conjugate containing an amidine-NH donor group cis to the chloride leaving group showed considerably reduced reactivity with N-acetylcysteine compared to the prototypical derivative containing a thiourea-S linkage. The opposite scenario has been observed previously in reactions with nucleobase nitrogen. Possible consequences of the unique target-selective tuning of the substitution chemistry for the pharmacodynamic properties and biological activity of these agents are discussed.