A series of Pt-IV anticancer complexes with different reduction potentials has been investigated for their reactivity toward 5'-guanosine monophosphate (5'-GMP). The Pt-IV complexes studied were Pt-IV(trans-d,l)(1,2-(NH2)(2)C6H10)Cl-4 (tetraplatin, Pt-IV(dach)Cl-4; dach = diaminocyclohexane), cis,trans,cis-[Pt-IV((CH3)(2)CHNH2)(2)(OH)(2)Cl-2] (iproplatin, Pt-IV(ipa)(2)(OH)(2)Cl-2; ipa = isopropylamine), cis, trans,cis-[Pt-IV(en)(OH)(2)Cl-2] (Pt-IV(en)(OH)(2)Cl-2; en = ethylenediamine), Pt-IV(en)Cl-4, and cis,trans,cis-[Pt-IV(en)(OCOCH3)(2)Cl-2] (Pt-IV(en)(OCOCH3)(2)Cl-2. The reactivity was monitored by the decreased H-1 NMR peak intensity at 8.2 ppm due to H8 of free 5'-GMP and the increased intensity of a new peak around 8.6 ppm due to H8 of 5'-GMP bound to Pt-II. The reactivity followed the order of cathodic reduction potentials of the Pt-IV complexes: Pt-IV(dach)Cl-4 (-90 mV) >> Pt-IV(en)Cl-4 (-160 mV) > Pt-IV(en)(OCOCH3)(2)Cl-2 (-546 mV) > Pt-IV(ipa)(2)(OH)(2)Cl-2 (-730 mV). The most reactive complex, Pt-IV(dach)Cl-4, showed an additional weak peak at 9.2 ppm due to H8 of the 5'-CMP bound to the Pt-IV complex, indicating the existence of a Pt-IV intermediate. H-1 NMR, UV/visible absorption spectra, and high-performance liquid chromatograms suggest that the final product is Pt-IV(dach)(5'-GMP)(ox5'-GMP), where ox5'-GMP is oxidized 5'-GMP. A plausible mechanism is that there is an initial substitution of one Pt-IV/ligand by a 5'-GMP molecule, followed by a two-electron reduction, and finally a second substitution by another 5'-GMP. In the presence of excess 5'-GMP (at least 20-fold), ox5'-GMP seems to be replaced by 5'-GMP to form Pt-II(dach)(5'-GMP)(2). UV/visible absorption spectroscopy shows that the formation of the Pt-IV intermediate by substitution is a very slow process followed by reduction. The reduction is characterized by a relatively fast exponential decay. The addition of a small amount of cis-[Pt-II(NH3)(2)Cl-2] shortened the slow formation time of the intermediate, implicating the occurrence of a Pt-II-assisted substitution reaction. These reactions may lead to a better understanding of the anticancer activity of Pt-IV complexes.