Formation of ternary complexes by coordination of (diethylenetriamine)-platinum(II) to N1 or N7 of the adenine moiety of the antiviral nucleotide analogue 9.

The synthesis of (Dien)Pt(PMEA-N1), where Dien = diethylenetriamine and PMEA2- = dianion of 9-[2-(phosphonomethoxy)ethyl]adenine, is described. The acidity constants of the threefold protonated H3[(Dien)Pt(PMEA-N1)]3+ complex were determined and in part estimated (UV spectrophotometry and potentiometric pH titration): The release of the proton from the (N7)H+ site in H4[(Dien)Pt(PMEA-N1)]3+ occurs with a rather low pKa (= 0.52+/-0.10). The release of the proton from the -P(O)2(OH) group (pKa = 6.69+/-0.03) in H[(Dien)Pt(PMEA-N1)]+ is only slightly affected by the N1-coordinated (Dien)Pt2+ unit. Comparison with the acidic properties of the H[(Dien)Pt(PMEA-N7)]+ species provides evidence that in the (Dien)Pt(PMEA-N7) complex in aqueous solution an intramolecular, outer-sphere macrochelate is formed through hydrogen bonds between the -PO3(2-) residue of PMEA2- and a PtII-coordinated (Dien)NH2 group; its formation degree amounts to about 40%. The stability constants of the M[(Dien)Pt(PMEA-N1)]2+ complexes with M2+ = Mg2+, Ca2+, Ni2+, Cu2+ and Zn2+ were measured by potentiometric pH titrations in aqueous solution at 25 degrees C and I = 0.1 M (NaNO3). Application of previously determined straight-line plots of log K(M(R-PO3))M versus pK(H(R-PO3)H for simple phosph(on)ate ligands. R-PO3(2-), where R represents a non-inhibiting residue without an affinity for metal ions, proves that the primary binding site of (Dien)Pt(PMEA-N1) is the phosphonate group with all metal ions studied; in fact, Mg2+, Ca2+ and Ni2+ coordinate (within the error limits) only to this site. For the Cu[(Dien)Pt(PMEA-N1)]2+ and Zn[(Dien)Pt(PMEA-N1)]2- systems also the formation of five-membered chelates involving the ether oxygen of the -CH2-O-CH2-PO3(2-) residue could be detected; the formation degrees are about 60% and 30%, respectively. The metal-ion-binding properties of the isomeric (Dien)Pt(PMEA-N7) species studied previously differ in so far that the resulting M[(Dien)Pt(PMEA-N7)]2+ complexes are somewhat less stable, but again Cu2+ and Zn2+ also form with this ligand comparable amounts of the mentioned five-membered chelates. In contrast, both M[(Dien)Pt(PMEA-N1/N7)]2+ complexes differ from the parent M(PMEA) complexes considerably; in the latter instance the formation of the five-membered chelates is of significance for all divalent metal ions studied. The observation that divalent metal-ion binding to the phosphonate group of (Dien)Pt(PMEA-N1) and (Dien)Pt(PMEA-N7) is only moderately inhibited (about 0.2-0.4 log units) by the twofold positively charged (Dien)Pt2+ unit at the adenine residue allows the general conclusion, considering that PMEA is a nucleotide analogue, that this is also true for nucleotides and that consequently participation of, for example, two metal ions in an enzymatic process involving nucleotides is not seriously hampered by charge repulsion.

Metal-modified nucleobase pairs and triplets as cytosine receptors.

A preorganized cationic receptor 2 for cytosine (C) is described which is composed of trans-a2PtII (a= NH3 or CH3NH2) cross-linked modules with adenine (A), guanine (G), and uracil (U) or thymine (T) model nucleobases. The functions of these three modules are as follows: i) Adenine orientates the two other bases at right angles, thus producing the L-shape of the receptor. ii) Guanine is the primary receptor. iii) Uracil or thymine act as coreceptors. Compared with the normal Watson-Crick pair between G and C, the association constant between 2 and C increases by a factor of 3 (in DMSO). As deduced from 1H NMR spectroscopy and confirmed by the X-ray crystal structure of the C adduct 4b, cytosine is fixed through five hydrogen bonds to the receptor, one of which involves the aromatic H(5) of C. A comparison of C binding is made with a structurally related linkage isomer receptor as well as the precursor molecule trans[alpha2PtAG]2+. The potential of modular, cationic receptors is illustrated.

Extreme (10(9)) acidification of adenine-NH2 in an open platinated nucleobase quartet. A pH switch with potential as a biological acid/base catalyst.

The combination of electronic effects (PtII binding to N7 and N1) and a favourable conformation permitting efficient stabilization of the anion brings about a 10(9) fold increase in the exocyclic amino group acidity of 9-ethyladenine.

Expression of 17-1A antigen and complement resistance factors CD55 and CD59 on liver metastasis in colorectal cancer.

Despite radical surgery, the prognosis for colorectal cancer patients with liver metastases has not changed markedly. Furthermore, no standard adjuvant therapeutic regimen has been developed. Adjuvant therapy with monoclonal antibodies (e.g., against 17-1A), which has been shown to be effective in preventing metastatic relapse in patients with Dukes' C colorectal cancer, might be a promising approach for these patients. However, the cytotoxic effects of monoclonal antibodies can be blocked by coexpression of complement resistance factors that inhibit antibody-dependent complement-mediated cytotoxicity. We therefore analyzed immunohistochemically the expression of 17-1A and the membrane-bound complement resistance factors CD55 and CD59 on metastatic tumor cells in the livers of 71 patients with colorectal carcinoma who had undergone resection of their metastases with curative intent. In 67 (94%) of 71 patients, liver metastases with homogeneous expression of 17-1A was seen. Heterogeneous expression of 17-1A was seen in four patients (6%). Heterogeneous expression of CD55 or CD59 was observed in 8 (11%) of 71 patients and 4 (6%) of 71 patients, respectively. None of the patients showed homogeneous expression of either CD55 or CD59. All patients with CD55 or CD59 expression showed homogeneous 17-1A expression, whereas none of the four patients with heterogeneous 17-1A expression was positive for CD55 or CD59. Our data indicate that 17-1A is widely expressed on liver metastases of patients with colorectal carcinoma. Therefore patients with completely resected liver metastases might be suitable candidates for adjuvant therapy with and-17-1A antibody since only a few of these lesions showed coexpression of complement resistance factors.

Isomeric equilibria in aqueous solution involving aromatic ring stacking in the sexternary complexes formed by the quaternary cis-(NH3)2Pt(2'-deoxyguanosine-N7)(dGMP-N7) complex and the binary Cu(2,2'-bipyridine)2+ or Cu(1,10-phenanthroline)2+ complexes (dGMP2- = 2'-deoxyguanosine 5'-monophosphate).

To the best of our knowledge, for the first time the stabilities of sexternary complexes are determined by potentiometric pH titrations in aqueous solution at 25 degrees C and I = 0.1 M (NaNO3). The sexternary complexes form by binding of the binary Cu(Arm)2+ complexes, where Arm = 2,2'-bipyridine (Bpy) or 1,10-phenanthroline (Phen), to the -PO3(2-) group present in the quaternary cis-(NH3)2Pt(dGuo)(dGMP) complex. It is shown by stability constant comparisons and spectrophotometric measurements (observation of charge-transfer bands for the Phen system) that the [cis-(NH3)2Pt(dGuo)(dGMP).Cu(Arm)]2+ complexes can fold in such a way that aromatic ring stacking between the aromatic rings of Bpy or Phen and a guanine residue (most probably the one of dGMP2-) becomes possible. The formation degree of the stacks reaches approximately 25 and 50% for the [cis-(NH3)2Pt-(dGuo)(dGMP).Cu(Bpy)]2+ and [cis-(NH3)2Pt(dGuo)(dGMP).Cu(Phen)]2+ species, respectively. By comparisons with Cu(Arm)(dGMP) complexes, it is shown that the cis-(NH3)2Pt2+ unit coordinated to N7 of the guanine residues in the sexternary complexes inhibits stacking but does not prevent it. This result is of general importance because it demonstrates that in aqueous solution purine residues of nucleotides or nucleic acids that carry a metal ion at N7 can still undergo stacking interactions with other suitable aromatic ring systems.