The overall efficacy of platinum based drugs is limited by metabolic deactivation through covalent drug–protein binding. In this study the factors affecting cytotoxicity in the presence of glutathione, human serum albumin (HSA) and whole serum binding with cisplatin, BBR3464, and TriplatinNC, a “noncovalent” derivative of BBR3464, were investigated. Upon treatment with buthionine sulfoximine (BSO), to reduce cellular glutathione levels, cisplatin and BBR3464-induced apoptosis was augmented whereas TriplatinNC-induced cytotoxicity was unaltered. Treatment of A2780 ovarian carcinoma cells with HSA-bound cisplatin (cisplatin/HSA) and cisplatin preincubated with whole serum showed dramatic decreases in cytotoxicity, cellular accumulation, and DNA adduct formation compared to treatment with cisplatin alone. Similar effects are seen with BBR3464. In contrast, TriplatinNC, the HSAbound derivative (TriplatinNC/HSA), and TriplatinNC pretreated with whole serum retained identical cytotoxic profiles and equal levels of cellular accumulation at all time points. Confocal microscopy of both TriplatinNC-NBD, a fluorescent derivative of TriplatinNC, and TriplatinNC-NBD/HSA showed nuclear/nucleolar localization patterns, distinctly different from the lysosomal localization pattern seen with HSA. Cisplatin-NBD, a fluorescent derivative of cisplatin, was shown to accumulate in the nucleus and throughout the cytoplasmwhile the localization of cisplatin-NBD/HSA was limited to lysosomal regions of the cytoplasm. The results suggest that TriplatinNCcan avoid high levels of metabolic deactivation currently seen with clinical platinum chemotherapeutics, and therefore retain a unique cytotoxic profile after cellular administration.

The interactions of polynuclear platinum complexes with human serum albumin were studied. The compounds examined were the "non-covalent" analogs of the trinuclear BBR3464 as well as the dinuclear spermidine-bridged compounds differing in only the presence or absence of a central -NH(2)-(+) (BBR3571 and analogs). Thus, closely-related compounds could be compared. Evidence for pre-association, presumably through electrostatic and hydrogen-bonding, was obtained from fluorescence and circular dichroism spectroscopy and Electrospray Ionization Mass Spectrometry (ESI-MS). In the case of those compounds containing Pt-Cl bonds, further reaction took place presumably through displacement by sulfur nucleophiles. The implications for protein pre-association and plasma stability of polynuclear platinum compounds are discussed.

Transplatinum planaramine (TPA) compounds possessing carboxylate ligands in the trans position have been shown to be potential antitumor drugs in a variety of cell types, including cisplatin and oxaliplatin-resistant cell lines. In this work, we ask whether the nature and stability of the carboxylate ligand can be tuned in an attempt to manipulate the extent of serum protein binding; and consequently influence cytotoxicity, cellular drug accumulation and DNA adduct formation. Monitoring the interactions of selected TPAs with N-acetyl-methionine (NAM) by (1)H and (195)Pt NMR spectroscopy shows significant differences in the rate of sulfur binding. TPA-containing acetate ligands show a much lower sulfur binding rate than those possessing formate leaving groups. The same trend was seen when acetate and formate TPA compounds were incubated with human serum albumin and the reaction monitored for 24 h. To understand whether these results could be translated into a cellular medium, MTT cytotoxicity assays were conducted for each compound, before and after incubation with whole serum. Both the formate and acetate compounds, t-[Pt(4-pic)NH(3)(OFm)(2)] and t-[Pt(4-pic)NH(3)(OAc)(2)], showed minimal losses in cytotoxic efficacy and outperformed cisplatin after pre-incubation with serum. The same trends were seen when monitoring the effects of protein binding on cellular uptake and DNA platination. The rate of protein binding/drug deactivation was shown to be directly related to the stability of the leaving group (OAc(-) > OFm(-) > Cl(-)). Thus, our results suggest that utilization of the 'carboxylate strategy' substantially enhances the cellular efficacy of TPA compounds over cisplatin by allowing for an optimal balance between cytotoxic and metabolic efficiency.

TriplatinNC, a “non-covalent” derivative of the phase II clinical drug BBR3464, exhibits a distinct mode of DNA binding mediated through “phosphate clamps”, which display structural analogy to the “polyarginine” (arginine fork) DNA/RNA recognition motif. TriplatinNC shows an improved pharmacokinetic profile and increased cellular accumulation compared to BBR3464. Furthering the “polyarginine” analogy, the cellular accumulation of TriplatinNC, unlike cisplatin or oxaliplatin, is dependent upon the presence of cell surface glycosaminoglycans (GAGs), shown through comparison of platinum level accumulation in wild-type Chinese Hamster Ovary (CHO) cell lines to those lacking heparan sulfate (CHO-pgsD-677) or both heparan sulfate and chondroitin sulfate (CHO-pgsA-745). The cellular localization of TriplatinNC and cisplatin was visualized in confocal microscopy experiments utilizing the fluorescently-tagged derivatives, TriplatinNC-NBD or cisplatin-NBD. TriplatinNC, but not cisplatin, shows distinct localization to the nucleolar region of human colon and ovarian carcinoma cell lines, HCT116 and A2780, respectively. It was hypothesized that this pattern of localization, combined with nucleic acid affinity, would allow TriplatinNC to disrupt rRNA synthesis, the primary function within the nucleolus. Using 32P-metabolic labeling to monitor the rate of pre-rRNA transcript formation after cellular treatment, it was observed that both 47S pre-RNA and processed rRNAs, 32S, 28S, and 18S levels decreased dramatically compared to the untreated control in a dose-dependent manner. While the localization and abundance of nucleolar proteins, RNA pol I and UBTF, were unaffected by treatments with TriplatinNC at early timepoints, levels of the clinical relevant proliferative marker Ki-67, decreased markedly within one hour post-treatment. The morphological characteristics of apoptosis, including reduction in cell size, membrane blebbing, and cytosolic vacuolization, become visible 10hrs after drug treatment. This apoptotic process is not dependent on p53, as western blot analysis showed cleavage of procaspases 8, 9, and 3, followed by Parp-1 in HCT116 p53+/+ and p53−/− cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C171.

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