In attempts to overcome the drawbacks of cisplatin - severe toxicity, drug resistance and poor oral bioavailability - the development of platinum-based drugs has progressed from carboplatin and oxaliplatin to the newest generation of drugs, such as satraplatin, picoplatin and the multinuclear platinum complex BBR3464 (triplatin). Despite encouraging preclinical in vitro and in vivo results, outcomes of clinical trials of these coordination complexes remained below expectations. Biased rationale underlying the drug design along with in vitro and in vivo preclinical tests with inadequate predictive power seem to have eventually resulted in the selection of drug candidates of limited clinical activity. The nature of the active species generated in vivo, uptake, efflux, intracellular trafficking and detailed mechanisms involved in chemoresistance of platinum drugs in vivo are topics that need further investigation to provide clues for the rational formulation of new drugs. Insufficient diffusion in tumor tissues is likely to constitute an important limiting step in the treatment of solid tumors with platinum compounds. Preclinical assays with improved predictive power for the clinical outcome of the compounds should be based on more representative tumor models, such as resistant primary cancer cell lines, spheroids and orthotopic xenograft models, respectively. Finally, use of new pharmaceutical formulations and bifunctional complexes, as well as their selection by decisive preclinical tests, are expected to result in the generation of platinum-based anticancer drugs with the potential to achieve clinical activity even in multidrug-resistant tumors.

Organic anion transporting polypeptides (OATP, SLCO genes) mediate the uptake of endobiotics and drugs. Thus, their expression levels and pattern could be of relevance for cancer therapy. This prompted us to investigate the expression of poorly characterized OATPs, namely OATP2A1, OATP3A1, OATP4A1 and OATP5A1 in hepatic cancer of different origin. First, mRNA levels of all eleven OATPs were determined in paired (cancerous and adjacent non-cancerous) specimens from 43 patients with primary liver cancer (hepatocellular carcinoma, HCC; cholangiocellular carcinoma, CCC) and liver metastases from colon tumors (MLT). Real-time RT-PCR analysis revealed that all OATPs, except OATP1C1 and OATP6A1, are extensively expressed in nearly all samples. In contrast to downregulated OATP1B1, OATP1B3, OATP1A2 and OATP2B1 in cancerous vs. non-cancerous samples, an increase in OATP2A1, OATP3A1, OATP4A1 and OATP5A1 mRNA levels was seen in tumors (up to 40-fold for OATP5A1 in the MLT group). Therefore, OATP2A1, OATP3A1, OATP4A1 and OATP5A1 were further investigated by immunofluorescence microscopy on paraffin-embedded cancerous and non-cancerous sections (seven per group). OATP-derived immunoreactivity was observed in plasma membranes and cytosol of hepatic tumor cells, and additionally, in various cytokeratin 19 positive bile ducts. An increased percentage of immunoreactive cells and a higher staining intensity in cancerous vs. non-cancerous paraffin sections paralleled higher mRNA levels of OATP2A1, OATP3A1, OATP4A1 and OATP5A1 in cancerous tissues of HCC, CCC and MLT patients. The extensive expression of OATP2A1, OATP3A1, OATP4A1 and OATP5A1 in hepatic tumors of different origin suggests that these transporters might be further exploited for the discovery of novel anticancer agents.

The organometallic compound titanocene (bis(cyclopentadienyl)titanium(IV) dichloride) showed promising preclinical anticancer activity in generally resistant tumors in vitro and in vivo but failed in clinical trials. A broad range of analogs with modified cyclopentadienyl ligands conferring increased stability and higher cytotoxicity were developed. Titanium was found to accumulate in the nucleus and inhibit DNA replication and transcription. The active species causing irreversible damage and exact mechanisms of action resulting in cell cycle arrest and apoptosis have not been identified to date. Our group investigated changes in global gene expression of NCI-H526 small cell lung cancer cells in response to the novel analog titanocene C (bis-N,N-dimethylamino-2-(N-methylpyrrolyl) methyl) titanium (IV)). Differences observed in transcript levels indicated downregulation of DNA unwinding by topoisomerases I and II alpha and activation of responses to DNA damage and cellular stress as well as shutdown of energy metabolism and, finally, apoptosis. Besides direct interaction of Ti(2+) with DNA, induction of the MT1 family of metallothionein genes and downregulation of cellular Zn(2+) uptake in response to titanocene C pointed to disturbed Zn(2+) homeostasis, which triggers cell cycle arrest and apoptosis due to defective transcription factors and metalloenzymes. In particular, histone H4 genes dependent on Zn(2+)-containing transcription factors H4TF-1/2 were specifically downregulated, and accumulation of defective metalloproteins in the endoplasmatic reticulum seemed to activate unfolded protein response. In conclusion, according to these results, we propose a model of titanocene-induced cytotoxicity, comprising direct DNA damage and perturbation of Zn(2+) homeostasis with impairment of the functions of cellular metalloproteome.

Platinum(IV) compounds like oxoplatin (cis, cis, trans-diammine-dichlorido-dihydroxido-platinum(IV))show increased stability and therefore can be applied orally. In a panel of 38 human cancer cell lines this drug induced S-phase arrest and cell death with IC 50 values 2.5-fold higher than cisplatin. Oxoplatin may be converted to cisplatin by intracellular reducing agents, however, exposure to 0.1 M HCl mimicking gastric acid yielded cis-diammine-tetrachlorido-platinum(IV) exhibiting twofold increased activity. Similar results were obtained for another platinum(IV) compound, JM 149 (ammine-dichlorido-(cyclohexylamine)-dihydroxido-platinum(IV)), but not for its parent drug JM 216/satraplatin. Genome-wide expression profiling of H526 small cell lung cancer cells treated with these platinum species revealed clear differences in the expression pattern of affected genes between oxoplatin and cisplatin. In conclusion, oxoplatin constitutes a potent oral agent that is either reduced or converted to distinct active compounds, for example, by gastric acid or acidic areas prevailing in solid tumors, in dependence of the respective pharmaceutical formulation.