Development of Zeise´s salt derivatives as antitumor agents

Intrinsic or acquired resistance to chemotherapeutics is a major problem in current cancer therapy. The mechanism behind this phenomenon is complex and can include altered drug transport, increased me- tabolism, mutation of drug targets or increased DNA damage repair. Therefore, we focused our inter- est on the design of antitumor-active compounds binding to enzymes, which are over-expressed in tumor cells and stimulate their growth. A suitable target is the cyclooxygenase 2 (COX-2), which is involved in inflammatory processes. Furthermore, it was already demonstrated that it has growth stim- ulatory potency in mammary or colon carcinoma cells. In a preliminary study, we demonstrated that the binding of Aspirin, a well known non-steroidal anti- inflammatory drug (NSAID), to metal cluster increased its COX inhibition and cytotoxicity against tumor cells. In this context, we could show for the first time that the eldest organometallic compound (Zeises salt) is pharmacologically active and inhibits the COX-1 due to the platination of the amino acids Thr385 and Ser516. In contrast to Cisplatin, which is the mostly used drug in cancer therapy, Zeises salt was not yet de- veloped as anticancer agent, because of its low stability. However, if Zeises salt is bound to Aspirin by an alkenol linker, the stability and the COX inhibitory potency strongly increased. The same is true for the cytotoxic effects, which are, however, 10 fold lower than that of Cisplatin. With these hopeful results, we are now in the position to optimize Zeises salts derivatives as selectively acting antitumor agents. Structural modifications should further increase the stability to avoid degradation under physiological conditions. A further aim is to increase the COX-2 selectivity. This should be realized by substituents at the Aspirin moiety or the exchange of Aspirin by selective NSAIDs (e.g. Celecoxib) to provide derivatives with higher subtype selectivity. It is expected that these complexes will damage tumor cells, which show an increased growing under the control of COX-2. Because COX enzymes are wide spread in human body, it is necessary to bind Zeises salt to a carrier molecule allowing an accumula- tion e.g. in hormone-dependent tumor cells. This can be achieved, if the carrier represents a selective estrogen receptor modulator. It is planned to use derivatives of Tamoxifen, which is an effective anti- estrogen used in the therapy of the hormone-dependent mammary carcinoma. Furthermore, with this drug design it could be possible to inhibit two cellular targets, the COX and the estrogen receptor, both responsible for increased tumor cell proliferation. All complexes will be tested for COX inhibition and cytotoxicity in cell culture experiments. Stability and cell uptake studies complete the experiments. Most effective complexes will then be selected for investigation on the mode of action (e.g. binding bio macromolecules) using mass spectrometry and atomic absorption spectroscopy.

The search for new cisplatin-like complexes with an alternative mode of action for the treatment of cisplatin-resistant tumors is an important topic in medicinal chemistry. So is the development of drugs that have efficacy against common hormone-dependent tumors (such as certain forms of breast cancer) via specific targets. This research project aims to develop compounds that inhibit tumor cell growth via multiple pharmacological targets simultaneously (multitargeting). An attractive lead structure for this is the so-called Zeise's salt, for which we were able to show in preliminary work that it interferes with the cyclooxygenase (COX) signaling pathway, which is involved in the regulation of inflammation and pain. However, this pathway has also been shown to play a role in the development and progression of cancer and in regulating the tumor microenvironment. However, Zeise's salt itself showed no cytotoxicity against tumor cells or healthy cells in vitro. This could be the result of rapid degradation in the cell culture medium or the formation of less potent DNA interactions compared to cisplatin. However, the biological activity can be optimized if the Zeise salt is chemically modified, e.g., linked to acetylsalicylic acid (ASA) via an alkyl spacer. The resulting ASA-Alk-PtCl3 complexes showed cytotoxicity and high COX-1 inhibitory activity. Therefore, the aim of this research project was to develop compounds with higher activity. Also, by coupling Zeise's salt to a selective estrogen receptor modulator, the effect of such compounds on hormone-dependent breast cancer cells was investigated. This work on the structure-activity relationship has shown that it is in principle possible to optimize Zeise's salt for therapeutic purposes. For example, replacement of the ethylene with an ASA alkenol ester increased stability in aqueous solution. In addition, substituents on the ASA moiety resulted in complexes with higher COX-2 inhibitory potency and cytotoxicity compared with Zeise's salt, depending on the nature of the substituent and its position on the aromatic ring. By replacing ASA with an antiestrogenic compound (GW7604), we observed high affinity for the estrogen receptor, selective accumulation in estrogen receptor-positive tumor cells, and potent inhibition of COX-1/2 isozymes. Compounds with such multitargeting properties are of great importance for the therapy of cancer.