Conclusion and Outlook

The aim of this study was the synthesis and characterisation of a multifunctional tool suitable for selective coordination of Cu⁺, which could be achieved in a five-stage plan. Besides the synthesis of Cu⁺ selective chelators, a new synthetic pathway to benzothiazoles was established. Coupling

9.2 Conclusion and Outlook

103 of the evaluated ligands with such a benzothiazole afford three multifunctional tools, L³BTA, L⁵BTA and L⁸BTA, which are not only metal selective but also contain an Aβ plaque sensitive unit.

In according with “Lipinski’s Rule of Five”, the final molecules are relatively nonpolar and should be able to cross the BBB and cell walls. However, this also resulted in poor aqueous solubility of the compounds, which complicates the analytics. Specifically, the most promising system L⁸BTA is poorly soluble in water and therefore could not be used in the cell viability tests. Thus, increasing the solubility would be the next step in the project progress. This could be achieved by the introduction of a water soluble group, e.g. a sulfonate group, on the ligand, the benzothiazole, or on the triazole.^[314] This would allow for cell viability tests and also binding studies in neutral buffer solution. Furthermore, water soluble compounds would allow studies on their alteration of the Aβ aggregation. Nevertheless, two multifunctional tools, L³BTA and L⁵BTA, could be synthesised, which show protective functionality against metal induced ROS and which are not toxic. Thus, these two compounds are suitable for preliminary in vivo testing.

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Chapter 10

CO-Releasing Molecules

10.1 Introduction

105

10.1 Introduction

Carbon monoxide (CO) is a stable, highly toxic gas, which is endogenously produced in plants, bacteria and animals by heme oxygenase enzymes.^[315] CO serves as a natural detoxicate and can protect cells against oxidative stress.^[316–318] In vivo CO can bind tightly to heme-containing targets (e.g. soluble guanylate cyclase, cytochrome c oxidase, NADPH oxidase, and BK potassium channels). It has been demonstrated that besides nitric oxide^[319] and hydrogen sulphide,^[320] CO is involved in the intracellular signal transduction.^[321] Furthermore, CO participation in important biological activities such as resolution of inflammatory states, vasorelaxation, anti-apoptotic, and cyto-protective actions could be shown.^[322,323] In low doses even a protective function against ROS could be proven.^[322] Thus, the growing interest for pharmaceutical applications of CO in the last decades is no surprise, but implementation is not a simple task, since overdoses are highly toxic. The discovery of CO-releasing molecules (CORMs), compounds that serve as carriers and can release CO in a controlled fashion finally opened a pathway to new therapies.^[324,325] In general two different types of CORMs can be distinguished. The first type of CORMs consists of boranocarbonates with sodium boranocarbonate (CORM-A1) as representative. These boranocarbonates can spontaneously release CO under physiological conditions, which make them superior for medical applications. ^[326–328] The second type comprises of metal carbonyl complexes with different transition metals, mainly ruthenium, rhodium, iron or manganese.^[322,329] The most famous representatives of this group are the lipid-soluble tricarbonyldichlororuthenium(II) dimer (CORM-2) and the water-soluble tricarbonylchloro(glycinato)ruthenium(II) (CORM-3) (Figure 45).^[324,325]

Figure 45 Literature known CO-releasing molecules (CORMs).

The aim of the current research mainly lies in the development of CORMs, which can under physiological conditions achieve a beneficial delivery of CO. To provide this, the compounds have to release CO in specific amounts over an extended time period or by an external trigger.

Photoactive CORMs fulfil these requirements and are announced as potential therapeutics.^[330–

333] Especially the manganese-containing systems have already shown remarkable cytotoxicity against cancer cells. Investigation of the take-out of HT-29 and MCF-7 human cancer cells with [CpMn(CO)3] (cymantrene) and its conjugate with cell-penetrating peptides (CPP) have shown a good activity. However, the long half-life of several hours and slow photolytic release alleviate the results (Figure 46).^[334,335] The next generation, the [Mn(CO)3(tpm)]⁺ (tpm = tris(pyrazolyl)methane) complex and its derivatives have a shorter half-life and feature by a higher activity, e.g. cytotoxicity on human colon cancer cells.^[244–246] Nevertheless, for pharmaceutical applications the irradiation has to be done at high wavelengths were the skin becomes permeable and so far no CORM is available, which combines all requirements.

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Figure 46 Manganese based light induced CORMs (tpm = tris(pyrazolyl)methylene, tpp = tris(pyridine-2yl)-phosphane).

Thus, most of the new developed CORMs consist of aromatic donor groups, such as pyridine or pyrazol. Fine-tuning of the physical and chemical properties of the CORMs is normally done by different substituent on the aromatic system. Variation of the donor set is only rarely done, since prognosis of the resulting CORMs behaviour is not easy, because it depends on various factors. Although there is a broad variation of reported CORMs in literature, up to date no mixed donor system with a {NS2} binding motif is known. The tripodal ligands L^3Me and L^5Me consist of two relatively weak thioether donors and one strong pyridine donor. The prepared Cu⁺ complex, presented in appendix A, shows that relatively few electron density is provided by the ligand. In case of manganese-based CORMs should this result in a weak Mn-CO bonding, which could induce a short half-life and a fast CO release. Thus the chelators stand out from previously reported systems. Furthermore, it could be demonstrated, that the synthesised ligands can be easily modified to precursors for click chemistry, allowing the coupling of the ligands to CPP or other molecules. L^3Me and L^5Me are therefore perfect candidates for the synthesis of CORMs. In the following chapter, the synthesis and characterisation of four new CORMs and first studies of their capabilities as CO suppliers will be presented. The main focus of this work lies in answering the question: Does the {NS2} moiety lead to a new type of CORMs? Besides L^3Me and L^5Me also L^6H wasusedto havea direct comparison. With its three pyridines, L^6H is similar to the previously reported tpp (tris(pyridine-2yl)-phosphane) chelator.^[336] However, in contrast to the literature known system, L^6H has the advantage, that it can be easily modified on the amine and thus coupling e.g. to CPPs is possible.