Macrocyclic nitrogen mustard prodrugs as hypoxia selective anti-cancer agents
The low selectivity of chemotherapy is an ongoing problem in the treatment of cancer. Prodrugs that are activated in vivo provide a therapeutic advantage for selective cytotoxicity. Here we have designed redox-active compounds that are electrochemically reduced in hypoxic (poorly oxygenated) tissue, resulting in release of a nitrogen mustard cytotoxin. This thesis describes the synthesis of novel macrocyclic N-mustard drugs and the development of their Cu(II) complexes as hypoxia-selective prodrugs. (Fig. 3736A) The (2-trimethylsilyl)ethanesulfonyl (SES) protecting group is very versatile. It is removed under mild conditions using fluoride. The published synthesis of the sulfonyl chloride A gave variable yield and purity, but we have improved the conditions to give consistently pure material in high yield (70-86% overall) (Scheme 1). (Fig. 3736B) Triamines with carbon bridges longer than three are difficult to prepare, often requiring multistep syntheses. A route was developed to synthesise linear triamines, using the SES-amide B. This route produces these triamines in relatively high yields (60-80% overall), via simple reactions with little purification necessary (Scheme 2). A variation on the Richman-Atkins synthesis has been exploited to reach known an novel triazamacrocyclic compounds D (Scheme 3), in order to explore their structure-activity relationship as N-mustard drugs E (made as shown in Scheme 4). Eight novel macrocyclic N-mustards E were found to be potent DNA cross-linking agents (nM range) by Prof. John Hartley at University College London. Three of the novel triazamacrocycles D were assessed in vitro for anti-parasitic activity by Dr. Michael Barrett at the University of Glasgow. They showed moderate activity against Leishmania mexicana and Trypanosoma brucei. (Fig. 3736C) Water soluble Cu(II) complexes of cytotoxic macrocyclic nitrogen mustards have been prepared and their structures have been determined using X-ray crystallography by Dr. Louis Farrugia in this department. The redox behaviour and reduction potentials (Cu[II] to Cu[I]) of the complexes in phosphate buffer were assessed using cyclic voltammetry. The thermodynamic stabilities of the Cu(II) complexes in aqueous solution were analysed qualitatively using UV-Vis spectroscopy. The mustard complexes F and G showed irreversible redox behaviour and low thermodynamic stability, and were not hypoxia-selective but behaved as typical mustard drugs. The cyclen-based mustard complex H showed reversible redox behaviour and had high thermodynamic stability under aqueous conditions. H exhibited excellent hypoxia selectivity (the best so far in the lung tumour cell line tested) and is an attractive lead compound for further development of this novel approach to cancer chemotherapy (Fig. 3736D).