STRUCTURE–ACTIVITY RELATIONSHIP STUDIES OF HYBRID ANTITUMOR AGENTS FOR THE TREATMENT OF NON-SMALL CELL LUNG CANCER (2009)

DNA-directed chemotherapies continue to play an important role in modern oncology. The cytotoxic complex, [PtCl(en)(ACRAMTU)](NO3)2 (en = ethane-1,2-diamine; ACRAMTU = 1-[2-(acridine-9-ylamino)ethyl]-1,3-dimethylthiourea) (PT-ACRAMTU, 31), is a dual platinating/intercalating DNA binder that, unlike clinical platinum agents, does not induce DNA cross-links. The research in this dissertation was concerned with establishing structure–activity relationships (SAR) in this novel class of DNA-targeted antitumor agents with the ultimate goal of improving the biological activity of the prototypical agent, PT-ACRAMTU. \n\tTwo ways were explored of tuning the reactivity and target interactions of platinum, which was considered critical in changing the pharmacological properties of this pharmacophore. The first approach involved a structurally minimally invasive modification of the prototype to enhance its chemical stability and reactivity with DNA. This was achieved by introducing a new inert nonleaving group (imino group) in place of thiourea. The first attempt involved the synthesis of a guanidine analogue of ACRAMTU, 1-[2-(acridine-9-ylamino)ethyl]-1,3-dimethylguanidine (38), by adding N-acridin-9-yl-N'-methylethane-1,2-diamine (36) to a Boc-activated carbodiimide (Me-N=C=N-Boc), obtained by desulfuration of N-methylthiourea (44) with HgCl2. While the study was able to delineate unusual pathways to novel cyclic and spirocyclic acridine derivatives, the ultimate goal of generating a guanidine analogue of PT-ACRAMTU was not reached due to the chelating properties of the guanidinato group (verified by 195Pt NMR). Instead, platinum-mediated amidination chemistry afforded two amidine derivatives of PT-ACRAMTU (54 and 55) with greatly enhanced activity (IC50 values of 26 nM and 28 nM, respectively) in H460 non-small-cell lung cancer (NSCLC). The amidination reaction involved addition of the secondary amine in 36 across the activated CN bond of platinum-bound propionitrile (EtCN). Complex 54 proved to be a more efficient DNA binder (t1/2 = 65 min) than PT-ACRAMTU (31) (t1/2 = 234 min), and showed considerably reduced reactivity with N-acetylcysteine compared to PT-ACRAMTU, based on a mechanistic study using time-dependent 1H NMR and 2-D HMQC NMR spectroscopy, as well as in-line liquid chromatography–electro-spray mass spectrometry (LC–ESMS). A cellular imaging study using confocal fluorescence microscopy was performed to study the subcellular distribution of complex 54. The results show higher cellular levels of 54 than of PT-ACRAMTU and suggest that the drug reaches the nucleus, although a significant amount of compound seems to be trapped in the lysosomes. A H460 mouse xenograft study showed complex 55 slows tumor growth by 40% when administered at a dose of 0.5 mg/kg, but the compound was quite toxic and caused weight loss in the animals. The concentrations of platinum detected using inductively-coupled plasma electrospray mass spectrometry (ICP–MS) in various biological tissues recovered from the euthanized mice reveal that, while compound 55 reaches the tumors, most of the detected platinum accumulated in the kidneys, the major organ of excretion. The high level of platinum in the kidneys most likely contributes to the toxicity observed in the treated animals. Complex 55 is the first non-cross-linking platinum agent endowed with promising activity in NSCLC. \n\tIn the second approach, novel thiourea- and guanidine-modified acridine-4-carboxamides and a corresponding platinum–intercalator conjugate have been synthesized and evaluated as cytotoxic agents. The point of attachment of the platinum-modified linker was changed from the 9-position to the 4-position of acridine to alter the DNA binding of the conjugate and its cytotoxicity. The IC50 value of 2.4 μM determined in H460 lung cancer cells for [PtCl(en)(N-(2-(1,3-dimethylthioureido)ethyl)acridine-4-carboxamide)](NO3)2 (58) indicates that this strategy has no advantage over the 9-aminoacridine-based prototype.