Anticancer Activity of Nitrogen Mustards: Effect of Metal Complexation and Nutrient Conjugation

Karmakar, Subhendu (2016) Anticancer Activity of Nitrogen Mustards: Effect of Metal Complexation and Nutrient Conjugation. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The The clinical efficacies of anticancer agents have been largely restricted by several factors, viz,. lack of specificity, severe toxicity and resistance. To fight with these issues, strategic changes became obvious towards the development of new anticancer agents. The alkylating agent containing the moiety – N(CH₂CH₂Cl)₂, commonly known as ‘nitrogen mustard’, was the first chemotherapeutic agent used to treat cancer. The mechanism of action of the nitrogen mustards are mainly through cross-linking of DNA, especially guanine nucleobase in an interstrand fashion. However, very early it was realized that further modification of the drugs were necessary since due to high reactivity the side effects were very high. Extensive further research rendered multiple handles for better control on the activity resulting in the current generation of clinical nitrogen mustards, e.g., cyclophosphamide, melphalan, bendamustine, estramustine etc. The most important objective was not only to reduce the reactivity to let the drugs reach target intact but also to increase their action selectively in certain cancers. In Chapter 1, a brief outline on clinical nitrogen mustards has been given which includes mechanism of action, clinical side effects and resistance mechanisms towards chemotherapy. The various adapted approaches included design of derivatives of nitrogen mustards to decrease the reactivity of lone pair on the nitrogen atom, viz., by conjugation of nitro group, quaternary salt formation and metal complex formation. Amongst several methodologies adapted to increase the efficacy and decrease side effects of nitrogen mustard although metal complex formation was one of the strategies but somehow complex formation is relatively less with the most successful clinical metal platinum. The chapter shows that surprisingly, even if complexes were formed they were mostly not studied for their anticancer activity. The reported metal complexes of nitrogen mustards in literature are reviewed in the Chapter 1. In order to fight with the known issues standing as a mountain in between an anticancer agent and its efficacy, efforts were directed towards generation of metal complexes of nitrogen mustards using the clinically favourable metal platinum. The existing platinum drugs, which represent an important class of metal based chemotherapeutic agents, also face the problem of resistance especially with the thiol containing bio-molecule like glutathione (GSH). It is to be mentioned here that both the nitrogen mustards and platinum drugs mostly have a common target, i.e., cross-linking of DNA. Chapter 2 describes a trans-[PtII(L¹)(Me₂SO)Cl₂] (1) of monodentate nitrogen mustard (L¹), in an attempt to observe the synergism of both the established DNA-cross-linking agents in a single molecule. The solution stability of nitrogen mustard increased owing to complexation though the complex itself is less stable in solution. Therefore, in spite of having better in vitro cytotoxic activity than FDA approved clinical drug cisplatin, the solution stability of the trans PtII complex with monodentate nitrogen mustard ligand (L¹) demerits its applicability as a better anticancer agent. The use of cis complex and chelating ligand may increase the solution stability of Pt-nitrogen mustard complex. Hence, in Chapter 3, cis-dichloridoplatinum(II) complex, cis-[PtII(L³)Cl₂] (3) of a chelated nitrogen mustard (L³) is reported along with a same cis-dichloridoplatinum(II) complex, cis-[PtII(L²)Cl₂] (2) of a non-mustard (L²) ligand to compare their cytotoxic activity. The in vitro data shows that 3 is more active than 2, which correlates with their solution stability. 3 shows better anticancer activity in hypoxic condition and in presence of excess GSH compared to cisplatin. The difference in activity profile between cisplatin and 3 in presence of GSH is explained by taking the help of various kinetic studies of 3 with GSH. Chapter 4 comprises of preparation of two tetrachloridoplatinum(IV) complexes, cis, cis, trans- [PtIV(L³)Cl₄] (5) & cis, cis, trans-[PtIV(L⁴)Cl₄] (6) containing the nitrogen mustard & half mustard moieties –N(CH₂CH₂Cl)₂ & –NHCH₂CH₂Cl, from the Pt(II) complexes containing –N(CH₂CH₂OH)2 (2) & –NHCH₂CH₂OH (4) moieties respectively. This is the first example of a reaction procedure where oxidative addition to the Pt(II) centre to form Pt(IV) and ligand’s functional group change from –OH to – Cl has concurred in a single step using thionyl chloride. The excellent kinetic stability of the Pt(IV) complex (5) does not restrict it to show in vitro cytotoxicity, rather it shows comparable cytotoxicity to its Pt(II) congener (3) against various tumor cell lines. The binding studies with model nucleobase and GSH indicated that in situ conversion of 5 to 3 might contribute towards anticancer activity of 5. Moreover 5 is found less toxic towards non-tumor cell line than 3 & cisplatin which is very much encouraging. The work reported in Chapter 5 attempts to control the reactivity of the nitrogen mustard group and enhanced uptake in cancer cells by exploiting its metabolism and reducing environment. Here three aromatic dinitrobenzoic acid derivatives of nitrogen mustards are functionalized with nutrients like methionine and glucosamine keeping in mind that cancer cell has more requirements of these nutrients than normal cell and hence that might help to achieve higher uptake and greater cytotoxicity in cancer cells. The in vitro cytotoxicity data indicated that out of the six dinitrobenzamide nitrogen mustard derivative of methionine and glucosamine, dinitrobenzamide derivatives synthesized from 2- chlorobenzoic acid (7 and 8) demonstrated higher cytotoxicity to cancer cell lines compared to non-tumor cell lines.

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