Synthesis and Characterisation of Novel 3-Chloropiperidines: Secondary Derivatives and Ligands for Cisplatin Analogues

Abstract

Since their discovery in the mid-20th century, alkylating agents have played a crucial role in cancer treatment. Nitrogen mustards, the earliest representatives, are characterized by their unique 2-chloroethylamine moiety. This structural motif facilitates the intramolecular formation of a highly electrophilic aziridinium intermediate, which is readily attacked by cellular nucleophiles, such as DNA bases. The resulting covalent adducts can cause depurination and DNA strand cleavage, thereby inhibiting DNA replication and eventually leading to apoptosis. However, the reactivity of these agents is not confined to malignant tissues, resulting in severe side effects and limiting their therapeutic application. Therefore, the development of improved derivatives is essential to enhance efficacy and tolerability. Consequently, a variety of drugs have been developed over the past decades. The reduction of the electron density on the central nitrogen atom has been shown to effectively lower the resulting reactivity, by decreasing the rate at of aziridinium ion formation. Prominent examples of this approach either incorporated the 2-chloroethylamine motif into a phosphonamide (Cyclophosphamide) or utilize electron withdrawing aromatic substituents (Chlorambucil). In the Göttlich group, the integrating the alkylating moiety into a cyclic structure has been investigated, aiming to introduce additional ring strain in the now bicyclic intermediate, similarly reducing the rate of aziridinium ion formation. The 3-chloropiperidines that emerged from this efforts have exhibited remarkable activity against multiple cancer cell lines in previous studies. This work focused on the synthesis and biological evaluation of novel 3-chloropiperidines incorporating a secondary amine in the cyclic core, contrary to the tertiary analogues exclusively examined in prior reports. Secondary nitrogen mustards and their stable aziridine intermediates are found in potent natural compounds and as an active metabolite of cyclophosphamide, the clinically most successful nitrogen mustard. A new synthetic method was developed revolving around a novel selective mono-chlorination protocol and in situ trapping of the unstable final product. Concluding this publication, these novel secondary 3-chloropiperidines and the respective aziridines both outperformed their tertiary counterparts in DNA-cleavage assays, demonstrating their potential as promising drug candidates. The second publication included in this work strived to combine our formerly reported cyclic nitrogen mustards with cisplatin based drugs, another invaluable class of anticancer agents. Direct coordination of the alkylating moiety to the metal centre significantly altered its reactivity, to prevent premature hydrolysation outside the nucleus. Additionally, this delivers two active chemotherapeutics simultaneously, possibly inducing clustered DNA-damage. Furthermore, the obtained complexes were able to largely overcome acquired cisplatin resistance in ovarian cancer cells, even surpassing the absolute potency of cisplatin in the resistant subline.