The design, synthesis and preliminary evaluation of prodrugs and hybrid drugs with tubulin polymerisation inhibitory activity and anti-proliferative activity

Abstract

The objective of this project was to design, synthesise and carry out preliminary evaluations on potential prodrugs and hybrid drugs with anti-proliferative effects. Chapter 1 gives an overview of current trends in cancer therapy, with a focus on drugs which target the tumour vasculature. Two prototype anti-proliferative drugs are introduced. The first is a tubulin-binding compound that was designed in-house and is closely related to the combretastatins and colchicine. The second is bestatin, a dipeptide with a range of pharmacological effects. The merits of these two drugs are discussed in relation to combination chemotherapy and hybrid drug design. The prodrug concept is introduced here as an increasingly topical strategy in the design of cytotoxic drugs. Chapter 2 describes the synthesis and characterisation of two prodrugs of the lead tubulin polymerisation inhibitor. One was designed simply with a view to improving the solubility of the otherwise relatively hydrophobic tubulin-binding agent. The second was designed as a conjugate of two separately functioning drugs which are shown in vitro to be generated in a 1:1 ratio in physiological medium and made available to act individually at their separate target proteins. The results of an in vitro spectrophotometric assay of APN inhibition indicate that esterification potentiates the efficacy of bestatin as an APN inhibitor. The primary objective of the work described in Chapter 3 was to investigate the incorporation of increased stability into the prodrug and hybrid drug design with a view to achieving slow release in vivo and generating a sustained low concentration of the active drugs. The synthesis and characterisation of amide conjugates are described as alternatives to the ester conjugates of Chapter 2. APA and APN are two enzymes whose expression is shown to be upregulated in malignancy. The amide prodrugs were designed as substrates for these enzymes. Conjugation at this site is shown by in vitro studies to enhance the efficacy of the bestatin component but to negatively impact on the cytotoxicity of the tubulin-binding component. The inclusion of a linker amino acid between the two differentially targeted drugs is determined to diminish the activity of both moieties.