Design and evaluation of novel anti-tubulin agents selectively targeting parasitic tubulin

Malaria is a parasitic infection which affects approximately 300 million people worldwide causing over one million deaths each year. There are few diseases that have the same impact on human social and economic development than malaria. In areas where malaria is prevalent, the disease can account for up to 40% of public health expenditure and 50% of all hospital visits. It is clear that in order to relieve the burden of malaria novel, cheap, effective treatments need to be investigated, as in this project. In order to suppress the resistance of the disease to drugs, antimalarials with novel mechanisms of action need to be developed.

In this research project, plasmodia1 tubulin has been investigated as a potential drug target. Microtubules play several critically important roles throughout the entire parasite life cycle. Most notably, they form the mitotic spindle during cell division and even a slight disruption of the microtubule dynamics can have a severe impact on the viability of the parasites Although tubulin is present in all eukaryotic cells, it may be sufficiently different from organism to organism to engage in selective targeting. Known antitubulin compounds, including current anticancer drugs such as paclitaxel, do not have selectivity to discriminate between parasite and human tubulin. Although most of the microtubule inhibitors studied to date are equally effective at poisoning parasite and human cells, two distinct classes of common herbicides, the dinitroanilines and the phosphorothioamidates, are potentially selective. The lead compounds for this project, amiprophosmethyl and butamifos, are off patent herbicides that have been tested for their anti-malarial activity Analogues of amiprophosmethyl, an antitubulin compound that appear to exhibit selective antitubulin activity, were designed with diverse architectures around the pentavalent phosphorus atom and many of these compounds have been synthesised and characterised. These modifications include:

• variations of substituents on the aromatic ring,
• replacement of the aromatic ring with other cycles, extension and branching of the amino chain,
• synthesis of cyclic ox-aza analogues,
• cyclic oxobenzodioxaphosphininylamines,
• thiophosphoryl and phosphoryl analogues,
• a series of hybrid molecules incorporating known antimalarial pharmacophores such as quinolines and chloroquinolines.

A library of 97 compounds was successfully synthesised based on several modifications of the lead structures. These compounds were tested against several parasite species and the results analysed in terms of their Structure Activity Relationships. When tested against leishmania and trypanosomes, the test compounds were not more active than standard reference therapies. However, several of the compounds showed interesting activity against plasmodia, with 6 showing higher biological activity than the lead compounds. Of these the hybrid drugs combining a chloroquine pharmacophore along with an organophosphate pharmacophore induced and improvement in activity over that observed with either drug component seperatly.