Design and Synthesis of Inhibitors of Dihydroorotate Dehydrogenase as Novel Anti-infectives

Malaria remains a large burden in many areas of the world, with millions of deaths caused by the Plasmodium parasite each year. There are several existing treatments for malaria but none are perfect, especially with the increasing prevalence of resistant strains.
Dihydroorotate dehydrogenase (DHODH) is an enzyme in the de novo pyrimidine biosynthesis pathway. It is an attractive target for Plasmodium parasites in particular, since these species lack a salvage pathway and therefore rely solely on the de novo pathway.
In this project, a potent and selective series of inhibitors based on a 1,8-naphthyridine scaffold has been developed. Once a reliable synthetic route to the desired 1,8-naphthyridines had been established, a library of compounds was synthesised, allowing the series to be optimised with special focus on pharmacokinetic properties.
Initial compounds showed excellent selectivity and potency against both DHODH and Plasmodium parasite but solubility in fasted-state simulated intestinal fluid (FaSSIF) was quite poor and metabolism by aldehyde oxidase was detected. Subsequently, compounds were made which overcame this metabolic liability while retaining excellent potency (IC50 < 20 nM on P. falciparum DHODH, EC50 < 10 nM on Plasmodium cells) and with an improvement in solubility (> 50 µM in FaSSIF). These lead compounds will be taken forward for further assays to assess whether they could be used as novel anti-malarial drugs.
Toxoplasma gondii, the causative agent of toxoplasmosis, was also targeted with this series of molecules. However, T. gondii DHODH has a mutation which results in lower levels of inhibition with these 1,8-naphthyridines. By using computational design, potency against T. gondii was increased, albeit not to as high a level as against P. falciparum, with the best inhibitors for this species having EC50 values around 500 nM. The tight binding site in this species made the design of potent inhibitors challenging.