Phosphodiesterase-inspired approaches in antiparasitic drug discovery
Authors:   , Zheng Yang 
Published: September 2023

With twenty Neglected tropical diseases on the list of WHO, the total infection worldwide comes to about one billion cases, with 1 out of every six persons worldwide experiencing one or more infections each year. In DALYs (Disability Adjusted Life Years), this adds up to 25 million, with over 500,000 deaths annually. The high disease burden combined with the threat of drug resistance calls for urgent and continuous drug discovery. The work presented in this thesis was performed under the scope of the EU-founded consortium PDE4NPD with an aim to develop novel treatments for two neglected tropical diseases (namely human African trypanosomiasis and schistosomiasis) and giardiasis.
Chapter 2 describes the discovery of a potent T. brucei inhibitor NPD-2975, starting from a known PDE inhibitor BIPPO. With modifications at the benzyl position of BIPPO, analogues with a 4-fluorophenyl substituent exhibited high potency against T. brucei. Although the mode of action turned out to be not related to PDE inhibition, the high potency and physiochemical properties of NPD-2975 resulted in successful parasite clearance in an acute mouse infection model.
Chapter 3 describes the optimization of NPD-2975. Taking NPD-2975 as a starting point, we went through a thorough SAR exploration of this compound. By introducing a methyl group at different positions of NPD-2975, we identified R2 as the only position to improve solubility without compromising antitrypanosomal activity. More importantly, modification of the R4 position yielded analogue NPD-3519, which shows improved activity against T. brucei and better in vitro metabolic stability. Further pharmacokinetic and in vivo results proved this compound as a promising candidate for further drug discovery programs.
Chapter 4 describes a scaffold merging project targeting G. lamblia. Since GlPDE has been proposed as a promising target for G. lamblia, in this project we hypothesized that merging the structural elements of phosphodiesterase inhibitors and a nitroimidazole scaffold would lead to increased (nitrosative) stress. Although most compounds exhibited nanomolar IC50 values against G. lamblia in a phenotypic assay, their mode of action turned out not to be GlPDE related after testing some compounds without nitro groups. However, their good drug-like properties together with nanomolar anti-Giardia activities prove 3-nitroimidazo[1,2-b]pyridazines as promising candidates for further lead optimization studies.
Chapter 5 describes a drug repurposing strategy targeting SmPDE4A. Following the target validation work of Thavy Long et al., we purified SmPDE4A protein, obtained X-ray structures for its catalytic domain and profiled our in-house library of human PDE inhibitors as SmPDE4A inhibitors (PDE toolbox). This screening identified two chemical classes, phthalazinone (NPD-0001) and benzamide (approved human PDE4 inhibitors piclamilast and roflumilast). As a follow-up, a detailed SAR exploration of 83 roflumilast analogues proves it challenging to improve the potency of roflumilast against SmPDE4A. Furthermore, an in vitro assay of roflumilast and NPD-0001 showed only little effect on worm killing but affected the egg-laying behavior at high micromolar concentrations. In total, the results with these SmPDE4A inhibitors suggest that SmPDE4A is not a suitable target for an effective anti-schistosomiasis therapy.