Human African Trypanosomiasis (HAT) is an endemic protozoan disease widespread in the sub-Saharan region that is caused by T. b. gambiense and T. b. rhodesiense. The development of molecules targeting rhodesain, the main cysteine protease of T. b. rhodesiense, has led to a panel of inhibitors endowed with micro/sub-micromolar activity towards the protozoa. However, whilst impressive binding affinity against rhodesain has been observed, the limited selectivity towards the target still remains a hard challenge for the development of antitrypanosomal agents. In this paper, we report the synthesis, biological evaluation, as well as docking studies of a series of reduced peptide bond pseudopeptide Michael acceptors (SPR10–SPR19) as potential anti-HAT agents. The new molecules show K_i values in the low-micro/sub-micromolar range against rhodesain, coupled with k_2nd values between 1314 and 6950 M⁻¹ min⁻¹. With a few exceptions, an appreciable selectivity over human cathepsin L was observed. In in vitro assays against T. b. brucei cultures, SPR16 and SPR18 exhibited single-digit micromolar activity against the protozoa, comparable to those reported for very potent rhodesain inhibitors, while no significant cytotoxicity up to 70 µM towards mammalian cells was observed. The discrepancy between rhodesain inhibition and the antitrypanosomal effect could suggest additional mechanisms of action. The biological characterization of peptide inhibitor SPR34 highlights the essential role played by the reduced bond for the antitrypanosomal effect. Overall, this series of molecules could represent the starting point for further investigations of reduced peptide bond-containing analogs as potential anti-HAT agents

Rhodesain is a cysteine protease that is crucial for the life cycle of Trypanosoma brucei rhodesiense, a parasite causing the lethal form of Human African Trypanosomiasis. CD24 is a recently developed synthetic inhibitor of rhodesain, characterized by a nanomolar affinity towards the trypanosomal protease (K_i = 16 nM), and acting as a competitive inhibitor. In the present work, we carried out a combination study of CD24 with curcumin, the multitarget nutraceutical obtained from Curcuma longa L., which we demonstrated to inhibit rhodesain in a non-competitive manner. By applying the Chou and Talalay method, we obtained an initial additive effect at IC₅₀ (f_a = 0.5, Combination Index = 1), while for the most relevant f_a values, ranging from 0.6 to 1, i.e., from 60% to 100% of rhodesain inhibition, we obtained a combination index < 1, thus suggesting that an increasingly synergistic action occurred for the combination of the synthetic inhibitor CD24 and curcumin. Furthermore, the combination of the two inhibitors showed an antitrypanosomal activity better than that of CD24 alone (EC₅₀ = 4.85 µM and 10.1 µM for the combination and CD24, respectively), thus suggesting the use of the two inhibitors in combination is desirable.

Bacteriophages (phages) play essential roles in microbial systems, yet most phage proteins remain poorly characterised. Protein tertiary and quaternary structure information contributes valuable information about protein function. As many phage proteins function as homooligomers, complexes that consist of multiple identical subunits, there is great interest in computationally predicting their configurations. Here we present a computational framework, the Phage Homomer Level Estimate and Generation Method (PHLEGM) for inferring homooligomeric states directly from the protein sequence by combining AlphaFold-Multimer modelling with inter-subunit interface quality assessment. We proceeded to experimentally validate two out of nine predicted homooligomers using size exclusion chromatography and complementary hydrodynamic techniques. These efforts confirmed our predictions for a dimer and a trimer, highlighting the value of experimentally benchmarked computational predictions and showing the challenges of heterologous phage protein production. Applied to >22,000 phage protein sequences in the PHROGs database, our approach revealed extensive diversity in phage homooligomeric protein complexes. Benchmarking against protein language model-based predictors on a curated reference set of known phage homooligomers demonstrated superior accuracy of our structure-based method, achieving robust performance in classifying protein homooligomeric states, with the highest accuracy observed for trimers and higher-order complexes. These results highlight the value of computational predictions to decipher the complexities of the vast viral sequence space. All predicted complex structures and functional inferences are made publicly available to support structural and functional studies of phage proteins.

In recent decades, neglected tropical diseases and poverty-related diseases have become a serious health problem worldwide. Among these pathologies, human African trypanosomiasis, and malaria present therapeutic problems due to the onset of resistance, toxicity problems and the limited spectrum of action. In this drug discovery process, rhodesain and falcipain-2, of Trypanosoma brucei rhodesiense and Plasmodium falciparum, are currently considered the most promising targets for the development of novel antitrypanosomal and antiplasmodial agents, respectively. Therefore, in our study we identified a novel lead-like compound, i.e., inhibitor 2b, which we proved to be active against both targets, with a K_i = 5.06 µM towards rhodesain and an IC₅₀ = 40.43 µM against falcipain-2.