Structure-Activity Relationship and Target Investigation of Thiophen-2-yl-Pyrimidines against Schistosoma species

Chemotherapeutic options for schistosomiasis, a prevalent infectious disease of poverty, are limited to just one drug, praziquantel (PZQ), and alternatives are needed. Our previous studies identified thiophen-2-yl pyrimidines (TPPs), which are structurally derived from microtubule (MT)-active phenylpyrimidines, as potent paralytics of Schistosoma mansoni. Although relatively non-toxic to mammalian cells, the progenitor compound, 3, had poor aqueous solubility and was lipophilic potentially hindering preclinical advancement. To address these issues and expand on the structure-activity and structure-property relationships, 43 new TPP analogs were designed and synthesized, their lipophilicity calculated (cLogP), and their anti-schistosomal activity evaluated in culture. This effort yielded compound 38, which possessed an oxetane-containing amine moiety at C5, and an ortho, ortho-difluoroaniline at C6 of the TPP scaffold. Compared to 3, compound 38 had better aqueous solubility (46 vs. < 0.5 μM), decreased lipophilicity (cLogP 4.48 vs. 6.81), a 14.5-fold increase in paralytic potency against adult S. mansoni (EC₅₀ = 37 vs. 538 nM), and limited toxicity (CC₅₀ > 20 μM) to three mammalian cell lines. In mice, 38 demonstrated a 3-fold longer plasma half-life (t_1/2; 1.51 vs. 0.48 h) for a 40% loss in maximum plasma concentration (C_max). In washout experiments, 38 produced a sustained paralysis of both juvenile and adult S. mansoni, possibly suggesting a broader in vivo efficacy spectrum compared to PZQ, which is inactive against the juvenile Parasite. The two Other medically important species, Schistosoma haematobium and Schistosoma japonicum, were also susceptible to 38. Finally, to identify potential protein targets, we synthesized a TPP photoaffinity labeling (PAL) probe that labeled several S. mansoni proteins by SDS-PAGE fluorescence analysis, although, notably, not tubulin, suggesting that the antischistosomal activity of 38 is a function of engaging Other targets. Future work with the TPP series will aim to decrease toxicity further and improve pharmacokinetic properties to better support in vivo efficacy testing.