Antimalarial imidazopyridazines and aminopyrazines: synthesis, physicochemical optimization and structure-activity relationships

According to the World Health Organization (WHO) world malaria report released in 2017, about 445,000 malaria deaths were recorded in 2016, a similar mortality as that recorded in the preceding year (446,000 deaths in 2015). Once effective and cheap drugs such as chloroquine and sulfadoxine-pyrimethamine have suffered widespread drug resistance. Additionally, despite the remarkable effectiveness of the currently recommended first line treatment, the artemisinin combination therapies (ACTs), resistance to artemisinin and the partner drugs is beginning to emerge in South East Asia. Furthermore, the current portfolio of medicines, both in clinical use and development has several other shortfalls which need redress. In addition, prevention of transmission and relapse with better safety profiles than current medicines are some of the important features that should be a prioritized characteristic of new medicines. Most importantly, these new regimens should be able to offer chemoprotection and prevent reinfection. Thus, there is need for constant research efforts aimed at identifying and developing novel chemotherapeutic agents for malaria, which are structurally diverse with novel mechanisms of action. In this PhD thesis, the medicinal chemistry optimization of two antimalarial chemotypes, the imidazopyridazines and aminopyrazines, is reported. In earlier studies, Le Manach and coworkers reported the impressive in vitro antiplasmodial activity and in vivo antimalarial efficacy of the imidazopyridazine lead compound 19 (Figure 1). However, this compound was plagued by poor solubility and a cardiotoxicity risk as shown from its inhibition of the hERG (human ether-a-go-go-related gene)-encoded potassium channel. Further medicinal chemistry optimization led to identification of other derivatives which, albeit exhibiting complete cure of P. berghei-infected mice, still displayed poor solubility and hERG inhibition issues. In this project, chemical modification approaches such as the introduction of water solubilizing groups, disruption of molecular planarity and making subtle changes (SAR 1 – 6, Figure 1) were adopted towards improving the solubility and countering hERG inhibition of this class of molecules. Through the thesis work undertaken, analogues with a combination of reduced hERG inhibition (IC50 = 7.8 – 32 μM) and submicromolar antiplasmodial activity (NF54, IC50 = 0.15 – 0.92 μM) were identified. Likewise, the modifications made delivered analogues with moderate to high solubility (60 – 200 μM) while exhibiting submicromolar antiplasmodial potency (NF54, IC50 = 0.14 – 0.99 μM). Furthermore, cytotoxicity assessment of selected analogues against the Chinese Hamster Ovarian (CHO) cell line revealed that most analogues were relatively noncytotoxic (selectivity indices in the range 72 - > 874). Selected compounds were also screened against gametocyte and liver stage parasites in order to assess transmission blocking and chemoprotection potential, respectively. In this regard, analogues with good gametocytocidal activity (IC50 = 0.098 – 0.75 μM) against late stage gametocytes and potent liver stage activity (IC50 = 0.045 μM) were identified. On the other hand, aminopyrazines have also recently shown potential as new antimalarial agents exhibiting promising in vivo efficacy in animal models of malaria infection with one analogue having progressed to an optimised late lead stage. However, this aminopyrazine lead compound 24 (Figure 2) as well as the first generation aminopyridine human Phase 2a clinical candidate MMV390048 showed sub-optimal solubility. In this aspect of the project, chemical modifications mainly focusing on replacing the two aromatic rings with fully and partially saturated heterocyclic systems, hypothesized to potentially disrupt intermolecular π – π stacking thereby improving aqueous solubility, were introduced. The first set of analogues corresponded to the replacement of the trifluoromethylpyridyl ring with partially and fully saturated heterocyclic rings as well as the 4-carboxyphenyl ring while keeping the 4- methylsulfonylphenyl group on the right-hand side portion of the aminopyrazine core scaffold fixed (SAR 1). In SAR 2, the trifluoromethylpyridyl group was fixed on the left-hand side of the aminopyrazine core scaffold while introducing partial and full saturation on the right-hand side of the core. SAR 3 analogues with both aromatic groups simultaneously replaced with partially and fully saturated heterocyclic rings were further generated. Compared to the lead compound 24 (NF54, IC50 = 0.008 μM), the introduced modifications drastically reduced antiplasmodial potency with only one analogue retaining submicromolar activity (NF54, IC50 = 0.51 μM). However, the introduced molecular features positively influenced solubility with the new analogues showing 4 - > 20-fold increase in aqueous solubility compared to the lead compound 24. For both imidazopyridazines and aminopyrazines, docking studies on a homology model of PfPI4K (P. falciparum phosphatidylinositol 4 kinase) were retrospectively undertaken. In both cases, the docking experiments showed that the introduction of the new molecular features was accompanied by loss of key binding interactions to the ATP binding pocket. This was in conformity with the generated parasite-based SAR