Apicomplexan parasites, including Plasmodium falciparum and Toxoplasma gondii, the causative agents of severe malaria and toxoplasmosis, respectively, undergo several critical developmental transitions during their lifecycle. Most important for human pathogenesis is the asexual cycle, in which parasites undergo rounds of host cell invasion, replication, and egress (exit), destroying host cell tissue in the process. Previous work has identified important roles for Protein Kinase G (PKG) and Protein Kinase A (PKA) in parasite egress and invasion, yet little is understood about the regulation of cyclic nucleotides, cGMP and cAMP, that activate these enzymes. To address this, we have focused upon the development of inhibitors of 3',5'-cyclic nucleotide phosphodiesterases (PDEs) to block the breakdown of cyclic nucleotides. This was done by repurposing human PDE inhibitors noting various similarities of the human and apicomplexan PDE binding sites. The most potent inhibitors blocked the in vitro proliferation of P. falciparum and T. gondii more potently than the benchmark compound zaprinast. 5-Benzyl-3-isopropyl-1H-pyrazolo[4,3-d]pyrimidin-7(6H)-one (BIPPO) was found to be a potent inhibitor of recombinant P. falciparum PfPDEα and activated PKG-dependent egress of T. gondii and P. falciparum, likely by promoting the exocytosis of micronemes, an activity that was reversed by a specific Protein Kinase G inhibitor. BIPPO also promotes cAMP-dependent phosphorylation of a P. falciparum ligand critical for host cell invasion, suggesting that the compound inhibits single or multiple PDE isoforms that regulate both cGMP and cAMP levels. BIPPO is therefore a useful tool for the dissection of signal transduction pathways in apicomplexan parasites.
The similarity between Plasmodium falciparum phosphodiesterase enzymes (PfPDEs) and their human counterparts have been examined and human PDE9A was found to be a suitable template for the construction of homology models for each of the four PfPDE isoforms. In contrast, the architecture of the active sites of each model was most similar to human PDE1. Molecular docking was able to model cyclic guanosine monophosphate (cGMP) substrate binding in each case but a docking mode supporting cyclic adenosine monophosphate (cAMP) binding could not be found. Anticipating the potential of PfPDE inhibitors as anti-malarial drugs, a range of reported PDE inhibitors including zaprinast and sildenafil were docked into the model of PfPDEα. The results were consistent with their reported biological activities, and the potential of PDE1/9 inhibitor analogues was also supported by docking.
Recent results using animal models of inflammatory skin conditions have shown that blockers of the voltage-gated potassium channel, Kv1.3 hold great promise for clinical utility. Kv1.3 blockers act as immunosuppressants by modulating the various subsets of inflammatory T and B cells involved in autoimmune disorders. While peptidic inhibitors based on naturally occurring venoms demonstrate potent and selective Kv1.3 blockade, these require parenteral administration and may face potential immunogenicity problems. Small molecule blockers show considerable diversity, however selectivity over other Kv1- family channels has been difficult to achieve. More recent advances have added to the evidence that Kv1.3 channels are a suitable therapeutic target and that the development of novel and selective agents will herald new drugs for inflammatory skin disorders.
Diphenoxylate, a well-known opioid agonist and anti-diarrhoeal agent, was recently found to block Kv1.3 potassium channels, which have been proposed as potential therapeutic targets for a range of autoimmune diseases. The molecular basis for this Kv1.3 blockade was assessed by the selective removal of functional groups from the structure of diphenoxylate as well as a number of other structural variations. Removal of the nitrile functional group and replacement of the C-4 piperidinyl substituents resulted in several compounds with submicromolar IC50 values.
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