Antimalarial activity of novel 4-cyano-3-methylisoquinoline inhibitors against Plasmodium falciparum: design, synthesis and biological evaluation.

Central to malaria pathogenesis is the invasion of human red blood cells by Plasmodium falciparum parasites. Following each cycle of intracellular development and replication, parasites activate a cellular program to egress from their current host cell and invade a new one. The orchestration of this process critically relies upon numerous organised phospho-signaling cascades, which are mediated by a number of central kinases. Parasite kinases are emerging as novel antimalarial targets as they have diverged sufficiently from their mammalian counterparts to allow selectable therapeutic action. Parasite protein kinase A (PfPKA) is highly expressed late in the cell cycle of the parasite blood stage and has been shown to phosphorylate a critical invasion protein, Apical Membrane Antigen 1. This enzyme could therefore be a valuable drug target so we have repurposed a substituted 4-cyano-3-methylisoquinoline that has been shown to inhibit rat PKA with the goal of targeting PfPKA. We synthesised a novel series of compounds and, although many potently inhibit the growth of chloroquine sensitive and resistant strains of P. falciparum, they were found to have minimal activity against PfPKA, indicating that they likely have another target important to parasite cytokinesis and invasion.

Exploration of 3-methylisoquinoline-4-carbonitriles as protein kinase A inhibitors of Plasmodium falciparum.

A series of isoquinolines have been evaluated in a homology model of Plasmodium falciparum Protein Kinase A (PfPKA) using molecular dynamics. Synthesis of these compounds was then undertaken to investigate their structure-activity relationships. One compound was found to inhibit parasite growth in an in vitro assay and provides a lead to further develop 3-methylisoquinoline-4-carbonitriles as antimalarial compounds. Development of a potent and selective PfPKA inhibitor would provide a useful tool to shed further insight into the mechanisms enabling malaria parasites to establish infection.

Specific phosphorylation of the PfRh2b invasion ligand of Plasmodium falciparum.

Red blood cell invasion by the malaria parasite Plasmodium falciparum relies on a complex protein network that uses low and high affinity receptor-ligand interactions. Signal transduction through the action of specific kinases is a control mechanism for the orchestration of this process. In the present study we report on the phosphorylation of the CPD (cytoplasmic domain) of P. falciparum Rh2b (reticulocyte homologue protein 2b). First, we identified Ser3233 as the sole phospho-acceptor site in the CPD for in vitro phosphorylation by parasite extract. We provide several lines of evidence that this phosphorylation is mediated by PfCK2 (P. falciparum casein kinase 2): phosphorylation is cAMP independent, utilizes ATP as well as GTP as phosphate donors, is inhibited by heparin and tetrabromocinnamic acid, and is mediated by purified PfCK2. We raised a phospho-specific antibody and showed that Ser3233 phosphorylation occurs in the parasite prior to host cell egress. We analysed the spatiotemporal aspects of this phosphorylation using immunoprecipitated endogenous Rh2b and minigenes expressing the CPD either at the plasma or rhoptry membrane. Phosphorylation of Rh2b is not spatially restricted to either the plasma or rhoptry membrane and most probably occurs before Rh2b is translocated from the rhoptry neck to the plasma membrane.

Evolution and architecture of the inner membrane complex in asexual and sexual stages of the malaria parasite.

The inner membrane complex (IMC) is a unifying morphological feature of all alveolate organisms. It consists of flattened vesicles underlying the plasma membrane and is interconnected with the cytoskeleton. Depending on the ecological niche of the organisms, the function of the IMC ranges from a fundamental role as reinforcement system to more specialized roles in motility and cytokinesis. In this article, we present a comprehensive evolutionary analysis of IMC components, which exemplifies the adaptive nature of the IMCs' protein composition. Focusing on eight structurally distinct proteins in the most prominent "genus" of the Alveolata-the malaria parasite Plasmodium-we demonstrate that the level of conservation is reflected in phenotypic characteristics, accentuated in differential spatial-temporal patterns of these proteins in the motile stages of the parasite's life cycle. Colocalization studies with the centromere and the spindle apparatus reveal their discriminative biogenesis. We also reveal that the IMC is an essential structural compartment for the development of the sexual stages of Plasmodium, as it seems to drive the morphological changes of the parasite during the long and multistaged process of sexual differentiation. We further found a Plasmodium-specific IMC membrane matrix protein that highlights transversal structures in gametocytes, which could represent a genus-specific structural innovation required by Plasmodium. We conclude that the IMC has an additional role during sexual development supporting morphogenesis of the cell, which in addition to its functions in the asexual stages highlights the multifunctional nature of the IMC in the Plasmodium life cycle.

Optimization of 2-Anilino 4-Amino Substituted Quinazolines into Potent Antimalarial Agents with Oral in Vivo Activity.

Novel antimalarial therapeutics that target multiple stages of the parasite lifecycle are urgently required to tackle the emerging problem of resistance with current drugs. Here, we describe the optimization of the 2-anilino quinazoline class as antimalarial agents. The class, identified from publicly available antimalarial screening data, was optimized to generate lead compounds that possess potent antimalarial activity against P. falciparum parasites comparable to the known antimalarials, chloroquine and mefloquine. During the optimization process, we defined the functionality necessary for activity and improved in vitro metabolism and solubility. The resultant lead compounds possess potent activity against a multidrug resistant strain of P. falciparum and arrest parasites at the ring phase of the asexual stage and also gametocytogensis. Finally, we show that the lead compounds are orally efficacious in a 4 day murine model of malaria disease burden.

Hierarchical phosphorylation of apical membrane antigen 1 is required for efficient red blood cell invasion by malaria parasites.

Central to the pathogenesis of malaria is the proliferation of Plasmodium falciparum parasites within human erythrocytes. Parasites invade erythrocytes via a coordinated sequence of receptor-ligand interactions between the parasite and host cell. One key ligand, Apical Membrane Antigen 1 (AMA1), is a leading blood-stage vaccine and previous work indicates that phosphorylation of its cytoplasmic domain (CPD) is important to its function during invasion. Here we investigate the significance of each of the six available phospho-sites in the CPD. We confirm that the cyclic AMP/protein kinase A (PKA) signalling pathway elicits a phospho-priming step upon serine 610 (S610), which enables subsequent phosphorylation in vitro of a conserved, downstream threonine residue (T613) by glycogen synthase kinase 3 (GSK3). Both phosphorylation steps are required for AMA1 to function efficiently during invasion. This provides the first evidence that the functions of key invasion ligands of the malaria parasite are regulated by sequential phosphorylation steps.