1NM-PP1 treatment of mice infected with Toxoplasma gondii

Drugs in development for toxoplasmosis: advances, challenges, and current status

Toxoplasma gondii causes fatal and debilitating brain and eye diseases. Medicines that are currently used to treat toxoplasmosis commonly have toxic side effects and require prolonged courses that range from weeks to more than a year. The need for long treatment durations and the risk of relapsing disease are in part due to the lack of efficacy against T. gondii tissue cysts. The challenges for developing a more effective treatment for toxoplasmosis include decreasing toxicity, achieving therapeutic concentrations in the brain and eye, shortening duration, eliminating tissue cysts from the host, safety in pregnancy, and creating a formulation that is inexpensive and practical for use in resource-poor areas of the world. Over the last decade, significant progress has been made in identifying and developing new compounds for the treatment of toxoplasmosis. Unlike clinically used medicines that were repurposed for toxoplasmosis, these compounds have been optimized for efficacy against toxoplasmosis during preclinical development. Medicines with enhanced efficacy as well as features that address the unique aspects of toxoplasmosis have the potential to greatly improve toxoplasmosis therapy. This review discusses the facets of toxoplasmosis that are pertinent to drug design and the advances, challenges, and current status of preclinical drug research for toxoplasmosis.

A Systematic Review of In vitro and In vivo Activities of Anti-Toxoplasma Drugs and Compounds (2006-2016)

The currently available anti-Toxoplasma agents have serious limitations. This systematic review was performed to evaluate drugs and new compounds used for the treatment of toxoplasmosis. Data was systematically collected from published papers on the efficacy of drugs/compounds used against Toxoplasma gondii (T. gondii) globally during 2006-2016. The searched databases were PubMed, Google Scholar, Science Direct, ISI Web of Science, EBSCO, and Scopus. One hundred and eighteen papers were eligible for inclusion in this systematic review, which were both in vitro and in vivo studies. Within this review, 80 clinically available drugs and a large number of new compounds with more than 39 mechanisms of action were evaluated. Interestingly, many of the drugs/compounds evaluated against T. gondii act on the apicoplast. Therefore, the apicoplast represents as a potential drug target for new chemotherapy. Based on the current findings, 49 drugs/compounds demonstrated in vitro half-maximal inhibitory concentration (IC50) values of below 1 μM, but most of them were not evaluated further for in vivo effectiveness. However, the derivatives of the ciprofloxacin, endochin-like quinolones and 1-[4-(4-nitrophenoxy) phenyl] propane-1-one (NPPP) were significantly active against T. gondii tachyzoites both in vitro and in vivo. Thus, these compounds are promising candidates for future studies. Also, compound 32 (T. gondii calcium-dependent protein kinase 1 inhibitor), endochin-like quinolones, miltefosine, rolipram abolish, and guanabenz can be repurposed into an effective anti-parasitic with a unique ability to reduce brain tissue cysts (88.7, 88, 78, 74, and 69%, respectively). Additionally, no promising drugs are available for congenital toxoplasmosis. In conclusion, as current chemotherapy against toxoplasmosis is still not satisfactory, development of well-tolerated and safe specific immunoprophylaxis in relaxing the need of dependence on chemotherapeutics is a highly valuable goal for global disease control. However, with the increasing number of high-risk individuals, and absence of a proper vaccine, continued efforts are necessary for the development of novel treatment options against T. gondii. Some of the novel compounds reviewed here may represent good starting points for the discovery of effective new drugs. In further, bioinformatic and in silico studies are needed in order to identify new potential toxoplasmicidal drugs.

Review of Experimental Compounds Demonstrating Anti-Toxoplasma Activity

Toxoplasma gondii is a ubiquitous apicomplexan parasite capable of infecting humans and other animals. Current treatment options for T. gondii infection are limited and most have drawbacks, including high toxicity and low tolerability. Additionally, no FDA-approved treatments are available for pregnant women, a high-risk population due to transplacental infection. Therefore, the development of novel treatment options is needed. To aid this effort, this review highlights experimental compounds that, at a minimum, demonstrate inhibition of in vitro growth of T. gondii When available, host cell toxicity and in vivo data are also discussed. The purpose of this review is to facilitate additional development of anti-Toxoplasma compounds and potentially to extend our knowledge of the parasite.

Identification of Direct Kinase Substrates Using Analogue-Sensitive Alleles

Identifying the substrates of protein kinases remains a major obstacle in the elucidation of eukaryotic signaling pathways. Promiscuity among kinases and their substrates coupled with the extraordinary plasticity of phosphorylation networks renders traditional genetic approaches or small-molecule inhibitors problematic when trying to determine the direct substrates of an individual kinase. Here we describe methods to label, enrich, and identify the direct substrates of analogue-sensitive kinases by exploiting their steric complementarity to artificial ATP analogues. Using calcium-dependent protein kinases of Toxoplasma gondii as a model for these approaches, this protocol brings together numerous advances that enable labeling of kinase targets in semi-permeabilized cells, quantification of direct labeling over background, and highly specific enrichment of targeted phosphopeptides.

Evaluation of the enzyme activity of protozoan protein kinases by using an in vitro kinase assay

The life cycles of parasites are more complicated than those of other biological species. Protein kinases (PKs) encoded by parasites are the main triggers of life stage conversions. Phosphorylation by cellular PKs regulates important cellular processes, and the protozoan genome contains many PKs. Some PK inhibitors inhibit specific parasite life cycle event. In this report, I present a practical approach to expressing and purifying protozoan PKs by using a wheat germ cell-free protein synthesis system and I assess the phosphorylation activities of protozoan PKs by using an in vitro kinase assay.

In Vitro and In Vivo Effects of the Bumped Kinase Inhibitor 1294 in the Related Cyst-Forming Apicomplexans Toxoplasma gondii and Neospora caninum

We report on the in vitro effects of the bumped kinase inhibitor 1294 (BKI-1294) in cultures of virulent Neospora caninum isolates Nc-Liverpool (Nc-Liv) and Nc-Spain7 and in two strains of Toxoplasma gondii (RH and ME49), all grown in human foreskin fibroblasts. In these parasites, BKI-1294 acted with 50% inhibitory concentrations (IC50s) ranging from 20 nM (T. gondii RH) to 360 nM (N. caninum Nc-Liv), and exposure of intracellular stages to 1294 led to the nondisjunction of newly formed tachyzoites, resulting in the formation of multinucleated complexes similar to complexes previously observed in BKI-1294-treated N. caninum beta-galactosidase-expressing parasites. However, such complexes were not seen in a transgenic T. gondii strain that expressed CDPK1 harboring a mutation (G to M) in the gatekeeper residue. In T. gondii ME49 and N. caninum Nc-Liv, exposure of cultures to BKI-1294 resulted in the elevated expression of mRNA coding for the bradyzoite marker BAG1. Unlike in bradyzoites, SAG1 expression was not repressed. Immunofluorescence also showed that these multinucleated complexes expressed SAG1 and BAG1 and the monoclonal antibody CC2, which binds to a yet unidentified bradyzoite antigen, also exhibited increased labeling. In a pregnant mouse model, BKI-1294 efficiently inhibited vertical transmission in BALB/c mice experimentally infected with one of the two virulent isolates Nc-Liv or Nc-Spain7, demonstrating proof of concept that this compound protected offspring from vertical transmission and disease. The observed deregulated antigen expression effect may enhance the immune response during BKI-1294 therapy and will be the subject of future studies.

Designing selective inhibitors for calcium-dependent protein kinases in apicomplexans

Apicomplexan parasites cause some of the most severe human diseases, including malaria (caused by Plasmodium), toxoplasmosis, and cryptosporidiosis. Treatments are limited by the lack of effective drugs and development of resistance to available agents. By exploiting novel features of protein kinases in these parasites, it may be possible to develop new treatments. We summarize here recent advances in identifying small molecule inhibitors against a novel family of plant-like, calcium-dependent kinases that are uniquely expanded in apicomplexan parasites. Analysis of the 3D structure, activation mechanism, and sensitivity to small molecules had identified several attractive chemical scaffolds that are potent and selective inhibitors of these parasite kinases. Further optimization of these leads may yield promising new drugs for treatment of these parasitic infections.

A single mutation in the gatekeeper residue in TgMAPKL-1 restores the inhibitory effect of a bumped kinase inhibitor on the cell cycle

Toxoplasma gondii is the causative pathogen for Toxoplasmosis. Bumped kinase inhibitor 1NM-PP1 inhibits the growth of T. gondii by targeting TgCDPK1. However, we recently reported that resistance to 1NM-PP1 can be acquired via a mutation in T. gondii mitogen-activated protein kinase like 1 (TgMAPKL-1). Further characterization of how this TgMAPKL-1 mutation restores the inhibitory effect of 1NM-PP1 would shed further light on the function of TgMAPKL-1 in the parasite life cycle. Therefore, we made parasite clones with TgMAPKL-1 mutated at the gatekeeper residue Ser 191, which is critical for 1NM-PP1 susceptibility. Host cell lysis of RH/ku80(-)/HA-TgMAPKL-1(S191A) was completely inhibited at 250 nM 1NM-PP1, whereas that of RH/ku80(-)/HA-TgMAPKL-1(S191Y) was not. By comparing 1NM-PP1-sensitive (RH/ku80(-)/HA-TgMAPKL-1(S191A)) and -resistant (RH/ku80(-)/HA-TgMAPKL-1(S191Y)) clones, we observed that inhibition of TgMAPKL-1 blocked cell cycle progression after DNA duplication. Morphological analysis revealed that TgMAPKL-1 inhibition caused enlarged parasite cells with many daughter cell scaffolds and imcomplete cytokinesis. We conclude that the mutation in TgMAPKL-1 restored the cell cycle-arresting effect of 1NM-PP1 on T. gondii endodyogeny. Given that endodyogeny is the primary mechanism of cell division for both the tachyzoite and bradyzoite stages of this parasite, TgMAPKL-1 may be a promising target for drug development. Exploration of the signals that regulate TgMAPKL-1 will provide further insights into the unique mode of T. gondii cell division.

Microplate assay for screening Toxoplasma gondii bradyzoite differentiation with DUAL luciferase assay

Toxoplasma gondii can differentiate into tachyzoites or bradyzoites. To accelerate the investigation of bradyzoite differentiation mechanisms, we constructed a reporter parasite, PLK/DLUC_1C9, for a high-throughput assay. PLK/DLUC_1C9 expressed firefly luciferase under the bradyzoite-specific BAG1 promoter. Firefly luciferase activity was detected with a minimum of 10(2) parasites induced by pH 8.1. To normalize bradyzoite differentiation, PLK/DLUC_1C9 expressed Renilla luciferase under the parasite's α-tubulin promoter. Renilla luciferase activity was detected with at least 10(2) parasites. By using PLK/DLUC_1C9 with this 96-well format screening system, we found that the protein kinase inhibitor analogs, bumped kinase inhibitors 1NM-PP1, 3MB-PP1, and 3BrB-PP1, had bradyzoite-inducing effects.

Bumped kinase inhibitor 1294 treats established Toxoplasma gondii infection

Toxoplasma gondii is a unicellular parasite that causes severe brain and eye disease. Current drugs for T. gondii are limited by toxicity. Bumped kinase inhibitors (BKIs) selectively inhibit calcium-dependent protein kinases of the apicomplexan pathogens T. gondii, cryptosporidia, and plasmodia. A lead anti-Toxoplasma BKI, 1294, has been developed to be metabolically stable and orally bioavailable. Herein, we demonstrate the oral efficacy of 1294 against toxoplasmosis in vivo.

Prevalence of Toxoplasma gondii infection in rabbits of Korea by serological tests and nested polymerase chain reaction

>This study surveyed the Toxoplasma (T.) gondii infection prevalence in the Korean rabbit population. Rabbits (n=142) were obtained from two breeding farms in the Gongju area, Chungnam Province, and in the Kochang area, Junbuk Province, Korea. Of 142 sera samples analyzed by enzyme-linked immunosorbent assay (ELISA), 15 (10.6%) exhibited T. gondii-specific IgG antibodies, and 1 (0.7%) rabbit harbored T. gondii-specific IgM. Female rabbits (9/84; 10.7%) had a similar T. gondii prevalence to males (6/58; 10.3%). When stratified by age, rabbits aged >1 year had a similar prevalence of T. gondii infection (7/66; 10.6%) to rabbits aged <1 year (8/76; 10.5%). Immunoblotting detected 6 major antigenic bands corresponding to T. gondii-positive sera at 20, 28, 30, 35, 63 and 77 kDa. Nested polymerase chain reaction (PCR) of whole-blood samples detected the T. gondii B1 gene in 23 rabbits (16.2%). All PCR-positive samples corresponded to partial T. gondii B1 gene sequences with 99% homology to a T. gondii sequence deposited in GenBank (accession number EU340874). Female rabbits (13/84; 15.5%) harbored a similar prevalence of T. gondii DNA to males (10/58; 17.2%). Rabbits aged >1 year had a similar prevalence (12/66; 18.2%) of T. gondii infection to rabbits aged <1 year (11/76; 14.5%). No statistically significant differences were observed regarding the prevalences of infection according to sex or age using molecular or serological tests. This study is the first survey using serological tests and nested PCR to analyze the T. gondii prevalence in rabbits in Korea.

Optimizing small molecule inhibitors of calcium-dependent protein kinase 1 to prevent infection by Toxoplasma gondii

Toxoplasma gondii is sensitive to bulky pyrazolo [3,4-d] pyrimidine (PP) inhibitors due to the presence of a Gly gatekeeper in the essential calcium dependent protein kinase 1 (CDPK1). Here we synthesized a number of new derivatives of 3-methyl-benzyl-PP (3-MB-PP, or 1). The potency of PP analogues in inhibiting CDPK1 enzyme activity in vitro (low nM IC(50) values) and blocking parasite growth in host cell monolayers in vivo (low μM EC(50) values) were highly correlated and occurred in a CDPK1-specific manner. Chemical modification of the PP scaffold to increase half-life in the presence of microsomes in vitro led to identification of compounds with enhanced stability while retaining activity. Several of these more potent compounds were able to prevent lethal infection with T. gondii in the mouse model. Collectively, the strategies outlined here provide a route for development of more effective compounds for treatment of toxoplasmosis and perhaps related parasitic diseases.