The development of drugs for neglected infectious diseases often uses parasite-specific enzymes as targets. We here demonstrate that parasite enzymes with highly conserved human homologs may represent a promising reservoir of new potential drug targets. The cyclic nucleotide-specific phosphodiesterases (PDEs) of Trypanosoma brucei, causative agent of the fatal human sleeping sickness, are essential for the parasite. The highly conserved human homologs are well-established drug targets. We here describe what is to our knowledge the first pharmacological validation of trypanosomal PDEs as drug targets. High-throughput screening of a proprietary compound library identified a number of potent hits. One compound, the tetrahydrophthalazinone compound A (Cpd A), was further characterized. It causes a dramatic increase of intracellular cyclic adenosine monophosphate (cAMP). Short-term cell viability is not affected, but cell proliferation is inhibited immediately, and cell death occurs within 3 days. Cpd A prevents cytokinesis, resulting in multinucleated, multiflagellated cells that eventually lyse. These observations pharmacologically validate the highly conserved trypanosomal PDEs as potential drug targets.

Trypanosomal phosphodiesterases B1 and B2 (TbrPDEB1 and TbrPDEB2) play an important role in the life cycle of Trypanosoma brucei, the causative parasite of human African trypanosomiasis (HAT), also known as African sleeping sickness. We used homology modeling and docking studies to guide fragment growing into the parasite-specific P-pocket in the enzyme binding site. The resulting catechol pyrazolinones act as potent TbrPDEB1 inhibitors with IC₅₀ values down to 49 nM. The compounds also block parasite proliferation (e.g., VUF13525 (20b): T. brucei rhodesiense IC₅₀ = 60 nM, T. brucei brucei IC₅₀ = 520 nM, T. cruzi = 7.6 μM), inducing a typical multiple nuclei and kinetoplast phenotype without being generally cytotoxic. The mode of action of 20b was investigated with recombinantly engineered trypanosomes expressing a cAMP-sensitive FRET sensor, confirming a dose-response related increase of intracellular cAMP levels in trypanosomes. Our findings further validate the TbrPDEB family as antitrypanosomal target.

No abstract

Protozoan infections remain a major unsolved medical problem in many parts of our world. A major obstacle to their treatment is the blatant lack of medication that is affordable, effective, safe and easy to administer. For some of these diseases, including human sleeping sickness, very few compounds are available, many of them old and all of them fraught with toxic side effects. We explore a new concept for developing new-generation antiprotozoan drugs that are based on phosphodiesterase (PDE) inhibitors. Such inhibitors are already used extensively in human pharmacology. Given the high degree of structural similarity between the human and the protozoan PDEs, the vast expertise available in the human field can now be applied to developing disease-specific PDE inhibitors as new antiprotozoan drugs.

It is well known that both histamine and dimaprit show moderate histamine H2-receptor agonistic activities on the guinea pig right atrium. Quantum chemical calculations on these two compounds showed similarities in electron distributions and molecular electrostatic potentials (MEP's), which could be extended to rigid analogues [2-amino-5-(2-aminoethyl)thiazoles] of the latter structure. On the base of these results a series of substituted 4- and 5-(2-aminoethyl)thiazoles was synthesized applying small alkyl substitution variations as reported for histamine. 2-Amino-5-(2-aminoethyl)-4-methylthiazole (Amthamine) proved to be the most potent full histamine H2-receptor agonist on the guinea pig right atrium, being with a pD2 value of 6.21 slightly more potent than histamine. This compound shows no affinity for H1-receptors and is a full but weak agonist on the histamine H3-receptor with a pD2 value of 4.70, thus showing a marked specificity for histamine H2-receptors. In the 5-(2-aminoethyl)thiazole series the presence of a 2-amino substituent proved to be not essential for stimulation of the histamine H2-receptor, leading to the important conclusion that in contrast to histamine, for this series, acceptance of a proton by the thiazole nucleus of the agonist from the active site of the receptor is sufficient for the stimulation of the histamine H2-receptor.