Structure, histone deacetylase, and antiprotozoal activities of apicidins B and C, congeners of apicidin with proline and valine substitutions.

[structure: see text]. Isolation and structure elucidation of two novel cyclic tetrapeptides that show a variety of potent antiprotozoal activities by reversibly inhibiting HDAC have been reported. These are the new members of a unique family of cyclic tetrapeptides that do not require the electrophilic alpha-epoxyketone moiety of HC-toxin, trapoxin A, or chlamydocin for their potent activities against HDAC and the malarial parasite.

Broad spectrum antiprotozoal agents that inhibit histone deacetylase: structure-activity relationships of apicidin. Part 1.

Apicidin, a natural product recently isolated at Merck, inhibits both mammalian and protozoan histone deacetylases (HDACs). The conversion of apicidin, a nanomolar inhibitor of HDACs, into a series of side-chain analogues that display picomolar enzyme affinity is described within this structure-activity study.

Broad spectrum antiprotozoal agents that inhibit histone deacetylase: structure-activity relationships of apicidin. Part 2.

Recently isolated at Merck, apicidin inhibits both mammalian and protozoan histone deacetylases (HDACs). The conversion of apicidin, a nonselective nanomolar inhibitor of HDACs, into a series of picomolar indole-modified and parasite-selective tryptophan-replacement analogues is described within this structure-activity study.

Synthesis of apicidin-derived quinolone derivatives: parasite-selective histone deacetylase inhibitors and antiproliferative agents.

Apicidin's indole was efficiently converted into a series of N-substituted quinolone derivatives by indole N-alkylation followed by a two-step, one-pot, ozonolysis/aldol condensation protocol. The new quinolones exhibited good parasite selectivity and potency both at the level of their molecular target, histone deacetylase, and in their whole cell antiproliferative activity in vitro.

Apicidin: a novel antiprotozoal agent that inhibits parasite histone deacetylase.

A novel fungal metabolite, apicidin [cyclo(N-O-methyl-L-tryptophanyl-L -isoleucinyl-D-pipecolinyl-L-2-amino-8-oxodecanoyl)], that exhibits potent, broad spectrum antiprotozoal activity in vitro against Apicomplexan parasites has been identified. It is also orally and parenterally active in vivo against Plasmodium berghei malaria in mice. Many Apicomplexan parasites cause serious, life-threatening human and animal diseases, such as malaria, cryptosporidiosis, toxoplasmosis, and coccidiosis, and new therapeutic agents are urgently needed. Apicidin's antiparasitic activity appears to be due to low nanomolar inhibition of Apicomplexan histone deacetylase (HDA), which induces hyperacetylation of histones in treated parasites. The acetylation-deacetylation of histones is a thought to play a central role in transcriptional control in eukaryotic cells. Other known HDA inhibitors were also evaluated and found to possess antiparasitic activity, suggesting that HDA is an attractive target for the development of novel antiparasitic agents.

Selective labeling of intracellular parasite proteins by using ricin.

Studies focused on the synthesis by intracellular parasites of developmentally regulated proteins have been limited due to the lack of a simple method for selectively labeling proteins produced by the parasite. A method has now been developed in which ricin is employed to selectively inhibit host-cell protein synthesis. Ricin is a heterodimer composed of two subunits, a lectin and a glycosidase, and it binds to terminal galactose residues on the cell surface via the lectin. Following endocytosis of the intact molecule, a disulfide bond linking the two subunits is cleaved, and only the glycosidase subunit enters the cytoplasm, where it inhibits cytoplasmic protein synthesis by catalyzing the cleavage of the 28S rRNA. Due to the loss of the receptor-binding lectin subunit, ricin cannot permeate host-cell mitochondria or intracellular parasites, and, therefore, protein synthesis within these compartments continues uninterrupted. This system has been used to selectively label parasite proteins from Eimeria tenella and Toxoplasma gondii by using the avian cell line DU-24. In these cells, mitochondrial protein synthesis was inhibited by using chloramphenicol. The use of the avian rho0 cell line DUS-3 provided an additional advantage, because these cells lack mitochondrial DNA. Therefore, those proteins radiolabeled with [35S]methionine/cysteine in ricin-treated, parasite-infected rho0 cells are exclusively those of the intracellular parasite. This technique should be applicable for studying protein synthesis by other intracellular parasites.

Evidence that a protein kinase enhances amsacrine mediated formation of topoisomerase II-DNA complexes in murine mastocytoma cell nuclei.

Cytoplasmic extracts of K21 murine mastocytoma cells contain a protein factor, distinct from topoisomerases I and II, that facilitates formation of amsacrine-induced topoisomerase II-DNA complexes (PDC) in isolated K21 cell nuclei (Darkin, S.J. and Ralph, R.K. (1988) Biochim. Biophys. Acta 1007, 295-300). The PDC enhancing activity was shown to reside in a protein kinase with specificity for a casein kinase II substrate and sensitive to heparin and anti-casein kinase II antiserum. This appears to be the first direct evidence of a protein factor that modulates amsacrine-induced topoisomerase II action.