Antimalarial Activity of Phenylthiazolyl-Bearing Hydroxamate-Based Histone Deacetylase Inhibitors

Abstract

The antimalarial activity and pharmacology of a series of phenylthiazolyl-bearing hydroxamate-based histone deacetylase inhibitors (HDACIs) was evaluated. In in vitro growth inhibition assays approximately 50 analogs were evaluated against four drug resistant strains of Plasmodium falciparum. The range of 50% inhibitory concentrations (IC 50 s) was 0.0005 to >1 M. Five analogs exhibited IC 50 s of <3 nM, and three of these exhibited selectivity indices of >600. The most potent compound, WR301801 (YC-2-88) was shown to cause hyperacetylation of P. falciparum histones, which is a marker for HDAC inhibition in eukaryotic cells. The compound also inhibited malarial and mammalian HDAC activity in functional assays at low nanomolar concentrations. WR301801 did not exhibit cures in P. berghei-infected mice at oral doses as high as 640 mg/kg/day for 3 days or in P. falciparum-infected Aotus lemurinus lemurinus monkeys at oral doses of 32 mg/kg/day for 3 days, despite high relative bioavailability. The failure of monotherapy in mice may be due to a short half-life, since the compound was rapidly hydrolyzed to an inactive acid metabolite by loss of its hydroxamate group in vitro (half-life of 11 min in mouse microsomes) and in vivo (half-life in mice of 3.5 h after a single oral dose of 50 mg/kg). However, WR301801 exhibited cures in P. berghei-infected mice when combined at doses of 52 mg/kg/day orally with subcurative doses of chloroquine. Next-generation HDACIs with greater metabolic stability than WR301801 may be useful as antimalarials if combined appropriately with conventional antimalarial drugs.Considerable research activity has focused on understanding the "histone code," and in particular on the design of hydroxamate-based histone deacetylase inhibitors (HDACIs) as novel therapeutics for the treatment of a wide range of disorders, including cancer and neurodegenerative diseases (26). HDACIs owe their action to their ability to reactivate silenced genes by modulating the condensation status of DNA. The posttranslational acetylation status of chromatin is determined by the competing activities of two classes of enzymes, histone acetyltransferases (HATs) and histone deacetylases (HDACs) which control the acetylation of lysine residues on histone tails. In general, HATs function to acetylate lysine groups in nuclear histones, resulting in neutralization of the charges on the histones and a more open, transcriptionally active chromatin structure, while the HDACs function to deacetylate and suppress transcription. A shift in the balance of acetylation on chromatin may result in changes in the regulation of patterns of gene expression (16,24,37,39). Since many cancers are associated with aberrant transcriptional activity, and HDACs can affect transcription factors and gene regulation, these enzymes have been identified as attractive targets for cancer therapy. Indeed, chemical inhibitors of HDACs have been shown to inhibit tumor cell growth and induce differentiation and cell death (28). Several such inhibitory agents,...

Lysine Acetylation in Sexual Stage Malaria Parasites Is a Target for Antimalarial Small Molecules

“…We and others have also demonstrated that hyperacetylation of parasite histone and nonhistone proteins and global transcriptional alterations are a functional consequence of HDAC inhibitor exposure in asexual stage parasites (31,32,58,63,69,73,75). This is consistent with a mode of action of this class of compounds that targets HDAC enzymes.…”

“…TSA and SAHA (Fig. 3) are canonical paninhibitors that act on class I/II eukaryotic HDACs 1 to 9 with roughly equal potency (60) and have previously been shown to have potent inhibitory activity against asexual blood-stage P. falciparum parasites (Table 1) (61)(62)(63). The third HDAC inhibitor tested, CAY10603 (Fig.…”

Small-molecule histone methyltransferase inhibitors display rapid antimalarial activity against all blood stage forms inPlasmodium falciparum

“…The HAT inhibitor anacardic acid is also effective against parasites in culture, although with an IC 50 of >30 µM (23). Inhibition of parasite HDACs has provided more potent compounds against parasite growth and proliferation (26)(27)(28)46). For example, parasite killing by apicidin is reported with an IC 90 of 90 nM, and the hydroxamate trichostatin A has been shown to inhibit parasite growth with an IC 50 of ∼10 nM (47).…”

“…For example, parasite killing by apicidin is reported with an IC 90 of 90 nM, and the hydroxamate trichostatin A has been shown to inhibit parasite growth with an IC 50 of ∼10 nM (47). Further refinement of the hydroxamates has produced increasingly efficacious compounds for parasite killing, with mixed consequences with regard to parasite specificity and in vivo activity (26)(27)(28). However, although HDAC inhibitors have progressed into clinical practice for the treatment of certain types of tumors, there are still significant questions over whether their strong antitumorogenic efficacy relates to a truly epigenetic effect-i.e., effect gene transcription although hyperacetylation of histones (21).…”

“…Aberrant histone methylation has been associated with a variety of human cancers, and as such protein methyltransferases are a current target class for the development for new cancer chemotherapies (20,21). As for targeting parasite epigenetic gene regulation through histone posttranslational modifications, the few studies present in the literature have focused exclusively on modulating histone acetylation via the histone acetyltransferase (HAT) inhibitors curcumin (22) or anacardic acid (23) or the histone deacetylase (HDAC) inhibitors nicotinamide (24), apicidin (25), or derivatives of hydroxamic acid (26)(27)(28). Although the characterization of P. falciparum protein arginine N-methyltransferase (PRMT1) activity and inhibition has been described (29), there are no reports of drug discovery efforts targeting the parasite histone lysine methyltransferases (HKMTs).…”