Evaluation of Nucleoside Hydrolase Inhibitors for Treatment of African Trypanosomiasis

Abstract

In this paper, we present the biochemical and biological evaluation of N-arylmethyl-substituted iminoribitol derivatives as potential chemotherapeutic agents against trypanosomiasis. Previously, a library of 52 compounds was designed and synthesized as potent and selective inhibitors of Trypanosoma vivax inosine-adenosine-guanosine nucleoside hydrolase (IAG-NH). However, when the compounds were tested against bloodstream-form Trypanosoma brucei brucei, only one inhibitor, N-(9-deaza-adenin-9-yl)methyl-1,4-dideoxy- were investigated using RNA interference. The findings from all these studies showed that it is probably not sufficient to target only the nucleoside hydrolase activity to block the purine salvage pathway of T. b. brucei and that, therefore, it is possible that UAMC-00363 acts on an additional target.In the search for new selective trypanocidal drugs, it has been proposed that the purine metabolism of Trypanosoma brucei provides a valuable target. In contrast to mammals, all parasites are unable to synthesize purines de novo and rely instead on the purine salvage pathway (PSP) to obtain purines, which are essential for their survival. The PSP is essential for all stages of T. brucei. The following enzymes of the PSP in T. brucei brucei are described in the literature (Fig. 1): inosine-adenosine-guanosine nucleoside hydrolase (IAG-NH; EC.3.2.2.1) (33, 38), inosine-guanosine nucleoside hydrolase (IG-NH; EC 3.2.2.1) (32), methylthioadenosine phosphorylase (MTAP; EC 2.4.2.28) (43), adenine phosphoribosyltransferase (APRT; EC 2.4.2.7) (5), hypoxanthine-guanine phosphoribosyltransferase (HGPRT; EC 2.4.2.8) (36), and adenosine kinase (AK; EC 2.7.1.20) (5). Key enzymes in the PSP of T. brucei are the nucleoside hydrolases (NH; EC 3.2.2.1). In these parasites, purine bases are obtained by cleavage of the N-glycosidic bond of nucleosides by NH. In T. b. brucei, two types of NH are present: the purine nucleoside-specific IAG-NH, which prefers inosine, adenosine, and guanosine as substrates, and the 6-oxopurine-specific IG-NH, with high affinities for inosine and guanosine (30,32,38). It should be noted that NH activity is absent in mammalian cells. Therefore, NH may provide a good target for the development of new antitrypanosomal compounds (19). Consequently, this target has been investigated during the past decade, and potent inhibitors of NH have been developed. The immucillins (20,29) and N-arylmethyl-substituted iminoribitol derivatives (4,22,23,39) (Fig. 2) are the strongest inhibitors of T. vivax IAG-NH (TvIAG-NH) and T. b. brucei IAG-NH (TbbIAG-NH) known to date, with K i values in the low nanomolar range. Additionally, many of these inhibitors show selectivity with respect to the human nucleoside-cleaving enzyme human pu-* Corresponding author. Mailing address:

Inhibitors of the Purine Salvage Pathway: A Valuable Approach for Antiprotozoal Chemotherapy?

“…Noteworthy, for a very long time no antiparasitic data have been published for the iminoribitol derivatives outside the genus Plasmodium. Very recently, results on a a panel of protozoan parasites (P. falciparum, T. b. brucei, L. infantum and T. cruzi) for a series of N-(arylmethyl-) substituted iminoribitols have been revealed by the authors of this publication, showing promising in vitro and in vivo activities for one compound against T. b. brucei [164]. However, it was hypothesised that single inhibition of T. b. brucei IAG-NH is insufficient and the antiparasitic activity of this compound is explained by unknown other targets.…”

“…Screening against TvIAG-NH, TbbIAG-NH and hPNP resulted in several selective nanomolar inhibitors, including the most potent and selective inhibitors of NHs known to date: UAMC-00115 (43), UAMC-00109 (44) and UAMC-00363 (45) shown in Fig. (18) [163,164]. Screening of these compounds against TbbIG-NH resulted in K i values in the micromolar range, showing IAG-NH and IG-NH enzymes clearly differ in active site features.…”

Synthesis and Structure–Activity Analysis of New Phosphonium Salts with Potent Activity against African Trypanosomes

“…In this series, replacement of the carbonyl group by an oxygen linker enhanced the in vitro activity 20 to 40-fold against T. b. rhodesiense (compare 56a vs 56c, 57a vs 57c). The diphenylether analogues 56c and 57c were the most active compounds of all the series with EC 50 values in the low nanomolar range against wild type and resistant T. b. brucei lines ( As far as the counterion is concerned, no significant difference in in vitro activity was observed between the bromide and the chloride salt of the compounds (Table 2, entries [16][17][18][19][20][21][22]. In addition, a longer linker (i.e., ethylene instead of methylene linker) between the phosphonium cation and the central diphenyl core hardly affected the activity as shown by the nanomolar EC 50 s of compounds 65 (73 nM vs 63 nM for 45g) and 64 (27 nM vs 50 nM for 45f) ( Table 5).…”

“…It should be noted that the small differences in phosphonium compounds EC 50 values observed between T. b. rhodesiense STIB900 and T. b. brucei s427 are most probably due to the slightly different assay procedures used in the two laboratories involved in this work. 16 Alternatively, they could reflect the minor biochemical differences between the two strains used.…”

Trypanosoma brucei Methylthioadenosine Phosphorylase Protects the Parasite from the Antitrypanosomal Effect of Deoxyadenosine

“…Thus the enzyme has a low K m (and a high catalytic efficiency) for MTA as a substrate compared with adenosine and deoxyadenosine. It is known from a previous study that knocking down TbMTAP leads to slightly slower growth and an increased proportion of anucleate and multinucleate T. brucei cells (31). The effect on the DNA content could possibly be a result of defective metabolism of polyamines, which are needed to stabilize DNA during replication.…”