Mutational analyses of Plasmodium falciparum and human S-adenosylhomocysteine hydrolases

Antiparasitic activity of FLLL-32 against four Babesia species, B. bovis, B. bigemina, B. divergens and B. caballi, and one Theileria species, Theileria equi in vitro, and Babesia microti in mice

Introduction: FLLL-32, a synthetic analog of curcumin, is a potent inhibitor of STAT3's constitutive activation in a variety of cancer cells, and its anticancer properties have been demonstrated both in vitro and in vivo. It is also suggested that it might have other pharmacological activities including activity against different parasites. Aim: This study therefore investigated the in vitro antiparasitic activity of FLLL-32 against four pathogenic Babesia species, B. bovis, B. bigemina, B. divergens, and B. caballi, and one Theileria species, Theileria equi. In vivo anti-Babesia microti activity of FLLL-32 was also evaluated in mice. Methods: The FLLL-32, in the growth inhibition assay with a concentration range (0.005-50 μM), was tested for it's activity against these pathogens. The reverse transcription PCR (RT-PCR) assay was used to evaluate the possible effects of FLLL-32 treatment on the mRNA transcription of the target B. bovis genes including S-adenosylhomocysteine hydrolase and histone deacetylase. Results: The in vitro growth of B. bovis, B. bigemina, B. divergens, B. caballi, and T. equi was significantly inhibited in a dose-dependent manner (in all cases, p < 0.05). FLLL-32 exhibits the highest inhibitory effects on B. bovis growth in vitro, and it's IC50 value against this species was 9.57 μM. The RT-PCR results showed that FLLL-32 inhibited the transcription of the B. bovis S-adenosylhomocysteine hydrolase gene. In vivo, the FLLL-32 showed significant inhibition (p < 0.05) of B. microti parasitemia in infected mice with results comparable to that of diminazene aceturate. Parasitemia level in B. microti-infected mice treated with FLLL-32 from day 12 post infection (pi) was reduced to reach zero level at day 16 pi when compared to the infected non-treated mice. Conclusion: The present study demonstrated the antibabesial properties of FLLL-32 and suggested it's usage in the treatment of babesiosis especially when utilized in combination therapy with other antibabesial drugs.

An efficient synthesis of the 4'-epimer of 2-fluoronoraristeromycin

The 4'-epimer of 2-fluoronoraristeromycin was synthesized by employing bis-t-butoxycarbonyl (Boc) protected 2-fluoroadenine as a superior substrate for the Mitsunobu reaction with the appropriate cyclopentenol. Unlike the unsubstituted counterpart 2-fluoroadenine, this substrate is completely soluble in THF and resulted in a very good yield in the Mitsunobu coupling reaction as well as subsequent steps.

The rationale for targeting the NAD/NADH cofactor binding site of parasitic S-adenosyl-L-homocysteine hydrolase for the design of anti-parasitic drugs

Trypanosomal S-adenoyl-L-homocysteine hydrolase (Tc-SAHH), considered as a target for treatment of Chagas disease, has the same catalytic mechanism as human SAHH (Hs-SAHH) and both enzymes have very similar x-ray structures. Efforts toward the design of selective inhibitors against Tc-SAHH targeting the substrate binding site have not to date shown any significant promise. Systematic kinetic and thermodynamic studies on association and dissociation of cofactor NAD/H for Tc-SAHH and Hs-SAHH provide a rationale for the design of anti-parasitic drugs directed toward cofactor-binding sites. Analogues of NAD and their reduced forms show significant selective inactivation of Tc-SAHH, confirming that this design approach is rational.

Synthesis of carbocyclic 2-substituted adenine nucleoside and related analogs

2-Iodonoraristeromycin, 2-iodoaristeromycin and related analogs were synthesized to investigate their inhibitory activities against human and Plasmodium falciparum S-adenosyl-L-homocysteine hydrolases.

S-Adenosyl-L-homocysteine Hydrolase as an Attractive Target for Antimicrobial Drugs

S-Adenosyl-L-homocysteine (SAH) hydrolase catalyzes breakdown of SAH, which arises after S-adenosylmethionine-dependent methylation, into adenosine and homocysteine. The enzyme activity is required for both metabolic pathway of sulfur-containing amino acids and a variety of biological methylations. Because of the essential roles of SAH hydrolase for living cells, inhibitors of SAH hydrolase are expected to be antimicrobial drugs, especially for viruses and malaria parasite. Our research focused on the development of new antimalarials based on the SAH hydrolase inhibition. Malaria parasite employs SAH hydrolase of itself for coping with the toxicity of SAH, so that the target offers opportunities for chemotherapy if structural differences are exploited between the parasite and human enzymes. In vitro screens of nucleoside analogs resulted in moderate but selective inhibition for recombinant SAH hydrolase of malaria parasite, Plasmodium falciparum, by 2-position substituted adenosine analogs. Similar selectivity was observed in the growth inhibition assay of cultured cells. Following crystal structure analysis of the parasite SAH hydrolase discovered an additional space, which is located near the 2-position of the adenine-ring, in the substrate binding pocket. Mutagenic analysis of the amino acid residue forming the additional space confirmed that the inhibition selectivity is due to the difference of only one amino acid residue, between Cys59 in P. falciparum and Thr60 in human. For developing antimalarial drugs, it might be suitable to select target from pathways that are present in the parasite but absent from humans; nevertheless, even if the target was common in parasite and host, slight structural difference such as single amino acid variation is likely to be available for improving inhibitor selectivity.