Synthesis of base-modified noraristeromycin derivatives and their inhibitory activity against human and Plasmodium falciparum recombinant S-adenosyl-l-homocysteine hydrolase

Diastereoselective Diboration of Cyclic Alkenes: Application to the Synthesis of Aristeromycin

The Pt-catalyzed diboration of cyclic alkenes is extended to unsaturated heterocycles and bicyclic compounds and can be accomplished in a diastereoselective fashion. The optimal procedures, substrate scope, and diastereoselectivity were investigated, and examples employing both homogeneous and heterogeneous catalysis were examined. Lastly, application to the construction of the nucleoside analog (±)-aristeromycin was conducted.

Structural insights into the reaction mechanism of S-adenosyl-L-homocysteine hydrolase

S-adenosyl-L-homocysteine hydrolase (SAH hydrolase or SAHH) is a highly conserved enzyme that catalyses the reversible hydrolysis of SAH to L-homocysteine (HCY) and adenosine (ADO). High-resolution crystal structures have been reported for bacterial and plant SAHHs, but not mammalian SAHHs. Here, we report the first high-resolution crystal structure of mammalian SAHH (mouse SAHH) in complex with a reaction product (ADO) and with two reaction intermediate analogues—3’-keto-aristeromycin (3KA) and noraristeromycin (NRN)—at resolutions of 1.55, 1.55, and 1.65 Å. Each of the three structures constitutes a structural snapshot of one of the last three steps of the five-step process of SAH hydrolysis by SAHH. In the NRN complex, a water molecule, which is an essential substrate for ADO formation, is structurally identified for the first time as the candidate donor in a Michael addition by SAHH to the 3’-keto-4’,5’-didehydroadenosine reaction intermediate. The presence of the water molecule is consistent with the reaction mechanism proposed by Palmer & Abeles in 1979. These results provide insights into the reaction mechanism of the SAHH enzyme.

A novel IKKα inhibitor, noraristeromycin, blocks the chronic inflammation associated with collagen-induced arthritis in mice

OBJECTIVES

To evaluate the therapeutic efficacy of a novel inhibitor for IκB kinase alpha (IKKα), noraristeromycin (NAM), for murine experimental model of rheumatoid arthritis, collagen- induced arthritis (CIA).

METHODS

NAM has been chemically synthesized as reported earlier. CIA was induced in DBA/1JNCrlj mice by intradermal inoculation of bovine type II collagen (col II) together with Freund Complete Adjuvant. Following the Day 21 booster injection of col II with Freund Incomplete Adjuvant, the animals were monitored for the development of arthritis and clinically evaluated. NAM was administered orally at different doses prior to induction (prophylactic protocol) or after the emergence of definitive arthritis (therapeutic protocol).

RESULTS

Here we demonstrate the experimental evidence that oral administration of NAM could completely prevent the occurrence of experimental arthritis in CIA mouse model at 0.3 mg/kg with ED50 value of approximately 0.1 mg/kg twice daily. Moreover, twice daily oral therapeutic dosage of 1 mg/kg of NAM significantly inhibited the paw swelling and disease progression even after the occurrence of experimental CIA. In addition, NAM exhibited an excellent pharmacokinetics in mice and oral administration of NAM could suppress the production of TNFα elicited by lipopolysaccharide (LPS) in a dose-dependent manner.

CONCLUSIONS

These results indicated that IKKα inhibition is an effective novel therapy for the treatment of chronic inflammatory processes such as those associated with RA and other related conditions.

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.

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.

Substrate-dependent dihydroxylation of substituted cyclopentenes: toward the syntheses of carbocyclic sinefungin and noraristeromycin

Carbocyclic nucleosides are of considerable interest for the development of new therapeutic agents. A key reaction in the preparation of many such nucleoside analogues is dihydroxylation of appropriately substituted cyclopentenes. Although often considered a routine reaction, in this paper, we report the dramatic influence of substituents on the facial selectivity of dihydroxylations. The substituted cyclopentene substrates are derived from acylnitroso cycloaddition reactions of cyclopentadiene, followed by N-O reduction and efficient enzymatic resolution. The results are directly utilized in a very efficient asymmetric synthesis of an antiviral carbocyclic nucleoside, noraristeromycin 5. Extensions toward the synthesis of carbocyclic sinefungin 7 document the importance of realizing the substituent dependence of the dihydroxylation reaction.

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.

4′-Fluorinated carbocyclic nucleosides: Synthesis and inhibitory activity against S-adenosyl-l-homocysteine hydrolase

4'-Fluorinated analogue of 9-[(1'R,2'S,3'R)-2',3'-dihydroxy-cyclopentan-1'-yl]adenine (DHCaA) and their related analogues were systematically synthesized under the Mitsunobu and palladium(0) coupling conditions followed by fluorination with inversion of the configuration by using diethylaminosulfur trifluoride, respectively. 4'-beta-Fluoro DHCaA and 2-amino-4'-alpha-fluoro DHCaA demonstrated slight inhibitory activity against human and Plasmodium falciparum S-adenosyl-L-homocysteine hydrolase, respectively.

An unusual occurrence on attempted purine C-8 electrophilic fluorination of 5'-noraristeromycin

Reaction of the in situ generated purine C-8 carbanion of a protected 5'-noraristeromycin derivative with N-fluorobenzenesulfonimide gave 8-phenylsulfonyl-5'-noraristeromycin rather than the expected 8-fluoro derivative. A single electron transfer (SET) mechanism is proposed for this occurrence. The phenylsulfonyl product offers a structural feature common to some anti-HIV agents. [reaction: see text]

Synthesis of 5′-methylenearisteromycin and its 2-fluoro derivative with potent antimalarial activity due to inhibition of the parasite S-adenosylhomocysteine hydrolase1

5'-methylenearisteromycin 5 and its 2-fluoro derivative 6, which were designed as antimalarial agents because of their AdoHcy hydrolase inhibition, were synthesized from D-ribose, using a stereoselective intramolecular radical cyclization as the key step to construct the carbocyclic structure. These compounds were evaluated as AdoHcy hydrolase inhibitors with the recombinant human and malarial parasite enzymes. Although 5 and 6 were both potent inhibitors of the malarial parasite AdoHcy hydrolase, the 2-fluoro derivative 6 proved to be superior due to its lower inhibitory effect on the human enzyme. In addition, 6 was identified as a potent antimalarial agent using an in vitro assay system with Plasmodium falciparum.

Crystal structure of S-adenosyl-L-homocysteine hydrolase from the human malaria parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is responsible for the death of more than a million people each year. The emergence of strains of malarial parasite resistant to conventional drug therapy has stimulated searches for antimalarials with novel modes of action. S-Adenosyl-L-homocysteine hydrolase (SAHH) is a regulator of biological methylations. Inhibitors of SAHH affect the methylation status of nucleic acids, proteins, and small molecules. P.falciparum SAHH (PfSAHH) inhibitors are expected to provide a new type of chemotherapeutic agent against malaria. Despite the pressing need to develop selective PfSAHH inhibitors as therapeutic drugs, only the mammalian SAHH structures are currently available. Here, we report the crystal structure of PfSAHH complexed with the reaction product adenosine (Ado). Knowledge of the structure of the Ado complex in combination with a structural comparison with Homo sapiens SAHH (HsSAHH) revealed that a single substitution between the PfSAHH (Cys59) and HsSAHH (Thr60) accounts for the differential interactions with nucleoside inhibitors. To examine roles of the Cys59 in the interactions with nucleoside inhibitors, a mutant PfSAHH was prepared. A replacement of Cys59 by Thr results in mutant PfSAHH, which shows HsSAHH-like nucleoside inhibitor sensitivity. The present structure should provide opportunities to design potent and selective PfSAHH inhibitors.

Synthesis of 2-fluoronoraristeromycin and its inhibitory activity against Plasmodium falciparum S-Adenosyl-l-homocysteine hydrolase

Palladium-coupling reaction of (1S, 4R)-cis-4-acetoxy-2-cyclopenten-1-ol with sodium salt of 2-fluoroadenine resulted in the formation of (1S,4R)-4-(6-amino-2-fluoro-9H-purin-9-yl)cyclopent-2-en-1-ol. Subsequent oxidation was carried out with osmium tetraoxide (OsO(4)) in the presence of 4-methylmorpholine N-oxide (NMO) to give 2-fluoronoraristeromycin, possessing significant inhibitory activity against recombinant Plasmodium falciparum SAH hydrolase.