Inhibition of the in vitro growth of Plasmodium falciparum by acyclic nucleoside phosphonates

Development of a continuous assay for high throughput screening to identify inhibitors of the purine salvage pathway in Plasmodium falciparum

Malaria, an infectious disease caused by protozoan parasites from the genus Plasmodium, represents a serious global health threat. The continued emergence of drug resistant strains has severely decreased current antimalarial drug efficacy and led to a perpetual race for drug discovery. Most protozoan parasites, including Plasmodium spp., are unable to synthesize purines de novo and instead rely on an essential purine salvage pathway for acquisition of purines from the infected host. Because purines are essential for Plasmodium growth and survival, the enzymes of the purine salvage pathway represent promising targets for drug discovery. Target-based high-throughput screening (HTS) assays traditionally focus on a single target, which severely limits the screening power of this type of approach. To circumvent this limitation, we have reconstituted the purine salvage pathway from Plasmodium falciparum in an assay combining four drug targets. This assay was developed for HTS and optimized to detect partial inhibition of any of the four enzymes in the pathway. Inhibitors of several enzymes in the pathway were identified in a pilot screen, with several compounds exhibiting effective inhibition when provided in micromolar amounts.

Plasmodium Purine Metabolism and Its Inhibition by Nucleoside and Nucleotide Analogues

Malaria still affects around 200 million people and is responsible for more than 400,000 deaths per year, mostly children in subequatorial areas. This disease is caused by parasites of the Plasmodium genus. Only a few WHO-recommended treatments are available to prevent or cure plasmodial infections, but genetic mutations in the causal parasites have led to onset of resistance against all commercial antimalarial drugs. New drugs and targets are being investigated to cope with this emerging problem, including enzymes belonging to the main metabolic pathways, while nucleoside and nucleotide analogues are also a promising class of potential drugs. This review highlights the main metabolic pathways targeted for the development of potential antiplasmodial therapies based on nucleos(t)ide analogues, as well as the different series of purine-containing nucleoside and nucleotide derivatives designed to inhibit Plasmodium falciparum purine metabolism.

The synthesis of the 8-C-substituted 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP) derivatives by diverse cross-coupling reactions

Diisopropyl 8-bromo-2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine was used as a starting material for the synthesis of the 8-C-substituted 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP) analogues. A systematic screening of diverse cross-coupling reactions was carried out. Stille, Suzuki–Miyaura, Negishi, and Sonogashira cross-couplings, as well as Pd-catalysed reactions with trialkylaluminiums, were employed for the introduction of various alkyl, alkenyl, alkynyl, aryl, and hetaryl substituents to the C-8 position of the 2,6-diaminopurine moiety. In contrast to the potent parent compound PMEDAP, which exhibits potent antiretroviral and antitumor activity, none of the sixteen newly synthesized 8-C-substituted analogues of PMEDAP showed any specific antiviral activity.

Synthesis and antiviral activity of N9-[3-fluoro-2-(phosphonomethoxy)propyl] analogues derived from N6-substituted adenines and 2,6-diaminopurines

An efficient method for the synthesis of N(9)-[3-fluoro-2-(phosphonomethoxy)propyl] (FPMP) derivatives of purine bases has been developed. Both (R)- and (S)-enantiomers of the N(6)-substituted FPMP derivatives of adenine and 2,6-diaminopurine were prepared and their anti-human immunodeficiency virus (HIV) and anti-Moloney murine sarcoma virus (MSV) activity was evaluated. Whereas none of the 6-substituted FPMPA derivatives showed any antiviral activity, several FPMPDAP derivatives had a moderate antiretroviral activity. Moreover, the data obtained from the study of the substrate activity of the active derivatives towards N(6)-methyl-AMP aminohydrolase support the notion that the studied N(6)-substituted FPMPDAP derivatives act as prodrugs of the antiretroviral FPMPG analogues.

Antischistosomal activity of hexadecyloxypropyl cyclic 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]adenine and other alkoxyalkyl esters of acyclic nucleoside phosphonates assessed by schistosome worm killing in vitro

9-(S)-[3-Hydroxy-2-(phosphonomethoxy)propyl]adenine [(S)-HPMPA] has been reported to have antischistosomal activity. Ether lipid esters of (S)-HPMPA and cidofovir (CDV) have greatly increased activities in antiviral assays and in lethal animal models of poxvirus diseases. To see if ether lipid esters of CDV and (S)-HPMPA enhance antischistosomal activity, we tested their alkoxyalkyl esters using Schistosoma mansoni worm killing in vitro. Hexadecyloxypropyl (HDP)-cyclic-(S)-HPMPA and HDP-cyclic-CDV exhibited significant in vitro antischistosomal activities and may offer promise alone or in combination with praziquantel.

Inhibition of hypoxanthine-guanine phosphoribosyltransferase by acyclic nucleoside phosphonates: a new class of antimalarial therapeutics

The purine salvage enzyme hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) is essential for purine nucleotide and hence nucleic acid synthesis in the malaria parasite, Plasmodium falciparum. Acyclic nucleoside phosphonates (ANPs) are analogues of the nucleotide product of the reaction, comprising a purine base joined by a linker to a phosphonate moiety. K(i) values for 19 ANPs were determined for Pf HGXPRT and the corresponding human enzyme, HGPRT. Values for Pf HGXPRT were as low as 100 nM, with selectivity for the parasite enzyme of up to 58. Structures of human HGPRT in complex with three ANPs are reported. On binding, a large mobile loop in the free enzyme moves to partly cover the active site. For three ANPs, the IC(50) values for Pf grown in cell culture were 1, 14, and 46 microM, while the cytotoxic concentration for the first compound was 489 microM. These results provide a basis for the design of potent and selective ANP inhibitors of Pf HGXPRT as antimalarial drug leads.

Cidofovir and (S)-9-[3-hydroxy-(2-phosphonomethoxy)propyl]adenine are highly effective inhibitors of vaccinia virus DNA polymerase when incorporated into the template strand

The acyclic nucleoside phosphonate drug (S)-9-[3-hydroxy-(2-phosphonomethoxy)propyl]adenine [(S)-HPMPA], is a broad-spectrum antiviral and antiparasitic agent. Previous work has shown that the active intracellular metabolite of this compound, (S)-HPMPA diphosphate [(S)-HPMPApp], is an analog of dATP and targets DNA polymerases. However, the mechanism by which (S)-HPMPA inhibits DNA polymerases remains elusive. Using vaccinia virus as a model system, we have previously shown that cidofovir diphosphate (CDVpp), an analog of dCTP and a related antiviral agent, is a poor substrate for the vaccinia virus DNA polymerase and acts to inhibit primer extension and block 3'-to-5' proofreading exonuclease activity. Based on structural similarities and the greater antiviral efficacy of (S)-HPMPA, we predicted that (S)-HPMPApp would have a similar, but more pronounced effect on vaccinia polymerase than CDVpp. Interestingly, we found that (S)-HPMPApp is a good substrate for the viral enzyme, exhibiting K(m) and V(max) parameters comparable to those of dATP, and certainly not behaving like CDVpp as a functional chain terminator. Metabolic experiments indicated that (S)-HPMPA is converted to (S)-HPMPApp to a much greater extent than CDV is converted to CDVpp, although both drugs cause identical effects on virus DNA replication at their 50% effective concentration. Subsequent studies showed that both compounds can be faithfully incorporated into DNA, but when CDV and (S)-HPMPA are incorporated into the template strand, both strongly inhibit trans-lesion DNA synthesis. It thus appears that nucleoside phosphonate drugs exhibit at least two different effects on DNA polymerases depending upon in what form the enzyme encounters the drug.

Synthesis and Antiviral Evaluation of Alkoxyalkyl Derivatives of 9-(S)-(3-Hydroxy-2-phosphonomethoxypropyl)adenine against Cytomegalovirus and Orthopoxviruses

9-(S)-(3-Hydroxy-2-phosphonomethoxypropyl)adenine [(S)-HPMPA] was one of the first acyclic nucleoside phosphonates described and has been reported to have good antiviral activity against most double-stranded DNA viruses, including the herpes group viruses and the orthopoxviruses. However, (S)-HPMPA is not orally bioavailable and has not been developed for clinical use. We have prepared orally bioavailable lipid esters of (S)-HPMPA and report their synthesis and antiviral evaluation against cytomegalovirus and orthopoxviruses. These esters were evaluated in vitro in cells infected with human cytomegalovirus (HCMV), murine cytomegalovirus (MCMV), vaccinia (VV), and cowpox viruses (CV). The most active compound, oleyloxyethyl-(S)-HPMPA, was found to have EC50 value of 0.003 microM against HCMV vs 1.4 microM for unmodified HPMPA. In cells infected with VV and CV, octadecyloxyethyl-(S)-HPMPA had EC50 values of 0.01-0.02 microM versus 2.7-4.0 microM for unmodified HPMPA. When compared with the alkoxyalkyl esters of cidofovir, the corresponding alkoxyalkyl esters of (S)-HPMPA were equally active against HCMV and MCMV but were 15-20-fold more active against VV and CV in vitro. The alkoxyalkyl esters of (S)-HPMPA are promising new compounds worthy of further investigation for treatment of infections caused by herpes viruses and orthopoxviruses.

The substrate activity of (S)-9-[3-hydroxy-(2-phosphonomethoxy)propyl]adenine diphosphate toward DNA polymerases alpha, delta and epsilon

In this study, we examined the substrate potency of (S)-9-[3-hydroxy-(2-phosphonomethoxy)propyl]- adenine diphosphate (HPMPApp) toward DNA polymerases alpha, delta and epsilon. The efficiency of HPMPApp incorporation decreased in the order pol epsilon >pol delta =pol alpha and was from 5.4- to 23-fold lower than that of dATP. Depending on which template-primer was used, the HPMPAppKm value was 3.3- and 8.3- (pol alpha), 3- and 0.82- (pol delta) or 2-fold (pol epsilon) higher than dATPKm. The ability of HPMPA to accumulate in DNA decreased in the order pol epsilon >pol alpha >pol delta. The difference between the elongation rate of DNA with and without one HPMPA molecule at 3' termini was about 1.1-2.3 fold. The 3'-5'-exonuclease activity of pol delta and epsilon can excise HPMPA from DNA. These observations indicate that interaction of HPMPApp with pol alpha, delta and epsilon may contribute to its cellular toxicity and explain its antiviral activity against polyomavirus.

Activity of 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]adenine against Schistosomiasis mansoni in mice

The activity of the acyclic nucleotide analogue 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]adenine [(S)-HPMPA] against Schistosoma mansoni was investigated in mice. The compound was injected intraperitoneally, usually on two or five consecutive days, at 10 to 20 mg/kg of body weight/day. The treatment started before, at the time of, and after the onset of egg laying (oviposition) by S. mansoni. The animals were killed from 7 to 40 days after the cessation of treatment. Significant reductions in the total numbers of female and coupled worms were found. Female fecundity and both hepatic and intestinal egg loads were suppressed. These effects were more pronounced with dosing regimens launched before the time of oviposition. The complete disappearance of immature eggs and a significant reduction to the complete absence of mature eggs, with 99 to 100% of the eggs being dead, were produced. No hepatic egg-induced granulomas were present in mice treated at the time of oviposition, and the granulomas were smaller in mice treated before S. mansoni oviposition. These preliminary findings extend the knowledge of the antiparasitic properties of (S)-HPMPA.

Synthesis of Acyclic Adenine 8,N-Anhydronucleosides

9-(4-Hydroxybutyl)adenine (10) was obtained by reaction of adenine with 4-[(2-tetrahydropyran-2-yl)oxy]butyl chloride (7) in the presence of DBU. 8-Bromo-9-(4-hydroxybutyl)adenine (13) was prepared by bromination of 10 or by alkylation of 8-bromoadenine (11) with 4-bromoethyl acetate followed by methanolysis. Tosylation of compound 13 afforded the 4-tosyloxy derivative 15 which gave on heating with methylamine or cyclopropylamine 6-methyl- (17a) or 6-cyclopropyl-7,8,9,10-tetrahydro-6H-[1,3]diazepino[1,2-e]purin-4-amine (17b), while the reaction with hydrazine afforded 7,8,9,10-tetrahydro-6H-[1,3]diazepino[1,2-e]purine-4,6-diamine (17d). Treatment of compound 13 with thionyl chloride gave 9-(4-chlorobutyl)-8-chloroadenine (18) as the main product which was transformed to 17b, 6-propyl-7,8,9,10-tetrahydro-6H-[1,3]diazepino[1,2-e]purin-4-amine (17c) or 7,8,9,10-tetrahydro-6H-[1,3]diazepino[1,2-e]purin-4-amine (17e) by reaction with cyclopropylamine, propylamine or ammonia, respectively. Compound 17e was quite stable both in acid and alkaline solutions, at room temperature or at 90 °C. Compound 13 was converted to 9-(4-hydroxybutyl)-8-methylaminoadenine (19) by reaction with methylamine. Compound 19 failed to undergo intramolecular cyclization to diazepine 17a on treatment with diphenyl carbonate, bis(4-nitrophenyl) carbonate or 1,1'-carbonyldiimidazole.