Activities of 11‐azaartemisinin and N‐sulfonyl derivatives against asexual and transmissible malaria parasites

Dihydroartemisinin (DHA), either used in its own right or as the active drug generated in vivo from the other artemisinins in current clinical use—artemether and artesunate—induces quiescence in ring‐stage parasites of Plasmodium falciparum (Pf). This induction of quiescence is linked to artemisinin resistance. Thus, we have turned to structurally disparate artemisinins that are incapable of providing DHA on metabolism. Accordingly, 11‐azaartemisinin 5 and selected N‐sulfonyl derivatives were screened against intraerythrocytic asexual stages of drug‐sensitive Pf NF54 and drug‐resistant K1 and W2 parasites. Most displayed appreciable activities against all three strains, with IC50 values 2000 toward asexual parasites. Overall, the readily accessible 11‐azaartemisinin 5 and the sulfonyl derivatives 11 and 16 represent potential candidates for further development, in particular for transmission blocking of artemisinin‐resistant parasites.http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1860-71872018-12-19hj2018Biochemistr

Syntheses of 8–(phenoxymethyl)caffeine analogues and their evaluation as inhibitors of monoamine oxidase and as antagonists of the adenosine A2A receptor

Thesis (MSc (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2013.Background and rationale: Parkinson’s disease (PD) is a progressive, degenerative disorder of the central nervous system and is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta. The loss of functional dopamine in the striatum is thought to be responsible for the typical symptoms of PD. Cardinal features of PD include bradykinesia, muscular rigidity, resting tremor and impairment of postural balance. This study focuses on the inhibition of monoamine oxidase B (MAO-B) and antagonism of A2A receptors as therapeutic strategies for PD. Monoamine oxidase (MAO) is a flavin adenine dinucleotide (FAD)-containing mitochondrial bound isoenzyme which consists of two isoforms namely MAO-A and MAO-B. The primary function of MAO is to catalyze the oxidative deamination of dietary amines, monoamine neurotransmitters and hormones. MAO-A is responsible for the oxidative deamination of serotonin (5-HT) and norepinephrine (NE), while MAO-B is responsible for the oxidative deamination of dopamine (DA). The formation of DA takes place in the presynaptic neuron where it is stored in vesicles and released into the presynaptic cleft. The released DA then either binds to D1 and D2 receptors which results in an effector response. The excess DA in the presynaptic cleft is metabolized by MAO-B which may result in the formation of free radicals and a decrease in DA concentrations. Under normal physiological conditions free radicals are removed from the body via normal physiological processes, but in PD these normal physiological processes are thought to be unable to remove the radicals and this may lead to oxidative stress. Oxidative stress is believed to be one of the leading causes of neurodegeneration in PD. The rationale for the use of MAO-B inhibitors in PD would be to increase the natural DA levels in the brain and also diminish the likelihood of free radicals to be formed. Adenosine is an endogenous purine nucleoside and yields a variety of physiological effects. Four adenosine receptor subtypes have been characterized: A1, A2A, A2B and A3. They are all part of the G-protein-coupled receptor family and have seven transmembrane domains. The A2A receptor is highly concentrated in the striatum. There are two important pathways in the basal ganglia (BG) through which striatal information reaches the globus pallidus, namely the direct pathway containing A1 and D1 receptors and the indirect pathway containing A2A and D2 receptors. The direct pathway facilitates willed movement and the indirect pathway inhibits willed movement. A balance of the two pathways is necessary for normal movement. In PD, there is a decrease in DA in the striatum, thus leading to unopposed A2A receptor signaling and ultimately resulting in overactivity of the indirect pathway. Overactivity of the indirect pathway results in the locomotor symptoms associated with PD. Treatment with an A2A antagonist will block the A2A receptor, resulting in the restoration of balance between the indirect and direct pathways, thus leading to a decrease in locomotor symptoms. Aim: In this study, caffeine served as a lead compound for the design of dual-targeted drugs that are selective, reversible MAO-B inhibitors as well as A2A antagonists. Caffeine is a very weak MAO-B inhibitor and a moderately potent A2A antagonist. Substitution on the C8 position of caffeine yields compounds with good MAO-B inhibition activities and A2A receptor affinities. An example of this behaviour is found with (E)-8-(3-chlorostyryl)caffeine (CSC), which is not only a potent A2A antagonist but also a potent MAO-B inhibitor. The goal of this study was to identify and synthesize dual-targeted xanthine compounds. Recently Swanepoel and co-workers (2012) found that 8-phenoxymethyl substituted caffeines are potent reversible inhibitors of MAO-B. Therefore, this study focused on expanding the 8-(phenoxymethyl)caffeine series and evaluating the resulting compounds as both MAO-A and -B inhibitors as well as A2A antagonists. Synthesis: Two series were synthesized namely the 8-(phenoxymethyl)caffeines and 1,3-diethyl-7-methyl-8-(phenoxymethyl)xanthines. The analogues were synthesized according to the literature procedure. 1,3-Dimethyl-5,6-diaminouracil or 1,3-diethyl-5,6-diaminouracil were used as starting materials and were acylated with a suitable substituted phenoxyacetic acid in the presence of N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDAC) as an activating reagent. The intermediary amide was treated with sodium hydroxide, which resulted in ring closure to yield the corresponding 1,3-dimethyl-8-phenoxymethyl-7Hxanthinyl or 1,3-diethyl-8-phenoxymethyl-7H-xanthinyl analogues. These xanthines were 7-N-methylated in the presence of an excess of potassium carbonate and iodomethane to yield the target compounds. In vitro evaluation: A radioligand binding assay was performed to determine the affinities of the synthesized compounds for the A2A receptor. The MAO-B inhibition studies were carried out via a fluorometric assay where the MAO-catalyzed formation of H2O2 was measured. Results: Both series showed good to moderate MAO-B inhibition activities, while none of the compounds had activity towards MAO-A. Results were comparable to that of a known MAOB inhibitor lazabemide. For example, lazabemide (IC50 = 0.091 μM) was twice as potent as the most potent compound identified in this study, 8-(3-chlorophenoxymethyl)caffeine (compound 3; IC50 = 0.189 μM). Two additional compounds, 8-(4-iodophenoxymethyl)caffeine and 8-(3,4-dimethylphenoxymethyl) caffeine, also exhibited submicromolar IC50 values for the inhibition of MAO-B. The structure-activity relationships (SARs) indicated that 1,3-diethyl substitution resulted in decreased inhibition potency towards MAO-B and that 1,3-dimethyl substitution was a more suitable substitution pattern, leading to better inhibition potencies towards MAO-B. The compounds were also evaluated for A2A binding affinity, and relatively weak affinities were recorded with the most potent compound, 1,3-diethyl-7-methyl-8-[4-chlorophenoxymethyl]xanthine (compound 16), exhibiting a Ki value of 0.923 μM. Compared to KW-6002 (Ki = 7.94 nM), a potent reference A2A antagonist, compound 16 was 35-fold less potent. Comparing compound 16 to CSC [Ki(A2A) = 22.6 nM; IC50(MAO-B) = 0.146 nM], it was found that compound 16 is 31-fold less potent as an A2A antagonist and 21-fold less potent as a MAO-B inhibitor. Loss of MAO-B inhibition potency may be attributed to 1,3-diethyl substitution which correlates with similar conclusions reached in earlier studies. In addition, the replacement of the styryl functional group (as found with CSC and KW-6002) with the phenoxymethyl functional group (as found with the present series) may explain the general reduction in affinity for the A2A receptor. This suggests that the styryl side chain is more appropriate for A2A antagonism than the phenoxymethyl functional group. Conclusion: In this study two series of xanthine derivatives were successfully synthesized, namely the 8-(phenoxymethyl)caffeines and 1,3-diethyl-7-methyl-8-(phenoxymethyl)xanthines (11 compounds in total). Three of the newly synthesized compounds were found to act as potent inhibitors of MAO-B, with IC50 values in the submicromolar range. None of the compounds were however noteworthy MAO-A inhibitors. The most potent A2A antagonist among the examined compounds, compound 16, proved to be moderately potent compared to the reference antagonists, CSC and KW-6002. It may be concluded that the styryl functional group (as found with CSC and KW-6002) is more optimal than the phenoxymethyl functional group (as found with the present series) for A2A antagonism. 1,3-Diethyl substitution of the xanthine ring was found to be less optimal for MAO-B inhibition compared to 1,3-dimethyl substitution. These results together with known SARs provide valuable insight into the design of 8-(phenoxymethyl)caffeines as selective and potent MAO-B inhibitors. Such drugs may find application in the therapy of PD.Master

The case for development of 11-aza-artemisinins for malaria

The current treatment regimens for uncomplicated malaria comprise an artemisinin in combination with another drug (ACT). However, the recent emergence of resistance to ACTs in South East Asia dramatically emphasizes the need for new artemisinins. The current artemisinins have been in use since the late 1970s and have relatively poor thermal, chemical and metabolic stabilities - all are metabolized or hydrolyzed in vivo to dihydroartemisinin (DHA) that itself undergoes facile decomposition in vivo. The current artemisinins possess neurotoxicity as demonstrated in animal models, an issue that mandates increased vigilance in view of trends to use of protracted treatment regimens involving sequential administration of different ACTs against the resistant disease. As artemisinins induce the most rapid reduction in parasitaemia of any drug, common sense dictates that any new artemisinin derivative, selected on the bases of more robust chemical and thermal stability, metabolic stability with respect to the generation of DHA in vivo, and relatively benign neurotoxicity should be used in any new ACT whose components are rationally chosen in order to counter resistant malaria and inhibit transmission. 11-Azaartemisinin and its N-substituted derivatives attract because of overall ease of preparation from artemisinin. Some derivatives also possess notable thermal stabilities and although metabolic pathways of the derivatives are as yet unknown, none can provide DHA. The azaartemisinins synthesized over the past 20 years are critically discussed on the basis of their synthetic accessibility and biological activities with the view to assessing suitability to serve as new artemisinin derivatives for treatment of malari

Activities of 11-azaartemisinin and N-sulfonyl derivatives against Neospora caninum and comparative cytotoxicities

Neosporosis caused by the apicomplexan parasite Neospora caninum is an economically important disease that induces abortion in dairy and beef cattle. There are no vaccines or drugs available on the market for control or treatment of the disease in bovines. The peroxide artemisinin and its derivatives used clinically for treatment of malaria are active against N. caninum and other apicomplexan parasites. We have now evaluated the activities of the readily accessible and chemically robust 11-azaartemisinin 5 and selected N-sulfonyl derivatives prepared as described in the accompanying paper against N. caninum tachyzoites grown in infected human foreskin fibroblasts. Azaartemisinin elicited an IC50 value of 150 nm, and the 2′,5′-dichloro-3′-thienylsulfonyl-11-azaartemisinin 17 was found to be the most active, with an IC50 value of 40 nm. Comparison with normal human fetal lung fibroblasts HFLF WI-38 revealed relatively benign cytotoxicity. The compounds were also screened in vitro against TK-10 (renal), UACC-62 (melanoma) and MCF-7 (breast) cancer cell lines; overall, in line with activities against HFLF cells, most compounds in the series were found to be inactiv

Activities of 11-azaartemisinin and N-sulfonyl derivatives against asexual and transmissible malaria parasites

Dihydroartemisinin (DHA), either used in its own right or as the active drug generated in vivo from the other artemisinins in current clinical use—artemether and artesunate—induces quiescence in ring-stage parasites of Plasmodium falciparum (Pf). This induction of quiescence is linked to artemisinin resistance. Thus, we have turned to structurally disparate artemisinins that are incapable of providing DHA on metabolism. Accordingly, 11-azaartemisinin 5 and selected N-sulfonyl derivatives were screened against intraerythrocytic asexual stages of drug-sensitive Pf NF54 and drug-resistant K1 and W2 parasites. Most displayed appreciable activities against all three strains, with IC50 values 2000 toward asexual parasites. Overall, the readily accessible 11-azaartemisinin 5 and the sulfonyl derivatives 11 and 16 represent potential candidates for further development, in particular for transmission blocking of artemisinin-resistant parasite