Synthesis of novel strobilurin–pyrimidine derivatives and their antiproliferative activity against human cancer cell lines

A series of new strobilurin–pyrimidine analogs were designed and synthesized based on the structures of our previously discovered antiproliferative compounds I and II. Biological evaluation with two human cancer cell lines (A549 and HL60) showed that most of these compounds possessed moderate to potent antiproliferative activity. Two potent candidates (8f, IC₅₀ = 2.2 nM and 11d, IC₅₀ = 3.4 nM) were identified with nanomolar activity against leukemia cancer cell line HL60 for further development. This activity represents a 1000- to 2500-fold improvement compared to the parent compounds I and II and is 20- to 30-fold better than the chemotherapy drug, doxorubicin. The present work provides strong incentive for further development of these strobilurin–pyrimidine analogs as potential antitumor agents for the treatment of leukemia.     

Anticancer activity of natural cytokinins: A structure–activity relationship study

Cytokinin ribosides (N⁶-substituted adenosine derivatives) have been shown to have anticancer activity both in vitro and in vivo. This study presents the first systematic analysis of the relationship between the chemical structure of cytokinins and their cytotoxic effects against a panel of human cancer cell lines with diverse histopathological origins. The results confirm the cytotoxic activity of N⁶-isopentenyladenosine, kinetin riboside, and N⁶-benzyladenosine and show that the spectrum of cell lines that are sensitive to these compounds and their tissues of origin are wider than previously reported. The first evidence that the hydroxylated aromatic cytokinins (ortho-, meta-, para-topolin riboside) and the isoprenoid cytokinin cis-zeatin riboside have cytotoxic activities is presented.Most cell lines in the panel showed greatest sensitivity to ortho-topolin riboside (IC₅₀ = 0.5–11.6 μM). Cytokinin nucleotides, some synthesized for the first time in this study, were usually active in a similar concentration range to the corresponding ribosides. However, cytokinin free bases, 2-methylthio derivatives and both O- and N-glucosides showed little or no toxicity. Overall the study shows that structural requirements for cytotoxic activity of cytokinins against human cancer cell lines differ from the requirements for their activity in plant bioassays. The potent anticancer activity of ortho-topolin riboside (GI₅₀ = 0.07–84.60 μM, 1st quartile = 0.33 μM, median = 0.65 μM, 3rd quartile = 1.94 μM) was confirmed using NCI₆₀, a standard panel of 59 cell lines, originating from nine different tissues. Further, the activity pattern of oTR was distinctly different from those of standard anticancer drugs, suggesting that it has a unique mechanism of activity. In comparison with standard drugs, oTR showed exceptional cytotoxic activity against NCI₆₀ cell lines with a mutated p53 tumour suppressor gene. oTR also exhibited significant anticancer activity against several tumour models in in vivo hollow fibre assays.     

Synthesis and structure–activity relationships of sinenxan A derivatives as multidrug resistance reversal agents

Two types of sinenxan A derivatives with different side chains at C-5 were synthesized and evaluated for their in vitro multidrug resistant reversal activities. Several derivatives exhibited better activities than the positive control verapamil. The structure–activity relationships of these derivatives suggested that a carbonyl group at C-13 and the length of side chain at C-5 are important for the activity.     

Synthesis and structure–activity relationships of 2-hydrazinyladenosine derivatives as A2A adenosine receptor ligands

A series of 2-hydrazinyladenosine derivatives was synthesized and investigated in radioligand binding studies for their affinity at the adenosine receptor subtypes with the goal to obtain potent and A_2AAR selective agonists and to explore the structure–activity relationships of this class of compounds at A_2AAR. Modifications included introduction of a second sugar moiety at position 2 of adenosine to form new bis-sugar nucleosides and/or modifications of the 2-position linker in different ways. The performed modifications were found to produce compounds with relatively high A_2AAR affinity and very high selectivity toward A_2AAR. The most potent bis-sugar nucleoside was obtained with the d-galactose derivative 16 which exhibited a K_i value of 329 nM at A_2AAR with marked selectivity against the other AR subtypes. In another set of compounds, compound 3 was modified via replacement of its cyclic structure with mono- and disubstituted phenyl moieties and the resulting hydrazones 10–14 were found to have low nanomolar affinity for A_2AAR. In addition to 3, compounds 10, 11 and 13 have been identified as the most potent compounds in the present series with K_i values of 16.1, 24.4, and 12.0 nM, respectively, at rat A_2AAR. Species differences were tested and found to exist in different rates. Functional properties of the most potent compounds 10, 11, 13 and 16 were assessed showing that the compounds acted as agonists at A_2AAR.     

Synthesis and structure–activity relationships study of dithiolethiones as inducers of glutathione in the SH-SY5Y neuroblastoma cell line

Parkinson’s disease is a neurodegenerative disorder that involves the degeneration of nigrostriatal dopaminergic neurons. Elevated levels of reactive oxygen species have been shown to deplete cellular levels of the ubiquitous antioxidant glutathione, leading to oxidative stress and eventual neuronal cell death. Dithiolethiones, a class of sulfur-containing heterocyclic molecules, have been shown to induce cellular production of glutathione in a variety of tissues, but have not been extensively evaluated in neurons. Herein, we report the synthesis and preliminary structure–activity relationships study of several substituted dithiolethiones. Three molecules were identified (D3T, CPDT, and 2d) that potently induced cellular glutathione in the SH-SY5Y neuroblastoma cell line. Furthermore, these compounds were found to provide neuroprotection in the 6-hydroxydopamine model of neurotoxicity. This study suggests that dithiolethione-mediated neuroprotection may have potential as a disease-modifying antiparkinsonian therapy.     

Synthesis and structure–activity relationships evaluation of benzothiazinone derivatives as potential anti-tubercular agents

N-Alkyl and heterocycle substituted 1,3-benzothiazin-4-one (BTZ) derivatives were synthesized. The anti-mycobacterial activities of these compounds were evaluated by determination of minimal inhibitory concentration (MIC) for Mycobacterium tuberculosis H37Ra and M. tuberculosis H37Rv. It was found that an extended or branched alkyl chain analog could enhance the potency, and activities of N-alkyl substituted BTZs were not affected by either nitro or trifluoromethyl at 6-position. Trifluoromethyl plays an important role in maintaining anti-tubercular activity in the piperazine or piperidine analogs. Compound 8o, which contains an azaspirodithiolane group, showed a MIC of 0.0001 μM against M. tuberculosis H37Rv, 20-fold more potent than BTZ043 racemate. These results suggested that the volume and lipophilicity of the substituents were important in maintaining activity. In addition, compound 8o was nontoxic to Vero cells and orally bioavailable in a preliminary pharmacokinetics study.     

Synthetic studies on selective adenosine A2A receptor antagonists: Synthesis and structure–activity relationships of novel benzofuran derivatives

A series of benzofuran derivatives were prepared to study their antagonistic activities to the A_2A receptor. Replacement of the ester group of the lead compound 1 with phenyl ring improved the PK profile, while modifications of the amide moiety showed enhanced antagonistic activity. From these studies, compounds 13c, 13f, and 24a showed good potency in vitro and were identified as novel A_2A receptor antagonists suitable for oral activity evaluation in animal models of catalepsy.     

Synthesis and structure–activity relationships of dehydroaltenusin derivatives as selective DNA polymerase α inhibitors

Herein, we describe the synthesis and structure–activity relationships of dehydroaltenusin derivatives as inhibitors of a mammalian DNA polymerase α. We have newly synthesized nine dehydroaltenusin derivatives modified at the side chains or benzoquinone moiety. We also achieved the first synthesis of desmethylaltenusin and desmethyldehydroaltenusin, metabolites of Alternaria sp. or Talaromyces flavus, respectively. Among all synthesized derivatives, demethoxydehydroaltenusin was the most selective inhibitor of DNA polymerase α. The o-hydroxy-p-benzoquinone (2-hydroxycyclohexa-2,5-dienone) moiety is essential for the inhibition of DNA polymerases. Substitution at the 5-position of dehydroaltenusin is important for the inhibitory potency. Because dehydroaltenusin is conjugated with N-acetylcysteine methyl ester at the o-hydroxy-p-benzoquinone moiety, one or more cysteine residues of DNA polymerase α may act as a target for this compound.     

Synthesis and structure–activity relationships of cassiarin A as potential antimalarials with vasorelaxant activity

Cassiarin A 1, a tricyclic alkaloid, isolated from the leaves of Cassia siamea (Leguminosae), shows powerful antimalarial activity against Plasmodium falciparum in vitro as well as P. berghei in vivo, which may be valuable leads for novel antimalarials. Interactions of parasitized red blood cells (pRBCs) with endothelium in aorta are especially important in the processes contribute to the pathogenesis of severe malaria. Nitric oxide (NO) reduces endothelial expression of receptors/adhesion molecules used by pRBC to adhere to vascular endothelium, and reduces cytoadherence of pRBC to vascular endothelium. Cassiarin A 1 showed vasorelaxation activity against rat aortic ring, which may be related with NO production. A series of a hydroxyl and a nitrogen-substituted derivatives and a dehydroxy derivative of 1 have been synthesized as having potent antimalarials against P. falciparum with vasodilator activity, which may reduce cytoadherence of pRBC to vascular endothelium. Cassiarin A 1 exhibited a potent antimalarial activity and a high selectivity index in vitro, suggesting that the presence of a hydroxyl and a nitrogen atom without any substituents may be important to show antimalarial activity. Relative to cassiarin A, a methoxy derivative showed more potent vasorelaxant activity, although it did not show improvement for inhibition of P. falciparum in vitro. These cassiarin derivatives may be promising candidates as antimalarials with different mode of actions.     

Anti-HBV agents. Part 3: Preliminary structure–activity relationships of tetra-acylalisol A derivatives as potent hepatitis B virus inhibitors

Thirty-two tetra-acylated derivatives of alisol A were synthesized and evaluated for their anti-hepatitis B virus (HBV) activities and cytotoxicities in vitro. Among the series of alisol A derivatives examined, five analogues were active against HBV surface antigen (HBsAg) and HBV e antigen (HBeAg) secretion in HepG 2.2.15 cells. These results also provide interesting structure–activity relationships of tetra-acylalisol A derivatives. Compounds tetra-acetyl alisol A (A1), tetra-methoxyacetyl alisol A (A23), and tetra-ethoxyacetyl alisol A (A24) exhibited high activities against secretion of HBsAg with IC₅₀ values of 0.0048, 0.0044, and 0.014 mM, respectively, HBeAg with IC₅₀ values of 0.011, 0.012, and 0.018 mM, respectively, and remarkable selective index values SI_HBsAg >333, SI_HBeAg >145; SI_HBsAg = 209, SI_HBeAg = 77; and SI_HBsAg >200, SI_HBeAg >156, respectively. Additional studies in rats showed that compound A1 has favorable pharmacokinetic prosperities for further development purpose, with elimination half-time (t_1/2) of 1.63 h and oral bioavailability (F) of 40.9%.     

Design,synthesis, and structure–activity relationships of pyrazole derivatives as potential FabH inhibitors

Fatty acid biosynthesis is essential for bacterial survival. FabH, β-ketoacyl-acyl carrier protein (ACP) synthase III, is a particularly attractive target, since it is central to the initiation of fatty acid biosynthesis and is highly conserved among Gram-positive and -negative bacteria. Fifty-six 1-acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives were synthesized and developed as potent inhibitors of FabH. This inhibitor class demonstrates strong antibacterial activity. Escherichia coli FabH inhibitory assay and docking simulation indicated that the compounds 1-(5-(4-fluorophenyl)-3-(4-methoxyphenyl)-4,5-dihydro-1H-pyrazol-1-yl)ethanone (12) and 1-(5-(4-chlorophenyl)-3-(4-methoxyphenyl)-4,5-dihydro-1H-pyrazol-1-yl)ethanone (13) were potent inhibitors of E. coli FabH.     

Antitrypanosomal structure–activity-relationship study of synthetic cynaropicrin derivatives

Cynaropicrin is a guaianolide sesquiterpene lactone with a 5-7-5 tricyclic skeleton, four exo-olefins, and two hydroxyl groups. Recently, it was found that the compound is a potent in vitro and in vivo inhibitor of the protozoan parasite Trypanosoma brucei, which causes human African trypanosomiasis (HAT; sleeping sickness). In this Letter, chemical derivatization of cynaropicrin and the structure–activity-relationship (SAR) study against T. brucei is described.     

Synthetic studies on selective adenosine A2A receptor antagonists. Part II: Synthesis and structure–activity relationships of novel benzofuran derivatives

Based on the previously reported lead compound, a series of benzofuran derivatives were prepared to study their antagonistic activities to A_2A receptor. The replacement of the phenyl group at the 4-position with a heterocyclic ring improved the PK profile and aqueous solubility. From these studies, we discovered a potent new A_2A antagonist, 12a, which has both a good oral bioavailability and in vivo efficacy on motor disability in MPTP-treated common marmosets.     

Synthesis and structure–activity relationships of 2-acylamino-4,6-diphenylpyridine derivatives as novel antagonists of GPR54

GPR54 is a G protein-coupled receptor (GPCR) which was formerly an orphan receptor. Recent functional study of GPR54 revealed that the receptor has an essential role to modulate sex-hormones including GnRH. Though antagonists of GPR54 are expected to be novel drugs for sex-hormone dependent diseases such as prostate cancer or endometriosis, small molecule GPR54 antagonists have not been reported. We have synthesized a series of 2-acylamino-4,6-diphenylpyridines to identify potent GPR54 antagonists. Detailed structure–activity relationship studies led to compound 9l with an IC₅₀ value of 3.7 nM in a GPR54 binding assay, and apparent antagonistic activity in a cellular functional assay.     

Synthesis and structure–activity relationships of carboxylic acid derivatives of pyridoxal as P2X receptor antagonists

Carboxylic acid derivatives of pyridoxal were developed as potent P2X₁ and P2X₃ receptor antagonists with modifications of a lead compound, pyridoxal-5′-phosphate-6-azophenyl-2′,5′-disulfonate (5b, iso-PPADS). The designing strategies included the modifications of aldehyde, phosphate or sulfonate groups of 5b, which may be interacted with lysine residues of the receptor binding pocket, to weak anionic carboxylic acid groups. The corresponding carboxylic acid analogs of pyridoxal-5′-phosphate (1), 13 and 14, showed parallel antagonistic potencies. Also, most of 6-azophenyl derivatives (24–28) of compound 13 or 14 showed potent antagonistic activities similar to that of 5b at human P2X₃ receptors with 100 nM range of IC₅₀ values in two-electrode voltage clamp (TEVC) assay system on the Xenopus oocyte. The results indicated that aldehyde and phosphoric or sulfonic acids in 5b could be changed to a carboxylic acid without affecting antagonistic potency at mouse P2X₁ and human P2X₃ receptors.     

Larkspur poisoning: toxicology and alkaloid structure–activity relationships

Systematic approaches to taxonomic classifications of the tall larkspur spp. have been developed using traditional chemical methods to profile alkaloids, comparison of relative toxicity of individual alkaloids, plant morphology/taxonomy and molecular genetics. Using these methods (papers published in this series) toxicology of three distinct species of tall larkspurs including Delphinium glaucum, Delphinium barbeyi and Delphinium occidentale is described. Tall larkspurs (Delphinium spp.) continue to be the most serious cause of cattle losses on mountain rangelands in the western US. Over 40 norditerpenoid alkaloids have been reported in species of larkspurs and toxicology data on 25 of these have been reported by the authors. These alkaloids can be classified into three general types based on their structural characteristics and toxicity: the N-(methylsuccinyl) anthranoyl lycoctonine (MSAL)-type, having high toxicity; the lycoctonine-type, with moderate toxicity; and the 7,8-methylenedioxylycoctonine (MDL)-type, of low toxicity. The structural importance of the methylsuccinimido anthranilic acid ester group at the C18 position is evident in the high toxicity of MSAL alkaloids, particularly methyllycaconitine (MLA), Nudicauline (NUD) and 14-deacetylnudicauline (14-DAN). Other structural aspects of these alkaloids such as the C14 functionality are also important, as demonstrated by the reduced toxicity of barbinine. MLA is the alkaloid of most importance in toxicity of larkspurs on mountain rangelands because of its prevalence in most larkspurs and high toxicity. While NUD and 14-DAN also possess high toxicity, they are relatively minor components in few larkspur species (generally the plains and low larkspurs), but when present at concentrations approaching 1 mg/g dry weight they contribute significantly to overall toxicity. Deltaline (DLT) is often found in high concentrations in many larkspurs but because of low toxicity, its contribution to larkspur poisoning in the field is relatively minor and it will probably not cause toxicosis in the absence of the MSAL-type alkaloids.     

Synthesis and structure–activity relationships of 1-benzylindane derivatives as selective agonists for estrogen receptor beta

The estrogen receptor beta (ERβ) selective agonist is considered a promising candidate for the treatment of estrogen deficiency symptoms in ERβ-expressing tissues, without the risk of breast cancer, and multiple classes of compounds have been reported as ERβ selective agonists. Among them, 6-6 bicyclic ring-containing structures (e.g., isoflavone phytoestrogens) are regarded as one of the cyclized analogues of isobutestrol 5b, and suggest that other cyclized scaffolds comprising 5-6 bicyclic rings could also act as selective ERβ ligands. In this study, we evaluated the selective ERβ agonistic activity of 1-(4-hydroxybenzyl)indan-5-ol 7a and studied structure–activity relationship (SAR) of its derivatives. Some functional groups improved the properties of 7a; introduction of a nitrile group on the indane-1-position resulted in higher selectivity for ERβ (12a), and further substitution with a fluoro or a methyl group to the pendant phenyl ring was also preferable (12b, d, and e). Subsequent chiral resolution of 12a identified that R-12a has a superior profile over S-12a. This is comparable to diarylpropionitrile (DPN) 5c, one of the promising selective ERβ agonists and indicates that this indane-based scaffold has the potential to provide better ERβ agonistic probes.