Definition of crucial structural factors of acetogenins, potent inhibitors of mitochondrial complex I

Uncovering the Mechanism of Action of Antiprotozoal Agents: A Survey on Photoaffinity Labeling Strategy

Plasmodium, Leishmania, and Trypanosoma parasites are responsible for infectious diseases threatening millions of people worldwide. Despite more recent efforts devoted to the search for new antiprotozoal agents, efficacy, safety, and resistance issues still hinder the development of suited therapeutic options. The lack of robustly validated targets and the complexity of parasite's diseases have made phenotypic screening a preferential drug discovery strategy for the identification of new chemical entities. However, via this approach, no information on biological target(s) and mechanisms of action of compounds are provided. Among the target deconvolution strategies useful to fill this gap, photoaffinity labeling (PAL) has emerged as one of most suited to enable investigation in a complex cellular environment. More recently, PAL has been exploited to unravel the molecular basis of bioactive compounds' function in live parasites, allowing elucidation of the mechanism of action of both approved drugs and new chemical entities. Besides highlighting new potential drug targets, PAL can provide valuable information on efficacy and liabilities of small molecules at the molecular level, which could be exploited to greatly facilitate the rational optimization of compounds in terms of potency and safety. In this review, we will report the most recent studies that have leveraged PAL to disclose the biological targets and mechanism of action of phenotypically active compounds targeting kinetoplastid diseases (i.e., human African trypanosomiasis, leishmaniasis, and Chagas disease) and malaria. Moreover, we will comment on potential perspectives that this innovative approach can provide in aiding the discovery and development of new antiprotozoal drugs.

Recent advances of mitochondrial complex I inhibitors for cancer therapy: Current status and future perspectives

Mitochondrial complex I (CI) as a critical multifunctional respiratory complex of electron transport chain (ETC) in mitochondrial oxidative phosphorylation has been identified as vital and essence in ATP production, biosynthesis and redox balance. Recent progress in targeting CI has provided both insight and inspiration for oncotherapy, highlighting that the development of CI-targeting inhibitors is a promising therapeutic approach to fight cancer. Natural products possessing of ample scaffold diversity and structural complexity are the majority source of CI inhibitors, although low specificity and safety hinder their extensive application. Along with the gradual deepening in understanding of CI structure and function, significant progress has been achieved in exploiting novel and selective small molecules targeting CI. Among them, IACS-010759 had been approved by FDA for phase I trial in advanced cancers. Moreover, drug repurposing represents an effective and prospective strategy for CI inhibitor discovery. In this review, we mainly elaborate the biological function of CI in tumor progression, summarize the CI inhibitors reported in recent years and discuss the further perspectives for CI inhibitor application, expecting this work may provide insights into innovative discovery of CI-targeting drugs for cancer treatment.

Cryo-electron microscopy reveals how acetogenins inhibit mitochondrial respiratory complex I

Mitochondrial complex I (NADH:ubiquinone oxidoreductase), a crucial enzyme in energy metabolism, captures the redox potential energy from NADH oxidation/ubiquinone reduction to create the proton motive force used to drive ATP synthesis in oxidative phosphorylation. High-resolution single-particle electron cryo-EM analyses have provided detailed structural knowledge of the catalytic machinery of complex I, but not of the molecular principles of its energy transduction mechanism. Although ubiquinone is considered to bind in a long channel at the interface of the membrane-embedded and hydrophilic domains, with channel residues likely involved in coupling substrate reduction to proton translocation, no structures with the channel fully occupied have yet been described. Here, we report the structure (determined by cryo-EM) of mouse complex I with a tight-binding natural product acetogenin inhibitor, which resembles the native substrate, bound along the full length of the expected ubiquinone-binding channel. Our structure reveals the mode of acetogenin binding and the molecular basis for structure-activity relationships within the acetogenin family. It also shows that acetogenins are such potent inhibitors because they are highly hydrophobic molecules that contain two specific hydrophilic moieties spaced to lock into two hydrophilic regions of the otherwise hydrophobic channel. The central hydrophilic section of the channel does not favor binding of the isoprenoid chain when the native substrate is fully bound but stabilizes the ubiquinone/ubiquinol headgroup as it transits to/from the active site. Therefore, the amphipathic nature of the channel supports both tight binding of the amphipathic inhibitor and rapid exchange of the ubiquinone/ubiquinol substrate and product.

Photo-affinity labelling and biochemical analyses identify the target of trypanocidal simplified natural product analogues

Current drugs to treat African sleeping sickness are inadequate and new therapies are urgently required. As part of a medicinal chemistry programme based upon the simplification of acetogenin-type ether scaffolds, we previously reported the promising trypanocidal activity of compound 1, a bis-tetrahydropyran 1,4-triazole (B-THP-T) inhibitor. This study aims to identify the protein target(s) of this class of compound in Trypanosoma brucei to understand its mode of action and aid further structural optimisation. We used compound 3, a diazirine- and alkyne-containing bi-functional photo-affinity probe analogue of our lead B-THP-T, compound 1, to identify potential targets of our lead compound in the procyclic form T. brucei. Bi-functional compound 3 was UV cross-linked to its target(s) in vivo and biotin affinity or Cy5.5 reporter tags were subsequently appended by Cu(II)-catalysed azide-alkyne cycloaddition. The biotinylated protein adducts were isolated with streptavidin affinity beads and subsequent LC-MSMS identified the FoF1-ATP synthase (mitochondrial complex V) as a potential target. This target identification was confirmed using various different approaches. We show that (i) compound 1 decreases cellular ATP levels (ii) by inhibiting oxidative phosphorylation (iii) at the FoF1-ATP synthase. Furthermore, the use of GFP-PTP-tagged subunits of the FoF1-ATP synthase, shows that our compounds bind specifically to both the α- and β-subunits of the ATP synthase. The FoF1-ATP synthase is a target of our simplified acetogenin-type analogues. This mitochondrial complex is essential in both procyclic and bloodstream forms of T. brucei and its identification as our target will enable further inhibitor optimisation towards future drug discovery. Furthermore, the photo-affinity labeling technique described here can be readily applied to other drugs of unknown targets to identify their modes of action and facilitate more broadly therapeutic drug design in any pathogen or disease model.

Acetogenins from Annonaceae

In recent decades, annonaceous acetogenins have become highly studied plant secondary metabolites in terms of their isolation, structure elucidation, synthesis, biological evaluation, mechanism of action, and toxicity. The aim of the present contribution is to summarize chemical and biological reports published since 1997 on annonaceous acetogenins and synthetic acetogenin mimics. The compounds are considered biologically in terms of their cytotoxicity for cancer cell lines, neurotoxicity, pesticidal effects, and miscellaneous activities.

Larvicidal and cytotoxic potential of squamocin on the midgut of Aedes aegypti (Diptera: Culicidae)

Acetogenins are secondary metabolites exclusively produced by Annonaceae, which have antitumor, cytotoxic, and pesticide activities. In this study, we evaluated the larvicidal and cytotoxic effect of squamocin from Annona squamosa on Aedes aegypti (Diptera: Culicidae) midgut. The compound was solubilized in 2% Tween 20 at 10, 20, 50, 80 and 100 ppm. The assay was conducted in a completely randomized design with four replications, each with 20 third-instar larvae. Larval mortality was assessed every hour until total mortality, and the data were subjected to Probit analysis. Cellular damage was evaluated every 30 min in groups comprising five larvae subjected to squamocin at 50 and 100 ppm for 240 min. The total larval mortality occurred after 360 min following application of 50, 80, and 100 ppm squamocin, and 600 min after applying other concentrations with LC₅₀ at 6.4 ppm. Both 50 and 100 ppm of squamocin showed cytotoxic activity in the midgut epithelium of A. aegypti after 240 min with 50 ppm resulting in midgut cells with light cytoplasm containing small vacuoles, whereas at 100 ppm were found cells with cytoplasm highly vacuolated, damaged apical surface and cell protrusion toward the gut lumen. In conclusion, squamocin has the potential to control A. aegypti.

Critical role of a methyl group on the γ-lactone ring of annonaceous acetogenins in the potent inhibition of mitochondrial complex I

C34-epi and C34-epi-C35-trifluoro analogues of solamin, a mono-THF annonaceous acetogenin, were synthesized. Their inhibitory activity, along with previously synthesized analogues (C35-fluoro, C35-difluoro, and C35-trifluorosolamins), against bovine mitochondrial NADH-ubiquinone oxidoreductase (complex I) was determined. The present study revealed that the methyl group on the γ-lactone moiety is critical to the potent inhibition of complex I by natural acetogenins.

Six cytotoxic annonaceous acetogenins from Annona squamosa seeds

Custard apple (Annona squamosa L.) is an edible tropical fruit, and its seeds had been used in south China as a folk medicine to treat "malignant sore" (cancer) and as an insecticide. Phytochemical investigation of the ethanol fraction of custard apple seeds led to the isolation of six new annonaceous acetogenins: annosquacins A-D (1-4), annosquatin A (5) and annosquatin B (6). Their structures were elucidated by spectroscopic analysis. Compounds 1-4 are adjacent bistetrahydrofuran annonaceous acetogenins. Compounds 5 and 6 are non-adjacent bistetrahydrofuran annonaceous acetogenins and the first examples in which the tetrahydrofuran ring system is located between C-9 and C-20. The absolute configurations of 1-6 were defined by the application of the Mosher method. Compounds 1-6 exhibited potent cytotoxic activity in vitro against five human tumour cell lines. Compounds 5 and 6 showed a high selectivity toward the MCF-7 and A-549 cell line respectively.

In vitro and in vivo antiproliferative activity of laherradurin and cherimolin-2 of Annona diversifolia Saff

Acetogenins from Annonaceae (ACG) are potent inhibitors of the mitochondrial complex I, they present cytotoxic activity on neoplasic lines, including those with multiresistance to drugs. In vivo antitumor activities of some ACG have been reported; however, no information is available regarding the relationship between their cytotoxic activity in cell cultures and their antiproliferative action in vivo. Laherradurin and cherimolin-2 acetogenins were isolated from Annona diversifolia seeds, and their inhibitory potential was analysed in vitro on HeLa and SW-480 cells. Doses containing 1, 10, 100 and 500 times the IC50 obtained from the proliferation assays and multiplied by the weight of the animal, were injected once daily into athymic mice bearing these cancer cell lines; their effect upon tumor growth was measured over a period of 20 days. Laherradurin was more active than cherimolin-2, and it showed in in vitro proliferation assays a similar IC50 in both neoplasic lines. In athymic mice, laherradurin administered in 10x, 100x and 500x doses, reduced the size of HeLa tumors, and with 100x and 500x doses, affected the SW-480 tumor development. These doses were similar to results found with the control drug doxorubicin (p < or = 0.05). On the other hand, cherimolin-2 had an effect on HeLa tumors cells at 100x and 500x doses (p < or = 0.05).

Medicinal chemistry of Annonaceous acetogenins: design, synthesis, and biological evaluation of novel analogues

Most Annonaceous acetogenins are characterized by between one and three THF ring(s) with one or two flanking hydroxyl group(s) in the center of a C32/34 fatty acid, and the terminal carboxylic acid is combined with a 2-propanol unit to form an alpha,beta-unsaturated gamma-lactone. While many studies have addressed the properties and synthesis of natural acetogenins due to their attractive biological activities and unique structural features, a number of analogues have also been described. This review covers the design, synthesis, and biological evaluation of acetogenin analogues.

Synthesis and antitumor activity of C-9 epimers of the tetrahydrofuran containing acetogenin 4-deoxyannoreticuin

A highly convergent synthesis of mono-tetrahydrofuran (THF) containing acetogenins, that is based on the cross-metathesis of THF and butenolide alkene precursors, was developed. This methodology was applied to the epimers of the C-9 alcohol of 4-deoxyannoreticuin, in an attempt to assign the configuration at this position in the naturally occurring material. Unfortunately, identification of one or the other epimeric structures with the natural product was not possible because of the closeness of the physical data for all three compounds. Both C-9 epimeric analogues showed similar cytotoxicity in the low micromolar range, against two human tumor cell lines PC-3 (prostate) and Jurkat (T-cell leukemia). This result contrasts to previous studies on closely related THF acetogenins, wherein configurational variation at analogous carbinol centers resulted in a significant effect on antitumor activity.

Goniothalamus species: a source of drugs for the treatment of cancers and bacterial infections?

Irrespective of the presence of cytotoxic acetogenins and styryl-lactones in the genus Goniothalamus, only 22 species in the genus Goniothalamus, out of 160 species (13.7%) have so far been investigated. In an effort to promote further research on the genus Goniothalamus which could represent a source of drugs for the treatment of cancers and bacterial infections, this work offers a broad analysis of current knowledge on Goniothalamus species. Therefore, it includes (i) taxonomy (ii) botanical description (iii) traditional medicinal uses and (iv) phytochemical and pharmacological studies. We discuss the molecular mechanisms of actions of acetogenins and styryl-lactones, with some emphasis on the possible involvement of protein kinase, Bax and TRAIL receptors in the cytotoxic effects of styryl-lactones. We also report (v) the growth inhibition of several nosocomial bacteria by Goniothalamus. scortechinii. The crude methanol extract of G. scortechinii showed a good and broad spectrum of antibacterial activity against both Gram-negative and Gram-positive bacteria.

Synthesis of Murisolin, (15R, 16R, 19R, 20S)-Murisolin A, and (15R, 16R, 19S, 20S)-16,19-cis-Murisolin and Their Inhibitory Action with Bovine Heart Mitochondrial Complex I

The asymmetric total synthesis of murisolin, (15R, 16R, 19R, 20S)-murisolin A, and (15R, 16R, 19S, 20S)-16,19-cis-murisolin was performed by using an epoxy alcohol as a versatile chiral building block for synthesizing the stereoisomers of mono-THF annonaceous acetogenins. The inhibitory activity of these murisolin compounds was examined with bovine heart mitochondrial complex I, and they showed almost the same activity.

Semisynthesis and screening of a small library of pro-apoptotic squamocin analogues: selection and study of a benzoquinone hybrid with an improved biological profile

Acetogenins of Annonaceae, including squamocin (1), exert spectacular cytotoxicity and the most potent inhibition of NADH:ubiquinone oxidoreductase known so far. Cell death induced by these natural products was identified as apoptosis and was thought to be linked to alterations in mitochondrial function. Quinone-squamocin hybrid compounds were semisynthesized and evaluated for their pro-apoptotic properties with a screening method based on dissipation of the mitochondrial transmembrane potential (DeltaPsim). Herein, we report a short one-step synthesis of a squamocin carboxylic acid analogue. For the first time on a natural product, the radical decarboxylation and quinone addition reaction has enabled preparation of a library of squamocin-quinone hybrids and four other analogues. Squamoquinone, tenfold more potent than squamocin as an inducer of apoptosis, emerged as a promising compound, as it induces apoptosis through a mitochondrial caspase-dependent pathway.

Acetogenins from Annonaceae: recent progress in isolation, synthesis and mechanisms of action

The aim of the present review is to summarise the knowledge about newly isolated acetogenins (ACGs) in the last six years. It will also report the total syntheses that have allowed either the confirmation or the revision of some structures, together with the biological activities and mechanism of action of such interesting natural products. In fact, of the 417 isolated compounds reviewed, over 176 have been added during the period from 1998 to 2004.

Synthesis and mitochondrial complex I inhibition of dihydroxy-cohibin A, non-THF annonaceous acetogenin analogue

To elucidate the inhibitory action of acetogenins, we synthesized an acetogenin derivative which possesses tetraol in place of the tetrahydrofuran ring and examined its inhibitory activity against bovine heart mitochondrial complex I. Our results indicate that these hydroxy groups are an essential structural factor though it is not effective as bis-THF hydroxy groups combination.

In vitro antileishmanial activity of acetogenins from Annonaceae

Twelve acetogenins from Annonaceae were evaluated in vitro for their antileishmanial activities in order to search for new lead-compounds having antileishmanial properties. The compounds were comparatively evaluated by the 50% inhibitory concentrations (IC50) determination on promastigote forms of wild-type and four drug-resistant lines of Leishmania donovani. In addition, after testing the toxicity on mouse peritoneal macrophages, the compounds were evaluated on amastigote infected macrophages and a therapeutic index was calculated. The IC50 of the acetogenins against promastigote forms of L. donovani was in a range 4.7-47.3 microM. The most active compound was Rolliniastatin 1 (IC50 at 4.7 microM). On the intramacrophage amastigote in vitro model, only seven compounds exhibited measurable antileishmanial activity with IC50 values in a range 2.5-29.7 microM. Rollinistatin 1 was the most interesting compound with IC50 of 2.5 microM and it appears as the most promising one on the basis of therapeutic index (18.08). Isoannonacin, which is active against intramacrophagic amastigotes (IC50 of 6.2 microM) with a therapeutic index of 2.05, exhibited a strong action on drug-resistant strains (IC50 from 5.1 to 9.8 microM). Acetogenins are a new chemical series with interesting in vitro antileishmanial activity and further studies will be focused on the understanding of this selectivity in regard to the membrane and mitochondrial action using specific probes.

Essential structural features of acetogenins: role of hydroxy groups adjacent to the bis-THF rings

The presence of two hydroxy groups adjacent to the THF ring(s) is a common structural feature of natural acetogenins. To elucidate the role of each hydroxy group in the inhibitory action of acetogenins, we synthesized three acetogenin analogues which lack either or both of the hydroxy groups, and investigated their inhibitory activities with bovine heart mitochondrial complex I. Our results indicate that the presence of either of the two hydroxy groups sufficiently sustains a potent inhibitory effect.

Synthesis and inhibitory activity of ubiquinone-acetogenin hybrid inhibitor with bovine mitochondrial complex I

To elucidate the inhibitory action of acetogenins, the most potent inhibitors of mitochondrial complex I, we synthesized an acetogenin analogue which possesses a ubiquinone ring (i.e., the physiological substrate of complex I) in place of the alpha,beta-unsaturated gamma-lactone ring of natural acetogenins, and named it Q-acetogenin. Our results indicate that the gamma-lactone ring of acetogenins is completely substitutable with the ubiquinone ring. This fact is discussed in light of the inhibitory action of acetogenins.

Inhibitory activities of three annonaceous acetogenins on NADH oxidase of chicken liver mitochondria

Annonaceous acetogenins (ACG) are natural products found in the plant family Annonaceae and which strongly inhibited mitochondrial complex I. The inhibition of NADH oxidase of chicken liver mitochondria by three different structural ACG was studied here, and ACG was shown to have potent inhibitory activities similar to rotenone for NADH oxidase. The IC(50) values indicated that bis-adjacent tetrahydrofuran (THF) type squamocin C was more potent than non-adjacent bis-THF type squamostatin B, and the latter was more potent than non-THF type compound 1 in the assay. The roles of structural factors of ACG such as the terminal gamma-lactone, the features of other ring moieties and hydroxyl groups, as well as the alkyl chain were simply discussed in this study.

Essential structural factors of acetogenins, potent inhibitors of mitochondrial complex I

To elucidate the role of the hydrophobic alkyl tail of acetogenins in the inhibitory action, we synthesized an acetogenin derivative possessing the shortest tail (i.e., methyl group) and examined its inhibitory activity against bovine heart mitochondrial complex I. Our results indicated that the alkyl tail, which is one of the common structural features of natural acetogenins, is not an essential structural factor required for the potent inhibition.

Probing the ubiquinone reduction site in bovine mitochondrial complex I using a series of synthetic ubiquinones and inhibitors

Studies of the structure-activity relationships of ubiquinones and specific inhibitors are helpful to probe the structural and functional features of the ubiquinone reduction site of bovine heart mitochondrial complex I. Bulky exogenous short-chain ubiquinones serve as sufficient electron acceptors from the physiological ubiquinone reduction site of bovine complex I. This feature is in marked contrast to other respiratory enzymes such as mitochondrial complexes II and III. For various complex I inhibitors, including the most potent inhibitors, acetogenins, the essential structural factors that markedly affect the inhibitory potency are not necessarily obvious. Thus, the loose recognition by the enzyme of substrate and inhibitor structures may reflect the large cavity like structure of the ubiquinone (or inhibitor) binding domain in the enzyme. On the other hand, several phenomena are difficult to explain by a simple one-catalytic site model for ubiquinone.