Synthesis and in vitro and in vivo evaluation of antimalarial polyamines

Polyamine-Drug Conjugates: Do They Boost Drug Activity?

Over the past two decades, the strategy of conjugating polyamine tails with bioactive molecules such as anticancer and antimicrobial agents, as well as antioxidant and neuroprotective scaffolds, has been widely exploited to enhance their pharmacological profile. Polyamine transport is elevated in many pathological conditions, suggesting that the polyamine portion could improve cellular and subcellular uptake of the conjugate via the polyamine transporter system. In this review, we have presented a glimpse on the polyamine conjugate scenario, classified by therapeutic area, of the last decade with the aim of highlighting achievements and fostering future developments.

α,ω-Diacyl-Substituted Analogues of Natural and Unnatural Polyamines: Identification of Potent Bactericides That Selectively Target Bacterial Membranes

In this study, α-ω-disubstituted polyamines exhibit a range of potentially useful biological activities, including antimicrobial and antibiotic potentiation properties. We have prepared an expanded set of diarylbis(thioureido)polyamines that vary in central polyamine core length, identifying analogues with potent methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli, Acinetobacter baumannii and Candida albicans growth inhibition properties, in addition to the ability to enhance action of doxycycline towards Gram-negative bacterium Pseudomonas aeruginosa. The observation of associated cytotoxicity/hemolytic properties prompted synthesis of an alternative series of diacylpolyamines that explored aromatic head groups of varying lipophilicity. Examples bearing terminal groups each containing two phenyl rings (15a-f, 16a-f) were found to have optimal intrinsic antimicrobial properties, with MRSA being the most susceptible organism. A lack of observed cytotoxicity or hemolytic properties for all but the longest polyamine chain variants identified these as non-toxic Gram-positive antimicrobials worthy of further study. Analogues bearing either one or three aromatic-ring-containing head groups were either generally devoid of antimicrobial properties (one ring) or cytotoxic/hemolytic (three rings), defining a rather narrow range of head group lipophilicity that affords selectivity for Gram-positive bacterial membranes versus mammalian. Analogue 15d is bactericidal and targets the Gram-positive bacterial membrane.

Valorisation of the diterpene podocarpic acid - Antibiotic and antibiotic enhancing activities of polyamine conjugates

As part of our search for new antimicrobials and antibiotic adjuvants, a series of podocarpic acid-polyamine conjugates have been synthesized. The library of compounds made use of the phenolic and carboxylic acid moieties of the diterpene allowing attachment of polyamines (PA) of different lengths to afford a structurally-diverse set of analogues. Evaluation of the conjugates for intrinsic antimicrobial properties identified two derivatives of interest: a PA3-4-3 (spermine) amide-bonded variant 7a that was a non-cytotoxic, non-hemolytic potent growth inhibitor of Gram-positive Staphylococcus aureus (MRSA) and 9d, a PA3-8-3 carbamate derivative that was a non-toxic selective antifungal towards Cryptococcus neoformans. Of the compound set, only one example exhibited activity towards Gram-negative bacteria. However, in the presence of sub-therapeutic amounts of either doxycycline (4.5 µM) or erythromycin (2.7 μM) several analogues were observed to exhibit weak to modest antibiotic adjuvant properties against Pseudomonas aeruginosa and/or Escherichia coli. The observation of strong cytotoxicity and/or hemolytic properties for subsets of the library, in particular those analogues bearing methyl ester or n-pentylamide functionality, highlighted the fine balance of structural requirements and lipophilicity for antimicrobial activity as opposed to mammalian cell toxicity.

Repurposing primaquine as a polyamine conjugate to become an antibiotic adjuvant

In our search for new antibiotic adjuvants as a novel strategy to deal with the emergence of multi-drug resistant (MDR) bacteria, a series of succinylprimaquine-polyamine (SPQ-PA) conjugates and derivatives of a cationic amphiphilic nature have been prepared. Evaluation of these primaquine conjugates for intrinsic antimicrobial properties and the ability to restore the antibiotic activity of doxycycline identified two derivatives, SPQ-PA3-8-3 and SPQ-PA3-10-3 that exhibited intrinsic activity against the Gram-positive bacteria Staphylococcus aureus and the yeast Cryptococcus neoformans. None of the analogues were active against the Gram-negative bacterium Pseudomonas aeruginosa. However, in the presence of a sub-therapeutic amount of doxycycline (4.5 µM), both SPQ-PA3-4-3 and SPQ-PA3-10-3 compounds displayed potent antibiotic adjuvant properties against P. aeruginosa, with MIC's of 6.25 µM. A series of derivatives were prepared to investigate the structure-activity relationship that explored the influence of both a simplified aryl lipophilic substituent and variation of the length of the polyamine scaffold on observed intrinsic antimicrobial properties and the ability to potentiate the action of doxycycline against P. aeruginosa.

Acylpolyamine Mygalin as a TLR4 Antagonist Based on Molecular Docking and In Vitro Analyses

Toll-like receptors (TLRs) are transmembrane proteins that are key regulators of innate and adaptive immune responses, particularly TLR4, and they have been identified as potential drug targets for the treatment of disease. Several low-molecular-weight compounds are being considered as new drug targets for various applications, including as immune modulators. Mygalin, a 417 Da synthetic bis-acylpolyamine, is an analog of spermidine that has microbicidal activity. In this study, we investigated the effect of mygalin on the innate immune response based on a virtual screening (VS) and molecular docking analysis. Bone marrow-derived macrophages and the cell lines J774A.1 and RAW 264.7 stimulated with lipopolysaccharide (LPS) were used to confirm the data obtained in silico. Virtual screening and molecular docking suggested that mygalin binds to TLR4 via the protein myeloid differentiation factor 2 (MD-2) and LPS. Macrophages stimulated by mygalin plus LPS showed suppressed gene expression of tumor necrosis factor (TNF-α), interleukine 6 (IL-6), cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS), as well as inhibition of signaling protein p65 of the nuclear factor κB (NF-κB), resulting in decreased production of nitric oxide (NO) and TNF-α. These results indicate that mygalin has anti-inflammatory potential, being an attractive option to be explored. In addition, we reinforce the importance of virtual screening analysis to assist in the discovery of new drugs.

Cu(I)- and Pd(0)-Catalyzed Arylation of Oxadiamines with Fluorinated Halogenobenzenes: Comparison of Efficiency

The comparison of the possibilities of Pd- and Cu-catalyzed amination reactions using fluorine-containing aryl bromides and iodides with oxadiamines to produce their N,N′-diaryl derivatives was carried out. The dependence of the reactivity of the aryl halides on the nature of the substituents and halogen atoms as well as on the structure of oxadiamines was investigated. It was found that the copper-catalyzed reactions were somewhat comparable with the palladium-mediated processes in the majority of cases, especially in the reactions with para-fluorine- and para-(trifluoromethyl)-substituted aryl halides, although the necessity to use aryl iodides in the Cu(I)-catalyzed amination was obvious. Pd catalysis was found inevitable for the successful amination of more sterically hindered ortho-(trifluoromethyl)aryl bromides.

Exploration of the antibiotic potentiating activity of indolglyoxylpolyamines

A series of substituted di-indolglyoxylamido-spermine analogues were prepared and evaluated for intrinsic antimicrobial properties and the ability to enhance antibiotic action. As a compound class, intrinsic activity was typically observed towards Gram-positive bacteria and the fungus Cryptococcus neoformans, with notable exceptions being the 5-bromo- and 6-chloro-indole analogues which also exhibited modest activity (MIC 34-50 μM) towards the Gram-negative bacteria Escherichia coli and Klebsiella pneumoniae. Several analogues enhanced the activity of doxycycline towards the Gram-negative bacteria Pseudomonas aeruginosa, E. coli, K. pneumoniae and Acinetobacter baumannii. Of particular note was the identification of five antibiotic enhancing analogues (5-Br, 7-F, 5-Me, 7-Me, 7-OMe) which also exhibited low to no cytotoxicity and red blood cell haemolytic properties. The mechanisms of action of the 5-Br and 7-F analogues were attributed to the ability to disrupt the integrity of, and depolarize, bacterial membranes.

Uptake and metabolism of arginine impact Plasmodium development in the liver

Prior to infecting erythrocytes and causing malaria symptoms, Plasmodium parasites undergo an obligatory phase of invasion and extensive replication inside their mammalian host's liver cells that depends on the parasite's ability to obtain the nutrients it requires for its intra-hepatic growth and multiplication. Here, we show that L-arginine (Arg) uptake through the host cell's SLC7A2-encoded transporters is essential for the parasite's development and maturation in the liver. Our data suggest that the Arg that is taken up is primarily metabolized by the arginase pathway to produce the polyamines required for Plasmodium growth. Although the parasite may hijack the host's biosynthesis pathway, it relies mainly upon its own arginase-AdoMetDC/ODC pathway to acquire the polyamines it needs to develop. These results identify for the first time a pivotal role for Arg-dependent polyamine production during Plasmodium's hepatic development and pave the way to the exploitation of strategies to impact liver infection by the malaria parasite through the modulation of Arg uptake and polyamine synthesis.

Interrogating alkyl and arylalkylpolyamino (bis)urea and (bis)thiourea isosteres as potent antimalarial chemotypes against multiple lifecycle forms of Plasmodium falciparum parasites

A new series of potent potent aryl/alkylated (bis)urea- and (bis)thiourea polyamine analogues were synthesized and evaluated in vitro for their antiplasmodial activity. Altering the carbon backbone and terminal substituents increased the potency of analogues in the compound library 3-fold, with the most active compounds, 15 and 16, showing half-maximal inhibitory concentrations (IC50 values) of 28 and 30 nM, respectively, against various Plasmodium falciparum parasite strains without any cross-resistance. In vitro evaluation of the cytotoxicity of these analogues revealed marked selectivity towards targeting malaria parasites compared to mammalian HepG2 cells (>5000-fold lower IC50 against the parasite). Preliminary biological evaluation of the polyamine analogue antiplasmodial phenotype revealed that (bis)urea compounds target parasite asexual proliferation, whereas (bis)thiourea compounds of the same series have the unique ability to block transmissible gametocyte forms of the parasite, indicating pluripharmacology against proliferative and non-proliferative forms of the parasite. In this manuscript, we describe these results and postulate a refined structure-activity relationship (SAR) model for antiplasmodial polyamine analogues. The terminally aryl/alkylated (bis)urea- and (bis)thiourea-polyamine analogues featuring a 3-5-3 or 3-6-3 carbon backbone represent a structurally novel and distinct class of potential antiplasmodials with activities in the low nanomolar range, and high selectivity against various lifecycle forms of P. falciparum parasites.

The antimicrobial activity of mono-, bis-, tris-, and tetracationic amphiphiles derived from simple polyamine platforms

A series of 34 amphiphilic compounds varying in both number of quaternary ammonium groups and length of alkyl chains has been assembled. The synthetic preparations for these structures are simple and generally high-yielding, proceeding in 1-2 steps without the need for chromatography. Antibacterial MIC data for these compounds were determined, and over half boast single digit MIC values against a series of gram-positive and gram-negative bacteria. MIC variation mostly hinged on the length of the alkyl chain, where a dodecyl group led to optimal activity; surprisingly, the number of cations and/or basic nitrogens was less important in dictating bioactivity. Additional structural variation was prepared in a trisamine series dubbed 12,3,X,3,12, providing a series of potent amphiphiles functionalized with varied allyl, alkyl, and benzyl groups. Tetraamines were also investigated, culminating in a two-step preparation of a tetracationic structure that showed only modestly improved bioactivity versus amphiphiles with two or three cations.

Investigation of indolglyoxamide and indolacetamide analogues of polyamines as antimalarial and antitrypanosomal agents

Pure compound screening has previously identified the indolglyoxylamidospermidine ascidian metabolites didemnidine A and B (2 and 3) to be weak growth inhibitors of Trypanosoma brucei rhodesiense (IC₅₀ 59 and 44 μM, respectively) and Plasmodium falciparum (K1 dual drug resistant strain) (IC₅₀ 41 and 15 μM, respectively), but lacking in selectivity (L6 rat myoblast, IC₅₀ 24 μM and 25 μM, respectively). To expand the structure–activity relationship of this compound class towards both parasites, we have prepared and biologically tested a library of analogues that includes indoleglyoxyl and indoleacetic “capping acids”, and polyamines including spermine (PA3-4-3) and extended analogues PA3-8-3 and PA3-12-3. 7-Methoxy substituted indoleglyoxylamides were typically found to exhibit the most potent antimalarial activity (IC₅₀ 10–92 nM) but with varying degrees of selectivity versus the L6 rat myoblast cell line. A 6-methoxyindolglyoxylamide analogue was the most potent growth inhibitor of T.brucei (IC₅₀ 0.18 μM) identified in the study: it, however, also exhibited poor selectivity (L6 IC₅₀ 6.0 μM). There was no apparent correlation between antimalarial and anti-T. brucei activity in the series. In vivo evaluation of one analogue against Plasmodium berghei was undertaken, demonstrating a modest 20.9% reduction in parasitaemia.

Discovery and evaluation of thiazinoquinones as anti-protozoal agents

Pure compound screening has identified the dioxothiazino-quinoline-quinone ascidian metabolite ascidiathiazone A (2) to be a moderate growth inhibitor of Trypanosoma brucei rhodesiense (IC₅₀ 3.1 μM) and Plasmodium falciparum (K1 dual drug resistant strain) (IC₅₀ 3.3 μM) while exhibiting low levels of cytotoxicity (L6, IC₅₀ 167 μM). A series of C-7 amide and Δ²⁽³⁾ analogues were prepared that explored the influence of lipophilicity and oxidation state on observed anti-protozoal activity and selectivity. Little variation in anti-malarial potency was observed (IC₅₀ 0.62–6.5 μM), and no correlation was apparent between anti-malarial and anti-T. brucei activity. Phenethylamide 7e and Δ²⁽³⁾-glycine analogue 8k exhibited similar anti-Pf activity to 2 but with slightly enhanced selectivity (SI 72 and 93, respectively), while Δ²⁽³⁾-phenethylamide 8e (IC₅₀ 0.67 μM, SI 78) exhibited improved potency and selectivity towards T. brucei rhodesiense compared to the natural product hit. A second series of analogues were prepared that replaced the quinoline ring of 2 with benzofuran or benzothiophene moieties. While esters 10a/10b and 15 were once again found to exhibit cytotoxicity, carboxylic acid analogues exhibited potent anti-Pf activity (IC₅₀ 0.34–0.035 μM) combined with excellent selectivity (SI 560–4000). In vivo evaluation of a furan carboxylic acid analogue against P. berghei was undertaken, demonstrating 85.7% and 47% reductions in parasitaemia with ip or oral dosing respectively.