Encyclopedia
Please note this is a comparison between Version 1 by jean Michel Brunel and Version 2 by Beatrix Zheng.
The series of 21 oleanolic acid derivatives containing di- and polyamine fragments at position C3 and C28 was synthesized and evaluated for their antimicrobial activities against both Gram-positive and Gram-negative bacterial. Almost all series presented good to moderate Minimum Inhibitory Concentrations (MIC) against Gram-positive S. aureus, S. faecalis and B. cereus bacteria, moreover compounds possess important antimicrobial activities against Gram-negative E. coli, P. aeruginosa, S. enterica, and EA289 bacteria with MICs ranging from 6.25 to 200 µg/mL. The SAR data showed that the nature of the polyamine fragment, as well as differences in the structure of oleanolic acid plays an important role in the potential activities of tested compounds. The testing of the ability to restore the antibiotic activity of doxycycline and erythromycin at a 2 µg/mL concentration in a synergistic assay showed that only Mannich base with spermine fragment 6 lead to a moderate improvement in terms of antimicrobial activities of the different selected combinations against both P. aeruginosa and E. coli. The study of the mechanism of action of the most important compound in this series (amide 2i derived from N-methyl-norspermidine) showed the effect of disruption of the outer bacterial membrane of P. aeruginosa PA01 cells. Computational ADMET profiling renders compound 2i as a suitable starting point for pharmacokinetic optimization.
- oleanolic acid
- triterpenic polyamine conjugates
- antimicrobial activities
- antibiotic enhancers
1. Introduction
Multidrug resistance (MDR) to antibiotics leads to serious issues in the treatment of microbial infections, which means an extremely high need for the search and development of new antimicrobial agents [1]. One solution consists of combining therapies of existing antibiotics with potentiating adjuvants (chemosensitizer agents), which re-empower the antibiotic agents to become efficacious against the resistant strains [2]. These compounds do not directly kill bacteria but enhance antibiotic activity by inhibiting antibiotic-modifying enzymes or by increasing the intracellular antibiotic accumulation through inhibiting efflux pumps, as well as facilitating the permeation of antibiotics entrance across membranes. Adjuvants can also target the biofilm formation by inhibiting the signaling and regulatory pathways that mediate antibiotic resistance as well as enhance the host defense by stimulating the immune cells [3]. The pharmaceutical market contains primarily antibiotics obtained from natural substances by means of semi-synthetic modifications and synthetic analogs of natural antibiotics [4].
Today, development of natural products for the prevention and treatment of diseases continue to attract attention worldwide [5][6][5,6]. Among them, plant-derived triterpenes represent an interesting class of molecules with a multitude of activities that make them references for drug-discovery programs as proven by the numerous ongoing clinical trials and drugs on the market. For example, the lupane-type triterpene betulin, which is the main component of birch bark extract, has medicinal properties for epidermolysis bullosa, wounds and burns [7]. Betulinic acid, which comes from oxidation of betulin, is under clinical trials for the treatment of dysplastic nevus syndrome and psychological stress [8], and boswellic acid, isolated from Boswellia serrata, is under trial for treatment of relapsing remitting multiple sclerosis, renal stones, joint pain and stiffness [9]. The synthetic derivative of oleanolic acid CDDO-Me demonstrated its efficacy as anticancer drug in different mouse models, and versus several types of cancer [10]. Ursolic acid, isolated from plants as Rosmarinus officinalis, Malus domestica, Salvia officinalis, and Thymus vulgarisis, is under clinical trials for use against metabolic syndrome and sarcopenia [11].
In recent years, triterpenes were identified as a promising class for the development of new antibiotics, as they are active against antibiotic-resistant strains [12]. The most complete picture of the antimicrobial properties of triterpenoids is presented for the Staphylococcus aureus culture, native and semi-synthetic derivatives of the ursane, oleanane, and lupane series being identified as the most promising [13]. Although there is no specific report on their modes of action, the inhibition of S. aureus growth through novel targets should stimulate research to develop triterpenic acids as effective agents against S. aureus bacteria [14]. Furthermore, it was shown that ursolic acid has a synergistic effect when associated with ampicillin and tetracycline against both Bacillus cereus and S. aureus [15]. Moreover, oleanolic and ursolic acids may be useful when administered in combination with β-lactam antibiotics to combat bacterial infections caused by some Gram-positive pathogens [16].
There are limited data about the antimicrobial activity of semi-synthetic derivatives derived from native triterpenoids [17]. On the other hand, polyamines conjugates are becoming important in all the biological and medicinal fields [18]. It is shown [19] that C3 or C28 functionalization of betulinic acid with the formation of polyamine or polyarginine derivatives (or their shorter analogs [20]) can significantly increase the MIC index ranging from 3.125–12.5 and 3.9–7.8 mg/L for S. aureus, respectively. Spermine derivatives of heterobetulonic and ursolic acids displayed antimicrobial activity on S. aureus, Streptococcus mutans and Listeria monocytogenes at a concentration of 6.25 mM, and cytotoxicity on different cancer cell lines and were characterized as supramolecular systems [21]. The amphiphile-like oleanolic acid-triazole-spermine conjugates self-assemble into highly entangled fibrous networks leading to gelation which can be used for multifunctional soft organic nanomaterials [22]. Triterpenic polyamines being analogs of steroidal squalamine and trodusquemine, were found as important substances for the search of new drugs with anticancer, antidiabetic and antimicrobial activities [23]. Oleanolic acid conjugates with homopyperazine, and diethylenetriamine fragments demonstrated a high inhibitory activity against C. trachomatis with a chemotherapeutic index (CTI) of 8 and >8, respectively [24].
2. Current Insights
This study was conducted on different bacterial strains including Gram-negative bacteria that have been proved to be the less sensitive strains than the Gram-positive ones, being resistant to many classes of antibiotics due to their outer lipidic membrane that restricts the access of molecules to the periplasm. Recently, we reported a great activity of polyaminosterol derivatives for their intrinsic antimicrobial activities [25][35] or for their use in combination [26][42]. Concerning the mechanism of action of our best derivative 2i (derived from N-methyl-norspermidine), our data suggest that this compound disrupts the outer membrane integrity of the P. aeruginosa Gram-negative bacteria by acting in a similar manner to PMBn. It is well admitted that disruption of Gram-negative bacterial cytoplasmic membrane constituted the main mechanism of action by polymyxin B [27][43]. Conversely, polymyxin B nonapeptide (PMBN) that lacks the fatty acyl tail does not possess activity but is able to compromise the outer membrane integrity [28][29][30][31][44,45,46,47]. Thus, an outer membrane permeabilization could occur because of the interactions between charges but the hydrophobic acyl tail could improve this phenomenon. The lack of activity of PMBN as well as our derivative 2i, however, tends to indicate that the outer membrane is a site of interaction, but it is not the killing target.
All of our data clearly suggest that the nature of the polyamine fragment, as well as the structure of the oleanane core plays an important role in the potential activities of tested compounds. Polyamine amides with 3β-hydroxy-group 7a and 7b did not show high activity, at the same time oxidation to the 3-oxo-group in case of analogs 2h and 2i was effective. However, 3β-hydroxy-derivative 7c showed higher activity against Gram-negative S. enterica (MIC 12.5 µg/mL) and EA289 (MIC 25 µg/mL) and against Gram-positive B. cereus (MIC 25 µg/mL) in comparison with 3-oxo-analog 2j. Diaminoderivatives 2a, 2b and 2c possess antimicrobial activity only against Gram-positive bacteria with MICs from 6.25 to 200 µg/mL. Among the 3-oxo-oleanane-polymethylenpolyamine amides only pentaamine 2j and spermine 2l derivatives presented antimicrobial activity against all the considered strains whereas aminopropoxy 2m analog of 2j did not lead to any activity against Gram-negative bacteria. Additionally, product 3 and N-propargyl amide of 3-oxo-oleanolic acid 5 were not active, while modification of the latter one in the Mannich base 6 with spermine fragment led to a high activity against all the bacterial strains. Among the derivatives possessing heterocyclic moieties such as 2-oxopyrrolidino- 2d and morpholino- 2e analogs they appeared not active, while piperazine containing derivative 2f possess antimicrobial activities with the highest value of MICs of 50 and 12.5 µg/mL, against Gram-negative E. coli ATCC 28922 and Gram-positive S. aureus, respectively. It is also noteworthy that methyl ester derivative 4 presenting a piperazine fragment at C3 was not active.
Increasing methylene group amounts in a series of triamine derivatives provided a positive effect: thus, compounds 2h and 2i possess higher antimicrobial activity than previously described for 2g [32][26]. Substituted secondary amine group with methyl fragment (compound 2i), provided increasing of activity compared with 2h. Additionally, derivatives 2h and 2i presenting a 3β-hydroxy group (7a, 7b) did not show activity.
On the other hand, the ADME study means that most of the administered 2i will be confined to the circulatory system. Generally, it indicates a low therapeutic index and necessitates a high loading dose to achieve therapeutic plasma concentration. High lipophilicity is known to contribute to human plasma protein binding the most [33][48]. In particular, α1-acid glycoprotein primarily binds basic and hydrophobic compounds, e.g., steroids [34][49], which renders it a suitable carrier for 2i. Blood-brain permeation for 2i cannot be ruled out unambiguously. All services predicted that it will be oxidized by cytochrome P450 3A4. The compound is unlikely to inhibit common cytochromes but CYP3A4 induction may affect the pharmacokinetic profiles of concomitantly administered CYP3A4-metabolized drugs [35][50]. Clearance is predicted to be relatively slow, however, due to low VD and high plasma protein binding elimination half-life T1/2 is also short. According to predicted moderate oral acute toxicity the compound can be attributed to Category 3 according to GHS classification with no toxicity towards heart, liver, mutagenic or carcinogenic properties.
Overall, computational ADMET profiling renders compound 2i as a suitable starting point for pharmacokinetic optimization. It is orally available and has favorable safety properties. Possible drawbacks include low solubility, volume of distribution and short plasma half-life. These issues might be addressed by lipophilicity management maintaining metabolic stability (e.g., introduction of unsaturated carbon-carbon bonds [36][51] and replacement of hydrogen with fluoride [37][52]).
