In vitro Antimicrobial Activity of Royleanone Derivatives Against Gram-Positive Bacterial Pathogens

* CorrespUniversidatical SciencE-mail: fas

PHYTOTHERAPY RESEARCHPhytother. Res. (2013)Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/ptr.4961

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In vitro Antimicrobial Activity of RoyleanoneDerivatives Against Gram-PositiveBacterial Pathogens

Patrícia Rijo,1,2 Aida Duarte,1 Ana Paula Francisco,1 Teresa Semedo-Lemsaddek3and Maria Fátima Simões1*1Faculdade de Farmácia da Universidade de Lisboa, Research Institute of Medicines and Pharmaceutical Sciences – iMed.UL, Av. Prof.Gama Pinto, 1649-003, Lisboa, Portugal2CBios – Research Center for Health Sciences & Technologies (Lab. Pharmacology and Therapeutics), Universidade Lusófona,Campo Grande 376, 1749-024, Lisboa, Portugal3Faculdade de Medicina Veterinária, Centro de Investigação Interdisciplinar em Sanidade Animal, Universidade Técnica de Lisboa,Av. da Universidade Técnica, 1300-477, Lisboa, Portugal

Infections caused by multiresistant bacterial pathogens are a significant problem worldwide, turning the searchfor natural compounds to act as alternatives to antibiotics of major importance. The aim of the present studywas to investigate the in vitro antimicrobial activity of 7a-acetoxy-6b-hydroxyroyleanone (1), isolated fromPlectranthus grandidentatus (Lamiaceae), and 11 additional royleanone abietane derivatives of 1 (2–12) againstimportant Gram-positive human bacterial pathogens. Results showed that the aromatic and alkylic esters 2, 3 and5 are more active than 1 against Enterococcus and Staphylococcus (minimum inhibitory concentration (MIC)values ranging from 0.98 to 62.50mg/mL). Moreover, 7a-acetoxy-6b-hydroxy-12-O-(4-chloro)benzoylroyleanone(2) gave rise to a new antibacterial-prototype (MIC values of 3.91–15.63mg/mL against Staphylococcus andof 0.98–3.91mg/mL against Enterococcus). The results showed that the compounds under analysis also pres-ent antimicrobial activity against resistant bacteria. The hydrophobic extra-interactions with bacterial tar-gets seem to play an important role on the activity of royleanones derivatives. Copyright © 2013 JohnWiley & Sons, Ltd.

Keywords: antimicrobial activity; abietane diterpene derivatives; royleanone esters; resistant bacterial pathogens.

INTRODUCTION

Hospital and community-acquired infections causedby the spread of bacteria resistant to multiple classes ofantibiotics are a serious health problem worldwide.Gram-positive bacteria, such as methicillin-resistantStaphylococcus aureus (MRSA) and vancomycin-resistant Enterococci (VRE), greatly contribute tothese concerns, mostly regarding immunosupressed,postsurgical, oncologic and organ transplanted patients(Maruyama et al., 2009; Namba et al., 2008; Mitchell,2007; Patel, 2003). The long-time established use ofplant extracts to treat human infections directedresearchers to search for antibacterial agents from plantsources, or their hemisynthezised derivates, a promis-ing approach, corroborated by the number of paperspublished in recent years (Newman and Cragg, 2012;van Vuuren and Viljoen, 2011). Among naturalcompounds, abietane diterpenes with phenolic andquinonic moieties are known to be interesting plantmetabolites with diverse pharmacological activities,namely, antimicrobial (Radulović et al., 2010; Hanson,2009; van Vuuren, 2008; Hanson, 2006; Hanson, 2004;Hanson, 1999; Hanson, 1998), antitubercular (Rijo et al.,

ondence to: Maria Fátima Simões, Faculdade de Farmácia dade de Lisboa, Research Institute of Medicines and Pharmaceu-es, iMed.UL, Av. Prof. Gama Pinto, 1649-003 Lisboa, [email protected]

© 2013 John Wiley & Sons, Ltd.

2010; Hanson, 1999) and cytotoxic (Fronza et al., 2012;Hanson, 2006; Hanson, 2005). However, a literature surveydoes not reveal significant attempts to modulate theseactivities on royleanone abietanes.

Royleanones are tricyclic abietane diterpenes with ahydroxyquinone moiety, vastly distributed throughLamiaceae being 7a-acetoxy-6b-hydroxyroyleanone (1)often isolated, namely, from Plectranthus species(Gaspar-Marques et al., 2006). Plants belonging to thisgenus are used for their medicinal properties, forinstance in the African continent (van Vuuren, 2008;Lukhoba et al., 2006). Our previous antimicrobial studieson royleanone metabolites revealed that they were activeagainst Staphylococcus and Enterococcus spp. Moreover,a preliminary structure activity relationship analysisof ten natural abietanes led to the conclusion that thepresence of a C ring with a 12-hydroxy-p-benzoquinonemoiety and an oxidized B ring at C-6/C-7 positions wassignificant for the activity (Gaspar-Marques et al., 2006).In addition, Young et al. (2001) reported that the presenceof additional benzyl groups influenced the anti-MRSAactivities of phenolic abietanoids. Along with other similarditerpene metabolites active against Gram-positivebacteria are totarol and abietic acid. These diterpeneshave the ability to transverse or damage the bacterialcytoplasmatic membrane due to their amphipathic char-acter. Besides that, it was described that antibacterialactivity may be modulated through an increase inlipophilicity and/or in hydrogen-bond donor/acceptorgroups of the hydrophilic moiety (Urzúa et al., 2008;

Received 24 May 2012Revised 01 February 2013

Accepted 04 February 2013

P. RIJO ET AL.

Bernabeu et al., 2002). However, neither the completemechanism of action nor their biological targets areentirely known.During the present investigation, we tried to identify

the structural requirements involved in the activity againstGram-positive bacteria. From P. grandidentatus, thenatural compound 7a-acetoxy-6b-hydroxyroyleanonewas obtained and used to hemisynthezise 11 additionalroyleanone abietane derivatives. Then, the microdilutionmethod was used to determine the minimum inhibitoryconcentration (MIC) against Gram-positive pathogensconsideredmain threats to humans, namely, Staphylococcusaureus and Enterococcus faecalis.

MATERIALS AND METHODS

Chemistry. 7a-Acetoxy-6b-hydroxyroyleanone (abietane 1),a metabolite produced in a relatively high concentrationby the easily cultivated plant Plectranthus grandidentatus,was isolated from the acetone extract as previouslydescribed (Gaspar-Marques et al., 2006). Subsequently,a set of 11 derivatives (abietanes 2 to 12), all esters ofHO-C6 and/or HO-C12, was hemisynthesized followingprocedures formerly described (Rijo et al., 2010).

Microorganisms and growth conditions.The antimicrobialassays were performed using both antibiotic sensitiveand resistant bacteria, namely, methicillin-sensitiveStaphylococcus aureus ATCC 25924, ATCC 43866 andATCC 700699; methicillin-resistant S. aureus CIP 106760and FFHB 29593 (MRSA); vancomycin-sensitiveEnterococcus faecalis FFHB 427483, E. casseliflavusATCC 49996, E. faecium FFHB 435628 and low-levelvancomycin-resistant E. faecalis ATCC 51299 (VRE).

Figure 1. Tautomers a and b of 7a-acetoxy-6b-hydroxyroyleanone(1). Horminone is the 7a-hydroxy analog of 1.

Determination of minimum inhibitory concentrations.The MICs were determined using the microplate brothmicrodilutionmethod according toClinical andLaboratoryStandards Institute (2006). Briefly, bacteria were culturedon Muller-Hinton agar; following bacterial growth, astandardized bacterial suspension, corresponding to0.5 McFarland, was prepared and used to inoculatemicrotiter plates with increasing concentrations of thecompounds under analysis; after incubation, turbidityof the broth in the wells was observed; MIC was definedas the lowest concentration of compound at which novisible growth could be detected. All assays wereperformed in triplicate, and positive (containing inoculumsbut no extracts) and negative (containing diterpenoidsbut no inoculums) controls were included at all times.

Figure 2. 12-O-Ester derivatives of 1: 7a-acetoxy-6b-hydroxy-12-O-(4-chloro)benzoylroyleanone (2), 7a-acetoxy-6b-hydroxy-12-O-(4-methoxy)benzoylroyleanone (3) and 7a-acetoxy-6b-hydroxy-12-O-(4-nitro)benzoylroyleanone (4).

Cytotoxicity assay. Cell growth-inhibition data towardVero cells, evaluated as the percent inhibition of cellgrowth and defined as GI50 (the concentration of thetested sample resulting in a 50% reduction of absorbancecompared with untreated controls), were those alreadyreported in a previous publication (Rijo et al., 2010).The selective index was estimated as SI =GI50(mg/mL)/MIC value (mg/mL).

Copyright © 2013 John Wiley & Sons, Ltd.

RESULTS AND DISCUSSION

Chemistry

The metabolite 7a-acetoxy-6b-hydroxyroyleanone (1),produced by P. grandidentatus (Gaspar-Marques et al.,2006), was isolated and used to hemisynthesize a set of11 derivatives (abietanes 2 to 12) (Rijo et al., 2010). Proto-type 1 has a lipophilic ring system, two hydroxyl groupsboth with potential hydrogen-bond donor/acceptor char-acteristics (at C6 and C12) but with different electronicand chemical behavior (Fig. 1) and three more oxygenatoms with hydrogen-bond acceptor abilities (carbonylgroups). Thus, the analogs 2–12 were prepared having inmind the plausible importance of the H-binding groups(hydroxyl, quinone and ester), van derWaals interactionsand hydrophobic binding moieties (hydrocarbon skele-ton, alkylic and aromatic features) in the interaction withthe bacterial targets. The derivatives 2–12, prepared from1, aremore lipophilic (Rijo et al., 2010) esters having extrahydrogen-bond acceptors atoms (mostly oxygen fromcarbonyl) and lower number of hydrogen-bond donoratoms. These derivatives may be chemically classified intothree groups: a set that preserved the alcoholic substituentat C6 (2–4) (Fig. 2), another without any free hydroxylgroups (5–8) (Fig. 3) and the remaining 9–12 all keepingthe more acidic OH group (at C12) (Fig. 4). This divisioncorresponds, nearly, to three levels of antibacterial activity(on the basis of simple arithmetic mean calculations).

Antimicrobial activity

During the present study, the effect of the substituent groupson the antibacterial activity of 1 against methicillin-sensibleand methicillin-resistant S. aureus, and vancomycin-sensibleand vancomycin-resistantEnterococcus species was inves-tigated. A previous assay showed that all derivatives(2–12) were inactive toward Gram-negative bacteria(MIC values >125 mg/mL), including Escherichia coli

Phytother. Res. (2013)

Figure 3. 6b,12-O-Diesters: 7a-acetoxy-6b-propionyloxy-12-O-propionylroyleanone (5), 7a-acetoxy-6b-benzoyloxy-12-O-benzo-ylroyleanone (6), 6b,7a-diacetoxy-12-O-acetylroyleanone (7) and 7a-acetoxy-6b-(4-chloro)benzoyloxy-12-O-(4-chloro)benzoylroyleanone (8).

Figure 4. 6b-O-Ester derivatives: 7a-acetoxy-6b-propionyloxyroy-leanone (9), 7a-acetoxy-6b-(4-nitro)benzoyloxyroyleanone (10),7a-acetoxy-6b-butyryloxyroyleanone (11) and 6b,7a-diacetoxyroy-leanone (12).

ANTIMICROBIAL ROYLEANONE DERIVATIVES

(ATCC 25922) and Pseudomonas aeruginosa (ATCC27853), in agreement with former observations(Gaspar-Marques et al., 2006; Tada and Ishimaru,2006; Yang et al., 2001).

Table 1. Minimum inhibitory concentration (MIC) values of compoundsmethicillin-resistant S. aureus (MRSA) strains.

Compoundsb

MSSA

ATCC25923 ATCC 43866

1 31.25 15.632 3.91 3.913 62.50 3.914 62.50 7.815 15.63 7.816 62.50 15.637 62.50 15.638 31.25 1259 31.25 31.2510 62.50 15.6311 62.50 31.2512 62.50 125DMSOd 250 250AMPe <0.49 >250METe 0.98 1.95OXYe 0.98 31.25TETe <0.49 125VANe 1.95 3.91

aMIC value was taken as the lowest concentration of compound inhibitiReader set to 630nm). MIC values are means of, at least, three experimbCompound concentration range: 500–0.49mg/mL (Müeller-Hinton brothcFFHB species are clinical isolates from Hospital do Barreiro and depLisbon University.dSolvent.eControl antibiotics: AMP, ampicillin; MET, methicillin; OXY, oxytetracyc


Despite template 1 to be almost equipotent againstboth Staphylococcus and Enterococcus, all derivatives(2–12) showed higher inhibitory growth propertiesagainst Enterococcus spp. than against Staphylococcusspp. (Tables 1 and 2). Compounds 2–11 inhibited thegrowth of VRE (Table 2) showing MIC values rangingfrom 1.95 to 62.50 mg/mL. Moreover, 2–10 inhibited thetested MRSA strains (MIC range 3.91–125 mg/mL).Nevertheless, the lowest MIC values observed for theresistant bacteria under analysis were exhibited by 2, 3and 5 (1.95–7.81 mg/mL), becoming them candidatesfor potential usage against the aforementioned bacteria.

The first group (2–4) and 5 showed the strongestactivity against Enterococcus spp. (MIC 0.98–15.63mg/mL)being also more active than 1 (MIC 7.81–15.63mg/mL)(Table 2). Besides, 2 and 5 (Figs 2 and 3) were alsothe most active derivatives against Staphylococcusspp. (MIC 3.91–15.63 mg/mL) and more active than1 (MIC 7.81–31.25 mg/mL).

The compounds 6–8 of the second group were lessactive against all analyzed bacteria (MIC 7.81–125mg/mL)than 2–5. Even so, the abietanes 6 and 7 are significantlyactive against the VRE strain with MIC values of7.81 mg/mL. Compounds of the third group (9–12)showed MIC values ranging from 15.63 to 125 mg/mLagainst the tested bacteria, revealing to be in most casesless active than 1.

The first partial conclusion is that oxygen atomattached to C12, most certainly, does not act as hydrogen-bond donor, and thus, this kind of interaction is notessential for activity. In fact, the more active compounds(2–5) have an ester functional group on C12. Consistently,

1–12 (mg/mL)a against methicillin-sensitive S. aureus (MSSA) and

MRSA

ATCC 700699 CIP 106760 FFHB 29593c

31.25 7.81 7.8115.63 3.91 7.8131.25 3.91 3.917.81 15.63 15.637.81 3.91 7.81

31.25 15.63 12531.25 15.63 62.5062.50 125 12531.25 15.63 31.2562.50 15.63 31.2531.25 31.25 31.2531.25 62.50 125

250 250 250<0.49 >250 12515.63 >250 >25062.50 31.25 <0.4962.50 31.25 <0.497.81 3.91 1.95

ng the bacterial growth (measured with an Absorbance Microplateents.medium).

osited on the Microbiology Laboratory of the Pharmacy Faculty,

line; TET, tetracycline; VAN, vancomycin.


Table 2. Minimum inhibitory concentration values of compounds 1–12 (mg/mL)a against Enterococcus species.

Compounds

Enterococcus faecalis Enterococcus casseliflavus Enterococcus faecium

ATCC 51299b FFHB 427483c ATCC 49996 FFHB 435628c

1 15.63 7.81 7.81 15.632 3.91 1.95 0.98 1.953 1.95 0.98 3.91 0.984 1.95 3.91 3.91 3.915 3.91 3.91 7.81 3.916 7.81 7.81 7.81 15.637 7.81 7.81 15.63 15.638 62.50 31.25 31.25 31.259 15.63 15.63 15.63 15.6310 15.63 7.81 15.63 31.2511 15.63 15.63 62.50 15.6312 31.25 15.63 62.50 15.63DMSOd 125 125 125 125AMPe <0.49 <0,49 <0.49 >125TETe <0.49 31.25 <0.49 <0.49VANe 62.50 1.95 3.91 0.98

aValues are means of, at least, three experiments.bLow level vancomycin-resistant Enterococcus.cFFHB species are clinical isolates from Hospital do Barreiro, deposited on the Microbiology Laboratory, Pharmacy Faculty of LisbonUniversity.dSolvent.eControl antibiotics: AMP, ampicillin; TET, tetracycline; VAN, vancomycin.

P. RIJO ET AL.

those with a free hydroxyl on C12 (as 9–12), which couldact as hydrogen-bond donor, are less active somehowsupporting similar previous assertions (Radulović et al.,2010). We believe that the prevalence of the tautomericform with a 1,2,4-quinone structure on the ring C (as thetautomer 1b represented on the Fig. 1) may contribute tothis behavior. Additionally, the importance for activity oftheOH-C6 (2–5), as hydrogen-bond donor, is questionablebecause of its low dissociation capacity and stereohindrance. However, the role of the hydroxyl groupsversus the carbonyl groups of the ester functions as hydro-gen-bond acceptors was not enough clarified, on the basisof the results obtained in this work. Most importantly, thesignificant increase of the activity revealed by compounds2–5, seems to result from the higher lipophilicity (Rijoet al., 2010) brought by the aromatic/aliphatic motif linkedto the ester groups (Figs 2 and 3). In fact, aromatic ringsmay improve the activity of antibacterial prototypesthrough additional hydrophobic and van der Waals inter-actions with both wall and membrane of bacterial cells.Moreover, 2–5may also interact with the ammonium ionsof the peptidoglycans and lipoteichoic acid constituents ofthe membrane system of Gram-positive bacteria, becauseof hydrogen bonding or induced dipole interactions(Patrick, 2009; Chopra and Roberts, 2001). The abietane2 showed the bestMIC values (0.98–15.63mg/mL) becauseof, must certainly, the extra hydrophobic interactionsof chlorine. Highlighting the difference betweenthe MIC values of the two 4-chlorobenzoic esters 2(12-O-(4-chloro)benzoyl monoester) and 8 (6b,12-O-(4-chloro)benzoyl diester) of, respectively, 0.98–15.63mg/mLand 31.25–125mg/mL allows further considerations. Itmay be inferred that 8 has a too high lipophilicity (Tables 1and 2) with an unfavorable shape and steric hindrance,and an incorrect spatial distribution of the hydrophobicmoieties, which may prevent the efficient binding tothe bacterial wall.


Spiridonov et al. (2003) suggested that the primary targetof the royleanones as antimicrobial (and antitumoral)agents is the cell membrane. Their mode of action shouldbe a consequence of the protonophoric activity due to thedissociation of the C12 hydroxyl group. Theoretical studies(Nicolás and Castro, 2006; Nicolás et al., 2003) postulatedthat the activity of horminone, which is structurally closeto 1 (Fig. 1), could be correlated with the activity of oxytet-racycline having a similar bacteriostatic mode of action. Infact, both have chelating abilities to form complexes withdivalent cations, with preference by Mg2+. Oxygen atomsattached to C7 and C14 behave as Lewis bases and coordi-nate withMg2+ ions. Thus, horminone (Fig. 1) should crossthe membrane system of bacteria (a ΔpH potential drivenprocess) and, within the cytoplasm, should inhibit proteinbiosynthesis by interaction of the hydrated horminone-Mg2+ complex with ribosomal RNA (Nicolás andCastro, 2006; Chopra and Roberts, 2001). But, contraryto oxytetracycline, both horminone, several royleanones(Gaspar-Marques et al., 2006; Tada and Ishimaru, 2006;Yang et al., 2001) and derivatives 2–12 are predominantlyactive againstGram-positive bacteria. Thismay be a conse-quence of the differences on bacterial cell walls. Thosesmall molecules may go across the porous cell wall of theGram-positive bacteria, through teichoic acids, whereasto cross the outer membrane of Gram-negative bacteria,they would need to move across the protein porins chan-nels, which would reduce their diffusion probability(Nelson et al., 2009). To the best of our knowledge, furtherinformation about the antimicrobial mode of action of theroyleanonic abietanes is unavailable. However, as all com-pounds (1–12) (Figs 1–4) show a decreased basic characterof the oxygen atoms of the acetyl group (C7), when com-pared with the Lewis basic character of the hydroxyl (C7)of horminone (Fig. 1) (Nicolás and Castro, 2006), it isdifficult to believe that their mode of action might involvethe coordination with Mg2+ cation as postulated for


Teichoic acid

Table 3. Selective indexa of compounds 1–3 calculated on the basis of cytotoxicity data toward Vero cellsb and on minimum inhibitoryconcentration (MIC) valuesc

Compounds

Staphylococcus aureus Enterococcus faecalisEnterococcuscasseliflavus

Enterococcusfaecium

ATCC ATCC ATCC CIP FFHB ATCC FFHB ATCC 49996 FFHB

25923 43866 700699 106760 29593 51299 427483 435628

1 <1 <1 <1 1.64 1.64 <1 1.64 1.64 <12 1.13 1.13 <1 1.13 <1 1.13 2.27 4.51 2.273 <1 <1 <1 <1 <1 1.38 2.76 <1 2.76

aSI=GI50 (mg/mL)/MIC value (mg/mL).bGI50 (Vero cells): 1=12.80; 2=4.42; 3=2.70mg/mL (Rijo et al., 2010).cMIC values on Tables 1 and 2.

ANTIMICROBIAL ROYLEANONE DERIVATIVES

horminone, unless esters hydrolysis occur. Indeed, furtherstudies are required to clarify how royleanones interactwith their biological targets.Unfortunately, selectivity against bacterial cells is not

quite satisfactory. Actually, selective index (Table 3),calculated on the basis of cytotoxicity data previouslyreported (Rijo et al., 2010) and on MIC values herepresented show that only template 1, and derivatives 2and 3 have SI values >1 (Table 3) toward some strains.Nevertheless, this first approach to find antimicrobialroyleanone-prototypes provided compounds with betterMIC values that may lead to potential antibacterialagents or, more probably, to synergistic-abietanes tobe used with common antibiotics (Gibbons, 2008;Hemaiswarya et al., 2008). However, to improve thepotency, selectivity and, at the end, to reach compoundswith attractive absorption, disposition, metabolism,excretion and toxicity properties, further efforts arerequired (Simpson et al., 2009).


The more relevant derivative as growth inhibitorof Gram-positive bacteria is 7a-acetoxy-6b-hydroxy-12-O-(4-chloro)benzoylroyleanone (2), which shows themore interesting SI values, is less cytotoxic than 1 (Rijoet al., 2010) and shows the lowest MIC values, mainly,against the Enterococcus species.

Acknowledgements

We would like to thank Prof. Benjamín Rodríguez (IQOG-CSIC,Madrid) for his unconditional expert advice, the Portuguese‘Fundação para a Ciência e a Tecnologia’ (FCT-MEC) for the financialsupport to iMed.UL and PhD grant no. SFRH/BD/19250/2004.

Conflict of Interest

The authors have declared that there is no conflict of interest.

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In vitro Antimicrobial Activity of Royleanone Derivatives Against Gram-Positive Bacterial Pathogens