Sesquiterpene Lactones from Gynoxys verrucosa and their Anti-MRSA Activity

Abstract

Ethnopharmacological relevance

Because of its virulence and antibiotic resistance, Staphylococcus aureus is a more formidable pathogen now than at any time since the pre-antibiotic era. In an effort to identify and develop novel antimicrobial agents with activity against this pathogen, we have examined Gynoxys verrucosa Wedd (Asteraceae), an herb used in traditional medicine in southern Ecuador for the treatment and healing of wounds.

Materials and Methods

The sesquiterpene lactones leucodine (1) and dehydroleucodine (2) were extracted and purified from the aerial parts of G. verrucosa, and their structure was elucidated by spectroscopic methods and single-crystal X-ray analysis. The in vitro anti-microbial activity of G. verrucosa extracts and its purified constituents was determined against six clinical isolates including S. aureus and Staphylococcus epidermidis strains with different drug-resistance profiles, using the microtiter broth method.

Results

Compound 1 has very low activity, while compound 2 has moderate activity with MIC₅₀s between 49 and195 μg/mL. The extract of G. verrucosa has weak activity with MIC₅₀s between 908 and 3290 μg/mL.

Conclusions

We are reporting the full assignment of the ¹H-NMR and ¹³C-NMR of both compounds, and the crystal structure of compound 2, for the first time. Moreover, the fact that compound 2 has antimicrobial activity and compound 1 does not, demonstrates that the exocyclic conjugated methylene in the lactone ring is essential for the antimicrobial activity of these sesquiterpene lactones. However, the weak activity observed for the plants extracts, does not explain the use of G. verrucosa in traditional medicine for the treatment of wounds and skin infections.

1. Introduction

Staphylococcus aureus is a devastating bacterial pathogen capable of causing a broad array of infections in both humans and animals. The ability to treat these infections is compromised by the continued emergence of antibiotic-resistant strains. This includes strains resistant to methicillin which in recent years have become predominant even among community-acquired isolates (Deurenberg and Stobberingh, 2009). For example, in 2005, invasive infections caused by methicillin resistant S. aureus (MRSA) affected 94,630 patients in the US alone, resulting in approximately 18,650 deaths – exceeding the national mortality rates for HIV/AIDS (Klevens et al., 2007).

Most ominous is the appearance of high-level vanA-mediated vancomycin resistance even among methicillin-resistant S. aureus (MRSA). Although these isolates remain rare (Zhu et al., 2008), this is likely to change given, the increasing reliance on vancomycin as a consequence of the continued emergence of MRSA. New agents with activity against these strains exist, but resistance to these has also begun to appear (Lee, 2008). This emphasizes the urgent need to identify and develop novel antimicrobial agents with activity against MRSA. One possible source of such agents is natural products derived from various biological sources including plants. In this context it has been recently emphasized that natural products, by acting through different mechanisms from that of conventional antibiotics, could be of clinical value in the treatment of resistant bacteria (Simoes et al., 2009).

G. verrucosa is a shrub belonging to the Senecionea tribe of the Asteraceae family. The aerial parts of this plant, commonly know as guangalo, are used in traditional medicine in southern Ecuador for the treatment of skin infections and wound healing by direct application to the skin (Tene et al., 2007).

Phytochemical investigation of the ethyl-acetate extract from the aerial parts of G. verrucosa led to the isolation of two known sesquiterpene lactones, of the guainolide group, leucodine (1) and dehydroleucodine (2) (Fig. 1). These compounds have not been previously reported in this plant species but they have been reported in other members of the Asteraceae family, including Artemisia douglasiana Besser (Wendel et al., 2008), Picris koreana Vorosch. (Michalska et al., 2007), Warionia saharae Benthem ex Coss. (Hilmi et al., 2003), Kaunia rufescens (P.W. Lund ex DC.) R.M. King & H. Rob. (Rucker et al., 1997), Achillea collina (Becker ex Rchb. f.) Heimerl (Trendafilova et al., 2006) and Stevia pilosa Lag. (Martínez and Muñoz-Zamora, 1988). However, the full assignment of the ¹H-NMR and ¹³C-NMR of either compound or the crystal structure of compound 2 have not been reported. Although, the biological activity of compound 2 has been demonstrated in various biological systems (Wendel et al., 2008; Vega et al., 2009), its activity against Gram-positive bacteria, in general, or Staphylococcus sp., in particular, has not been previously reported.

Fig. 1.

Structures of leucodine (1), dehydrolecudodine (2) and 3-D-ORTEP projection of the X-ray crystal structure of dehydroleucodine with 50% probability ellipsoids (3).

2. Materials and Methods

2.1. General Experimental Procedures

Melting points were measured with a capillary melting point apparatus and are uncorrected. IR spectra and optical rotation were measured on a Magna IR 550 spectrometer and an Autpol III (Rudolph) polarimeter, respectively. UV spectra were recorded on a Hewlett-Packard 8452A diode array spectrophotometer. NMR spectra were recorded in CDCl₃, at 25°C, on a Varian Unity NMR spectrometer, operating at 500 MHz for ¹H and 125 MHz for ¹³C spectra, equipped with a Nalorac 3 mm inverse detection probe with a z axis gradient coil. Each sample consisted of ca. 5 mg of compound dissolved in 1 mL of CDCl₃ containing TMS as an internal reference. All 1D and 2D spectra were acquired and processed with standard Varian software. Mass spectral data was obtained in an Agilent 5975 GC/MSD instrument operating under EI conditions at 70 eV. Column chromatography was carried out using 60 – 230 mesh silica gel. Preparative TLC was performed on pre-coated silica gel 60 F₂₅₄ plates (0.2 mm thick, Merck).

2. 2. Plant material

The aerial parts of G. verrucosa were collected in June 2007 in the locality of Yangana, in the Loja province of Ecuador. Voucher specimens (PPN-as-11) are deposited at the Herbarium of the Applied Chemistry Institute of the Universidad Técnica Particular de Loja, Loja Ecuador, and at the Herbarium Reynaldo Espinoza of the Universidad Nacional de Loja.

2.3. Extraction and isolation

The air-dried plant material (200g) was extracted with ethyl acetate (dynamic maceration for 5 h) at room temperature and concentrated under reduced pressure. The extract (14 g) was filtered through a reverse phase C₁₈ column (LiChroprep Merck 25–40 μm) with a mixture MeOH/H₂O 85:15, for the removal of chlorophylls. The filtrate (7g) was fractioned by column chromatography using a hexane-EtOAc gradient. Compound 2 was eluted in the hexane: EtOAc (85:15) fraction (0.7g) and recrystallized from EtOAc as a white crystalline solid. Compound 1 was isolated from the mother liquid using preparative TLC hexane: EtOAc (70:30) and recrystallized from EtOAc.

2.4. Physical and spectroscopic data

Leucodine (1): colorless crystals (EtOAc), mp 199–200°C; [α]²⁵_D +61.7 (c 0.41, CH₂Cl₂); UV (MeOH) λ_sh (log ε) 214 (3.78) nm, λ_max (log ε) 256 (4.22) nm; IR (KBr) v_max 2940, 1780, 1686, 1637, 1620 cm⁻¹; EI-MS: m/z (rel. int.) 246 (100), 231 (10), 217 (26), 173 (31), 159 (16), 145 (19), 135 (14), 115 (9), 105 (16), 91 (42), 77 (19); ¹H NMR (CDCl₃, 500 MHz) see Table 1; ¹³C NMR (CDCl₃, 125 MHz) see Table 1.

Table 1.

NMR Spectroscopy data at 500 MHz for ¹H and 125 MHz for ¹³C for compounds 1 and 2.

	Leucodine (1)				Dehydroleucodine (2)
Position	δH	δC	HMBC	COSY	δH	δC	HMBC	COSY
1		169.92				169.4
2	6.16 (q,1.2)	135.55	195.9, 169.9, 131.9, 52.6, 19,8	3.41, 2.29	6.17 (q, 1.2)	135.8	195.7, 131.9, 53.0, 169.4, 19.8	2.33
3		195.91				195.7
4		131.85				131.9
5		152.10				151.8
6	2.42 (ddd, 14.5, 12.5, 2) 2.33 (ddd, 14.5, 6, 2)	37.55	152.1, 131.9, 56.4, 26.0, [21.6]a	2.33, 1.99, 1.36 2.42, 1.99, 1.36	2.52 (ddd, 1.5, 12.5, 2) 2.39 (ddd, 14.5, 6, 2)	37.3	151.9, 131.9, 53.1, 24.5 151.9, 131.9, 53,1, 21.8	2.39, 2.22, 1.44 2.52, 2.22, 144
7	1.99 (m) 1.36 (dddd, 14, 12,10, 2)	25.98	152.1, 84.2, 56.4, 37,5	2.42, 2.33, 1.95, 1.36 2.42, 2.33, 1.99, 1,95	2.22 (dddd, 14, 6, 3,2) 1.44 (dddd, 14, 12, 10, 2)	24.5	151.9, 84.4, 53.1, 37.3 151.9, 84.4, 53.1, 37.3	2.89, 2.52, 2.39, 1.44 2.89, 2.52, 2.39, 2.22
8	1.96 (m)	56.37	84.2, 52.6, 41.1, 12.3	2.25, 1.99, 1.36	2.89 (ddd, 12, 10, 3)	53.1	138.5, 119.0, 84.4, 37.3	6.18, 5.47, 3.62, 2.22, 1.44
9	3.62 (t,10.0)	84.20	163.9, 131.9, [56.4], 41.1, 26.0	3.41, 1.95	3.62 (t,10)	84.4	[138.5], 131.9, 53.0, 24.5, [169.4]	3.52, 2.89
10	3.41 (bd, 10)	52.56	[195.9], 169.9, 152.1, 135.6, 131.9, 84,6	6.16, 3.62, [2.44], 2.29	3.52 (bd, 10)	53.0	[151.9], 135.8, 131.9, 84.4, 53.1, 169.4	3.62, 2.44, 2.33
11		177.54				169.1
12	2.25 (dq, 12.5, 6.9)	41.13	177.5, 56.4, 26.0, 12.3	1.95, 1.27		138.5
13	1.27 (d,6.9, 3H)	12.28	177.5, 56.4, 41.1	2.25	6.18 (d,3.3) 5.47 (d,3.3)	119.0	169.1, 138.5, 53,1 169.1, [138.5], 53.1	5.47, 2.89 6.18, 2.89
14	2.29 (t, 1.2, 3H)	19.80	169.9, 135.6, 52.6	6.16, 3.41	2.33 (t,1.2, 3H)	19.8	135.8, 53.0, 169.4	6.17, 3.52
15	2.44 (bs, 3H)	21.61	152.1, 131.9, 37.6	[3.4]	2.44(bs,3H)	21.8	151.9, 131.9, 37.3	3.52

^aFigures in brackets detonate weak long range correlations

Dehydroleucodine (2): colorless crystals (EtOAc), mp 128–130°C; [α]²⁵_D +48.98 (c 1.42, CH₂Cl₂); UV (MeOH) λ_max (log ε) 216 (3.91) nm, 256 (4.20) nm; IR (KBr) v_max 3098, 2970, 1780, 1681, 1631, 1615 cm⁻¹; EI-MS: m/z (rel. int.) 244 (100), 183 (13), 173 (13), 159 (15), 145 (18), 133 (10), 115 (13), 91 (46), 77 (22), 53 (22);. ¹H NMR (CDCl₃, 500 MHz) see Table 1; ¹³C NMR (CDCl₃, 125 MHz) see Table 1.

X-ray Data of compound 2: Crystal data: C₁₅H₁₆O_3. Mw = 244.29, orthorhombic, space group P2₁2₁2₁, a = 7.5784(5) Å, b = 11.1203(7)Å, c = 15.3702(11)Å, V = 1295.31(15) ų, Z = 4, crystal dimensions: 0.22 × 0.37 × 0.6 mm, T = 223 K Rigaku SCXmini system, Mercury 2 CCD detector, Mo-Kα radiation (λ = 0.71075A), 9029 total measured reflections, 2969 unique measured reflections, Rsym = 0.025, exposure time:10 sec/image, total collection time: 4hrs 12min, crystal-to-detector distance:50.1 mm, 2θSwing:30.0°, number of Scans: 2, data Images:360, 2θmax = 55.0°. R_factor (all unique reflections) = 0.054. CCDC 730479 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk).

2.5. Antimicrobial activity

2.5.1. Bacteria and Culture Conditions

Six staphylococcal isolates were used in the determination of bacteriostatic activity. Two strains of S. epidermidis (UAMS-302 & UAMS-1037) and four strains of S. aureus (UAMS-1, UAMS-1625, UAMS-1864, & UAMS-1865) were included in the study. The antibiotic resistance profile for each strain is detailed in Table 2. Staphylococcal isolates were grown in tryptic soy broth (TSB) overnight at 37°C and diluted to the equivalent of OD_600nm = 0.05 in TSB for MIC₅₀ testing.

Table 2.

MIC₅₀ and MIC₉₀ values for compounds 1, 2, and the ethyl acetate extract of Gynoxys verrucosa against various strains of methicillin resistant Staphylococcus aureus (MRSA), methicillin sensitive Staphylococcus aureus (MSSA), methicillin resistant Staphylococcus epidermidis (MRSE), and methicillin resistant Staphylococcus epidermidis (MSSE).

Organism		S. aureus (UAMS-1625)	S. aureus (UAMS-1864, ATCC 33593)	S. aureus (UAMS-1865, NRSa 385)	S. (UAMS-1, ATCCb 49230)	S. epidermidis (UAMS-1037, NRSa 101, ATCCd 35984)	S. epidermidis (UAMS-302, O-47c)
Phenotype		MRSA	MRSA	MRSA	MSSA	MRSE	MSSE
Source		Brain abscess (Sifri et al., 2007)	Bloodstream	Bloodstream	Osteomyelitis (bone)	Catheter sepsis	Unknown
Antibiotic Resistance Profiled		AMP; CIP; ERY; KAN; LVX; NEO; OX; PEN	AMP; CLI; ERY; GM; KAN; OX; PEN; SXT; TET	AMP; CIP; CLI; CZe; ERY; GM; KAN; LVX; OX; PEN; SXT; TET	AMP; PEN	AMP; CIP; CLI; ERY; GM; KAN; LVXe; OX; PEN	AMP; PEN
MIC50 (μg/mL)	Extract	908	1460	3290	980	2190	1300
	1	>246	>246	>246	>246	>246	>246
	2	49	49	147	98	195	195
	OX	36	26	92	3	19	0.08
	VAN	2.7	3.2	4	3.2	2.8	2.3
MIC90 (μg/mL)	2	450	280	400	440	680	490
	OX	48	80	132	5	88	0.75
	VAN	4.9	4.7	5	4.8	4.6	4.5

^aNRS: Network for Antibiotic Resistant Staphylococcus aureus

^bATCC: American Type Culture Collection

^cO-47 was kindly provided by the F. Gotz’s collection of clinical isolates from Germany

^dAMP: Ampicillin; CIP: Ciprofloxacin; CLI: Clindamycin; CZ: Cefazolin; ERY: Erythromycin; GM: Gentamicin; KAN: Kanomycin; LVX: Levofloxacin; NEO: Neomycin; OX: Oxacillin (Methicillin); PEN: Penicillin; SXT: Sulfamethoxazole/Trimethroprim; TET: Tetracycline; VAN: Vancomycin

^eIntermediate resistance

2.5.2. Determination of Antibiotic Resistance Profile

Staphylococcal isolates included in this study were examined for their antibiotic resistance profile using the disc diffusion method following established protocols (Isenberg, 2004). Briefly, Mueller-Hinton agar was seeded with bacteria and antibiotic disks: 1 μg Oxacillin, 10 μg Ampicillin, 5 μg Ciprofloxacin, 2 μg Clindamycin, 30 μg Cefazolin, 15 μg Erythromycin, 10 μg Gentamicin, 30 μg Kanomycin, 30 μg Neomycin, 10 units Penicillin, 25 μg Sulfamethoxazole/Trimethroprim, 30 μg Tetracycline and 30 μg Vancomycin (Becton Dickinson, BBL Antibiotic Susceptibility Discs, NJ, USA) were applied to the agar. Following incubation, the diameter of the zone of inhibition was measured and compared to the antibiotic kit’s susceptibility chart for S. aureus and S. epidermidis.

2.5.3. Determination of MICs

Minimum inhibitory concentrations (MICs) were determined by the microtiter broth method (Amsterdam, 1996) in sterile flat-bottom 96-well polystyrene plates. Bacteria were suspended in TSB and added to microtiter plates. Serial dilution techniques were employed to examine the activity of compounds 1 and 2 (purity > 99%), and extract at concentrations ranging from 3–1000 μM. In addition, negative (growth) controls (cells + TSB), vehicle controls (cells + TSB + DMSO), and positive controls (TSB+ antibiotic; vancomycin or oxacillin) were included. All tests were performed in triplicate. Microtiter plates were placed in a 37°C incubator on a shaker for 18 hours. Optical density readings (λ = 600 nm) were taken using a Biotek microplate reader at 0 and 18 hours post-inoculation. Results are reported as the MIC₅₀ and MIC₉₀ for growth at 18 hours post-inoculation.

3. Results and discussion

From the ethyl acetate extract of G. verrucosa, two sesquiterpene lactones were isolated and identified by spectroscopy methods and by single-crystal X-ray analysis. Table 1 shows the NMR analysis of both isolated compounds.

The X-ray crystal structure of compound 2, was established and reported for the first time for this molecule, and is fully consistent with the previously reported structure (Wendel et al., 2008). The molecular structure of 2 is illustrated in Fig. 1, the crystal structure shows that both H-8 and H-10 are below the approximate plane of the seven membered ring, while H-9 is above the plane, hence, the configuration is confirmed as being 8α-H, 9β-H, and 10α-H. The seven-membered ring shows an approximated chair conformation with the atoms C-4, C-5, C-6, and C-10 approximately coplanar, and the atoms C-7, C-8, and C-9 above that plane. The full assignment of the ¹H-NMR and ¹³C-NMR spectra for compounds 1 and 2 are reported for the first time and are fully consistent with the proposed structures and the X-ray crystal structure of compound 2. For compound 1 we observed a large (anti) H-8/H-12 coupling, a ROESY peak between H-12 and H-9 indicating that these hydrogens are on the same face and a ROESY peak between C(13)H₃ and H-8 indicating that these hydrogens are also on the same face.

The isolated constituents were tested in vitro against six staphylococcal isolates strains, compound 2 was found to have antimicrobial activity against all staphylococcal isolates, including four methicillin-resistant strains. The antimicrobial activities and the characteristics of the strains tested are presented in Table 2, and range from 49 to 195 μg/mL for the MIC₅₀ and 280–680 for the MIC₉₀. Although, the MRSA activity of compounds 2 is not very high, these results suggest its potential use as a template for further development. Furthermore, the significant activity of compound 2 against the Gram-negative Helicobacter pylori recently reported by Vega et al (2009) suggests that this compounds has a wide spectrum of antimicrobial activity. The fact that compound 2 showed activity against clinical isolates from brain abscesses, suggest that this compound may have a potential clinical use in this area. This is relevant because Staphylococcal brain abscesses are difficult to treat.

Although, no toxic effects have been reported for dehydroleucodine or G. verrucosa, caution should be advised, given that some sesquiterpene lactones have been reported to produce toxic effects to humans and animals, including contact dermatitis and neurotoxicity (Schmidt, 1999).

Compound 2 is a non-ionizable molecule, has a molecular weight smaller than 300, and showed the highest activity against two MRSA clinical isolates resistant to a broad range of antibiotics. All of these facts suggest that compound 2 may have potential for clinical applications as an antimicrobial or may serve as a template for the development of more potent antimicrobials. The establishment of the X-ray structure of compound 2 (Fig. 1) should assist in such efforts.

At a maximum test concentration of 1 mM, compound 1 showed very low level of activity, ranging from 0 – 5% inhibition against strains of S. aureus and 5% and 33% inhibition for the strains of S. epidermidis. Thus, these results demonstrate that the exocyclic conjugated methylene in the lactone ring is essential for the antimicrobial activity of these sesquiterpene lactones. These results are in contrast with the previously reported assertion that the exocyclic α-β-unsaturated methylene moiety is not required for the antimicrobial activity of the sesquiterpene lactones (Lee et al., 1977).

It is significant that three related sesquiterpene lactones structurally similar to compound 2 (arnicolide C, xanthatin and a sesquiterpene ketolactone) have also been found to have significant antimicrobial activity against MRSA and MSSA (Gibbons, 2004). Compound 2 and these compounds have, in addition to the carbonyl of the lactone ring, a carbonyl in the opposite side of the cycloheptane ring, suggesting that perhaps a secondary hydrogen binding point is necessary for the activity.