DEVELOPMENT AND VALIDATION OF A HPLC METHOD FOR DETERMINATION AND QUANTIFICATION OF RUBRAXANTHONE IN STEM BARK EXTRACT OF MANGOSTEEN

Email : meri_susanti008@yahoo.com ABSTRACT Rubraxanthone is one of antibacterial and antioxidant compound which is isolated from stem bark of mangosteen (Garcinia mangostana Linn.). The precise method for analysis of plant constituents is normally a reverse-phase high performance liquid chromatographic (RP-HPLC). Development and validation of a RP-HPLC method for chromatographic separation was carried out on a reversed-phase column Shimadzu ® Shimp-pack VP – ODS (4.6x250mm) using an isocratic mobile phase of 0.4% formic acid acetonitrile at a flow rate 1mL min, and detection was with a UV detector. The linearity of the proposed method was found in the range of 2.5-25μg/mL with regression coefficient 0.999. Intraday precision studies showed the relative standard deviation ≤1.58% and inter-day ≤3.20%. Accuracy of the method was determined by a recovery study conducted at 3 different levels, and the average recovery was 102.18%. The Limit of Detection Limit of Quantitation (LOQ) and were 0.47 and 1.56μg/mL.The contents of rubraxanthon in the crude was 0.23±0.07%w/w.


INTRODUCTION
Garcinia mangostana Linn. Guttiferae, known as "queen of fruits" is cultivated in the tropical rainforest of some Southeast Asian nations like Indonesia, Malaysia, Sri Lanka, Philippines, and Thailand. The bark of G. mangostana is one of the sources for the extraction of rubraxanthone. Rubraxanthone is one class of xanthone compounds that have pharmacology activity as antibacterial agent, where these compounds have been shown to inhibit the growth of Staphylococcus aureus (Iimuna, 1996), Trichophyton mentagrophytes, and Microsporum gypseum, Staphylococcus epidermidis (Pattalung, 1988), Micrococcus luteus, Pseudomonas aeruginosa , Escherichia coli and Helicobacter pyroli (Dachriyanus, 2003). In addition, rubraxanthone also been reported as antiplatelet (Jantan, 2002), and an antioxidant (Dachriyanus, 2003).
It is well accepted that for evaluating the quality of herbal medicines, it is necessary to standardize the content of the active principles. Standardization and analysis of the active principles of the herbal medicine is always difficult. Quantitative determination of active principles in the herbal medicine preparation required optimal separation techniques by which these compounds are separated with the highest resolution and the least interferences from each other. Herbal medicine has been enjoying renaissance among the customers throughout the world. However, one of the impediments in the acceptance of herbal products worldwide is the lack of standard quality control profiles (Shinde, 2009).
Chemical and chromatographic techniques may be used to aid in the identification of an herbal material or extract. Chromatographic techniques such as HPLC, TLC, GC and capillary electrophoresis and spectroscopic methods such as IR, NMR and UV may also be used for fingerprinting (Patra, 2010). In order to control the quality of herbal drugs in a better way, we must develop new techniques and terms to the maximum extent. The development and validation of an efficient analytical method is an integral part of the quality control of the source material, to guarantee the safety and effectiveness of the resulting compound (Hefnawy, 2006). Thus, considering the pharmacological potential of bark extract of G. mangostana and the lack of specifications for the quality control of this plant raw material, which is a prerequisite for the production and registration of phytomedicines. The objective of the present study was to develop and validate a method for the separation and quantitative analysis of rubraxanthone by RP-HPLC. Furthermore, no previous study described for the analysis of Rubraxanthone in stem bark extract of G.mangostana. So, it will provide the scientific basis for the quality control of extracts prepared from the bark of this plant. There was no rubraxanthone available for standard compound, therefore in this study, rubraxanthone ( Figure 1) was isolated from metanol extract of bark of G. cowa using chromato-graphic techniques (column chromatography, TLC,) and its structure was confirmed on the basis of spectral data.

Chemical and reagents
All reagents and solvents were analytical and HPLC grades (merck), except formic acid. Pure isolated Rubraxanthon were used as the external standards.

Plant materials
Stem bark of G. mangostana was collected from Lubuk Alung, West Sumatra. The plants sample were identified by taxonomist from Herbarium ANDA Andalas University. The bark were cut into small pieces and dried in a hot oven at 50°C for 72h. The dried samples were ground into powder and passed through a sieve (20 meshes).

Isolation of rubraxanthone
Ground air-dried stem bark (2.2kg) was percolated with methanol (10L). The extract was evaporated to dryness under reduced pressure and the dark mass (113.54g) was extracted in turn with hexane (4x100mL) which on evaporation yielded a gum (5.7g), ethyl acetate (11x600mL), which on evaporation yielded a gum (43.0g) and butanol (8x150mL) which on evaporation yielded a gum (45.0g).

Instrumentation and chromatographic condition
HPLC method was performed on a Shimadzu (Kyoto, Japan) liquid chromatograph system, equipped with a model LC-20 AD pump, UV-Vis SPD M-20A Diode detector. Separation was performed in a reversed-phase column Shimadzu ® Shimp-pack VP-ODS ( 4.6x250mm).The elution was carried out with isocratic solvent using 0.4% v/v formic acidacetonitrile with a flow rate 1mL menit -1 . The mobile phase was prepared daily, filtered through a 0.45μm and sonicated before use. Total running time was 20min and the sample injection volume was 10μL while the wavelength of the UV-VIS detector was set at 243nm. The compound was quantified using CLASS VP software.

Preparation of standard solutions
A stock solution of rubraxanthone reference standard was prepared by dissolving an accurately weighed 10mg of rubraxanthon in 10mL of methanol in a volumetric flask. Various concentrations of the standard solution were diluted to obtain final concentrations at 2.5: 5: 10: 12.5: and 25μg/mL -1 with methanol.

Preparation of sample solutions
The powder bark of G. mangostana (100g) was separately placed into a thimble and was extracted with 1000mL of 95% ethanol at room temperature for 18h. Each extract was filtered through a Whatman no. 1 filter paper by suction. The filtrate was concentrated under reduced pressure at 50ºC using a rotary vacuum evaporator. The final weight of the crude extract was weighed and calculated for the yield. The extraction of each sample was done in triplicate.
Each dried extract (10mg) was accurately weighed and transferred to a 10mL volumetric flask. Methanol was added to volume (final concentration 1.000μg/mL -1 ). Aliquot of the solution (2.5mL) was diluted with methanol in a 10mL volumetric flask to make a concentration of 250μg/mL -1 . Prior to analysis, the solutions were filtered through 0.45μm membrane filters.

Quantitative analysis of rubraxanthon content
Determination of rubraxanthon content was carried out by HPLC under the same condition as the proposed method. Rubraxanthon content in the extract was calculated using its calibration curve with regarding to the dilution factor. The contents of rubraxanthon in the extract was expressed as gram per 100g of the extract. Each determination was carried out in triplicate.

Validation of the method
Validation of the analytical method was done according to the International Conference on Harmonization guideline (ICH, 1995). The method was validated for linearity, precision, and accuracy, limit of detection (LOD) and limit of quantitation (LOQ).

Linearity
Linearity was determined by using rubraxanthon standard solution in the concentration range were of 2.5-25ug/mL -1 in methanol, each of the standard solution was prepared (n=3). The calibration curves were obtained by plotting the peak area versus the concentration of the standard solutions. The precision was determined by analyzing 2.5:10 and 25μg/mL -1 of standard solution of rubraxanthon (n=3). Intraand inter-day assay precision were determined as relative standard deviation (RSD). Intraday precission (repeatability) involved here replicates per day and inter-day (intermediate) precission were prformed on three separate days.

Accuracy
The accuracy of the method was tested by performing recovery studies. Three different levels concentration (2.5:10 and 25ug/mL -1 ) of the standard solution in methanol) were added to the sample solution (10.43μg/mL -1 ) and analyzed by the proposed RP HPLC method. The recovery and average recovery were calculated.
Three determinations were performed for each concentration level.

LOD and LOQ
According to ICH (1995), technical requirements for the registration of pharmaceuticals for human use recommend-dations, the approach based on SD of the response and the slope were used for determining the detection and quantitation limits.

RESULTS AND DISCUSSION
HPLC method with isocratic elution was developed for the quantification of rubraxanthon in stem bark extracts of G. mangostana.. Optimization of mobile phase was performed based on asymmetric factor and peak area obtained. Different mobile phases were used but satisfactory separation, well resolved and good symmetrical peaks were obtained with the mobile phase 0.4% formic acid and acetonitrile (25:75%v/v). The retention time of rubraxanthon was found to be 12.121 min in relation to asymmetry, the peaks showed an asymmetric factor was at 0.815 (<2.5), which indicates a good baseline (Figure 6). The number of theoretical plates was found to be 14296 (>2500), which indicates efficient performance of the column. The UV spectra of Rubraxanthone showed the maximum absorption at 243nm. Thus, it was chosen as detection wave length in liquid chromatography.
The calibration curve for rubraxanthon was obtained by plotting the peak area ratio versus the concentration of rubraxanthon over the range of 2.5-25μg/mL -1 , and it was found to be linear with r=0.999. The regression equation of rubraxanthon concentration over its peak area ratio was found to be y=156554 x -6185, where x is the concentration of rubraxanthon and y is the respective peak area. The limit of detection and limit of quantification for rubraxanthon was found to be 0.47 and 1.56μg/mL -1 , indicates the sensitivity of the method. The system suitability and validation parameters were given in (Table II) The intra-day precision (repeatability) of the assay was determined by analysis of 3 different concentration (2.5, 10, and 25μg/mL -1 ) of standard rubraxanthon on the same day. For determination of inter-day (intermediate) precision, the samples were analysed on 3 different days. In the precision study, percentage relative standard deviation of the peak area of rubraxanthon are shown in table II. These values were within limits <5% (AOAC, 2002).
The accuracy of the method was determined by adding accurate amount of rubraxanthon standard to quantified rubraxanthone in extract samples. The mean values of the percentage analytical recoveries for the concentration of 2.5, 10 and 25μg/mL -1 of rubraxanthon were 99.39, 104.47 and 102.69% respectively (Table III). AOAC (2002) recommended that percentage of recovery in the range of 80-120%. The results showed the percentage of recovery fulfilled this recommendation. The high percentage of recovery of rubraxanthon indicates that the proposed method is highly accurate.
The RSD values for accuracy and precision studies obtained were less than 5%. It can be concluded that developed method was accurate and precise. The limit of detection and limit of quantification for rubraxanthon was found to be 0.47 and 1.56μg/mL -1 , indicates the sensitivity of the method.
Rubraxanthon content in the samples of stem bark extracts G. mangostana which obtained from Lubuk Alung, West Sumata determined by the developed HPLC was 0.23±0.07 %W/W respectively. HPLC chromatograms of extracts showed similar pattern with a peak rubraxanthon at retention time of 12.12min ( Figure 6). The identity of the peak of rubraxanthon in the sample chromatograms was confirmed by spiking with its standard and determination of retention time.    The results are mean ± SD of 9 determination. * 3 replicates were assayed on the same day; **3 replicates were assayed on 3 different days

CONCLUSIONS
The proposed HPLC method promoted high precision, sensitivity and accuracy for quality control of extract of bark G. mangostana This proposed method will be useful for quantitative analysis in standardization and quality assessment of extract of bark G. mangostana for pharmaceutical uses.