In vitro anti-inflammatory effects of naturally-occurring compounds from two Lauraceae plants

The in vitro anti-inflammatory effects of seven known lignans and one dihydrochalcone isolated from the leaves of two Lauraceae species (Pleurothyrium cinereum and Ocotea macrophylla), were evaluated through the inhibition of COX-1, COX-2, 5-LOX and the aggregation of rabbit platelets induced by PAF, AA and ADP. (+)-de-4′′-O-methylmagnolin 4 was found to be a potent COX-2/5-LOX dual inhibitor and PAF-antagonist (COX-2 IC50 2.27 μM; 5-LOX IC50 5.05 μM; PAF IC50 2.51 μM). However, all compounds exhibited an activity at different levels, indicating good anti-inflammatory properties to be considered in further structural optimization studies.


INTRODUCTION
Prostaglandin H 2 synthase (also named cyclooxygenase, COX), which has three isoforms, COX-1, COX-2 and COX-3 (Simmons et al. 2004), and lipoxygenase (LOX) catalyze the rate-limiting step in the formation of prostaglandins (PGs) and leukotrienes (LTs), respectively.The delineation of their distinct functions in physiological and pathological processes has promoted plenty of studies that are focused on the search for potential anti-inflammatory agents.In this way, COX-2 inhibitors were developed as a safer alternative to traditional nonsteroidal anti-inflammatory drugs (NSAIDs) because there was less risk of gastrointestinal ulceration (Charlier and Michaux 2003).
Platelets, being the smallest cellular component in the blood, change shape from disc-shaped into spiny spheres, in the presence of stimuli such as arachidonic acid (AA).Activated platelets bind to fibrinogen, aggregate and release adenosine diphosphate (ADP) and serotonin, among others, contained in their intracellular granules (Rao 1993).Thromboxane A 2 (TxA 2 ), an AA metabolite, ADP and platelet activating factor (PAF) intensify the extent of platelet aggregation as an autocatalytic process rather than promote an equilibrium response (Armstrong 1996).Under pathophysiological conditions, platelet activation may result in peripheral, cardiovascular or cerebrovascular thrombosis with severe consequences.The relationship between platelet aggregation and acute inflammation had been previously discussed (Prescott et al. 2002), as well as the possible risk of suffering cardiovascular problems (myocardial infarction and thromboembolic events) by using COX-2-specific inhibitors, since the healthy balance between COX-2-mediated prostacyclin and COX-1-dependent thromboxane is disturbed (Konstam et al. 2001).Therefore, the search for COX inhibitory compounds with antiplatelet effects, which possess a different mechanism from that of NSAIDS, appeared to be a relevant way to provide protection against harmful cardiovascular events in treating inflammatory problems (Bombardier et al. 2000).

GENERAL EXPERIMENTAL PROCEDURES
Melting points were determined on a Fisher-Johns melting point apparatus without correction.Ultraviolet (UV) spectra and colorimetric measurements were performed on a Cary 50 Scan (Varian instruments) spectrophotometer. 1 H and 13 C NMR spectra were recorded on a Bruker Avance 400 spectrometer, using deuterated chloroform (CDCl 3 ) as solvent, and tetramethylsilane (TMS) as internal shift reference.High resolution mass spectra (HRMS) were determined on a micromass quadrupoletime of flight (qToF) (Waters Inc.) [with an electrospray ionization (ESI) source and in the positive ion mode] mass spectrometer.Platelet aggregation was measured on a Chrono-Log (Chrono-Log Corporation) optical aggregometer.(Coy andCuca, 2008a, b, Coy et al. 2010).

CELLS AND REAGENTS
COX-(ovine) inhibitor screening and 5-LOX-(potato) inhibitor screening kits were purchased from Cayman Chemical Company, Ann Arbor, MI, USA (Catalogs No. 560101 and No. 760700,respectively). AA, PAF, ADP and ginkgolide B were acquired from Sigma-Aldrich.Platelet-rich plasma (PRP) was obtained from blood collected from the central ear artery of female White New Zealand rabbits with a body weight between 3 and 4 kg.

COX-1, COX-2, AND 5-LOX INHIBITION ASSAYS
The ability of the test compounds listed in Table I to inhibit ovine COX-1, COX-2, and potato 5-LOX were determined using enzyme immunoassay (EIA) kits, according to the reported methods (Rao et al. 2003, Chowdhury et al. 2008).
PLATELET AGGREGATION INDUCED BY PAF, AA AND ADP Anti-platelet activity was carried out according to the reported method (Koch 2005), which was approved by  the ethics committee at Universidad Nacional de Colombia.Briefly, convenient PRP was stirred at 800 rpm and maintained at 37 • C. Samples of PRP were preincubated for 5 min at 37 • C with test compounds (selected concentrations according to the previous work of Koch 2005) in dimethyl sulfoxide (DMSO).Aggregation was induced by the addition of 10 μL diluted PAF, AA or ADP, according to the test.The final concentration was 7.20 nM for PAF, 100 μM for AA, and 4.00 μM for ADP on PRP.In order to eliminate the effect of the solvent on the aggregation, the final concentration of DMSO was fixed at 0.5%, which did not affect the aggregation measurement.The inhibition of platelet aggregation versus a solvent control was calculated as percentage.

STATISTICS
Half-maximal inhibition concentrations (IC 50 , μM) were determined by a non-linear regression analysis using GraphPad prism 5.00 (GraphPad software, San Diego, CA, USA). a The data were expressed as means 95% confidence intervals of 4 rabbits.

RESULTS AND DISCUSSION
The in vitro abilities of the compounds 1-8 to inhibit the isozymes COX-1 and COX-2 (which appeared to be dose-dependent) were determined in the COX-catalyzed transformation of AA into PGH 2 , which was then reduced to PGF 2α and detected by the enzyme immunoassay (EIA), following the reported methodology (Rao et al. 2003), whose results are shown in Table I.Whereas lignans 1, 4 and 7-8 showed an activity against COX-2 (IC 50 7.21-49.6μM range), the dihydrochalcone 6 displayed a very low activity.However, 6 was the most potent COX-1 inhibitor (IC 50 1.22 μM) among test compounds, being three times less active than the control.Aryltetralin (1-3) and dibenzylbutane (7-8) lignans exhibited selective inhibition against COX-2, and the dibenzylbutane lignans were significantly more active than the aryltetralins, and threo-isomer 8 was a better inhibitor than the meso-isomer 7. The furofuran lignans (3-5) inhibited the action of both COX isozymes with lower selectivity, although 4 was almost fifteen times more selective than 5.
The in vitro inhibitory effect of compounds 1-8 against 5-LOX was determined in the lipoxygenation reaction using a purified lipoxygenase to transform AA into hydroperoxides, which are detected and measured by the addition of a chromogen, following the reported methodology (Chowdhury et al. 2008).As expected, compounds having a free phenolic OH group (1-2, 4-5) inhibited 5-LOX (IC 50 4.22-18.2μM range), 5 being the most potent 5-LOX inhibitor that had a comparable activity (IC 50 4.22 μM) to that of the positive control that was used (caffeic acid, IC 50 3.73 μM).Compounds 3 and 6-8 exhibited weak 5-LOX inhibitory effect (IC 50 46.4 to 127 μM).Regarding the diastereomeric lignans 7-8, the meso-isomer 7 was displayed a lower IC 50 value than its threo-isomer 8, in contrast to their COX-inhibition.By comparing the COX and 5-LOX inhibition results, 4 appeared to be the best candidate as a COX-2/5-LOX dual inhibitor, which is a current matter of interest in the development of anti-inflammatory agents (Chowdhury et al. 2008, Charlier andMichaux 2003).
In addition, the capability to inhibit the aggregation of rabbit platelets induced by PAF, AA and ADP was evaluated in vitro, according to the reported method (Koch 2005), whose results are shown in Table II.Compounds 1-3 showed no significant antiplatelet effect in the test.Compound 6 had the highest IC 50 value for AA-induced platelet aggregation, indicating that the antiplatelet activity is likely a consequence of COX-1 inhibition.Lignans 4-5 were found to be the most active compounds, since they inhibited the platelet aggregation induced by the three agonists, showing important selectivity for PAF-induced aggregation.The diastereomeric dibenzylbutane lignans 7-8 also displayed selectivity towards PAF-inhibition, the threo-isomer being more active than the meso-isomer.All compounds showed lower activity than the positive controls; how-An Acad Bras Cienc (2011) 83 (4) ever, compound 5 presented a valuable pharmacological profile, because of its COX-2/5-LOX dual inhibition and antiplatelet activities.Recently, a very closely related compound isolated from Magnolia fargesii (Magnoliaceae) was found to be a good inhibitor of the production of nitric oxide (NO) and PGE 2 , as well as the expression of inducible nitric oxide synthase (iNOS) and COX-2, respectively, through the suppression of IκBα degradation and NFκB activation (Kim et al. 2009).
In conclusion, these results imply that aryltetralin and furofuran lignans might be excellent candidates for further studies aiming for structural optimization to improve their activity, and delineate their anti-inflammatory effects.Our study clearly demonstrated that these lignans have good anti-inflammatory properties that are in agreement with the literature (Lin et al. 2007, da Silva Filho et al. 2004, Ballabeni et al. 2007, Kim et al. 2009, Kim and Yun-Choi 2007, Chen et al. 1996, Morais et al. 1999, Jesus-Morais et al. 2000).However, further structure-activity studies and biological analyses are required to clarify the underlying mechanism and to draw unambiguous conclusions regarding the antiinflammatory potential of aryltetralin and furofuran lignans.
PLANT MATERIAL, EXTRACTION AND ISOLATION Whole plants of P. cinereum van der Werff and O. macrophylla Kunth were collected in November 2005 in the Awá indigenous reservation in Alto Albí, Tumaco county, department of Nariño, Colombia, and in June 2006 by the side of Nocaima-Vergara road in Nocaima county, department of Cundinamarca, Colombia, respectively.Voucher specimens (numbers COL518334 for P. cinereum and COL517191 for O. macrophylla, which were identified by the botanist Adolfo Jara Muñoz) were deposited at Herbario Nacional Colombiano, Universidad Nacional de Colombia.Detailed extraction and isolation processes for compounds 1-8 have been previously reported

TABLE I COX-1, COX-2 and 5-LOX enzyme inhibition results of compounds 1-8.
a Values are means of two experiments, standard deviation from the mean is <10% of the mean value.

TABLE II Inhibitory effects of compounds 1-8 on the aggregation of rabbit platelets induced by PAF, AA and ADP.
An Acad BrasCienc (2011) 83 (4)