Synthesis and biological evaluation of helicid analogues as mushroom tyrosinase inhibitors

A series of helicid analogues were synthesized and evaluated as tyrosinase inhibitors. The results demonstrated that some compounds had more potent inhibitory activities than arbutin (IC(50) 7.3 mM). In particular, compound 1c bearing 4,6-O-benzylidene substituent on the sugar moiety was found to be the most potent inhibitor with IC(50) value of 0.052 mM. The inhibition kinetics analyzed by Lineweaver-Burk plots revealed that helicid analogues were competitive inhibitors. The Circular dichroism spectra indicated that those compounds induced conformational changes of mushroom tyrosinase upon binding.     

Design and synthesis of acyclic triaryl (Z)-olefins: a novel class of cyclooxygenase-2 (COX-2) inhibitors

A group of acyclic 2-alkyl-1,1-diphenyl-2-(4-methylsulfonylphenyl)ethenes was designed for evaluation as selective cyclooxygenase-2 (COX-2) inhibitors. In vitro COX-1 and COX-2 isozyme inhibition structure-activity studies identified 1,1-diphenyl-2-(4-methylsulfonylphenyl)hex-1-ene as a highly potent (IC(50) = 0.014 microM), and an extremely selective [COX-2 selectivity index (SI) > 7142], COX-2 inhibitor that showed superior anti-inflammatory (AI) activity (ID(50) = 2.5 mg/kg) relative to celecoxib (ID(50) = 10.8 mg/kg). This initial study was extended to include the design of a structurally related group of acyclic triaryl (Z)-olefins possessing an acetoxy (OAc) substituent at the para-position of the C-1 phenyl ring that is cis to a C-2 4-methylsulfonylphenyl substituent. COX-1 and COX-2 inhibition studies showed that (Z)-1-(4-acetoxyphenyl)-1-phenyl-2-(4-methylsulfonylphenyl)but-1-ene [(Z)-13b] is a potent (COX-1 IC(50) = 2.4 microM; COX-2 IC(50) = 0.03 microM), and selective (COX-2 SI = 81), COX-2 inhibitor which is a potent AI agent (ID(50) = 4.1mg/kg) with equipotent analgesic activity to celecoxib. A molecular modeling (docking) study showed that the SO(2)Me substituent of (Z)-13b inserts deep inside the 2 degrees -pocket of the COX-2 active site, where one of the O-atoms of SO(2) group undergoes a H-bonding interaction with Phe(518). The p-OAc substituent on the C-1 phenyl ring is oriented in a hydrophobic pocket comprised of Met(522), Gly(526), Trp(387), Tyr(348), and Tyr(385), and the C-2 ethyl substituent is oriented towards the mouth of the COX-2 channel in the vicinity of amino acid residues Arg(120), Leu(531), and Val(349). Structure-activity data acquired indicate that a (Z)-olefin having cis C-1 4-acetoxyphenyl (phenyl) and C-2 4-methylsulfonylphenyl substituents, and a C-1 phenyl substituent in conjunction with either a C-2 hydrogen or short alkyl substituent provides a novel template to design acyclic olefinic COX-2 inhibitors that, like aspirin, have the potential to acetylate COX-2.     

Design, synthesis, and biological evaluation of N-acetyl-2-carboxybenzenesulfonamides: a novel class of cyclooxygenase-2 (COX-2) inhibitors

N-Acetyl-2-carboxybenzenesulfonamide (11), and a group of analogues possessing an appropriately substituted-phenyl substituent (4-F, 2,4-F(2), 4-SO(2)Me, 4-OCHMe(2)) attached to its C-4, or C-5 position, were synthesized for evaluation as selective cyclooxygenase-2 (COX-2) inhibitors. In vitro COX-1/COX-2 inhibition studies showed that 11 is a more potent inhibitor (COX-1 IC(50)=0.06microM; COX-2 IC(50)=0.25microM) than aspirin (COX-1 IC(50)=0.35microM; COX-2 IC(50)=2.4microM), and like aspirin [COX-2 selectivity index (S.I.)=0.14], 11 is a nonselective COX-2 inhibitor (COX-2 S.I.=0.23). Regioisomers having a 2,4-difluorophenyl substituent attached to the C-4 (COX-2 IC(50)=0.087microM; COX-2 S.I. >1149), or C-5 (COX-2 IC(50)=0.77microM, SI>130), position of 11 exhibited the most potent and selective COX-2 inhibitory activity relative to the reference drug celecoxib (COX-1 IC(50)=33.1microM; COX-2 IC(50)=0.07microM; COX-2 S.I.=472). N-Acetyl-2-carboxybenzenesulfonamide (11, ED(50)=49 mg/kg), and its C-4 2,4-difluorophenyl derivative (ED(50)=91 mg/kg), exhibited superior antiinflammatory activity (oral dosing) in a carrageenan-induced rat paw edema assay compared to aspirin (ED(50)=129 mg/kg). These latter compounds exhibited comparable analgesic activity to the reference drug diflunisal, and superior analgesic activity compared to aspirin, in a 4% NaCl-induced abdominal constriction assay. A molecular modeling (docking) study indicated that the SO(2)NHCOCH(3) substituent present in N-acetyl-2-carboxy-4-(2,4-fluorophenyl)benzenesulfonamide, like the acetoxy substituent in aspirin, is suitably positioned to acetylate the Ser(530) hydroxyl group in the COX-2 primary binding site. The results of this study indicate that the SO(2)NHCOCH(3) pharmacophore present in N-acetyl-2-carboxybenzenesulfonamides is a suitable bioisostere for the acetoxy (OCOMe) group in aspirin.     

Design, synthesis and biological evaluation of novel acrylamide analogues as inhibitors of BCR-ABL kinase

A series of acrylamide analogues were designed and synthesized from Imatinib and Nilotinib as novel BCR-ABL inhibitors by application of the principle of nonclassical electronic isostere. All new compounds were evaluated for their inhibitory effects on the activity of BCR-ABL kinase and the proliferation of K562 leukemia cancer cells in vitro. The acrylamide analogues in which the substituent in C ring was trifluoromethyl group were identified as highly potent BCR-ABL kinase inhibitors. Compound 13f exhibited an IC(50) value as low as 20.6nM in ABL kinase inhibition and an IC(50) value of 32.3nM for antiproliferative activity, about 10.5-fold and 12-fold lower than those of Imatinib respectively. These results suggest that compound 13f is a promising candidate as a novel BCR-ABL kinase inhibitor for further development.     

Synthesis and structure-activity relationship studies on tryprostatin A, an inhibitor of breast cancer resistance protein

Tryprostatin A is an inhibitor of breast cancer resistance protein, consequently a series of structure-activity studies on the cell cycle inhibitory effects of tryprostatin A analogues as potential antitumor antimitotic agents have been carried out. These analogues were assayed for their growth inhibition properties and their ability to perturb the cell cycle in tsFT210 cells. SAR studies resulted in the identification of the essential structural features required for cytotoxic activity. The absolute configuration L-Tyr-L-pro in the diketopiperazine ring along with the presence of the 6-methoxy substituent on the indole moiety of 1 was shown to be essential for dual inhibition of topoisomerase II and tubulin polymerization. Biological evaluation also indicated the presence of the 2-isoprenyl moiety on the indole scaffold of 1 was essential for potent inhibition of cell proliferation. Substitution of the indole N(a)-H in 1 with various alkyl or aryl groups, incorporation of various L-amino acids into the diketopiperazine ring in place of L-proline, and substitution of the 6-methoxy group in 1 with other functionality provided active analogues. The nature of the substituents present on the indole N(a)-H or the indole C-2 position influenced the mechanism of action of these analogues. Analogues 68 (IC(50)=10 microM) and 67 (IC(50)=19 microM) were 7-fold and 3.5-fold more potent, respectively, than 1 (IC(50)=68 microM) in the inhibition of the growth of tsFT210 cells. Diastereomer-2 of tryprostatin B 8 was a potent inhibitor of the growth of three human carcinoma cell lines: H520 (IC(50)=11.9 microM), MCF-7 (IC(50)=17.0 microM) and PC-3 (IC(50)=11.1 microM) and was equipotent with etoposide, a clinically used anticancer agent. Isothiocyanate analogue 71 and 6-azido analogue 72 were as potent as 1 in the tsFT210 cell proliferation and may be useful tools in labeling BCRP.     

Evaluation of the toxicity of substituted benzthioanilides by using in vitro tests

The cytotoxicity of 12 benzthioanilides substituted in the N-aromatic ring, and of two commercial preparations (imaverol and thiuram) for comparison, was studied with clone 81 cat cells, by determining the highest tolerated dose, and by using the neutral red uptake assay and the kenacid blue assay for total protein. The concentrations that induced 20%, 50% and 80% (IC20, IC50 and IC80) inhibition relative to controls were calculated from dose-response curves. For some compounds, rat LD50 values were also determined. All the benzthioanilide preparations showed in vitro toxicities lower than those of the fungicides imaverol and thiuram. It was confirmed that the cytotoxicities of the compounds depend on the type of substituent. The least toxic compound contained a CONHCH(2)CO(2)H substituent in the para position of the N-aromatic ring, and the most toxic compounds contained chloro and fluoro, or three chloro substituents in the anilide moiety. All the benzthioanilides tested showed fungistatic activity for dermatophytes; two of the compounds (compound 5 and compound 12) also inhibited the development of yeasts at concentrations lower than those which caused toxicity in vitro. The LD50 values and the cytotoxic concentrations in vitro were linearly related.     

Synthesis and antienteroviral activity of a series of novel, oxime ether-containing pyridyl imidazolidinones

A series of novel, oxime ether-containing pyridyl imidazolidinones were synthesized and their antiviral activity was evaluated in a plaque reduction assay. It is very interesting that this class of compounds is specific for human enteroviruses, in particular, enterovirus 71 (EV71). Some derivatives strongly inhibited enterovirus replication with activities higher or comparable to those of the reference compounds such as A1 and A2. Preliminary SAR studies revealed that the chain length of the alkyl linker and the alkyl substituent at the oxime ether group largely influenced the in vitro anti-EV71 activity of this new class of potent antiviral agents. Among this series of compounds synthesized, the pyridyl imidazolidinone with an ethyl oxime ether group located at the para position of the phenoxyl ring (8b) was identified as the most potent enterovirus 71 inhibitor (IC50=0.001 microM) with no apparent cytotoxic effect toward RD (rhabdomyosarcoma) cell lines (CC50>25 microM). Furthermore, this compound has been shown broad-spectrum activity against most of the serotypes of enteroviruses tested in the nanomolar range.     

In vitro kinetics of the rat brain succinate dehydrogenase inhibition by hexachlorophene.

Brain succinate dehydrogenase (SDH) activity was inhibited by in vitro hexachlorophene (HCP) with a half inhibitory concentration (IC50) of 0.65 x 10(-3) M. The HCP exerted noncompetitive inhibition at 0.5 mM (IC50) on SDH activity. The brain SDH demands more energy of activation (deltaE) in the presence of HCP. The ionizable groups of SDH such as the sulfhydral group of cysteine and alpha-amino groups of cysteine were not altered qualitatively in the presence of HCP.     

Degradation-promoters of cellular inhibitor of apoptosis protein 1 based on bestatin and actinonin

A series of hybrid compounds of bestatin (1) and actinonin (3), which promote degradation of cellular inhibitor of apoptosis protein 1 (cIAP1), were designed and synthesized. Structure-activity relationship studies indicated that absolute configuration, hydrophobicity at the alpha-position of the internal amide carbonyl group, and the presence of a small substituent at the alpha-position of the ester group are important factors for the expression of potent cIAP1 degradation-promoting activity. HAB-5A (30b) showed the most potent activity (IC(50)=0.53 microM) among the compounds prepared.     

Design, synthesis, and biological evaluation of 1,3-diarylprop-2-en-1-ones : a novel class of cyclooxygenase-2 inhibitors

A group of regioisomeric (E)-1,3-diarylprop-2-en-1-one derivatives possessing a COX-2 SO2Me pharmacophore at the para position of the C-1 or C-3 phenyl ring, in conjunction with a C-3 or C-1 phenyl (4-H) or substituted-phenyl ring (4-F, 4-OMe and 4-Me), were designed for evaluation as selective cyclooxygenase-2 (COX-2) inhibitors. These target (E)-1,3-diarylprop-2-en-1-ones were synthesized via a Claisen-Schmidt condensation reaction. In vitro COX-1/COX-2 isozyme inhibition structure-activity studies identified (E)-1-(4-methanesulfonylphenyl)-3-(4-methylphenyl)prop-2-en-1-one (9f) as a potent COX-2 inhibitor (IC50=0.3 microM) with a high COX-2 selectivity index (SI=106) comparable to that of the reference drug rofecoxib (COX-2 IC50=0.5 microM; COX-2 SI>200). A molecular modeling study where 9f was docked in the binding site of COX-2 showed that the para-SO2Me substituent on the C-1 phenyl ring is oriented in the vicinity of the secondary COX-2 binding site near Val523. The structure-activity data acquired indicate that the propenone moiety constitutes a suitable scaffold to design novel acyclic 1,3-diarylprop-2-en-1-ones with selective COX-2 inhibitory activity.     

Inactivation of the intestinal uptake system for beta-lactam antibiotics by diethylpyrocarbonate

The uptake system for beta-lactam antibiotics in the rabbit small intestine was investigated using brush-border membrane vesicles. After treatment of membrane vesicles with the reagent diethylpyrocarbonate (DEP), the uptake of orally active beta-lactam antibiotics with an alpha-amino group in the substituent at position 6 or 7 of the penam or cephem nucleus was significantly inhibited, whereas DEP-treatment had no inhibitory effect on the uptake of beta-lactam antibiotics without an alpha-amino group. The kinetic analysis revealed an apparent competitive inhibition indicating a decreased affinity of the transport system for alpha-amino-beta-lactam antibiotics. Substrates of the intestinal dipeptide transport system - dipeptides and alpha-amino-beta-lactam antibiotics - could protect the transport system from irreversible inhibition by DEP, whereas beta-lactam antibiotics without an alpha-amino group as well as amino acids or bile acids had no effect. Incubation of DEP-treated vesicles with hydroxylamine led to a partial restoration of the transport activity indicating that DEP may have led to a modification of a histidine residue of the transport protein. From the data presented we conclude that a specific interaction of the alpha-amino group in the substituent at position 6 or 7 of the penam or cephem nucleus presumably with a histidine residue of the transport protein is involved in the translocation process of orally active alpha-amino-beta-lactam antibiotics across the intestinal brush-border membrane.     

1,1-Diarylalkenes as anticancer agents: dual inhibitors of tubulin polymerization and phosphodiesterase 4

A series of 1,1-diarylalkene derivatives were prepared to optimize the properties of CC-5079 (1), a dual inhibitor of tubulin polymerization and phosphodiesterase 4 (PDE4). By using the 3-ethoxy-4-methoxyphenyl PDE4 pharmacophore as one of the aromatic rings, a significant improvement in PDE4 inhibition was achieved. Compound 28 was identified as a dual inhibitor with potent PDE4 (IC(50)=54 nM) and antitubulin activity (HCT-116 IC(50)=34 nM and tubulin polymerization IC(50) ～1 μM). While the nitrile group at the alkene terminus was generally required for potent antiproliferative activity, its replacement was tolerated if there was a hydroxyl or amino group on one of the aryl rings. Conveniently, this group could also serve as a handle for amino acid derivatization to improve the compounds' solubility. The glycinamide analog 45 showed significant efficacy in the HCT-116 xenograft model, with 64% inhibition of tumor growth upon dosing at 20 mg/kg qd.     

Cloning, characterization and evaluation of potent inhibitors of Shigella sonnei acetohydroxyacid synthase catalytic subunit

Acetohydroxyacid synthase (AHAS) is a thiamin diphosphate (ThDP)- and flavin adenine dinucleotide (FAD)-dependent plant and microbial enzyme that catalyzes the first common step in the biosynthesis of essential amino acids such as leucine, isoleucine and valine. To identify strong potent inhibitors against Shigella sonnei (S. sonnei) AHAS, we cloned and characterized the catalytic subunit of S. sonnei AHAS and found two potent chemicals (KHG20612, KHG25240) that inhibit 87-93% S. sonnei AHAS activity at an inhibitor concentration of 100uM. The purified S. sonnei AHAS had a size of 65kDa on SDS-PAGE. The enzyme kinetics revealed that the enzyme has a K(m) of 8.01mM and a specific activity of 0.117U/mg. The cofactor activation constant (K(s)) for ThDP and (K(c)) for Mg(++) were 0.01mM and 0.18mM, respectively. The dissociation constant (K(d)) for ThDP was found to be 0.14mM by tryptophan fluorescence quenching. The inhibition kinetics of inhibitor KHG20612 revealed an un-competitive inhibition mode with a K(ii) of 2.65mM and an IC(50) of 9.3μM, whereas KHG25240 was a non-competitive inhibitor with a K(ii of) 5.2mM, K(is) of 1.62mM and an IC(50) of 12.1μM. Based on the S. sonnei AHAS homology model structure, the docking of inhibitor KHG20612 is predicted to occur through hydrogen bonding with Met 257 at a 1.7? distance with a low negative binding energy of -9.8kcal/mol. This current study provides an impetus for the development of a novel strong antibacterial agent targeting AHAS based on these potent inhibitor scaffolds.     

Synthetic tyrosyl gallate derivatives as potent melanin formation inhibitors

Three tyrosyl gallate derivatives (1-3) with variable hydroxyl substituent at the aromatic ring of tyrosol were synthesized and evaluated as potent inhibitors on tyrosinase activity and melanin formation in melan-a cells. Among three tyrosyl gallate derivatives, 4-hydroxyphenethyl 3,4,5-trihydroxybenote (1) (IC(50)=4.93 microM), 3-hydroxyphenethyl 3,4,5-trihydroxybenote (2) (IC(50)=15.21 microM), and 2-hydroxyphenethyl 3,4,5-trihydroxybenote (3) (IC(50)=14.50 microM) exhibited significant inhibitory effect on tyrosinase activity. Compound 1 was the most active compound, though it did not show the inhibitory effect on melanin formation in melan-a cells. However, compounds 2 (IC(50)=8.94 microM) and 3 (IC(50)=13.67 microM) significantly suppressed the cellular melanin formation without cytotoxicity. This study shows that the position of hydroxyl substituent at the aromatic ring of tyrosol plays an important role in the intracellular regulation of melanin formation in cell-based assay system.     

Structure-activity relationship of HIV-1 protease inhibitors containing alpha-hydroxy-beta-amino acids. Detailed study of P1 site.

The structure-activity relationship of HIV-1 protease (HIV-1 PR) inhibitors containing alpha-hydroxy-beta-amino acids is discussed. We demonstrated that substituent groups on the P1 aromatic rings of the inhibitors exert significant influence on their biological activity. Inhibitors bearing an alkyl or a fluorine atom at the meta and para position on their P1 benzene ring were found to be good inhibitors. We also discovered that the substitution positions of the P2 benzamides were crucial for good antiviral potency. In this study, inhibitor 48 was the most potent [IC90 (CEM/HIV-1 IIIB) 27 nM] and showed good pharmacokinetics in rats.     

Structural requirements for potent human spermicidal activity of dual-function aryl phosphate derivative of bromo-methoxy zidovudine (compound WHI-07)

WHI-07, a novel bromo-methoxy-substituted aryl phosphate derivative of zidovudine (ZDV), is a potent dual-function contraceptive agent. Although the bromo-methoxy functional groups in the thymine ring of its ZDV are very important for its sperm-immobilizing activity (SIA), the importance of the esterification of the phosphate group with an amino acid side chain and the identity of the para substituent in the aryl moiety remain unclear. In the present study, we have synthesized 23 new analogues of WHI-07 by replacing the alanine (Ala) side chain with different amino acids containing nonpolar side chains, namely tryptophan (Trp), proline (Pro), phenylalanine (Phe), leucine (Leu), methionine (Met), valine (Val), or glycine (Gly). The para substituents on the aryl moiety included bromo, chloro, fluoro, nitro, or methoxy groups. The SIA of each of the 23 WHI-07 analogues was evaluated by computer-assisted sperm analysis. The potential cytotoxicity of these compounds against normal human ectocervical and endocervical epithelial cells was evaluated using MTT (3-[4, 5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) cell viability assays. The replacement of the Ala side chain of WHI-07 with Val, Leu, or Phe led to a complete loss of SIA (EC(50) values > 500 microM), whereas replacement with Trp reduced the SIA by 4-fold. The presence of para substituents on the phenyl moiety led to significant alterations in SIA. The anti-human immunodeficiency virus (HIV) activity of Trp-containing WHI-07 analogues was also diminished. Our finding highlights the necessity of Ala side chain and the presence of electron-withdrawing para-bromo substituent on the phenyl moiety in addition to bromo-methoxy functionalization groups on the thymine ring in order for the phosphoramidate derivatives of ZDV to be effective dual-function spermicidal agents. Unlike the detergent-type microbicide, nonoxynol-9, which was cytotoxic to normal human ectocervical and endocervical epithelial cells (IC(50) values of 22 microM and 16 microM, respectively) at spermicidal concentrations (EC(50) = 81 microM), WHI-07 and its active analogues were selectively spermicidal without cytotoxicity against female genital tract epithelial cells. WHI-07 and its Trp analogues hold particular clinical promise for the development of novel, nondetergent-type prophylactic contraceptives for the prevention of heterosexual HIV/acquired immunodeficiency syndrome transmission.     

Tyrosinase inhibition kinetics of anisic acid

Anisic acid (p-methoxybenzoic acid) was characterized as a tyrosinase inhibitor from ani-seed, a common food spice. It inhibited the oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA) catalyzed by tyrosinase with an IC50 of 0.60 mM. The inhibition of tyrosinase by anisic acid is a reversible reaction with residual enzyme activity. This phenolic acid was found to be a classical noncompetitive inhibitor and the inhibition constant K(I) was obtained as 0.603 mM. Anisic acid also inhibited the hydroxylation of L-tyrosine catalyzed by tyrosinase. The lag phase caused by the monophenolase activity was lengthened and the steady-state activity of the enzyme was decreased by anisic acid.     

New benzimidazole derivatives as antiplasmodial agents and plasmepsin inhibitors: synthesis and analysis of structure-activity relationships

The newly synthesized benzimidazole compounds were suggested to be inhibitors of Plasmodium falciparum plasmepsin II and human cathepsin D by virtual screening of an internal library of synthetic compounds. This was confirmed by enzyme inhibition studies that gave IC(50) values in the low micromolar range (2-48μM). Ligand docking studies with plasmepsin II predicted binding of benzimidazole compounds at the center of the extended substrate-binding cleft. According to the plausible mode of binding, the pyridine ring of benzimidazole compounds interacted with S1' subsite residues whereas the acetophenone moiety was in contact with S1-S3 subsites of plasmepsin II active center. The benzimidazole derivatives were evaluated for capacity to inhibit the growth of intraerythrocytic P. falciparum in culture. Four benzimidazole compounds inhibited parasite growth at ?3μM. The most active compound 10, 1-(4-phenylphenyl)-2[2-(pyridinyl-2-yl)-1,3-benzdiazol-1-yl]ethanone showed an IC(50) of 160nM. The substitution of a phenyl group and a chlorine atom at the para position of the acetophenone moiety were shown to be crucial for antiplasmodial activity.     

Inhibitory effect of dalbergioidin isolated from the trunk of Lespedeza cyrtobotrya on melanin biosynthesis

Tyrosinase is a key enzyme for melanin biosynthesis, and hyperpigmentation disorders are associated with abnormal accumulation of melanin pigments, which can be reduced by treatment with depigmenting agents. The methanol extract of Lespedeza cyrtobotrya MIQ showed inhibitory activity against mushroom tyrosinase. The active compound was purified from the methanol extract of L cyrtobotrya, followed by several chromatographic methods, and identified as dalbergioidin (DBG) by spectroscopic methods. The results showed that DBG exhibited tyrosinase inhibitory activity with an IC50 of 20 mM. The kinetic analysis tyrosinase inhibition revealed that DBG acted as noncompetitive inhibitor. In addition, DBG showed a melanin biosynthesis inhibition zone in the culture plate of Streptomyces bikiniensis that has commonly been used as an indicator organism. Furthermore, 27 mM DBG decreased more than 50% of melanin contents on the pigmentation using immortalized mouse melanocyte, melan-a cell.     

Inhibition of lipoxygenase activity and HL60 leukemic cell proliferation by ursolic acid isolated from heather flowers (Calluna vulgaris).

A compound was isolated and purified from heather flowers (Calluna vulgaris) based on its ability to inhibit lipoxygenase activity. This molecule was characterized as ursolic acid by GC-MS. Ursolic acid was found to be an inhibitor of both potato tuber 5-lipoxygenase and soybean 15-lipoxygenase with IC50 values of 0.3 mM. Ursolic acid also inhibits lipoxygenase activity in mouse peritoneal macrophages at 1 microM and HL60 leukemic cells growth (IC50 = 0.85 microM) as well as their DNA synthesis (IC50 = 1 microM). The possible role of lipoxygenase inhibition in the proliferation of leukemic cells is discussed.