Urogenital distribution of a mouse membrane-associated prostaglandin E(2) synthase.

PGE(2) plays a critical role in regulating renal function and facilitating reproduction. One of the rate-limiting biosynthetic enzymes in PGE(2) synthesis is the terminal PGE(2) synthase (PGES). In the present studies, we report the functional expression of a membrane-associated murine PGES (mPGES) and its expression in urogenital tissues. Two independent cDNA clones sharing an identical open reading frame of 459 bp and encoding a peptide of 153 amino acids, but differing in the 3'-untranslated region, were identified. Assays for enzymatic activity, using microsomes prepared from cells transfected with mPGES cDNA, showed that these cDNA sequences encode a functional protein that catalyzes the conversion of PGH(2) to PGE(2). Constitutive expression of mPGES was highest in the mouse kidney, ovary, and urinary bladder but was also expressed at lower levels in uterus and testis. Renal mPGES expression was predominantly localized to epithelia of distal tubules and medullary collecting ducts. High expression was also seen in transitional epithelial cells of bladder and ureter and in the primary and secondary follicles in the ovary. In conclusion, mPGES is constitutively expressed along the urogenital tract, where it may have important roles in normal physiology and disease.

A high level of cyclooxygenase-2 inhibitor selectivity is associated with a reduced interference of platelet cyclooxygenase-1 inactivation by aspirin.

Both nonsteroidal anti-inflammatory drugs, such as ibuprofen, and the prototypical selective cyclooxygenase (Cox)-2 inhibitors DuP-697 and NS-398 block the inhibition of Cox-1 by aspirin in vitro. However, clinical studies have shown that the Cox-2 selective drugs (or coxibs) rofecoxib and etoricoxib, at therapeutic doses, do not interfere with the antiplatelet effect of aspirin, in contrast to ibuprofen. Here, we have evaluated the relative potential of ibuprofen and various coxibs to interfere with the inactivation of Cox-1 by aspirin by using purified enzyme and calcium ionophore-activated human platelets. The irreversible inactivation of Cox-1 by aspirin can be antagonized by ibuprofen and coxibs, albeit with widely different potencies. The rank order of potencies for this process (ibuprofen > celecoxib > valdecoxib > rofecoxib > etoricoxib) parallels that obtained for the inhibition of Cox-1-mediated thromboxane B(2) production by calcium ionophore-stimulated platelets. The antagonism of aspirin therefore likely involves a competition at the enzyme active site. The EC(50) value for the antagonism against 10 microM aspirin for each drug is approximately 10- to 40-fold lower than the corresponding IC(50) value for inhibition of platelet Cox-1 activity, consistent with the much weaker initial binding of aspirin to Cox-1 as compared with arachidonic acid. These results show that a low affinity for Cox-1 and a high degree of Cox-2 selectivity confers a low potential to block aspirin inhibition of platelet Cox-1, consistent with the results of clinical studies.

Effects of COX-2 inhibitors on aortic prostacyclin production in cholesterol-fed rabbits.

Prostacyclin (PGI(2)) is a potent vasodilator and inhibitor of platelet aggregation that is produced by prostacyclin synthase via the cyclooxygenase (COX) pathway of arachidonic acid metabolism. We investigated the potential role of COX-2 in the production of vasoactive prostanoids by aortic tissue in a rabbit model of dietary cholesterol-induced atherosclerosis. COX-1 was detected as the major isoform by immunoblot analysis in extracts from aortas of normal and 8 week cholesterol-fed animals with COX-2 being induced in atherosclerotic plaques from cholesterol-fed animals. Aortic tissue from cholesterol-fed animals showed decreased levels of basal 6-keto-PGF(1 alpha) and PGE(2) production as compared to the normal controls but showed no difference with respect to their ability to synthesize these prostanoids in response to exogenous arachidonic acid. The highly selective COX-2 inhibitors rofecoxib and the furanone DFP at concentrations of up to 10 micromol/l had no effect on the arachidonic acid-dependent production of 6-keto-PGF(1 alpha), in contrast to indomethacin, which caused a complete inhibition at 0.5 micromol/l. Celecoxib caused a significant inhibition of 6-keto-PGF(1 alpha) at 10 micromol/l but had little effect when the dose was lowered to 1 micromol/l. Similar effects of these inhibitors were observed with respect to the production of PGE(2) and no major difference was observed between aortic tissues from normal and cholesterol-fed animals with regard to inhibitor sensitivity. These results indicate that in a rabbit model of early stage cardiovascular disease, the basal production of 6-keto-PGF(1 alpha) and PGE(2) by aortic tissue is decreased. Furthermore, COX-2 expression is induced in atherosclerotic plaques and may play a role in altering localized synthesis of prostanoids in these lesions but does not appear to significantly impact the arachidonic acid-dependent prostacylin production of aortic tissues, which is largely mediated by COX-1.

In vitro metabolism considerations, including activity testing of metabolites, in the discovery and selection of the COX-2 inhibitor etoricoxib (MK-0663).

Characterization of the metabolites of the COX-2 inhibitor etoricoxib (MK-0663 and L-791,456) produced in vitro indicate formation of an N-oxide pyridine and hydroxymethyl pyridine that can further be glucuronidated or oxidized to an acid. Significant turnover is observed in human hepatocytes. Several CYPs are involved in the oxidative biotranformations and, from in vitro studies, etoricoxib is not a potent CYP3A4 inducer or inhibitor. Based on an in vitro whole blood assay, none of the metabolites of etoricoxib inhibits COX-1 or contributes significantly to the inhibition of COX-2.

Etoricoxib (MK-0663): preclinical profile and comparison with other agents that selectively inhibit cyclooxygenase-2.

We report here the preclinical profile of etoricoxib (MK-0663) [5-chloro-2-(6-methylpyridin-3-yl)-3-(4-methylsulfonylphenyl) pyridine], a novel orally active agent that selectively inhibits cyclooxygenase-2 (COX-2), that has been developed for high selectivity in vitro using whole blood assays and sensitive COX-1 enzyme assays at low substrate concentration. Etoricoxib selectively inhibited COX-2 in human whole blood assays in vitro, with an IC(50) value of 1.1 +/- 0.1 microM for COX-2 (LPS-induced prostaglandin E2 synthesis), compared with an IC(50) value of 116 +/- 8 microM for COX-1 (serum thromboxane B2 generation after clotting of the blood). Using the ratio of IC(50) values (COX-1/COX-2), the selectivity ratio for the inhibition of COX-2 by etoricoxib in the human whole blood assay was 106, compared with values of 35, 30, 7.6, 7.3, 2.4, and 2.0 for rofecoxib, valdecoxib, celecoxib, nimesulide, etodolac, and meloxicam, respectively. Etoricoxib did not inhibit platelet or human recombinant COX-1 under most assay conditions (IC(50) > 100 microM). In a highly sensitive assay for COX-1 with U937 microsomes where the arachidonic acid concentration was lowered to 0.1 microM, IC(50) values of 12, 2, 0.25, and 0.05 microM were obtained for etoricoxib, rofecoxib, valdecoxib, and celecoxib, respectively. These differences in potency were in agreement with the dissociation constants (K(i)) for binding to COX-1 as estimated from an assay based on the ability of the compounds to delay the time-dependent inhibition by indomethacin. Etoricoxib was a potent inhibitor in models of carrageenan-induced paw edema (ID(50) = 0.64 mg/kg), carrageenan-induced paw hyperalgesia (ID(50) = 0.34 mg/kg), LPS-induced pyresis (ID(50) = 0.88 mg/kg), and adjuvant-induced arthritis (ID(50) = 0.6 mg/kg/day) in rats, without effects on gastrointestinal permeability up to a dose of 200 mg/kg/day for 10 days. In squirrel monkeys, etoricoxib reversed LPS-induced pyresis by 81% within 2 h of administration at a dose of 3 mg/kg and showed no effect in a fecal 51Cr excretion model of gastropathy at 100 mg/kg/day for 5 days, in contrast to lower doses of diclofenac or naproxen. In summary, etoricoxib represents a novel agent that selectively inhibits COX-2 with 106-fold selectivity in human whole blood assays in vitro and with the lowest potency of inhibition of COX-1 compared with other reported selective agents.

Mutations of the C-terminal end of cathepsin K affect proenzyme secretion and intracellular maturation.

Transfection of the human cathepsin K cDNA into CHO cells results in the expression of mature catalytically active 27-kDa protein and in cells secreting the 39-kDa proenzyme form. Monensin, which neutralizes the pH of acidic organelles, was found to inhibit intracellular processing of the proenzyme and to stimulate its secretion into the culture medium. Brefeldin A caused alterations in immunofluorescence staining consistent with interference of lysosomal targeting and inhibited both intracellular processing and secretion of cathepsin K. Inhibition of glycosylation by tunicamycin also abolished cathepsin K maturation. Furthermore, the processing of the proenzyme to the mature form was abolished by a single mutation of the terminal Met(329) to Ala. The triple mutation of Ser(325), Pro(327), and Met(329) (all to Ala) inhibited both maturation and secretion, using either transient or stable expression systems. The results indicate that intracellular maturation and secretion of cathepsin K can be affected differentially by various treatments and by mutations of the C-terminal end of the protein. These results are consistent with the involvement of both the secreted proenzyme and the intracellularly processed enzyme in cathepsin K-mediated processes.

Novel, nonpeptidic cyanamides as potent and reversible inhibitors of human cathepsins K and L.

Compounds containing a 1-cyanopyrrolidinyl ring were identified as potent and reversible inhibitors of cathepsins K and L. The original lead compound 1 inhibits cathepsins K and L with IC(50) values of 0. 37 and 0.45 M, respectively. Modification of compound 1 by replacement of the quinoline moiety led to the synthesis of N-(1-cyano-3-pyrrolidinyl)benzenesulfonamide (2). Compound 2 was found to be a potent inhibitor of cathepsins K and L with a K(i) value of 50 nM for cathepsin K. Replacement of the 1-cyanopyrrolidine of compound 2 by a 1-cyanoazetidine increased the potency of the inhibitor by 10-fold. This increase in potency is probably due to an enhanced chemical reactivity of the compound toward the thiolate of the active site of the enzyme. This is demonstrated when the assay is performed in the presence of glutathione at pH 7.0 which favors the formation of a GSH thiolate anion. Under these assay conditions, there is a loss of potency in the 1-cyanoazetidine series due to the formation of an inactive complex between the GSH thiolate and the 1-cyanoazetidine inhibitors. 1-Cyanopyrrolidinyl inhibitors exhibited time-dependent inhibition which allowed us to determine the association and dissociation rate constants with human cathepsin K. The kinetic data obtained showed that the increase of potency observed between different 1-cyanopyrrolidinyl inhibitors is due to an increase of k(on) values and that the association of the compound with the enzyme fits an apparent one-step mechanism. (13)C NMR experiments performed with the enzyme papain showed that compound 2 forms a covalent isothiourea ester adduct with the enzyme. As predicted by the kinetic analysis, the addition of the irreversible inhibitor E64 to the enzyme-cyanopyrrolidinyl complex totally abolished the signal of the isothiourea bond as observed by (13)C NMR, thereby demonstrating that the formation of the covalent bond with the active site cysteine residue is reversible. Finally, compound 2 inhibits bone resorption in an in vitro assay involving rabbit osteoclasts and bovine bone with an IC(50) value of 0.7 M. 1-Cyanopyrrolidine represents a new class of nonpeptidic compounds that inhibit cathepsin K and L activity and proteolysis of bone collagen.

Cloning, expression, and up-regulation of inducible rat prostaglandin e synthase during lipopolysaccharide-induced pyresis and adjuvant-induced arthritis.

We have cloned and expressed the inducible form of prostaglandin (PG) E synthase from rat and characterized its regulation of expression in several tissues after in vivo lipopoylsaccharide (LPS) challenge. The rat PGE synthase is 80% identical to the human enzyme at the amino acid level and catalyzes the conversion of PGH(2) to PGE(2) when overexpressed in Chinese hamster ovary K1 (CHO-K1) cells. PGE synthase activity was measured using [(3)H]PGH(2) as substrate and stannous chloride to terminate the reaction and convert all unreacted unstable PGH(2) to PGF(2alpha) before high pressure liquid chromatography analysis. We assessed the induction of PGE synthase in tissues from Harlan Sprague-Dawley rats after LPS-induced pyresis in vivo. Rat PGE synthase was up-regulated at the mRNA level in lung, colon, brain, heart, testis, spleen, and seminal vesicles. Cyclooxygenase (COX)-2 and interleukin 1beta were also up-regulated in these tissues, although to different extents than PGE synthase. PGE synthase and COX-2 were also up-regulated to the greatest extent in a rat model of adjuvant-induced arthritis. The RNA induction of PGE synthase in lung and the adjuvant-treated paw correlated with a 3.8- and 16-fold induction of protein seen in these tissues by immunoblot analysis. Because PGE synthase is a member of the membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG) family, of which leukotriene (LT) C(4) synthase and 5-lipoxygenase-activating protein are also members, we tested the effect of LTC(4) and the 5-lipoxygenase-activating protein inhibitor MK-886 on PGE synthase activity. LTC(4) and MK-886 were found to inhibit the activity with IC(50) values of 1.2 and 3.2 microm, respectively. The results demonstrate that PGE synthase is up-regulated in vivo after LPS or adjuvant administration and suggest that this is a key enzyme involved in the formation of PGE(2) in COX-2-mediated inflammatory and pyretic responses.

Synthesis, characterization, and activity of metabolites derived from the cyclooxygenase-2 inhibitor rofecoxib (MK-0966, Vioxx).

Metabolites of the COX-2 inhibitor rofecoxib (MK-0966, Vioxx) were prepared by synthetic or biosynthetic methods. Metabolites include products of oxidation, glucuronidation, reduction and hydrolytic ring opening. Based on an in vitro whole blood assay, none of the known human metabolites of rofecoxib inhibits COX-1 nor contributes significantly to the inhibition of COX-2.

Expression, maturation, and rhodamine-based fluorescence assay of human cathepsin K expressed in CHO cells.

Cathepsin K is a cysteine protease that degrades type I human collagen during bone resorption. We have expressed the recombinant human cathepsin K in Chinese hamster ovary (CHO) cells as a pre-proenzyme and demonstrated that it is processed intracellularly to an active enzyme form and that only the proenzyme form is secreted. Immunofluorescence detection of cathepsin K in CHO cells resulted in discrete punctate distribution consistent with a lysosomal localization of the enzyme. With both extract and cell preparations of CHO cells expressing cathepsin K, [Z-Leu-Arg](2)-rhodamine was the best substrate for analyzing cathepsin K activity over background proteases. We have established a cellular-based assay to analyze cell-permeable inhibitors of cathepsin K and validated the assay with detection of intracellular versus extracellular activity, fluorescence-assisted cell sorter (FACS) analysis, and a selective cathepsin K inhibitor. The intracellular activity of cathepsin K was monitored by FACS analysis using the rhodamine substrate, which demonstrated an increased fluorescence over mock-transfected cells that was also inhibitable by (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester (E64d). A selective cathepsin K inhibitor, 1, 3-bis(CBZ-Leu-NH)-2-propanone, had an IC(50) of 134 nM in the CHO/Cat K cells, which is the same potency as that measured against a purified enzyme preparation of cathepsin K. Therefore, we have established a system to evaluate intracellular cathepsin K activity and inhibition by cell-permeable inhibitors of this thiol protease.

LTA(4)-derived 5-oxo-eicosatetraenoic acid: pH-dependent formation and interaction with the LTB(4) receptor of human polymorphonuclear leukocytes.

5-oxo-(7E,9E,11Z,14Z)-eicosatetraenoic acid (5-oxo-ETE) has been identified as a non-enzymatic hydrolysis product of leukotriene A(4) (LTA(4)) in addition to 5,12-dihydroxy-(6E,8E,10E, 14Z)-eicosatetraenoic acids (5,12-diHETEs) and 5,6-dihydroxy-(7E,9E, 11Z,14Z)-eicosatetraenoic acids (5,6-diHETEs). The amount of 5-oxo-ETE detected in the mixture of the hydrolysis products of LTA(4) was found to be pH-dependent. After incubation of LTA(4) in aqueous medium, the ratio of 5-oxo-ETE to 5,12-diHETE was 1:6 at pH 7.5, and 1:1 at pH 9.5. 5-Oxo-ETE was isolated from the alkaline hydrolysis products of LTA(4) in order to evaluate its effects on human polymorphonuclear (PMN) leukocytes. 5-Oxo-ETE induced a rapid and dose-dependent mobilization of calcium in PMN leukocytes with an EC(50) of 250 nM, as compared to values of 3.5 nM for leukotriene B(4) (LTB(4)500 nM for 5(S)-hydroxy-(6E,8Z,11Z,14Z)-eicosatetraenoic acid (5-HETE). Pretreatment of the cells with LTB(4) totally abolished the calcium response induced by 5-oxo-ETE. In contrast, the preincubation with 5-oxo-ETE did not affect the calcium mobilization induced by LTB(4). The calcium response induced by 5-oxo-ETE was totally inhibited by the specific LTB(4) receptor antagonist LY223982. These data demonstrate that 5-oxo-ETE can induce calcium mobilization in PMN leukocyte via the LTB(4) receptor in contrast to the closely related analog 5-oxo-(6E,8Z,11Z, 14Z)-eicosatetraenoic acid which is known to activate human neutrophils by a mechanism independent of the receptor for LTB(4).

A new structural variation on the methanesulfonylphenyl class of selective cyclooxygenase-2 inhibitors.

By inserting an oxygen link between the 3-fluorophenyl and the lactone ring of 5,5-dimethyl-3-(3fluorophenyl)-4-(4-methanesulfonylphenyl)-2 (5H)-furanone 1 (DFU), analogs with enhanced in vitro COX-2 inhibitory potency as well as in vivo potency in models of inflammation were obtained.

Synthesis and biological evaluation of 3-heteroaryloxy-4-phenyl-2(5H)-furanones as selective COX-2 inhibitors.

A series of 3-heteroaryloxy4-phenyl-2-5H)-furanones were prepared and evaluated for their potency and selectivity as COX-2 inhibitors. This led to the identification of L-778,736 as a potent, orally active and selective inhibitor of the COX-2 enzyme.

Substituted indoles as potent and orally active 5-lipoxygenase activating protein (FLAP) inhibitors.

This paper reports on the SAR investigation of inhibitors of 5-lipoxygenase activating protein (FLAP) based on MK-0591. Emphasis was made on modifications to the nature of the link between the indole and the quinoline moieties, to the substitution pattern around the two heterocycles and to possible replacements of the quinoline moiety. Lead optimization culminated in (3-[1-(4-chlorobenzyl)-3-(t-butylthio)-5-(pyridin-2-ylmethoxy)-ind ol-2-yl]-2,2-dimethylpropanoic acid (18k), as a potent inhibitor of leukotriene biosynthesis that is well absorbed and active in functional models.

Cloning and expression of rhesus monkey cathepsin K.

Cathepsin K is a cysteine protease involved in degradation of human type I collagen and plays a primary role in bone resorption. We have cloned rhesus monkey cathepsin K by reverse transcriptase-polymerase chain reaction (RT-PCR) from rhesus ovary poly A+ RNA. The sequence for the rhesus enzyme is 98% identical to that of the human with 100% identity within the mature active form of cathepsin K. Rhesus monkey cathepsin K was transiently expressed in Chinese hamster ovary (CHO) cells and found to be secreted as the proenzyme in the culture media and 50% activated to the mature form intracellularly. The substrate specificity preference of aminomethylcoumarin and rhodamine peptide substrates was Leu > Phe > Pro in the P2 position when tested with constant arginine at P1. The enzyme activity expressed in CHO cell extracts was sensitive to inhibition by E-64 and cystatin with IC50s of 3.5 nmol/L and 13 ng/mL, respectively. The apparent second order rate constants of inactivation by E-64 were 66,000 M(-1) s(-1) and 130,000 M(-1) s(-1) for the recombinantly expressed rhesus monkey and human cathepsin K, respectively. The high similarity between the sequences and the kinetic properties of rhesus monkey and human cathepsin K establishes this monkey species as a suitable animal model for development of novel cathepsin K inhibitors as antiresorptive agents.

SAR in the alkoxy lactone series: the discovery of DFP, a potent and orally active COX-2 inhibitor.

Extensive SAR has been established in the alkoxy lactone series and this has lead to the discovery of DFP (5,5-dimethyl-3-(2-propoxy)-4-methanesulfonylphenyl)-2(5H)-furanon e), a potent COX-2 inhibitor exhibiting in vivo efficacy in all models studied.

2-heterosubstituted-3-(4-methylsulfonyl)phenyl-5-trifluoromethyl pyridines as selective and orally active cyclooxygenase-2 inhibitors.

A series of novel 2-alkoxy, 2-thioalkoxy and 2-amino-3-(4-methylsulfonyl)phenylpyridines has been synthesized and shown to be highly potent and selective cyclooxygenase-2 (COX-2) inhibitors. Structure-activity relationship studies have demonstrated that central pyridine ring substituents play an important role in the COX-2 potency, selectivity vs the COX-1 enzyme, and oral activity.

The discovery of rofecoxib, [MK 966, Vioxx, 4-(4'-methylsulfonylphenyl)-3-phenyl-2(5H)-furanone], an orally active cyclooxygenase-2-inhibitor.

The development of a COX-2 inhibitor rofecoxib (MK 966, Vioxx) is described. It is essentially equipotent to indomethacin both in vitro and in vivo but without the ulcerogenic side effect due to COX-1 inhibition.

2,3-Diarylcyclopentenones as orally active, highly selective cyclooxygenase-2 inhibitors.

Cyclopentenones containing a 4-(methylsulfonyl)phenyl group in the 3-position and a phenyl ring in the 2-position are selective inhibitors of cyclooxygenase-2 (COX-2). The selectivity for COX-2 over COX-1 is dramatically improved by substituting the 2-phenyl group with halogens in the meta position or by replacing the phenyl ring with a 2- or 3-pyridyl ring. Thus the 3,5-difluorophenyl derivative 7 (L-776,967) and the 3-pyridyl derivative 13 (L-784,506) are particularly interesting as potential antiinflammatory agents with reduced side-effect profiles. Both exhibit good oral bioavailability and are potent in standard models of pain, fever, and inflammation yet have a much reduced effect on the GI integrity of rats compared to standard nonsteroidal antiflammatory drugs.

Phosphodiesterase 4-dependent regulation of cyclic AMP levels and leukotriene B4 biosynthesis in human polymorphonuclear leukocytes.

Several selective phosphodiesterase 4 inhibitors were found to be potent inhibitors of the N-formyl-Met-Leu-Phe (fMLP)-induced leukotriene B4 biosynthesis by human polymorphonuclear leukocytes with IC50s in the nanomolar range (0.09-26 nM). The rank order of potency was 6-(4-pyridylmethyl)-8-(3-nitrophenyl)quinoline (RS-14203) > 3-benzyl-5-phenyl-3H-imidazo[4,5-c][1,8]naphthyridin-4(5H)-one (KF18280) > 8-aza-1-(3-nitrophenyl)-3-(4-pyridylmethyl)-2,4-quinazoline dione (RS-25344) > 3-cyclo-pentyloxy-N-[3,5-dichloro-4-pyridyl]-4-methoxybenzamide (RP-73401) > R-rolipram > R-4-[2-(3-cyclopentyloxy-4-methoxyphenyl)-2-phenylethyl] pyridine (CDP840)> S-rolipram. Isoproterenol (IC50 = 350 nM) and prostaglandin E2 (IC50 = 59 nM) also suppressed leukotriene B4 biosynthesis. Inhibitors of the phosphodiesterase 1 (8-methoxymethyl-1-methyl-3-(2-methylpropyl)xanthine (8-MeOMe-IBMX)), phosphodiesterase 2 (erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA)), phosphodiesterase 3 (quazinone and milrinone) and phosphodiesterase 5 (zaprinast and dipyridamole) had no inhibitory effects on the fMLP-induced leukotriene B4 biosynthesis (IC50s > 20 microM). All phosphodiesterase 4 inhibitors caused an accumulation of cellular cyclic AMP to 140-185% over the basal level of fMLP-treated control cells, comparable to that observed with high concentrations of isoproterenol and prostaglandin E2. In contrast, the complete inhibition of leukotriene B4 production by 5-lipoxygenase and 5-lipoxygenase-activating protein (FLAP) inhibitors had no effect on cyclic AMP levels. Phosphodiesterase 1, 2, 3 and 5 inhibitors had little effect on the level of cellular cyclic AMP (89-126% of the basal cyclic AMP level). Dose-dependencies for R-rolipram, RS-14203 and CDP840 indicated that the maximal accumulation of cyclic AMP occurred at concentrations of phosphodiesterase 4 inhibitors higher than those required for the inhibition of leukotriene B4 production. The presence of a mixture of 8-MeOMe-IBMX, EHNA, milrinone and zaprinast to inhibit phosphodiesterase 1, 2, 3 and 5 had little effect on the dose-dependence of R-rolipram for the inhibition of leukotriene B4 biosynthesis or cyclic AMP accumulation. These data demonstrate that selective phosphodiesterase 4 inhibitors can inhibit the fMLP-induced leukotriene B4 biosynthesis in human polymorphonuclear leukocytes with a potency similar or greater than that of potent 5-lipoxygenase or FLAP inhibitors. This inhibition is accompanied by small variations in the levels of cellular cyclic AMP and appears to proceed independently of the other phosphodiesterases.