Terphenyl cyclooxygenase-2 (COX-2) inhibitors: optimization of the central ring and o-biphenyl analogs

The discovery of terphenyl derivatives as highly selective COX-2 inhibitors resulted from our efforts to overcome poor pharmacokinetics demonstrated by the COX-2 selective diarylthiophene DuP 697 [2-bromo-4-(4'-sulfonylmethyl)phenyl-5-(4'-fluoro)phenylthiophe ne]. Detailed SAR related to the ortho-biphenyls and variants of the central ring are described herein.

MEK inhibitors: the chemistry and biological activity of U0126, its analogs, and cyclization products

In search of antiinflammatory drugs with a new mechanism of action, U0126 was found to functionally antagonize AP-1 transcriptional activity via noncompetitive inhibition of the dual specificity kinase MEK with an IC50 of 0.07 microM for MEK 1 and 0.06 microM for MEK 2. U0126 can undergo isomerization and cyclization reactions to form a variety of products, both chemically and in vivo, all of which exhibit less affinity for MEK and lower inhibition of AP-1 activity than parent, U0126.

Identification of a novel inhibitor of mitogen-activated protein kinase kinase

The compound U0126 (1,4-diamino-2,3-dicyano-1, 4-bis[2-aminophenylthio]butadiene) was identified as an inhibitor of AP-1 transactivation in a cell-based reporter assay. U0126 was also shown to inhibit endogenous promoters containing AP-1 response elements but did not affect genes lacking an AP-1 response element in their promoters. These effects of U0126 result from direct inhibition of the mitogen-activated protein kinase kinase family members, MEK-1 and MEK-2. Inhibition is selective for MEK-1 and -2, as U0126 shows little, if any, effect on the kinase activities of protein kinase C, Abl, Raf, MEKK, ERK, JNK, MKK-3, MKK-4/SEK, MKK-6, Cdk2, or Cdk4. Comparative kinetic analysis of U0126 and the MEK inhibitor PD098059 (Dudley, D. T., Pang, L., Decker, S. J., Bridges, A. J., and Saltiel, A. R. (1995) Proc. Natl. Acad. Sci U. S. A. 92, 7686-7689) demonstrates that U0126 and PD098059 are noncompetitive inhibitors with respect to both MEK substrates, ATP and ERK. We further demonstrate that the two compounds bind to deltaN3-S218E/S222D MEK in a mutually exclusive fashion, suggesting that they may share a common or overlapping binding site(s). Quantitative evaluation of the steady state kinetics of MEK inhibition by these compounds reveals that U0126 has approximately 100-fold higher affinity for deltaN3-S218E/S222D MEK than does PD098059. We further tested the effects of these compounds on the activity of wild type MEK isolated after activation from stimulated cells. Surprisingly, we observe a significant diminution in affinity of both compounds for wild type MEK as compared with the deltaN3-S218E/S222D mutant enzyme. These results suggest that the affinity of both compounds is mediated by subtle conformational differences between the two activated MEK forms. The MEK affinity of U0126, its selectivity for MEK over other kinases, and its cellular efficacy suggest that this compound will serve as a powerful tool for in vitro and cellular investigations of mitogen-activated protein kinase-mediated signal transduction.

Heteroatom- and carbon-linked biphenyl analogs of Brequinar as immunosuppressive agents

Structure-activity relationships were explored for some analogs of Brequinar having a linking atom between the 2-biphenyl substituent and the quinoline ring. Activities as inhibitors of dihydroorotate dehydrogenase and the mixed lymphocyte reaction were related to the overall shape and lipophilicity of the 2-substituent.

Competitive inhibition of MAP kinase activation by a peptide representing the alpha C helix of ERK

On the basis of the crystal structure of the MEK substrate ERK, we have synthesized a 15 amino acid peptide representing the alpha C helix of human ERK1. We find this peptide to be an inhibitor of ERK phosphorylation by its upstream activator MEK. Circular dichroic spectroscopy indicates that the peptide has little secondary structure in aqueous buffer, but can readily adopt an alpha-helical structure in aprotic solvent. Steady-state kinetic analysis indicates that the peptide serves as a competitive inhibitor of ERK binding to MEK, with a dissociation constant, Ki, of 0.84 microM. Together with ATP-competitive inhibitors of MEK, we have used this peptide to define the kinetic mechanism of MEK catalysis. These studies reveal that MEK operates through a bi-bi random-ordered sequential mechanism. The synthetic peptide inhibits also the phosphorylation of p38 and ERK by the upstream activator MKK3, but is at least 3-fold less potent as an inhibitor of SEK activation of JNK1. Interestingly, the peptide also showed some ability to inhibit ERK-mediated phosphorylation of myelin basic protein, but was inactive as an inhibitor of the unrelated kinases Raf, Abl, and PKA. These results imply that the alpha C helix is an important locus of interaction for the formation of a MEK-ERK complex. The alpha C helix cannot, however, be the sole determinant of activator selectivity among the MAP kinases. Molecules designed to target the alpha C helix binding pocket of MAP kinase activators may provide a novel means of inhibiting these signal transducers.

Kinetic effects due to nonspecific substrate-inhibitor interactions in enzymatic reactions

Nonspecific protein binding is a commonly encountered problem with synthetic molecules designed as enzyme inhibitors. When the substrate for the enzymatic reaction is itself a protein, such nonspecific protein binding can also occur. Here, we demonstrate that this phenomenon can have a dramatic effect on the steady-state kinetic evaluation of such inhibitors.

Structure-based design and synthesis of a series of hydroxamic acids with a quaternary-hydroxy group in P1 as inhibitors of matrix metalloproteinases

Examination of the S1 area of the active site of pro-stromelysin has led us to the design of novel and potent inhibitors of matrix metalloproteinases containing constrained quaternary-hydroxy group at P1. The synthesis and biological activity of these compounds with variations at P1', P2', and P3' will be described.

Structure-activity relationships (SAR) of some tetracyclic heterocycles related to the immunosuppressive agent Brequinar Sodium

The structure-activity relationships of some tetracyclic heterocycles related to Brequinar were explored. Activities as inhibitors of dihydroorotate dehydrogenase and the mixed lymphocyte reaction are related to ring system, heteroatom placement, and pendant ring substitution.

Spectroscopic characterization of arrestin interactions with competitive ligands: study of heparin and phytic acid binding

A combination of intrinsic fluorescence and circular dichroic (CD) spectroscopy has been used to characterize the complexes formed between bovine retinal arrestin and heparin or phytic acid, two ligands that are known to mimic the structural changes in arrestin attending receptor binding. No changes in the CD spectra were observed upon ligand binding, nor did the degree of tryptophan fluorescence quenching change significantly in the complexes. These data argue against any large-scale changes in protein secondary or tertiary structure accompanying ligand binding. The change in tyrosine fluorescence intensity was used to determine the dissociation constants for the heparin and phytic acid complexes of arrestin. The only change observed was a saturable diminution of tyrosine fluorescence signal from the protein. For both ligands, the data suggest two distinct binding interactions with the protein--a high--affinity interaction with Kd between 200 and 300 nM, and a lower affinity interaction with Kd between 2 and 8 microM. Study of collisional quenching of tyrosine fluorescence in free arrestin and the ligand-replete complexes indicates that 10 of the 14 tyrosine residues of the protein are solvent-exposed in the free protein; this value drops to between 5 and 6 solvent-exposed residues in the high-affinity complexes of the two ligands. These data suggest that ligand binding leads to direct occlusion of between 4 and 5 tyrosine residues on the solvent-exposed surface of the protein, but not to any large-scale changes in protein structure. The large activation energy previously reported to be associated with arrestin-receptor interactions may therefore reflect localized movements of the N- and C-termini of arrestin, which are proposed to interact in the free protein through electrostatic interactions. Binding of the anionic ligands heparin, phytic acid, or phosphorylated rhodopsin may compete with the C-terminus of arrestin for these electrostatic interactions, thus allowing the C-terminus to swing out of the binding region.

Histidine to alanine mutants of human dihydroorotate dehydrogenase. Identification of a brequinar-resistant mutant enzyme

Dihydroorotate dehydrogenase (DHODase) is the rate-limiting enzyme of the mammalian de novo pyrimidine biosynthesis pathway, and is the molecular target of the antiproliferative, immunosuppressive compound brequinar sodium (BQR). We have shown previously that the activity of the recombinant human enzyme displays pH and diethylpyrocarbonate sensitivities that implicate a critical role for one or more histidine residues in catalysis [Copeland et al., Arch Biochem Biophys 323: 79-86, 1995.]. Here we report the results of alanine scanning mutagenesis for each of the 8 histidine residues of the recombinant human enzyme. In most cases, the replacement of histidine by alanine had little effect on the Km values of the two substrates, dihydroorotate and ubiquinone, or on the overall kcat of the enzymatic reaction. Replacement of H71, H129, and H364 by alanine, however, completely abolished enzymatic activity. The loss of activity for the H71A mutant was unexpected, since this residue is not conserved in the homologous rat enzyme; in the rodent enzyme this residue is an asparagine. Replacement of H71 by asparagine in the human enzyme led to a full recovery of enzymatic activity, indicating that a histidine is not required at this position. Replacement of H26 by alanine led to about a 10-fold reduction in catalytic activity relative to the wild-type enzyme, with no significant perturbation of the substrate Km values. This mutant was, however, at least 167-fold less sensitive to inhibition by the noncompetitive inhibitor BQR. While the wild-type and other mutant enzymes displayed IC50 values for BQR inhibition between 6 and 10 nM, the H26A mutant was inhibited less than 25% at concentrations of BQR as high as 150 nM. These data suggest that H26 plays an important role in BQR binding to the enzyme.

Detection of an Fe2+-protoporphyrin-IX intermediate during aspirin-treated prostaglandin H2 synthase II catalysis of arachidonic acid to 15-HETE

Spectral intermediates associated with the dioxygenase and peroxidase activities of prostaglandin H2 (PGH2) synthase I and II were monitored by stopped-flow spectrometry. During reactions of PGH2 synthase I with arachidonic acid (AA) and ethyl hydrogen peroxide (EtOOH), compound I (Fe5+; formally (protoporphyrin-IX) x +Fe4+=O) and compound II (Fe4+; formally (protoporphyrin-IX)Fe4+=O) were detected. These intermediates were observed sooner with EtOOH (within 50 ms) than with AA (within 200 ms). Compound I and compound II were found to be kinetically competent with respect to AA-dependent O2 uptake. These findings are consistent with a mechanism in which peroxidative cleavage precedes AA dioxygenation. During reactions with PGH2 synthase II with AA, compound I and compound II were again observed within 200 ms and were kinetically competent to participate in dioxygenation. However, during reactions of PGH2 synthase II with EtOOH, compound I and compound II were detected much later (after 10 s). These findings would be inconsistent with a mechanism in which peroxidative cleavage precedes AA dioxygenation. When aspirin-treated PGH2 synthase II was reacted with EtOOH, a normal peroxidase cycle occurred with compound I and compound II formation occurring over 10 s. However, when aspirin-treated PGH2 synthase II was reacted with AA, a unique spectral intermediate with lambda(max) at 446 nm was detected within 3 ms and was strikingly similar to ferrous (Fe2+) protoporphyrin-IX. Aspirin-treated PGH2 synthase II was found to produce 15-HETE, and the appearance of the Fe2+ intermediate (within 3 ms) indicated that it was kinetically competent to participate in the 15-dioxygenation event. The detection of this Fe2+ intermediate and the slow formation of compound I and compound II observed with EtOOH in PGH2 synthase II suggest that peroxidative cleavage is not the initiating event in dioxygenation. Instead, it is proposed that the reduction of Fe3+ in heme to Fe2+ oxidizes a peroxide to yield an initiating peroxy radical. Since it is unlikely that 11- and 15-dioxygenation occurs via different mechanisms, our findings question mechanisms of catalysis in both PGH2 synthases.

Recombinant human 92-kDa type IV collagenase/gelatinase from baculovirus-infected insect cells: expression, purification, and characterization

Human 92-kDa type IV collagenase/gelatinase (MMP9) has been expressed in insect cells and secreted into the cell medium via a baculovirus expression system. The expression level of the proenzyme from Trichoplusia ni cells was estimated to be > = 300 mg/L of cell medium. The recombinant protein was purified in a single step using heparin-affinity chromatography with an overall yield of ca. 70%. The purified zymogen could be activated in vitro using 4-aminophenylmercuric acetate to yield an active protease. Kinetic analysis of the activated recombinant enzyme demonstrates that this material is comparable to activated MMP9 from natural human sources. The recombinant enzyme provides a useful source of protein for a variety of biochemical and biophysical studies aimed at elucidating the structure and function of human MMP9.

Ratio of oxygen and nitrogen Raman cross sections in the ultraviolet

We report measurements of the ratio of oxygen and nitrogen Raman cross sections for excitation wavelengths between 220 and 290 nm. These measurements confirm strong enhancement of the oxygen Raman cross section in this region.

Wavelength dependence of the Raman cross section for liquid water

We report measurements of the wavelength dependence of the 3400-cm(-1) Raman scattering cross section of liquid water for excitation wavelengths between 215 and 550 nm. Using previous measurements of the absolute Raman scattering cross section, we have determined an expression for the wavelength dependence of the absolute Raman cross section for water.

Spectrally resolved absolute fluorescence cross sections for bacillus spores

Absolute fluorescence cross sections for Bacillus subtilis and B. cereus bacterial spores as both aqueous suspensions and aerosols were measured at a number of excitation wavelengths between 228 and 303 nm. The fluorescence was spectrally resolved at each excitation wavelength. We found that the optimum excitation wavelength for spore fluorescence is between 270 and 280 nm. The fluorescence cross section for aqueous suspensions is four times larger than for dry aerosols when measured under similar conditions. Measurements on wet aerosols showed an increase in fluorescence cross section over dry aerosols, indicating an enhancement of the fluorescence when the bacterial spores are wet. Mie scattering cross sections at 90 degrees to the direction of the incident radiation and extinction cross sections as a function of wavelength for B. subtilis suspensions and fluorescence cross sections for tryptophan are also reported.

Field evaluation of arthropod repellents, deet and a piperidine compound, AI3-37220, against Anopheles funestus and Anopheles arabiensis in western Kenya

A field evaluation of the repellents N,N-diethyl-3-methylbenzamide (deet) and 1-(3-cyclohexen-1-yl-carbonyl)-2-methylpiperidine (AI3-37220, a piperidine compound) was conducted against Anopheles funestus and An. arabiensis in Kenya. Both repellents provided significantly more protection (P < 0.001) than the ethanol control. AI3-37220 was significantly more effective (P < 0.001) than deet in repelling both species of mosquitoes. After 9 h, 0.1 mg/cm2 of AI3-37220 provided 89.8% and 71.1% protection against An. arabiensis and An. funestus, respectively. Deet provided > 80% protection for only 3 h, and protection rapidly decreased after this time to 60.2% and 35.1% for An. arabiensis and An. funestus, respectively, after 9 h. Anopheles funestus was significantly less sensitive (P < 0.001) to both repellents than An. arabiensis. The results of this study indicate that AI3-37220 is more effective than deet in repelling anopheline mosquitoes in western Kenya.

Expression purification and characterization of recombinant human inducible prostaglandin G/H synthase from baculovirus-infected insect cells

The inducible isoform of human prostaglandin G/H synthase (human cyclooxygenase; hCOX2) has been produced in Sf21 insect cells using the baculovirus expression system. The full-length gene for hCOX2 was placed under the control of the hybrid pCap/PolH promoter and recombinant virus generated by homologous recombination. Insect cells infected with recombinant virus synthesized active hCOX2 at levels exceeding 5% of total cellular protein 72 h postinfection. Optimal production on a preparative scale and high activity yields were attained in 8-liter spinner flasks using a supplemented Grace's medium containing 10% FCS. The apo-enzyme was purified to homogeneity by detergent extraction and ion exchange chromatography and functionally reconstituted with heme to form the holo-enzyme. The purified enzyme from insect cells was identified as hCOX2 by enzymatic activity, Western immunoassay, and N-terminal sequence analysis; the latter also indicated correct processing of the hCOX2 signal sequence. Insect recombinant hCOX2 displays high specific activity for both cyclooxygenase and peroxidase activities at levels indistinguishable from mammalian derived enzyme. Spectroscopic analysis suggests that the recombinant enzyme adopts native-like secondary and tertiary structure. The data presented here demonstrate that this system is capable of providing high yields of active enzyme for biochemical, biophysical, and pharmacological investigations.

The immunosuppressive metabolite of leflunomide is a potent inhibitor of human dihydroorotate dehydrogenase

The active metabolite of leflunomide. A771726, is a novel immunosuppressive compound that has been shown to be a powerful antiproliferative agent for mononuclear and T-cells. The molecular mechanism of action for this compound has not been clearly established. In vitro cellular and enzymatic assays, however, demonstrate that leflunomide is an inhibitor of several protein tyrosine kinases, with IC50 values between 30 and 100 microM. The in vivo properties of A771726 are reminiscent of another immunosuppressive agent, brequinar sodium, which has been shown to be a nonomolar inhibitor (Ki = 10-30 nM) of the enzyme dihydroorotate dehydrogenase (DHODase). On the basis, we have investigated the effects of leflunomide and A771726 on the activity of purified recombinant human DHODase. We find that A771726 is a potent inhibitor of DHODase (Ki = 179 +/- 19 nM), while the parent compound, leflunomide, had no inhibitory effect at concentrations as high as 1 microM. Studies of the dependence of inhibition on the concentrations of the substrates ubiquinone and dihydroorotate demonstrate that A771726 is a competitive inhibitor of the ubiquinone binding site and is noncompetitive with respect to dihydroorotate. The potency of A771726 as a DHODase inhibitor is thus 100-100-fold greater than that reported for its inhibition of protein tyrosine kinases. These data suggest that an alternative explanation for the immunosuppressive efficacy of A771726 may be the potent inhibition of DHODase by this compound.

Comparative effects of selective cyclooxygenase 1 and cyclooxygenase 2 inhibitors on myeloperoxidase and 3 alpha-hydroxysteroid dehydrogenase

The clinical efficacy of non-steroidal anti-inflammatory drugs (NSAIDs) is believed to result from the ability of these compounds to inhibit the inducible isoform of the enzyme cyclooxygenase, COX2. The gastrointestinal and renal side effects of these drugs, in contrast, are thought to relate to their ability to inhibit the constitutive isozyme, COX1. There is structural and pharmacological evidence that suggests that NSAIDs may also inhibit two unrelated enzymes, myeloperoxidase (MP) and 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD), potentially with untoward consequences for the patient. Our laboratories have been investigating a new structural class of potential COX inhibitors, the tri-cyclic aromatics. In this study we have examined the inhibitory potency of selected compounds for the enzymes human COX1, human COX2, human MP, and rat liver 3 alpha-HSD. The compounds selected span a range of COX isoform selectivities, from specific for COX2 to selective for COX1 only, and include three representative tri-cyclic aromatics. We found that compounds within the tri-cyclic aromatic class do not act as potent inhibitors of either myeloperoxidase or 3 alpha-HSD. These results demonstrate the unique inhibitor selectivity that can be achieved with the tri-cyclic aromatics. Examples of COX1 selective, and COX2 selective inhibitors within this structural class are presented.

Recombinant human dihydroorotate dehydrogenase: expression, purification, and characterization of a catalytically functional truncated enzyme

An N-terminally truncated cDNA for human dihydroorotate dehydrogenase (DHODase) was placed under the control of the inducible T7 lac promoter in a pyrimidine auxotrophic strain of Escherichia coli lacking the endogenous enzyme. Induction of gene expression rescued growth in media lacking exogenous pyrimidines. The recombinant enzyme was purified to homogeneity from detergent extracts of bacterial membranes by two chromatographic steps. The purity of the resulting enzyme was judged to be > 95% based on SDS-PAGE with Coomassie staining. The enzyme displays an apparent molecular weight of ca. 40 kDa on SDS-PAGE and ca. 120 kDa on native size-exclusion chromatography, suggesting that the native enzyme is multimeric. Recombinant DHODase displayed a specific activity and Km for dihydroorotate that were similar to those for the enzymes from bovine and human liver tissue. The pH dependence of the activity of the recombinant enzyme was likewise similar to that of the enzyme from human liver and may indicate the involvement of a critical histidine residue in catalytic turnover; only eight histidine residues remain in the truncated version of DHODase used here. The catalytic activity of the recombinant enzyme is inhibited in a dose-dependent fashion by the histidine-selective modifying agent diethylpyrocarbonate. These results further suggest a potential role for histidine in enzyme turnover. Brequinar sodium, an experimental drug which has been shown to be a nanomolar noncompetitive inhibitor of mammalian DHODases, inhibited the activity of the purified recombinant enzyme with a Ki value similar to that for enzyme derived from human liver tissue. The recombinant DHODase thus displays enzymatic behavior similar to the 50-kDa full-length human liver enzyme, illustrating that the catalytically essential structural features of the enzyme, as well as the site of Brequinar binding, are contained within the 40-kDa truncated version of the enzyme that was expressed here.

Antiinflammatory 4,5-diarylpyrroles. 2. Activity as a function of cyclooxygenase-2 inhibition

The antiinflammatory activity of a series of 2-substituted- and 2,3-disubstituted-4-(4-fluorophenyl)-5-[4-(methylsulfonyl)phenyl]-1H- pyrroles was previously shown by quantitative structure-activity relationship (QSAR) studies to be correlated with the molar refractivity and inductive field effect of the 2-substituent and the lipophilicity of the 3-substituent. The present study demonstrates that much of the antiinflammatory activity of these pyrroles could be correlated with the inhibition of the inducible isoform of cyclooxygenase (COX2). Additional QSAR studies have been used to identify the molecular parameters necessary for maximizing COX2 inhibition while simultaneously minimizing the inhibition of constitutively expressed cyclooxygenase-1. Such an effort should facilitate the discovery and development of selective COX inhibitors that should lead to safer nonsteroidal antiinflammatory drugs.

Consumer product-related ocular trauma

Leading causes of consumer product-related ocular trauma have not been well described. To delineate these causes in a nationally representative sample, data collected by the US Consumer Product Safety Commission (USCPSC) were reviewed. Data were obtained from the National Electronic Injury Surveillance System (NEISS), a national probability sample survey conducted by USCPSC that continuously monitors consumer product-related injuries treated in hospital emergency rooms across the United States. These data formulated the product summary report for the calendar year 1991. The sampling frame for the NEISS consisted of hospitals listed on the Center for Health Statistics Master Inventory File stratified geographically by size of hospital and number of emergency-room visits. During the calendar year 1991, there were a nationally estimated 298,852 consumer product-related eye injuries treated in hospital emergency rooms. Appoximately 500 different products were implicated in these injuries, with the leading cause being contact lenses (hard and soft), accounting for an estimated 26,490 emergency-room visits. This is followed by welding equipment (12,771 visits), hair curlers/curling irons (5946 visits), and workshop power grinders (5476 visits). Consumer products account for a sizable number of ocular injuries requiring treatment in hospital emergency rooms. Research on outcomes and prevention strategies of the injuries is warranted.