A new hypothesis based on suicide substrate inhibitor studies for the mechanism of action of aromatase.

Recently, it was discovered that 4-hydroxy-4-androstene-3,17-dione, 4-androstene-3,6,17-trione, and 1,4,6-androstatriene-3,17-dione, compounds previously reported to be competitive inhibitors of aromatase, cause a time-dependent loss of aromatase activity in human placental microsomes. We report here that 1,4-androstadiene 3,17-dione (Ki 0.32 microM; kinact 0.91 X 10(-3)/sec) and testolactone (Ki 35 microM; kinact 0.36 X 10(-3)/sec) also cause a similar loss of aromatase activity. The mechanism which explains the unexpected loss of activity caused by these five inhibitors is neither established nor apparent from current theories of the enzyme mechanism of action of aromatase. We propose an inactivation mechanism based on a new hypothesis for estrogen biosynthesis in which the third enzyme oxidation carried out by aromatase results in the formation of an enzyme-bound intermediate. This intermediate is released as an aromatized product via a facile elimination reaction which simultaneously regenerates the unaltered active enzyme. Various structural modifications made in these five inhibitors are hypothesized to redirect this elimination reaction so that the steroid intermediate remains covalently attached to the enzyme instead of being released as an aromatized product.

Bivalent binding and signaling characteristics of Leridistim, a novel chimeric dual agonist of interleukin-3 and granulocyte colony-stimulating factor receptors.

Leridistim is a member of a novel family of engineered chimeric cytokines, myelopoietins, that contain agonists of both interleukin-3 (IL-3) receptors (IL-3R) and granulocyte colony-stimulating factor (G-CSF) receptors (G-CSFR). To more clearly understand Leridistim's function at the molecular level, binding to both IL-3R and G-CSFR and subsequent signaling characteristics have been delineated. The affinity of Leridistim for the human G-CSFR was found to be comparable to that of native G-CSF (IC(50) = 0.96 nM and 1.0 nM, respectively). Both Leridistim and G-CSF induced receptor tyrosine phosphorylation to a similar maximal level. Compared with native recombinant human IL-3 (rhIL-3), Leridistim was found to possess higher affinity for the IL-3R alpha chain (IL-3Ralpha) (IC(50) = 85 nM and 162 nM, respectively). However, the increase in Leridistim binding affinity to the functional, high-affinity heterodimeric IL-3Ralphabeta(c) receptor is lower than that observed with rhIL-3 (85 nM and 14 nM vs 162 nM and 3.5 nM, respectively). Leridistim induced tyrosine phosphorylation of beta(c) to a level comparable to native IL-3, and the level of JAK2 tyrosine phosphorylation in cells expressing both IL-3R and G-CSFR was comparable to that observed with IL-3 or G-CSF alone. The ability of Leridistim to interact with IL-3R and G-CSFR simultaneously was demonstrated using surface plasmon resonance analysis. These studies were extended to demonstrate that Leridistim exhibited a higher affinity for the IL-3R on cells that express both the IL-3Ralphabeta(c) and the G-CSFR (IC(50) = 2 nM) compared with cells that contain the IL-3Ralphabeta(c) alone (IC(50) = 14 nM). Leridistim binds to both IL-3R and G-CSFR simultaneously and has been shown to activate both receptors. The bivalent avidity may explain the unique biologic effects and unexpected potency of Leridistim in hematopoietic cells compared with rhIL-3 or G-CSF alone or in combination.

Progenipoietins: biological characterization of a family of dual agonists of fetal liver tyrosine kinase-3 and the granulocyte colony-stimulating factor receptor.

The progenipoietins, a class of engineered proteins containing both fetal liver tyrosine kinase-3 and granulocyte colony-stimulating factor receptor agonist activities, were functionally characterized in vitro and in vivo. Four representative progenipoietins were evaluated for receptor binding, receptor-dependent cell proliferation, colony-forming unit activity, and their effects on hematopoiesis in the C57BL/6 mouse.The progenipoietins bound to fetal liver tyrosine kinase-3 and the granulocyte colony-stimulating factor receptor with affinities within twofold to threefold of the native ligands, and each progenipoietin bound simultaneously to both fetal liver tyrosine kinase-3 and the granulocyte colony-stimulating factor receptor. The progenipoietins exhibited different levels of activity in receptor-dependent cell proliferation assays. The fetal liver tyrosine kinase-3-dependent cell proliferation activity of three of four progenipoietins was decreased sixfold to 33-fold relative to native fetal liver tyrosine kinase-3 ligand, while granulocyte colony-stimulating factor receptor-dependent activity of the progenipoietins was within twofold to threefold of native granulocyte colony-stimulating factor. At nonsaturating concentrations, the progenipoietins stimulated colony formation to a greater extent than the equimolar combination of fetal liver tyrosine kinase-3 and granulocyte colony-stimulating factor. Treatment of mice with the progenipoietins yielded dramatic increases in peripheral blood and splenic white blood cells, polymorphonuclear leukocytes, and dendritic cells. These preclinical results demonstrate that the progenipoietins are potent hematopoietic growth factors that stimulate cells in a receptor-dependent manner. When administered in vivo, the progenipoietins effectively promote the generation of multiple cell lineages. Thus, in both in vitro and in vivo settings, the progenipoietins as single molecules exhibit the synergistic activity of the combination of fetal liver tyrosine kinase-3 and granulocyte colony-stimulating factor.

Use of combinatorial mutagenesis to select for multiply substituted human interleukin-3 variants with improved pharmacologic properties.

A combinatorial mutagenesis strategy was used to create a collection of nearly 500 variants of human interleukin 3 (IL-3), each with four to nine amino acid substitutions clustered within four linear, nonoverlapping regions of the polypeptide. The variants were secreted into the periplasm of Escherichia coli and supernatants were assayed for IL-3 receptor-dependent cell proliferation activity. Sixteen percent of the variants, containing "region-restricted" substitutions, retained substantial proliferative activity through two rounds of screening. A subset of these was combined to yield variants with substitutions distributed through approximately half of the polypeptide. With one exception, "half-substituted" variants exhibited proliferative activity within 3.5-fold of native IL-3. A subset of the "half-substituted" variants was combined to yield "fully substituted" IL-3 variants having 27 or more substitutions. The combination of the substitutions resulted in a set of polypeptides, some of which exhibit increased proliferative activity relative to native IL-3. The elevated hematopoietic potency was confirmed in a methylcellulose colony-forming unit assay using freshly isolated human bone marrow cells. A subset of the multiply substituted proteins exhibited only a modest increase in inflammatory mediator (leukotriene C4) release. The molecules also exhibited 40- to 100-fold greater affinity for the alpha subunit of the IL-3 receptor and demonstrated a 10-fold faster association rate with the alpha-receptor subunit. The multiply substituted IL-3 variants described in this study provide a unique collection of molecules from which candidates for clinical evaluation may be defined and selected.

Multiple conformations of a human interleukin-3 variant.

Interleukin-3 (IL-3) is a cytokine that stimulates the proliferation and differentiation of hematopoietic cells. The hyperactive hIL-3 variant SC-55494 was shown to have at least two major conformations by high-resolution NMR spectroscopy. Mutants of SC-55494 were constructed in which alanine was substituted for proline in order to test the hypothesis that proline cis-trans isomerization is the source of the observed conformational heterogeneity, as well as to evaluate the effect of prolyl peptide bond configuration on biological activity. NMR spectra of four single proline-to-alamine mutants (P30A, P31A, P33A, and P37A) retain doubled resonances, while spectra of the double mutant P30A/P31A and the quadruple mutant P30A/P31A/P33A/ P37A are substantially free of heterogeneity. These observations suggest that the two major conformations in SC-55494 correspond to cis and trans isomers of either or both of the R29-P30 and P30-P31 peptide bonds. All six mutants had somewhat lower cell proliferative activity than SC-55494, with relative activities ranging from 40 to 80%. The P37A mutant has a binding affinity to the low-affinity IL-3 receptor alpha-subunit statistically equivalent to SC-55494, while P30A, P31A, and P33A each had about two-fold decreases, and P30A/P31A and P30A/P31A/P33A/P37A had four-fold decreases. These findings suggest an important role for the cis configuration of either or both of the R29-P30 and P30-P31 peptide bonds in IL-3 for optimal interaction with the receptor alpha-subunit.

Enzyme-generated intermediates derived from 4-androstene-3,6,17-trione and 1,4,6-androstatriene-3,17-dione cause a time-dependent decrease in human placental aromatase activity.

Kinetic evidence is presented for a time-dependent decrease in human placental aromatase activity by enzyme-generated intermediates derived from two widely used steroids previously described as competitive inhibitors of estrogen biosynthesis. Thus, 4-androstene-3,6,17-trione binds to the enzyme with an apparent Ki of 0.43 microM and has a pseudo-first order overall rate constant for decrease in activity of 4.03x10(-3)sec-1, while 1,4,6-androstatriene-3,17-dione has an apparent Ki of 0.18 microM and a pseudo-first order overall rate constant for decrease in activity of 1.10x10(-3)sec-1. These findings imply that the potent inhibition of estrogen biosynthesis caused by these steroids results primarily from a decrease in enzyme activity caused by enzyme-generated intermediates from the parent steroids.

Novel indole-2-carboxylates as ligands for the strychnine-insensitive N-methyl-D-aspartate-linked glycine receptor.

A series of indole-2-carboxylates were prepared and evaluated for their ability to inhibit the binding at the strychnine-insensitive glycine receptor that is associated with the NMDA-PCP-glycine receptor complex. All of the compounds were selective for the glycine site relative to other sites on the receptor macrocomplex and several of the compounds in this series were found to have submicromolar affinity for this receptor. The lead compound, 2-carboxy-6-chloro-3-indoleacetic acid (Ki = 1.6 microM vs [3H]glycine), was also found to noncompetitively inhibit the binding of MK-801, a ligand for the phencyclidine site on the receptor macrocomplex. These latter data suggest that the compound functions as an antagonist at the strychnine-insensitive glycine receptor. The structural activity relationships within this series of indole-2-carboxylates is discussed and several key pharmacophores are identified for this series of glycine ligands. In general, the most potent compounds were the C-3 acetamides, with N-propyl-2-carboxy-6-chloro-3-indoleacetamide having the highest receptor affinity.

Studies of the inactivation of human placental aromatase by 17 alpha-ethynyl-substituted 10 beta-hydroperoxy and related 19-nor steroids.

The inactivation of human placental aromatase by 17 alpha-ethynyl-10 beta-hydroperoxy-17 beta-hydroxy-4-estren-3-one (SCH 10015) was investigated. In either the presence or absence of added NADPH, SCH 10015 (Ki = 41 microM) caused a time-dependent loss of aromatase activity (e.g. 50% loss after 20 min with 20 microM SCH 10015). Evidence for the oxidation of an active site sulfhydryl group as the molecular basis for SCH 10015 inactivation is presented. The contraceptive 17 alpha-ethynyl-substituted 19-nor steroids, norethisterone (Ki = 48 microM) and norethynodrel (Ki = 38 microM), were evaluated and found not to inactivate aromatase, suggesting that the potential conversion of either compound to SCH 10015 did not occur to a significant extent in these microsomal incubations. It is speculated that the previously observed potent contraceptive effects of SCH 10015 may have been the result of irreversible inhibition of estrogen biosynthesis.

Glycine modulation of the phencyclidine binding site in mammalian brain.

Neurophysiological studies have shown that glycine potentiates the NMDA response in cultured neurons by a strychnine-insensitive mechanism. Autoradiographic data have demonstrated a correspondence between strychnine-insensitive [3H]glycine binding sites and NMDA-sensitive [3H]glutamate binding sites. Here we report that in synaptic plasma membranes from rat brain, the binding of a PCP analog, [3H]TCP, was enhanced more than 5-fold by 1 microM glycine. This glycine stimulation of binding of [3H]TCP was blocked by the competitive NMDA-receptor antagonist, D-AP7. These data provide support for the hypothesis that a unique amino acid recognition site is associated with the proposed NMDA/PCP receptor complex in brain.

Evidence for a functional coupling of the NMDA and glycine recognition sites in synaptic plasma membranes.

Activation of the N-methyl-D-aspartate (NMDA) receptor complex is subject to modulation via interactions at a coupled [3H]glycine recognition site in rat brain synaptic plasma membranes (SPM). We examined the effect of the potent and specific glycine site antagonists, 1-hydroxy-3-amino-2-pyrrolidone (HA-966) and 1-aminocyclobutane-1-carboxylate (ACBC), on the NMDA recognition site. These glycine analogs were found to significantly stimulate the binding of the competitive NMDA antagonist, [3H]3-(2-carboxypiperazin-4-y1)propyl-1-phosphonate ([3H]CPP) in a dose-dependent fashion, whereas both compounds inhibited NMDA-specific L-[3H]glutamate (agonist) binding. Additionally, both glycine antagonists reduced the binding of [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) to SPM, a functional assessment of activation of the NMDA receptor-channel complex. The glycine site agonists, glycine and serine reversed these effects in a dose-dependent manner, with the serine reversal being stereospecific for D-serine. The relative potencies of these compounds in reversing the glycine antagonist effects on the NMDA recognition site corresponded with their ability to competitively displace strychnine-insensitive [3H]glycine binding. These results provide evidence for a functional coupling between the glycine and NMDA recognition sites and further, may provide a mechanism by which compounds interacting at the glycine recognition site may modulate NMDA receptor activity.

Guanine nucleotide modulation of [3H]TCP binding to the NMDA receptor complex.

Guanine nucleotides have been examined for their effect on [3H]1-[1-(2-thienyl)-cyclohexyl]-piperidine ([3H]TCP) binding to rat forebrain synaptic plasma membranes (SPM). We report that of the series of guanine nucleotides tested, GTP, GDP, 5'-guanylylimidodiphosphate (Gpp(NH)p) and 5'-guanylylmethylenediphosphate (Gpp(CH2)p) are significantly more potent at decreasing [3H]TCP binding than GMP, cyclic GMP, and guanosine. GTP, the most potent compound tested, inhibited basal [3H]TCP binding with an IC50 of 38.7 microM. Stimulation of [3H]TCP binding with either the N-methyl-D-aspartate (NMDA) agonist, L-glutamate, or Mg2+ was also inhibited by GTP. Addition of GTP resulted in a rightward shift in the glutamate dose-response curve and a decrease in the maximum level of stimulation. The Mg2+ stimulation of [3H]TCP binding was completely blocked by the addition of GTP. These results, coupled with the previous findings that guanine nucleotides inhibit the binding of L-[3H]glutamate to the NMDA recognition site (Monahan et al., 1988), indicate that guanine nucleotides antagonize NMDA receptor-mediated neurotransmission, at least in part, through their action (direct or indirect) on the NMDA recognition site and thus may be endogenous negative modulators of the NMDA receptor.

Identification of a novel structural class of positive modulators of the N-methyl-D-aspartate receptor, with actions mediated through the glycine recognition site.

This study describes a new structural class of compounds which interact at the N-methyl-D-aspartate (NMDA) receptor-associated glycine recognition site. These E-gamma-substituted vinylglycine derivatives were active in displacing [3H]glycine binding from the NMDA receptor-associated recognition site in rat forebrain synaptic plasma membranes, with Ki values in the range of 0.24-8.7 microM. Functional analyses of these compounds indicate that they positively modulate basal [3H](+)-5-methyl-10,11-dihydro-5H- [a,d]cyclohepaten-5,10-imine ([3H]MK-801) binding, consistent with their having agonist characteristics. Little stereospecificity is observed with the gamma-substituted methyl and propyl derivatives while the L-isomer of the hexyl analog is significantly more potent than the D-isomer. The D- and L-hydroxyethyl gamma-substituted vinylglycines were the most potent inhibitors of [3H]glycine binding with Ki values of 0.75 +/- 0.06 microM and 0.24 +/- 0.02 microM, respectively. The 3,4-double bond was necessary for activity in that the saturated hexyl derivative (2-aminodecanoate) was inactive. Based on the results reported herein, the hypothesis that there is a distinct size restriction for functional agonists which interact with the glycine recognition site, should be altered to include these larger extensions of vinylglycine.

Cis-2,4-methanoglutamate is a potent and selective N-methyl-D-aspartate receptor agonist.

Cis- and trans-2,4-methanoglutamate were compared with L-glutamate as acidic amino acid ligands. Cis-2,4-methanoglutamate had a Ki of 0.052 microM against N-methyl-D-aspartate (NMDA)-specific L-[3H]glutamate binding compared with 0.050 microM for L-glutamate. Cis-2,4-methanoglutamate exhibited no significant affinity against [3H]kainate or [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate ([3H]AMPA) binding. Trans-2,4-methanoglutamate had no significant affinity for any of these sites. Cis-2,4-methanoglutamate increased [3H]N-1[2-thienyl]cyclohexyl-3,4-piperidine [( 3H]TCP) binding with EC50 of 0.35 +/- 0.14 microM. It produced an inward current in rat brain mRNA-injected Xenopus oocytes which was blocked by the NMDA antagonist, D-2-amino-7-phosphonoheptanoate (D-AP7). Cis-2,4-methanoglutamate (EC50 = 15.9 microM) was 100-fold more potent than L-glutamate (EC50 = 1,584 microM) in reducing the excitatory postsynaptic potential in CA1 of hippocampal slices. Cis-2,4-methanoglutamate is the most potent, selective NMDA agonist known.

Determination of the pharmacokinetics of 2-amino-7-phosphonoheptanoate in rat plasma and cerebrospinal fluid.

The pharmacokinetics of 2-amino-7-phosphonoheptanoate (AP7) in rat plasma and appearance in cerebrospinal fluid (CSF) are reported. Using high-performance liquid chromatography (HPLC) with fluorescence detection, concentrations of AP7 can be detected as low as 1.0 microM. Peak CSF concentrations (12-15 microM) for both the D-AP7 and D,L-AP7 are observed 10-15 min after i.v. administration and amount to approximately 0.1% of a 1 mmol/kg dose. Significant quantities (3 microM) are present in CSF at 2 h and no AP7 is detectable at 4 h. Following i.v. administration, a monoexponential clearance was observed for D-AP7 clearance from plasma, 15.4 +/- 0.8 S.E.M. ml/min/kg with a t1/2 of 38.9 +/- 0.8 S.E.M. min. However, a biexponential clearance from plasma was observed for D,L-AP7.

D-cycloserine: a ligand for the N-methyl-D-aspartate coupled glycine receptor has partial agonist characteristics.

We have previously shown that D-cycloserine displaces [3H]glycine binding to a recognition site with properties consistent with an N-methyl-D-aspartate (NMDA) receptor modulatory site. Additionally, D-cycloserine positively modulates the NMDA receptor as evidenced by its dose-dependent enhancement of [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to the NMDA receptor-coupled ionophore. Further evaluation of this compound indicates that the maximal stimulation of [3H]TCP binding induced by D-cycloserine is lower than that produced by other compounds acting at the NMDA receptor associated glycine modulatory site (glycine and D-serine). Moreover, the stimulation of [3H]TCP binding induced by D-cycloserine in the presence of various fixed concentrations of glycine results in a family of dose-response curves which asymptotically converge to 40-50% of the maximal stimulation induced by glycine alone. These results are consistent with D-cycloserine acting as a partial agonist of the NMDA receptor via its interaction with the coupled glycine modulatory site.

Characterization of D-3,4-cyclopropylglutamates as N-methyl-D-aspartate receptor agonists.

The 4 configurational isomers of D-3,4-cyclopropylglutamate (D-CGA) have been synthesized and analyzed for their interactions as excitatory amino acid recognition sites. Additionally, functional assessment of the action of these compounds at the N-methyl-D-aspartate (NMDA) receptor was performed. All 4 analogs function as agonists at the NMDA receptor as evidenced by their ability to stimulate [3H]MK-801 binding to the coupled PCP recognition site. Furthermore, the rank order of potency of these compounds in stimulating [3H]MK-801 binding corresponds with their Ki values for the displacement of NMDA-selective L-[3H]glutamate and [3H]CGS-19755 binding (D-CGA-C greater than D-CGA-B greater than D-CGA-D greater than D-CGA-A). The D-CGA-C isomer has affinity and potency at the NMDA receptor similar to the endogenous agonist, L-glutamate. This high potency coupled with greater specificity than L-glutamate, makes D-CGA-C a potentially useful pharmacological tool for the study of this receptor.

Characterization of 3-carboxy-5-phosphono-1,2,3,4-tetrahydroisoquinoline (SC-48981), a potent competitive N-methy-D-aspartate (NMDA) receptor antagonist, in vitro and in vivo.

(+/-)-3-Carboxy-5-phosphono-1,2,3,4-tetrahydroisoquinoline (SC-48981), a conformationally restricted analog of the potent competitive N-methyl-D-aspartate (NMDA) antagonist, 2-amino-5-phosphonopentanoate (AP-5), potently inhibited the binding of [3H]glutamate to the N-methyl-D-aspartate (NMDA) receptors with a Ki of 1.6 mcM, but with minimal affinity for kaininate and quisqualate receptors (Ki greater than 50 mcM), in vitro. Consistent with its ability to antagonize the NMDA receptor, SC-48981 decreased the binding of [3H]glycine and [3H]TCP [1-(2-thienyl)cyclohexylpiperidine] to the NMDA-associated glycine and phencyclidine (PCP) recognition sites, in vitro. SC-48981 attenuated levels of basal cGMP and harmaline-induced increases in levels of cGMP in the mouse cerebellum, in vivo, in a competitive manner, with ED50 values of 5.5 and 8.7 mg/kg, i.p. Direct intracerebellar injection of SC-48981 (0.5 microgram) attenuated increases in levels of cGMP induced by central injection of the NMDA-associated glycine receptor agonist, D-serine and by NMDA itself. Parenteral administration of SC-48981 (25 mg/kg, s.c.) decreased basal levels of cGMP for up to 3 h. These results indicate that SC-48981 represents a novel bioavailable competitive NMDA antagonist with a long duration of action.

D-cycloserine, a positive modulator of the N-methyl-D-aspartate receptor, enhances performance of learning tasks in rats.

Glycine has recently been shown to positively modulate the N-methyl-D-aspartate (NMDA) subclass of acidic amino acid receptors which are important in neural pathways involved in learning and memory. We report that d-cycloserine (DCS), an antimycobacterial agent known to cross the blood-brain barrier, binds with high affinity to this glycine modulatory site, functions as a positive modulator, and facilitates performance of learning tasks in rats. In addition, DCS appears to be a potent cognitive enhancer at doses lower than those required for antibacterial activity. Based on these data, we propose that modulation of NMDA receptors via glycinergic mechanisms may be a means of influencing cognitive processes.

Synthesis, absolute configuration and activity at N-methyl-D-aspartic acid (NMDA) receptor of the four D-2-amino-4,5-methano-adipate diastereoisomers.

The four D-2-amino-4,5-methano-adipates 26, 27, 32, 33 were synthesized and their biological activity at the N-methyl-D-aspartate (NMDA) receptor was assessed. The synthesis involved as a key step a rhodium acetate dimer catalyzed addition of ethyl diazoacetate to the protected D-allylglycine (17). In vitro receptor binding using L-[3H]glutamate as the radioligand provided affinity data, while modulation of [3H]TCP binding was used as a functional assay. The analogues were also evaluated in [3H]kainate and [3H]AMPA binding to assess selectivity over non-NMDA glutamate receptors. Three of the four diastereoisomer, D-CAA B (27), C (32) and D (33) were shown to have agonist properties at the NMDA-site, while the fourth, (2R,4R,5R) D-CAA A (26) was characterized as an NMDA-site atypic antagonist.