Prostaglandin prodrugs. 5.1 Prostaglandin E2 ethylene ketal.

In order to improve the chemical stability of prostaglanding E2 (2), prostaglanding E2 ethylene ketal (1) was prepred by direct ketalization of 2 with ethylene glycol in benzene. To establish a quantitative assessment of 1 as a chemically stable and orally active prodrug of 2, the hydrolysis of 1 to 2 and the subsequent dehydration of 2 to prostaglandin A2 (3) were followed at 25 degrees C and six pH's ranging from 2.0 to 6.5 by means of a high-pressure liquid chromatographic procedure. Kinetic results clearly indicate that 1 should be quantitively hydrolyzed back to the parent drug 2 dehydration of 2 to 3 are in the order of 1 h and 14 days, respectively. The preliminary data on the biological response after oral administration of 1 appeared to indicate the 1 is bioequivalent to 2.

Synthesis and platelet aggregation inhibiting activity of prostaglandin D analogues.

Several prostaglandin D (PGD) analogues have been synthesized, incorporating the following variations: (a) varying degrees of side-chain unsaturation, (b) C-9 hydroxy removed or in the unnatural 9 beta configuration, (c) metabolically stabilized analogues (e.g., 15-methyl, 16,16-dimethyl, 17-phenyl, etc.), and (d) delta 12 isomers resulting from decomposition of PGD2. With regard to their ability to inhibit adenosine diphosphate (ADP) induced human platelet aggregation: (a) PGD3 greater than or equal to PGD2 greater than PGD1 greater than 13,14-dihydro-PGD1, (b) the 9 beta- and 9-deoxy-PGD2 analogues are more potent than PGD2, (c) metabolically stabilized analogues with bulky substituents at or near C-15 have substantially reduced antiaggregatory activity relative to PGD2 and (d) the delta 12 isomers of PGD2 are much less active than PGD2.

Potent renin inhibitory peptides containing hydrophilic end groups.

A previously reported renin inhibitor, Boc-Pro-Phe-N(Me)His-Leu psi [CHOHCH2]Val-Ile-Amp (U-71038), was altered by the incorporation of polar, hydrophilic moieties at either end, e.g., tris(hydroxymethyl)aminomethane (THAM) or glucosamine urea groups at the N-terminus, and the THAM amide or aminomethylpyridine N-oxide at the C-terminus. These modified analogues, with dramatically improved water solubility, all retained the potent renin inhibitory activity of U-71038 in vitro. The fact that good activity was maintained in these new analogues, which possess hydrophilicity and steric bulk considerably different from the parent compound, suggests that neither end of these molecules is critical for recognition and binding of the inhibitors by renin. These modified analogues were evaluated in a rat model, and several exhibited hypotensive activity after both oral and iv administration which was greater in magnitude and longer in duration than that caused by equimolar doses of U-71038. Furthermore, unlike U-71038, the oral activity of these analogues was not dependent upon administration in a citric acid vehicle.

Synthesis and biological activity of prostaglandin lactones.

Most of the primary prostaglandins and several biologically important prostaglandin analogues were converted to 1,9-, 1,11- or 1,15-lactones, in order to investigate the biological profiles of these internal esters and to assess their potential as prodrugs for the corresponding open-chain hydroxy acids. In each case, the key lactonization step was done using Corey's "double activation" procedure (cyclization of omega-hydroxy-2-pyridinethiol esters). In general, the 1,9-lactones exhibited less than 1% of the biological activity of the parent hydroxy acids in the standard prostaglandin test systems. The 1,11- and 1,15-lactones, on the other hand, were essentially equal to the parent hydroxy acids as antifertility agents (a 4-day assay which would allow time for in vivo enzymatic lactone hydrolysis). The 1,11- and 1,15-lactones exhibited very low activity in acute or in vitro screens (e.g., rat blood pressure and gerbil colon stimulation), assays which more closely reflect the intrinsic activity of the lactones themselves. These results are consistent with the observed relative ease of enzymatic hydrolysis of the prostaglandin lactones (1,15 greater than or equal to 1,11 much greater than 1,9). Several of the lactones whose parent hydroxy acids are resistant to metabolic inactivation (e.g., 15-methyl, 16-phenoxy, and 17-phenyl) exhibited potent abortifacient activity in the hamster. These lactones, with greatly diminished activity in the blood pressure and smooth muscle assays (indicators of potential side effects), represent a therapeutically useful class of antifertility agents.

PNU-107484A with alpha isoform-dependent functional changes in alpha(x)beta2gamma2 subtypes of rat recombinant GABA(A) receptors.

1. We discovered a novel gamma-aminobutyric acidA (GABA(A)) receptor ligand displaying seemingly opposite functionalities, depending on the alpha isoform of the alpha(x)beta2gamma2 subtypes. PNU-107484A enhanced GABA-induced Cl- currents in the alpha1beta2gamma2 subtype, but inhibited the currents in the alpha3beta2gamma2 and alpha6beta2gamma2 subtypes, and its half-maximal concentrations in the subtypes were 3.1 +/- 0.5, 4.2 +/- 1, and 3.5 +/- 0.2 microM, respectively, without showing much dependency on alpha isoforms. 2. In the alpha1beta2 subtype, the drug at concentrations up to 40 microM showed no effect on GABA-induced Cl- currents, suggesting the requirement of the gamma subunit for its action. 3. PNU-107484A behaved like a positive allosteric modulator of the alpha1beta2gamma2 subtype with its binding site distinct from those for benzodiazepines, barbiturates and neurosteroids. With the alpha3beta2gamma2 subtype, the drug behaved like a non-competitive inhibitor of GABA, thus blocking Cl- currents by GABA alone or in the presence of pentobarbitone and neurosteroids. 4. It appears that PNU-107484A is a unique GABA(A) receptor ligand with alpha isoform-dependent functionalities, which may provide a basis for development of alpha isoform-selective ligands, and it could be useful as a probe to investigate the physiological roles of the various alpha isoform subtypes.

Novel membrane localized iron chelators as inhibitors of iron-dependent lipid peroxidation.

Attachment of various iron chelating moieties to hydrophobic steroids greatly enhanced their abilities to inhibit iron-dependent lipid peroxidation. Using whole rat brain homogenates, lipid peroxidation initiated by the addition of 200 microM Fe2+ was assessed by the formation of thiobarbituric acid reactive products (TBAR). Under these conditions, 50% inhibitory concentrations of Fe3+ chelators such as desferrioxamine or N1,N8-bis(2,3-dihydroxybenzoyl) spermidine hydrobromide (compound II) were around 170 and 50 microM respectively. Coupling desferrioxamine or compound II to a steroid at the D ring increased their potency in lipid peroxidation assays by 5- to 10-fold. Evidence that inhibition of lipid peroxidation by the steroid-chelator adducts was due to iron chelation was suggested by the fact that methylation of the catechol oxygens of compound II, which are essential for chelation, completely eliminated activity of the steroid adduct. A series of 21-aminosteroids which complex Fe2+ iron and potently inhibit iron-dependent lipid peroxidation has also been synthesized. Coupling Fe2+ chelators to hydrophobic steroids increased their inhibitory potencies by as much as 10- to 100-fold. Some steroid-based Fe2+ chelators stimulated lipid peroxidation at low concentrations in the presence of Fe3+. The degree of stimulation was related to the affinity of a compound for Fe2+ with the stronger chelators causing greater stimulation. The most potent inhibitors of lipid peroxidation in the 21-aminosteroid series were found to be those compounds forming the weakest Fe2+ complexes. The findings suggest that it is iron at or near the membrane that is responsible for the catalysis of lipid peroxidation. The compounds described should provide useful tools for studies of the involvement of iron in the lipid peroxidation process.

Contribution of serum protein association to discrepancy between the in vivo and in vitro UDS results for 6,7-dimethyl-2,4-di-1-pyrrolidinyl-7H-pyrrolo[2,3-d]pyrimidine (U-89843).

U-89843 has been shown to undergo biotransformation, both in vitro and in vivo, to form U-97924 as a major primary metabolite. U-89843 was found to be positive in an in vitro UDS mutagenesis screen conducted with primary rat hepatocytes in serum-free media. In contrast to in vitro results, no evidence of genetic toxicity of U-89843 was observed in rats in the in vivo/in vitro version of the UDS test with single oral doses up to 1400 mg/kg. The negative results may be related to more robust in vivo detoxification mechanisms or relatively lower exposure to reactive metabolites formed by bioactivation of U-89843 as compared to that observed in the serum-free in vitro hepatocyte test system. Further studies showed rat serum suppressed the in vitro metabolism of U-89843 as well as the formation of the corresponding hydroxylated metabolite, U-97924, the putative precursor of proposed reactive electrophilic metabolite. The measured in vivo systemic clearance of U-89843 (0.53 l/h/kg) in rats was about 1000-fold slower than the in vitro intrinsic clearance (606 l/h/kg) estimated by measuring the formation of U-97924 in rat liver microsomal incubations. Since U-89843 is extensively associated with serum proteins a poor extraction ratio into the liver may account for the slower biotransformation of U-89843 in vivo as compared to that exhibited in in vitro serum-free hepatocyte incubations. Addition of bovine serum albumin (1-40 mg/ml) to the in vitro UDS assay medium decreased the UDS mean net grains per nucleus response of U-89843. These results suggest that the effect of serum protein should be considered when comparing serum-free in vitro UDS and in vivo UDS results for highly serum protein bound compounds.

Neuroprotective efficacy of microvascularly-localized versus brain-penetrating antioxidants.

The 21-aminosteroid (lazaroid) tirilazad mesylate has been demonstrated to be a potent inhibitor of lipid peroxidation and to reduce traumatic and ischemic damage in a number of experimental models. Currently, tirilazad is being actively investigated in phase III clinical trials in head and spinal cord injury, ischemic stroke and subarachnoid hemorrhage. This compound acts in large part to protect the microvascular endothelium and consequently to maintain normal blood-brain barrier (BBB) permeability and cerebral blood flow autoregulatory mechanisms. However, due to its limited penetration into brain parenchyma, tirilazad has generally failed to affect delayed neuronal damage to the selectively vulnerable hippocampal CA1 and striatal regions. Recently, we have discovered a new group of antioxidant compounds, the pyrrolopyrimidines, which possess significantly improved ability to penetrate the BBB and gain direct access to neural tissue. Several compounds in the series, such as U-101033E, have demonstrated greater ability to protect the CA1 region in the gerbil transient forebrain ischemia model with a post-ischemic therapeutic window of at least four hours. In addition, U-101033E has been found to reduce infarct size in the mouse permanent middle cerebral artery occlusion model in contrast to tirilazad which is minimally effective. These results suggest that antioxidant compounds with improved brain parenchymal penetration are better able to limit certain types of ischemic brain damage compared to those which are localized in the cerebral microvasculature. On the other hand, microvascularly-localized agents like tirilazad appear to have better ability to limit BBB damage.

Nonmammalian sources of eicosanoids.

Despite the considerable effort expended in the exploration of alternative, large-scale sources of eicosanoids, the first-discovered nonmammalian source--the gorgonian Plexaura homomalla--remains by far the most impressive. The evolutionary reasons for the huge concentrations of prostaglandins in this coral and the origin of the dependence of C-15 configuration on geographical location still constitute fascinating areas for speculation and research. Although the coral provided an extremely valuable interim source of prostaglandins between the eras of biosynthesis and total chemical synthesis, the best of the many synthetic routes now available offer the advantages of versatility and unlimited scale-up potential, and are economically competitive as well. Partial synthesis from various readily available iridoid glycoside intermediates still appears to be an attractive alternative for the preparation of selected eicosanoids. The discovery of unusual eicosanoids such as hybridalactone and the clavulones demonstrates that the oxidative metabolism of C20 unsaturated fatty acids may be considerably more complex than yet realized. The discovery of additional structurally novel, biologically significant eicosanoids from both animal and plant sources can be predicted with virtual certainty. Given the low levels of C20 unsaturated fatty acids in most plants, it is not surprising that the levels of oxidized eicosanoids (e.g., prostaglandins) observed to date in plants have also been very low. The levels were sufficient, however, to dispel the long-popular notion that prostaglandins existed only in the animal kingdom. Plants such as the mosses and ferns mentioned earlier (3,55), which contain unusually large amounts of arachidonic acid, seem to be particularly promising species in which to search for additional novel eicosanoids. In addition to new structures, further information regarding the physiological role of eicosanoids in plants will undoubtedly be forthcoming.

Hapten mimic elicits antibodies recognizing prostaglandin E2.

Antibodies that recognized the prostaglandin (PG) E structure were elicited from rabbits. 9-Deoxy-9-methylene-PGF2alpha, a stable isosteric mimic of PGE2, was conjugated to two different protein immunogens and the immune response system was duped into producing antibodies with poor recognition for prostaglandins other than the hapten mimic (9-deoxy-9-methylene-PGF2alpha) and its isosteric counterparts (PGE1 and PGE2). With this procedure, crossreaction that would ordinarily arise from the chemical or metabolic instability of an authentic PGE2 immunogen was avoided. Antibodies raised against a keyhole limpet hemocyanin conjugate of 9-deoxy-9-methylene-PGF2alpha had an average intrinsic association constant, Ko = 2.6 X 10(9) liters.mole-1, for PGE2. Crossreaction was low for a number of related prostaglandins, and a sensitive radioimmunoassay procedure with a detection limit of 6 pg was developed.

Synthesis and biological properties of 9-deoxo-16,16-dimethyl-9-methylene-PGE2.

The in vivo monkey uterine stimulating potency of 9-deoxy-16,16-dimethyl-9-methylene-PGE2 is similar to that of 16,16-dimethyl-PGE2 and approximately 15 times that of PGE2. Low doses of this compound stimulated uterine contractions when administered vaginally. Pregnancy was terminated prematurely following subcutaneous, intramuscular or vaginal suppository treatment. Estimates of potential for gastrointestinal side effects using the rat enteropooling assay and in vivo monkey effects indicate that diarrhea will be substantially reduced with retention of uterine stimulating potency.

Analysis of the biological activity of azoprostanoids in human platelets.

A structure-activity analysis of seven different azoprostanoids in human platelets and rat aortas has shown that discrete changes in the prostanoid skeleton induce marked changes in biological activity. The 15S-hydroxyl group is important for agonist activity in the platelet (pro-aggregatory) and in the aorta (vasoconstriction). Removal of the 15S-hydroxyl results in a compound that inhibits the thromboxane synthetase and platelet aggregation, and is a much weaker constrictor of the aorta. The 13,14 double bond is more important for agonist activity on the aorta than the 5,6 double bond of the prostanoid skeleton. Completely saturated molecules that are 15-deoxy still retain considerable activity on the aorta, but are two orders of magnitude less active as inhibitors of the thromboxane synthetase and aggregation than the azoprostanoid with both 5,6 and 13,14 double bonds. Compounds that exhibit no agonist activity in platelets still retain considerable agonist activity on the aorta, suggesting that the thromboxane A2 receptor in platelets may be different from the thromboxane A2 receptor in blood vessels.

Biosynthesis and metabolism of 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid by human umbilical vein endothelial cells.

Incubation of cultured human umbilical vein endothelial cells with [1-14C]arachidonic acid, followed by reverse-phase high-pressure liquid chromatography analysis, results in the appearance of two principal radioactive products besides 6-keto-prostaglandin F1 alpha. The first peak is 12-L-hydroxy-5,8,10-heptadecatrienoic acid, a hydrolysis product of the prostaglandin endoperoxide. The second peak was esterified, converted to the trimethylsilyl ether derivative, and analyzed by gas chromatography-mass spectrometry and shown to be the lipoxygenase product 15(S)-hydroxy-5,8,11,13-eicosatetraenoic acid (15-HETE). Incubation of the 15-HETE precursor 15(S)-hydroperoxy-5,8,11,13-eicosatetraenoic acid (15-HPETE) with endothelial cells results in the formation of four distinct UV absorbing peaks. UV and gas chromatography-mass spectrometry analysis showed these peaks to be 8,15(S)-dihydroxy-5,8,11,13-eicosatetraenoic acids (8,15-diHETE) differing only in their hydroxyl configuration and cis trans double-bond geometry. Formation of 8,15-diHETE molecules suggests the prior formation of the unstable epoxide molecule 14(S),15(S)-trans-oxido-5,8-Z-14,15-leukotriene A4 or an attack at C-10 of 15-HPETE by an enzyme with mechanistic features in common with a 12-lipoxygenase. The observation that endothelial cells can synthesize both 15-HETE and 8,15-diHETE molecules suggests that this cell type contains both a 15-lipoxygenase and a system that can synthesize 14,15-leukotriene A4.

Evidence for 15-HETE synthesis by human umbilical vein endothelial cells.

Incubation of cultured human umbilical vein endothelial cells with [1-14C]-arachidonic acid, followed by RP-HPLC analysis, resulted in the appearance of two principal radioactive products besides 6-keto-PGF1 alpha. The first peak was HHT, a hydrolysis product of the prostaglandin endoperoxide. The second peak was esterified, converted to the trimethylsilyl ether derivative, and analyzed by GC/MS and was shown to be the lipoxygenase product 15-HETE. Stimulation of endothelial cells with thrombin enhanced 15-HETE synthesis from arachidonate. Subsequent experiments showed that 5-HETE and 12-HETE were also synthesized by endothelial cells, but no evidence of leukotriene synthesis was found. Incubation of the 15-HETE precursor 15-HPETE with endothelial cells resulted in the formation of four distinct ultraviolet light-absorbing peaks. Ultraviolet and GC/MS analysis showed these peaks to be 8,15-diHETEs that differed only in their hydroxyl configuration and cis-trans double-bond geometry. Formation of 8,15-diHETE molecules suggests the prior formation of the unstable epoxide molecule 14,15-LTA4 or an attack at C-10 of 15-HPETE by an enzyme with mechanistic features in common with a 12-lipoxygenase. The observation that endothelial cells can synthesize both 15-HETE and 8,15-diHETE molecules suggest that this cell type contains both a 15-lipoxygenase and a system that can synthesize 14,15-LTA4.

Plasma concentrations of 9-deoxo-16-dimethyl-9-methylene-PGE2 in rhesus monkeys after administration by various routes.

A method is described for the estimation of 9-deoxo-16, 16-dimethyl-9-methylene-PGE2 by double antibody radioimmunoassay. Plasma samples obtained from animals treated with 9-methylene-16, 16-dimethyl-PGE2, 1-adamantanamic salt were extracted with diethyl ether to recover the prostaglandin. The validation of sample preparation and assay procedure are presented. Rhesus females were treated by several routes of administration and the samples assayed for drug content. Maximum blood levels were probably reached 30 minutes following subcutaneous injection and within 30 seconds of an intravenous injection. Results of the acute intravenous injection indicate an initial half-life of approximately one minute in peripheral circulation. Continuous intravenous infusion at 3 increasing doses of this compound resulted in a stepwise increase in plasma drug concentrations. Vaginal administration of 9-methylene-16, 16-dimethyl-PGE2, 1-adamantanamine salt in suppositories produced a dose dependent increase in plasma drug concentration. Higher plasma drug concentrations were produced when the prostaglandin was delivered in H-15 base suppositories than in E-76 base suppositories.

Receptor-coupled signal transduction in human polymorphonuclear neutrophils: effects of a novel inhibitor of phospholipase C-dependent processes on cell responsiveness.

1-[6-[[17 beta-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]- 1H-pyrrole-2,5-dione (U-73122), an inhibitor of phospholipase C (PLC)-dependent processes in human platelets, was found to be a potent inhibitor of human polymorphonuclear neutrophil (PMN) activation by structurally unrelated receptor-specific agonists. U-73122 caused a time- and concentration-dependent (0.1-1 microM) inhibition of myeloperoxidase and vitamin B12-binding protein release from PMNs exposed to N-formyl-methionyl-leucyl-phenylalanine, recombinant human C5a, leukotriene B4 and platelet-activating factor. Activation of the respiratory burst, as measured by superoxide anion production, in PMNs stimulated with these agonists was also suppressed by U-73122. These data suggested that U-73122 inhibited a component of signal transduction that was common to the mechanisms of action of these stimuli. Production of inositol 1,4,5-trisphosphate and 1,2-diacylglycerol and the rise in the cytosolic free calcium concentration, which are early postreceptor events in PMN activation, were all suppressed in U-73122-treated PMNs stimulated with the agonists. These signal transduction events require activation of PLC. Receptor-coupled activation of PLC in membranes isolated from PMNs was potently inhibited by U-73122. U-73122, however, had no direct effect on PMN protein kinase C activity. 1-[6-[[17 beta-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl] -2,5- pyrrolidine-dione (U-73343), a close analog of U-73122 that does not suppress PLC activity, did not inhibit receptor-specific agonist-induced PMN responsiveness. U-73122, therefore, is a novel reagent that is useful in investigating PLC function in receptor-mediated PMN activation.

Prostaglandin d2 as a potential antithrombotic agent.

Prostaglandin D2 was found to be a potent inhibitor of platelet aggregation. Aggregation of human platelets by ADP, collagen and prostaglandin G2 was inhibited more strongly by PGD2 than by PGE1. Although ADP-induced aggregation of rabbit platelets was inhibited more strongly by PGE1 than by PGD2 the latter prostaglandin gave a more long-lasting inhibitory effect on platelet aggregation following intravenous or oral administration. These results coupled with the finding that PGD2 has less hypotensive effects on the cardiovascular system than PGE1 suggest the possible use of PGD2 as an antithrombotic agent.