Studies on the luteolytic activity of MDL-646, a new gastroprotective PGE1 analogue, in the hamster

MDL-646, 11,15-dihydroxy-16-methyl-16-methoxy-9-oxo- prost -13-en-1-oic acid methyl ester, is one of the most active members of a new class of PGE1 analogues with potent gastric cytoprotective and antisecretory activity. The potential luteolytic activities of MDL-646 and its corresponding PGE2 derivative, L 14224 were assessed from their ability to terminate pregnancy and to reduce plasma progesterone levels in the hamster. PGE1 and PGE2 were used as reference compounds. The biological and biochemical data clearly demonstrate that these 16-methyl-16-methoxy PGE derivatives, given s.c. or p.o. either once or for 3 days, have no luteolytic effects up to a daily dose of 2-2.5 mg/kg, and are therefore at most 1/2 to 1/4 as luteolytic as the parent natural PGEs. The dissociation between gastroprotective and luteolytic activity was interpreted to indicate that these new PGE derivatives have a specific action.     

MDL-646, a new synthetic E1-prostaglandin with local protective effects on the gastric mucosa

The gastric protection, diarrheogenic and arterial hypotensive effects of MDL-646, a PGE₁ derivative, have been studied in rats. The compound administered p.o. or i.v. was able to inhibit the maroscopic damage to gastric mucosa produced by noxious stimuli (ethanol and indomethacin). In the stomach perfusion test with the anesthetized rat, intravenously administered MDL-646 reduced histamine- or pentagastrin-stimulated gastric secretion. After intraduodenal administration (i.d.) doses at least 40–50 times greater were necessary for an antisecretory effect. In conscious rats with chronic gastric fistulas, intragastrically administered (i.g.) MDL-646 affected both acid concentration and volume of unstimulated gastric secretion. In experimental models for gastric lesions, DML-646 was much more potent after oral (p.o.) (15–30 times) than after i.v. administration. (ED₅₀ μg/kg: vs. alcohol lesions, 0.05 p.o. and 0.7 i.v.; vs. indomethacin ulcers, 7.0 p.o. and 195 i.v.). Our data would fit the hypothesis that it was a local effect on the gastric mucosa. The mechanism of this effect is not known. The supposed local activity coupled with the antisecretory effects and the good tolerability make it interesting to test MDL-646 as an anti-ulcer agent in man.     

Effects of PGE1 or PGE2 on luteal function in pseudopregnant rats

Effects of PGE₁ or PGE₂ on luteal function were studied in 163 pseudopregnant rats. PGE₁ (10, 100, or 300μg) given intrauterine every 6 hr did not shorten pseudopregnancy (P < 0.05), however, the same doses of PGE₂ given intrauterine every 6 hr advanced luteolysis (P < 0.05). PGE₁ (100 or 300μg) given every 4 hr intramuscular maintained levels of progesterone in peripheral blood above controls (P < 0.05) while 100 or 300μg of PGE₂ hastened the decline in progesterone (P < 0.05). The antiluteolytic effect of PGE₁ was not via an inhibition of PGF secretion (P < 0.05) by the uterus or by induction of ovulation in treated animals. Moreover, PGE₁ (100, 200, or 500μg) given intramuscular every 4 hr from day 4 of pseudopregnancy until the next proestrus delayed luteal regression around 3 days (P < 0.05). PGE₂ at doses of 100, 200, or 500μg every 4 hr given intramuscular consistently shortened pseudopregnancy (P < 0.05). Lower doses were without effect (P < 0.05). Based on the above data it is concluded that PGE₂ is consistently luteolytic whereas PGE₁ is not luteolytic in pseudopregnant rats and that PGE₁ may be an antiluteolysin.     

Utility of d4-PGE2 as an internal standard to quantify endogenous levels of PGE1, PGE2, 190H PGE1 and 190H PGE2 in human seminal fluid by GC-MS-SIM

Four prostaglandins-PGE₁, PGE₂, 190H PGE₁ and 190H PGE₂-were quantified in human seminal fluid by GC-MS-SIM using only the internal standard, d₄-PGE₂. Methods and calculations were developed to minize errors inherent in using only one internal standard for quantifying four closely related prostaglandins. Preliminary data concerning the statistical significance of the differences found between PGE and 190H PGE levels in fertile, azospermic and oligospermic men are reported.     

MDL-646, a new synthetic E1-prostaglandin with local protective effects on the gastric mucosa

The gastric protection, diarrheogenic and arterial hypotensive effects of MDL-646, a PGE1 derivative, have been studied in rats. The compound administered p.o. or i.v. was able to inhibit the macroscopic damage to gastric mucosa produced by noxious stimuli (ethanol and indomethacin). In the stomach perfusion test with the anesthetized rat, intravenously administered MDL-646 reduced histamine- or pentagastrin-stimulated gastric secretion. After intraduodenal administration (i.d.) doses at least 40-50 times greater were necessary for an antisecretory effect. In conscious rats with chronic gastric fistulas, intragastrically administered (i.g.) MDL-646 affected both acid concentration and volume of unstimulated gastric secretion. In experimental models for gastric lesions, MDL-646 was much more potent after oral (p.o.) (15-30 times) than after i.v. administration. (ED50 micrograms/kg: vs. alcohol lesions, 0.05 p.o. and 0.7 i.v.; vs. indomethacin ulcers, 7.0 p.o. and 195 i.v.). Our data would fit the hypothesis that it was a local effect on the gastric mucosa. The mechanism of this effect is not known. The supposed local activity coupled with the antisecretory effects and the good tolerability make it interesting to test MDL-646 as an anti-ulcer agent in man.     

A dose-response comparison of PGA2, PGE1 and PGE2 using the phenylephrine-stimulated perfused oviduct of the rabbit

The phenylephrine-stimulated perfused oviduct of the rabbit was evaluated as a model for studying the activity of prostaglandins that produce inhibition of the oviducal smooth muscle. Elevation of the normal “tone” of the oviduct by perfusing phenylephrine through the lumen permitted quantitation of the responses to PGA₂, PGE₁ and PGE₂ by measuring the magnitude of the inhibitory response produced by the agents. PGE₂ was relatively more potent, efficacious and specific for the oviduct than PGA₂ or PGE₁. It was concluded that the model was suitable for comparative dose-response studies of PGA₂, PGE₁ and PGE₂ and their analogs.     

Selective antagonism of the systemic vasodepressor response to PGE1 by d,1–11, 15-bisdeoxy PGE1

Several bisdeoxy PGE₁ analogs are potent, competitive antagonists of PGE₁-induced colonic contractions in the gerbil. The efficacy of these analogs in antagonizing PGE₁-mediated systemic vasodepression has not been previously demonstrated. In this study, serial doses of PGs were administered before, during and after infusion of d,1–11, 15-bisdeoxy PGE₁. Bolus injections of PGE₁ (3.0 μk/kg), PGE₂ (3.0 μg/kg) and PGI₂ (0.3 μg/kg) were administered via the right external jugular vein to male Wistar rats. PGE₁, PGE₂ and PGI₂ decreased systemic arterial pressure 41%, 38% and 38%, respectively. The PGE₁ analog was infused (200 μg/kg/min) through the right common carotid artery. The analog itself had no effect on mean systemic arterial pressure, but maximum reversible inhibition (51%) of PGE₁-mediated vasodepression occurred following a 50 minute infusion. No significant effect of the PGE₁ analog was observed on PGE₂ or PGI₂-mediated vasodepression. These data demonstrate the ability to antagonize PGE₁-mediated vasodepression, and to differentiate the vascular responses to PGE₁ and PGE₂ or PGI₂.     

Nasal vasoconstrictor activity of a novel PGE2 analog

A novel PGE₂ analog (CL 116,069) was shown to be effective in dogs as a nasal decongestant. Threshold doses were approximately 0.1 μg/kg with intravenous administration and between 0.08 and 4 μg with topical administration. CL 116,069 was compared to 17-phenyl-trinor PGE₂ (CL 116,147), a compound recently studied in humans, and xylometazoline, a well-known nasal decongestant. When given i.v., efficacious doses of xylometazoline tended to raise blood pressure and be shorter acting than the PGs, which did not affect blood pressure. When given topically, all three were long-acting. CL 116,069 usually had the lowest threshold and CL 116,147 usually induced the smallest response. All three agents were more effective than PGE₁ or PGE₂. A 30-day (b.i.d., topical) toxicity test with CL 116,069 produced no inflammation or nasal pathology and no loss in tissue sensitivity. $\text{In}$$\text{vitro}$ examination of xylometazoline and CL 116,069 for vascoconstrictor activity on dog isolated mucosa revealed a response profile similar to that observed with these agents $\text{in}$$\text{vivo}$; i.e., the magnitude of response was comparable for both agents but the t $\text{1}\text{2}$ was only 74 minutes for xylometazoline and greater that 6.5 hours for CL 116,069. The data suggest that CL 116,069 may provide a therapeutic alternative in which constriction of the nasal blood vessels need not be associated with a generalized vasoconstrictor liability.     

Plasma levels of 13,14-dihydro-15-keto PGE2 after vaginal application of a new PGE2 film

To determine the release and absorption profile of prostaglandin E₂ from a new vaginal film formulation containing 850 μg PGE₂, serial plasma levels of 13,14-dihydro-15-keto PGE₂ were measured by radioimmunoassay in pregnant women between 16 and 18 weeks gestation. A control group, using placebo vaginal film was included in the study. There was a somewhat uniform increase in the plasma levels of the PGE₂ metabolite, reaching peak levels between 4 and 6 hours after application of the film. The findings suggest that this drug formulation could be used clinically when slow constant release of the prostaglandin is required over a period of hours such as in pre-induction cervical ripening of term pregnancy.     

Prostaglandin E1 or E2 (PGE1, PGE2) prevents premature luteolysis induced by progesterone given early in the estrous cycle in ewes

The objective of this study was to determine whether prostaglandin E₁ (PGE₁) or prostaglandin E₂ (PGE₂) prevents premature luteolysis in ewes when progesterone is given during the first 6 days of the estrous cycle. Progesterone (3 mg in oil, im) given twice daily from Days 1 to 6 (estrus = Day 0) in ewes decreased (P < 0.05) luteal weights on Day 10 postestrus. Plasma progesterone concentrations differed (P < 0.05) among the treatment groups; toward the end of the experimental period, concentrations in jugular venous blood decreased (P < 0.05) compared with the other treatment groups. Plasma progesterone concentrations in ewes receiving PGE₁ or PGE₁ + progesterone were greater (P < 0.05) than in vehicle controls or in ewes receiving PGE₂ or PGE₂ or PGE₂ + progesterone. Chronic intrauterine treatment with PGE₁ or PGE₂ prevented (P < 0.05) decreases in plasma progesterone concentrations, luteal weights, and the proportion of luteal unoccupied and occupied LH receptors on Day 10 postestrus in ewes given exogenous progesterone, but did not affect (P > 0.05) concentrations of PGF_2α in inferior vena cava blood. Progesterone given on Days 1 to 6 in ewes advanced (P < 0.05) increases in PGF_2α in inferior vena cava blood. We concluded that PGE₁ or PGE₂ prevented progesterone-induced premature luteolysis by suppressing loss of luteal LH receptors (both unoccupied and occupied).     

Inotropic effect of PGE1 and PGE2 on isolated rat atria. Influence of adrenergic mechanisms

The effects of a wide range of PGE₁ and PGE₂ concentrations on the isometric developed tension of isolated rat atria beating spontaneously or paced at a fixed rate, were explored. PGE₁ only produced a negative inotropic effect (NIE), whereas PGE₂ elicited a biphasic inotropic action; negative at low concentrations and positive (PIE) at higher ones. Phenoxybenzamine and phentolamine failed to modify either the NIE or the PIE, but subthreshold exogenous norepinephrine abolished the NIE, suggesting a presynaptic inhibitory effect of PGEs on the adrenergic neurotransmitter release. Auricles pretreated with subthreshold norepinephrine react with a PIE to PGE₁, but not to PGE₂. On the contrary in the presence of subthreshold methoxamine the PIE of PGE₂ was increased whereas the action of PGE₁ was not modified.     

Basal level of human platelet prostaglandins: PGE1 is more elevated than PGE2

Radioimmunoassays of platelet prostaglandins E₁ and F_1α in platelet rich plasma or platelet suspension, demonstrate that both PGE₁ and PGF_1α are present at higher concentrations than prostaglandins E₂ and F_2α. Gas chromatography — mass spectrometry determinations of prostaglandins E₁ and E₂ in resting washed platelets confirm this difference. Lastly, there is a greater incorporation of [1-¹⁴C] acetate into prostaglandins E₁ and F_1α compared to that into prostaglandins E₂ and F_2α.     

A new PGE1 analogue (CL115,574) III. Effects on gastric acid and mucus secretion in man

CL115,574, an analogue of PGE₁, is a potent inhibitor of gastric acid secretion in animals. The effects of this compound on gastric acid and mucus secretion were studied in 8 male volunteers. The compound was well tolerated, and its maximally effective antisecretory dose (750 μg) inhibited pentagastrin stimulated acid secretion by approximately 40% over a 2-hour period, with stimulation beginning one hour after the drug was orally administered. CL115,574 proved to have a significant and sustained effect upon the stimulation of mucus secretion into gastric juice. Considering the possible role that mucuse may play in mucosal cytoprotection, CL115,574 because of its antisecretory and mucogenic actions may prove to be an important clinical anti-ulcer compound.     

Antilipolytic activity of viprostol, a transdermally active antihypertensive PGE2 analog

Viprostol, a novel prostaglandin E₂ congener, was assessed for antilipolytic activity in the spontaneously obese rat. In isolated epididymal adipocytes, viprostal exhibited a dose-dependent inhibition of catecholamine-stimulated lipolysis at concentrations ranging from 10 μM to 1 mM, but was ineffective at lower concentrations. Additionally, viprostal exhibited approximately 50% of the antilipolytic activity of naturally-occurring PGE₁ and PGE₂ at similar concentrations, but was as potent as PGF_2α. At 10 μM, viprostol inhibited maximum catecholamine-stimulated lipolysis by approximately 35% of the total, hormone-stimulated glycerol release. The results of these experiments indicate that viprostol exhibits antilipolytic activity , but is less potent than the naturally-occurring PGE's to which it is most closely related structurally.     

PGE1 metabolism by the perfused rat liver

The hepatic and biliary metabolites of PGE₁ have been isolated and identified after infusions of PGE₁ into isolated rat liver preparations. The results demonstrate that in general PGE₁ undergoes metabolism similar to that of PGE₂ in the rat and reveals the possibility of a selective PG metabolite transport system across the biliary canalicular membrane.     

Bronchodilator activity of a PGE2 analog in animals and in man

A C-11 substituted PGE₂ analog, DHET-PGE₂ [$\text{?}$-11-deoxy-11α-(2-hydroxyethylthio)-PGE₂ methyl ester], was demonstrated to exert potent bronchodilator activity in three $\text{in vivo}$ models of augmented airway resistance: (1) acute bronchospasms, induced by 5-hydroxytryptamine, histamine and acetylcholine in the anesthetized guinea pig, (2) acute bronchospasm, induced by pilocarpine, in the anesthetized dog, and (3) chronic bronchospasm, induced by SO₂ exposure, in the unanesthetized dog. In acute and 30-day toxicological studies in the dog, no cardiovascular, respiratory or gastrointestinal side effects were observed at aerosol doses at least 1,000 times those required for efficacy. $\text{In vitro}$, DHET-PGE₂ effectively relaxed isolated preparations of dog bronchus that had been contracted with carbachol. In clinical studies, human asthmatics and bronchitics responded consistently to β-agonist bronchodilators but variably to DHET-PGE₂. Overall, increases in pulmonary resistance or decreases in FEV₁ were observed with DHET-PGE₂. Subsequent evaluation in isolated carbachol-contracted human bronchus revealed that, in contrast to the bronchodilator activity of PGE₁ and β-agonists, DHET-PGE₂ and PGE₂ induced contraction. Considered along with results from previous clinical studies on other PGs, these data underscore the difficulties in making extrapolations on this class of compounds from animal models to humans and suggest that human bronchial tissue may provide the only appropriate preclinical test system for predicting the clinical efficacy of PG bronchodilators.     

Desensitization of PGE1 receptors in neuroblastoma-glioma hybrid cells

Prostaglandin E₁ receptor sites were measured in homogenates of NG108-15 neuroblastoma-glioma hybrid cells after exposure of intact cells to PGE₁. Scatchard analysis of competitive binding studies showed that incubation of NG108-15 cells in the presence of 2.5 μM PGE₁ for 16 h resulted in a loss of PGE₁ receptors and an increase in the dissociation constant of the remaining receptors. Thus, cells challenged with PGE₁ not only lose adenylate cyclase activity, but also lose PGE₁ receptors and decreased the affinity of the remaining receptors for PGE₁.     

Interactions of prostaglandin E1 (PGE1) and LRH on anterior pituitary function

Numerous biochemical pathways influence the synthesis and release of anterior pituitary hormones. Releasing factors extracted from the hypothalamus and prostaglandins (PGs) appear to alter a common biochemical activity, adenyl cyclase, in pituitary cells. Luteinizing hormone releasing hormone (LRH), prostaglandin (PGE₁), 7 oxa-13-prostynoic acid and cycloheximide were tested for individual and interacting effects on the in vitro release of FSH, LH and prolactin from hemipituitaries of 15 day old female rats. LRH (10 ng/ml) consistently released both LH and FSH in all in vitro experiments and inhibited prolactin release in 1 of 2 experiments. Lower concentrations (5 and 1 ng/ml) also stimulated LH and FSH release but did not influence prolactin release. Concurrent depletion of stored LH and FSH in the gland was observed. PGE₁ in a 6.5 hour incubation increased the storage of LH within the gland in the absence of LRH. In a 1.5 hour incubation in the presence of LRH, storage of LH was also increased. PGE₁ had no effect on LH and FSH release; however, in 1 of 2 experiments it stimulated prolactin release in the absence of LRH. Prostynoic acid stimulated LH and FSH release but did not synergize with LRH action in the same tissue. Cycloheximide did not affect LH release during the first 30 minutes of incubation; however, the release during the subsequent 1 hour was significantly inhibited. Similar tissue also exposed to cycloheximide was still responsive to LRH during the latter 1 hour incubation period. Cycloheximide had no effect on prolactin storage and release from the same tissue.