Metabolic oxidation of the pyrrole ring: structure and origin of some urinary metabolites of the anti-hypertensive pyrrolylpyridazinamine, mopidralazine. III: Studies with the 13C-labelled drug.

The metabolism of the anti-hypertensive drug, mopidralazine, N-(2',5'-dimethyl-1H-pyrrol-1-yl)-6-(4"-morpholinyl)-3-pyridazinamine, was reinvestigated in rats using the [2'(5')-13CH3]-labelled drug to determine the significance of the pharmacologically active intermediate 3-hydrazino-6-(4-morpholinyl)pyridazine. The previously proposed mesonic structure of the major metabolite I, i.e., 5'-hydroxy-3',6'-dimethyl-1'-[6-(4"-morpholinyl)-3-pyridazinyl]pyrida zinium hydroxide inner salt, was confirmed by chemical synthesis, X-ray diffraction analysis and 1H n.m.r. of the [3',6'-13CH3]-labelled metabolite I. Metabolite II, 3-methyl-6-(4-morpholinyl)-triazolo [4,3-6 b]pyridazine and metabolite VII, 3-methyl-7-(4-morpholinyl)-3H-pyridazino[1,6-c]pyridazine, were shown to retain the 13CH3 labelling of mopidralazine, whereas metabolite X, 3-acetyl-hydrazino-6-(4-morpholinyl)-pyridazine, loses the labelling, indicating that their formation involves two different pathways. It is hypothesized that the oxidation of the pyrrole leads to ring opening followed by a chemical rearrangement giving rise directly to metabolites II and VII or, with the intermediacy of the pharmacologically active 3-hydrazino-6-(4-morpholinyl) derivative and an enzymic acetylation or conjugation with pyruvic acid, to metabolites X, II and VII.

Angiotensin converting enzyme inhibitors as antihypertensive agents: 1-[(2-mercaptocycloalkyl)carbonyl]-L-prolines.

The synthesis of 1-[(2-mercaptocyclopentyl)carbonyl]-L-prolines, 1-[(2-mercaptocyclobutyl)carbonyl]-L-prolines and related benzoyl derivatives as pure isomers is described. The abilities of all the compounds to inhibit angiotensin converting enzyme (ACE) in vitro and in vivo and to lower the systolic blood pressure in renal hypertensive dogs were determined. Three of them, namely 1-[[2-(benzoylthio)cyclopentyl]carbonyl]-L-proline (10f(R,S], 1-[(2-mercaptocyclopentyl)carbonyl]-L-proline (10g(R,S], and 1-[[2-(benzoylthio)cyclobutyl]carbonyl]-L-proline (16f(R,S], were found to be as potent as captopril in reducing blood pressure. The influence of chirality and ring size on the ACE inhibition is described.

Metabolic pathways of the anti-hypertensive agent, N-(2,5-dimethyl-1H-pyrrol-1-yl)-6-(4-morpholinyl)-3-pyridazinamine+ ++ hydrochloride. I. Studies in the rat.

The metabolic fate of a new anti-hypertensive, 1-pyrrolyl pyridazinamine, was studied in male Wistar rats after both p.o. and i.v. administration (1 mg/kg). The compound undergoes rapid metabolism, disappearing from the central compartment with a half-life of about 0.5 h. Plasma concn. of the parent drug and its major metabolite I following i.v. and p.o. administration suggest a route-dependent first-pass metabolism. Ten metabolites were isolated from the urine and identified by u.v., i.r., mass and 1H-n.m.r. spectroscopy. The structure of some was confirmed by 13C-n.m.r. and chemical synthesis. All biotransformations are restricted to the pyrrole ring which undergoes oxidative cleavage followed by a series of chemical rearrangements. A minor pathway leads to the formation of methyl sulphinyl and methyl sulphonyl pyrroles. It is suggested that, as with natural indoles, the pyrrole might be oxidized by a 2,3-dioxygenase. The three major metabolites, I, II and IX, along with two minor ones, VI and VII, were inactive when tested i.v. for antihypertensive activity.

Metabolic pathways of the anti-hypertensive agent, N-(2,5-dimethyl-1H-pyrrol-1-yl)-6-(4-morpholinyl)-3-pyridazinamine hydrochloride. II: Studies in the dog.

The metabolic fate of a new anti-hypertensive, 1-pyrrolyl-pyridazinamine, was studied in male Beagle dogs given both p.o. and i.v. doses of the 14C-labelled drug (1 mg/kg). The compound given as a single i.v. injection disappeared from the central compartment with a half-life of about 0.9 h. Plasma levels of total 14C were represented mostly by metabolites. Eight urinary metabolites designated as metabolites I, II and XI-XVI were purified and their structures assigned by means of u.v., i.r., n.m.r. and mass spectrometry. Quantitatively the primary metabolic attack involved the morpholine moiety of the molecule which undergoes oxidative opening. A minor pathway afforded the cleavage of the pyrrole followed by chemical rearrangements to form six-membered sidnone-like products or a triazole derivative. The major (XIII) and three minor metabolites were studied for their antihypertensive activity in rats and were shown to be inactive.

Potential antihypertensives. Synthesis of 6-substituted-N-(4H-1,2,4-triazol-4-yl)-3-pyridazinamines and 3-substituted-6-(3,5-dimethyl-1H-1,2,4-triazol-1-yl)pyridazines.

A series of 6-substituted-N-(4H-1,2,4-triazol-4-yl)-3-pyridazinamines [(II), Scheme 1] and 3-substituted-6-(3,5-dimethyl-1H-1,2,4-triazol-1-yl) pyridazines [(XII), Scheme 3-5] were synthesized. The compounds were evaluated for their oral antihypertensive activity in rats (SHR) and only some compounds of structure (XII) induced a moderate decrease in systolic blood pressure.

Pharmacokinetics of a new antihypertensive pyrrolyl pyridazinamine (MDL-899) in the rat and the dog.

The pharmacokinetics of N-(2,5-dimethyl-1H-pyrrol-1-yl)-6-(4-morpholinyl)-3-pyridazinamine hydrochloride (MDL-899), a new antihypertensive agent, was studied in rats and dogs. The 14C-labelled compound was synthesized by a microscale procedure with 45% chemical yield and 98% radiochemical purity. In both animal species, MDL-899 was rapidly absorbed from the gastro-intestinal tract, achieving peak plasma levels in 0.5-2 h. The ratio between the plasma concentrations of 14C and of unchanged MDL-899 indicates rapid metabolic transformation and, especially in the rat, a marked first-pass effect. MDL-899 binds to serum proteins with a very low affinity and rapidly enters the tissue compartment, with large distribution volumes (1.6 l/kg rat, 2.0 l/kg dog). Target tissues in the rat were the liver, kidneys, adrenals, lungs, ovaries, uterus and the arterial walls, which constitute a deep-compartment. The plasma half-life of unchanged MDL-899 was 0.5 h in the rat and 1.4 h in the dog, while the terminal half-lives for total radioactivity were much longer (two elimination phases). Within the range of doses tested (1 and 40 mg/kg) there is evidence of non-linear kinetics (dog). The plasma kinetics profiles of both MDL-899 and 14C were the same in both males and females (rat). In both rats and dogs, elimination of the test dose was preferentially via the kidneys, as metabolites. The time course of the pharmacological response seems to be correlated to the kinetics of the active species in deep-compartment(s) rather than to the plasma concentration.

Antihypertensives. N-1H-Pyrrol-1-yl-3-pyridazinamines.

The hypothesis that the side effects of hydralazine, such as mutagenicity and lupus erythematosus like syndrome, might be due to the NHNH2 group prompted us to incorporate part of this moiety into a pyrrole ring. Therefore, we prepared a series of N-1H-pyrrol-1-yl-3-pyridazinamines and a limited number of N-1H-pyrrol-1-yl-1-phthalazinamines by reaction of 3-hydrazinopyridazines and 1-hydrazinophthalazines with gamma-diketones. Most of these compounds, especially in the pyridazine series, showed moderate to strong antihypertensive activity in spontaneously hypertensive rats. The decrease in blood pressure generally had a slow onset after either oral or intravenous administration. N-(2,5-Dimethyl-1H-pyrrol-1-yl)-6-(4-morpholinyl)-3-pyridazinamine hydrochloride (30) (MDL 899) showed no mutagenic activity in several tests and is now in clinical trials in patients.

Synthesis of potential antihypertensive compounds: N-(4,5-dihydroimidazol-2-yl-amino)pyrroles.

A series of N-(4,5-dihydroimidazol-2-yl-amino)pyrroles (V) were prepared by reaction of the 2-hydrazino-2-imidazolines with 2,5-diketones. Noticeable oral antihypertensive activity in renal hypertensive dogs was shown by N-(4,5-dihydroimidazol-2-yl-amino)-2,5-dimethyl pyrrole.

Synthesis of three metabolites of 3-hydrazino-6-[bis-(2-hydroxyethyl)amino]pyridazine. A new rearrangement of 3-[1-methylethyliden)hydrazino] derivative.

The synthesis of three metabolites (I), (II), (III) , isolated from rat urine, of the hypotensive compound 3-hydrazino-6-[bis-(2-hydroxyethyl)amino]pyridazine dihydrochloride (IV), is described. The first metabolite, 3-[bis-(2-hydroxyethyl)amino]pyridazine (I) (Scheme 1) was obtained either by reaction of 6(3)-chloro-3(6)-[bis-(2-hydroxyethyl)amino]pyridazine-N-oxide (V) with hydrazine hydrate or by catalytic hydrogenation of 3-chloro-6-[bis(2-hydrxyethyl)amino]pyridazine (VI). 3-Methyl-6-[bis-(2-hydroxyethyl)amino]-2-triazolo [4,3-b]pyridazine (II) was prepared (Scheme 2) by reaction of (IV) with acetic anhydride and subsequent hydrolysis of the resutling 6-[bis-(2-acetyloxyethyl)amino]-s-triazolo[4,3-b]pyridazine (VII). The third metabolite, 3-[bis-(2-hydroxyethyl)amino]-6-isopropyloxypyridazine (III) Scheme 3), was synthesized from 3-[(1-methylethyliden)hydrazino]-6-[bis-(2-hydroxyethyl)amino]pyridazine (VIII) which in phosphate buffer (pH 7.38) or in sodium hydrogen carbonate solution, underwent a rearrangement of new type to give (III).

Synthesis of 1-hydroxy and 1-acyloxypyrrolidin-2-ones.

A series of 1-hydroxypyrrolidin-2-ones (v) has been prepared by reduction of gamma-nitrocarboxylic acid esters with Zn dust in the presence of ammonium chloride. Acylation of compounds (v) gave the corresponding 1-acyloxypyrrolidin-2-ones (VI). None of the synthesized compounds showed any significant antiflammatory activity.

The reaction of phthalazino(2,3-b)phthalazine-5,12(7H, 14H)-diones with nitrous acid.

3,4-Dihydrophthalazin-1(2H)-one (I) was oxidized to phthalazin-1(2H)-one (III) with nitrous acid or with ferric chloride . Phthalazino [2,3-b] phthalazine-5,12(7H, 14H)-diones (IV) did not react with ferric chloride but they were oxidized with nitrous acid to 2-[1(2H)-oxo-2-phthalazinyl] methylbenzoic acids (V) and (VI). The formation of (V) or (VI) depends upon the substituents of compounds (IV). Strucutres (V) and (VI) were established by pKa measurements in methylcellosolve and by mass and N.M.R. spectra.