555-30-6

Colorless or almost colorless crystals or white to yellowish-white fine powder. Almost tasteless. In the sesquihydrate form. pH (saturated aqueous solution) about 5.0.

METHYL DOPA(555-30-6) undergoes catalytic oxygenation in the presence of magnesium, cupric, cobalt, nickel and ferric ions . A weakly acidic amino acid.

Very hygroscopic. Slightly water soluble. May be sensitive to prolonged exposure to air and light. The stability of aqueous solutions is markedly dependent on pH, oxygen and the amount of initial reactant. Aqueous solutions are stable for up to 50 hours in acid and neutral pH (6.2). At pH 8.0, decomposition products are formed in 3 to 5 hours. Solutions develop a red tint that becomes progressively darker (eventually forming a black precipitate).

Flash point data for this chemical are not available; however, METHYL DOPA is probably combustible.

Methyldopa is an α-methoxylated derivative of levodopa that exhibits hypotensive action by reducing overall peripheral vascular resistance and reducing heart work. Antihypertensive action of methyldopa consists of the biotransformation of methyldopa into methylnoradrenaline (methylnorepinephrine), which acts as a “pseudo neurotransmitter.” The current, universally accepted point of view is that the action of methyldopa is carried out through the CNS, where methylnorepinephrine, a powerful stimulant of α-adrenergic receptors of the medulla, inhibits the vasomotor center.

Aldometil,MSD,W. Germany,1962

Antihypertensor;L-aromatic aminoacid decarboxylase inhibitor

It is prescribed for arterial hypertension and hypertensive crises.

vitamin, coenzyme B12

ChEBI: A derivative of L-tyrosine having a methyl group at the alpha-position and an additional hydroxy group at the 3-position on the phenyl ring.

The dl-α-methyl-3,4-dihydroxyphenylalanine may be made as described in US Patent 2,868,818. Five-tenths of a gram of 3-hydroxy-4- methoxyphenylalaninewas dissolved in 20 ml of concentrated hydrochloric acid, the solution saturated with hydrogen chloride and heated in a sealed tube at 150°C for 2 hours. The dark reaction mixture was concentrated to dryness in vacuo, excess acid removed by flushing several times with ethanol. On dissolving the dark residue in a minimum amount of water and adjusting the clarified solution to pH 6.5 with ammonium hydroxide the compound separated in fine crystals which were filtered, washed with alcohol and ether. The crystalline product had a MP of 299.5 to 300°C with decomposition.
Then, as described in US Patent 3,158,648, the optical isomers may be resolved as follows. 37 g of racemic α-methyl-3,4-dihydroxyphenylalanine are slurried at 35°C in 100 cc of 1.0 N hydrochloric acid. The excess solids are filtered leaving a saturated solution containing 34.6 g of racemic amino acid of which about 61% is present as the hydrochloride. The solution is then seeded at 35°C with 7 g of hydrated L-α-methyl-3,4-dihydroxyphenylalanine (6.2 g of anhydrous material). The mixture is then cooled to 20°C in 30 minutes and aged one hour at 20°C. The separated material is isolated by filtration, washed twice with 10 cc of cold water and dried in vacuo. The yield of product is 14.1 g of L-α-methyl-3,4-dihydroxyphenylalanine in the form of a sesquihydrate of 100% purity as determined by the rotation of the copper complex.

Antihypertensive

The spectrum of activity of α-methyldopa (Aldomet) lies between those of the more potent agents, such as guanethidine, and the milder antihypertensives, such as reserpine. α-Methyldopa is a structural analogue of dihydroxyphenylalanine (dopa) and differs from dopa only by the presence of a methyl group on the -carbon of the side chain.

L-aromatic amino acid decarboxylase inhibitor. Antihypertensive.

A number of theories have been put forward to account for the hypotensive action of α-methyldopa. Current evidence suggests that for α-methyldopa to be an antihypertensive agent, it must be converted to α-methylnorepinephrine; however, its site of action appears to be in the brain rather than in the periphery. Systemically administered α-methyldopa rapidly enters the brain, where it accumulates in noradrenergic nerves, is converted to α-methylnorepinephrine, and is released. Released α-methylnorepinephrine activates CNS α- adrenoceptors whose function is to decrease sympathetic outflow. Why α-methylnorepinephrine decreases sympathetic outflow more effectively than does the naturally occurring transmitter is not entirely clear.

The oral bioavailability of methyldopa ranges from 20 to 50% and varies among individuals. Optimum blood pressure response occurs in 12 to 24 hours in most patients. After withdrawal of the drug, blood pressure returns to pretreatment levels within 24 to 48 hours. Methyldopa and its metabolites are weakly bound to plasma proteins. Although 95% of a dose of methyldopa is eliminated in hypertensive patients with normal renal function, with a plasma half-life of approximately 2 hours, in patients with impaired renal function the half-life is doubled to approximately 3 to 4 hours, with about 50% of it excreted. Orally administered methyldopa undergoes presystemic first-pass metabolism in the gastrointestinal (GI) tract to its 3-O-monosulfate metabolite. Sulfate conjugation occurs to a greater extent when the drug is given orally than when it is given intravenously (IV). Its rate of sulfate conjugation is decreased in patients with renal insufficiency. Methyldopa is excreted in urine as its mono-O-sulfate conjugate. Any peripherally decarboxylated α-methylnorepinephrine is metabolized by catecho-o-methyltransferase (COMT) and monoamine oxidase (MAO). Methyldopate is slowly hydrolyzed in the body to form methyldopa. The hypotensive effect of IV methyldopate begins in 4 to 6 hours and lasts 10 to 16 hours.

The primary hemodynamic alteration responsible for the hypotensive effects of α-methyldopa remains in dispute. When the patient is supine, the reduction in blood pressure produced by α-methyldopa correlates best with a decrease in peripheral vascular resistance, cardiac output being only slightly reduced. When the patient is upright, the fall in blood pressure corresponds more closely with a reduced cardiac output.
An important aspect of α-methyldopa’s hemodynamic effects is that renal blood flow and glomerular filtration rate are not reduced. As occurs with most sympathetic depressant drugs and vasodilators, long-term therapy with α-methyldopa leads to fluid retention, edema formation, and plasma volume expansion.While data conflict somewhat, it is generally thought that - methyldopa suppresses plasma renin activity.

α-Methyldopa is not generally believed to be suitable for monotherapy of primary hypertension. Because plasma volume increases as the duration of α-methyldopa therapy is extended, the drug should be used in conjunction with a diuretic; this will produce a significantly greater fall in blood pressure than would occur with either drug used alone. Because α-methyldopa lowers blood pressure without compromising either renal blood flow or the glomerular filtration rate, it is particularly valuable in hypertension complicated by renal disease. However, if end-stage renal failure accompanies severe hypertension,α-methyldopa may not be effective.
The presence of α-methyldopa and its metabolites in the urine reduces the diagnostic value of urinary catecholamine measurements as an indicator of pheochromocytoma, since these substances interfere with the fluorescence assay for catecholamines.

The most commonly encountered side effects of α- methyldopa are sedation and drowsiness.These CNS effects are probably the result of reductions in brain catecholamine levels. Other side effects, also typical of sympathetic depression, are dry mouth, nasal congestion, orthostatic hypertension, and impotence.
Autoimmune reactions associated with α-methyldopa treatment include thrombocytopenia and leukopenia. Since a few cases of an α-methyldopa–induced hepatitis have occurred, the drug is contraindicated in patients with active hepatic disease. Flulike symptoms also are known to occur.

Methyldopa, (-)-3-(3,4-dihydroxyphenyl)-2-methylalanine (22.2.5), is synthesized by a few methods that are only slightly different. The first method is from 3,4- dimethoxyphenylacetone, which undergoes a Strecker¨CZelinski reaction using potassium cyanide and ammonium carbonate, to give 4-methyl-4-(3,4-dimethoxybenzylhydantoine (22.3.3), which is further hydrolyzed in the presence of barium hydroxide to give (
)-3-(3,4-dimethoxyphenyl)-2-methylalanine (22.3.4). This undergoes acetylation at the amino group, and the racemic mixture is then separated using (-)-1-phenylethylamine. The isolated isomer is hydrolyzed using hydrobromic acid, which simultaneously removes the methoxy- and acetyl groups to give the desired (-)-3-(3,4-dihydroxyphenyl)-2-methylalanine (22.3.5) [8¨C10]. Alternative syntheses have been proposed.

Potentially hazardous interactions with other drugs
Anaesthetics: enhanced hypotensive effect.
Antidepressants: avoid concomitant use with MAOIs.
Lithium: neurotoxicity (without increased plasma lithium concentrations).
Salbutamol: acute hypotension reported with
salbutamol infusions.

Approximately 50% of an orally administered dose of α-methyldopa is absorbed from the gastrointestinal tract. Both peak plasma drug levels and maximal blood pressure–lowering effects are observed 2 to 6 hours after oral administration. A considerable amount of unchanged α-methyldopa and several conjugated and decarboxylated metabolites can be found in the urine.

Room temperature

Recrystallise methyldopa from H2O. [Reinhold et al. J Org Chem 33 1209 1968.] The L-isomer forms a sesquihydrate from H2O m 302-304o (dec), and the anhydrous crystals are hygroscopic,[] 23D -4.0o (c 1, 0.1N HCl), []546 +154.5o (c 5, CuSO4 solution). It has max at 281nm ( 2780). Its solubility in H2O at 25o is ~10mg/mL and the pH of an aqueous solution is ~5.0. It is insoluble in most organic solvents. [Stein et al. J Am Chem Soc 77 700 1955, Beilstein 4 IV 2505.]