Prolopa - Scientific Information
|Condition:||Parkinson's Disease, Restless Legs Syndrome|
|Ingredients:||Levodopa, Benserazide, Gelatin, Indigotine, Iron oxide, Magnesium stearate, Mannitol (50-12.5 capsule only), Microcrystalline cellulose, Povidone, Talc, Titanium dioxide|
|Proper Name:||levadopa and benserazide combination|
|Chemical Name:||PROLOPA contains levodopa and benserazide in a 4:1 ratio.
Levodopa is chemically L-(-3-(3,4-dihydroxy-phenyl)-alanine, a
metabolic precursor of dopamine.
Benserazide, an inhibitor of aromatic amino acid decarboxylase, is
chemically DL- (-) seryl-2(2,3,4-trihydroxybenzyl) hydrazine
Pharmacological experiments in various species of animals have shown that levodopa produces increased motor activity, aggressive behaviour and electroencephalographic alerting behaviour. However, occasional sedation and ataxia have also been reported in some species. Levodopa also reverses the reserpine induced Parkinson-like effects in animals. Cardiovascular studies in dogs and cats have shown that levodopa increases the catecholamine levels in the brain which was evident as an initial increase in blood pressure followed by a secondary decrease in blood pressure. The changes in blood pressure appear to correlate with changes in renal function.
Biochemical studies in vivo as well as in vitro have demonstrated that levodopa is decarboxylated to dopamine in many tissues. Levodopa crosses the blood-brain barrier and elevates the dopamine concentration in the brain, although probably less than 1% penetrates the central nervous system. The level of aromatic acid decarboxylase activity in the striatum is reduced, but apparently sufficient enzymatic action remains to convert levodopa to the active moiety, dopamine. Levodopa is extensively decarboxylated in its first passage through the liver. The dopamine formed can be degraded to dihydroxyphenylacetic acid and homovanillic acid, which are the two major metabolites in the urine. Dopamine may also be converted to noradrenaline, in which case the major metabolites are 4-hydroxy-3-methoxy-mandelic acid and mandelic acid.
Dopamine is a pharmacologically active catecholamine with effects on alpha and beta adrenergic receptors. Its cardiac stimulating properties are produced by an action on beta adrenergic receptors but the orthostatic hypotension produced by levodopa is thought to be related to a central effect. The cardiac effect of levodopa can usually be prevented by beta-adrenergic blockers. Dopamine also stimulates the extracerebral chemotherapy trigger zone (CTZ) in the area postrama of the medulla producing nausea and vomiting. Levodopa stimulates the secretion of growth hormone and can inhibit the secretion of prolactin.
Benserazide belongs to the hydroxyphenylalkylhydrazine groups of aromatic L-amino acid decarboxylase inhibitors. Benserazide also inhibits other enzymes found in the periphery, e.g., in vivo tryptophan hydroxylase, aromatic amino acid transaminase, monoamine oxidase, diamine oxidase, dopamine-beta-hydroxylase and catechol-0-methyltransferase. However, benserazide is significantly more specific for decarboxylase. In the rat, benserazide penetrates into the brain at high doses (300 mg/kg) whereas at lower dosages (as high as 50 mg/kg) in the mouse, guinea pig and rabbit there is a complete and selective inhibition of extracerebral decarboxylase without inhibition of this enzyme in cerebral tissues. This results in a diminution of catecholamines and an accumulation of levodopa in peripheral tissues. Dopa decarboxylase forms a major barrier to the entry of levodopa into the central nervous system at the level of the brain capillaries and it has been suggested that benserazide inhibits this enzymatic mechanism in the capillaries of the extrapyramidal area which in turn provides levodopa more efficiently for conversion to dopamine within the basal ganglia. High doses of benserazide cause a slight decrease of endogenous monoamines in various peripheral tissues, e.g., noradrenaline in the heart.
When administered alone, the effects of benserazide vary from species to species, e.g., blood pressure, blood flow, and heart rate increase in the cat, although there are only slight cardiovascular effects in the dog. In normotensive and renal hypertensive rats, benserazide, over the range 50 to 500 mg/kg, has little or no effect on blood pressure. High pre-treatment doses of benserazide (200 mg/kg) inhibit the hypotensive effect of alpha-methyldopa in renal hypertensive rats. The respiratory minute volume also increases, especially in the cat. Benserazide administered alone shows no diuretic, anti-inflammatory or analgesic activity in rats. The hypnotic activity of ethyl alcohol, sodium pentobarbital or hexobarbital is not affected by administration of benserazide. Benserazide has no effect on experimentally induced cough in guinea pigs, apart from a weak protection from the effects of a histamine spray. It has no protective effect against anaphylactic shock produced in experimental animals. Thus, in the usually administered doses there are no appreciable effects of benserazide on the cardiovascular, renal, gastrointestinal or central nervous systems.
Levodopa and Benserazide Combination
Because of the selective inhibition of extracerebral aromatic acid decarboxylase, the degradation of levodopa in the heart, kidney, liver, gastrointestinal tract and peripheral adrenergic terminals, as well as in the capillaries of the brain is diminished. Therefore, a considerably larger proportion of levodopa is available for the cerebral parenchyma where it is converted to dopamine. In addition, there is improved intestinal absorption of levodopa as a consequence of decarboxylase inhibition in the gastrointestinal tract. Benserazide has not been shown to further raise the levodopa-induced increase of homovanillic acid in the CSF and can result in a reduced concentration of dopamine metabolites. The combination of levodopa and benserazide has slight or no effect on sleeping time when given with other hypnotics and no analgesic or antitussive properties. The combination stimulated conditioned-avoidance response rates in the rat. Small doses of benserazide markedly enhance the increase in locomotor activity resulting from administration of levodopa, possibly as a result of greater dopaminergic transmission in the basal ganglia. This combination was also shown to partially antagonize the cataleptic effect of chlorpromazine and butyrophenone. In addition, benserazide improved the efficacy of levodopa in reversing the reserpine syndrome. All of these effects are presumably due to enhancement of the levodopa-induced rise in catecholamines, mainly dopamine, in the brain as a result of benserazide pre-treatment. Administration of the combination to dogs reduced catecholamine formation in peripheral organs and has been shown in some studies to significantly diminish levodopa-induced cardiac arrhythmias, arterial hypertension, nausea and vomiting.
In rats, mice, dogs and humans, the major proportion of benserazide is absorbed following oral administration. The main biotransformation of benserazide takes place in the gut and consists in splitting off the serine moiety to liberate the trihydroxybenzylhydrazine aromatic acid decarboxylase inhibitor. In rats, following oral administration, plasma levels peak approximately 30 to 60 minutes after dosing, level off after 2 hours and remain fairly constant up to 24 hours. At the usual doses recommended in humans distribution of benserazide and its metabolites in rat tissues is consistent with peripheral, but not central, decarboxylase inhibition. In studies in mice, benserazide and/or its metabolites showed a great affinity for tissues with a high content of elastic fibres, i.e., large arteries and to a lesser extent, the corium. The highest concentrations of radioactivity were found respectively in the intestine, kidney, bladder, liver and no radioactive material was detected in the central nervous system following oral administration. Similar findings have been reported for rats with the highest radioactivity being found in liver and kidney tissues.
In mice and rats, the majority of benserazide is eliminated in the bile and urine and approximately similar proportions of a given dose are excreted in the feces and urine. Most of the dose is excreted within 24 hours. In dogs, excretion in urine accounted for two thirds of an oral dose of 100 mg/kg benserazide.
|Species||LD50(mg/kg)||Signs of Toxicity|
|Rabbits||609||increased activity, convulsions, vasodilation|
|Mice||6317||sedation, depressed respiration, tonic clonic spasms|
|Rats||6489||sedation, depressed respiration|
|Rabbits||1720||increased and decreased activity, ataxia|
|Neonatal Rats||500||respiratory failure, cyanosis, pallor|
|Mice||>8000||excitation, sedation, depressed respiration|
|Rats||>4000||decreased motor activity|
|Rabbits||1920||decreased motor activity|
|Mice||>8000||excitation, sedation, depressed respiration|
The minimum lethal oral dose for the squirrel monkey of the 4:1 combination of levodopa plus benserazide was 2000 mg/kg. For benserazide and levodopa given separately, the minimum lethal dose was 400 mg/kg.
Thirteen-week studies in dogs treated with benserazide in oral doses ranging from 40 to 120 mg/kg indicated that 40 mg/kg was well tolerated except for a slight loss of weight and some fatty degeneration of the liver. Higher doses resulted in anorexia and severe loss of weight, severe fatty degeneration of the liver and testicular changes, the latter possibly owing to inanition. No bone deformation or neurological damage was observed.
Oral studies (13 weeks) in the rat with benserazide alone indicated that 75 mg/kg/day were well tolerated, except for the development of skeletal alterations. Skeletal alterations were increasingly severe with doses of 150 and 240 mg/kg/day; these doses were also associated with paralysis of the hind limbs. The bone changes were characterized by severe deformations in the thoracic region (vertebral column, ribs, sternum) and the extremities, observable macroscopically. Histologically, increased bone resorption and pronounced proliferation of periosteal connective tissue were also observed.
Slightly raised hematocrit and hemoglobin levels and elevated leucocyte counts were seen in the two higher dosage groups of rats (240 and 150 mg/kg/day). In some rats of both groups, serum alkaline phosphatase activity levels were increased, and the blood glucose levels were slightly reduced. Mild fatty degeneration of the liver was observed in all three groups, but was more pronounced in the highest dosage group.
A study in rats to determine the effect of benserazide alone on possible skeletal alterations showed that skeletal formation where the epiphyseal discs had already closed was not affected by benserazide. Normal ossification also occurred in locations where the epiphyseal discs were due to close shortly, and no skeletal alterations were observed. Only those epiphyseal discs which were not due to close for a long time were severely altered. In all groups dosage started at 125 mg/kg/day of benserazide and 500 mg/kg/day of levodopa. After three weeks, the dosage level was lowered to 100 and 400 mg/kg/day and the study was continued for another nine weeks.
A further study was made on the effect of benserazide alone on possible hepatic changes in dogs. A severe fatty degeneration of the liver was produced by high doses (up to 70 mg/kg/day) of benserazide. However, this degeneration was shown to be reversible within 21⁄2 weeks following withdrawal of benserazide.
One-year chronic toxicity studies in dogs with a 4:1 combination of levodopa plus benserazide indicated that an oral daily dose of 60 mg/kg levodopa + 15 mg/kg benserazide was well tolerated except for frequent vomiting during the first two weeks. Doses of 120 mg/kg of levodopa + 30 mg/kg of benserazide were moderately well tolerated. Doses of 240 mg/kg of levodopa + 60 mg/kg of benserazide were poorly tolerated, and none of the animals could continue with these high doses beyond three weeks.
In the high dosage group (240 mg/kg of levodopa + 60 mg/kg of benserazide), the prothrombin time of all dogs was increased; some animals showed moderate to definite leucocytosis. BSP elimination in all dogs was seriously delayed; SGPT (but not SGOT) levels in some dogs were greatly increased; and BUN values in nearly all dogs were higher. All dogs had fatty degeneration of the liver and all showed a decrease in hematopoietic tissue in the bone marrow, possibly due to malnutrition.
In the other two groups of dogs (120 + 30 mg/kg/day, and 60 + 15 mg/kg/day) the red blood cell values, were slightly decreased and there was a slight increase in alkaline phosphatase and a temporary delay in BSP elimination in individual animals. In the group with 120 + 30 mg/kg/day the weights of the liver, adrenals and pituitary gland were increased, no histological abnormality was noted in these two groups.
In the 18 month oral toxicity test in rats treated daily with 400 mg/kg levodopa + 100 mg/kg benserazide, 200 mg/kg levodopa + 50 mg/kg benserazide or 100 mg/kg levodopa + 25 mg/kg benserazide, the findings were essentially the same as in the 13 week test with benserazide alone. The combination was very poorly tolerated by the highest dosage group, in which the experiment had to be terminated after 12 to 15 weeks. It was also sufficiently poorly tolerated in the middle dosage group and required termination of the experiment after approximately 12 months. The same types of skeletal abnormalities were observed in all groups as had been noted in the study with benserazide alone, and they were again dose related. Unlike the experiment with benserazide alone, no evidence of hepatic dysfunction was observed.
Teratologic and Reproductive Studies
Malformations of the heart and great vessels were observed in fetuses from rabbits fed 125 mg/kg and 250 mg/kg of levodopa during days 8 to 15 of gestation. Anomalies observed included septal defects, constricted or missing ductus arteriosus, enlarged aortic arches, fused aortas and pulmonary arches, and transposition. A low level of fetal-toxicity was also observed. A similar heart malformation was observed in one mouse fetus from a dam who was fed 500 mg/kg of levodopa from day 6 through 15 of gestation.
In experiments with rats and mice, levodopa plus benserazide in a 4:1 combination (up to 320 + 80 mg/kg/day) and benserazide alone (up to 200 mg/kg/day) were well tolerated by mothers and fetuses. Reproduction physiology data, the detailed examination for fetal skeletal abnormalities, and the rearing and fertility experiments yielded no indication of any detrimental effects.
In the rabbit experiment with benserazide alone, the lower doses (10 and 30 mg/kg/day) had no effect on the reproductive process, or on the viability of the young (24 hour test). Doses of 100 mg/kg/day were toxic for the females: severe loss of weight, raised mortality rate, fatty degeneration of the liver and resorption of 95% of already implanted embryos, usually in the early stage of embryonic development. The fetuses delivered did not exhibit any deformities.
In the rabbit experiment with levodopa + benserazide 4:1, 16 + 4 mg/kg/day had no adverse effect on the course of reproduction. Studies at 48 + 12 mg/kg/day and 120 + 30 mg/kg/day dosage levels caused a marked increase in the resorption rate and a reduction in the mean fetal weight; the other recorded reproduction parameters remained within the normal range. The viability of the neonates (24 hour test) was not impaired. The toxic threshold dose for pregnant rabbits would appear to be 120 + 30 mg/kg/day.