Antiparkinsonian Drugs

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Chapter: Essential pharmacology : Antiparkinsonian Drugs

These are drugs that have a therapeutic effect in parkinsonism.


ANTIPARKINSONIAN DRUGS

 

These are drugs that have a therapeutic effect in parkinsonism.


Belladonna alkaloids had been empirically used in PD. A breakthrough was made in 1967 when levodopa was found to produce dramatic improvement. Its use was based on sound scientific investigations made in the preceding 10 years that—DA is present in the brain; it (along with other monoamines) is depleted by reserpine; reserpine induced motor defect is reversed by DOPA (the precursor of DA); striatum of patients dying of PD was deficient in DA. Thus, parkinsonism was characterized as a DA deficiency state and levodopa was used to make good this deficiency, because DA itself does not cross the bloodbrain barrier. In the subsequent years, a number of levodopa potentiators and DA agonists have been developed as adjuvants/ alternatives.

 

Classification

 

1. Drugs affecting brain dopaminergic system

 

a.              Dopamine Precursor : Levodopa (ldopa)

 

b.             Peripheral Decarboxylase Inhibitors : Carbidopa, Benserazide.

 

c.              Dopaminergic Agonists: Bromocriptine, Ropinirole, Pramipexole

 

d.             MAO-B Inhibitor: Selegiline

 

e.              COMT Inhibitors: Entacapone, Tolcapone

 

f.               Dopamine Facilitator: Amantadine.

 

2. Drugs affecting brain cholinergic system

 

a. Central Anticholinergics: Trihexyphenidyl (Benzhexol), Procyclidine, Biperiden.

 

b. Antihistaminics : Orphenadrine, Promethazine.

 

 

LEVODOPA

 

Levodopa has a specific salutary effect in PD: efficacy exceeding that of any other drug used alone. It is inactive by itself, but is the immediate precursor of the transmitter DA. More than 95% of an oral dose is decarboxylated in the peripheral tissues (mainly gut and liver). DA thus formed acts on heart, blood vessels, other peripheral organs and on CTZ (though located in the brain, i.e. floor of IV ventricle, it is not bound by blood-brain barrier). About 1–2% of administered levodopa crosses to the brain, is taken up by the surviving dopaminergic neurones, converted to DA which is stored and released as a transmitter. Brains of parkinsonian patients treated with levodopa till death had DA levels higher than those not so treated. Further, those patients who had responded well had higher DA levels than those who had responded poorly.

 

Actions

 

1. CNS

 

Levodopa hardly produces any effect in normal individuals or in patients with other neurological diseases. Marked symptomatic improvement occurs in parkinsonian patients. Hypokinesia and rigidity resolve first, later tremor as well. Secondary symptoms of posture, gait, handwriting, speech, facial expression, mood, self care and interest in life are gradually normalized.

 

The effect of levodopa on behaviour has been described as a ‘general alerting response’. In some patients this progresses to excitement— frank psychosis may occur. Embarrassingly disproportionate increase in sexual activity has also been noted. Dementia, if present, does not improve; rather it predisposes to emergence of psychiatric symptoms.

 

Levodopa has been used to produce a nonspecific ‘awakening’ effect in hepatic coma.

 

Two subtypes of DA receptors (D1, D2) were originally described. Three more (D3, D4, D5) have now been identified and cloned. All are G protein coupled receptors and are grouped into two families:

 

D1 like (D1, D5) Are excitatory: act by increasing cAMP formation and PIP2 hydrolysis thereby mobilizing intracellular Ca2+ and activating protein kinase C through IP3 and DAG.

 

D2 like (D2, D3, D4) Are inhibitory: act by inhibiting adenylyl cyclase/opening K+ channels/depressing voltage sensitive Ca2+ channels.

 

The various subtypes of DA receptors are differentially expressed in different areas of the brain, and appear to play distinct roles. Both D1 and D2 receptors are present in the striatum and are involved in the therapeutic response to levodopa. They respectively regulate the activity of two pathways having opposite effects on the thalamic input to the motor cortex (Fig. 31.1). Thus, stimulation of excitatory D1 as well as inhibitory D2 receptors in the striatum achieves the same net effect of smoothening movements and reducing muscle tone.

 

Dopamine receptor in SNPC and in pituitary is also of D2 type. The D3 receptors predominate in nucleus accumbans and hypothalamus, but are sparse in caudate and putamen, while D4 and D5 are mostly distributed in neocortex, midbrain, medulla and hippocampus.

 


 

2. CVS

 

The peripherally formed DA can cause tachycardia by acting on β adrenergic receptors. Though DA can stimulate vascular adrenergic receptors as well, rise in BP is not seen. Instead, postural hypotension is quite common. This may be a central action—DA and NA formed in the brain decrease sympathetic outflow; also DA formed in autonomic ganglia can impede ganglionic transmission.

 

Gradual tolerance develops to both cardiac stimulant and hypotensive actions.

 

3. CTZ

 

Dopaminergic receptors are present in this area and DA acts as an excitatory transmitter. The DA formed peripherally gains access to the CTZ without hindrance—elicits nausea

 

4. Endocrine

 

DA acts on pituitary mammotropes to inhibit prolactin release and on somatotropes to increase GH release. Though prolactin levels in blood fall during levodopa therapy, increased GH levels are not noted in parkinsonian patients. Probably the mechanisms regulating GH secretion are altered in these patients.

 

Pharmacokinetics

 

Levodopa is rapidly absorbed from the small intestines by utilizing the active transport process meant for aromatic amino acids. Bioavailability of levodopa is affected by:

 

1.   Gastric emptying: if slow, levodopa is exposed to degrading enzymes present in gut wall and liver for a longer time—less is available to penetrate bloodbrain barrier.

 

2.   Amino acids present in food compete for the same carrier for absorption: blood levels are lower when taken with meals.

 

Levodopa undergoes high first pass metabolism in g.i. mucosa and liver. The peripheral and central pathway of metabolism of levodopa is depicted in Fig. 31.2.

 


 

About 1% of administered levodopa that enters brain, aided by amino acid carrier mediated active transport across brain capillaries, also undergoes the same transformation. The plasma t½ of levodopa is 1–2 hours. Pyridoxal is a cofactor for the enzyme dopa-decarboxylase. The metabolites are excreted in urine mostly after conjugation.

 

Adverse Effects

 

Side effects of levodopa therapy are frequent and often troublesome. Most are dose-related and limit the dose that can be administered, but are usually reversible. Some are prominent in the beginning of therapy while others appear late.

 

At The Initiation Of Therapy

 

These can be minimized by starting with a low dose.

 

1. Nausea And Vomiting It occurs in almost every patient. Tolerance gradually develops and then the dose can be progressively increased.

 

2. Postural Hypotension It occurs in about 1/3 of patients, but is mostly asymptomatic; some patients experience dizziness, few have fainting attacks; more common in patients receiving antihypertensives. Tolerance develops with continued treatment and BP normalizes.

 

3.  Cardiac Arrhythmias Due to β adrenergic action of peripherally formed DA; more in patients with pre existing heart disease.

 

4. Exacerbation  Of Angina  Due to β adrenergic action of peripherally formed DA; more in patients with pre existing heart disease.

 

5. Alteration In Taste Sensation

 

After Prolonged Therapy

 

1. Abnormal Movements Facial tics, grimacing, tongue thrusting, choreoathetoid movements of limbs start appearing after a few months of use of levodopa at optimum therapeutic dose and progress with time to include practically all patients. No tolerance develops to this adverse effect, but dose reduction decreases severity. Abnormal movements may become as disabling as the original disease itself—are the most important dose-limiting side effects.

 

2. Behavioral Effects Range from mild anxiety, nightmares, etc. to severe depression, mania, hallucinations, mental confusion or frank psychosis. Excessive DA action in the limbic system is probably responsible (antidopaminergic drugs are antipsychotic).

 

3. Fluctuation In Motor Performance After 2–5 years of therapy, the level of control of parkinsonian symptomatology starts showing fluctuation. ‘End of dose’ deterioration (wearing off) which is initially gradual, develops into rapid ‘switches’ or ‘onoff’ effect. With time ‘all or none’ response develops, i.e. the patient is alternately well and disabled. Abnormal movements may jeopardise even the ‘on’ phase. This is probably a reflection of progression of the disorder: with progressive degeneration of DA neurones the ability to regulate storage and release of DA may be largely lost: DA is then synthesized in the striatum on a moment to moment basis resulting in rapid and unpredictable fluctuations in motor control. Dose fractionation and more frequent administration tends to diminish these for a time.

 

Cautious use is needed in elderly; patients with ischaemic heart disease; cerebrovascular, psychiatric, hepatic and renal disease; peptic ulcer; glaucoma, gout.

 

Dose: Start with 0.25 g BD after meals, gradually increase till adequate response is obtained. Usual dose is 2–3 g/ day.

 

LEVOPA,  BIDOPAL  0.5  g  tab.

 

Interactions

 

Pyridoxine: Abolishes therapeutic effect by enhancing peripheral decarboxylation of levodopa; less is available to cross to the brain.

 

Phenothiazines, butyrophenones, metoclopramide reverse therapeutic effect of levodopa by blocking DA receptors. The antidopaminergic domperidone blocks levodopa induced nausea and vomiting without abolishing its antiparkinsonian effect, because domperidone does not cross bloodbrain barrier. Reserpine abolishes levodopa action by preventing entry of DA into synaptic vesicles.

 

Nonselective MAO inhibitors: prevent degradation of peripherally synthesized DA and NA—hypertensive crisis can occur.

 

Antihypertensives: postural hypotension is accentuated, reduce their dose if levodopa is started.

 

Atropine, and other anticholinergic drugs have additive antiparkinsonian action with low doses of levodopa, but retard its absorption— more time is available for peripheral degradation —efficacy of levodopa may be reduced.

 

PERIPHERAL DECARBOXYLASE INHIBITORS

 

Carbidopa and benserazide are extracerebral dopa decarboxylase inhibitors; they do not penetrate bloodbrain barrier and do not inhibit conversion of levodopa to DA in the brain. Administered along with levodopa, they increase its t½ in the periphery and make more of it available to cross bloodbrain barrier to reach its site of action.

 

Benefits of the combination are

 

1.    The plasma t½ of levodopa is prolonged and its dose is reduced to approximately 1/4th.

2.   Systemic concentration of DA is reduced, nausea and vomiting are not prominent— therapeutic doses of levodopa can be attained quickly.

3.   Cardiac complications are minimized.

4.   Pyridoxine reversal of levodopa effect does not occur.

5.   ‘Onoff’ effect is minimized since cerebral DA levels are more sustained.

6.   Degree of improvement may be higher; some patients, not responding adequately to levodopa alone, also improve.

 

Problems not resolved or accentuated are

 

1.   Involuntary movements  

2.   Behavioral abnormalities

3.   Postural hypotension.

 

Currently, levodopa is practically always used along with a decarboxylase inhibitor, except in patients who develop marked involuntary movements with the combination.

 

Combination of levodopa with carbidopa has been given the name ‘Cocareldopa’.

 

BENSPAR, MADOPAR: Benserazide 25 mg + levodopa 100 mg cap.

 

Usual daily maintenance dose of levodopa is 0.4–0.8 g along with 75–100 mg carbidopa or 100–200 mg benserazide, given in 3–4 divided doses. Therapy is started at a low dose and suitable preparations are chosen according to the needs of individual patients, increasing the dose as required.

 

 

DOPAMINERGIC AGONISTS

 

The DA agonists can act on striatal DA receptors even in advanced patients who have largely lost the capacity to synthesize, store and release DA from levodopa. Moreover, they are longer acting, can exert subtype selective activation of DA receptors involved in parkinsonism and not share the concern expressed about levodopa of contributing to dopaminergic neuronal damage by oxidative metabolism.

 

Bromocriptine

 

It is an ergot derivative which acts as potent agonist on D2, but as partial agonist or antagonist on D1 receptors. Improvement in parkinsonian symptoms occurs within ½–1 hr of an oral dose of bromocriptine and lasts for 6–10 hours. If used alone, doses needed in parkinsonism are high, expensive and often produce intolerable side effects—vomiting, hallucinations, hypotension, nasal stuffiness, conjunctival injection. Marked fall in BP with the ‘first dose’ has occurred in some patients, especially those on antihypertensive medication.

 

In parkinsonism, bromocriptine is used only in late cases as a supplement to levodopa: starting with low doses (1.25 mg once at night) and gradually increasing as needed upto 5–10 mg thrice daily. It serves to improve control and smoothen ‘end of dose’ and ‘onoff’ fluctuations. Dyskinesias are less prominent with bromocriptine compared to levodopa.

 

PROCTINAL, SICRIPTIN, PARLODEL, 1.25, 2.5 mg tabs, ENCRIPT 2.5, 5 mg tabs.

 

Ropinirole and Pramipexole

 

These are two recently developed non-ergoline, selective D2/D3 receptor agonists with negligible affinity for D1 and nondopaminergic receptors. Pramipexole has relatively greater affinity for D3 receptors. Therapeutic effect as supplementary drugs to levodopa in advanced cases of PD as well as side effect profile is similar to bromocriptine, but they are better tolerated with fewer g.i. symptoms. Consequently dose titration for maximum improvement can be achieved in 1–2 weeks, while the same may take several months with bromocriptine.

 

Ropinirole and pramipexole are now frequently used as monotherapy for early PD as well. Trials have found them to afford symptom relief comparable to levodopa. Fewer cases treated with ropinirole needed supplemental levodopa than those treated with bromocriptine. The Parkinson Study Group and other multicentric trials have noted lower incidence of dyskinesias and motor fluctuations among patients treated with these drugs than those treated with levodopa. There is some indirect evidence that use of these DA agonists may be associated with slower rate of neuronal degeneration. Such encouraging findings indicate that the newer DA agonists are effective alternatives to levodopa and afford longer symptom free life to PD patients.

 

Ropinirole is rapidly absorbed orally, 40% plasma protein bound, extensively metabolized, mainly by hepatic CYP1A2, to inactive metabolites, and eliminated with a terminal t½ of 6 hrs. It is thus longer acting than levodopa, useful in the management of motor fluctuations and reducing frequency of on off effect.

 

Side effects are nausea, dizziness, hallucinations, and postural hypotension. Episodes of day time sleep have been noted with ropinirole as well as pramipexole. The higher incidence of hallucinations and sleepiness may disfavour their use in the elderly.

 

Ropinirole has recently been approved for use in ‘restless leg syndrome’.

 

Ropinirole: Starting dose is 0.25 mg TDS, titrated to a maximum of 4–8 mg TDS. Early cases generally require 1–2 mg TDS.

 

ROPITOR 0.25, 0.5, 1.0, 2.0 mg tabs. ROPITOR, ROPARK, ROPEWAY 0.25, 0.5, 1.0, 2.0 mg tabs.

 

Pramipexole: Starting dose 0.125 mg TDS, titrate to 0.5–1.5 mg TDS.

 

Pergolide (bromocriptinelike) and Piribedil (apomorphinelike) are other DA agonists used in parkinsonism.

 

MAO-B INHIBITOR

 

Selegiline (Deprenyl)

 

It is a selective and irreversible MAOB inhibitor. Two isoenzyme forms of MAO, termed MAOA and MAOB are recognized; both are present in peripheral adrenergic structures and intestinal mucosa, while the latter predominates in the brain and blood platelets. Unlike nonselective MAO inhibitors, selegiline in low doses (10 mg/day) does not interfere with peripheral metabolism of dietary amines; CA accumulation and hypertensive reaction does not develop, while intracerebral degradation of DA is retarded. This is responsible for the therapeutic effect in parkinsonism. Higher doses can produce hypertensive interactions.

 

Selegiline alone has mild antiparkinsonian action in early cases. Administered with levodopa, it prolongs levodopa action, attenuates motor fluctuations and decreases ‘wearing off’ effect. As an adjuvant to levodopa, it is beneficial in 50–70% patients and permits 20–30% reduction in levodopa dose. However, advanced cases with ‘on-off’ effect are not improved and the peak dose levodopa side effects such as dyskinesias, mental confusion or hallucinations may be worsened. Moreover, clinical benefits derived from selegiline are short lived (6–26 months).

 

Based on the hypothesis that oxidation of DA and/or environmental toxins (MPTP-like) in the striatum by MAO to free radicals was causative in parkinsonism, it was proposed that early therapy with selegiline might delay progression of the disorder. However, no difference in the course of the disease has been detected on follow up of selegiline treated patients in large multicentric studies.

 

Adverse Effects

 

Postural hypotension, nausea, confusion, accentuation of levodopa induced involuntary movements and psychosis.

Contraindicated in patients with convulsive disorders.

 

Selegeline interacts with pethidine causing excitement, rigidity, hyperthermia, respiratory depression. It may also interact with tricyclic antidepressants and selective serotonin reuptake inhibitors.

 

ELDEPRYL 5, 10 mg tab; SELERIN, SELGIN 5 mg tab; Dose: 5 mg with breakfast and with lunch, either alone (in early cases) or with levodopa/carbidopa. Reduce by 1/4th levodopa dose after 2–3 days of adding selegiline.

 

COMT  INHIBITORS

 

Two selective, potent and reversible COMT inhibitors Entacapone and Tolcapone have been introduced as adjuvants to levodopa-carbidopa for advanced PD. When peripheral decarboxylation of levodopa is blocked by carbidopa/benserazide, it is mainly metabolized by COMT to 3Omethyldopa (see Fig. 31.2). Blockade of this pathway by entacapone/tolcapone prolongs the t½ of levodopa and allows a larger fraction of administered dose to cross to brain. Since COMT plays a role in the degradation of DA in brain as well, COMT inhibitors could preserve DA formed in the striatum and supplement the peripheral effect. However, entacapone acts only in the periphery (probably because of short duration of action ~2 hr). For tolcapone also, the central action is less important.

 

Both entacapone and tolcapone enhance and prolong the therapeutic effect of levodopa-carbidopa in advanced and fluctuating PD. They may be used to smoothen ‘wearing off’, increase ‘on’ time, decrease ‘off’ time, improve activities of daily living and allow levodopa dose to be reduced. They are not indicated in early PD cases. Entacapone: 200 mg with each dose of levodopa-carbidopa.

 

Tolcapone:  100–200 mg BD or TDS.

 

Worsening of levodopa adverse effects such as nausea, vomiting, dyskinesia, postural hypotension, hallucinations, etc. occurs often when a COMT inhibitor is added. However, this can be minimised by adjustment of levodopa dose. Other prominent side effect is diarrhoea in 10–18% patients (less with entacapone) and yellow-orange discolouration of urine. Because of reports of acute fatal hepatitis and rhabdomyolysis, tolcapone has been suspended in Europe and Canada, while in USA its use is allowed only in those not responding to entacapone. Entacapone is not hepatotoxic.

 

 

DOPAMINE FACILITATOR

 

Amantadine

 

Developed as an antiviral drug for prophylaxis of influenza A2, it was found serendipitiously to benefit parkinsonism. It acts rapidly but has lower efficacy than levodopa, though higher than anticholinergics. About 2/3rd patients derive some benefit. However, tolerance develops over months and the efficacy is lost. Amantadine appears to act by promoting presynaptic synthesis and release of DA in brain. Action on NMDA type of glutamate receptors, through which the striatal dopaminergic system exerts its influence is now considered to be more important.

 

Amantadine can be used in milder cases, or in short courses to supplement levodopa for advanced cases. In the latter situation, it serves to suppress motor fluctuations and abnormal movements. Fixed dose of 100 mg BD is used (not titrated according to response). Effect of a single dose lasts 8–12 hours;

 

AMANTREL,  COMANTREL  100  mg  tab.

 

Side Effects

 

These are generally not serious: insomnia, dizziness, confusion, nightmares, anticholinergic effects and rarely hallucinations. A characteristic side effect due to local release of CAs resulting in vasoconstriction is livedo reticularis and edema of ankles.

 

Side effects are accentuated when it is combined with anticholinergics.

 

CENTRAL ANTICHOLINERGICS

 

These are drugs having a higher central : peripheral anticholinergic action ratio than atropine, but the pharmacological profile is similar to it. In addition, certain H1 antihistaminics have significant central anticholinergic property. There is little to choose clinically among these drugs, though individual preferences vary.

 

They act by reducing the unbalanced cholinergic activity in the striatum of parkinsonian patients. All anticholinergics produce 10–25% improvement in clinical features, lasting 4–8 hours after a single dose. Generally, tremor is benefited more than rigidity; hypokinesia is affected the least. Sialorrhoea is controlled by their peripheral action. The overall efficacy is much lower than levodopa. However, they are cheap and produce less side effects than levodopa. They may be used alone in mild cases and when levodopa is contraindicated. In others, they can be combined with levodopa in an attempt to lower its dose.

 

Anticholinergics are the only drugs effective in drug (phenothiazine) induced parkinsonism.

 

The side effect profile is similar to atropine. Impairment of memory and organic confusional states are more common in the elderly. The antihistaminics are less efficacious than anticholinergics, but are better tolerated by older patients. Their sedative action also helps. Orphenadrine has mild euphoriant action.

 

Trihexyphenidyl It is the most commonly used drug. Start with the lowest dose in 2–3 divided portions per day and gradually increase till side effects are tolerated.

 

1. Trihexyphenidyl (benzhexol): 2–10 mg/day; PACITANE, PARBENZ 2 mg tab.

 

2. Procyclidine: 5–20 mg/day; KEMADRIN 2.5, 5 mg tab.

 

3. Biperiden:   2–10   mg/day  oral, i.m. or i.v.: DYSKINON 2 mg tab., 5 mg/ml inj.

 

4. Orphenadrine: 100–300 mg/day; DISIPAL, ORPHIPAL 50 mg tab.

 

5. Promethazine: 25–75 mg/day; PHENERGAN 10, 25 mg tab.

 

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