Comparison of central and peripheral pharmacologic effects of biperiden and trihexyphenidyl in human volunteers

[Central anticholinergic, neuroleptic malignant and serotonin syndromes]

Impaired consciousness is a frequent phenomenon after general anesthesia. In addition to the classical causes (e.g., overhang of sedatives), an impairment of consciousness can also be an adverse side effect of drugs. Many drugs used in anesthesia can trigger these symptoms. Alkaloids, such as atropine can trigger a central anticholinergic syndrome, opioids can promote the occurrence of serotonin syndrome and the administration of a neuroleptic can lead to neuroleptic malignant syndrome. These three syndromes are difficult to diagnose due to the individually very heterogeneous symptoms. Mutual symptoms, such as impaired consciousness, tachycardia, hypertension and fever further complicate the differentiation between the syndromes; however, more individual symptoms, such as sweating, muscle tension or bowl sounds can be helpful in distinguishing these syndromes. The time from the trigger event can also help to differentiate the syndromes. The central anticholinergic syndrome is the fastest to appear, usually taking just a few of hours from trigger to clinical signs, serotonin syndrome takes several hours up to 1 day to show and neuroleptic malignant syndrome usually takes days. The clinical symptoms can range from mild to life-threatening. Generally, mild cases are treated with discontinuation of the trigger and extended observation. More severe cases can require specific antidotes. The specific treatment recommended for central anticholinergic syndrome is physostigmine with an initial dose of 2 mg (0.04 mg/kg body weight, BW) administered over 5 min. For serotonin syndrome an initial dose of 12 mg cyproheptadine followed by 2 mg every 2 h is recommended (maximum 32 mg/day or 0.5 mg/kgBW day-1) but this medication is only available in Germany as an oral formulation. For neuroleptic malignant syndrome 25-120 mg dantrolene (1-2.5 mg/kgBW maximum 10 mg/kgBW day-1) is the recommended treatment.

[Central anticholinergic, neuroleptic malignant and serotonin syndromes : Important differential diagnoses in postoperative impairment of consciousness]

Impaired consciousness is a frequent phenomenon after general anesthesia. In addition to the classical causes (e.g., overhang of sedatives), an impairment of consciousness can also be an adverse side effect of drugs. Many drugs used in anesthesia can trigger these symptoms. Alkaloids, such as atropine can trigger a central anticholinergic syndrome, opioids can promote the occurrence of serotonin syndrome and the administration of a neuroleptic can lead to neuroleptic malignant syndrome. These three syndromes are difficult to diagnose due to the individually very heterogeneous symptoms. Mutual symptoms, such as impaired consciousness, tachycardia, hypertension and fever further complicate the differentiation between the syndromes; however, more individual symptoms, such as sweating, muscle tension or bowl sounds can be helpful in distinguishing these syndromes. The time from the trigger event can also help to differentiate the syndromes. The central anticholinergic syndrome is the fastest to appear, usually taking just a few of hours from trigger to clinical signs, serotonin syndrome takes several hours up to 1 day to show and neuroleptic malignant syndrome usually takes days. The clinical symptoms can range from mild to life-threatening. Generally, mild cases are treated with discontinuation of the trigger and extended observation. More severe cases can require specific antidotes. The specific treatment recommended for central anticholinergic syndrome is physostigmine with an initial dose of 2 mg (0.04 mg/kg body weight, BW) administered over 5 min. For serotonin syndrome an initial dose of 12 mg cyproheptadine followed by 2 mg every 2 h is recommended (maximum 32 mg/day or 0.5 mg/kgBW day^-1) but this medication is only available in Germany as an oral formulation. For neuroleptic malignant syndrome 25-120 mg dantrolene (1-2.5 mg/kgBW maximum 10 mg/kgBW day^-1) is the recommended treatment.

Cholinergic models of memory impairment in animals and man: scopolamine vs. biperiden

Scopolamine has been used as a pharmacologic model for cognitive impairments in dementia and Alzheimer's disease. The validity of this model seems to be limited because findings in animals do not readily translate to novel treatments in humans. Biperiden is also a cholinergic deficit model for cognitive impairments but specifically blocks muscarinic M1 receptors. The effects of scopolamine and biperiden (and pirenzepine) are compared in animal studies and related to findings in humans. It is concluded that the effects on cognitive functions are different for scopolamine and biperiden, and they should be considered as different cognitive deficit models. Scopolamine may model more advanced stages of Alzheimer's disease whereas biperiden may model the early deficits in declarative memory in aging and mild cognitive impairment.

Biperiden Challenge Model in Healthy Elderly as Proof-of-Pharmacology Tool: A Randomized, Placebo-Controlled Trial

Selective M1 muscarinic acetylcholine receptor (mAChR) agonists are being developed as symptomatic treatment for neurodegenerative and neuropsychiatric disorders that lead to cognitive dysfunction. Demonstrating cognition-enhancing effects in early-phase clinical development in healthy subjects is difficult. A challenge with the M1 mAChR antagonist biperiden could be used to demonstrate procognitive and pharmacological effects of selective M1 mAChR agonists. The aim of this study was to develop such a model. To this end, 12 healthy elderly subjects participated in a randomized, placebo-controlled, 3-way crossover study investigating tolerability, pharmacokinetic (PK) and pharmacodynamic (PD) effects of 2 and 4 mg biperiden. Repeated PD assessments were performed using neurocognitive tasks and electrophysiological measurements. A population PK-PD model was developed. Four milligrams of biperiden showed significant impairment of sustained attention (-2.1 percentage point in adaptive tracking [95%CI, -3.043 to -1.148], verbal memory (2-3 fewer words recalled [95%CI, -5.9 to -0.2]) and working memory (up to a 50-millisecond increase in the n-back task reaction time [95%CI, 21.854-77.882]) compared with placebo. The PK data were best fitted by a 2-compartment model and showed high interoccasion and intersubject variability. Population PK-PD analysis quantified significant concentration-effect relationships for the n-back reaction time, n-back accuracy, and adaptive tracking. In conclusion, biperiden caused M1 mAChR-related dose- and concentration-dependent temporary declines in cognitive functioning. Therefore a biperiden pharmacological challenge model can be used for proof-of-pharmacology studies and to demonstrate cognition-enhancing effects of new cholinergic compounds that are being developed.

Cholinergic and dopaminergic effects on prediction error and uncertainty responses during sensory associative learning

Navigating the physical world requires learning probabilistic associations between sensory events and their change in time (volatility). Bayesian accounts of this learning process rest on hierarchical prediction errors (PEs) that are weighted by estimates of uncertainty (or its inverse, precision). In a previous fMRI study we found that low-level precision-weighted PEs about visual outcomes (that update beliefs about associations) activated the putative dopaminergic midbrain; by contrast, precision-weighted PEs about cue-outcome associations (that update beliefs about volatility) activated the cholinergic basal forebrain. These findings suggested selective dopaminergic and cholinergic influences on precision-weighted PEs at different hierarchical levels. Here, we tested this hypothesis, repeating our fMRI study under pharmacological manipulations in healthy participants. Specifically, we performed two pharmacological fMRI studies with a between-subject double-blind placebo-controlled design: study 1 used antagonists of dopaminergic (amisulpride) and muscarinic (biperiden) receptors, study 2 used enhancing drugs of dopaminergic (levodopa) and cholinergic (galantamine) modulation. Pooled across all pharmacological conditions of study 1 and study 2, respectively, we found that low-level precision-weighted PEs activated the midbrain and high-level precision-weighted PEs the basal forebrain as in our previous study. However, we found pharmacological effects on brain activity associated with these computational quantities only when splitting the precision-weighted PEs into their PE and precision components: in a brainstem region putatively containing cholinergic (pedunculopontine and laterodorsal tegmental) nuclei, biperiden (compared to placebo) enhanced low-level PE responses and attenuated high-level PE activity, while amisulpride reduced high-level PE responses. Additionally, in the putative dopaminergic midbrain, galantamine compared to placebo enhanced low-level PE responses (in a body-weight dependent manner) and amisulpride enhanced high-level precision activity. Task behaviour was not affected by any of the drugs. These results do not support our hypothesis of a clear-cut dichotomy between different hierarchical inference levels and neurotransmitter systems, but suggest a more complex interaction between these neuromodulatory systems and hierarchical Bayesian quantities. However, our present results may have been affected by confounds inherent to pharmacological fMRI. We discuss these confounds and outline improved experimental tests for the future.

DARK Classics in Chemical Neuroscience: Atropine, Scopolamine, and Other Anticholinergic Deliriant Hallucinogens

Anticholinergic drugs based on tropane alkaloids, including atropine, scopolamine, and hyoscyamine, have been used for various medicinal and toxic purposes for millennia. These drugs are competitive antagonists of acetylcholine muscarinic (M-) receptors that potently modulate the central nervous system (CNS). Currently used clinically to treat vomiting, nausea, and bradycardia, as well as alongside other anesthetics to avoid vagal inhibition, these drugs also evoke potent psychotropic effects, including characteristic delirium-like states with hallucinations, altered mood, and cognitive deficits. Given the growing clinical importance of anti-M deliriant hallucinogens, here we discuss their use and abuse, clinical importance, and the growing value in preclinical (experimental) animal models relevant to modeling CNS functions and dysfunctions.

Trihexyphenidyl rescues the deficit in dopamine neurotransmission in a mouse model of DYT1 dystonia

Trihexyphenidyl, a nonselective muscarinic receptor antagonist, is the small molecule drug of choice for the treatment of DYT1 dystonia, but it is poorly tolerated due to significant side effects. A better understanding of the mechanism of action of trihexyphenidyl is needed for the development of improved treatments. Because DTY1 dystonia is associated with both abnormal cholinergic neurotransmission and abnormal dopamine regulation, we tested the hypothesis that trihexyphenidyl normalizes striatal dopamine release in a mouse model of DYT1 dystonia using ex vivo fast scan cyclic voltammetry and in vivo microdialysis. Trihexyphenidyl increased striatal dopamine release and efflux as assessed by ex vivo voltammetry and in vivo microdialysis respectively. In contrast, ʟ-DOPA, which is not usually effective for the treatment of DYT1 dystonia, did not increase dopamine release in either Dyt1 or control mice. Trihexyphenidyl was less effective at enhancing dopamine release in Dyt1 mice relative to controls ex vivo (mean increase WT: 65% vs Dyt1: 35%). Trihexyphenidyl required nicotinic receptors but not glutamate receptors to increase dopamine release. Dyt1 mice were more sensitive to the dopamine release decreasing effects of nicotinic acetylcholine receptor antagonism (IC₅₀: WT = 29.46 nM, Dyt1 = 12.26 nM) and less sensitive to acetylcholinesterase inhibitors suggesting that nicotinic acetylcholine receptor neurotransmission is altered in Dyt1 mice, that nicotinic receptors indirectly mediate the differential effects of trihexyphenidyl in Dyt1 mice, and that nicotinic receptors may be suitable therapeutic targets for DYT1 dystonia.

Frequency and correlates of anticholinergic use among patients with schizophrenia in Denmark: A Nation-wide pharmacoepidemiological study

Anticholinergic medications are used to treat extrapyramidal adverse effects induced by antipsychotics. Anticholinergics are associated with adverse effects: constipation, dry mouth and worsening of cognitive function. Anticholinergics have potential for abuse and are not recommended for long term-treatment. We aimed to investigate the use of anticholinergics in patients with schizophrenia. The national health registers in Denmark were used to examine: The prevalence of anticholinergics in 1996-2012 using a cross-sectional design; geographic variations in the prescription of anticholinergics in 2012; correlates of treatment with anticholinergics. The proportion of patients using anticholinergics decreased significantly from 11.7% in 1996 to 5.7% in 2012. The prescription pattern varied considerably between national regions in 2012, ranging from 4.0% in the Capital Region to 8.1% in the Northern Denmark Region. Long-term use of anticholinergics was predicted by older age, age at debut of schizophrenia, receiving early retirement pension, typical antipsychotic use, antipsychotic polypharmacy, typical + atypical antipsychotics, antidepressant treatment, high doses of antipsychotics measured in defined-daily-dose, physical comorbidity and psychiatrists` greater caseload. Use of anticholinergics declined during the study period, and showed substantial variation across the regions in 2012. Long-term use was linked to typical antipsychotic use and variables that are associated with greater illness severity.

Pharmacological management of anticholinergic delirium - theory, evidence and practice

The spectrum of anticholinergic delirium is a common complication following drug overdose. Patients with severe toxicity can have significant distress and behavioural problems that often require pharmacological management. Cholinesterase inhibitors, such as physostigmine, are effective but widespread use has been limited by concerns about safety, optimal dosing and variable supply. Case series support efficacy in reversal of anticholinergic delirium. However doses vary widely and higher doses commonly lead to cholinergic toxicity. Seizures are reported in up to 2.5% of patients and occasional cardiotoxic effects are also recorded. This article reviews the serendipitous path whereby physostigmine evolved into the preferred anticholinesterase antidote largely without any research to indicate the optimal dosing strategy. Adverse events observed in case series should be considered in the context of pharmacokinetic/pharmacodynamic studies of physostigmine which suggest a much longer latency before the maximal increase in brain acetylcholine than had been previously assumed. This would favour protocols that use lower doses and longer re-dosing intervals. We propose based on the evidence reviewed that the use of cholinesterase inhibitors should be considered in anticholinergic delirium that has not responded to non-pharmacological delirium management. The optimal risk/benefit would be with a titrated dose of 0.5 to 1 mg physostigmine (0.01-0.02 mg kg(-1) in children) with a minimum delay of 10-15 min before re-dosing. Slower onset and longer acting agents such as rivastigmine would also be logical but more research is needed to guide the appropriate dose in this setting.

Innovative mechanisms of action for pharmaceutical cognitive enhancement: A systematic review

Pharmacological cognitive enhancement refers to improvement in cognitive functions after drug use in healthy individuals. This popular topic attracts attention both from the general public and the scientific community. The objective was to explore innovative mechanisms of psychostimulant's action, whose potential effectiveness was assessed in randomized placebo-controlled trials (RCTs). A systematic review was carried out, using the words "attention", "memory", "learning", "executive functions", and "vigilance/wakefulness" combined to "cognitive enhancer" or "smart drug". Methylphenidate, amphetamines, modafinil, nicotine, acetylcholine esterase inhibitors and antidepressants were extensively studied in previous meta-analyses and were not included in the present work. Drugs were classified according to their primary mode of action, namely catecholaminergic drugs (tolcapone, pramipexole, guanfacine), cholinergic drugs (anticholinergics), glutamatergic drugs (ampakines), histaminergic drugs, and non-specified (glucocorticoids). Overall, 50 RCTs were included in the present review. In conclusion, a number of new active drugs were found to improve some cognitive functions, in particular verbal episodic memory. However the number of RCTs was limited, and most of the studies found negative results. Future studies should assess both effectiveness and tolerance of repeated doses administration, and individual variability in dose response (including baseline characteristics and potential genetic polymorphisms). One explanation for the limited number of recent RCTs with new psychostimulants seems to be the ethical debate surrounding pharmaceutical cognitive enhancement in healthy subjects.

Hippocampus and basal forebrain volumes modulate effects of anticholinergic treatment on delayed recall in healthy older adults

Introduction

Volumes of hippocampus and cholinergic basal forebrain are associated with delayed recall performance and may modulate the effect of a muscarinic receptor antagonist on delayed recall in healthy volunteers.

Methods

We studied 15 older adults before and after the oral administration of a single dose of 1 or 2 mg of the preferential M1 muscarinic receptor antagonist trihexyphenidyl (Artane™) or placebo in a double-blind randomized cross-over design. Hippocampus and basal forebrain volumes were measured using magnetic resonance imaging.

Results

We found a significant interaction between treatment and hippocampus volume and a trend level effect between treatment and anterior basal forebrain volume on task performance, with an attenuation of the association between volume size and performance with trihexyphenidyl.

Discussion

These findings suggest a reduction of delayed recall performance with increasing doses of the muscarinic antagonist that is related to an uncoupling of the association of task performance with cholinergic basal forebrain and hippocampus volumes.

Benefits and limits of anticholinergic use in schizophrenia: focusing on its effect on cognitive function

All currently available antipsychotic drugs are the dopamine D2 receptor antagonists and are capable of producing extrapyramidal side-effects (EPS). Anticholinergic drugs are primarily used to treat EPS or prevent EPS induced by antipsychotics in the treatment of psychosis and schizophrenia. However, they can cause a variety of distressing peripheral side-effects (e.g. dry mouth, urinary disturbances, and constipation) and central adverse effects (e.g. cognitive impairment, worsening of tardive dyskinesia, and delirium). Disturbances in cognitive abilities are cardinal features of schizophrenia from its earliest phases and account for much of the functional disability associated with the illness. It is likely that long-term concomitant administration of anticholinergics exacerbates the underlying cognitive impairment in patients with schizophrenia and subsequently affects patients' quality of life. Thus, current treatment guidelines for schizophrenia generally do not recommend the prophylactic and long-term use of anticholinergics. However, the high use of long-term anticholinergic drugs with antipsychotics has been identified as an important issue in the treatment of schizophrenia in several countries. To assess the benefits and limits of anticholinergic use in psychosis and schizophrenia, this article will provide a brief review of the pharmacology and clinical profiles of anticholinergic drugs and will focus on their effects on cognitive function in schizophrenia, particularly during the course of the early phase of the illness. In addition, we will address the effects of discontinuation of anticholinergics on cognitive function in patients with schizophrenia and provide a strategy for adjunctive anticholinergic use in patients treated with long-acting injectable antipsychotics.

Human electrophysiological correlates of learned irrelevance: effects of the muscarinic M1 antagonist biperiden

Learned irrelevance (LIrr) refers to a reduction in associative learning after pre-exposure of the conditioned and unconditioned stimulus in a non-contingent fashion. This paradigm might serve as a translational model for (pre)attentive information processing deficits in schizophrenia. This is the first study to investigate the event-related potentials (ERPs) of a within-subject LIrr paradigm in humans. Furthermore, the effects of the muscarinic M1 antagonist biperiden on LIrr were assessed. As expected, LIrr was found to be intact in young healthy volunteers after placebo. Furthermore, in the placebo condition P3b latency was decreased for target stimuli, which were pre-cued. This suggests that the predictability of the occurrence of these stimuli is mainly reflected by this ERP component. Biperiden had no effect on the behavioural LIrr measures, although prolonged reaction times were evident. Biperiden increased the N1 amplitude of the pre-exposed predictor letters, suggesting an effect of this drug on early perceptual processing. In conclusion, the within-subject paradigm used in the current study in combination with electroencephalography can reveal brain mechanisms involved in LIrr. M1 antagonism did not affect LIrr performance but seemed to influence early information processing.

Modulation of memory and visuospatial processes by biperiden and rivastigmine in elderly healthy subjects

RATIONALE

The central cholinergic system is implicated in cognitive functioning. The dysfunction of this system is expressed in many diseases like Alzheimer's disease, dementia of Lewy body, Parkinson's disease and vascular dementia. In recent animal studies, it was found that selective cholinergic modulation affects visuospatial processes even more than memory function.

OBJECTIVE

In the current study, we tried to replicate those findings. In order to investigate the acute effects of cholinergic drugs on memory and visuospatial functions, a selective anticholinergic drug, biperiden, was compared to a selective acetylcholinesterase-inhibiting drug, rivastigmine, in healthy elderly subjects.

METHODS

A double-blind, placebo-controlled, randomised, cross-over study was performed in 16 healthy, elderly volunteers (eight men, eight women; mean age 66.1, SD 4.46 years). All subjects received biperiden (2 mg), rivastigmine (3 mg) and placebo with an interval of 7 days between them. Testing took place 1 h after drug intake (which was around Tmax for both drugs). Subjects were presented with tests for episodic memory (wordlist and picture memory), working memory tasks (N-back, symbol recall) and motor learning (maze task, pursuit rotor). Visuospatial abilities were assessed by tests with high visual scanning components (tangled lines and Symbol Digit Substitution Test).

RESULTS

Episodic memory was impaired by biperiden. Rivastigmine impaired recognition parts of the episodic memory performance. Working memory was non-significantly impaired by biperiden and not affected by rivastigmine. Motor learning as well as visuospatial processes were impaired by biperiden and improved by rivastigmine.

CONCLUSIONS

These results implicate acetylcholine as a modulator not only of memory but also of visuospatial abilities.

Trihexyphenidyl (Artane): a Brazilian study of its abuse

In Brazil, the medicinal misuse of trihexyphenidyl (Artane) has been observed among several segments of society. The present study was conducted in the city of São Paulo during 2002 to characterize this abuse. A sample of 21 users and 16 ex-users was interviewed using qualitative methodology; the subjects were single, unemployed, male polydrug users, who used trihexyphenidyl in order to attain states of mental alterations, mainly hallucinations and deliriums. Trihexyphenidyl is consumed in association with alcohol, other licit drugs (benzodiazepines), or illicit drugs, impairing cognitive functions such as memory, attention, and learning, intervening with some activities of users' daily life.

Increased anticholinergic challenge-induced memory impairment associated with the APOE-epsilon4 allele in the elderly: a controlled pilot study

The degree to which elderly adults experience cognitive impairments from centrally acting anticholinergic drugs is variable, but the cause of this variability is unknown. The present study examined the epsilon4 allele as a possible modulator of the effects of trihexyphenidyl hydrochloride (Artane( trade mark )), an anticholinergic drug, on memory functioning. Of the 24 cognitively intact, elderly participants (age range 62-76), 12 who possessed the epsilon4 allele, participated in a double-blind, randomized, placebo-controlled, crossover, three-way study. All participants were tested after receiving a single oral dose of trihexyphenidyl (1 or 2 mg) or placebo, with a 7-day washout period between sessions. Memory and psychomotor tests were administered at baseline, and at 1, 2.5, and 5 h post-treatment. Results showed that participants with the epsilon4 allele demonstrated significant impairments in delayed recall after both 1 and 2 mg doses of trihexyphenidyl while the non-epsilon4 group did not. Additionally, while acute administration of the 2 mg dose significantly impaired total recall in both epsilon4 and non-epsilon4 carriers, the epsilon4 carriers showed a more persistent impairment. These findings held when participants with the epsilon2 allele were excluded from the analyses. The epsilon4 groups did not differ with respect to psychomotor performance or plasma drug levels. These results provide evidence suggesting that the epsilon4 allele plays a significant role in increasing cognitive sensitivity to trihexyphenidyl and that a temporal component of memory consolidation may be especially vulnerable. A larger study is warranted to confirm these preliminary findings.

Effects of anticholinergics on serial-probe recognition accuracy of rhesus macaques (Macaca mulatta)

Potential deleterious behavioral effects of the anticholinergics biperiden and scopolamine were examined via the performance of rhesus monkeys on a serial-probe recognition (SPR) procedure. On each trial, six unique stimuli (list items) were presented sequentially followed by a choice phase. In the choice phase, two stimuli were presented, a standard or 'default' stimulus (a white rectangle) and a 'probe' stimulus that differed with each choice trial. Choosing the probe stimulus was considered correct if the probe matched one of the list items; otherwise, choosing the default stimulus was considered correct. Behavior was examined under a range of doses of biperiden (0.001-1.0 mg/kg) and scopolamine (0.0056-0.03 mg/kg). Scopolamine (0.01-0.03 mg/kg) and biperiden (0.3-1.0 mg/kg) reduced overall accuracy. At the highest dose, scopolamine, but not biperiden, reduced the number of trials completed per session. The results suggest that doses of scopolamine and biperiden necessary to prevent or eliminate organophosphate induced seizures may affect performance adversely. However, because the degree of impairment from biperiden was modest, further examination of this anticonvulsant may be warranted.

Clinical pharmacokinetic and pharmacodynamic properties of drugs used in the treatment of Parkinson's disease

Current research in Parkinson's disease (PD) focuses on symptomatic therapy and neuroprotective interventions. Drugs that have been used for symptomatic therapy are levodopa, usually combined with a peripheral decarboxylase inhibitor, synthetic dopamine receptor agonists, centrally-acting antimuscarinic drugs, amantadine, monoamine oxidase-B (MAO-B) inhibitors and catechol-O-methyltransferase (COMT) inhibitors. Drugs for which there is at least some evidence for neuroprotective effect are certain dopamine agonists, amantadine and MAO-B inhibitors (selegiline). Levodopa remains the most effective drug for the treatment of PD. Several factors contribute to the complex clinical pharmacokinetics of levodopa: erratic absorption, short half-life, peripheral O-methylation and facilitated transport across the blood-brain barrier. In patients with response fluctuations to levodopa, the concentration-effect curve becomes steeper and shifts to the right compared with patients with stable response. Pharmacokinetic-pharmacodynamic modelling can affect decisions regarding therapeutic strategies. The dopamine agonists include ergot derivatives (bromocriptine, pergolide, lisuride and cabergoline), non-ergoline derivatives (pramipexole, ropinirole and piribedil) and apomorphine. Most dopamine agonists have their specific pharmacological profile. They are used in monotherapy and as an adjunct to levodopa in early and advanced PD. Few pharmacokinetic and pharmacodynamic data are available regarding centrally acting antimuscarinic drugs. They are characterised by rapid absorption after oral intake, large volume of distribution and low clearance relative to hepatic blood flow, with extensive metabolism. The mechanism of action of amantadine remains elusive. It is well absorbed and widely distributed. Since elimination is primarily by renal clearance, accumulation of the drug can occur in patients with renal dysfunction and dosage reduction must be envisaged. The COMT inhibitors entacapone and tolcapone dose-dependently inhibit the formation of the major metabolite of levodopa, 3-O-methyldopa, and improve the bioavailability and reduce the clearance of levodopa without significantly affecting its absorption. They are useful adjuncts to levodopa in patients with end-of-dose fluctuations. The MAO-B inhibitor selegiline may have a dual effect: reducing the catabolism of dopamine and limiting the formation of neurotoxic free radicals. The pharmacokinetics of selegiline are highly variable; it has low bioavailability and large volume of distribution. The oral clearance is many-fold higher than the hepatic blood flow and the drug is extensively metabolised into several metabolites, some of them being active. Despite the introduction of several new drugs to the antiparkinsonian armamentarium, no single best treatment exists for an individual patient with PD. Particularly in the advanced stage of the disease, treatment should be individually tailored.