Hepatotoxic profile of catechol-O-methyltransferase inhibitors in Parkinson's disease

Entacapone and tolcapone are selective catechol-O-methyltransferase (COMT) inhibitors developed recently as adjuncts to levodopa for the treatment of Parkinson's disease (PD). They extend the duration of action of levodopa. As a result, they increase 'on' time, decrease 'off' time and improve motor scores in patients with motor fluctuations. Both benefits and main side effects are related to increased dopaminergic activity. This paper reviews the use of those COMT inhibitors in PD with particular focus on the issue of hepatotoxicity. Neither tolcapone nor entacapone caused hepatotoxicity in preclinical studies. However, in 1998, four patients who were using tolcapone presented with serious liver dysfunction; three of them died due to acute liver failure. Tolcapone is now known to have the potential to cause hepatotoxicity in clinical use and experimental studies. It is now recommended that tolcapone be administered only in patients with motor fluctuations who are no longer satisfactorily treated with other medications for PD. Routine liver monitoring is now mandatory with this agent. Entacapone has been described as a well-tolerated and safe drug in recent experimental studies, human clinical trials and postmarketing surveillance. It can be offered to any patient with motor fluctuations and routine liver monitoring is not required.

Optimizing levodopa pharmacokinetics: intestinal infusion versus oral sustained-release tablets

Continuous duodenal infusion of carbidopa/levodopa has been shown to control motor fluctuations in advanced Parkinson's disease (PD). The authors compared the pharmacokinetics of levodopa and 3-O-methyldopa in patients with advanced PD after administration of an oral sustained-release levodopa preparation and after continuous intestinal levodopa infusion with a new formulation as a gel suspension. A randomized crossover trial was carried out in 12 patients. Carbidopa/levodopa was administered as an oral sustained-release tablet and by nasoduodenal continuous infusion for 3-week periods for each treatment. Plasma levodopa concentrations and motor performance were evaluated every 30 minutes during 3 test days of each treatment period. The average intraindividual coefficient of variation for the plasma levodopa concentrations after oral therapy was 34% and was significantly lower (14%, p < 0.01) during continuous infusion. Hourly video evaluations showed a significant increase in ON time during infusion and a significant decrease in OFF time and dyskinesia. Continuous intraduodenal delivery of a new carbidopa/levodopa formulation offers a means for markedly improved control of motor fluctuations in late stages of PD.

Non-oral drug delivery in Parkinson's disease: a summary from the symposium at the 7th International Congress of Parkinson's Disease and Movement Disorders. 10-14 November 2002, Miami, FL, USA

This symposium reviewed the issues of non-oral therapy in the late stage Parkinson's disease (PD). The accepted standard treatment of PD is oral levodopa or oral dopamine agonists. However, the long-term complications and limitations of this treatment might be improved by changing therapy from the present pulsatile stimulation to a more constant stimulation of central dopamine receptors. Stimulation of these receptors may be possible with non-oral drug delivery treatments. Many of these non-oral options have been evaluated during the last few decades to find a more continuous drug delivery. The non-oral treatment options include invasive measures such as intraduodenal levodopa, subcutaneous apomorphin and most recently, the non-invasive transdermal (patch) delivery system, with the novel dopamine agonist rotigotine (Aderis Pharmaceuticals Inc.). The benefits of the non-oral, more continuous dopaminergic treatment of PD needs to be demonstrated in clinical trials and long-term clinical practice, before they can be considered as potential replacements of the standard oral therapy.

Comparative single- and multiple-dose pharmacokinetics of levodopa and 3-O-methyldopa following a new dual-release and a conventional slow-release formulation of levodopa and benserazide in healthy volunteers

The objective was to assess the single- and multiple-dose pharmacokinetics of levodopa and 3-O-methyldopa following administration of a new dual-release and conventional slow-release formulation of levodopa/benserazide in the dose ratio of 4:1. In an open-label, two-way cross-over study, 20 healthy volunteers were randomized to receive first either Madopar DR or Madopar HBS for 8 days. Then they crossed over to the other formulation. A first dose of 200 mg levodopa and 50 mg benserazide ('250' mg) was given on day 1, '125' mg t.i.d. on the subsequent 6 days (days 2-7), followed by '250' mg on day 8. The two treatment periods of 8 days were separated by a wash-out period of at least 7 days. Blood samples were taken at specific times over a 12-hour period (day 1) or a 36-hour period (day 8). Plasma concentrations of levodopa and 3-O-methyldopa were measured by high-performance liquid chromatography for pharmacokinetic evaluation. The pharmacokinetics of levodopa after a single-dose administration (day 1) of Madopar DR and Madopar HBS were significantly different as reflected by the respective mean values of maximum plasma concentration (C(max) 1.99 vs. 0.82 mg x l-1), time to reach maximum concentration (t(max) 0.7 vs. 2.6 h) and area under the plasma concentration-time curve (AUC(0- infinity ) 4.52 vs. 3.18 mg x h x l-1). The respective values after multiple doses (day 8) were: C(max) 1.98 vs. 0.93 mg x l-1, t(max) 0.7 vs. 2.3 h and AUC(0-infinity ) 4.84 vs. 3.96 mg x h x l-1. The relative bioavailability (Madopar DR vs. Madopar HBS) was 1.73 on day 1 and 1.32 on day 8. Bioequivalence could not be demonstrated for log-transformed data of AUC and C(max) within a predefined range of 80-125 and 70-143%, respectively. In conclusion, the observed differences in C(max), t(max) and AUC are consistent with a faster rate and higher extent of levodopa absorption after administration of Madopar DR. Statistical evaluation of these kinetic data showed that Madopar DR is not bioequivalent to Madopar HBS.

Novel levodopa gastroretentive dosage form: in-vivo evaluation in dogs

Due to its narrow absorption window, levodopa has to be administered continuously to the upper parts of the intestine in order to maintain sustained therapeutic levels. This may be achieved by a controlled release (CR) gastroretentive dosage form (GRDF). The aim of this work was to develop a novel GRDF, based on unfolding polymeric membranes, that combines extended dimensions with high rigidity, and to examine the pharmacokinetics of levodopa compounded in the GRDF. Levodopa CR-GRDFs were administered to beagle dogs pretreated with carbidopa. The CR-GRDF location in the gastrointestinal tract was determined by X-ray, and serial blood samples were collected and assayed for levodopa. Optimization of the pharmacokinetic profile of levodopa from the CR-GRDFs was carried out based on the in-vitro in-vivo correlation following modifications of the release rates (adjusted by various membrane thicknesses) and drug loads. The successful CR-GRDF maintained therapeutic levodopa concentrations (>500 ng ml(-1)) over 9 h. In comparison to non-gastroretentive CR-particles and oral solution, mean absorption time was significantly extended. These outcomes demonstrate that the CR-GRDF may be used to improve levodopa therapy and can be applied to extend the absorption of other narrow absorption window drugs that require continuous input.

[Apomorphine hyperpigmentation]

A 72 year old bedridden, disoriented man presented with a continuously increasing number of blue nodules on his abdomen and both thighs. In addition, he had a melanoma on his left forearm (SSM, Clark level III, Breslow 0.75 mm), which lead to the clinical diagnosis of melanoma metastases. Biopsy of one of the blue nodules showed CD68 positive histiocytic cells loaded with brownish pigment granules and a lymphocytic infiltrate within the deep dermis and upper subcutis. The pigment reacted histochemically similarly to melanin. Melanocytes were absent at these sites. Because of the unexplained clinical and histopathological picture, the patient's history was reassessed and it was learned that the patient had received subcutaneous infusions of apomorphine for the past 10 years for the treatment of Parkinson's disease. By oxidation, apomorphine may be converted into tetrahydroisoquinoline-melanin, which apparently is the cause for the accumulation of pigment within the deep dermis.

Effects of food on the pharmacokinetics of levodopa in a dual-release formulation

The objective was to assess the effect of food on the pharmacokinetics of levodopa and 3-O-methyldopa after administration of a new levodopa/benserazide formulation with a dual-release drug delivery profile (Madopar DR). In an open-label, two-way cross-over study, 19 healthy volunteers who had fasted overnight were randomized to receive a single oral dose of levodopa/benserazide (200/50 mg) in the absence or presence of a standardized, high-fat breakfast, administered 30 min before drug administration. The treatment periods (fasting, non-fasting) were preceded by a baseline regimen of levodopa/benserazide (100/25 mg t.i.d. for 6 or 7 days). Blood samples were taken at specific times over a 12-hour period. Plasma concentrations of levodopa and 3-O-methyldopa were determined by high-performance liquid chromatography for pharmacokinetic evaluation. The parameter C(max) of levodopa was significantly lower and t(max) longer under postprandial conditions than under fasting conditions (mean C(max) 1.41 vs. 2.09 mg l(-1); mean t(max) 3.1 vs. 1.0 h). With food, the area under the curve (AUC) of levodopa was equivalent to that following an overnight fast. Compared with volunteers who had fasted, food did not alter t(1/2). Estimates of C(max), t(max) and AUC of 3-O-methyldopa under non-fasting conditions were not significantly different from those under fasting conditions. In conclusion, food decreases the rate of levodopa absorption, but had no effect on the systemic exposure to levodopa and the degree of 3-O-methyldopa formation. Standardization of levodopa/benserazide administration with respect to meal times is recommended.

Iontophoretic delivery of apomorphine: from in-vitro modelling to the Parkinson patient

Apomorphine is a mixed dopamine D1/D2 receptor agonist which is potentially useful in the treatment of Parkinson's disease. The delivery of apomorphine is however complicated because it is not absorbed orally and other delivery routes with the exception of the intravenous route seem to fail. The most interesting route for controlled delivery of apomorphine is transdermal iontophoresis because this could enable the Parkinson patient to directly control the needed amount of apomorphine by increasing or decreasing the drug input in order to achieve optimal drug therapy ('on-demand') with a minimum of toxic side effects. The typical features of Parkinson's disease could be used to monitor the needed drug input and even more elegantly by means of suitable chip sensors which are able to directly measure bradykinesia, akinesia and/or tremor and to regulate in such a way the drug input. Such a chip-controlled iontophoretic system would be the first closed-loop system monitoring not pharmacokinetic data (blood levels) but more importantly externally measurable pharmacodynamic effects of Parkinson's disease. This scenario is more feasible as skin irritation and toxicity studies have proven that iontophoresis is a safe route of treatment. This review describes the basics of iontophoresis and the development of a transdermal iontophoretic delivery system on the basis of integrated pharmacokinetic/pharmacodynamic (PK/PD) investigations in patients with idiopathic Parkinson's disease. Transdermal iontophoretic transport of apomorphine was studied both in vitro with human stratum corneum using a newly developed iontophoretic continuous flow-through transport cell and in vivo in a first exploratory study in patients with Parkinson's disease. These studies showed that the delivery of apomorphine is feasible and furthermore the rate of delivery can be controlled by variation of the current densities. Additionally the pretreatment of the skin either with a mono-surfactant or a vesicular suspension of elastic liquid-state vesicles may be useful to further increase the apomorphine flux across the skin in combination with iontophoresis.

Study of the biodistribution of the amantadine labelled with technetium-99m in Wistar female rats

Amantadine (AMA) has been described as dopamine stimulant and norepineprhine release, capable to block the N-methyl-D aspartate (NMDA) glutamatergic and nicotinic receptors, enhancing the sexual behavior of the male rats and inducing hypersexuality in humans. The use of technetium-99m (99mTc) can be justified for its physical and chemical properties. The aim of this study was to label and evaluate the bioavailability of the AMA labelled with 99mTc (99mTc-AMA) in Wistar female rats. The solution of 99mTc-AMA was administered by intraperitoneal way and the animals were sacrificed in CO2 chamber 10 min after the administration of the radiotracer. Various organs were removed, weighted, their radioactivity was determined using an auto-gamma counter and the results were expressed as the percentage of the injected activity per gram of tissue (%ATI/g). In the control group only Na99mTcO4 was administered. The analysis of results shows that the highest uptakes 99mTc-AMA treated group were: ovary (7.11 +/- 1.43), spleen (3.54 +/- 1.05), thyroid (2.67 +/- 0.15), stomach (1.56 +/- 1.10), duodenum (0.87 +/- 0.52), muscular tissue (0.57 +/- 0.06), liver (0.52 +/- 0.25), and at control group: thyroid (16.45 +/- 2.57), ovary (1.28 +/- 0.12), liver (1.10 +/- 0.04), spleen (0.57 +/- 0.07) and muscular tissue (0.26 +/- 0.03). The results obtained suggest that 99mTc-AMA may be used to study the bioavailability of amantadine and evaluate its effect in sexual behavior in female rats.

Cabergoline plasma concentration is increased during concomitant treatment with itraconazole

We report on 2 patients with idiopathic Parkinson's disease who experienced marked improvement in symptoms following the addition of itraconazole to current cabergoline treatment. Plasma levels of cabergoline were analyzed in one of the patients and increased to approximately 300% during treatment with itraconazole, which paralleled major clinical improvement.

Entacapone improves the availability of L-dopa in plasma by decreasing its peripheral metabolism independent of L-dopa/carbidopa dose

AIMS

Entacapone is a peripherally acting catechol-O-methyltransferase (COMT) inhibitor. To improve the benefits of oral L-dopa in the treatment of Parkinson's disease (PD), entacapone is administered as a 200 mg dose with each daily dose of L-dopa. This study evaluated the effects of entacapone 200 mg on the pharmacokinetics and metabolism of L-dopa given as standard release L-dopa/carbidopa.

METHODS

Six different doses of l-dopa/carbidopa were investigated in this placebo-controlled, double-blind (regarding entacapone), randomized, single-dose study in 46 young healthy males. The subjects were divided into three groups (n = 14-16). Two different L-dopa/carbidopa doses were administered to each subject (50/12.5 mg and 150/37.5 mg, or 100/10 mg and 100/25 mg, or 200/50 mg and 250/25 mg). Each dose was given on two occasions; simultaneously with entacapone or with placebo, in random order, on two consecutive study visits, separated by a washout period of at least 3 weeks (four-way crossover design). Serial blood samples were drawn before dosing and up to 24 h after the dose and pharmacokinetic parameters of L-dopa, its metabolites, carbidopa, and entacapone were determined.

RESULTS

Entacapone increased the AUC(0,12 h) of L-dopa to a similar extent at all doses of L-dopa/carbidopa, that is by about 30-40% compared with placebo (P < 0.001, 95% CI 0.15, 0.40). When evaluated as the ratio of geometric means, entacapone slightly decreased the mean C(max) values for L-dopa at all L-dopa/carbidopa doses compared with placebo. When given with entacapone, higher plasma concentrations of L-dopa were maintained for a longer period at all doses of L-dopa/carbidopa. Entacapone also decreased the peripheral formation of 3-O-methyldopa (3-OMD) to about 55-60% of the placebo treatment level (P < 0.001, 95% CI -0.72, -0.35) and increased the mean AUC(0,12 h) of 3,4-dihydroxy-phenylacetic acid (DOPAC) 2-2.6-fold compared with placebo (P < 0.001, 95% CI 0.60, 1.10). The mean AUC(0,12 h) of 3-methoxy-4-hydroxy-phenylacetic acid (HVA) following entacapone was approximately 65-75% of that observed with placebo (P < 0.001-0.05, 95% CI -0.76, -0.01) at each L-dopa/carbidopa dose except the 50/12.5 mg dose (P > 0.05, 95% CI -0.59, 0.05). The metabolic ratios (MR, AUC metabolite/AUC L-dopa) also confirmed that entacapone significantly decreased the proportion of 3-OMD (P < 0.001, 95% CI -0.85, -0.68) and HVA (P < 0.001, 95% CI -1.01, -0.18) in plasma at each L-dopa/carbidopa dose, whereas the AUC DOPAC/AUC L-dopa ratio was increased again at all doses (P < 0.001, 95% CI 0.26, 0.90). Entacapone did not significantly affect the pharmacokinetics of carbidopa at any of the doses, nor did L-dopa/carbidopa affect the pharmacokinetics of entacapone.

CONCLUSIONS

The 200 mg dose of entacapone similarly and significantly increases the AUC of L-dopa by changing the metabolic balance of L-dopa independent of the L-dopa/carbidopa dose and therefore entacapone is likely to have a similar L-dopa potentiating effect independent of L-dopa dose.

Evaluation of EMD 128 130 occupancy of the 5-HT1A and the D2 receptor: a human PET study with [11C]WAY-100635 and [11C]raclopride

The use of so-called, atypical antipsychotic medication is becoming more widespread in the treatment of psychotic disorders. EMD 128 130 is a novel compound acting as an agonist at the 5-HT1A receptor, and as an antagonist at the dopamine-2 (D2) receptor. This dual action may confer additional benefits over selective D2 antagonists in the treatment of psychotic disorders. In this study, we investigated the occupancy of EMD 128 130 in vivo at the human D2 and 5-HT1A receptors with positron emission tomography using the radiotracers [11C]raclopride and [11C]WAY-100635. Seven healthy volunteers were examined before and after 5 days of treatment with EMD 128 130, administered in an incremental dose building up to 50 mg, b.d. A significant occupancy was demonstrated at the human D2 receptor (40% following a dose of 50 mg, b.d.) while there was no consistent effect observed at the 5-HT1A receptor, despite a similar affinity of EMD 128 130 for cloned human D2 and 5-HT1A receptors, and the presence of typical, central 5-HT1A agonist side-effects. The differential effects of EMD 128 130 at the D2 and the 5-HT1A receptor (antagonist at D2 receptor, agonist at the 5-HTIA receptor) may explain the differences in occupancy observed.

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.

Pharmacodynamic response of entacapone in rats after administration of entacapone formulations and prodrugs with varying bioavailabilities

The aim of this in vivo study was to assess the effect of improved oral bioavailability of entacapone on its actual pharmacodynamic response, COMT inhibition in erythrocytes. Rats were administered entacapone orally as a suspension, as a plain solution, an entacapone/HP-beta-CD solution, two N-alkyl-carbamate ester prodrugs and intravenously as a solution. Also the relationship between pharmacodynamic and pharmacokinetic responses of entacapone was investigated. The administration of entacapone as a solution (plain solution pH 7.4; F=34.8% or entacapone/HP-beta-CD solution pH 3.0; F = 18.5%) resulted in significantly higher degree of COMT inhibition in erythrocytes than could be achieved by administering entacapone as a suspension (pH 3.0; F=8.9%). The inhibitory Emax model did not reveal any significant differences in EC50 estimates of entacapone suspension, entacapone/HP-beta-CD solution or entacapone solution. The overall pharmacodynamic response of entacapone (AUE; area under effect-time curve) was dependent on the pharmacokinetic response (AUC; area under concentration-time curve) irrespective of the entacapone formulation and dosage form. However, this dependency did not extend to formulations producing very high peak concentrations of entacapone in plasma; high plasma concentrations reached transiently after administration of entacapone solution had only a minor effect on the overall pharmacodynamic response (AUE). The inhibitory Emax model revealed that a plateau of COMT inhibition near to Emax is attained by plasma concentrations under 2000 ng/ml, irrespective of the formulation. This supports the results concerning the dependence of AUE on AUC.

Gender and pramipexole effects on levodopa pharmacokinetics and pharmacodynamics

The authors studied the pharmacokinetics of levodopa (LD) with and without pramipexole (PPX) in men and postmenopausal women with PD. Patients on stable dose of carbidopa/LD were randomized to receive escalating doses of placebo or PPX over 7 weeks. LD and PPX pharmacokinetics were performed after a single test dose 25/100 of carbidopa/LD, before initiation of PPX or placebo, at 1.5 mg/d and 4.5 mg/d of PPX or placebo. Compared to men, women had greater LD bioavailability. PPX did not alter LD bioavailability, and PPX pharmacokinetics were equivalent in men and women.

Quantitative analysis of memantine in human plasma by gas chromatography/negative ion chemical ionization/mass spectrometry

A sensitive and specific method for the determination of memantine in human plasma is presented. Memantine was extracted from plasma and derivatized to the pentafluorobenzoyl derivative in a one-step procedure avoiding any sample concentration steps. Amantadine was used as an internal standard. The compounds were measured by gas chromatography/negative ion chemical ionization mass spectrometry without any further processing. Using this detection mode, the fragment ions at m/z 353 and 325 were obtained at high relative abundance. Calibration graphs were linear over the range 0.117-30 ng ml(-1). At the limit of quantification (LOQ), the inter-assay precision was 2.00% and the intra-assay variability was 3.22%. The accuracy at the LOQ showed deviations of -1.42% (intra-assay) and -2.47% (inter-assay). The method is rugged, rapid and robust and was applied to the batch determination of memantine during pharmacokinetic profiling of the drug.

Role of integrative pharmacokinetic and pharmacodynamic optimization strategy in the management of Parkinson"s disease patients experiencing motor fluctuations with levodopa

Parkinson's disease is a progressively debilitating motor neuron disease that affects the dopaminergic neurons within the nigral-striatal and surrounding pathways and which is characterized clinically by rigidity, resting tremor and bradykinesia with or without postural imbalance. Levodopa is the "gold standard" for the treatment and management of Parkinson's disease worldwide. However, following prolonged use of the drug, the "honey-moon" which was once enjoyed by patients on levodopa begins to wane. The clinical as well as the socio-economic costs associated with such failure in response to levodopa is enormous. Various approaches in the management of Parkinson's disease patients experiencing motor fluctuations with levodopa treatment have been suggested and include both pharmacologic and non-pharmacologic strategies involving invasive surgical intervention. Currently, the non-pharmacological approach, which is invasive, remains to be fully perfected and is associated with high morbidity and mortality. The use of the non-invasive, pharmacological approach is currently the most widely accepted approach but would require a review of all possible drug regimens used. This entails evaluating the pharmacokinetics and pharmacodynamic actions of the drug regimens used and possibly, dosage form and route of administration of the drugs. The use of levodopa formulated for transdermal or intranasal administration might help improve the ease of use and compliance. Controversy abounds as to the role of plasma pharmacokinetics of levodopa in the management of Parkinson's patients, vis a vis its dynamics at the central nerve terminal and its receptor site. However, it is worthy of mention that an integrated optimal pharmacological approach involving the peripheral, and central pharmacokinetics of levodopa as well as its central pharmacodynamics would ensure better treatment and management of this disease. In addition, the choice of alternate formulations and routes of administration will not only improve on the bioavailability and overall pharmacokinetics of levodopa, but also increase compliance. Furthermore, monitoring of both plasma and central concentrations of levodopa and its metabolites might play a major role in individualization of pharmacotherapy in special Parkinsonian patients experiencing motor fluctuations with levodopa.

Oral solution of levodopa ethylester for treatment of response fluctuations in patients with advanced Parkinson's disease

Levodopa ethylester (LDEE), a highly soluble prodrug of levodopa, may overcome the impaired absorption of regular levodopa, due mainly to a combination of levodopa's poor solubility and delayed gastric emptying. We conducted a double-blind, levodopa-controlled, multicenter study of oral LDEE solution compared with standard levodopa-carbidopa (LD-CD) tablets. Sixty-two patients with Parkinson's disease who had "delayed on" and "no-on" subtypes of response fluctuations were randomly assigned for treatment with LDEE-CD or LD-CD 250/25 mg for 4 weeks (phase A). Only the first morning and first post-lunch dose of LD were replaced. This was followed by a 2-week extension with a supplementation of carbidopa (25 mg) to each replaced dose (phase B). Patients filled home diaries 2 weeks before and during the trial period in which times of turning on and off for the two doses were reported. In phase A, mean latency to turning on was reduced by 21% (morning dose) and 17% (post-lunch dose) in the LDEE-CD group. Percentage of no-on episodes after the post-lunch dose was decreased by 21% in the LDEE-CD group but increased by 36% in the LD-CD group (P < 0.01). In phase B, LDEE-CD decreased latencies to on after the morning and post-lunch doses and no-on episodes after the post-lunch dose. The beneficial effects of LDEE were supported by the pharmacokinetic data. Results indicate that LDEE solution is beneficial in ameliorating delayed on and no-on response fluctuations. This effect of LDEE is due to more rapid levodopa absorption.

Levodopa pharmacokinetics and motor performance during activities of daily living in patients with Parkinson's disease on individual drug combinations

Pharmacokinetics and pharmacodynamics of levodopa were evaluated at a high-resolution level in a heterogeneous group of 10 patients with idiopathic Parkinson's disease during their normal daily activity. A physician and a nurse spent 10 hours with each patient from the first morning dose of levodopa during daily activities at home and at work. Plasma samples were obtained every 20 minutes for analysis of levodopa and 3-O-methyldopa by high-performance liquid chromatography. To assess clinical response, mobility was rated on every test occasion by patients and by investigators. Food and fluid intake and physical activity were also monitored. There was a large intra- and interindividual variability in the pharmacokinetics of levodopa regardless of the different drug combinations used. Mean plasma levodopa concentration ranged between 0.45 to 7.07 microg/mL and peak concentrations between 0.95 to 13.75 microg/mL. In 44 of 58 dosing events, an oral dose of levodopa was related to a peak in plasma concentration. Assessment of the clinical effects was more sensitive when given by patients than when given by the investigators. The fluctuations of the levodopa concentration in plasma had a clear effect on the clinical parameters assessed, even during early disease stages. Variation in levodopa concentration is the determining factor for motor fluctuations also in patients on clinically optimized combinations with dopamine agonists and enzyme inhibitors.

Influence of layer position on in vitro and in vivo release of levodopa methyl ester and carbidopa from three-layer matrix tablets

A versatile oral controlled release system for the simultaneous delivery of levodopa methyl ester and carbidopa, consisting of a three-layer matrix tablet, has been studied and developed. Each individual layer of the matrix exhibited a different release mechanism, i.e. the first layer was swellable (S), the second one was erodible (E) and the third one was disintegrating (D). The three layers have been assembled in the monolithic matrix in different relative positions. It was found that in the monolith the three layers could interact, producing in vitro release profiles depending on their relative position. The monoliths having the configurations DSE and SDE were administered to human volunteers in order to determine the plasma profiles. The pharmacokinetic data showed a significant difference between the early time plasma curves: the monolith DSE, having the fast release profile, gave rise to a rapid appearance of a high levodopa plasma level, whereas the slower releasing monolith SDE produced a smoothed plasma concentration profile.

Body weight influences pharmacokinetics of levodopa in Parkinson's disease

Changes in body weight may induce substantial variations in peripheral pharmacokinetics of drugs, but the relation between body weight and levodopa (LD) pharmacokinetics has never been investigated in Parkinson's disease. To address this issue, we conducted a pharmacokinetic study with 164 patients with sporadic Parkinson's disease. Patients underwent an oral acute LD test with 250 mg of LD, and pharmacokinetic variables were assessed at baseline and at 30, 60, 120, and 240 minutes after LD administration. Plasmatic-LD areas under the curve and body weight were significantly and inversely correlated as well as the elimination of the half-life of LD and body weight. In our sample, women were significantly lighter and had a significantly greater area under the curve than men. Moreover, a greater percentage of women showed LD peak-dose dyskinesias compared with men. Our findings suggest that lighter patients with Parkinson's disease probably receive a greater cumulative dosage of LD per kilogram of body weight during long-term treatment, because in clinical practice, LD is administered without any adjustment of the dose to body weight. This could explain gender differences for the development of LD-induced peak-dose dyskinesias observed during the course of the disease.