Absorption and metabolism of L-dopa by the human stomach

Long-Term PEG-J Tube Safety in Patients With Advanced Parkinson's Disease

OBJECTIVES:

The objectives of this study were to present procedure- and device-associated adverse events (AEs) identified with long-term drug delivery via percutaneous endoscopic gastrojejunostomy (PEG-J). Levodopa-carbidopa intestinal gel (LCIG, also known in US as carbidopa-levodopa enteral suspension, CLES) is continuously infused directly to the proximal small intestine via PEG-J in patients with advanced Parkinson's disease (PD) to overcome slow and erratic gastric emptying and treat motor fluctuations that are not adequately controlled by oral or other pharmacological therapy.

METHODS:

An independent adjudication committee of three experienced (>25 years each) gastroenterologists reviewed gastrointestinal procedure- and device-associated AEs reported for PD patients (total n=395) enrolled in phase 3 LCIG studies. The rate, clinical significance, and causality of the procedure/device events were determined.

RESULTS:

The patient median exposure to PEG-J at the data cutoff was 480 days. Procedure- and device-associated serious AEs (SAEs) occurred in 67 (17%) patients. A total of 42% of SAEs occurred during the first 4 weeks following PEG-J placement. SAEs of major clinical significance with the highest procedural incidence were peritonitis (1.5%), pneumonia (1.5%), and abdominal pain (1.3%). The most common non-serious procedure- and device-associated AEs were abdominal pain (31%), post-operative wound infection (20%), and procedural pain (23%). In all, 17 (4.3%) patients discontinued treatment owing to an AE.

CONCLUSIONS:

In conclusion, incidences of PEG-J AEs with the LCIG delivery system and PEG-J longevity were compared favorably with ranges described in the PEG/PEG-J literature. A low discontinuation rate in this study suggests acceptable procedural outcomes and AE rates in PD patients treated with this PEG-J drug delivery system.

Pharmacokinetics and pharmacodynamics of gastroretentive delivery of levodopa/carbidopa in patients with Parkinson disease

OBJECTIVE

To investigate, in patients with Parkinson disease, the pharmacokinetics and pharmacodynamics of levodopa/carbidopa delivered via 3 different extended-release (ER) tablet formulations.

METHODS

This was a randomized, crossover study in patients with stable idiopathic Parkinson disease comparing a conventional ER tablet (C-ER) administered orally 3 times daily with 2 levodopa/carbidopa gastroretentive ER formulations administered orally twice daily, one with an immediate release (IR) component (IR/ER) and one without (ER). Blood samples were collected for pharmacokinetic (PK) analysis, and a finger-tapping test was performed to assess pharmacodynamics. Tolerability was evaluated by monitoring adverse events and measuring vital signs. PK modeling was performed to estimate steady-state levodopa concentrations.

RESULTS

Fourteen patients completed the study. Compared with C-ER, both gastroretentive ER tablets significantly extended the first maximum time (6.0 vs 2.5 h; P < 0.025) and had smoother plasma concentration-time profiles while achieving a similar maximum plasma concentration and area under the curve. The IR/ER formulation exhibited a significantly longer duration of concentration above the presumed efficacious threshold of 300 ng/mL (21 vs 18 h; P = 0.0027) compared with C-ER. PK modeling predicts a steady-state levodopa peak/trough ratio of 4 for both IR/ER and ER formulations and a ratio of 21 for C-ER. Furthermore, superior response in the finger tapping test was observed for the IR/ER and ER formulations compared with the C-ER formulation.

CONCLUSIONS

This study demonstrated that the gastroretentive ER formulations achieved more constant plasma levodopa concentrations and better pharmacodynamics with reduced dose frequency, potentially reducing the on-off phenomena that have been associated with fluctuations in plasma levodopa concentrations.

Plasma dopa concentrations after different preparations of levodopa in normal subjects

The concurrent administration of levodopa with a decarboxylase inhibitor produced a plasma concentration/time curve comparable with 1/4 to 1/5 of the dose of levodopa given alone. There was no evidence to suggest that the decarboxylase inhibitor slowed the rate of elimination of levodopa from plasma. Metoclopramide (Maxolon) increased the rate of levodopa absorption. Higher plasma concentrations of levodopa during the first 2 h after dosing were followed by lower plasma concentrations during the third and fourth hours. The amount of levodopa absorbed after Larodopa as indicated by the AUC was not altered by adding metoclopramide. None of the current preparations of levodopa produced sustained plasma concentrations. In vitro testing confirmed that Brocadopa Temtabs tablets disintegrate and dissolve slowly. In vivo, Brocadopa Temtabs behaved as a slow release preparation but it did not produce sustained plasma concentrations of levodopa.

Pharmacokinetics of levodopa/carbidopa delivered from gastric-retentive extended-release formulations in patients with Parkinson's disease

The investigators conducted a single-dose pharmacokinetic (PK) study of levodopa/carbidopa delivered from novel gastric-retentive extended-release (ER) tablets versus a comparator ER tablet (M-ER) in patients with Parkinson's disease. Two levodopa/carbidopa (200 mg/50 mg) gastric-retentive ER formulations (4 hours and 6 hours) and M-ER were administered orally with food. Blood samples were collected for up to 24 hours post dose to determine levodopa and carbidopa concentrations. Tolerability was assessed by monitoring adverse events and measuring vital signs. PK modeling was conducted to estimate the release characteristics for future gastric-retentive ER formulations to achieve a less fluctuating plasma concentration profiles. Compared with M-ER, both gastric-retentive ER formulations exhibited a longer time to reach a lower maximal plasma concentration for levodopa and carbidopa. The 4-hour formulation demonstrated a similar area under the concentration-time curve compared with M-ER, whereas the 6-hour formulation demonstrated a lower area under the concentration-time curve. All formulations were well tolerated. Modeling suggests that a gastric-retentive ER formulation with a longer release duration administered twice daily may achieve a less fluctuating levodopa concentration profile than M-ER administered 3 times daily. This study demonstrates that gastric-retentive ER dosage forms may reduce dose frequency while minimizing the plasma peak-to-trough fluctuation and consequently reduce motor fluctuation in patients with Parkinson's disease.

Duodenal levodopa infusion for the treatment of Parkinson’s disease

Motor fluctuations are a common problem in the long-term management of Parkinson's disease (PD), resulting in disability and impaired quality of life. The relatively short serum half-life (approximately 90 min) of oral levodopa/carbidopa and its erratic absorption due to delayed and inconsistent gastric emptying (a non-motor feature of PD) are thought to be important factors in the development of motor fluctuations. Continuous infusion of levodopa/carbidopa directly into the small intestine of PD patients results in marked reduction of motor fluctuations by reducing plasma levodopa variability by an order of magnitude over oral therapy. Previously, the use of long-term intraduodenal infusion of levodopa/carbidopa was limited by the relatively large volumes of infusate necessitated by the low solvency of levodopa. The development of a micronized levodopa (20 mg/ml) and carbidopa (5 mg/ml) suspension utilizing a methylcellulose gel provides the high levodopa concentration and physical and chemical stability necessary for long-term enteral therapy. Clinical evidence indicates that a marked reduction of motor fluctuations and dyskinesias can be achieved and maintained by intraduodenal administration of this suspension. This article reviews the published data describing the efficacy and safety of duodenal levodopa, and discusses its current and potential role in meeting the needs of PD patients.

Pharmacokinetic optimisation in the treatment of Parkinson's disease : an update

Pharmacotherapy for Parkinson's disease is focused on dopaminergic drugs, mainly the dopamine precursor levodopa and dopamine receptor agonists. The elimination half-life (t(1/2)) of levodopa from plasma (in combination with a decarboxylase inhibitor) of about 1.5 hours becomes more influential as the disease progresses. The long-duration of response to levodopa, which is evident in early Parkinson's disease, diminishes and after a few years of treatment motor performance is closely correlated to the fluctuating plasma concentrations of levodopa. Absorption of levodopa in the proximal small intestine depends on gastric emptying, which is erratic and may be slowed in Parkinson's disease. The effects of levodopa on motor function are dependent on gastric emptying in patients in the advanced stages of disease. The current treatment concept is continuous dopaminergic stimulation (CDS). Sustained-release formulations of levodopa may provide more stable plasma concentrations. Oral liquid formulations shorten the time to reach peak concentration and onset of effect but do not affect plasma levodopa variability. The t(1/2) of levodopa can be prolonged by adding a catechol-O-methyltransferase inhibitor (entacapone or tolcapone), which may reduce fluctuations in plasma concentrations, although both peak and trough concentrations are increased with frequent administration. Intravenous and enteral (duodenal/jejunal) infusions of levodopa yield stable plasma levodopa concentrations and motor performance. Enteral infusion is feasible on a long-term basis in patients with severe fluctuations. Among the dopamine receptor agonists the ergot derivatives bromocriptine, cabergoline, dihydroergocryptine and pergolide, and the non-ergot derivatives piribedil, pramipexole and ropinirole, have longer t(1/2) compared with levodopa. Thus, they stimulate dopamine receptors in a less pulsatile manner, yet pharmacokinetic studies of repeated doses of dopamine receptor agonists are few. Optimisation of these drugs is often performed with standardised titration schedules. Apomorphine and lisuride have short t(1/2) and are suitable for subcutaneous infusion, with results similar to those of levodopa infusion. Transdermal administration of dopamine receptor agonists such as rotigotine might be an alternative in the future. In general, initial dopamine receptor agonist monotherapy is associated with poorer motor performance and lower incidence of motor complications compared with levodopa. Buccal administration of the monoamine oxidase-B inhibitor selegiline (deprenyl) provides better absorption and less formation of metabolites compared with standard tablets. To conclude, several new drugs, formulations and routes of administration have been introduced in the treatment of Parkinson's disease during the last decade, mainly with CDS as the aim. CDS can be approached by optimising the use of dopaminergic drugs based on pharmacokinetic data.

Impact of gastric emptying on levodopa pharmacokinetics in Parkinson disease patients

Adjunction of the catechol-O-methyltransferase (COMT) inhibitor entacapone (EN) to levodopa/carbidopa (LD/CD) improves motor symptoms in patients with Parkinson disease (PD) by a prolonged elimination of LD. But it is not known whether EN addition influences gastric emptying and thus LD pharmacokinetics and pharmacodynamics. Objectives were to simultaneously determine plasma LD elimination, gastric emptying, and clinical response after a single intake of the same LD dosage as LD/CD--or as (LD/CD/EN) formulation on 2 consecutive days. In both groups, PD patients with delayed gastric emptying had significant lower LD plasma concentrations. Addition of EN did not influence gastric emptying but significantly improved motor response, which was not different for patients with delayed gastric emptying. However, with and without EN adjunction gastric emptying distinctly contributes to the variability of plasma LD bioavailability. This may impact LD delivery to the brain and thus motor response in PD patients. Therefore, fine tuning of LD application, which considers gastric emptying, becomes more and more essential in advanced PD stages with a reduced striatal neuronal dopamine capacity, which is responsible for maintenance of motor response in early PD patients.

Increased gastric motility during 5-HT4 agonist therapy reduces response fluctuations in Parkinson's disease

We investigated the clinical efficacy and tolerability of 45 mg/day mosapride, a selective 5-hydroxytryptamine type 4 (5-HT4) agonist, in an open-label study involving five patients with Parkinson's disease (PD) who had response fluctuations (RFs). 'On' time and motor function scores were determined, and gastric motility was measured by a radionuclide gastric emptying (GE) test, the most reliable quantitative method available. We found that mosapride therapy significantly shortened GE half-time, reduced RFs, and improved motor functions in all patients. There were no adverse reactions. We conclude that selective 5-HT4 agonist therapy is beneficial for patients with PD who have RFs.

Levodopa Infusion Therapy in Parkinson Disease

Orally administered levodopa, in combination with a decarboxylase inhibitor, is the gold standard therapy for Parkinson disease (PD). The problems in management of motor fluctuations in the advanced stages of the disorder are due to the close relationship between plasma levodopa levels and availability of dopamine at striatal receptor sites. The fluctuating levodopa concentrations are mainly explained by the fact that levodopa absorption only occurs in the proximal small intestine. The patient's motor function thus depends on gastric emptying, which is erratic and may even be delayed in PD. Oral therapy with sustained-release formulations and COMT inhibitors have not solved the problems satisfactorily. Therefore, infusions of levodopa by intravenous and enteral (duodenal/jejunal) routes of administration have been studied. In this review of the literature on clinically relevant levodopa infusion studies, it is shown that improvements regarding fluctuations in both plasma levodopa levels and motor performance have been repeatedly reported. The results acquired so far suggest that levodopa infusion is a safe and efficacious therapy. Recent drug delivery development and long-term studies have shown that infusion is a clinically feasible alternative to treat advanced PD.

A pulmonary formulation of L-dopa enhances its effectiveness in a rat model of Parkinson's disease

The efficacy of oral L-dopa becomes problematic with the progression of Parkinson's disease, due in large part to a lost ability to accommodate L-dopa's inherently poor pharmacokinetics. Pulmonary delivery represents a novel approach to reducing this problem. L-dopa was formulated into inhalable (Alkermes AIR) particles, and its pharmacokinetics and pharmacodynamics compared with those of an oral formulation. Pulmonary administration of L-dopa (2 mg) to rats resulted in a rapid elevation of plasma levels (C(max) = 4.8 +/- 1.10 microg/ml at 2 min), whereas oral administration of L-dopa produced a much delayed and lower C(max) (1.8 +/- 0.40 microg/ml at 30 min). In a rat model of Parkinson's disease (unilateral 6-hydroxydopamine lesion), the pulmonary formulation of L-dopa (0.5-2.0 mg) yielded more rapid and robust elevations in striatal L-dopa, dopamine, and dihydroxyphenylacetic acid levels, as well as 2.5 to 3.7 times as many c-fos-expressing striatal neurons. Moreover, motor function was significantly improved by 10 min after administration, with peak improvements occurring within 15 to 30 min. In contrast, considerably higher doses (6.8-10 mg) of orally administered L-dopa took over three times longer to produce similar effects. These results suggest that an inhalable formulation of l-dopa has superior pharmacokinetic properties and may provide patients with a more effective form of rescue therapy as well as being a reliable adjuvant or replacement for first-line oral therapy.

Role of sphingosine synthesis inhibition in transcutaneous delivery of levodopa

The present study was designed to investigate the role of skin sphingosine synthesis inhibition in enhancing the in vitro permeation of levodopa (LD), a hydrophilic drug, across rat skin. beta-Chloroalanine (beta-CA), a selective inhibitor of serine palmitoyl transferase was used for inhibiting sphingosine synthesis in viable skin. The sphingosine content in viable skin perturbed by acetone treatment and immediately treated with beta-CA (600 or 1200 microg/7 cm(2)) was significantly less than that of perturbed viable skin after 36 h of treatment (P<0.001). The in vitro permeation of LD across perturbed-beta-CA treated skin was significantly greater than that across perturbed skin (P<0.001). This indicates an inverse relationship between in vitro permeation of LD and skin sphingosine content. The systemic delivery of percutaneously applied LD across normal rat skin was negligible. Higher C(max), lower T(max) and maintenance of effective plasma concentration of LD over 28 h was achieved by a single topical application of carbidopa-LD combination (1:4) to perturbed-beta-CA treated skin. Hence, skin sphingosine synthesis inhibition can be used as a novel means of enhancing systemic delivery of LD.

A pharmacokinetic model to predict the PK interaction of L-dopa and benserazide in rats

PURPOSE

To study the PK interaction of L-dopa/benserazide in rats.

METHODS

Male rats received a single oral dose of 80 mg/kg L-dopa or 20 mg/kg benserazide or 80/20 mg/kg L-dopa/benserazide. Based on plasma concentrations the kinetics of L-dopa, 3-O-methyldopa (3-OMD), benserazide, and its metabolite Ro 04-5127 were characterized by noncompartmental analysis and a compartmental model where total L-dopa clearance was the sum of the clearances mediated by amino-acid-decarboxylase (AADC), catechol-O-methyltransferase and other enzymes. In the model Ro 04-5127 inhibited competitively the L-dopa clearance by AADC.

RESULTS

The coadministration of L-dopa/benserazide resulted in a major increase in systemic exposure to L-dopa and 3-OMD and a decrease in L-dopa clearance. The compartmental model allowed an adequate description of the observed L-dopa and 3-OMD concentrations in the absence and presence of benserazide. It had an advantage over noncompartmental analysis because it could describe the temporal change of inhibition and recovery of AADC.

CONCLUSIONS

Our study is the first investigation where the kinetics of benserazide and Ro 04-5127 have been described by a compartmental model. The L-dopa/benserazide model allowed a mechanism-based view of the L-dopa/benserazide interaction and supports the hypothesis that Ro 04-5127 is the primary active metabolite of benserazide.

Monoamine oxidase and catechol-O-methyltransferase inhibitors

Despite advances in the treatment of PD, there remain significant unmet therapeutic needs. This is particularly true at the later stages of the disease when dopaminergic therapy is complicated by motor fluctuations and dyskinesias. Inhibition of dopamine metabolism is a valuable adjunct to exogenous dopaminergic replacement. Inhibitors of MAO-B have been used to treat early and advanced PD for a number of years. Although controversy remains, existing evidence still raises the possibility that MAO-B inhibition may confer a protective effect in PD, delaying the progression of the underlying pathology. More recently, clinically useful inhibitors of COMT have become available. These medications largely act peripherally to increase the pool of available dopamine precursor and prolong the duration of effect of L-dopa. They are indicated primarily for control of motor fluctuations.

Antiparkinsonian Agents : Clinically Significant Drug Interactions and Adverse Effects, and Their Management

The treatment of Parkinson's disease for most patients entails long term exposure to multiple agents, including anticholinergics, levodopa, amantadine, dopamine receptor agonists, catechol-O-methyltransferase inhibitors, selegiline (deprenyl) and clozapine. Patients with Parkinson's disease require medication for the control of the motor symptoms of their condition, for related medical or psychiatric symptoms of the disorder, and for concurrent medical problems, such as hypertension or cardiac disease.All these agents may cause adverse effects. There is a potential for drug-drug interactions between different antiparkinsonian agents and between antiparkinsonian medication and the other drugs a patient may be taking. Clinicians caring for patients with Parkinson's disease must be knowledgable about the potential adverse effects and drug interactions of an expanding array of medications for this condition.

The influence of protein containing meals on the pharmacokinetics of levodopa in healthy volunteers

1. The pharmacokinetics of levodopa and paracetamol after single oral doses have been investigated in eight healthy young volunteers in the fasted state and following isocaloric meals containing either 10.5 g or 30.5 g of protein. 2. The initial peak and maximum plasma drug concentrations and the times at which these occurred were not affected by food. 3. The mean area under the plasma concentration-time curve (AUC) for paracetamol following an overnight fast did not differ significantly from that observed following the low and high protein meals. 4. By contrast, the AUC for levodopa following the low protein meal (193.9 +/- 15.7 micrograms ml-1 min) was significantly lower compared with administration in the fasted state (216.5 +/- 26.1 micrograms ml-1 min). However, there were no significant differences in the kinetics of levodopa between the fasting state and following the high protein meal. 5. There was no evidence that consumption of a meal containing 30.5 g of protein impaired either the rate or extent of absorption of levodopa. Therefore the reported beneficial effects of a low protein diet in the treatment of patients with Parkinson's disease probably result from reduced competition for levodopa transport across the blood-brain barrier.

Controlled-release Sinemet (CR-4): a double-blind crossover study in patients with fluctuating Parkinson's disease

Patients with moderate to advanced Parkinson's disease may have prominent levodopa-related motor fluctuations. In a double-blind crossover study, we compared the anti-Parkinson effects of standard Sinemet with a controlled-release formulation (Sinemet CR-4) in 23 patients with short-duration responses to standard Sinemet. With Sinemet CR-4, approximately half the patients who completed the study displayed a prolongation of their "on" response (optimal response to treatment), as assessed by monitoring individual drug-response cycles. A few patients experienced prolonged delays before the peak anti-Parkinson response developed to Sinemet CR-4. End-of-dose "wearing off" was favorably affected by Sinemet CR-4, but patients still had unpredictable "off" (parkinsonian) periods. Subjective ratings of Sinemet CR-4 varied, and 39% of patients who completed the study actually preferred standard Sinemet to the new formulation. We conclude that Sinemet CR-4 may benefit some patients with Parkinson's disease with a short-duration response to standard Sinemet; however, not all patients found it preferable to the standard formulation.

On-off phenomenon: relation to levodopa pharmacokinetics and pharmacodynamics

Motor function in the moderately to severely affected parkinsonian patient is critically dependent upon delivery of levodopa to the striatum. This, in turn, is influenced by the fluctuating plasma concentrations of levodopa produced by the drug's short half-life and erratic absorption, and by modifiable transport at the blood-brain barrier. Duration of response to a single dose of levodopa is proportional to peak plasma drug levels, and paradoxical responses may occur when plasma concentrations are in the vicinity of minimum effective concentrations. Thus the strategy of administering frequent, small doses of levodopa may contribute additional unpredictability to a fluctuating clinical response imposed by pharmacokinetic factors.

Drug-antacid interactions: assessment of clinical importance

Antacids and adsorbents are commonly used preparations that are generally considered to be pharmacologically inert and free from adverse effects. They may, however, interact with a diverse range of primary drugs and the sequelae can be disadvantageous to the efficacy of the primary medication. Many such reports in the literature are based on animal experiments, or on single-dose studies in healthy subjects. Some reports are anecdotal and are unconfirmed; others are based solely on in vitro evidence. Potentially important interactions have been suggested for a relatively small group of drugs: tetracyclines, phenytoin, digoxin, chloroquine, cimetidine, quinidine, nonsteroidal antiinflammatory drugs, and beta-blocking agents. The evidence for these has been critically evaluated, as well as for antacid-anticoagulant and antacid-nitrofurantoin interactions that have been wrongly emphasized in the literature. The majority of literature reports on interactions with antacids have been overemphasized; only ferrous sulfate-, isoniazid-, and tetracycline-antacid interactions fall into a category I importance (scale I-III of descending importance). This category is for those interactions with good evidence of actual or potential importance in patients or in relevant studies on normal subjects.

Interactions affecting drug absorption

The influence of drug-drug and drug-food interactions affecting the absorption of orally administered medication is reviewed. Drug-drug interactions can be classified in terms of indirect effects by one drug on gastrointestinal tract physiology influencing the absorption of other drugs, or direct interactions involving altered pH, adsorption, absorption, or chelation. Most, but not all, drug-drug interactions result in reduced or delayed systemic drug availability. Drug-food interactions may result in reduced, delayed, or increased systemic drug availability. The absorption of only a small number of drugs is unaffected by concomitant food intake. The degree of interaction and whether it positively or negatively affects drug absorption depends on a number of factors including the physical and chemical nature of the drug, the formulation, the type of meal, and the time interval between eating and dosing. Mechanisms of drug-food interactions are not well characterised. They clearly involve both direct and indirect factors in a similar fashion to drug-drug interactions, but indirect factors probably predominate. Reduced or delayed drug absorption is generally attributed, at least in part, to delayed stomach-emptying due to food. Increased absorption may also result from delayed stomach-emptying facilitating greater drug dissolution before it passes from the stomach into the small intestine. Increased bioavailability of some drugs, e.g. propranolol, metoprolol and labetalol, may be related to reduced presystemic clearance. The potential clinical implications of drug-drug and drug-food interactions must be taken into account with oral medications in order to minimise variations in systemic drug availability and hence in clinical efficacy.

Dosage form design for improvement of bioavailability of levodopa V: Absorption and metabolism of levodopa in intestinal segments of dogs

Plasma levels of levodopa, total dopamine, and residual amounts of levodopa and its metabolites at the administered site were analyzed following administration of single 100-mg doses of levodopa in solution into isolated segments of the duodenum, jejunum, and ileum of the dog. The largest area under the plasma concentration-time curve (AUC) of levodopa during the 1.0-hr study was obtained following administration in the duodenum, followed by the jejunum and ileum. In addition, the residual amounts of levodopa and its metabolites detected at the administration sites were: ileum, 23%; jejunum, 7% and duodenum, less than 1%. The largest AUC of total dopamine was obtained following administration in the jejunum, followed by the ileum and duodenum. This order was consistent with the order of levodopa decarboxylase enzyme activity reported previously. Therefore, it can be concluded that the major absorption site of levodopa in the intestine resides in the upper small intestine. Levodopa in 10-, 50-, and 100-mg doses was administered into isolated duodenal segments. The AUC of levodopa increased nonlinearly with increasing dose. Negligible amounts of both levodopa and its metabolites were observed in the segment at 1.0 hr after administration, indicating that the duodenal absorption of levodopa was not saturable within the dose range tested.

Dosage form design for improvement of bioavailability of levodopa II: bioavailability of marketed levodopa preparations in dogs and parkinsonian patients

To estimate the absolute bioavailability of oral levodopa, plasma concentrations and urinary excretion of levodopa and its metabolites were determined in beagle dogs and in parkinsonian patients after intravenous and oral drug administration. The absolute bioavailability of orally administered levodopa was estimated to be about 35% in both dogs and patients; however, the total amount absorbed of intact drug and levodopa metabolites was estimated to be 80--90% of the administered dose. Due to the similarities of the pharmacokinetic characteristics of levodopa found in beagle dogs and in humans, beagle dogs can serve as a model to study bioavailability, absorption, and metabolic mechanisms.

Effects of 3,4-dihydroxyphenylpyruvic acid and L-glutamic acid on some pharmacokinetic parameters of L-dopa in the rat

In the rat, various oral doses of 3,4-dihydroxyphenylpyruvic acid (DHPPA) administered simultaneously with L-dopa were shown to elevate the serum L-dopa and cerebral dopamine concentrations dose-dependently. With increasing DHPPA: L-dopa ratio the L-dopa-sparing effect became greater. Although oral and intravenous doses of DHPPA showed that it was readily converted to L-dopa in the serum, only small amounts of dopamine (DA) were detected in the brain. The high 3-O-methyldopa concentrations, measured in the serum after DHPPA administration, might be responsible for the failure of the L-dopa formed to pass the blood-brain barrier. Concomitant administration of L-dopa and glutamic acid gave significantly higher L-dopa values in the serum and DA and homovanillic acid values in the brain than did the same dose of L-dopa alone. This indicates that the transamination of L-dopa can also be effected by coadministration of L-dopa and an amino group donor in vivo.

Plasma O-methyldopa in levodopa-induced dyskinesias. A bioclinical investigation

The peripheral metabolism of Dopa has been studied in correlation with the clinical occurrence of Leyodopa-induced dyskinesias in Parkinson patients. Within the group of patients treated with a peripheral decarboxylase inhibitor (PDI), the combination of all the plasma levels of O-Methyldopa from patients with dyskinesias shows significantly higher values than those from patients without dyskinesias. For Dopa itself, no significant differnece can be detected. Such high O-Methyldopa levels seem to be due to a progressive accumulation of this compound and not to a higher degree of formation. In contrast, no significantly different Dopa or O-Methyldopa levels are found within the group of patients treated with L-Dopa alone. These results are discussed in relation to some of the suspected mechanisms involved in Levodopa-induced dyskinesias.

An improved fluorimetric method for assay of dopa in urine and tissues and its use for determination of urinary dopa, at endogenous level, in different species

An optimized fluorimetric method is presented which permits the analysis of DOPA in urine or tissues, at endogenous levels. A wide variety of eluates can be analyzed by applying the manual or the automated schedule. The automated manifold, developed for DOPA assay, may be used to estimate norepinephrine (NE) in other samples, by only changing the nature of reagents to be pumped. Amounts of DOPA as low as 0.3 ng/ml of eluate can be detected. Determinations of endogenous DOPA are reported in urinary samples of Humans, Rats, Dogs and Sheeps, and in brain of Rats. The pattern of changes in DOPA urinary excretion has been investigated as a function of time in Rats. Dietary influences have been studied in Man, Rat and Dog. It is concluded that the greatest part of free and conjugated DOPA excreted in urine of these animals has an endogenous origin.

Dose-dependent pharmacokinetics of laevodopa and its metabolites in the rat

1. Plasma levels of total radioactivity, unchanged laevodopa, dopamine, 3-O-methyldopa and non-amino phenolic acids were measured in rats treated orally with [3H]laevodopa at total laevodopa doses of 1, 10, 20 and 100mg/kg while the radioactive dose was maintained at 100muCi/kg for all animal groups. 2. Within the dosage range studied, non-linear pharmacokinetics were observed for unchanged laevodopa, non-amino phenolic acids and dopamine but not for 3-O-methyldopa. 3. The area under the plasma concentration-time curve for laevodopa increased exponentially with linear increments in the oral dose. This non-linear increase in laevodopa bioavailability is consistent with the hypothesis that high oral doses of laevodopa are required to saturate gastro-intestinal metabolism of the drug. 4. At the lower doses (1 and 20 mg/kg), only 2-3% of total radioactivity could be attributed to dopamine. At 100mg/kg, the dopamine metabolite fraction amounted to about 1/3 of total radioactivity in the plasma.

Impaired absorption of chlorpromazine in rats given trihexyphenidyl

1 The absorption and tissue distribution of orally administered [14C]-chlorpromazine (CPZ) were compared in trihexyphenidyl (THP; Artane)-treated and control rats. 2 Total radioactivity (CPZ) in the plasma and brain of rats treated with THP was significantly lower whereas total radioactivity in the stomach was significantly higher than in rats not previously treated with THP. 3 Gastric emptying in rats treated with THP was significantly delayed as measured by gastric clearance of a marker [14C]-polyethylene glycol. 4 Transport of [14C]-CPZ in everted sacs was not affected by treatment with THP. 5 Metabolism of [14C]-CPZ by liver homogenates was not affected by treatment with THP. 6 The relationship of delayed gastric emptying in THP-treated rats and their lower plasma and brain levels of [14C]-CPZ after oral administration is discussed.

Absorption, metabolism and distribution of (14C)-O-methyldopa and (14C)-L-dopa after oral administration to rats

1 The absorption, tissue distribution, and metabolism of [(14)C]-O-methyldopa were compared with those of [(14)C]-L-DOPA after oral administration to rats.2 Total radioactivity in the plasma and brain of rats treated with [(14)C]-O-methyldopa was significantly higher (2 fold and 30-50 fold, respectively) than that of rats treated with [(14)C]-L-DOPA.3 Total radioactivity in the gut washings and intestinal tissue 2 h after oral administration was significantly higher in rats treated with [(14)C]-L-DOPA than in rats treated with [(14)C]-O-methyldopa. The reverse was observed in the stomach tissues.4 Peripheral metabolism of [(14)C]-O-methyldopa was much lower than that of [(14)C]-L-DOPA; the major metabolite of [(14)C]-O-methyldopa in the plasma is L-DOPA, whereas L-DOPA is mainly metabolized to phenylcarboxylic acids.

Effect of chronic drug treatment on intestinal membrane transport of 14 C-L-dopa

1. By the use of the everted jejunal sac it was shown that chronic oral treatment of rats with various drugs can either increase or decrease the mucosal transport of (14)C-L-DOPA or alter its serosal/tissue ratio.2. (14)C-L-DOPA transport was significantly increased in rats that were chronically treated with L-DOPA and diminished in those that were treated with chlorpromazine and phenobarbitone.3. Chronic treatment with amantadine and neomycin did not affect (14)C-L-DOPA intestinal transport, although direct addition of amantadine to the medium, significantly increased (14)C-L-DOPA transport in everted sacs of nontreated rats. Addition of neomycin directly to the medium did not affect (14)C-L-DOPA transport.4. The possible mechanisms of these findings and their clinical significance are discussed.