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

Perioperative Use of Intravenous Levodopa as an Anti-Parkinsonian Drug: A Propensity Score Analysis

Background

Perioperative discontinuation of oral anti-parkinsonian medication can negatively impact the prognosis of abdominal surgery in patients with Parkinson's disease. Although intravenous levodopa may be an alternative, its efficacy has not yet been investigated.

Objectives

To determine the efficacy of intravenous levodopa as an alternative to oral anti-Parkinsonian drugs during gastric or colorectal cancer surgery.

Methods

We identified patients with Parkinson's disease who underwent surgery for gastric or colorectal cancer between April 2010 and March 2020, using the Diagnosis Procedure Combination database, a nationwide inpatient database in Japan. Patients were divided into two groups: those who received intravenous levodopa during the perioperative period and those who did not. We compared in-hospital mortalities, major complications, and postoperative length of stay between the groups after adjusting for background characteristics with overlap weights based on propensity scores.

Results

We identified 648 patients who received intravenous levodopa and 1207 who did not receive levodopa during the perioperative period. In the adjusted cohort, the mean postoperative length of stay was 24.7 and 29.0 days (percent difference, -7.7%; 95% confidence interval, -13.1 to -1.5); in-hospital death was 3.2% and 3.3% (adjusted odds ratio, 0.95; 95% CI: 0.54-1.67); and incidence of major complications were 21.4% and 19.3% (adjusted odds ratio, 0.89; 95% confidence interval, 0.70-1.13) in those with and without intravenous levodopa, respectively.

Conclusions

Intravenous levodopa was associated with a shorter postoperative length of stay, but not with mortality or morbidity. Intravenous levodopa may improve perioperative care in patients with Parkinson's disease.

Exosomes in Parkinson: Revisiting Their Pathologic Role and Potential Applications

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by bradykinesia, rigidity, and tremor. Considerable progress has been made to understand the exact mechanism leading to this disease. Most of what is known comes from the evidence of PD brains' autopsies showing a deposition of Lewy bodies-containing a protein called α-synuclein (α-syn)-as the pathological determinant of PD. α-syn predisposes neurons to neurotoxicity and cell death, while the other associated mechanisms are mitochondrial dysfunction and oxidative stress, which are underlying precursors to the death of dopaminergic neurons at the substantia nigra pars compacta leading to disease progression. Several mechanisms have been proposed to unravel the pathological cascade of these diseases; most of them share a particular similarity: cell-to-cell communication through exosomes (EXOs). EXOs are intracellular membrane-based vesicles with diverse compositions involved in biological and pathological processes, which their secretion is driven by the NLR family pyrin domain-containing three proteins (NLRP3) inflammasome. Toxic biological fibrils are transferred to recipient cells, and the disposal of damaged organelles through generating mitochondrial-derived vesicles are suggested mechanisms for developing PD. EXOs carry various biomarkers; thus, they are promising to diagnose different neurological disorders, including neurodegenerative diseases (NDDs). As nanovesicles, the applications of EXOs are not only restricted as diagnostics but also expanded to treat NDDs as therapeutic carriers and nano-scavengers. Herein, the aim is to highlight the potential incrimination of EXOs in the pathological cascade and progression of PD and their role as biomarkers and therapeutic carriers for diagnosing and treating this neuro-debilitating disorder.

Improved Flux of Levodopa via Direct Deposition of Solid Microparticles on Nasal Tissue

Epithelial flux and permeability across bovine olfactory tissue were compared when levodopa (L-DOPA) was loaded in different physical states. Aqueous solution of L-DOPA was prepared in Krebs-Ringer buffer (KRB), at a concentration (0.75 mg/mL) verified to be less than the saturation solubility at both 25 and 37°C. Sodium metabisulfite was added to solution to minimize L-DOPA oxidation; chemical stability of aqueous L-DOPA was evaluated using HPLC-UV. Solid-state characterization of unprocessed, dry, crystalline L-DOPA powder was performed using TGA, DSC, PXRD, and optical microscopy to ensure that preparation of L-DOPA microparticles used for diffusion experiments did not elicit a phase change. Measurements of in vitro flux were made for all preparations, using freshly excised bovine olfactory mucosal membrane. Samples obtained from transport studies were analyzed by HPLC-UV. Tissue viability was measured before and after experiments using transdermal epithelial electrical resistance (TEER). The average steady-state flux (J _ss ) of L-DOPA from solid microparticles directly deposited on nasal epithelial tissue was 6.08 ± 0.69 μg/cm²/min, approximately three times greater than the J _ss measured for L-DOPA from solution (2.13 ± 0.97 μg/cm²/min). The average apparent permeability coefficient (P _app ) of L-DOPA was calculated to be 4.73 × 10^-5 cm/s. These findings suggest that nasal delivery of L-DOPA by administration of solid microparticles not only benefits from improved chemical and microbiological stability by avoiding the use of aqueous formulation vehicle but also does not compromise cumulative mass transport across the olfactory membrane.

The Human Experience with Intravenous Levodopa

Objective: To compile a comprehensive summary of published human experience with levodopa given intravenously, with a focus on information required by regulatory agencies.

Background: While safe intravenous (IV) use of levodopa has been documented for over 50 years, regulatory supervision for pharmaceuticals given by a route other than that approved by the U.S. Food and Drug Administration (FDA) has become increasingly cautious. If delivering a drug by an alternate route raises the risk of adverse events, an investigational new drug (IND) application is required, including a comprehensive review of toxicity data.

Methods: Over 200 articles referring to IV levodopa were examined for details of administration, pharmacokinetics, benefit, and side effects.

Results: We identified 142 original reports describing IVLD use in humans, beginning with psychiatric research in 1959–1960 before the development of peripheral decarboxylase inhibitors. At least 2760 subjects have received IV levodopa, and reported outcomes include parkinsonian signs, sleep variables, hormone levels, hemodynamics, CSF amino acid composition, regional cerebral blood flow, cognition, perception and complex behavior. Mean pharmacokinetic variables were summarized for 49 healthy subjects and 190 with Parkinson's disease. Side effects were those expected from clinical experience with oral levodopa and dopamine agonists. No articles reported deaths or induction of psychosis.

Conclusion: At least 2760 patients have received IV levodopa with a safety profile comparable to that seen with oral administration.

Orthostatic stability with intravenous levodopa

Intravenous levodopa has been used in a multitude of research studies due to its more predictable pharmacokinetics compared to the oral form, which is used frequently as a treatment for Parkinson's disease (PD). Levodopa is the precursor for dopamine, and intravenous dopamine would strongly affect vascular tone, but peripheral decarboxylase inhibitors are intended to block such effects. Pulse and blood pressure, with orthostatic changes, were recorded before and after intravenous levodopa or placebo-after oral carbidopa-in 13 adults with a chronic tic disorder and 16 tic-free adult control subjects. Levodopa caused no statistically or clinically significant changes in blood pressure or pulse. These data add to previous data that support the safety of i.v. levodopa when given with adequate peripheral inhibition of DOPA decarboxylase.

Hydrosoluble fiber (Plantago ovata husk) and levodopa I: experimental study of the pharmacokinetic interaction

Fiber therapy could be used in patients with Parkinson disease to reduce the symptoms of gastrointestinal disorders; however, it could interact with levodopa reducing its effectiveness. In this experimental study we have investigated whether the presence of Plantago ovata husk (water-soluble fiber) modifies in rabbits the bioavailability and other pharmacokinetic parameters of levodopa (20 mg/kg) when administered by the oral route at the same time. We have also studied whether pharmacokinetic modifications are fiber-dose dependent (100 and 400 mg/kg). The extent of levodopa absorbed when administering 100 mg/kg of fiber (AUC=43.4 mug min ml(-1)) is approximately the same as when levodopa is administered alone (AUC=47.1 microg min ml(-1)); however, Cmax is lower (1.04 versus 1.43 microg ml(-1)). Results obtained indicate that fiber at the higher dose increases the extent of levodopa absorbed (AUC=62.2 microg min ml(-1)), being the value of Cmax similar (1.46 microg ml(-1)). The value of tmax increases from 10 min when levodopa is administered alone to 20 min when the animals receive fiber. On the other hand, since certain time on, levodopa concentrations are always higher in the groups that receive fiber: 60 min with 100 mg/kg fiber and 20 min with 400 mg/kg fiber. Fiber also increases the mean residence time (MRT). P. ovata husk administration with levodopa could be beneficial, not only in patients with constipation, due to: lower adverse reactions (lower values of Cmax) and longer and more stable effects (higher final concentrations and more time in the body).

Classification of orally administered drugs on the World Health Organization Model list of Essential Medicines according to the biopharmaceutics classification system

Since its inception in 1995, the biopharmaceutical classification system (BCS) has become an increasingly important tool for regulation of drug products world-wide. Until now, application of the BCS has been partially hindered by the lack of a freely available and accurate database summarising solubility and permeability characteristics of drug substances. In this report, orally administered drugs on the Model list of Essential Medicines of the World Health Organization (WHO) are assigned BCS classifications on the basis of data available in the public domain. Of the 130 orally administered drugs on the WHO list, 61 could be classified with certainty. Twenty-one (84%) of these belong to class I (highly soluble, highly permeable), 10 (17%) to class II (poorly soluble, highly permeable), 24 (39%) to class III (highly soluble, poorly permeable) and 6 (10%) to class IV (poorly soluble, poorly permeable). A further 28 drugs could be provisionally assigned, while for 41 drugs insufficient or conflicting data precluded assignment to a specific BCS class. A total of 32 class I drugs (either certain or provisional classification) were identified. These drugs can be further considered for biowaiver status (drug product approval based on dissolution tests rather than bioequivalence studies in humans).

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.

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.

New, selective catechol-O-methyltransferase inhibitors as therapeutic agents in Parkinson's disease

Levodopa remains the most effective drug for Parkinson's disease (PD). However, its benefits are limited owing to extensive metabolism by catechol-O-methyltransferase (COMT), especially if levodopa is used in combination with peripheral dopa-decarboxylase inhibitors. A new generation of potent, orally active, selective, and reversible COMT inhibitors has become available recently. Among these, tolcapone and entacapone have been best characterised. Preclinical and clinical studies have shown that COMT inhibitors markedly enhance levodopa availability and prolong its plasma half-life. In recent large clinical trials they proved to be able to ameliorate motor fluctuations, reduce disability, and decrease levodopa requirements in PD patients. The tolerability profiles of entacapone and tolcapone are good. COMT inhibition promises to become an important means of extending the benefits of levodopa therapy in PD.

Improvement of L-dopa absorption by dipeptidyl derivation, utilizing peptide transporter PepT1

In the present study, possible enhancement of intestinal absorption of L-dopa by utilizing intestinal peptide transporter was examined using Caco-2 cells and Xenopus oocytes expressing human peptide transporter (hPepT1). To see whether this peptide transporter could be utilized for the improvement of L-dopa absorption, we employed a dipeptide-mimetic derivative of L-dopa, L-dopa-L-Phe. L-Dopa-L-Phe inhibited the uptake of [14C]Gly-Sar, but not that of L-[3H]-dopa by Caco-2 cells. Uptake of L-dopa-L-Phe was increased by expression of hPepT1 in Xenopus oocytes. The appearance of L-dopa and its metabolite, dopamine, on the basolateral side of Caco-2 cells was significantly higher after addition of L-dopa-L-Phe than after that of L-dopa and was reduced by the presence of Gly-Sar on the apical side. These results indicate that the L-dopa-L-Phe is absorbed more efficiently than L-dopa and is taken up via the peptide transporter, but not via the amino acid transporter, demonstrating the possibility of targeting the peptide transporter as a means for improving intestinal absorption of peptide-like drugs.

Simultaneous in vivo microdialysis in plasma and skeletal muscle: a study of the pharmacokinetic properties of levodopa by noncompartmental analysis

This in vivo study compared the pharmacokinetics of intravenously (iv) administered levodopa (L-dopa) in plasma and skeletal muscle. For this purpose, a single iv dose of L-dopa (25 mg/kg) was given to an anesthetized beagle dog, and L-dopa as well as its O-methyl metabolite, 3-O-methyldopa (3-OMD), were monitored in plasma and skeletal muscle simultaneously by microdialysis. The plasma and muscle dialysates were continuously collected during a 3-h period after the iv administration of the drug. The pharmacokinetic variables were then compared in both tissues with noncompartmental modeling. The mean maximum concentration (Cmax) for L-dopa in plasma was 173.10 +/- 9.85 ng/mL, whereas in skeletal muscle extracellular fluid, it was 14.56 +/- 2.27 ng/mL. The area under the curve of concentration versus time from time zero to infinity (AUC0- > inf) values for L-dopa were 20 times higher in plasma compared with muscle. The difference in half-life between the two tissues probably indicated the large contribution of the distribution phase in either or both tissues over the 3-h time interval. Interestingly enough, the AUC0- > 3h values for 3-OMD were within the same range in both tissues. These data demonstrated that over a period of 3 h, no distribution equilibrium for L-dopa was reached over the two tissues. The very low L-dopa/3-OMD ratios suggested that, in contrast to L-dopa, 3-OMD is accumulating in skeletal muscle. Whether these findings have any implication for the therapeutic response to L-dopa in Parkinson's disease remains to be determined.

The effect of carbidopa on the pharmacokinetics and metabolism of intravenously administered levodopa in blood plasma and skeletal muscle

The effect of carbidopa on the pharmacokinetics and metabolism of levodopa (L-dopa) in blood plasma and skeletal muscle extracellular fluid (ECF) has been studied by repeated measurements in one beagle dog. The administration of a single dose of L-dopa (25 mg/kg i.v.) without carbidopa pretreatment (controls) resulted in an increase in the concentrations of L-dopa and 3-O-methyldopa (3-OMD) in blood plasma and skeletal muscle ECF dialysates. This effect was clearly potentiated for L-dopa in blood plasma (186% increase in AUC) and 3-OMD in skeletal muscle dialysates (108% increase in AUC) after pretreatment with carbidopa (100 mg/day). In addition, carbidopa prolonged the half-life of the elimination of L-dopa in blood plasma by 48% and in skeletal muscle ECF by 66% but did not influence its blood plasma distribution half-life (t 1/2 alpha = 0.17 h). The elimination half-life of L-dopa in the controls was higher in muscle (t 1/2 beta = 1.76 h) than in blood plasma (t 1/2 beta = 0.50 h). Carbidopa pretreatment resulted in a relatively small increase (29%) in the L-dopa content of skeletal muscle ECF as indicated by the AUC. The accumulation of 3-OMD in muscle dialysates, in contrast to that in plasma, was significantly enhanced after the administration of L-dopa following treatment with carbidopa. In the control experiments, dopamine (DA) was detectable only in the dialysates from muscle ECF.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo pharmacokinetics of levodopa and 3-O-methyldopa in muscle. A microdialysis study

In the present study in vivo microdialysis sampling coupled to high-performance liquid chromatography with electrochemical detection, was used to study the pharmacokinetics of levodopa and 3-O-methyldopa in skeletal muscle in dog, after intravenous administration of levodopa. For comparison, the pharmacokinetic parameters of both compounds were simultaneously determined in plasma using blood collection. Muscle microdialysis samples and blood were continuously collected for 4 h after the administration of levodopa (25 mg/kg). Pharmacokinetic profiles of levodopa in plasma and muscle were different. The mean Tmax value of levodopa in plasma and muscle was 0.16 h and 1.0 h, respectively. The AUC0----inf for levodopa in plasma was nearly 18-fold higher in plasma than in muscle. The 3-O-methyldopa concentration increased very rapidly after the administration of levodopa, to reach a plateau after 2.5 h and 3 h in plasma and muscle, respectively. The AUC0----4 for 3-O-methyldopa was 3.6-fold higher in plasma than in muscle. The ratio levodopa/3-O-methyldopa, reflecting the metabolic rate of levodopa, was 3.5 times higher in plasma than in muscle, at the peak value of levodopa, and then rapidly declined to values lower than 1, one hour after administration of the drug. We compared our results with literature data from postmortem studies done in rat experiments. We concluded that levodopa is not accumulating in muscle as such, but is converted to 3-O-methyldopa probably before leaving the plasma compartment.

Peripheral pharmacokinetic handling and metabolism of L-dopa in the rat: the effect of route of administration and carbidopa pretreatment

The effect of carbidopa (L-alpha-methyldopa hydrazine; 25 mg kg-1 i.p.) pretreatment on the pharmacokinetics and peripheral metabolism of orally and intra-aortically administered L-3,4-dihydroxyphenylalanine (L-dopa; 50 mg kg-1) has been examined in rats. Following intra-aortic (i.a.) administration, plasma levels of the drug declined biexponentially. Pretreatment with carbidopa resulted in higher plasma concentrations after i.a. administration of L-dopa, but had no effect on the half-life (t1/2) for its distribution or elimination. Oral L-dopa gave peak plasma concentrations at 1.5 h and then a log-linear decline between 1.5 and 6 h. Pretreatment with carbidopa also produced higher plasma concentrations of L-dopa given orally, and the t1/2 for its elimination tended to be increased compared with values achieved after the drug alone. Pretreatment with carbidopa decreased volume of distribution and total plasma clearance and increased area under the curve (0-infinity) after L-dopa i.a. and increased AUC0-infinity after L-dopa p.o. The fraction of the oral dose absorbed through the gut was not affected. Carbidopa pretreatment enhanced the accumulation of 3-O-methyldopa and decreased dopamine levels in plasma after both i.a. and oral administration of L-dopa. Higher plasma concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid (HVA) were detected in the plasma after i.a. rather than oral administration of L-dopa and pretreatment with carbidopa greatly reduced these plasma concentrations. However, following oral L-dopa, only HVA levels were reduced by carbidopa pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Continuous intravenous monitoring of levodopa and 3-O-methyldopa by microdialysis and high-performance liquid chromatography with electrochemical detection

Microdialysis, in association with ion-pair reversed-phase high-performance liquid chromatography with electrochemical detection, was applied in vitro for the determination and quantification of levodopa and 3-O-methyldopa in blank spiked plasma and blood. The method presented gave accurate results; the calibration graphs for plasma were linear over the range of the expected values for both compounds. When using a dialysis probe with a membrane length of 1.6 cm at a 5 microliters min-1 perfusion speed, the recovery rate in plasma for levodopa was 30.1% and 68.5% for 3-O-methyldopa. However, less reproducible results were obtained for plasma levodopa levels in the range of 0.5 microliter ml-1 and lower. The microdialysis technique was subsequently successfully applied for the continuous intravenous monitoring of levodopa and 3-O-methyldopa in a levodopa-treated dogs.

Design and early clinical evaluation of selective inhibitors of monoamine oxidase

1. Selective inhibitors of the monoamine oxidase (MAO) isoenzymes, types A and B, are of potential therapeutic utility. Brain selectivity would overcome the risk of tyramine interactions which have been shown to occur with selective MAO-A but not MAO-B inhibitors. 2. (E)-3-Fluoroallylamines of general structure, FHC = C(R)CH2NH2 have been designed as enzyme-activated, irreversible inhibitors of these enzymes. Two compounds, MDL 72145 (R = 3,4 dimethoxyphenyl) and MDL 72974 (R = 4-fluorophenethyl), are selective and irreversible inhibitors of MAO type B which in vivo show high inhibitory potency against the rat brain enzyme (ED50 0.35 and 0.18 mg/kg p.o., respectively). In animals, these inhibitors do not potentiate the cardiovascular effects of tyramine and have no amphetamine-like effects. However, they do potentiate the central effects of L-Dopa and prevent the neurotoxic effects of MPTP in both mice and monkeys. 3. In early clinical studies, MDL 72145 has been shown to be a potent, long-acting inhibitor of MAO type B. Doses of 16 mg per patient totally inhibit platelet enzyme without potentiating the cardiovascular effects of oral tyramine. Compounds of this type should prove useful in Parkinson's disease. 4. Selective inhibition of brain MAO-A can be achieved by using the bioprecursor amino acid MDL 72394 (E-beta-fluoromethylene-m-tyrosine). This amino acid is decarboxylated by aromatic L-amino acid decarboxylase (AADC) to liberate MDL 72392 (R = 3-hydroxyphenyl), a potent irreversible inhibitor of MAO-A. Combination of MDL 72394 with a peripherally selective inhibitor of AADC (e.g., carbidopa) restricts MAO inhibition to the brain. Consequently, under these conditions, there is a greatly reduced propensity to potentiate the cardiovascular effects of tyramine. 5. This has been confirmed in human volunteers; MDL 72394 (8 mg), combined with carbidopa, substantially decreased urinary MHPG and plasma DHPG concentrations with minimal potentiation of the cardiovascular effects of i.v. tyramine. These results predict that such therapy has potential in the treatment of affective disorders.

Clinical pharmacokinetics of anti-parkinsonian drugs

Of the neurological disorders, none can claim a battery of therapeutic agents based upon as rational a pharmacology as can Parkinson's disease. In this review, the clinical pharmacokinetics of the major classes of anti-Parkinsonian drugs is discussed. Although they are the oldest drugs in the anti-Parkinsonian armamentarium, little pharmacokinetic data are available regarding the anticholinergic and antihistaminic agents. Based on elimination half-lives of 10 to 18 hours, most could probably be effectively given on a twice-daily schedule. Amantadine is unique among anti-Parkinsonian agents both in lacking a clearly defined mechanism of action and in being eliminated from the body exclusively by renal excretion of unchanged drug. Thus the normal decline of renal function in the elderly Parkinsonian population becomes an important factor in avoiding potential drug toxicity. The pharmacokinetics and pharmacodynamics of levodopa are complex. Since it is an amino acid, it follows metabolic pathways and must compete for absorption and brain uptake with a number of large neutral amino acids. It has a short elimination half-life and, as Parkinson's disease progresses, the brain loses its capacity to store the drug and becomes dependent in a moment-to-moment fashion on plasma levodopa concentrations, creating therapeutic response fluctuations in over 50% of patients. Pharmacokinetic considerations in the management of these response fluctuations are discussed. The newest class of anti-Parkinsonian agents are the direct acting dopamine receptor agonists. These drugs, all derivatives of ergot, have more prolonged durations of anti-Parkinsonian action than levodopa. However, other than bromocriptine, clinical experience with members of this class of drugs is still limited.

Parkinsonism--drug treatment: Part I

The purpose of this two-part review is to explain current drug treatment in part I and discuss investigational drug therapy and miscellaneous drugs in the management of parkinsonism in part II. The medical approach to this disease is still based on the imbalance between a deficiency of dopamine and a functional increase in acetylcholine. Anticholinergic agents are used to treat the tremors in the early stages of the disease.

The pharmacokinetics of intravenous and oral levodopa in patients with Parkinson's disease who exhibit on-off fluctuations

We have studied the clinical effects and pharmacokinetics of levodopa infusions and oral therapy in seven patients with Parkinson's disease. They all showed on-off fluctuations whilst receiving long-term treatment with levodopa in combination with a peripheral decarboxylase inhibitor. Intravenous infusion at a constant rate for up to 16 h resulted in a smoother clinical response, and maintained plasma levodopa concentrations within narrower limits compared with conventional oral therapy. Following infusion rates of 32-80 mg h-1 (0.5-1.3 mg kg-1 h-1) the plasma concentration associated with optimum therapeutic response lay between 0.3 and 1.6 mg l-1. There was considerable variation in the oral absorption and elimination of levodopa, both within and between subjects. The concentration of 3-OMe dopa in plasma hardly increased during each day's levodopa therapy. In all cases levels were greater than the maximum concentrations of levodopa, sometimes by as much as a factor of 10. In contrast to most previous reports on the pharmacokinetics of levodopa, the data presented here are consistent with a two-compartment kinetic model. It is not known whether the difference in pharmacokinetics is due to chronic therapy or whether it is specific to those patients who show on-off phenomena, but such changes might be related in some way to the development of fluctuations in clinical response.

Inhibition of monoamine oxidase selectively in brain monoamine nerves using the bioprecursor (E)-beta-fluoromethylene-m-tyrosine (MDL 72394), a substrate for aromatic L-amino acid decarboxylase

(E)-beta-Fluoromethylene-m-tyrosine (FMMT) is a dual-enzyme-activated inhibitor of monoamine oxidase (MAO). The compound is not an inhibitor per se but is decarboxylated by aromatic L-amino acid decarboxylase (AADC) to yield a potent enzyme-activated irreversible inhibitor of MAO, (E)-beta-fluoromethylene-m-tyramine, which shows some selectivity for inhibition of MAO type A. Decarboxylation of FMMT was demonstrated in vitro using hog kidney AADC and in vivo in rats by the ability of alpha-monofluoromethyldopa (MFMD), a potent inhibitor of AADC, to prevent MAO inhibition produced by FMMT. In isolated synaptosomes, FMMT was decarboxylated by AADC, and, furthermore, the compound was actively transported into these isolated nerve endings. An active transport into the CNS has also been demonstrated in vivo by performing competition experiments with leucine. To demonstrate that FMMT is preferentially decarboxylated within monoamine nerves of the CNS, the nigrostriatal 3,4-dihydroxyphenylethylamine (dopamine) pathway of rats was unilaterally lesioned with 6-hydroxydopamine or infused with MFMD. Under these conditions, MAO inhibition produced by orally administered FMMT in the striatum ipsilateral to the lesion or infusion was markedly attenuated. Combination of FMMT with an inhibitor of extracerebral AADC, such as carbidopa, protected peripheral organs against the MAO inhibitory effects and concomitantly enhanced MAO inhibition in the CNS. Such combinations had a greatly reduced propensity to augment the cardiovascular effects of intraduodenally administered tyramine, when compared with FMMT given alone or with clorgyline, a selective inhibitor of MAO type A. The results obtained with FMMT suggest the possibility of achieving selective inhibition of MAO within monoamine nerves of the CNS and, further, suggest that combination of FMMT with an inhibitor of extracerebral AADC will reduce the propensity of this inhibitor to produce adverse interactions with tyramine.

The effect of carbidopa on the pharmacokinetics of intravenously administered levodopa: the mechanism of action in the treatment of parkinsonism

The pharmacokinetics of short and long intravenous infusions of levodopa with and without concurrent oral administration of carbidopa was studied in 9 parkinsonian patients. Carbidopa reduced by 50% both the infusion rate required to produce a clinical response and the time required for plasma clearance of levodopa. Using this value for clearance, it is estimated that carbidopa doubles the bioavailability of orally administered levodopa. Carbidopa did not alter the therapeutically effective plasma concentration of levodopa, suggesting that carbidopa does not modify the so-called enzymatic blood-brain barrier. The decline of the plasma levodopa concentration was biphasic; carbidopa modestly increased half-lives of both phases. The apparent volume of distribution was not significantly altered. Carbidopa did not change the duration of the clinical response after the discontinuation of short infusions. From these observations we infer that the therapeutic effects of carbidopa can be attributed to doubling the bioavailability of orally administered levodopa and halving its plasma clearance.

Pharmacokinetics and toxicity testing

The pharmacokinetic basis for the design of toxicity tests is discussed with reference to the absorption and clearance of drugs. The absorption and clearance of a wide range of drugs by laboratory animals and man has been examined and reviewed to provide a firm basis against which new drugs can be compared. Some pitfalls in either the empirical approach to toxicology or the incorrect interpretation of kinetic data are highlighted. An approach is outlined for the rational application of animal pharmacokinetic data in the assessment of the safety in man of a new therapeutic agent.

Intestinal absorption of levodopa in man

In four healthy subjects the intestinal absorption of levodopa (l-dopa) was investigated by measuring the plasma concentration of the amino acid following the administration of l-dopa at three different sites in the small intestine. In order to minimize presystemic clearance of l-dopa, the subjects were pretreated with the peripheral decarboxylase inhibitor benserazide 3 X 50 mg every 8 h on the previous day and 1 X 50 mg 2 h prior to administration of the l-dopa. L-dopa 100 mg dissolved in 0.05 N HCl and 50 mg benserazide dissolved in 0.05 N HCl were coadministered. Under these conditions no difference in tmax, cmax or AUC of l-dopa was observed between administration of the drug into the proximal or the distal part of duodenum, or into the upper part of jejunum. The results indicate that in healthy subjects, during inhibition of peripheral decarboxylase, the rate and extent of l-dopa absorption does not differ at any site in the upper small intestine.

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 IV: Possible causes of low bioavailability of oral levodopa in dogs

Several potential mechanisms for reduced levodopa bioavailability following oral administration to dogs and humans were investigated by studying the influence of the administration route on plasma levodopa levels after intravenous, hepatoportal, and duodenal administrations to dogs. The observed average areas under the plasma concentration-time curves (AUC) of levodopa following hepatoportal injection and intravenous injection were virtually identical; but following duodenal administration a decrease in the AUC of levodopa was observed with a concomitant increase in the AUC of total dopamine. The possible involvement of intestinal microorganisms in levodopa metabolism was explored in dogs that had been administered a combination of paromomycin and kanamycin to reduce intestinal microflora. Similar patterns of plasma level profiles and urinary excretion were observed between control and treated dogs. As measured by the release of [14C]carbon dioxide from [14C]levodopa, the distribution of levodopa decarboxylase enzyme activity in various parts of the intestine was studied in homogenates prepared from isolated intestinal segments of the duodenum and upper, middle, and lower parts of the jejunum and ileum. The jejunum showed the highest decarboxylase activity followed by the ileum and duodenum. These data indicate that the reduced bioavailability of orally administered levodopa occurs as a result of metabolism by levodopa decarboxylase enzyme in the gut wall.

Dosage form design for improvement of bioavailability of levodopa III: Influence of dose on pharmacokinetic behavior of levodopa in dogs and Parkinsonian patients

The relationship between the dose of levodopa and its pharmacokinetic behavior following intravenous and oral administration was investigated in dogs and parkinsonian patients. Six beagle dogs received single doses of 2.4, 4.8, and 9.6 mg of levodopa/kg iv and single doses of 4.8, 9.6, and 19.2 mg of levodopa/kg po in a crossover fashion on separate occasions. Three parkinsonian patients received single oral doses of approximately 3.8, 7.7, and 15.4 mg of levodopa/kg in a crossover test. Plasma samples were analyzed for intact levodopa and total dopamine. The relationship between the area under the plasma concentration-time curve (AUC) of levodopa and the intravenous dose to dogs was linear. However, in both dogs and patients, the relationship after oral dosing was nonlinear, with the relative AUC increasing with increasing dose. Therefore, the pharmacokinetic behavior of levodopa after oral administration to dogs and patients was dose dependent.