Population pharmacokinetics and pharmacodynamics of oral levodopa in parkinsonian patients

Model-based optimization of controlled release formulation of levodopa for Parkinson's disease

Levodopa is currently the standard of care treatment for Parkinson's disease, but chronic therapy has been linked to motor complications. Designing a controlled release formulation (CRF) that maintains sustained and constant blood concentrations may reduce these complications. Still, it is challenging due to levodopa's pharmacokinetic properties and the notion that it is absorbed only in the upper small intestine (i.e., exhibits an "absorption window"). We created and validated a physiologically based mathematical model to aid the development of such a formulation. Analysis of experimental results using the model revealed that levodopa is well absorbed throughout the entire small intestine (i.e., no "absorption window") and that levodopa in the stomach causes fluctuations during the first 3 h after administration. Based on these insights, we developed guidelines for an improved CRF for various stages of Parkinson's disease. Such a formulation is expected to produce steady concentrations and prolong therapeutic duration compared to a common CRF with a smaller dose per day and a lower overall dose of levodopa, thereby improving patient compliance with the dosage regime.

Levodopa-Induced Dyskinesias in Parkinson's Disease: An Overview on Pathophysiology, Clinical Manifestations, Therapy Management Strategies and Future Directions

Since its first introduction, levodopa has become the cornerstone for the treatment of Parkinson's disease and remains the leading therapeutic choice for motor control therapy so far. Unfortunately, the subsequent appearance of abnormal involuntary movements, known as dyskinesias, is a frequent drawback. Despite the deep knowledge of this complication, in terms of clinical phenomenology and the temporal relationship during a levodopa regimen, less is clear about the pathophysiological mechanisms underpinning it. As the disease progresses, specific oscillatory activities of both motor cortical and basal ganglia neurons and variation in levodopa metabolism, in terms of the dopamine receptor stimulation pattern and turnover rate, underlie dyskinesia onset. This review aims to provide a global overview on levodopa-induced dyskinesias, focusing on pathophysiology, clinical manifestations, therapy management strategies and future directions.

An integrative model of Parkinson's disease treatment including levodopa pharmacokinetics, dopamine kinetics, basal ganglia neurotransmission and motor action throughout disease progression

Levodopa is considered the gold standard treatment of Parkinson's disease. Although very effective in alleviating symptoms at their onset, its chronic use with the progressive neuronal denervation in the basal ganglia leads to a decrease in levodopa's effect duration and to the appearance of motor complications. This evolution challenges the establishment of optimal regimens to manage the symptoms as the disease progresses. Based on up-to-date pathophysiological and pharmacological knowledge, we developed an integrative model for Parkinson's disease to evaluate motor function in response to levodopa treatment as the disease progresses. We combined a pharmacokinetic model of levodopa to a model of dopamine's kinetics and a neurocomputational model of basal ganglia. The parameter values were either measured directly or estimated from human and animal data. The concentrations and behaviors predicted by our model were compared to available information and data. Using this model, we were able to predict levodopa plasma concentration, its related dopamine concentration in the brain and the response performance of a motor task for different stages of disease.

Dopamine Buffering Capacity Imaging: A Pharmacodynamic fMRI Method for Staging Parkinson Disease

We propose a novel pharmacological fMRI (phMRI) method for objectively quantifying disease severity in Parkinson disease (PD). It is based on the clinical observation that the benefit from a dose of levodopa wears off more quickly as PD progresses. Biologically this has been thought to represent decreased buffering capacity for dopamine as nigrostriatal cells die. Buffering capacity has been modeled based on clinical effects, but clinical measurements are influenced by confounding factors. The new method proposes to measure the effect objectively based on the timing of the known response of several brain regions to exogenous levodopa. Such responses are robust and can be quantified using perfusion MRI. Here we present simulation studies based on published clinical dose-response data and an intravenous levodopa infusion. Standard pharmacokinetic-pharmacodynamic methods were used to model the response. Then the effect site rate constant k e was estimated from simulated response data plus Gaussian noise. Predicted time - effect curves sampled at times consistent with phMRI differ substantially based on clinical severity. Estimated k e from noisy input data was recovered with good accuracy. These simulation results support the feasibility of levodopa phMRI hysteresis mapping to measure the severity of dopamine denervation objectively and simultaneously in all brain regions with a robust imaging response to exogenous levodopa.

Pharmacokinetics of Rytary®, An Extended-Release Capsule Formulation of Carbidopa-Levodopa

Parkinson's disease (PD) is a chronic progressive neurological disorder characterized by resting tremor, rigidity, bradykinesia, gait disturbance, and postural instability. Levodopa, the precursor to dopamine, coadministered with carbidopa or benserazide, aromatic amino acid decarboxylase inhibitors, is the most effective and widely used therapeutic agent in the treatment of PD. With continued levodopa treatment, a majority of patients develop motor complications such as dyskinesia and motor 'on-off' fluctuations, which are, in part, related to the fluctuations in plasma concentrations of levodopa. A new extended-release (ER) carbidopa-levodopa capsule product (also referred to as IPX066) was developed and approved in the US as Rytary^® and in the EU as Numient^®. The capsule formulation is designed to provide an initial rapid absorption of levodopa comparable to immediate-release (IR) carbidopa-levodopa, and to subsequently provide stable levodopa concentrations with reduced peak-to-trough excursions in plasma concentrations in order to reduce motor fluctuations associated with pulsatile stimulation of dopamine receptors and to minimize dyskinesia. Phase III studies of this ER carbidopa-levodopa capsule formulation in patients with PD have shown a significant reduction in 'off' time compared with IR carbidopa-levodopa and carbidopa-levodopa-entacapone. We present a review of the clinical pharmacokinetics and pharmacodynamics of this ER product of carbidopa-levodopa in healthy subjects and in patients with PD.

Population pharmacokinetics of levodopa in subjects with advanced Parkinson's disease: levodopa-carbidopa intestinal gel infusion vs. oral tablets

AIMS

Levodopa-carbidopa intestinal gel (LCIG) provides continuous levodopa-carbidopa delivery through intrajejunal infusion. This study characterized the population pharmacokinetics of levodopa following a 16 h jejunal infusion of LCIG or frequent oral administration of levodopa-carbidopa tablets (LC-oral) in subjects with advanced Parkinson's disease (PD).

METHODS

A non-linear mixed-effects model of levodopa pharmacokinetics was developed using serial plasma concentrations from an LCIG phase 1 study and a phase 3 double-blind, double-dummy study of the efficacy and safety of LCIG compared with LC-oral in advanced PD patients (n = 68 for model development; 45 on LCIG and 23 on LC-oral). The final model was internally evaluated using stochastic simulations and bootstrap and externally evaluated using sparse pharmacokinetic data from 311 subjects treated in a long term safety study of LCIG.

RESULTS

The final model was a two compartment model with a transit compartment for absorption, first order elimination, bioavailability for LCIG (97%; confidence interval = 95% to 98%) relative to LC-oral, different first order transit absorption rate constants (LCIG = 9.2 h(-1) vs. LC-oral = 2.4 h(-1) ; corresponding mean absorption time of 7 min for LCIG vs. 25 min for LC-oral) and different residual (intra-subject) variability for LCIG (15% proportional error, 0.3 μg ml(-1) additive error) vs. LC-oral (29% proportional error, 0.59 μg ml(-1) additive error). Estimated oral clearance and steady-state volume of distribution for levodopa were 24.8 l h(-1) and 131 l, respectively.

CONCLUSIONS

LCIG administration results in faster absorption, comparable levodopa bioavailability and significantly reduced intra-subject variability in levodopa concentrations relative to LC-oral administration.

Population pharmacodynamics of IPX066: an oral extended-release capsule formulation of carbidopa-levodopa, and immediate-release carbidopa-levodopa in patients with advanced Parkinson's disease

A pharmacodynamic model is presented to describe the motor effects (tapping rate, Unified Parkinson's Disease Rating Scale [UPDRS] Part III, and investigator-rating of ON/OFF, including dyskinesia) of levodopa (LD) in patients with advanced idiopathic Parkinson's disease (PD) treated with immediate-release (IR) carbidopa-levodopa (CD-LD) or an extended-release (ER) formulation of CD-LD (IPX066). Twenty-seven patients participated in this open-label, randomized, single- and multiple-dose, crossover study. The pharmacodynamic models included a biophase effect site with a sigmoid E(max) transduction for tapping and UPDRS and an ordered categorical model for dyskinesia. The pharmacodynamics of LD was characterized by a conduction function with a half-life of 0.59 hours for tapping rate, and 0.4 hours for UPDRS Part III and dyskinesia. The LD concentration for half-maximal effect was 1530 ng/mL, 810 ng/mL, and 600 ng/mL for tapping rate, UPDRS Part III, and dyskinesia, respectively. The sigmoidicity of the transduction was 1.53, 2.5, and 2.1 for tapping rate, UPDRS Part III, and dyskinesia, respectively. External validation of the pharmacodynamic model using tapping rate indicated good performance of the model.

Integrated pharmacokinetics and pharmacodynamics in drug development

Drug development is a complex, lengthy and expensive process. Pharmaceutical companies and regulatory authorities have recognised that the drug development process needs optimisation for efficiency in view of the return on investments. Pharmacokinetics and pharmacodynamics are the two main principles determining the relationship between dose and response. This article provides an update on integrated approaches towards drug development by linking pharmacokinetics, pharmacodynamics and disease aspects into mathematical models. Gradually, a transition is taking place from a rather empirical approach towards a modelling- and simulation-based approach to drug development. The main learning phases should be phases 0, I and II, whereas phase III studies should merely have a confirmatory purpose. In model-based drug development, mechanism-based mathematical models, which are iteratively refined along the path of development, incorporate the accumulating knowledge of the investigational drug, the disease and their mutual interference in different subsets of the target population. These models facilitate the design of the next study and improve the probability of achieving the projected efficacy and safety endpoints. In this article, several theoretical and practical aspects of an integrated approach towards drug development are discussed, together with some case studies from different therapeutic areas illustrating the application of pharmacokinetic/pharmacodynamic disease models at different stages of drug development.

Importance of within subject variation in levodopa pharmacokinetics: a 4 year cohort study in Parkinson's disease

The purpose of the study was to describe the population pharmacokinetics of levodopa in patients with Parkinson's disease studied in 5 trials (10 occasions) over 4 years. Twenty previously untreated Parkinsonian patients were investigated. Each trial consisted of a 2-hr IV infusion of levodopa (1 mg/kg/h) with concomitant oral carbidopa given on two occasions separated by 72 hr with no levodopa in between. This trial design was repeated at 6, 12, 24 and 48 months. A two-compartment pharmacokinetic model with central volume (V1), peripheral volume (V2), clearance (CL) and inter-compartmental clearance (CL(ic)) was used to fit plasma levodopa concentrations. The model accounted for levodopa dosing prior to each trial and endogenous levodopa synthesis. Population parameter estimates (geometric mean) and population parameter variability (PPV; SD of normal distribution) were V1 11.4 l/70 kg (0.44), CL 30.9 l/h/70 kg (0.25), V2 27.3 l/70 kg (0.27), and CL(ic) 34.6 l/h/70 kg (0.48). PPV was partitioned into between subject variability (BSV) which was 0.12 V1, 0.13 CL, 0.15 V(2), 0.28 CL(ic), within trial variability (WTV) which was 0.16 V1, 0.13 CL, 0.08 V2, 0.18 CL(ic) and between trial variability (BTV) which was 0.40 V1, 0.17 CL, 0.21 V2, 0.34 CL(ic.) Neither structural nor random levodopa pharmacokinetic parameters were associated with the time course of development of fluctuation in motor response. Variability in levodopa pharmacokinetic parameters (particularly V1) may result in variability in plasma levodopa concentrations that could contribute to fluctuations in motor response.

Modeling the short- and long-duration responses to exogenous levodopa and to endogenous levodopa production in Parkinson's disease

Clinicians recognize levodopa has a short-duration response (measured in hr) and a long-duration response (measured in days) in Parkinson's disease. In addition there is a diurnal pattern of motor function with better function in the morning. Previous pharmacokinetic-pharmacodynamic modeling has quantified only the short-duration response. We have developed a pharmacokinetic-pharmacodynamic model for the short- and long-duration responses to exogenous levodopa and the effects of residual endogenous levodopa synthesis in patients with Parkinson's disease. Thirteen previously untreated (de novo) patients with Parkinson's disease and twelve patients who had received levodopa orally for 9.7+/-4.0 years (chronic) were investigated. A 2 hr IV infusion of levodopa with concomitant oral carbidopa was given on two occasions separated by 3 days with no levodopa in between. A two compartment pharmacokinetic model was used to fit plasma levodopa concentrations. A sigmoid Emax model was used to relate concentrations from endogenous and exogenous sources to tapping rate (a measure of motor response). A model incorporating three effect compartments (fast equilibration (half life, Teqf). slow equilibration (Teqs) and dopa synthesis (Teqd)), yielded the most descriptive model for levodopa pharmacokinetics and pharmacodynamics. Baseline tapping rate reflected endogenous levodopa synthesis and the long-duration response. Partial loss of the long-duration response during the 3 days without levodopa in the chronic group lowered baseline tapping (36+/-7%, mean+/-SEM) and increased maximum levodopa induced response above baseline (112+/-31%). The maximum levodopa induced response after the drug holiday is a result of lowered baseline tapping due to the loss of long-duration response and not due to a change in levodopa pharmacokinetics or pharmacodynamics.

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.

Levodopa therapy monitoring in patients with Parkinson disease: a kinetic-dynamic approach

The authors assessed differences in both therapeutic and dyskinesia-matched concentrations of levodopa by kinetic-dynamic modeling in a large cohort of patients with Parkinson disease grouped by severity of symptoms. The goal was to provide a kinetic-dynamic approach to levodopa therapy monitoring to assist treating physicians in rationalizing patients' drug schedules in line with disease progression. Eighty-six patients, grouped according to Hoehn & Yahr (H&Y) clinical stage (H&Y I, n = 23; II, n = 25; III; n = 25; IV, n = 13) enrolled in the study. After a 12-hour levodopa washout each patient was examined using a standard oral levodopa test, based on simultaneous serial measurements of plasma levodopa concentrations, finger-tapping motor effects, and dyskinesia ratings. The kinetic-dynamic modeling for both effects was carried out according to the "link" effect compartment model and sigmoidal pharmacodynamic model. Levodopa plasma kinetics did not differ among patient groups. Duration of motor response was significantly (p < 0.001) curtailed in patients in advanced clinical stages whereas dyskinesia duration showed minor changes among the three affected groups (H&Y II, III, and IV). Median effective concentrations (EC 50 ) were increased at the more advanced clinical stage (p < 0.001), from a median 0.2 microg/mL in patients at H&Y stage I to 0.9 microg/mL in patients at H&Y stage IV, whereas the maximum effect showed less consistent changes among the four groups. Intrasubject levodopa therapeutic concentrations were lower than values for dyskinesias in patients at the moderate stage of the disease, equaling dyskinesia-matched drug concentrations in the more affected patients. These findings are in line with previous observations of major changes in levodopa concentration-effects relationship with disease progression and support a stratification of patients with Parkinson disease according to kinetic-dynamic modeling. From a practical point of view, knowledge of individual patients' kinetic-dynamic variables can help the physician assess patients' clinical needs objectively and optimize levodopa dosing according to disease progression.

Pharmacokinetic-pharmacodynamic relationship of levodopa with and without tolcapone in patients with Parkinson's disease

OBJECTIVE

To investigate the effect of administration of the catechol-Omethyltransferase (COMT) inhibitor tolcapone on the concentration-effect relationship of levodopa in patients with advanced Parkinson's disease and on-off fluctuations.

DESIGN

Nonblind single-group 2-period pharmacokinetic-pharmacodynamic study.

PATIENTS AND PARTICIPANTS

12 patients, mean age 59 years, with idiopathic Parkinson's disease and response fluctuations.

METHODS

The pharmacokinetics [plasma concentrations of levodopa and 3-O-methyldopa (3-OMD)] and motor effects [global score of the Columbia University Rating Scale (CURSsigma)] of levodopa (plus the peripheral decarboxylase inhibitor benserazide 1:4) were determined for 4 consecutive dosage intervals (4 hours each, starting at 8.00am) in 12 patients before (day 1) and during (day 8) coadministration of tolcapone 100 mg 3 times daily for 7 days.

RESULTS

Under tolcapone, exposure to levodopa [area under the plasma concentration-time for the dosage interval (AUCt)] observed for the separate doses increased by 1.6- to 2.2-fold, and peak plasma drug concentrations (Cmax) increased by 1.1 - to 2.1 -fold. 3-OMD concentrations at day 8 were reduced to about 20% of the values at day 1. At baseline (day 1, before the first levodopa dose), CURSsigma averaged 40 +/- 10 points. After the first levodopa dose. CURSsigma declined to 20 +/- 9 points. At day 8. the predose CURSsigma decreased to a final score of 31 +/-13 points, and the maximal decline after the first levodopa dose was to a final score of 16 +/- 8 points. Population analysis (NONMEM) of the concentration-effect relationship of levodopa according to a sigmoidal Emax model and over all dosage intervals did not show differences in levodopa responsiveness with or without tolcapone. The population mean of the 50% effective concentration (EC50) of levodopa was 1350 microg/L with an standard error of the population parameter estimate of 18%: adding tolcapone treatment as a covariate did not significantly change the population fit. Circadian influences on levodopa respon- siveness were not evaluable by the NONMEM model due to overparametrisation, but visual inspection of plotted data did not suggest differences in the concentration-effect relationship between the 4 consecutive dosage intervals on days 1 and 8.

CONCLUSIONS

The gain in clinical improvement with levodopa under tolcapone can be fully explained by tolcapone-induced changes of peripheral levodopa pharmacokinetics. We suggest that this interaction study, performed in patients and using clinical data, excludes any central effects of tolcapone or any inhibiting effect of 3-OMD on levodopa permeation through the blood-brain barrier, which otherwise would have led to a decrease in the EC50 of levodopa.

Concentration-response relationship of levodopa in patients at different stages of Parkinson's disease

OBJECTIVE

To assess differences in the pharmacokinetic and pharmacodynamic relations of levodopa in clinically defined groups and to prove that pharmacokinetic and pharmacodynamic parameters are associated with duration of disease and length of treatment.

METHODS

We studied the pharmacokinetic and pharmacodynamic relations of levodopa after a single dose (100 mg levodopa with 25 mg benserazide) among four groups of patients with Parkinson's disease. Group 1 was levodopa-naive patients (n = 8); group 2 was patients in stable condition taking levodopa (n = 10); group 3 was patients with on-and-off fluctuations (n = 11); and group 4 was patients with on-and-off fluctuations and peak-dose dyskinesia (n = 8). The Columbia University Rating Scale was used for clinical assessment. The pharmacokinetic-pharmacodynamic analysis was based on an estimate of the maximal response model with a semiparametric approach to effect-site equilibrium (equilibration half-life).

RESULTS

The mean concentration at half-maximal effect estimated for the different groups was as follows (mean value +/- SD): group 1, 389 +/- 138 ng x ml(-1); group 2, 346 +/- 203 ng x ml(-1); group 3, 543 +/- 245 ng x ml(-1); group 4, 711 +/- 215 ng x ml(-1). Estimate of the maximal response was determined to be the following: group 1, 10 +/- 3; group 2, 12 +/- 5; group 3, 24 +/- 13; group 4, 18 +/- 7. A significant correlation was observed between duration of Parkinson's disease and mean concentration at half-maximal effect (p < 0.001), estimate of maximal response (p < 0.05), and, inversely, equilibration half-life (p < 0.05).

CONCLUSIONS

The data suggested that levodopa-naive patients and patients in stable condition taking levodopa do not differ in pharmacokinetic-pharmacodynamic relations, whereas patients with fluctuations, especially patients with peak-dose dyskinesia, exhibit a larger threshold level (mean concentration at half-maximal effect). It was concluded that progression of the disease (loss of endogenous dopamine synthesis and reduced dopamine storage) is reflected by pharmacokinetic and pharmacodynamic parameters that characterize the demand for exogenous dopamine provided by levodopa.

Population pharmacodynamic modeling of levodopa in patients with Parkinson's disease receiving entacapone

OBJECTIVE

To assess the pharmacodynamics of levodopa among patients with Parkinson's disease showing end-of-dose fluctuations at different doses of entacapone.

METHODS

Nineteen patients participated in a randomized, double-blind phase II study with a crossover design. Doses of 50, 100, 200, or 400 mg entacapone or placebo were given with the patient's individual levodopa-dopa decarboxylase inhibitor dose. Blood samples were withdrawn for pharmacokinetic analysis, and the clinical response was measured using the motor part of the Unified Parkinson's Disease Rating Scale. A population pharmacodynamic model was developed with the NONMEM program.

RESULTS

A sigmoidal Emax model with an effect compartment was used to relate plasma concentrations of levodopa with clinical response. In the population analysis two covariate relationships were found. The first was E0 = 55.2, [1 + 0.012. (Dur-13)], where E0 is the initial motor Unified Parkinson's Disease Rating Scale score, and Dur is the duration of disease in years. The second was C50(carbidopa) = 951 ng/ml; C50(benserazide) = 1238 ng/ml, where C50 is the steady-state plasma concentration of levodopa eliciting half of maximum attainable effect, and carbidopa and benserazide are the dopa decarboxylase inhibitors given in the study. No effect of entacapone on clinical response beyond its influence on levodopa pharmacokinetics was found. Interindividual and interoccasion variabilities were estimated.

CONCLUSIONS

A population pharmacodynamic model for levodopa was built that took into account interindividual and intraindividual variability. The main finding was that entacapone does not alter the concentration-effect curve of levodopa, suggesting that entacapone acts at the level of peripheral pharmacokinetics of levodopa and that plasma levels of 3-O-methyldopa have a negligible role in the pharmacodynamics of levodopa.

NONMEM population pharmacokinetic modeling of orally administered cyclosporine from routine drug monitoring data after heart transplantation

The population pharmacokinetics of cyclosporine (CsA) in adult recipients of cardiac transplants were determined from sparse, retrospective drug monitoring data accumulated for at least 3 months after surgery. All were receiving oral CsA twice daily, and morning trough levels in whole-blood were measured by high-performance liquid chromatography. Additional data included height, weight, gender, age, ethnicity, hematocrit, total bilirubin, and concurrent drug use. Population modeling was performed using NONMEM on 36 randomly selected patients, assuming a one-compartment model with first-order absorption and elimination. Improved fits were obtained by incorporating the following expression in the model to adjust oral bioavailability as a function of postoperative day (POD): F = 0.2 + 10 x ABS (POD - 7)/([POD + 10] x 60). Interpatient variability (CV%) in clearance (CL) was 20.2%. There was a mean bias of 8.5% at the average CsA concentration of 250 ng/ml when the predictive performance was assessed statistically in a reserved subset of 33 patients who received cardiac transplants. For the entire population (n = 69 patients), the average CsA CL and terminal half-life (T1/2) were, respectively: CL (l/h) = 0.256 x weight (kg); T1/2 = 11.0 hours, or CL (l/h) = 0.184 x weight (kg); T1/2 = 14.7 hours, if there was concomitant diltiazem administration. These results compared favorably with those reported elsewhere for studies of postcardiac transplant kinetics using the traditional multiple blood sampling approach.

Effect of oral levodopa treatment on articulatory function in Parkinson's disease: preliminary results

To quantify lip function in 16 patients with Parkinson's disease, a computerized semiconductor lip pressure transducer system was used prior to subjects being administered oral levodopa and at approximately 0.5 hr, 1.5 hr, and 3.0 hr postmedication. Two blood samples were taken from each subject at varying times during the levodopa dosage interval, and the exact time and dosage of levodopa were noted. Lip function measurements were expressed as percentage changes from baseline and were plotted for each subject against time and levodopa concentrations to determine the effects of levodopa therapy on articulatory function. The results supported the effectiveness of levodopa therapy in improving lip function. In particular, lip pressures recorded during both speech and nonspeech tasks tended to improve after levodopa administration, the lip measures improving somewhat in parallel with the rise and fall of blood plasma levodopa concentrations. Evidence of a hysteresis effect was noted.