Comparative Bioavailability of 2 Tablet Formulations of Levodopa/Benserazide in Healthy, Fasting Volunteers: A Single-Dose, Randomized-Sequence, Open-Label Crossover Study

Dopamine regulates decision thresholds in human reinforcement learning in males

Dopamine fundamentally contributes to reinforcement learning, but recent accounts also suggest a contribution to specific action selection mechanisms and the regulation of response vigour. Here, we examine dopaminergic mechanisms underlying human reinforcement learning and action selection via a combined pharmacological neuroimaging approach in male human volunteers (n = 31, within-subjects; Placebo, 150 mg of the dopamine precursor L-dopa, 2 mg of the D2 receptor antagonist Haloperidol). We found little credible evidence for previously reported beneficial effects of L-dopa vs. Haloperidol on learning from gains and altered neural prediction error signals, which may be partly due to differences experimental design and/or drug dosages. Reinforcement learning drift diffusion models account for learning-related changes in accuracy and response times, and reveal consistent decision threshold reductions under both drugs, in line with the idea that lower dosages of D2 receptor antagonists increase striatal DA release via an autoreceptor-mediated feedback mechanism. These results are in line with the idea that dopamine regulates decision thresholds during reinforcement learning, and may help to bridge action selection and response vigor accounts of dopamine.

Effects of Generic Exchange of Levodopa Medication in Patients With Parkinson Disease

OBJECTIVES

Generic exchange is common practice in most healthcare systems. This study investigated how patients with Parkinson disease (PD) perceived a switch of their levodopa medication and the resulting effects on their PD symptoms.

METHODS

A questionnaire was developed, piloted, and finally distributed to 13,857 members of the national PD patient support group. It was designed to be completed by patients and their pharmacies. χ 2 tests for independence statistics with or without Monte Carlo simulation were performed. Cramér φ and Cramér V were calculated. McNemar test was used to investigate whether a generic switch of a levodopa-containing medication had an impact on PD symptoms.

RESULTS

Analyses were done with 410 finalized respondents of 13,857 distributed questionnaires. More than half of the responders were 75 years or older and rated themselves Hoehn and Yahr stages 3 to 5. Most patients were confused by a change of their medication. A total of 54.7% of the switchers (n = 148) reported swallowing difficulties with medication, which was significantly more frequent than with nonswitchers (37.3% of 204, P = 0.001). Adverse effects related to the switch were reported by 26.6% of all switchers (switchback rate, 20.5%). The patients at higher Hoehn and Yahr stages were affected to a greater extend.

CONCLUSIONS

Patients, who experienced any brand switch of their levodopa medication, frequently expressed distrust and confusion. Swallowing difficulties and negative effects on their symptoms were problems, which were more pronounced in advanced disease stages. It remains unclear whether the detrimental impact on therapy was caused by the brand switch or by a nocebo effect.

Dopaminergic modulation of the exploration/exploitation trade-off in human decision-making

Involvement of dopamine in regulating exploration during decision-making has long been hypothesized, but direct causal evidence in humans is still lacking. Here, we use a combination of computational modeling, pharmacological intervention and functional magnetic resonance imaging to address this issue. Thirty-one healthy male participants performed a restless four-armed bandit task in a within-subjects design under three drug conditions: 150 mg of the dopamine precursor L-dopa, 2 mg of the D2 receptor antagonist haloperidol, and placebo. Choices were best explained by an extension of an established Bayesian learning model accounting for perseveration, directed exploration and random exploration. Modeling revealed attenuated directed exploration under L-dopa, while neural signatures of exploration, exploitation and prediction error were unaffected. Instead, L-dopa attenuated neural representations of overall uncertainty in insula and dorsal anterior cingulate cortex. Our results highlight the computational role of these regions in exploration and suggest that dopamine modulates how this circuit tracks accumulating uncertainty during decision-making.

Effects of generic exchange of solid oral dosage forms in neurological disorders: a systematic review

Background Generic drug exchange is common practice in most healthcare systems. While generics certainly contribute to economic savings, the altered drug formulation might be associated with potential therapeutic problems. Given the narrow therapeutic windows in neurologic indications, any detrimental effect on the therapy can lead to significant consequences. Aim of the review This review aims to investigate potential problems related to a switch from brand-name to generic or from generic to generic drug products in patients with neurologic diseases. Method The review was conducted following the PICO framework and the PRISMA guidelines. MEDLINE and Scopus databases were searched for articles published in English and German language between January 1, 1995 and October 17, 2018. Studies included in this review were randomized controlled studies, reviews, systematic reviews, overviews, cohort studies and case-control studies. Studies excluded were letters, comments, authors view, congress or seminar papers and studies with a focus on economic impact or costs. Results were synthesized qualitatively. The primary outcomes were pharmacokinetic parameters such as the area under the curve (AUC), the peak serum concentration (c_max) or the time at which c_max is observed (t_max). Results The search identified 67 studies with a great variety of endpoints and study designs. The leading indication was epilepsy. Two small RCTs were found on lamotrigine switch. Analysis of the other studies found no significant differences in pharmacokinetic parameters when switching to generic drugs. A more heterogeneous picture was revealed regarding hospitalizations, breakthrough seizures, failure of therapy, adherence and patient concerns. Conclusion While most reports were of poor quality, lamotrigine was the drug with the best available data. Summarizing the results of the available studies, pharmacokinetic parameters of antiepileptic drugs show low deviation. In contrast, data on clinical parameters are less consistent. Some studies found increased seizure frequencies and adverse-drug events, while others showed no complications. Adherence and patient satisfaction seemed to be impaired. In daily practice, generic exchange in epilepsy should be a carefully balanced decision, conducted with great caution. Further research is needed, especially regarding neurologic indications other than epilepsy.

Clinical and pharmacokinetics equivalence of multiple doses of levodopa benserazide generic formulation vs the originator (Madopar)

AIMS

While several generic preparations of levodopa/carbidopa and levodopa/benserazide (LBD) are currently available, pharmacokinetic (PK) equivalence and therapeutic equivalence studies with levodopa generics are not available in Italy. Lack of data on generic formulations is a critical factor for their limited use in this country and often lead patients to refuse the generic version of the branded drug.

METHODS

An experimental, 2-centre, randomized, double-blind, 2-sequence, noninferiority cross-over study was designed to evaluate both the PK equivalence and clinical equivalence of multiple doses of the generic preparation of LDB, Teva Italia, compared to the originator (Madopar). Forty-three out-patients with a diagnosis of idiopathic Parkinson's disease on LDB, were recruited and randomly assigned to 1 of 2 study sequences: generic-originator or originator-generic. Clinical evaluations were performed at the end of each study period. A PK study with an LDB fixed dose (100 + 25 mg) was performed in a subpopulation of 14 subjects.

RESULTS

Clinical data showed a reduction of 0.49 and 1.54 in the mean UPDRS III scores for the LDB and the originator, respectively. The 95% CIs [-2.21: 0.11] of the mean difference original vs LDB are smaller than the clinically significant difference of 3 UPDRS III points, supporting the conclusion that the treatment with LDB is not inferior to the originator. No statistically significant differences were found with respect to area under the curve to last dose, half-life, maximum concentration, time to maximum concentration and last observed concentration.

CONCLUSION

These findings prove the therapeutic clinical equivalence as well the PK equivalence of the generic LDB and the originator (Madopar).

Baseline impulsivity may moderate L-DOPA effects on value-based decision-making

Research has indicated a major role of dopamine in decision-making processes, but the underlying mechanisms remain largely unknown due to inconsistency in effects of dopaminergic drugs. To clarify the impact of dopamine on impulsive choice, we administered 150 mg L-DOPA to 87 healthy adults in a randomized, placebo-controlled, double-blind, crossover study, evaluating performance in four value-based decision-making tasks. We predicted that baseline impulsivity would moderate L-DOPA effects. In support of our hypothesis, L-DOPA had no main effect on impulsive choice, but reduced risk-seeking for gains in more-impulsive subjects. Because L-DOPA effects may be influenced by body weight, we repeated our analyses on data from half of the sample (n = 44) with lower weight, anticipating a stronger effect. In addition to the effect on risk-seeking for gains, low-weight participants also exhibited baseline-dependent effects of L-DOPA on loss aversion and delay discounting. Our results are consistent with the hypothesis of an inverted U-shaped dopamine function in which both low and high extremes of dopamine signaling are associated with high-impulsive choice. Consideration of differential baseline impulsivity and body weight may resolve previous seemingly paradoxical pharmacological results and might deepen our understanding of dopaminergic mechanisms underlying impulsivity.

Sensitive LC–MS/MS method for quantitation of levodopa and carbidopa in plasma: application to a pharmacokinetic study

Aim: The combination of levodopa with carbidopa has been used for treatment of Parkinson's disease being an important therapy in dopamine level control in the brain. Both are very polar compounds becoming a challenge for analysis by LC–MS/MS.Materials & methods: In this work, it was developed and validated a sensitive bioanalytical method by UHPLC–MS/MS for simultaneous levodopa and carbidopa quantification in human plasma using a fast protein precipitation method. Moreover, the bioanalytical method was applied to a pharmacokinetic study in healthy volunteers. Results/Conclusion: The results demonstrated a sensitive and adequate method for application to pharmacokinetic/bioequivalence studies.

Model-based dietary optimization for late-stage, levodopa-treated, Parkinson's disease patients

Levodopa has been the gold standard for Parkinson’s disease treatment for more than 40 years. Its bioavailability is hindered by dietary amino acids, leading to fluctuations in the motor response particularly in late-stage (stage 3 and 4 on Hoehn and Yahr scale) patients. The routine dietary intervention consists of low-protein (<0.8 g/kg) diets or the redistribution of daily protein allowance to the last meal. Computational modeling was used to examine the fluctuation of gastrointestinal levodopa absorption under consideration of the diet by (i) identifying the group of patients that could benefit from dietary interventions, (ii) comparing existing diet recommendations for their impact on levodopa bioavailability, and (iii) suggesting a mechanism-based dietary intervention. We developed a multiscale computational model consisting of an ordinary differential equations-based advanced compartmentalized absorption and transit (ACAT) gut model and metabolic genome-scale small intestine epithelial cell model. We used this model to investigate complex spatiotemporal relationship between dietary amino acids and levodopa absorption. Our model predicted an improvement in bioavailability, as reflected by blood concentrations of levodopa with protein redistribution diet by 34% compared with a low-protein diet and by 11% compared with the ante cibum (a.c.) administration. These results are consistent with the reported better outcome in late-stage patients. A systematic analysis of the effect of different amino acids in the diet suggested that a serine-rich diet could improve the bioavailability by 22% compared with the a.c. administration. In addition, the slower gastric emptying rate in PD patients exacerbates the loss of levodopa due to competition. Optimizing dietary recommendations in quantity, composition, and intake time holds the promise to improve levodopa efficiency and patient’s quality of life based on highly detailed, mechanistic models of gut physiology endowed with improved extrapolative properties, thus paving the way for precision medical treatment.

Simultaneous determination of levodopa, carbidopa, entacapone, tolcapone, 3-O-methyldopa and dopamine in human plasma by an HPLC-MS/MS method

BACKGROUND

In this study, we developed and validated a HPLC-MS/MS method capable of simultaneously determining levodopa, carbidopa, entacapone, tolcapone, 3-O-methyldopa and dopamine in human plasma. RESULTS & METHODOLOGY: Chromatographic separation was achieved using a C8 column with a mobile phase consisting of a gradient of water and acetonitrile:methanol (90:10 v/v), both containing 0.1% formic acid. The developed method was selective, sensitive (LD<7.0 ng ml(-1)), linear (r>0.99), precise (RSD<11.3%), accurate (RE<11.8%) and free of residual and matrix effects. The developed method was successfully applied in plasma patients with Parkinson's disease using Stalevo®.

CONCLUSION

The new method can be used for the clinical monitoring of these substances and applied to adjustments in drug dosages.