O-Methyl-DL-tyrosine

Introduction: Parkinson's disease is a chronic, neurodegenerative disease entity with heterogeneous features and course. Levodopa is the most efficacious dopamine substituting drug. Particularly, long-term application of oral levodopa/decarboxylase inhibitor formulations sooner or later supports onset of fluctuations of movement. It also shifts levodopa turnover to O-methylation, which impairs human methylation capacity and increases oxidative stress.Areas covered: This narrative review summarizes pharmacokinetic and pharmacodynamic features of available levodopa cotherapies on the basis of a literature search with the terms L-dopa, inhibitors of catechol-O-methyltransferase and monoamine oxidase-B.Expert opinion: Long-term levodopa/dopa decarboxylase inhibitor application with concomitant inhibition of both, catechol-O-methyltransferase and monoamine oxidase-B supports a more continuous dopamine substitution, which ameliorates fluctuations of motor behavior. This triple combination also enhances both, antioxidative defense and methylation capacity. Inhibition of monoamine oxidase-B reduces generation of oxidative stress in the brain. Constraint of catechol-O-methyltransferase reduces homocysteine synthesis due to diminished consumption of methyl groups for levodopa turnover at least in the periphery. An additional nutritional supplementation with methyl group donating and free radical scavenging vitamins is recommendable, when future drugs are developed for long-term levodopa/dopa decarboxylase treated patients. Personalized medicine treatment concepts shall also consider nutritional aspects of Parkinson's disease.

Background: Parkinson's disease is an aggressive and progressive neurodegenerative disorder that depletes dopamine (DA) in the central nervous system. Dopamine replacement therapy, mainly through actual dopamine and its original prodrug l-dopa (LD), faces many challenges such as poor blood brain barrier penetration and decreased response to therapy with time. Methods: The prodrugs described herein are ester, amide, dimeric amide, carrier-mediated, peptide transport-mediated, cyclic, chemical delivery systems and enzyme-models prodrugs designed and made by chemical means, and their bioavailability was studied in animals. Results: A promising ester prodrug for intranasal delivery has been developed. LD methyl ester is currently in Phase III clinical trials. A series of amide prodrugs were synthesized with better stability than ester prodrugs. Both amide and dimeric amide prodrugs offer enhanced blood brain barrier (BBB) penetration and better pharmacokinetics. Attaching LD to sugars has been used to exploit glucose transport mechanisms into the brain. Conclusions: Till now, no DA prodrug has reached the pharmaceutical market, nevertheless, the future of utilizing prodrugs for the treatment of PD seems to be bright. For instance, LD ester prodrugs have demonstrated an adequate intranasal delivery of LD, thus enabling the absorption of therapeutic agents to the brain. Most of the amide, cyclic, peptidyl or chemical delivery systems of DA prodrugs demonstrated enhanced pharmacokinetic properties.

In recent years, PET using radiolabelled amino acids has gained considerable interest as an additional tool besides MRI to improve the diagnosis of cerebral gliomas and brain metastases. A very successful tracer in this field is O-(2-[18F]fluoroethyl)-L-tyrosine (FET) which in recent years has replaced short-lived tracers such as [11C]-methyl-L-methionine in many neuro-oncological centers in Western Europe. FET can be produced with high efficiency and distributed in a satellite concept like 2- [18F]fluoro-2-deoxy-D-glucose. Many clinical studies have demonstrated that FET PET provides important diagnostic information regarding the delineation of cerebral gliomas for therapy planning, an improved differentiation of tumor recurrence from treatment-related changes and sensitive treatment monitoring. In parallel, a considerable number of experimental studies have investigated the uptake mechanisms of FET on the cellular level and the behavior of the tracer in various benign lesions in order to clarify the specificity of FET uptake for tumor tissue. Further studies have explored the effects of treatment related tissue alterations on tracer uptake such as surgery, radiation and drug therapy. Finally, the role of blood-brain barrier integrity for FET uptake which presents an important aspect for PET tracers targeting neoplastic lesions in the brain has been investigated in several studies. Based on a literature research regarding experimental FET studies and corresponding clinical applications this article summarizes the knowledge on the uptake behavior of FET, which has been collected in more than 30 experimental studies during the last two decades and discusses the role of these results in the clinical context.