L-3,4-Dihydroxy-6-[F]fluorophenylalanine

Parkinson’s disease (PD) is associated with a loss of dopamine-containing neurons in striatum of the brain (1, 2). PD is caused by a shortage of dopamine. Dopamine, a neurotransmitter, plays an important role in the mediation of movement, cognition and emotion. Dopamine also plays a role in various neuropsychiatric disorders, such as schizophrenia, autism, attention deficit hyperactivity disorder, and drug abuse. Dopamine is synthesized within nerve cells (3). L-tyrosine is converted to dihydroxyphenylalanine (L-DOPA) and then to dopamine in a two-step process. The first, rate limiting step is catalyzed by tyrosine 3-monoxygenase (tyrosine hydroxylase or TH). The second step is catalyzed by aromatic L-amino acid decarboxylase (L-DOPA decarboxylase, AAAD). In parts of the nervous system that release dopamine as a neurotransmitter (dopaminergic neurons), no further metabolism occurs and dopamine is stored in vesicles in the presynaptic nerve terminals by virtue of the dopamine reuptake transporter, DAT. 6-[F]Fluoro-L-DOPA (FDOPA) is a radiolabeled analog of L-DOPA used to evaluate the central dopaminergic function of pre-synaptic neurons using positron emission tomography (PET) (4, 5). FDOPA PET reflects DOPA transport into the neurons, DOPA decarboxylation and dopamine storage capacity. The tracer is converted to 6-[F]fluorodopamine (FDA) by AAAD and retained in the striatum. FDA can be O-methylated by catechol-O-methyltransferase (COMT) to 3-O-methyl-6-[F]fluoro-L-dopa (3-OMFD), which is uniformly distributed throughout the brain. FDA is also metabolized via monoamine oxidase to yield [F]6-fluoro-3,4-dihydroxyphenylacetic acid (FDOPAC) and subsequently by COMT to yield [F]6-fluorochomovanillic acid (FHVA). AAAD and COMT are also present in peripheral tissues such as liver, kidneys, and lung. In clinical studies, AAAD is commonly inhibited with carbidopa, whereas COMT is blocked by entacapone and nitecapone. These two types of inhibitors enhance the availability of FDOPA in the brain.