Mechanism Of Action
In dopaminergic nerve terminals in the brain, Fluorodopa (FDOPA) F 18 is decarboxylated by amino acid decarboxylase to Fluorodopamine (FDA) F 18 and stored in presynaptic vesicles in the brain. The accumulation of F 18 FDA in the striatum is visually detected in the PET scan.
Optimal PET imaging is achieved between 75 to 90 minutes after administration of Fluorodopa F 18 Injection based on its pharmacokinetics. The relationship between Fluorodopa F 18 dose and plasma concentration is not fully characterized.
Following the intravenous administration, Fluorodopa F 18 is cleared from the blood with a biologic half-life of about 1 to 3 hours. The time course of “background” brain radioactivity after Fluorodopa F 18 was evaluated. F 18 activity in the cerebellum was greater than the parietal or occipital cortex during the first 30 minutes after Fluorodopa F 18 suggesting regional differences in amino acid transport.
Fluorodopa F 18 is cleared from the blood and tissue within 24 hours.
Fluorodopa F 18 is decarboxylated by aromatic amino acid decarboxylase in the striatum to Fluorodopamine F 18. Fluorodopamine F 18 is also metabolized via monoamine oxidase to yield [18F] 6-fluoro- 3,4-dihydroxyphenylacetic acid (18FDOPAC) and subsequently by COMT to yield [18F]6-fluorohomovanillic acid (18FHVA).
80% of the radioactivity is eliminated through the urine. Urine radioactivity peaks at about 30 minutes post-injection. The radiation absorbed dose to the bladder wall is reduced by emptying the bladder just before scanning.
The safety and efficacy of F-18 FDOPA were evaluated in a prospective single-arm study conducted at a single center that enrolled 68 adult patients with possible Parkinsonian syndrome (PS). In the study, F-18 FDOPA image reads were compared to a reference clinical diagnostic standard of Parkinsonian syndrome or non- Parkinsonian syndrome (non-PS) established at 6 to 9 months after the F-18 FDOPA PET scan by a movement disorder specialist blinded to F-18 FDOPA PET results. The reference clinical diagnostic standard for PS included Parkinson's disease (PD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and multiple system atrophy (MSA). These conditions have been associated with dopaminergic neurodegeneration and F-18 FDOPA PET imaging was not designed to distinguish among the conditions. The reference clinical diagnostic standard for non-PS included essential tremor (ET) diagnosis, vascular parkinsonism, drug-induced parkinsonism, and other non-PS diagnoses.
Carbidopa (150 mg) was administered orally to patients approximately 60 minutes prior to the administration of Fluorodopa F 18 Injection. Each patient was injected 5 mCi F-18 FDOPA (range 4.5-5.5 mCi), and PET images were acquired 80-100 minutes post injection.
F-18 FDOPA PET images were evaluated independently by 3 readers blinded to clinical information who had no other role in patient assessment. Patients with at least one cardinal feature of PS were referred to the study by physicians who were uncertain of a diagnosis. Thirty-three of these patients had symptoms for 2 years or less. Fifty-six of these patients completed F-18 FDOPA PET imaging and at least 6 month clinical follow-up. Among these 56 patients, 32% were female and 68% were male. Mean patient age was 66 years. Thirty-three patients were categorized as PS, and twenty-three patients were categorized as non-PS based on minimum 6 month clinical follow-up by a movement disorder specialist.
Table 5 shows the positive percent agreement and negative percent agreement of F-18 FDOPA Injection PET image results with the reference clinical diagnostic standard. Positive percent agreement represents the percent of patients with positive F-18 FDOPA PET images among all the patients with a clinical diagnostic reference standard of PS. Negative percent agreement represents the percent of patients with negative F-18 FDOPA PET images among the patients with a non-PS clinical diagnostic reference standard.
Table 5. Positive and Negative Percent Agreement for F-18 FDOPA Injection PET Imaging
|n=56 Patients||Positive percent agreement||Negative percent agreement|
|Positive test/Clinical PS|
(95 % CI)
|Negative test/Clinical non-PS|
(95 % CI)
73% (55, 87)
91% (72, 99)
49% (31, 67)
91% (72, 99)
58% (39, 75)
83% (61, 94)
The effectiveness of F-18 FDOPA PET as a screening or confirmatory test and for monitoring disease progression or response to therapy has not been established.