Clinical Pharmacology for DaTscan
Mechanism Of Action
The active drug substance in DATSCAN is N-ω-fluoropropyl-2β-carbomethoxy-3β-(4Â[123I]iodophenyl)nortropane or ioflupane I 123.
In vitro, ioflupane binds reversibly to the human dopamine transporter (DaT) (Ki = 0.62 nM; IC50 = 0.71nM). Autoradiography of post-mortem human brain slices exposed to radiolabeled ioflupane shows concentration of the radiolabel in striatum (caudate nucleus and putamen). The specificity of the binding of ioflupane I 125 to dopamine transporter was demonstrated by competition studies with the DaT inhibitor GBR 12909 (a dopamine reuptake inhibitor), the serotonin reuptake inhibitor citalopram, and the norepinephrine reuptake inhibitor desipramine in post-mortem human brain slices exposed to radiolabeled ioflupane. Citalopram reduced binding in the neocortex and thalamus with only minor effects in the striatum. This indicated that the binding in the cortex and thalamus is mainly to the serotonin reuptake sites. Desipramine showed no effect on the level of striatal binding of ioflupane I 125, but reduced extrastriatal binding by 60 to 85%. The binding of ioflupane I 125 to the striatum was abolished in the presence of high concentrations of GBR 12909, indicating selectivity of ioflupane binding for the pre-synaptic DaT.
Following administration of DATSCAN to humans, radioactive decay of the iodine-123 emits gamma radiation which can be detected externally using gamma detectors, allowing visualization of the brain striata through SPECT imaging [see CLINICAL PHARMACOLOGY].
Pharmacodynamics
As DATSCAN contains a very small quantity of ioflupane, no ioflupane pharmacologic effects are expected [see DESCRIPTION].
Pharmacokinetics
The pharmacokinetics of ioflupane I 123 were studied by monitoring radioactivity following intravenous injection; only 5% of the administered radioactivity remained in whole blood at 5 minutes post-injection. Uptake in the brain reached approximately 7% of injected radioactivity at 10 minutes post-injection and decreased to 3% after 5 hours; striata to background ratios were relatively constant between 3 hours and 6 hours post-injection. About 30% of the whole brain radioactivity was attributed to striatal uptake. By 48 hours post-injection, approximately 60% of the injected radioactivity has been excreted in the urine, with fecal excretion estimated to be approximately 14%.
Animal Toxicology And/Or Pharmacology
Single-and repeated-dose intravenous toxicity studies have been performed using ioflupane in rats, rabbits, and dogs. Additionally, single-dose acute toxicity studies have been performed in cynomolgus monkeys. No mortality or other toxicity was observed at doses up to 5,500 times the maximum clinical dose of DATSCAN; at doses greater than 1,500 times the maximum clinical dose, pharmacological responses such as mydriasis and hyperactivity were seen in some species.
Clinical Studies
Suspected Parkinsonian Syndromes
The safety and efficacy of DATSCAN were evaluated in two multicenter, single-arm, open-label studies (Study 1 and Study 2) that enrolled 284 adult patients with suspected PS.
In these studies, DATSCAN image interpretation was compared to a reference clinical diagnostic standard of “PS” or “non-PS”. PS consisted of Parkinson’s disease (PD), multiple system atrophy (MSA), and progressive supranuclear palsy (PSP). These conditions have been associated with dopaminergic neurodegeneration and DATSCAN imaging was not designed to distinguish among the conditions. Among the non-PS population, subjects with a history of the following conditions were included: vascular parkinsonism, psychogenic parkinsonism, essential tremor (ET), various forms of tremor, and other non-PS diagnoses; DATSCAN imaging was not designed to distinguish among these conditions.
Patients underwent SPECT imaging with a variety of multi-headed cameras or multi-detector single-slice systems 3 to 6 hours after DATSCAN administration. The DATSCAN activity in these two clinical studies ranged from 111 MBq to 201 MBq (3 mCi to 5.4 mCi) [median 167.8 MBq (4.5 mCi)]. Readers blinded to clinical information interpreted DATSCAN imaging as normal or abnormal [see DOSAGE AND ADMINISTRATION]. A normal image is inconsistent with clinical conditions associated with PS and an abnormal image is consistent with clinical conditions associated with PS.
Study 1 consisted of 99 patients with early features of Parkinsonism; patients with features suggestive of MSA or PSP were excluded. The baseline clinical diagnoses consisted of: probable PD (44%), possible PD (31%), “benign” PD (6%), possible ET (11%), and other diagnoses (7%). The patients were 44% female, 42% of age 65 years or over, and all were White. Study 1 readers had no other role in patient assessment. The reference clinical diagnostic standard consisted of the clinical diagnoses established by a consensus panel of movement disorder specialists that evaluated data inclusive through 36 months of follow-up.
Study 2 consisted of 185 patients with clinically established diagnosis of PS (PD, MSA, PSP) or ET. The baseline clinical diagnoses consisted of: PD (70%), MSA (10%), PSP (5%), and ET (15%). The patients were 35% female, 48% of age 65 years or over, and 99% were White. Study 2 readers included site investigators. The reference clinical diagnostic standard consisted of the investigator-determined baseline clinical diagnosis.
Table 4 shows the positive percent agreement and negative percent agreement of the DATSCAN image results with the reference clinical diagnostic standard. Positive percent agreement represents the percent of patients with abnormal DATSCAN images among all the patients with a clinical diagnostic reference standard of PS. Negative percent agreement represents the percent of patients with normal DATSCAN images among the patients with a non-PS clinical diagnostic reference standard.
Table 4: Positive and Negative Percent Agreements for Studies 1 and 2
|
Positive percent agreement (95 % CI) |
Negative percent agreement (95 % C I) |
| Study 1 (patients with early signs and/or symptoms of PS) |
| Reader A, n = 99 |
77 (66, 87) |
96 (82, 100) |
| Reader B, n = 96 |
78 (66, 87) |
96 (82, 100) |
| Reader C, n = 98 |
79 (67, 87) |
96 (82, 100) |
| Study 2 (patients with established diagnoses of PS or ET) |
| Reader A, n = 185 |
93 (88, 97) |
96 (81, 100) |
| Reader B, n = 185 |
97 (93, 99) |
74 (54, 89) |
| Reader C, n = 185 |
96 (92, 99) |
85 (66, 96) |
| Reader D, n = 185 |
92 (87, 96) |
93 (76, 99) |
| Reader E, n = 185 |
94 (90, 97) |
93 (76, 99) |
Suspected Dementia With Lewy Bodies
The evidence for the efficacy of DATSCAN in visualizing striatal dopamine transporters in adult patients with DLB is derived from the scientific literature. In 55 patients clinically diagnosed with DLB or with Alzheimer’s disease, the results of consensus blinded visual assessment of the DATSCAN images were compared to neuropathological diagnosis made using standard international criteria at autopsy. The patients were 38% female and of ages 63 to 95 years (mean 77 years) at the time of DATSCAN imaging. The median administered DATSCAN activity was 150 MBq to 185 MBq (4.1 mCi to 4.9 mCi). DATSCAN images were rated as normal or abnormal [see DOSAGE AND ADMINISTRATION]. Based on autopsy diagnosis, the sensitivity and specificity of DATSCAN for DLB was 80% (95% CI 61% to 92%) and 92% (95% CI 74% to 99%), respectively.
The effectiveness of DATSCAN as a screening or confirmatory test and for monitoring disease progression or response to therapy has not been established.