As previously described, human doses of D6-[18F]FP-(+)-DTBZ were synthesized using a one-step fluorination and SPE purification process, resulting in high radiochemical yields (30%-40%) within 35 minutes[25]. The SPE method was effectively employed for routine production, eliminating the requirement for high-performance liquid chromatography (HPLC) purification. Importantly, no adverse reactions to D6-[18F]FP-(+)-DTBZ were observed in any of the subjects following intravenous injection.
The biodistribution and dosimetry investigation of D6-[18F]FP-(+)-DTBZ revealed the highest accumulation in the liver (289 µSv/MBq), characterized by an increase in SUVmean from 7.9 g/mL at 10 min to 13.7 g/mL at 180 min (Figure S2). Consequently, the observed radiation dose of D6-[18F]FP-(+)-DTBZ was only about half that of [18F]FP-(+)-DTBZ (289 vs. 541 µSv/MBq). The deuterated D6-[18F]FP-(+)-DTBZ may have a reduced hepatic dealkylation[28–30]. The slower clearance observed in humans between deuterated and non-deuterated tracers was consistent with the higher values reported in rats (3.10 vs. 2.12%ID/g at 2 h after injection)[23].
Additionally, significantly lower red marrow uptake of D6-[18F]FP-(+)-DTBZ were found with the deuterated radioligand dose being almost ten times less than the non-deuterated counterpart (28.8 vs. 272 µSv/MBq). The level of uptake in red marrow only slightly increased at 3 hours, with SUVmean values of 1.8 and 2.4 g/mL at 10 min and 180 min, respectively (Figure S2). This reduction in bone uptake was also comparable to what was observed in rats[23]. Moreover, the substitution of four ethyl hydrogens with deuterium in [18F]FE-(+)-DTBZ significantly reduced the defluorination rate from 0.012 to 0.0016[24]. A prolonged uptake and retention of D6-[18F]FP-(+)-DTBZ was observed in the lungs in comparison to other blood-containing organs such as the heart wall, thyroid, and spleen, which was similar to that of [18F]FP-(+)-DTBZ[28]. Moreover, the radioactivity dose in the lung of D6-[18F]FP-(+)-DTBZ was significantly lower than that of [18F]FP-(+)-DTBZ (31.7 vs 108 µSv/MBq).
D6-[18F]FP-(+)-DTBZ showed the highest initial uptake in the pancreas in all organs (SUVmean = 10.69 g/mL, Figure S2) and its accumulation was twice that of [18F]FP-(+)-DTBZ (185 vs 77.8 µSv/MBq). These values align with those reported in rat studies (5.01 vs 4.48%ID/g at 1 h after injection)[23]. The high VMAT2 expression by β cells in the pancreatic endocrine islets, coupled with their small proportion (1–2%) and widespread distribution in the pancreas[2, 31–32]. D6-[18F]FP-(+)-DTBZ may hold promise for sensitively detecting subtle changes in diabetes mellitus (DM) patients through PET imaging, attributed to its strong binding capability.
SUVR values for D6-[18F]FP-(+)-DTBZ showed the highest uptake in the putamen, followed by the caudate and nucleus accumbens, with moderate uptakes in the raphi nucleus and substantia nigra in the midbrain area. The thalamus displayed uptake similar to non-specific levels, consistent with that of [18F]FP-(+)-DTBZ[17]. Notably, the correlation between SUVR and BPnd was consistently robust and stable for all VOIs from 30 to 130 min (R2 > 0.99, Figure S4a). Beyond 70 min, SUVR was only slightly lower than BPnd (0.91 < slope < 1.03). A specific correlation was observed between SUVR and BPnd at the 90 min time window with an R2 value of 0.9967 (Figure S4b). This uptake profile mirrors that of [18F]FP-(+)-DTBZ, and the practical time window for routine diagnostic studies appeared to be 70–130 min post-injection (R2 > 0.99, 0.91 < slope < 1.03)[33].
The direct comparison between D6-[18F]FP-(+)-DTBZ and [18F]FP-(+)-DTBZ demonstrated that the SUVRs of the deuterated DTBZ radioligand were notably higher in VMAT2 high-density areas (caudate, putamen, and nucleus accumbens) than those of the non-deuterated DTBZ radioligand, which were consistent with findings in rat studies (the uptake in the striatum were 6.05 and 5.66%ID/g, respectively)[23]. Meanwhile, D6-[18F]FP-(+)-DTBZ showed significantly lower brain uptake than [18F]FP-(+)-DTBZ (39 vs 128 µSv/MBq), which matches rat study findings, partly due to reduced uptake in non-specific brain areas like the occipital and cerebellum.
In post-mortem studies of human brain, there appears to be a broad agreement on the age-related loss of dopaminergic neurons, although the degree of dopamine production and storage decline remained controversial. The age effect of nigrostriatal uptake was observed by PET studies using [11C]-(+)-DTBZ[15, 34]. D6-[18F]FP-(+)-DTBZ PET imaging of the brain showed a similar reduction of uptake for elderly subjects. The SUVR of caudate and putamen for older healthy participants (53–75 years old) were nearly 30% lower than that of young subjects (20–22 years old). Therefore, future studies are warranted to investigate the age effect of VMAT2 binding with D6-[18F]FP-(+)-DTBZ and its relationship with the specificity and sensitivity in detecting the integrity of monoamine neurons in the brain.
There are several limitations of this current study. A primary limitation of this dosimetry and brain dynamic PET imaging study was the relatively young age of the participants (22–34 years old). However, conducting a head-to-head comparison of brain imaging between D6-[18F]FP-(+)-DTBZ and [18F]FP-(+)-DTBZ in the same subjects may have enhanced the study's specificity in target VMAT2 binding while minimizing confounding factors. Second, the absence of blood sampling in this study limited the investigation of metabolism in blood and plasma-to-blood ratio correction, which could enhance quantification accuracy, like BPND.