Radiation dosimetry of N-([11C]methyl)benperidol as determined by whole-body PET imaging of primates

Challenges in the development of dopamine D2- and D3-selective radiotracers for PET imaging studies

The dopamine D2-like receptors (ie, D2/3 receptors) have been the most extensively studied CNS receptor with Positron Emission Tomography (PET). The 3 different radiotracers that have been used in these studies are [¹¹ C]raclopride, [¹⁸ F]fallypride, and [¹¹ C]PHNO. Because these radiotracers have a high affinity for both dopamine D2 and D3 receptors, the density of dopamine receptors in the CNS is reported as the D2/3 binding potential, which reflects a measure of the density of both receptor subtypes. Although the development of D2- and D3-selective PET radiotracers has been an active area of research for many years, this by and large presents an unmet need in the area of translational PET imaging studies. This article discusses some of the challenges that have inhibited progress in this area of research and the current status of the development of subtype selective radiotracers for imaging D3 and D2 dopamine receptors with PET.

Absorbed radiation dosimetry of the D3-specific PET radioligand [18F]FluorTriopride estimated using rodent and nonhuman primate

[¹⁸F]FluorTriopride ([¹⁸F]FTP) is a dopamine D₃-receptor preferring radioligand with potential for investigation of neuropsychiatric disorders including Parkinson disease, dystonia and schizophrenia. Here we estimate human radiation dosimetry for [¹⁸F]FTP based on the ex-vivo biodistribution in rodents and in vivo distribution in nonhuman primates. Biodistribution data were generated using male and female Sprague-Dawley rats injected with ~370 KBq of [¹⁸F]FTP and euthanized at 5, 30, 60, 120, and 240 min. Organs of interest were dissected, weighed and assayed for radioactivity content. PET imaging studies were performed in two male and one female macaque fascicularis administered 143-190 MBq of [¹⁸F]FTP and scanned whole-body in sequential sections. Organ residence times were calculated based on organ time activity curves (TAC) created from regions of Interest. OLINDA/EXM 1.1 was used to estimate human radiation dosimetry based on scaled organ residence times. In the rodent, the highest absorbed radiation dose was the upper large intestines (0.32-0.49 mGy/MBq), with an effective dose of 0.07 mSv/MBq in males and 0.1 mSv/MBq in females. For the nonhuman primate, however, the gallbladder wall was the critical organ (1.81 mGy/MBq), and the effective dose was 0.02 mSv/MBq. The species discrepancy in dosimetry estimates for [¹⁸F]FTP based on rat and primate data can be attributed to the slower transit of tracer through the hepatobiliary track of the primate compared to the rat, which lacks a gallbladder. Out findings demonstrate that the nonhuman primate model is more appropriate model for estimating human absorbed radiation dosimetry when hepatobiliary excretion plays a major role in radiotracer elimination.

Radiation dosimetry of [(18)F]VAT in nonhuman primates

Background

The objective of this study is to determine the radiation dosimetry of a novel radiotracer for vesicular acetylcholine transporter (−)-(1-((2R,3R)-8-(2-[¹⁸F]fluoro-ethoxy)-3-hydroxy-1,2,3,4-tetrahydronaphthalen-2-yl)piperidin-4-yl)(4-fluorophenyl)-methanone ([¹⁸F]VAT) based on PET imaging in nonhuman primates. [¹⁸F]VAT has potential for investigation of neurological disorders including Alzheimer’s disease, Parkinson’s disease, and dystonia.

Methods

Three macaque fascicularis (two males, one female) received 185.4–198.3 MBq [¹⁸F]VAT prior to whole-body imaging in a MicroPET-F220 scanner. Time activity curves (TACs) were created from regions of interest (ROIs) that encompassed the entire small organs or samples with the highest activity within large organs. Organ residence times were calculated based on the TACs. We then used OLINDA/EXM 1.1 to calculate human radiation dose estimates based on scaled organ residence times.

Results

Measurements from directly sampled arterial blood yielded a residence time of 0.30 h in agreement with the residence time of 0.39 h calculated from a PET-generated time activity curve measured in the left ventricle. Organ dosimetry revealed the liver as the critical organ (51.1 and 65.4 μGy/MBq) and an effective dose of 16 and 19 μSv/MBq for male and female, respectively.

Conclusions

The macaque biodistribution data showed high retention of [¹⁸F]VAT in the liver consistent with hepatobiliary clearance. These dosimetry data support that relatively safe doses of [¹⁸F]VAT can be administered to obtain imaging in humans.

Radiation dosimetry and biodistribution in non-human primates of the sodium/iodide PET ligand [(18)F]-tetrafluoroborate

Background

[¹⁸F]-tetrafluoroborate is a PET radiotracer taken up by the sodium/iodide symporter (NIS). Albeit the in vivo behavior in rodents is similar to the ^99mTc-pertechnetate, no studies exist in primates or in humans. The aims of this study were to evaluate the biodistribution of [¹⁸F]-tetrafluoroborate in non-human primates with PET and to estimate the absorbed dose in organs.

Methods

Whole-body PET imaging was done in a Siemens ECAT HR+ scanner in two male Macaca fascicularis monkeys. After an i.v. injection of 24.93 ± 0.05 MBq/kg of [¹⁸F]-tetrafluoroborate, prepared by isotopic exchange of sodium tetrafluoroborate with [¹⁸F]-fluoride under acidic conditions, eight sequential images from the head to the thigh (five beds) were collected for a total duration of 132 min. The whole-body emission scan was reconstructed applying attenuation and scatter corrections. After image reconstruction, three-dimensional volumes of interest (VOIs) were hand-drawn on the PET transaxial or coronal slices of the frame where the organ was most conspicuous. Time-activity curves for each VOI were obtained, and the organ residence times were calculated by integration of the time-activity curves. Human absorbed doses were estimated using the OLINDA/EXM software and the standard human model.

Results

[¹⁸F]-tetrafluoroborate was able to discriminate clearly the thyroid gland with an excellent signal-to-noise ratio. Most of the radiotracers (residence time) are localised in the organs that express NIS (stomach wall, salivary glands, thyroid, olfactory mucosa), are involved in excretion (kidneys and bladder), or reflect the vascular phase (heart and lungs). Considering the OLINDA source organs, the critical organs were the stomach wall, thyroid and bladder wall, with absorbed doses lower than 0.078 mGy/MBq. The effective dose was 0.025 mSv/MBq.

Conclusions

[¹⁸F]-tetrafluoroborate is a very useful radiotracer for PET thyroid imaging in primates, with a characteristic biodistribution in organs expressing NIS. It delivers an effective dose slightly higher than the dose produced by ^99mTc-pertechnetate but much lower than that produced by radioiodine in the form of ¹³¹INa, ¹²³INa, or ¹²⁴INa.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0148-5) contains supplementary material, which is available to authorized users.

Whole-body distribution and radiation dosimetry of (68)Ga-NOTA-RGD, a positron emission tomography agent for angiogenesis imaging

(68)Ga labeled NOTA-RGD was a recently developed positron emission tomography (PET) radiotracer for the visualization of angiogenesis, and is regarded as a promising imaging agent for cancer and several other disorders. In this study, we investigated the whole-body distribution and radiation dosimetry of (68)Ga-NOTA-RGD in humans. Ten cancer patients (53.7 ± 13.5 years; 61.5 ± 7.4  kg) participated in this study. PET scans were performed using a PET/computed tomography (scanner in three-dimensional mode). After an intravenous injection of 172.4 ± 20.5  MBq of (68)Ga-NOTA-RGD, eight serial whole-body scans were performed during 90 minutes. Volumes of interest were drawn manually over the entire volumes of the urinary bladder, the gallbladder, heart, kidneys, liver, lungs, pancreas, spleen, and stomach. Time-activity curves were obtained from serial PET scan data. Residence times were calculated from areas under curve of time-activity curves and used as input to the OLINDA/EXM 1.1 software. The uptake of (68)Ga-NOTA-RGD was highest in the kidneys and urinary bladder. Radiation doses to kidneys and urinary bladder were 71.6 ± 28.4  μ Gy/MBq and 239.6 ± 56.6  μ Gy/MBq. Mean effective doses were 25.0 ± 4.4  μ Sv/MBq using International Commission of Radiation Protection (ICRP) publication 60 and 22.4 ± 3.8  μ Sv/MBq using ICRP publication 103 weighting factor. We evaluated the radiation dosimetry of (68)Ga labeled NOTA-RGD, which has an acceptable effective radiation dose.