Biodistribution and dosimetry of the PET radioligand [18F]CHDI-650 in mice for detection of mutant huntingtin aggregates

doi:10.21203/rs.3.rs-5152047/v1

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Biodistribution and dosimetry of the PET radioligand [¹⁸F]CHDI-650 in mice for detection of mutant huntingtin aggregates

https://doi.org/10.21203/rs.3.rs-5152047/v1

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Background

Huntington’s disease (HD) is a rare neurodegenerative disorder caused by an expansion of the CAG trinucleotide repeat in the huntingtin gene which encodes the mutant huntingtin protein (mHTT) that is associated with HD-related neuropathophysiology. Noninvasive visualization of mHTT aggregates in the brain, with positron emission tomography (PET), will allow to reliably evaluate the efficacy of therapeutic interventions in HD. This study aimed to assess the radiation burden of [¹⁸F]CHDI-650, a novel fluorinated mHTT radioligand, in humans based on both in vivo and ex vivo biodistribution in mice and subsequent determination of dosimetry for dosing in humans.

Results

Wild-type male and female CD-1 Swiss mice (n = 15/sex) were used to assess in vivo PET imaging-based and ex vivo biodistribution-based tracer distribution of [¹⁸F]CHDI-650 at 30-, 60-, 120-, 240- and 360-minutes post-injection. Three-dimensional volumes of interest of the organs were drawn on the co-registered PET/CT image and organs were collected after dissection. Organ radioactivity levels were determined using both modalities. The residence time was calculated and extrapolated to human phantoms. The absorbed and effective doses were computed with OLINDA/EXM 2.2 and IDAC-Dose2.1. Ex vivo and PET-imaging biodistribution of [¹⁸F]CHDI-650 showed rapid washout after 30 minutes in most of the organs with the highest uptake in the gallbladder and urine in mice. Extrapolation of the data to human phantoms with OLINDA showed a total mean in vivo based effective dose of 14.7 µSv/MBq with the highest effective organ dose in the urinary bladder wall (4.29 µSv/MBq). The total mean ex vivo based effective dose was calculated to be 20.6 µSv/MBq. The highest effective organ dose ex vivo in the urinary bladder wall was estimated to be 4.22 µSv/MBq. The predicted exposure in humans using IDAC-Dose correlated well to those obtained with OLINDA for both in vivo and ex vivo measurements (r = 0.9801 and r = 0.9364, respectively).

Conclusions

Dosimetry analysis indicated absorbed and effective doses of [¹⁸F]CHDI-650 are well below the recommended limits, suggesting that the radioligand is suitable for clinical assessment. Based on the highest effective dose estimates, an injection of 370 MBq in humans would result in a radiation dose of 7.62 mSv.

Nuclear Medicine & Medical Imaging

Huntington’s disease

Positron emission tomography

In vivo dosimetry

Ex vivo dosimetry

Mouse

Huntington’s disease (HD) is a rare neurodegenerative disorder caused by an expansion of the CAG trinucleotide repeat in exon 1 of the huntingtin (HTT) gene located on the short arm of chromosome 4 (1). This expansion leads to an elongation of the polyglutamine tract encoding the mutant huntingtin (mHTT) protein (2). Based on the toxic gain-of-function hypothesis, mHTT-lowering interventions are currently among the most promising therapeutic approaches (3, 4). To evaluate the efficacy of these therapeutic interventions, it is essential to accurately quantify mHTT aggregates in the brain.

Two ¹¹C positron emission tomography (PET) radioligands, [¹¹C]CHDI-626 and [¹¹C]CHDI-180R (5–9) were developed and validated to noninvasively visualize mHTT aggregates in preclinical models (5, 6). However, [¹¹C]CHDI-626 displayed suboptimal kinetics in rodents and in the first-in-human evaluation, it was deemed not suitable for clinical use due to an apparent accumulation of a brain-associated metabolite resulting in inadequate tracer kinetics (10). On the contrary, longitudinal assessment of [¹¹C]CHDI-180R found significant levels of mHTT aggregates in the striatum of heterozygous zQ175DN mice as early as 3 months (5). With the goal of clinical translatability, the utility of [¹¹C]CHDI-180R in quantifying mHTT aggregates in vivo was further corroborated in a non-human primate model of HD (11). The [¹¹C]CHDI-180R first-in-human dosimetry study confirmed the acceptable radiation burden and suitable reversible kinetics for quantification (10). Fluorinated radioligands are often preferred to ¹¹C ligands due to the longer half-life (¹⁸F: 109.8 min; ¹¹C: 20.4 min) which facilitates the use for imaging centers without a cyclotron on site as well as the ability to scan more patients with a single radiosynthesis (12). As part of the effort to identify a 2nd generation mHTT PET ligand, a novel fluorinated radioligand, namely [¹⁸F]CHDI-650, was recently developed (13). [¹⁸F]CHDI-650 displayed suitable reversible kinetics in wild-type (WT) mice and non-human primates as well as specific binding to mHTT aggregates in post-mortem human HD brain (13). In order to ensure the suitability of [¹⁸F]CHDI-650 in humans, a dosimetry study is generally recommended in a preclinical species to estimate the radiation exposure due to the radioligand (14).

In this study, we aimed to assess and compare the in vivo and ex vivo biodistribution profile of [¹⁸F]CHDI-650 in male and female CD-1 Swiss mice and extrapolate the equivalent dose to standardized human phantoms. A radiation exposure assessment of [¹⁸F]CHDI-650 in these phantoms was performed for both in vivo and ex vivo measurements. To compare their ability to calculate mean absorbed and total mean effective doses, we performed a head-to-head comparison between the commercial FDA/CE approved software package, OLINDA/EXM 2.2 developed by the RADAR group and the freely available IDAC-Dose2.1, which is based on published recommendations of the international Commission on Radiological Protection (ICRP).

2.1 Experimental design

Three-month old WT male and female CD-1 Swiss mice (n = 15 per sex) were obtained from Janvier (Le Genest Saint Isle, France). Animals were group-housed (six per cage) in individually ventilated cages under a 12 h light/dark cycle in a temperature- and humidity-controlled environment with food and water ad libitum. The animals were acclimatized to the facility at least one week before the start of procedures. All experiments were approved by the Ethical Committee for Animal Testing (ECD 2019-39) at the University of Antwerp (Belgium) and the European Committee Guidelines (decree 2010/63/CEE) for the care and use of animals were followed.

The mice were allocated to five cohorts, each containing three males and three females (Fig. 1). The in vivo PET/CT data was used to assess the activity concentration in the brain, lungs, heart, liver, kidneys, intestines, bladder, and muscle (hind leg). In addition to the same organs delineated in in vivo scans, the ex vivo data was used to assess tracer distribution in organs that are difficult to delineate on the PET/CT images. The organs or fluids collected for ex vivo assessment were brain, eyes, lungs, heart, liver, gallbladder, spleen, pancreas, stomach, small intestine, large intestine, kidneys, urinary bladder, gonads, thyroid, adipose tissue, bone (tibia), muscle (hind leg) and skin, blood and urine.

2.2 Radiosynthesis

[¹⁸F]CHDI-650 was synthesized using an automated radiosynthesis module (Trasis AllInOne, Belgium) available at the Antwerp University Hospital, adapting the previously described process (9) to the system (15). Radiochemical purity was greater than 98% and molar activity (A_m) was 185.2 ± 27.7 GBq/µmol (range 157.3–231.9 GBq/µmol) with a decay-corrected radiochemical yield of 18.02 ± 5.82% at end of synthesis.

2.3 PET image acquisition

MicroPET/CT imaging was performed on two Siemens Inveon PET-CT scanners (Siemens Preclinical Solution, USA). Animals were placed on a single PET bed with the full body of the animals in the scanner’s field of view. The animals of the static imaging cohorts were given an intravenous (i.v.) bolus injection in the tail vein and kept separately in a cage for tracer uptake. The animals were anesthesized and placed on the scanner bed at their assigned time point. The animals of the dynamic imaging cohort were anesthesized, catheterized in the tail vein and placed on the scanner bed. For this cohort, the PET scan was initiated simultaneously with the i.v. bolus injection of the tracer. Anesthesia was induced by inhalation of isoflurane (5% for induction, and 1.5-2% for maintenance during preparation and scanning) supplemented with oxygen. Respiration and heart rate of the animal were constantly monitored using the Monitoring Acquisition Module (Minerve, France) during the entire scanning period. The core body temperature of the animals was maintained using a warm air flow. A 10 min 80 kV/500 µA CT scan was performed for attenuation and scatter correction before initiation of both the static and dynamic PET scans. PET data were acquired in list mode. An overview of the animal dosing information for both sexes is shown in Supplemental Table 1.

2.4 PET image reconstruction

Static PET data were reconstructed into one frame of either 1 x 600 s or 1 x 1200 s depending on the timepoint while dynamic 120 min PET data were reconstructed into 45 frames of increasing length (12 × 10 s, 3 × 20 s, 3 × 30 s, 3 × 60 s, 3 × 150 s, and 21 × 300 s). All images were reconstructed in 8 iterations and 16 subsets using the 3D ordered subset expectation maximization (OSEM 3D) algorithm utilizing a list-mode iterative reconstruction with proprietary spatially variant resolution modeling for quantitative analysis (16). Corrections for normalization, dead time, and CT-based attenuation were applied. PET image frames were reconstructed using 0.776 × 0.776 × 0.796 mm³ voxels on a 128 × 128 × 159 grid.

2.5 Data analysis

For in vivo data, the co-registered PET/CT image was used to delineate the organs with three-dimensional volumes of interest (VOIs) in PMOD 3.6 software (Pmod Technologies, Zurich, Switzerland). The organ activity inside these VOIs was determined and normalized by the injected dose. The data were pooled together (n = 3/sex/timepoint) to form time activity curves (TACs) for each organ.

2.6 Biodistribution and dosimetry

For the ex vivo part of the study, at the end of the scan, the animals were sacrificed under isoflurane anesthesia, with cervical dislocation. Blood was collected via cardiac puncture; urine was collected after the voiding of the bladder during cervical dislocation. After dissection, the organs were collected, rinsed with a phosphate-buffered saline solution (PBS), weighed and radioactivity was measured using a calibrated gamma counter (Wizard2 2480, Perkin Elmer). For each organ, the counts per minute (CPM) were determined and decay corrected to the time of sacrifice. Based on the calibration curve prepared with [¹⁸F]FDG for ¹⁸F radiotracers, the CPM values were converted to kBq. The radioactivity level was normalized to the injected dose and the volume of the organ in cubic centimeters (cc) and expressed as the ratio of percentage injected dose (%ID) to cc (%ID/cc). The density of the organs was assumed to be 1 g/cc except for bone which was assumed to be 1.4 g/cc (17). The data were pooled together to obtain a TAC for each organ.

The residence time (RT) per organ was calculated by integrating the area under the curve of the TACs. For the majority of the organs, exponentials were fitted and the data was extrapolated to infinity to calculate the total residence time. The trapezoid rule was used in case no exponential could be fitted to the TAC. For these organs, physical decay of the activity was assumed after the last measured time point. The residence time of the large intestines was assumed to be equally distributed in the left and right colon (18). Since no input source organs were available for the skin, muscle and fat measurements in OLINDA, these measurements were summed and put into the ‘total body/remainder’ source organ. The RT for each organ was extrapolated to humans based on the following formula (19):

RT_humans = [(\(\:\frac{\text{R}\text{T}\:\text{m}\text{o}\text{u}\text{s}\text{e}}{\text{O}\text{r}\text{g}\text{a}\text{n}\:\text{m}\text{o}\text{u}\text{s}\text{e}\:\left(\text{g}\right)}\)*(Total bodyweight mouse (kg))] *(\(\:\frac{\text{O}\text{r}\text{g}\text{a}\text{n}\:\text{h}\text{u}\text{m}\text{a}\text{n}\:\left(\text{g}\right)}{\text{T}\text{o}\text{t}\text{a}\text{l}\:\text{b}\text{o}\text{d}\text{y}\text{w}\text{e}\text{i}\text{g}\text{h}\text{t}\:\text{h}\text{u}\text{m}\text{a}\text{n}\:\left(\text{k}\text{g}\right)}\)).

Human organ masses and body weights were based on models proposed in ICRP publication 89 (20). The absorbed and total mean effective dose of [¹⁸F]CHDI-650 were estimated in humans with the dosimetry programs OLINDA/EXM 2.2 (HERMES Medical Solutions, Stockholm, Sweden) (21) and IDAC-Dose 2.1 (22). Absorbed organ doses (Gy/MBq) were converted to effective organ doses (Sv/MBq) based on the weighting factors presented in ICRP 103 (23).

2.6 Statistical analysis

The in vivo and ex vivo based absorbed dose estimates measured with OLINDA and IDAC were tested for normality with the Shapiro-Wilk test. The Spearman’s rho test was used to determine the correlation between the two dosimetry programs and between the two modalities used for collection of biodistribution data. Statistical analyses were performed with GraphPad Prism (v10) statistical software. Data are represented as mean ± SD unless specified otherwise. All tests were two-tailed, and statistical significance was set at p value < 0.05.

3.1 PET-imaging based-dosimetry

The whole-body distribution and the TACs for the delineated organs are presented in Fig. 2 and Fig. 3, respectively. The highest initial uptake was found in the liver, heart and urinary bladder. The muscle, brain, lungs, and heart showed a rapid washout followed by the kidneys and liver. The bladder still retained a portion of the tracer until 240 minutes, while the intestines showed retention at 360 minutes. The slower washout profile of radioactivity from the kidneys and the liver and the high uptake in the intestines and bladder appears to be due to clearance of [¹⁸F]CHDI-650 by renal and hepatobiliary pathways.

The residence times for adult human phantoms based on in vivo imaging are presented in Table 1. For males, the highest residence time was observed in the intestines (0.290 Bq*h/Bq) followed by muscles (0.182 Bq*h/Bq) and liver (0.140 Bq*h/Bq). For females the highest residence time was found in the bladder (0.278 Bq*h/Bq) followed by intestines (0.275 Bq*h/Bq) and muscles (0.133 Bq*h/Bq).

Table 1

**Residence time of [**¹⁸**F]CHDI-650 calculated for humans based on in vivo** **PET-imaging**.
	Residence time (Bq*h/Bq)
	Males	Females
Heart	4.56E-03	5.45E-03
Lungs	1.07E-02	1.27E-02
Liver	1.40E-01	8.94E-02
Kidneys	6.09E-03	9.80E-03
Bladder	1.11E-01	2.78E-01
Muscle	1.82E-01	1.33E-01
Brain	6.19E-03	5.92E-03
Intestines	2.90E-01	2.75E-01

The sex averaged absorbed and effective doses based on in vivo PET-imaging of [¹⁸F]CHDI-650 calculated using OLINDA and IDAC are shown in Table 2, with separate male and female doses presented in Supplemental Table 2 and Supplemental Table 3. Using OLINDA, the highest absorbed dose as well as the highest effective organ dose was found in the urinary bladder wall with values estimated at 107 µGy/MBq and 4.29 µSv/MBq, respectively. However, IDAC computed the highest absorbed dose and effective organ dose in the left colon wall with values estimated at 59.0 µGy/MBq and 3.54 µSv/MBq, respectively. The total mean effective dose based on in vivo PET-imaging determined was 14.7 µSv/MBq based on the OLINDA values while IDAC calculated a lower total mean effective dose of 11.2 µSv/MBq.

Table 2

**In vivo based absorbed and effective dose estimates for [**¹⁸**F]CHDI-650 with OLINDA/EXM 2.2 and IDAC-Dose2.1.**
Organs	Absorbed dose (mGy/MBq)		Effective dose (mSv/MBq)
Organs	OLINDA	IDAC	OLINDA	IDAC
Adrenals	6.33E-03	5.55E-03	5.85E-05	4.78E-05
Brain	1.26E-03	1.09E-03	1.26E-05	1.09E-05
Breasts	1.27E-03	1.34E-03	1.53E-04	1.61E-04
Esophagus	2.76E-03	2.09E-03	1.10E-04	8.36E-05
Eyes/Eye lenses	8.57E-04	2.34E-04	0.00	0.00
Gallbladder wall	9.41E-03	9.3E-03	8.68E-05	8.00E-05
Heart wall	5.32E-03	4.60E-03	4.91E-05	3.96E-05
Kidneys	9.29E-03	9.00E-03	8.57E-05	7.74E-05
Left colon wall	7.82E-02	5.90E-02	3.79E-03	3.54E-03
Liver	1.84E-02	1.44E-02	7.36E-04	5.76E-04
Lungs	3.62E-03	3.17E-03	4.35E-04	3.80E-04
Ovaries	1.00E-02	1.35E-02	4.01E-04	5.40E-04
Pancreas	7.68E-03	7.95E-03	7.09E-05	6.68E-04
Prostate	6.08E-03	8.26E-03	2.81E-05	7.10E-05
Red marrow	2.90E-03	3.63E-03	3.48E-04	4.40E-04
Right colon wall	4.60E-02	5.81E-02	2.23E-03	3.49E-03
Salivary glands	9.78E-04	4.73E-04	9.78E-06	4.73E-06
Small intestine wall	4.57E-02	3.05E-02	4.22E-04	2.62E-04
Spleen	3.60E-03	2.72E-03	3.32E-05	2.23E-05
Stomach wall	4.12E-03	5.18E-03	4.94E-04	6.12E-05
Testes	1.86E-03	1.10E-03	7.44E-05	4.40E-05
Thyroid	1.23E-03	7.96E-04	4.92E-05	3.18E-05
Urinary bladder wall	1.07E-01	3.73E-02	4.29E-03	1.49E-03
Uterus/cervix	1.90E-02	2.29E-02	8.80E-05	1.97E-04
Effective dose (mSv/MBq)	N.A.		1.47E-02	1.12E-02

3.2 Biodistribution based-dosimetry

The ex vivo biodistribution data are presented in Fig. 4, Supplemental Table 4 and Supplemental Table 5. Rapid washout of [¹⁸F]CHDI-650 was seen in most of the organs after 30 minutes except for the small intestines, urine and gallbladder, which showed a slower clearance of the radioactivity over time. On the other hand, the large intestines displayed a relatively low levels of radioactivity between 30–120 minutes, followed by a substantial increase at 240 minutes. The high radioactivity in both the gallbladder/intestines and urine/bladder is likely due to a mixed hepatobiliary and renal clearance pattern for [¹⁸F]CHDI-650.

The residence times for adult human phantoms based on ex vivo organ harvesting are presented in Table 3. The highest residence time for males was found in the fat (0.334 Bq*h/Bq), gallbladder (0.271 Bq*h/Bq) and small intestines (0.186 Bq*h/Bq). The highest residence time for females was observed and urine (0.268 Bq*h/Bq), fat (0.250 Bq*h/Bq) and small intestines (0.228 Bq*h/Bq).

Table 3

Residence time of [¹⁸F]CHDI-650 in humans calculated based on CD-1 mice ex vivo biodistribution data.
Organ	Residence time (Bq*h/Bq)
Organ	Males	Females
Blood	7.93E-02	8.29E-02
Heart	2.50E-03	2.71E-03
Lungs	1.01E-02	8.82E-03
Liver	6.68E-02	4.92E-02
Spleen	7.00E-04	2.12E-03
Pancreas	6.54E-04	7.45E-04
Stomach	8.01E-03	6.69E-03
Small intestine	1.86E-01	2.28E-01
Large intestine	1.36E-01	1.22E-01
Kidneys	7.15E-03	8.71E-03
Bladder	6.04E-03	3.01E-03
Urine	9.84E-02	2.68E-01
Muscle	6.50E-02	8.99E-02
Fat	3.34E-01	2.50E-01
Bone	2.82E-03	3.90E-03
Brain	2.32E-03	2.86E-03
Skin	1.96E-02	2.24E-02
Eyes	1.28E-06	1.78E-06
Testes/Ovaria	1.35E-04	7.90E-05
Gallbladder	2.71E-01	1.29E-01
Thyroid	1.09E-05	2.62E-05

Using the ex vivo data, the sex-averaged absorbed and effective doses of [¹⁸F]CHDI-650 computed with OLINDA and IDAC are shown in Table 4, separate male and female doses are presented in Supplemental Table 2 and Supplemental Table 3. The highest absorbed dose, based on ex vivo data, calculated with OLINDA and IDAC was estimated to be in the gallbladder wall at 361 µGy/MBq and 114 µGy/MBq, respectively. The highest effective organ dose computed with OLINDA was seen in the urinary bladder wall at 4.22 µSv/MBq, followed by the left colon wall at 3.75 µSv/MBq. Based on application of IDAC, the highest effective organ dose in both the left and right colon wall was determined to be 3.49 and 3.59 µSv/MBq, respectively. The total mean effective dose based on ex vivo biodistribution was determined to be 20.6 µSv/MBq estimated by OLINDA while IDAC computed a lower total mean effective dose of 15.8 µSv/MBq.

Table 4

Ex vivo based absorbed and effective dose estimates for [¹⁸F]CHDI-650 with OLINDA/EXM 2.2 and IDAC-Dose2.1.
Organs	Absorbed dose (mGy/MBq)		Effective dose (mSv/MBq)
Organs	OLINDA	IDAC	OLINDA	IDAC
Adrenals	1.08E-02	1.33E-02	9.93E-05	1.14E-04
Brain	1.02E-03	1.01E-03	1.02E-05	1.01E-05
Breasts	3.10E-03	2.58E-03	3.72E-04	3.10E-04
Esophagus	6.43E-03	4.34E-03	2.57E-04	1.74E-04
Eyes/Eye lenses	1.00E-03	1.26E-03	0.00	0.00
Gallbladder wall	3.61E-01	1.14E-01	3.34E-03	9.80E-04
Heart wall	2.65E-02	6.57E-03	2.45E-04	5.68E-05
Kidneys	1.26E-02	1.48E-02	1.16E-04	1.27E-04
Left colon wall	7.73E-02	5.82E-02	3.75E-03	3.49E-03
Liver	1.75E-02	1.90E-02	7.01E-04	7.60E-04
Lungs	5.38E-03	5.22E-03	6.54E-04	6.26E-04
Ovaries	1.32E-02	1.75E-02	5.26E-04	7.0E-04
Pancreas	1.30E-02	2.48E-02	1.20E-04	2.13E-04
Prostate	7.77E-03	9.29E-03	3.59E-05	7.99E-05
Red marrow	5.06E-03	6.85E-03	6.07E-04	8.22E-04
Right colon wall	5.09E-02	5.99E-02	2.47E-03	3.59E-03
Salivary glands	2.22E-03	9.65E-04	2.22E-05	9.65E-06
Small intestine wall	6.83E-02	4.53E-02	6.31E-04	3.90E-04
Spleen	6.88E-03	7.23E-03	6.35E-05	6.22E-05
Stomach wall	1.05E-02	1.44E-02	1.26E-03	1.73E-03
Testes	2.72E-03	2.81E-03	1.09E-04	1.12E-04
Thyroid	3.54E-03	3.35E-03	1.41E-04	1.34E-04
Urinary bladder wall	1.06E-01	5.05E-02	4.22E-03	2.02E-03
Uterus/cervix	2.21E-02	2.48E-02	1.02E-04	2.13E-04
Effective dose (mSv/MBq)	N.A.		2.06E-02	1.58E-02

3.3 Comparison of OLINDA/EXM 2.2 and IDAC-Dose2.1

We compared the outcome of two different software for dosimetry evaluation, namely OLINDA and IDAC, based on a correlation analysis of the computed sex-averaged absorbed doses. The correlation plot of the absorbed dose estimates based on in vivo imaging displayed a higher correlation (r = 0.9801) for most organs compared to the estimates based on ex vivo biodistribution (r = 0.9368) (Fig. 5). When performing an ex vivo vs. in vivo absorbed dose correlation, computed with OLINDA and IDAC, a high correlation (r = 0.9879 and r = 0.9697, respectively) between both modalities was seen when the same input organs are used, with the liver and small intestine wall being the only noticeable outliers for both sexes. Interestingly, the uterus was an outlier based on IDAC estimates which was not seen with the OLINDA estimates (Fig. 6, Supplemental Table 6, Supplemental Table 7). This high correlation was also reflected in the residence time, used as input variables in OLINDA and IDAC, which were comparable for both modalities in the kidneys, lungs, intestines and bladder (e.g., kidneys, for ex vivo: 7.93E-03 Bq*h/Bq; for in vivo: 7.95E-03 Bq*h/Bq). Other organs such as the heart showed a higher in vivo based residence time compared to ex vivo (in vivo: 5.01E-03 Bq*h/Bq; ex vivo: 2.61E-03 Bq*h/Bq) but did not have any noticeable effects on the absorbed dose correlation between modalities. (Table 1 and Table 3). Nonetheless, few differences in the absorbed dose estimates, between programs, were observed for high exposure values, with the most notable difference seen ex vivo in the gallbladder wall which was about 3 times higher with OLINDA (361 µGy/MBq) compared to IDAC (114 µGy/MBq) (Fig. 5 and Table 4); similarly, the heart wall exhibited higher mean values with OLINDA (26.5 µGy/MBq) compared to values obtained with IDAC (6.6 µGy/MBq). For the in vivo based measurements, where no direct measurements for the gallbladder was possible, the computed absorbed dose for the heart wall was comparable between programs. Both ex vivo and in vivo sex-averaged estimation of the urinary bladder wall found twice or almost three times as high absorbed dose values with OLINDA compared to IDAC (e.g., ex vivo: 106 µGy/MBq vs 50.5 µGy/MBq; in vivo: 107 µGy/MBq vs 37.5 µGy/MBq, respectively). Additional differences were found for the intestinal organs, where the small intestine wall showed 1.5 times higher absorbed dose values by OLINDA than by IDAC for both in vivo imaging based and ex vivo biodistribution based estimates. Moreover, the absorbed dose for left colon wall was estimated higher in OLINDA, while the right colon wall was estimated higher with IDAC based on in vivo and ex vivo data. Lastly, the absorbed dose values, based on ex vivo biodistribution, for the pancreas was estimated to be almost two times higher with IDAC (OLINDA: 13.0 µGy/MBq vs IDAC: 24.8 µGy/MBq) (Fig. 5, Table 2 and Table 4).

In this study, we report the in vivo and ex vivo biodistribution and dosimetry profile of [¹⁸F]CHDI-650, a novel fluorinated radioligand for imaging of mHTT aggregates. The highest uptake in vivo was seen in the urinary bladder with the highest uptake ex vivo in the gallbladder and urine, hinting at a mixed hepatobiliary and renal clearance. The mouse data was extrapolated to standardized adult human phantoms (20) in order to perform an exposure assessment of the radioligand based on the absorbed and effective doses. The total mean effective dose computed with OLINDA and IDAC based on in vivo imaging was 14.7 µSv/MBq and 11.2 µSv/MBq, respectively, and based on ex vivo biodistribution was 20.6 µSv/MBq and 15.8 µSv/MBq, respectively, which is comparable to other [¹⁸F] radioligands (average [¹⁸F] effective dose 20.6 ± 6.8 µSv/MBq) (24).

The absorbed and effective doses of [¹⁸F]CHDI-650 were estimated based on in vivo PET-imaging and ex vivo biodistribution residence times. The residence times for the organs with the highest absorbed and effective dose estimates such as the bladder and intestines were in the same range for both in vivo and ex vivo. We did observe some differences in the residence time in the heart, liver, muscle and brain, however, these organs had a relative low impact on the absorbed dose estimates and the correlation between modalities. This was corroborated by a high correlation of OLINDA and IDAC estimates (r = 0.9879 and r = 0.9697, respectively) when the same input organs are used to correlate in vivo and ex vivo values. Similar correlation was also found in previous studies exploring both in vivo and ex vivo biodistribution with other radioligands (25, 26).

[¹⁸F]CHDI-650 displayed a high uptake in the gallbladder and urinary bladder wall as well as high estimated absorbed and effective doses in the intestinal organs. Similar observation was made for previous mHTT aggregate ligands, [¹¹C]CHDI-180R and [¹¹C]CHDI-626 in a dosimetry study conducted in humans (10). However, the overall radiation burden of both [¹¹C]CHDI-180R (4.58 µSv/MBq) and [¹¹C]CHDI-626 (5.09 µSv/MBq) was lower compared to that estimated for [¹⁸F]CHDI-650 (ex vivo: 20.6 µSv/MBq; in vivo: 14.7 µSv/MBq), likely due to the shorter half-life of the ¹¹C isotope relative to the ¹⁸F isotope.

An important aspect we wanted to address with this study is the performance comparison of two dosimetry programs, OLINDA/EXM 2.2 and IDAC-Dose2.1. OLINDA/EXM is an FDE/CE certified software which has been extensively utilized in research and clinical settings for almost two decades, establishing itself as the gold standard within the field. On the other hand, IDAC-Dose2.1 is a more recently developed, non-commercial software package based on the ICRP recommendations without any formal European or American regulatory approval. We found an overall high agreement utilizing both programs for both in vivo (r = 0.9801) and ex vivo (r = 0.9368) based absorbed dose estimates. The largest difference using ex vivo based biodistribution data was seen in the absorbed dose of the gallbladder wall which was up to three times as high in OLINDA (361 µGy/MBq) compared to IDAC (114 µGy/MBq). Absorbed dose calculations are influenced by the chosen phantom model and the specific absorbed fractions (SAF). IDAC utilizes adult male and female voxel phantoms proposed in ICRP 110 (27), which are limited by voxel resolution having a less accurate representation of the gallbladder wall and urinary bladder wall (28). The non-uniform rational b-spline (NURBS) model used by OLINDA improves on the voxel phantoms by having more accurate and smoother organ surfaces (29). Furthermore, the SAF calculation of OLINDA is based on the GEANT4 code (29, 30) while IDAC utilized the EGS4 code presented in ICRP 133 (31, 32). The total mean effective dose computed with OLINDA was higher (in vivo 14.7 µSy/MBq; ex vivo 20.6 µSy/MBq) compared to IDAC (in vivo 11.2 µSy/MBq; ex vivo 15.8 µSy/MBq). The effective dose estimations for both programs are based on the ICRP 103 recommendation (23); however, OLINDA deviates by incorporating different tissue weighting factors for selected organs, most notably for the gonads (OLINDA: 0.04 vs ICRP 103: 0.08) (33).

Both dosimetry programs and modalities displayed effective doses comparable to other [¹⁸F] ligands (24). High absorbed and effective organ doses were consistently found in the gallbladder wall and urinary bladder wall with the highest effective organ doses being 3.34 µSv/MBq and 4.29 µSv/MBq, respectively. If necessary, the gallbladder wall dose can be reduced by consumption of a fatty meal after completion of the PET scan (34). Likewise, the urinary bladder wall dose can be decreased by voiding the bladder after injection. An effective dose radiation exposure limit based on the Radioactive Drug Research Committee (RDRC) Program under 21 CFR 361.1 is 30 mSv for a single exposure and 50 mSv for an annual exposure for whole body, active blood-forming organs, eye lens, and gonads. For all other organs the Radioactive Drug Research Committee Program limits established are 50 mSv and 150 mSv for single and annual exposure, respectively meaning that the gallbladder and urinary bladder wall are well below these limits when using typical injections of approximately 370 MBq (10 mCi) (35). Based on the highest effective dose estimates (20.6 µSv/MBq) of [¹⁸F]CHDI-650, an injection of 370 MBq would result in a radiation dose of 7.62 mSv. These estimates indicate that [¹⁸F]CHDI-650 can be administered multiple times per year, based on maximum total dose recommendations by the RDRC in the United States of America (50 mSv/year). However, based on the category II B classification of the radioligand established by the European Commission in Europe (10 mSv/year, for patients below 50 years of age) the administration of 370 MBq is limited to once a year (36, 37). Collectively, these findings suggest that administration of [¹⁸F]CHDI-650 poses low radioactivity exposure related risk to humans during clinical investigation.

We evaluated the biodistribution and radiation dosimetry profile of [¹⁸F]CHDI-650, a novel mHTT radioligand. In vivo and ex vivo yielded comparable residence times, however, more organs were measured ex vivo leading to a higher estimation of the mean effective dose. OLINDA/EXM 2.2 and IDAC-Dose2.1 calculated similar doses for most organs, nevertheless, OLINDA/EXM 2.2 calculated higher doses in organs with a greater tissue weighting factor resulting in an increased mean effective dose compared to IDAC-Dose2.1. The effective dose of [¹⁸F]CHDI-650 ranged from 11.2 µSv/MBq to 20.6 µSv/MBq. Based on the highest effective dose estimate an injection of 370 MBq (10 mCi) in humans would result in an effective dose of 7.62 mSv. This supports the clinical investigation of [¹⁸F]CHDI-650.

A_m

Molar activity

Becquerel

Cubic centimetre

CPM

Counts per minute

Gray

Huntington’s disease

HTT

Huntingtin gene

ICRP

International Commission on Radiological Protection

i.v.

Intravenous

mHTT

Mutant huntingtin

NURBS

non-uniform rational b-spline

PBS

Phosphate-buffered saline

PET

Positron emission tomography

p.i

post-injection

RDRC

Radioactive Drug Research Committee

Residence time

SAF

Specific absorbed fractions

Sievert

TACs

Time-activity curves

TEAB

Tetra-ethyl ammonium bicarbonate

VOIs

Volumes of interest

Wild-type

Ethics approval and consent to participate

All experiments were approved by the Ethical Committee for Animal Testing (ECD 2019-39) at the University of Antwerp (Belgium) and the European Committee Guidelines (decree 2010/63/CEE) for the care and use of animals were followed.

Consent for publication

Not applicable

Availability of data and material

The datasets generated and/or analysed during the current study will be made available in an online repository upon acceptance.

Competing interests.

LL, JB, VK and RD are employed by CHDI Management, Inc. the company that manages the scientific activities of CHDI Foundation, Inc. No other potential conflicts of interest relevant to this article exist.

Funding

This work was funded by CHDI Foundation, Inc. (www.chdifoundation.org), a privately-funded nonprofit biomedical research organization exclusively dedicated to collaboratively developing therapeutics that improve the lives of those affected by Huntington’s disease. DB acknowledges support by the Research Foundation Flanders (FWO) (ID: 1229721N). University of Antwerp supported the work through a partial assistant professor position for JV, a tenure-track assistant professor position for DB, a full professor position for SSta as well as funding to DB (FFB210050).

CRediT Author Contribution Statement

Conceptualization: JV, JB, LL, VK, SS, DB

Methodology: JA, AM, JV, SS, DB

Tracer synthesis: FE

Data acquisition: JA

Formal analysis and investigation: JA, AM

Data interpretation: JA, AM, JV, FZ, JB, LL, VK, RD, SS, DB

Writing – original draft preparation: JA, SS, DB

Writing – review and editing: JA, AM, JV, FE, FZ, JB, LL, VK, RD, SS, DB

Acknowledgements

The authors thank Philippe Joye, Caroline Berghmans, Eleni Van der Hallen and Annemie van Eetveldt of the Molecular Imaging Center Antwerp (MICA) for their valuable assistance. We also thank Brenda Lager (CHDI) for animal supply and Maya Bader (CHDI) for program management and Dr Simon Noble for reviewing and providing constructive feedback.

Supplementary Materials

Supplementary material associated with this article can be found in the online version.

ORCiD

Jordy Akkermans https://orcid.org/0000-0002-1533-9504

Alan Miranda https://orcid.org/0000-0002-5381-015X

Filipe Elvas https://orcid.org/0000-0001-9790-3369

Franziska Zajicek https://orcid.org/0000-0001-5410-687X

Jonathan Bard https://orcid.org/0000-0002-5097-1361

Longbin Liu https://orcid.org/0000-0003-3995-0196

Vinod Khetarpal https://orcid.org/0000-0002-2175-5117

Robert Doot https://orcid.org/0000-0003-1747-239X

Steven Staelens https://orcid.org/0000-0003-3376-0519

Daniele Bertoglio https://orcid.org/0000-0003-4205-5432

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The authors declare potential competing interests as follows: LL, JB, VK and RD are employed by CHDI Management, Inc. the company that manages the scientific activities of CHDI Foundation, Inc. No other potential conflicts of interest relevant to this article exist.

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Biodistribution and dosimetry of the PET radioligand [¹⁸F]CHDI-650 in mice for detection of mutant huntingtin aggregates

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Abstract

Background

Results

Conclusions

Figures

1. Background

2. Methods

2.1 Experimental design

2.2 Radiosynthesis

2.3 PET image acquisition

2.4 PET image reconstruction

2.5 Data analysis

2.6 Biodistribution and dosimetry

2.6 Statistical analysis

3. Results

3.1 PET-imaging based-dosimetry

3.2 Biodistribution based-dosimetry

3.3 Comparison of OLINDA/EXM 2.2 and IDAC-Dose2.1

4. Discussion

5. Conclusion

Abbreviations

Declarations

References

Additional Declarations

Supplementary Files

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