Whole-Body Biodistribution and Radiation Dosimetry of the Human Cannabinoid Type-1 Receptor Ligand 18F-MK-9470 in Healthy Subjects

Sex Differences in Neuropsychological Functioning are Domain-Specific in Adolescent and Young Adult Regular Cannabis Users

OBJECTIVE

Adolescence into young adulthood represents a sensitive period in which brain development significantly diverges by sex. Regular cannabis use by young people is associated with neuropsychological vulnerabilities, but the potential impact of sex on these relationships is unclear.

METHOD

In a cross-sectional study, we examined sex differences in multi-domain neuropsychological functioning using the Cambridge Neuropsychological Test Automated Battery (CANTAB) and tested whether sex moderated the relationship between cognitive performance and age of initiation, frequency of cannabis use, amount of cannabis use, and withdrawal symptoms in at least weekly adolescent and young adult cannabis users (n = 171; aged 13-25 years; 46.2% female).

RESULTS

Male cannabis users had poorer visual recognition memory and female cannabis users showed worse attention and executive functions, with medium to large effect sizes. These sex effects persisted, when controlling for age, IQ, amount of alcohol and nicotine use, mood and anxiety symptoms, emotional stability and impulsive behavior. Earlier age of initiated use and more use were associated with worse attentional functions in females, but not males. More use was more strongly associated with worse episodic memory in males than in females. More use was associated with poorer learning in males only.

CONCLUSIONS

Domain-specific patterns of neuropsychological performance were found by sex, such that males showed poorer visual memory and females showed worse performance on measures of attention (sustained visual, multitasking) and executive functioning (spatial planning/working memory subdomains). Larger studies including healthy controls are needed to determine if the observed sex differences are more exaggerated relative to non-users.

Biodistribution and radiation dosimetry of the positron emission tomography probe for AMPA receptor, [11C]K-2, in healthy human subjects

[¹¹C]K-2, a radiotracer exhibiting high affinity and selectivity for α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs), is suitable for the quantification of AMPARs in living human brains and potentially useful in the identification of epileptogenic foci in patients. This study aimed to estimate the radiation doses of [¹¹C]K-2 in various organs and calculate the effective dose after injection of [¹¹C]K-2 in healthy human subjects. Twelve healthy male subjects were registered and divided into two groups (370 or 555 MBq of [¹¹C]K-2), followed by 2 h whole-body scans. We estimated the radiation dose of each organ and then calculated the effective dose for each subject. The highest uptake of [¹¹C]K-2 was observed in the liver, while the brain also showed relatively high uptake. The urinary bladder exhibited the highest radiation dose. The kidneys and liver also showed high radiation doses after [¹¹C]K-2 injections. The effective dose of [¹¹C]K-2 ranged from 5.0 to 5.2 μSv/MBq. Our findings suggest that [¹¹C]K-2 is safe in terms of the radiation dose and adverse effects. The injection of 370–555 MBq (10 to 15 mCi) for PET studies using this radiotracer is applicable in healthy human subjects and enables serial PET scans in a single subject.

Is There a Role for GPCR Agonist Radiotracers in PET Neuroimaging?

Positron emission tomography (PET) is a molecular imaging modality that enables in vivo exploration of metabolic processes and especially the pharmacology of neuroreceptors. G protein-coupled receptors (GPCRs) play an important role in numerous pathophysiologic disorders of the central nervous system. Thus, they are targets of choice in PET imaging to bring proof concept of change in density in pathological conditions or in pharmacological challenge. At present, most radiotracers are antagonist ligands. In vitro data suggest that properties differ between GPCR agonists and antagonists: antagonists bind to receptors with a single affinity, whereas agonists are characterized by two different affinities: high affinity for receptors that undergo functional coupling to G-proteins, and low affinity for those that are not coupled. In this context, agonist radiotracers may be useful tools to give functional images of GPCRs in the brain, with high sensitivity to neurotransmitter release. Here, we review all existing PET radiotracers used from animals to humans and their role for understanding the ligand-receptor paradigm of GPCR in comparison with corresponding antagonist radiotracers.

Impact of Region-of-Interest Delineation Methods, Reconstruction Algorithms, and Intra- and Inter-Operator Variability on Internal Dosimetry Estimates Using PET

PURPOSE

Human dosimetry studies play a central role in radioligand development for positron emission tomography (PET). Drawing regions of interest (ROIs) on the PET images is used to measure the dose in each organ. In the study aspects related to ROI delineation methods were evaluated for two radioligands of different biodistribution (intestinal vs urinary).

PROCEDURES

PET images were simulated from a human voxel-based phantom. Several ROI delineation methods were tested: antero-posterior projections (AP), 3D sub-samples of the organs (S), and a 3D volume covering the whole-organ (W). Inter- and intra-operator variability ROI drawing was evaluated by using human data.

RESULTS

The effective dose estimates using S and W methods were comparable to the true values. AP methods overestimated (49 %) the dose for the radioligand with intestinal biodistribution. Moreover, the AP method showed the highest inter-operator variability: 11 ± 1 %.

CONCLUSIONS

The sub-sampled organ method showed the best balance between quantitative accuracy and inter- and intra-operator variability.

Radiation dosimetry estimates of (18)F-alfatide II based on whole-body PET imaging of mice

We estimated the dosimetry of (18)F-alfatide II with the method established by MIRD based on biodistribution data of mice. Six mice (three females and three males) were scanned for 160min on an Inveon MicroPET/CT scanner after injection of (18)F-alfatide II via tail vein. Eight source organs were delineated on the CT images and their residence times calculated. The data was then converted to human using scaling factors based on organ and body weight. The absorbed doses for human and the resulting effective dose were computed by OLINDA 1.1 software. The highest absorbed doses was observed in urinary bladder wall (male 0.102mGy/MBq, female 0.147mGy/MBq); and the lowest one was detected in brain (male 0.0030mGy/MBq, female 0.0036). The total effective doses were 0.0127mSv/MBq for male and 0.0166 mSv/MBq for female, respectively. A 370-MBq injection of (18)F-alfatide II led to an estimated effective dose of 4.70mSv for male and 6.14mSv for female. The potential radiation burden associated with (18)F-alfatide II/PET imaging therefore is comparable to other PET examinations.

Advances in CNS Imaging Agents: Focus on PET and SPECT Tracers in Experimental and Clinical Use

The physiological functioning of the brain is not well-known in current day medicine and the pathologies of many neuropsychiatric disorders are still not yet fully understood. With our aging population and better life expectancies, it has become imperative to find better biomarkers for disease progression as well as receptor target engagements. In the last decade, these major advances in the field of molecular CNS imaging have been made available with tools such as functional magnetic resonance imaging (fMRI), magnetic resonance spectroscopy (MRS), single photon emission computed tomography (SPECT), and neuroreceptor-targeted positron emission tomography (PET). These tools have given researchers, pharmaceutical companies, and clinical physicians a better method of understanding CNS dysfunctions, and the ability to employ improved therapeutic agents. This review is intended to provide an update on brain imaging agents that are currently used in clinical and translational research toward treatment of CNS disorders. The review begins with amyloid and tau imaging, the former of which has at least three [(18)F] agents that have been recently approved and will soon be available for clinical use for specific indications in the USA and elsewhere. Other prevalent PET and SPECT neurotransmitter system agents, including those newly US FDA-approved imaging agents related to the dopaminergic system, are included. A review of both mature and potentially growing PET imaging agents, including those targeting serotonin and opiate receptor systems, is also provided.

Synthesis and Preliminary Evaluation of a 2-Oxoquinoline Carboxylic Acid Derivative for PET Imaging the Cannabinoid Type 2 Receptor

Cannabinoid receptor subtype 2 (CB2) has been shown to be up-regulated in activated microglia and therefore plays an important role in neuroinflammatory and neurodegenerative diseases such as multiple sclerosis, amyotrophic lateral sclerosis and Alzheimer’s disease. The CB2 receptor is therefore considered as a very promising target for therapeutic approaches as well as for imaging. A promising 2-oxoquinoline derivative designated KP23 was synthesized and radiolabeled and its potential as a ligand for PET imaging the CB2 receptor was evaluated. [¹¹C]KP23 was obtained in 10%–25% radiochemical yield (decay corrected) and 99% radiochemical purity. It showed high stability in phosphate buffer, rat and mouse plasma. In vitro autoradiography of rat and mouse spleen slices, as spleen expresses a high physiological expression of CB2 receptors, demonstrated that [¹¹C]KP23 exhibits specific binding towards CB2. High spleen uptake of [¹¹C]KP23 was observed in dynamic in vivo PET studies with Wistar rats. In conclusion, [¹¹C]KP23 showed promising in vitro and in vivo characteristics. Further evaluation with diseased animal model which has higher CB2 expression levels in the brain is warranted.

Whole-body biodistribution and radiation dosimetry of the cannabinoid type 2 receptor ligand [11C]-NE40 in healthy subjects

PURPOSE

The type 2 cannabinoid receptor (CB2R) is part of the human endocannabinoid system and is involved in central and peripheral inflammatory processes. In vivo imaging of the CB2R would allow study of several (neuro)inflammatory disorders. In this study we have investigated the safety and tolerability of [11C]-NE40, a CB2R positron emission tomography (PET) ligand, in healthy human male subjects and determined its biodistribution and radiation dosimetry.

PROCEDURE

Six healthy male subjects (age 20-65 years) underwent a dynamic series of nine whole-body PET/CT scans for up to 140 min, after injection of an average bolus of 286 MBq of [11C]-NE40. Organ absorbed and total effective doses were calculated through OLINDA.

RESULTS

[11C]-NE40 showed high initial uptake in the spleen and a predominant hepatobiliary excretion. In the brain, rapid uptake and swift washout were seen. Organ absorbed doses were largest for the small intestine and liver, with 15.6 and 11.5 μGy/MBq, respectively. The mean effective dose was 3.64±0.81 μSv/MBq. There were no changes with aging observed. No adverse events were encountered.

CONCLUSIONS

This first-in-man study of [11C]-NE40 showed an expected biodistribution compatible with lymphoid tissue uptake and appropriate fast brain kinetics in the healthy human brain, underscoring the potential of this tracer for further application in central and peripheral inflammation imaging. The effective dose is within the typical expected range for 11C ligands.

18F-MK-9470 PET imaging of the type 1 cannabinoid receptor in prostate carcinoma: a pilot study

Background

Preclinical and histological data show overexpression of the type 1 cannabinoid receptor (CB1R) in prostate carcinoma (PCa). In a prospective study, the feasibility of ¹⁸F-MK-9470 positron emission tomography (PET) imaging in patients with primary and metastatic PCa was evaluated.

Methods

Eight patients were included and underwent ¹⁸F-MK-9470 PET/CT imaging. For five patients with primary PCa, dynamic PET/CT imaging was performed over three acquisition intervals (0 to 30, 60 to 90 and 120 to 150 min post-injection). In malignant and benign prostate tissue regions, time activity curves of the mean standardized uptake value (SUV_mean) were determined as well as the corresponding area under the curve to compare ¹⁸F-MK-9470 uptake over time. Muscle uptake of ¹⁸F-MK-9470 was used as reference for non-specific binding. Magnetic resonance imaging (MRI) was used as anatomical reference and for delineating intraprostatic tumours. Histological and immunohistochemical (IHC) examination was performed on the whole-mount histopathology sections of four patients who underwent radical prostatectomy to assess the MRI-based tumour versus benign tissue classification. For three patients with proven advanced metastatic disease, two static PET/CTs were performed 1 and 3 h post-injection. ¹⁸F-MK-9470 uptake was evaluated in bone lesions of metastatic PCa by comparing SUV_mean values of metastases with these of the contralateral bone tissue.

Results

¹⁸F-MK-9470 uptake was significantly higher in benign and malignant prostate tissue compared to muscle, but it did not differ between both prostate tissue compartments. IHC findings of corresponding prostatic histopathological sections indicated weak CB1R expression in locally confined PCa, which was not visualized with ¹⁸F-MK-9470 PET. Metastases in the axial skeleton could not be detected while some metastases in the appendicular skeleton showed higher ¹⁸F-MK-9470 uptake as compared to the uptake in contralateral normal bone.

Conclusions

¹⁸F-MK-9470 PET could not detect local PCa or bone metastases in the axial skeleton but was able to visualize metastases in the appendicular skeleton. Based on these pilot observations, it seems unlikely that CB1R PET will play a significant role in the evaluation of PCa.

In vivo SPECT and ex vivo autoradiographic brain imaging of the novel selective CB1 receptor antagonist radioligand [125I]SD7015 in CB1 knock-out and wildtype mouse

We aimed to evaluate the novel high-affinity and relatively lipophilic CB(1) receptor (CB(1)R) antagonist radioligand [(125)I]SD7015 for SPECT imaging of CB(1)Rs in vivo using the multiplexed multipinhole dedicated small animal SPECT/CT system, NanoSPECT/CT(PLUS) (Mediso, Budapest, Hungary), in knock-out CB(1) receptor knock-out (CB(1)R-/-) and wildtype mice. In order to exclude possible differences in cerebral blood flow between the two types of animals, HMPAO SPECT scans were performed, whereas in order to confirm the brain uptake differences of the radioligand between knock-out mice and wildtype mice, in vivo scans were complemented with ex vivo autoradiographic measurements using the brains of the same animals. With SPECT/CT imaging, we measured the brain uptake of radioactivity, using %SUV (% standardised uptake values) in CB(1)R-/- mice (n=3) and C57BL6 wildtype mice (n=7) under urethane anaesthesia after injecting [(125)I]SD7015 intravenously or intraperitoneally. The Brookhaven Laboratory mouse MRI atlas was fused to the SPECT/CT images by using a combination of rigid and non-rigid algorithms in the Mediso Fusion™ (Mediso, Budapest, Hungary) and VivoQuant (inviCRO, Boston, MA, USA) softwares. Phosphor imager plate autoradiography (ARG) was performed on 4 μm-thin cryostat sections of the excised brains. %SUV was 8.6±3.6 (average±SD) in CB(1)R-/- mice and 22.1±12.4 in wildtype mice between 2 and 4 h after injection (p<0.05). ARG of identically taken sections from wildtype mouse brain showed moderate radioactivity uptake when compared with the in vivo images, with a clear difference between grey matter and white matter, whereas ARG in CB(1)R(-/-) mice showed practically no radioactivity uptake. [(125)I]SD7015 enters the mouse brain in sufficient amount to enable SPECT imaging. Brain radioactivity distribution largely coincides with that of the known CB(1)R expression pattern in rodent brain. We conclude that [(125)I]SD7015 should be a useful SPECT radioligand for studying brain CB(1)R in mouse and rat disease models.

Comparison of Cannabinoid CB(1) Receptor Binding in Adolescent and Adult Rats: A Positron Emission Tomography Study Using [F]MK-9470

Despite the important role of cannabinoid CB(1) receptors (CB(1)R) in brain development, little is known about their status during adolescence, a critical period for both the development of psychosis and for initiation to substance abuse. In the present study, we assessed the ontogeny of CB(1)R in adolescent and adult rats in vivo using positron emission tomography with [(18)F]MK-9470. Analysis of covariance (ANCOVA) to control for body weight that would potentially influence [(18)F]MK-9470 values between the two groups revealed a main effect of age (F(1,109)=5.0, P = 0.02) on [(18)F]MK-9470 absolute binding (calculated as percentage of injected dose) with adult estimated marginal means being higher compared to adolescents amongst 11 brain regions. This finding was confirmed using in vitro autoradiography with [(3)H]CP55,940 (F(10,99)=140.1, P < 0.0001). This ontogenetic pattern, suggesting increase of CB(1)R during the transition from adolescence to adulthood, is the opposite of most other neuroreceptor systems undergoing pruning during this period.

Biodistribution and radiation dosimetry of the glycine transporter-1 ligand 11C-GSK931145 determined from primate and human whole-body PET

PURPOSE

(11)C-GSK931145 is a novel radioligand suitable for imaging the glycine transporter 1 (GlyT-1) in brain. In the present study, human dosimetry is estimated from baboon and human biodistribution data.

PROCEDURES

Three baboons and eight healthy human volunteers underwent whole-body positron emission tomography (PET) scans. Human dosimetry was estimated using three different region-of-interest (ROI) delineation methods that ranged in their complexity and execution time: ROIs drawn on anterior-posterior compressed PET images, on subsamples of the organs, and covering the whole-organ. Residence times for each organ were calculated as the area under the time-activity curves divided by the injected activity. Radiation dose estimates were calculated from organ residence times using the OLINDA/EXM software package.

RESULTS

The overall distribution of activity was similar in baboons and humans. Early scans presented high activity in the liver, and moderate activity in the lungs and kidneys. The principal route of clearance was intestinal and no urinary excretion was observed. The limiting organ with the highest radiation-absorbed dose was the liver. The mean effective dose in humans was 4.02 μSv/MBq (male phantom) and 4.95 μSv/MBq (female phantom) (ROIs drawn on subsamples of the organs). The human effective dose estimated from baboon data was ~15% larger than the effective dose estimated from human data.

CONCLUSION

Human PET imaging of the glycine transporter-1 with (11)C-GSK931145 results in a moderate effective human radiation dose, which allows for multiple PET examinations in the same individual. Among the three methods compared to delineate ROIs, the organ subsampling method shows the best balance between quantitative accuracy and practical application.

Abnormal cerebellar morphometry in abstinent adolescent marijuana users

Functional neuroimaging data from adults have, in general, revealed frontocerebellar dysfunction associated with acute and chronic marijuana (MJ) use. The goal of this study was to characterize cerebellar volume in adolescent chronic MJ users following 1 month of monitored abstinence. Participants were MJ users (n=16) and controls (n=16) aged 16-18 years. Extensive exclusionary criteria included history of psychiatric or neurologic disorders. Drug use history, neuropsychological data, and structural brain scans were collected after 28 days of monitored abstinence. Trained research staff defined cerebellar volumes (including three cerebellar vermis lobes and both cerebellar hemispheres) on high-resolution T1-weighted magnetic resonance images. Adolescent MJ users demonstrated significantly larger inferior posterior (lobules VIII-X) vermis volume than controls, above and beyond effects of lifetime alcohol and other drug use, gender, and intracranial volume. Larger vermis volumes were associated with poorer executive functioning. Following 1 month of abstinence, adolescent MJ users had significantly larger posterior cerebellar vermis volumes than non-using controls. These greater volumes are suggested to be pathological based on linkage to poorer executive functioning. Longitudinal studies are needed to examine typical cerebellar development during adolescence and the influence of marijuana use.

Quantification of cerebral cannabinoid receptors subtype 1 (CB1) in healthy subjects and schizophrenia by the novel PET radioligand [11C]OMAR

Several studies have examined the link between the cannabinoid CB1 receptor and several neuropsychiatric illnesses, including schizophrenia. As such, there is a need for in vivo imaging tracers so that the relationship between CB1 and schizophrenia (SZ) can be further studied. In this paper, we present our first human studies in both healthy control patients and patients with schizophrenia using the novel PET tracer, [(11)C]OMAR (JHU75528), we have shown its utility as a tracer for imaging human CB1 receptors and to investigate normal aging and the differences in the cannabinoid system of healthy controls versus patients with schizophrenia. A total of ten healthy controls and nine patients with schizophrenia were included and studied with high specific activity [(11)C]OMAR. The CB1 binding (expressed as the distribution volume; V(T)) was highest in the globus pallidus and the cortex in both controls and patients with schizophrenia. Controls showed a correlation with the known distribution of CB1 and decline of [(11)C]OMAR binding with age, most significantly in the globus pallidus. Overall, we observed elevated mean binding in patients with schizophrenia across all regions studied, and this increase was statistically significant in the pons (p<0.05), by the Students t-test. When we ran a regression of the control subjects V(T) values with age and then compared the patient data to 95% prediction limits of the linear regression, three patients fell completely outside for the globus pallidus, and in all other regions there were at least 1-3 patients outside of the prediction intervals. There was no statistically significant correlations between PET measures and the individual Brief Psychiatry Rating Score (BPRS) subscores (r=0.49), but there was a significant correlation between V(T) and the ratio of the BPRS psychosis to withdrawal score in the frontal lobe (r=0.60), and middle and posterior cingulate regions (r=0.71 and r=0.79 respectively). In conclusion, we found that [(11)C] OMAR can image human CB1 receptors in normal aging and schizophrenia. In addition, our initial data in subjects with schizophrenia seem to suggest an association of elevated binding specific brain regions and symptoms of the disease.

Biodistribution and dosimetry in humans of two inverse agonists to image cannabinoid CB1 receptors using positron emission tomography

PURPOSE

Cannabinoid subtype 1 (CB(1)) receptors are found in nearly every organ in the body, may be involved in several neuropsychiatric and metabolic disorders, and are therefore an active target for pharmacotherapy and biomarker development. We recently reported brain imaging of CB(1) receptors with two PET radioligands: (11)C-MePPEP and (18)F-FMPEP-d (2). Here we describe the biodistribution and dosimetry estimates for these two radioligands.

METHODS

Seven healthy subjects (four men and three women) underwent whole-body PET scans for 120 min after injection with (11)C-MePPEP. Another seven healthy subjects (two men and five women) underwent whole-body PET scans for 300 min after injection with (18)F-FMPEP-d (2). Residence times were acquired from regions of interest drawn on tomographic images of visually identifiable organs for both radioligands and from radioactivity excreted in urine for (18)F-FMPEP-d (2).

RESULTS

The effective doses of (11)C-MePPEP and (18)F-FMPEP-d (2) are 4.6 and 19.7 microSv/MBq, respectively. Both radioligands demonstrated high uptake of radioactivity in liver, lung, and brain shortly after injection and accumulated radioactivity in bone marrow towards the end of the scan. After injection of (11)C-MePPEP, radioactivity apparently underwent hepatobiliary excretion only, while radioactivity from (18)F-FMPEP-d (2) showed both hepatobiliary and urinary excretion.

CONCLUSION

(11)C-MePPEP and (18)F-FMPEP-d (2) yield an effective dose similar to other PET radioligands labeled with either (11)C or (18)F. The high uptake in brain confirms the utility of these two radioligands to image CB(1) receptors in brain, and both may also be useful to image CB(1) receptors in the periphery.

Prefrontal cortex morphometry in abstinent adolescent marijuana users: subtle gender effects

Adult human studies suggest frontal dysfunction associated with chronic marijuana (MJ) use, but due to continued neuromaturation, adult studies may not generalize to adolescents. This study characterized prefrontal cortex (PFC) morphometry in chronic MJ-using adolescents following 1 month of monitored abstinence. Data were collected from MJ users (n = 16) and controls (n = 16) aged 16-18. Extensive exclusionary criteria included co-morbid psychiatric and neurologic disorders. Substance use and anatomical measures were collected after 28 days of monitored abstinence. PFC volumes were ascertained from manual tracing by reliable raters on high-resolution magnetic resonance images. After controlling for lifetime alcohol use, gender and intracranial volume, MJ users did not differ from controls in PFC volume. However, marginal group-by-gender interactions were observed (P < 0.09): female MJ users demonstrated comparatively larger PFC volumes while male MJ users had smaller volumes compared with same-gender controls. Further, group status and total PFC volume interacted in predicting executive functioning (P < 0.05). Among MJ users, smaller PFC total volume was associated with better executive functioning while the opposite pattern was seen among the controls. These preliminary results indicate that gender may moderate the relationship between MJ use and PFC morphometry. Given the relationship between larger PFC total volumes and poorer executive functioning among MJ users, female MJ users may be at increased risk for neurocognitive consequences. Future research will measure PFC gray and white matter separately and follow boys and girls over adolescence to examine the influence of MJ use on neurodevelopment.

Whole-body biodistribution and radiation dosimetry of 18F-GE067: a radioligand for in vivo brain amyloid imaging

We have characterized the biodistribution and dosimetry of (18)F-3'-F-6-OH-BTA1 ((18)F-GE067), a newly developed radioligand to visualize and quantify amyloid burden, in healthy elderly human subjects.

METHODS

Six subjects (5 men and 1 woman; age range, 51-74 y) underwent dynamic whole-body PET/CT for 6 h after a bolus injection of (18)F-GE067. Source organs were delineated on PET/CT. Individual organ doses and effective doses were determined.

RESULTS

No adverse events or clinically significant changes were observed. (18)F-GE067 is excreted predominantly through the hepatobiliary system. The gallbladder, upper large intestine, and small intestine are the organs with the highest absorbed dose (average, 287, 173, and 155 microGy/MBq, respectively). The mean effective dose was 33.8 +/- 3.4 microSv/MBq, a dose comparable to that of many other (18)F-labeled radiopharmaceuticals.

CONCLUSION

The estimated effective dose of (18)F-GE067 for PET amyloid imaging was acceptable (class II-b defined by the World Health Organization), and relatively low variability between subjects was observed.