Neydher Berroterán-Infante

The organometallic technetium-99m tricarbonyl core, [^99mTc][Tc(CO)₃(H₂O)₃]⁺, is a versatile precursor for the development of radiotracers for single photon emission computed tomography (SPECT). A drawback of the ^99mTc-tricarbonyl core is its lipophilicity, which can influence the pharmacokinetic properties of the SPECT imaging probe. Addition of polar pharmacological modifiers to ^99mTc-tricarbonyl conjugates holds the promise to counteract this effect and provide tumor-targeting radiopharmaceuticals with improved hydrophilicities, e.g., resulting in a favorable fast renal excretion in vivo. We applied the “Click-to-Chelate” strategy for the assembly of a novel ^99mTc-tricarbonyl labeled conjugate made of the tumor-targeting, modified bombesin binding sequence [Nle¹⁴]BBN(7–14) and the carbohydrate sorbitol as a polar modifier. The ^99mTc-radiopeptide was evaluated in vitro with PC-3 cells and in Fox-1^nu mice bearing PC-3 xenografts including a direct comparison with a reference conjugate lacking the sorbitol moiety. The glycated ^99mTc-tricarbonyl peptide conjugate exhibited an increased hydrophilicity as well as a retained affinity toward the Gastrin releasing peptide receptor and cell internalization properties. However, there was no significant difference in vivo in terms of pharmacokinetic properties. In particular, the rate and route of excretion was unaltered in comparison to the more lipophilic reference compound. This could be attributed to the intrinsic properties of the peptide and/or its metabolites. We report a novel glycated (sorbitol-containing) alkyne substrate for the “Click-to-Chelate” methodology, which is potentially of general applicability for the development of ^99mTc-tricarbonyl based radiotracers displaying an enhanced hydrophilicity.

Schizophrenia is characterized by increased behavioral and neurochemical responses to dopamine-releasing drugs. This prompted the hypothesis of psychosis as a state of “endogenous” sensitization of the dopamine system although the exact basis of dopaminergic disturbances and the possible role of prefrontal cortical regulation have remained uncertain. To show that patients with first-episode psychosis release more dopamine upon amphetamine-stimulation than healthy volunteers, and to reveal for the first time that prospective sensitization induced by repeated amphetamine exposure increases dopamine-release in stimulant-naïve healthy volunteers to levels observed in patients, we collected data on amphetamine-induced dopamine release using the dopamine D_2/3 receptor agonist radioligand [¹¹C]-(+)-PHNO and positron emission tomography. Healthy volunteers (n = 28, 14 female) underwent a baseline and then a post-amphetamine scan before and after a mildly sensitizing regimen of repeated oral amphetamine. Unmedicated patients with first-episode psychosis (n = 21; 6 female) underwent a single pair of baseline and then post-amphetamine scans. Furthermore, T1 weighted magnetic resonance imaging of the prefrontal cortex was performed. Patients with first-episode psychosis showed larger release of dopamine compared to healthy volunteers. After sensitization of healthy volunteers their dopamine release was significantly amplified and no longer different from that seen in patients. Healthy volunteers showed a negative correlation between prefrontal cortical volume and dopamine release. There was no such relationship after sensitization or in patients. Our data in patients with untreated first-episode psychosis confirm the “endogenous sensitization” hypothesis and support the notion of impaired prefrontal control of the dopamine system in schizophrenia.

Background: Several MR-based attenuation correction (AC) approaches were developed to conquer the challenging AC in hybrid PET/MR imaging. These AC methods are commonly evaluated on standardized uptake values or tissue concentration. However, in neurotransmitter system studies absolute quantification is more favorable due to its accuracy. Therefore, our aim was to investigate the accuracy of segmentation- and atlas-based MR AC approaches on serotonin transporter (SERT) distribution volumes and occupancy after a drug challenge. Methods: 18 healthy subjects (7 male) underwent two [¹¹C]DASB PET/MRI measurements in a double-blinded, placebo controlled, cross-over design. After 70 min the selective serotonin reuptake inhibitor (SSRI) citalopram or a placebo was infused. The parameters total and specific volume of distribution (V_T, V_S = BP_P) and occupancy were quantified. All subjects underwent a low-dose CT scan as reference AC method. Besides the standard AC approaches DIXON and UTE, a T1-weighted structural image was recorded to estimate a pseudo-CT based on an MR/CT database (pseudoCT). Another evaluated AC approach superimposed a bone model on AC DIXON. Lastly, an approach optimizing the segmentation of UTE images was analyzed (RESOLUTE). PET emission data were reconstructed with all 6 AC methods. The accuracy of the AC approaches was evaluated on a region of interest-basis for the parameters V_T, BP_P, and occupancy with respect to the results of AC CT. Results: Variations for V_T and BP_P were found with all AC methods with bias ranging from −15 to 17%. The smallest relative errors for all regions were found with AC pseudoCT (<|5%|). Although the bias between BP_P SSRI and BP_P placebo varied markedly with AC DIXON (<|12%|) and AC UTE (<|9%|), a high correlation to AC CT was obtained (r²∼1). The relative difference of the occupancy for all tested AC methods was small for SERT high binding regions (<|4%|). Conclusion: The high correlation might offer a rescaling from the biased parameters V_T and BP_P to the true values. Overall, the pseudoCT approach yielded smallest errors and the best agreement with AC CT. For SERT occupancy, all AC methods showed little bias in high binding regions, indicating that errors may cancel out in longitudinal assessments.

(+)-[¹¹C]PHNO, a dopamine D_2/3 receptor agonistic radiotracer, is applied for investigating the dopaminergic system via positron emission tomography (PET). An improved understanding of neuropsychiatric disorders associated with dysfunctions in the dopamine system and the underlying mechanism is a necessity in order to promote the development of new potential therapeutic drugs. In contrast to other broadly applied ¹¹C-radiopharmaceuticals, the production of this radiotracer requires a challenging four-step radiosynthesis involving harsh reaction conditions and reactants as well as an inert atmosphere. Consequently, the production is prone to errors and troubleshooting after failed radiosyntheses remains time consuming. Hence, we aimed to optimize the radiosynthesis of (+)-[¹¹C]PHNO for achieving better activity yields without loss of product quality. Therefore, we synthesized (+)-[¹¹C]PHNO and omitted all heating and cooling steps leading to higher activity yields. As a result, radiosynthesis fully conducted at room temperature led to a time-reduced production procedure that saves about 5 min, which is an appreciable decay-prevention of around 15% of the activity yield. Additionally, we established a troubleshooting protocol by investigating reaction intermediates, byproducts, and impurities. Indeed, partial runs enabled the assignment of byproducts to their associated error source. Finally, we were able to generate a decision tree facilitating error detection in (+)-[¹¹C]PHNO radiosynthesis.

Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental disorder with a robust genetic influence. The norepinephrine transporter (NET) is of particular interest as it is one of the main targets in treatment of the disorder. As ADHD is a complex and polygenetic condition, the possible regulation by epigenetic processes has received increased attention. We sought to determine possible differences in NET promoter DNA methylation between patients with ADHD and healthy controls. DNA methylation levels in the promoter region of the NET were determined in 23 adult patients with ADHD and 23 healthy controls. A subgroup of 18 patients with ADHD and 18 healthy controls underwent positron emission tomography (PET) with the radioligand (S,S)-[¹⁸F]FMeNER-D₂ to quantify the NET in several brain areas in vivo. Analyses revealed significant differences in NET methylation levels at several cytosine–phosphate–guanine (CpG) sites between groups. A defined segment of the NET promoter (“region 1”) was hypermethylated in patients in comparison with controls. In ADHD patients, a negative correlation between methylation of a CpG site in this region and NET distribution in the thalamus, locus coeruleus, and the raphe nuclei was detected. Furthermore, methylation of several sites in region 1 was negatively associated with the severity of hyperactivity–impulsivity symptoms. Our results point to an epigenetic dysregulation in ADHD, possibly due to a compensatory mechanisms or additional factors involved in transcriptional processing.

Introduction: In-vivo quantification of the serotonin transporter (SERT) guided our understanding of many neuropsychiatric disorders. A recently introduced bolus plus constant infusion protocol has been shown to allow the reliable determination of SERT binding with reduced scan time. In this work, the outcomes of two methods, a bolus injection paradigm on a GE PET camera, and a bolus plus infusion paradigm on a combined Siemens PET/MR camera were compared. Methods: A total of seven healthy subjects underwent paired PET and paired PET/MR scans each with intravenous double-blind application of 7.5 mg citalopram or saline in a randomized cross-over study design. While PET scans were performed according to standard protocols and non-displaceable binding potentials (BP_ND) were calculated using the multi-linear reference tissue model, during PET/MR measurements [¹¹C]DASB was applied as bolus plus constant infusion, and BP_ND was calculated using the steady state method and data acquired at tracer equilibrium. Occupancies were calculated as the relative decrease in BP_ND between saline and citalopram scans. Results: During placebo scans, a mean difference in BP_ND of −0.08 (−11.71%) across all ROIs was found between methods. PET/MR scans resulted in higher BP_ND estimates than PET scans in all ROIs except the midbrain. A mean difference of −0.19 (−109.40%) across all ROIs between methods was observed for citalopram scans. PET/MR scans resulted in higher BP_ND estimates than PET scans in all ROIs. For occupancy, a mean difference of 23.12% (21.91%) was observed across all ROIs. PET/MR scans resulted in lower occupancy compared to PET scans in all ROIs except the temporal cortex. While for placebo, BP_ND of high-binding regions (thalamus and striatum) exhibited moderate reliability (ICC = 0.66), during citalopram scans ICC decreased (0.36–0.46). However, reliability for occupancy remained high (0.57–0.82). Conclusion: Here, we demonstrated the feasibility of reliable and non-invasive SERT quantification using a [¹¹C]DASB bolus plus constant infusion protocol at a hybrid PET/MR scanner, which might facilitate future pharmacological imaging studies. Highest agreement with established methods for quantification of occupancy and SERT BP_ND at baseline was observed in subcortical high-binding regions.