Synthesis of 15-(4-[11C]methylphenyl)pentadecanoic acid (MePPA) via Stille cross-coupling reaction

For experimental studies by animal PET [¹¹C]-labeled 15-(4-methylphenyl)pentadecanoic acid (MePPA) is an attractive alternative to the radioiodinated 15-(4-iodophenyl)pentadecanoic acid (IPPA) which has widely been used for imaging of fatty acid metabolism. The important physiological aspect is that the iodine atom and the methyl substituent have similar steric and lipophilic properties. For preparation of [¹¹C]MePPA, Stille cross-coupling reaction was applied since the same tin precursor as for the radiosynthesis of IPPA and readily available [¹¹C]CH₃I can be used. Unsaturated tris(dibenzylideneacetone)dipalladium(0)/tri(o-tolyl)phosphine [Pd₂(dba)₃/P(o-tolyl)₃] was taken as the catalytic system. The reaction conditions were optimized with respect to temperature, time, solvent and amount of precursor. The best radiochemical yields of 73 ± 2.8% (decay corr.) were obtained using 0.525 mg tin precursor in DMF at 80 °C already after a reaction time of 10 min. The labeled methyl ester was hydrolyzed by 1 M NaOH/EtOH at 80 °C within 3 min to give [¹¹C]IPPA in a RCY of 62 ± 3.0%. The radiochemical purity of the product assured by HPLC was >99% and the overall preparation time including HPLC purification and formulation was 40 min.     

Radiosynthesis of 10-(2-[18F]fluoroethoxy)-20(S)-camptothecin as a potential positron emission tomography tracer for the imaging of topoisomerase I in cancers

The preparation of 10-(2-[¹⁸F]fluoroethoxy)-20(S)-camptothecin, a potential positron emission tomography tracer for the imaging of topoisomerase I in cancers, is described. 10-(2-[¹⁸F]Fluoroethoxy)-20(S)-camptothecin was synthesized by the [¹⁸F]fluoroalkylation of the corresponding hydroxy precursor molecule with 2-[¹⁸F]fluoroethyl bromide ([¹⁸F]FEtBr) in dimethylsulfoxide (DMSO) at 55 °C for 20 min; this was followed by purification using high performance liquid chromatography (HPLC) with a total preparation time of 60 min. The overall radiochemical yield was approximately 5.4–12 % (uncorrected), and the radiochemical purity was above 96 %.     

A one-step fully automated radio-synthesis of a dopamine transporter PET imaging agent 18F-FECNT and its in vivo evaluations

2β-Carbomethoxy-3β-(4-chlorophenyl)-8-(2-[¹⁸F]fluoroethyl)nortropane ([¹⁸F]-FECNT) is a potential dopamine transporter PET imaging agent. However, its current radio-synthesis is tedious and time consuming. In this article, we reported a fully automatic method for the synthesis of [¹⁸F] FECNT through only one step, using a TRACERlab FX_N module, with decay corrected radiochemical yield of 25 ± 5 % (n = 5). The total synthesis time was 50–55 min. The synthesized [¹⁸F]FECNT was evaluated in vivo in Parkinson’s disease model rats.     

In vivo biodistribution of pancreatic-derived factor using 18F-labeled PANDER PET imaging

The purpose of this study was to investigate in vivo biodistribution and potential target tissues of pancreatic-derived factor (PANDER, FAM3B) using ¹⁸F-labeled PANDER positron emission tomography (PET) imaging. ¹⁸F-Labeled PANDER ([¹⁸F]FB-PANDER) was prepared by reaction of PANDER and N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB). The uncorrected radiochemical yield of [¹⁸F]FB-PANDER was 15.2 ± 3.4 % (n = 4) based on [¹⁸F]SFB within the total synthesis time of 30 min. In vivo biodistribution of [¹⁸F]FB-PANDER in nomal mice and PET imaging demonstrated high uptake of the radiotracer in urinary bladder, kidneys and gall bladder, and fast clearance from kidneys and gall bladder. Also, moderate uptake in blood, liver, pancreas, small intestine and bone, low uptake in brain and muscle, and almost no uptake in S180 fibrosarcoma tissue were observed. The results indicated that the major excretion route of PANDER was through renal-urinary bladder and biliary system, and no obvious binding targets of PANDER in the main organs and S180 fibrosarcoma tissue were found.     

Radiosynthesis and biodistribution studies of [62Zn/62Cu]–plerixafor complex as a novel in vivo PET generator for chemokine receptor imaging

In order to develop a possible C-X-C chemokine receptor type 4 (CXCR4) imaging agent for oncological scintigraphy, [⁶²Zn]labeled 1,1′-[1,4-phenylenebis(methylene)]bis-1,4,8,11-tetraazacyclotetradecane ([⁶²Zn]-AMD3100) was prepared using in-house made [⁶²Zn]ZnCl₂ and AMD-3100 for 1 h at 50 °C (radiochemical purity: >97 % ITLC, >96 % HPLC, specific activity: 20–22 GBq/mmol) in acetate buffer. The complex showed highly hydrophilic properties (log P = ?1.114). Stability of the complex was checked in presence of human serum (37 °C) and in final formulation for 1 day. The biodistribution of the labeled compound in vital organs of wild-type Sprague–Dawdley rats were determined and compared with that of free Zn²⁺ cation up to 6 h. Co-incidence imaging of the complex was consistent with the distribution data up to 3 h. The complex can be a possible in vivo generator compound for PET imaging in CXCR4 positive tumors.     

Radiosynthesis of New Carbon-11-labeled Nimesulide Analogs as Potential PET SAER Tracers for Imaging of Aromatase Expression in Breast Cancer

Carbon-11-labeled nimesulide analogs, N-[¹¹C]methyl-N-(2-benzyloxy-4-nitrophenyl)methanesulfonamide ([¹¹C]4a), N-[¹¹C]methyl-N-[2-(4′-methylbenzyloxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]4b), N-[¹¹C]methyl-N-[2-(4′-fluorobenzyloxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]4c), N-[¹¹C]methyl-N-[2-(4′-nitrobenzyloxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]8a), N-[¹¹C]methyl-N-[2-(β-naphthylmethoxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]8b), and N-[¹¹C]methyl-N-[2-(2′-phenylbenzyloxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]8c), have been synthesized as new potential positron emission tomography (PET) selective aromatase expression regulator (SAER) radiotracers for imaging of aromatase expression in breast cancer. The target tracers were prepared by N-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic conditions (NaH) and isolated by reversed-phase high-pressure liquid chromatography (HPLC) method in 30–50% radiochemical yields decay corrected to end of bombardment (EOB) with 25–30 min overall synthesis time and 111–148 GBq/μmol specific activity at end of synthesis (EOS).     

Facile radiosynthesis of <Superscript>18</Superscript>F-labeled β-glutamic acid as a new PET tracer for imaging lung cancer

In this paper, N-(2-[¹⁸F]fluoropropionyl)-β-glutamic acid 8 ([¹⁸F]FP-β-Glu), a new N-substituted ¹⁸F-labeled amino acid tracer, was synthesized from the precursor 4 (diethyl 3-(2-bromopropanamido)pentanedioate) via a two-step reaction on the modified FDG synthesizer. The radiochemical yield was 20 ± 5% (n = 10, decay-corrected) from [¹⁸F]fluoride within 40 min, the radiochemical purity was 98%. Moreover, microPET studies showed that [¹⁸F]FP-β-Glu 8 exhibited rapid tumor uptake and good tumor-to-lung ratio in SPC-A-1 tumor-bearing mice. A high accumulation of radioactivity was found in the kidneys and bladder, which suggested that the tracer was mainly eliminated through the urinary system.     

Full automatic synthesis of [<Superscript>18</Superscript>F]THK-5351 for tau protein PET imaging in Alzheimer’s disease patients: 1 year experience

[¹⁸F]THK-5351, a new candidate for tau protein imaging, is based on an aryl quinoline structure. We report the full automatic synthesis using disposable cassettes under pH controlled [¹⁸F]fluorination. After the trapping of 88.5 ± 21.9 GBq of [¹⁸F]fluoride, it was eluted with potassium methansulfonate (KOMs) (pH 7.8)/K₂₂₂. After drying, 3 mg of the precursor was added to 1 mL DMSO and subjected to [¹⁸F]fluorination at 110 °C for 10 min. After hydrolysis, the final product was purified by HPLC. The overall radiochemical yield was 31.9 ± 11.1% (n = 22), satisfying all quality control criteria. It was stable for up to 6 h with high radiochemical purity as 99.8 ± 0.5%.     

Preparation, nano purification, quality control and labeling optimization of [64Cu]-5,10,15,20-tetrakis (penta fluoro phenyl) porphyrin complex as a possible imaging agent

Cu-64 was produced via the ⁶⁸Zn (p,αn)⁶⁴Cu nuclear reaction (≈200 mCi, >95 % chemical yield at 180 μA for 1.1 h irradiation, (radionuclidic purity >96 %, copper-67 as impurity) followed by purification with amino functionalized nano magnetic oxide, Fe₃O₄ aiming to remove trace amount of heavy metal ions from aqueous media due to achieve ultra pure [⁶⁴Cu] CuCl₂ for labeling step. [⁶⁴Cu] labeled 5,10,15,20-tetrakis(penta fluoro phenyl) porphyrin ([⁶⁴Cu]-TFPP) was prepared using freshly prepared [⁶⁴Cu] CuCl₂ (Cu-64; T _1/2 = 12.7 h) and 5,10,15,20-tetrakis(penta fluoro phenyl)porphyrin (H₂TFPP) for 60 min at 100 °C under reflux condition (radiochemical purity: >97 % ITLC, >98 % HPLC, specific activity: 14–16 GBq/mmol). Stability of the complex was checked in final formulation and human serum for 24 h. The partition coefficient was calculated for the compound (log P = 0.73). The biodistribution of the labeled compound in vital organs of wild-type rats was studied using scarification studies and PET imaging up in 2 and 4 h after injection. A detailed comparative pharmacokinetic study performed for ⁶⁴Cu cation and [⁶⁴Cu]-TFPP. The complex is mostly washed out from the circulation through kidneys and liver and can be an interesting tumor imaging/targeting agent due to high specific uptake and rapid excretion through the urinary tract.     

Radioiodination of 4-benzyl-1-(3-iodobenzylsulfonyl)piperidine, 4-(3-iodobenzyl)-1-(benzylsulfonyl)piperazine and their derivatives via isotopic and non-isotopic exchange reactions

A mild and simple technique for preparing of 4-benzyl-1-(3-[¹²⁵I]iodobenzylsulfonyl)piperidine, 4-(3-[¹²⁵I]iodobenzyl)-1-(benzylsulfonyl)piperazine and their derivatives, as sigma-1 receptor ligands, with relatively high radiochemical yields via nucleophilic substitution reaction by means of isotopic and non-isotopic exchange reactions is described. Some factors affecting the radiochemical yield were commonly studied in presence of acidic medium at elevated temperature. Unfortunately, the radiochemical yields were weak. Some attempts were carried out in presence of polar aprotic solvents to enhance the radiochemical yield. N,N-Dimethylformamide was proved highly efficient for preparing of radioiodinated 4-benzyl-1-(3-iodobenzylsulfonyl)piperidine (4-B-[¹²⁵I]-IBSP, 70 ± 5.7 %) and 4-(3-iodobenzyl)-1-(benzylsulfonyl)piperazine (4-[¹²⁵I]-IBBSPz, 72 ± 6.0 %) at moderate temperature (100–105 °C) within 8 h. The specific activities of 4-B-[¹²⁵I]-IBSP and 4-[¹²⁵I]-IBBSPz (6,534.2 and 5,927.4 MBq/mmol) were obtained respectively.     

A facile direct nucleophilic synthesis of O-(2-[18F]fluoroethyl)-l-tyrosine ([18F]FET) without HPLC purification

Due to favourable in vivo characteristics, its high specificity and the longer half-life of ¹⁸F (109.8 min) allowing for remote-site delivery, O-(2-[¹⁸F]fluoroethyl)-l-tyrosine ([¹⁸F]FET) has gained increased importance for molecular imaging of cerebral tumors. Consequently, the development of simple and efficient production strategies for [¹⁸F]FET could be an important step to further improve the cost-effective availability of [¹⁸F]FET in the clinical environment. In the present study [¹⁸F]FET was synthesized via direct nucleophilic synthesis using an earlier developed chiral precursor, the Ni^II complex of an alkylated (S)-tyrosine Schiff base, Ni-(S)-BPB-(S)-Tyr-OCH₂CH₂OTs. The purification method has been developed via solid phase extraction thereby omitting cumbersome HPLC purification. The suggested SPE purification using combination of reverse phase and strong cation exchange cartridges provided [¹⁸F]FET in high chemical, radiochemical and enantiomeric purity and 35 % radiochemical yield (decay-corrected, 45 min synthesis time). The method was successfully automated using a commercially available synthesis module, Scintomics Hotbox^one. Based on the current results, the proposed production route appears to be well suited for transfer into an automated cassette-type radiosynthesizers without using HPLC.     

An Improved Synthesis of N-(4-[18F]Fluorobenzoyl)-Interleukin-2 for the Preclinical PET Imaging of Tumour-Infiltrating T-cells in CT26 and MC38 Colon Cancer Models

Positron emission tomography (PET) imaging of activated T-cells with N-(4-[¹⁸F]fluorobenzoyl)-interleukin-2 ([¹⁸F]FB-IL-2) may be a promising tool for patient management to aid in the assessment of clinical responses to immune therapeutics. Unfortunately, existing radiosynthetic methods are very low yielding due to complex and time-consuming chemical processes. Herein, we report an improved method for the synthesis of [¹⁸F]FB-IL-2, which reduces synthesis time and improves radiochemical yield. With this optimized approach, [¹⁸F]FB-IL-2 was prepared with a non-decay-corrected radiochemical yield of 3.8 ± 0.7% from [¹⁸F]fluoride, 3.8 times higher than previously reported methods. In vitro experiments showed that the radiotracer was stable with good radiochemical purity (>95%), confirmed its identity and showed preferential binding to activated mouse peripheral blood mononuclear cells. Dynamic PET imaging and ex vivo biodistribution studies in naïve Balb/c mice showed organ distribution and kinetics comparable to earlier published data on [¹⁸F]FB-IL-2. Significant improvements in the radiochemical manufacture of [¹⁸F]FB-IL-2 facilitates access to this promising PET imaging radiopharmaceutical, which may, in turn, provide useful insights into different tumour phenotypes and a greater understanding of the cellular nature and differential immune microenvironments that are critical to understand and develop new treatments for cancers.     

Molecular and Functional Imaging Studies of Psychedelic Drug Action in Animals and Humans

Hallucinogens are a loosely defined group of compounds including LSD, N,N-dimethyltryptamines, mescaline, psilocybin/psilocin, and 2,5-dimethoxy-4-methamphetamine (DOM), which can evoke intense visual and emotional experiences. We are witnessing a renaissance of research interest in hallucinogens, driven by increasing awareness of their psychotherapeutic potential. As such, we now present a narrative review of the literature on hallucinogen binding in vitro and ex vivo, and the various molecular imaging studies with positron emission tomography (PET) or single photon emission computer tomography (SPECT). In general, molecular imaging can depict the uptake and binding distribution of labelled hallucinogenic compounds or their congeners in the brain, as was shown in an early PET study with N¹-([¹¹C]-methyl)-2-bromo-LSD ([¹¹C]-MBL); displacement with the non-radioactive competitor ketanserin confirmed that the majority of [¹¹C]-MBL specific binding was to serotonin 5-HT_2A receptors. However, interactions at serotonin 5HT_1A and other classes of receptors and pleotropic effects on second messenger pathways may contribute to the particular experiential phenomenologies of LSD and other hallucinogenic compounds. Other salient aspects of hallucinogen action include permeability to the blood–brain barrier, the rates of metabolism and elimination, and the formation of active metabolites. Despite the maturation of radiochemistry and molecular imaging in recent years, there has been only a handful of PET or SPECT studies of radiolabeled hallucinogens, most recently using the 5-HT_2A/2C agonist N-(2[¹¹CH₃O]-methoxybenzyl)-2,5-dimethoxy- 4-bromophenethylamine ([¹¹C]Cimbi-36). In addition to PET studies of target engagement at neuroreceptors and transporters, there is a small number of studies on the effects of hallucinogenic compounds on cerebral perfusion ([¹⁵O]-water) or metabolism ([¹⁸F]-fluorodeoxyglucose/FDG). There remains considerable scope for basic imaging research on the sites of interaction of hallucinogens and their cerebrometabolic effects; we expect that hybrid imaging with PET in conjunction with functional magnetic resonance imaging (fMRI) should provide especially useful for the next phase of this research.     

Preparation and evaluation of a new neurotensin analog labeled with 99mTc for targeted imaging of neurotensin receptor positive tumors

Neurotensin receptors are overexpressed in several human tumors and can be targets for tumors diagnosis and therapy. In this study, a new neurotensin analogue was labeled with ^99mTc via HYNIC and tricine/EDDA as coligands and investigated further. [HYNIC⁰, Gly⁷, Lys⁹, d-Tyr¹¹]-Neurotensin (7–13) was synthesized using a standard Fmoc strategy. Labeling with ^99mTc was performed at 100 °C for 10 min and radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was checked in the presence of humane serum at 37 °C up to 24 h. The receptor bound internalization and externalization rates were studied in neurotensin receptor expressing HT-29 cells. Biodistribution of radiopeptide was studied in nude mice bearing HT-29 tumor. Labeling yield of 98.6 ± 0.54 % (n = 3) was obtained corresponding to a specific activity of 81 MBq/nmol. Peptide conjugate showed good stability in the presence of human serum. The radioligand showed specific internalization into HT-29 cells (12.43 ± 0.52 % at 4 h). In biodistribution studies, a receptor-specific uptake was observed in neurotensin receptor positive organs so that after 1 h the uptakes in mouse intestine and tumor were 0.87 ± 0.16 and 0.63 ± 0.12 % ID/g respectively.     

Stability of L-[S-methyl-11C]methionine solutions

For clinical PET studies L-[S-methyl-¹¹C]methionine ([¹¹C]MET) solutions have been prepared in both high doses and specific activities (up to 48.1 GBq and 370 GBq/mmol, respectively) with high radiochemical purity (>99%). The stability of these preparations was investigated by HPLC to ensure the radiopharmaceutical efficacy during the usable shelf life. Under our routine conditions the observed radiochemical purity loss never exceeded 3.5% one hour after EOS. The decomposition rate was affected by total activity and chemical composition of the solutions.     

An efficient synthesis of ([F]fluoropropyl)quinoline-5,8-diones by rapid radiofluorination-oxidative demethylation

Since many molecules bearing quinoline-5,8-dione or fused 1,4-quinone moieties possess a wide spectrum of biological activities, efficient methods for incorporation of fluorine-18 (F-18) into quinoline-5,8-diones have received considerable attention in positron emission tomography (PET) molecular imaging studies. In this paper, we describe an efficient synthetic route for the regioselective preparation of fluoropropyl-substituted quinoline-5,8-diones on the C3, C4, and C6 positions by tert-alcohol media fluorination, followed by oxidative demethylation of the corresponding dimethoxy compound using N-bromosuccinimide (NBS) in the presence of catalytic amounts of sulfuric acid. Moreover, F-18 labeled [¹⁸F]fluoropropylquinoline-5,8-diones [¹⁸F]21-23 were prepared from the corresponding mesylate precursors by a method of rapid and efficient one-pot, two-step reactions: radiofluorination using TBA [¹⁸F]F generated under no-carrier-added (NCA) conditions; oxidative demethylation, resulting in a 45% radiochemical yield of [¹⁸F]21-23 (decay-corrected) with a total synthesis time (including HPLC purification) of 75 min and high radiochemical purity (>99%), as well as high specific activity (∼230 GBq/μmol).     

Automated synthesis of [18F]FMISO on IBA Synthera®

In cancer cells hypoxia can cause resistance to both radio- and chemo-therapy. Being able to quantify, the degree of hypoxia in the cells is a useful tool in therapy planning. The positron emitting 1-[¹⁸F]fluoro-3-(2-nitro-1H-imidazol-1-yl)propan-2-ol ([¹⁸F]FMISO) is the most extensively used tracer for imaging hypoxia. Automated synthesis of [¹⁸F]FMISO was set up on IBA Synthera^®. The precursor 1-(2′-nitro-1′-imidazolyl)-2-O-tetrahydropyranyl-3-O-p-toluenesulfonyl propanediol (NITTP) was heated at 100 °C for 10 min with [K/K 2.2.2.]⁺[¹⁸F]^? and thereafter hydrolyzed with 0.1 M hydrochloric acid at 90 °C for 2 min. Purification was performed on solid-phase extraction (SPE) cartridges. [¹⁸F]FMISO was obtained in 50 ± 3 % (n = 6) radiochemical yield (decay-corrected) in 35 min synthesis time with radiochemical purity of ≥98 %. The use of disposable Integrated Fluid Processors (IFP?:s) and cartridge purification simplifies the handling and shortens the synthesis time. This is a no frills setup based on all commercially available materials and the synthesis is performed with minor changes from the FDG time-list.     

Synthesis of novel PEG-modified nitroimidazole derivatives via “hot-click” reaction and their biological evaluation as potential PET imaging agent for tumors

Ten novel PEG-modified nitroimidazole derivatives labeled with ¹⁸F were synthesized via “hot-click” reaction and evaluated as PET tumor tracers. The radiolabeling method was convenient and efficient, all compounds displayed high radiochemical purity (>95%), high yield (30–50%) and good stability in saline and human serum. The biodistribution studies in EMT-6 tumor-bearing mice demonstrated their tumor uptake values at 60 min postinjection were 1.46–2.99%ID/g, which were lower than [¹⁸F]FMISO (5.43%ID/g), their tumor-to-liver ratios were 0.73–1.07, higher than [¹⁸F]FMISO (0.69). In vitro MCF-7 cell uptake studies showed their uptake values have no significant difference between hypoxic and aerobic cells.     

99mTc-labeling of a dithiocarbamate-DWAY fragment using [99mTcN]2+ core for the preparation of potential 5HT1A receptor imaging agents

1-(2-methoxy phenyl) piperazine fragment of WAY100635 or its phenolic analogue, derived from DWAY is used to design the desired structure of 5HT_1A receptor imaging agents. In this study a DWAY analogue was labeled with ^99mTc-nitrido ([^99mTcN]²⁺) core via dithiocarbamate. 2-(piperazin-1-yl) phenol dithiocarbamate was synthesized by the reaction of 2-(piperazin-1-yl) phenol with an equivalent amount of carbon disulfide in KOH solution then radiolabeled with ^99mTc-nitrido core. The final complex was characterized by HPLC and its radiochemical purity was more than 90 %. In vitro stability studies have shown the complex was stable at least 4 h after labeling at room temperature. The n-octanol/water partition coefficient experiment demonstrated log p = 1.34 for ^99mTcN–OHPP–DTC. Biodistribution results showed that radio tracer had moderate brain uptake (0.39 ± 0.03 %ID/g at 15 min and 0.29 ± 0.02 %ID/g at 120 min) and good retention, suggesting that this complex may lead to a further development of a radiotracer with specific binding to 5-HT_1A receptor.     

N-[11C]methyl-3,4-methylenedioxyamphetamine (Ecstasy) and 2-methyl-N-[11C]methyl-4,5-methylenedioxyamphetamine: Synthesis and biodistribution studies

In order to evaluate the neurobiological mechanism causing the psychogenic effects of methylenedioxy-derivatives of amphetamine, the carbon-11 labeled analogues of 3,4-methylenedioxymethamphetamine (MDMA),2 and 2,N-dimethyl-4,5-methylenedioxyamphetamine (MADAM-6)4 were prepared for application in in-vivo PET studies by methylation of 3,4-methylenedioxyamphetamine (MDA)1 and 2-methyl-4,5-methylenedioxyamphetamine3 with [¹¹C]CH₃I. The radiochemical yield was determined in dependence on time, temperature and amount of precursor. The best conditions for a fast labeling reaction with carbon-11 on a preparative scale were found to be a reaction time of 10 min using 1 mg of the corresponding dimethyl-precursors1 or3, thus obtaining radiochemical yields of 60% (based on produced [¹¹C]CH₃I). Biodistribution studies were performed in rats, a high brain to blood ratio of 7.5 was observed for [¹¹C]MDMA in contrast to a ratio of 3.7 for [¹¹C]MADAM-6.