A method for labelling the octapeptide octreotide (D-Phe-Cys-Phe-D-TrpLys-Thr-Cys-Thr(ol)) with the positron emitting Br-76 (T-1/2 = 16 h) is presented. epsilon -Boc-protected octreotide was conjugated to N-succinimidyl 4-[Br-76]bromobenzoate 1 and N-succinimidyl 5-[Br-76]bromo-3-pyridinecarboxylate 3 using microwave heating. The conjugates 4 and 5 were isolated in 50-55% radiochemical yield after the removal of the protecting Boc-group. Compound 3 was synthesised from the corresponding trimethylstannyl-precursor in 25% radiochemical yield. The synthesis of methyl-4-[Br-76] bromobenzimidate 8 in 40% radiochemical yield from the precursor methyl -4-trimethylstannylbenzimidate is also described. Experiments were performed to react 8 with Boc-octreotide but no product was obtained. The binding-properties of Br-76-conjugates 4 and 5 to meningiomas were investigated using frozen section autoradiography. Compound 5 showed better binding properties than 4.
The alpha 7 nicotinic acetylcholine receptor (alpha 7-nAChR) is implicated in a variety of neurodegenerative and neuropsychiatric disorders, such as Alzheimer's disease (AD) and schizophrenia. The progress of these disorders can be studied using positron emission tomography (PET) with radiotracers for alpha 7-nAChR. [F-18]ASEM and [F-18] para-ASEM (also referred to as [F-18]DBT-10) are novel and potent alpha 7-nAChR PET radiotracers which have successfully been used in human subjects and nonhuman primates, though further improvement of them is still a pressing task in the community of neurodegeneration research. In this work, we demonstrate the use of modern in silico techniques to predict the binding modes, binding strengths, and residence times for molecular PET tracers binding to proteins, using ASEM and DBT-10 as a showcase of the predictive and interpretational power of such techniques, in particular free energy perturbation theory. The corresponding compounds were synthesized and further tested by in vitro binding experiment for validation. Encouragingly, our in silico modeling can correctly predict the binding affinities of the ASEM analogues. The structure-activity relationships for the ortho- and para-substitutions are well explained at the atomistic level and provide structure-based guiding for the future development of PET tracers for alpha 7-nAChR. A discussion is presented on the complementary use of in silico rational methods based on atomic and electronic principles for in vitro characterization of PET tracers.
Understanding unbinding kinetics of protein-ligand systems is of great importance for the design of ligands with desired specificity and safety. In recent years, enhanced sampling techniques have emerged as effective tools for studying unbinding kinetics of protein-ligand systems at the atomistic level. However, in many protein-ligand systems, the ligand unbinding processes are strongly coupled to protein conformational changes and the disclosure of the hidden degrees of freedom closely related to the protein conformational changes so that sampling is enhanced over these degrees of freedom remains a great challenge. Here, we show how potential-scaled molecular dynamics (sMD) and infrequent metadynamics (InMetaD) simulation techniques can be combined to successfully reveal the unbinding mechanism of 3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-[F-18]fluorodibenzo[b,d]thiophene 5,5-dioxide ([F-18]ASEM) from a chimera structure of the alpha 7-nicotinic acetylcholine receptor. By using sMD simulations, we disclosed that the "close to "open" conformational change of loop C plays a key role in the ASEM unbinding process. By carrying out InMetaD simulations with this conformational change taken into account as an additional collective variable, we further captured the key states in the unbinding process and clarified the unbinding mechanism of ASEM from the protein. Our work indicates that combining sMD and InMetaD simulation techniques can be an effective approach for revealing the unbinding mechanism of a protein-ligand system where protein conformational changes control the unbinding process.
The amyloid β (Aβ) fibril is a hallmark of Alzheimer's disease (AD) and has therefore served as an important target for early diagnosis of AD. The Pittsburgh Compound-B (PiB) is one of the most famous positron emission tomography (PET) tracers commonly used for in vivo detection of Aβ fibrils. Many theoretical studies have predicted the existence of various core binding sites with different microenvironments for probes binding to the Aβ fibril. However, little attention has been devoted to how the probes actually penetrate into the different core binding sites. In this study, an integrated molecular modeling scheme is used to study the penetration of PiB into the core binding sites of the Aβ1-42 fibril structure recently obtained by cryogenic electron microscopy. We find that there are two core binding sites for PiB with dramatic differences in cavity size and microenvironment properties, and furthermore that the penetration of PiB into site-1 is energetically prohibitive, whereas the penetration into site 2 is much more favorable. Therefore, the binding capacity at site-2 may be larger than that at site-1 despite its lower binding affinity. Our results thus suggest that site-2 may be a major binding site for PiB binding to Aβ fibril and emphasize the importance to adopt a full dynamical picture when studying tracer fibril binding problems in general, something that in turn can be used to guide the development of tracers with higher affinity and selectivity for the Aβ fibril.
Essential hypertension occurs in approximately 25% of the adult population and one cause of hypertension is primary aldosteronism. Targeting the angiotensin II AT1 receptor using PET and an appropriate tracer may offer a diagnostic method for adrenocortical tissue. This report describes the synthesis of the selective AT1 receptor antagonist [carboxyl-11C]eprosartan 10, 4-[2-butyl-5-((E)-2-carboxy-3-thiophen-2-yl-propenyl)-imidazol-1-ylmethyl]-[carboxyl-11C]benzoic acid, and its precursor (E)-3-[2-butyl-3-(4-iodo-benzyl)-3H-imidazol-4-yl]-2-thiophen-2-ylmethyl-acrylic acid 9. 11C-carboxylation of the iodobenzyl moiety was performed using a palladium-mediated reaction with [11C]carbon monoxide in the presence of tetra-n-butyl-ammonium hydroxide in a micro-autoclave using a temperature gradient from 25 to 140°C over 5 min. After purification by semipreparative HPLC, [carboxyl-11C]eprosartan 10 was obtained in 37–54% decay-corrected radiochemical yield (from [11C]carbon monoxide) with a radiochemical purity >95% within 35 min of the end of bombardment (EOB). A 5-µAh bombardment gave 2.04 GBq of 10 (50% rcy from [11C]carbon monoxide) with a specific activity of 160 GBq µmol−1 at 34 min after EOB. Frozen-section autoradiography shows specific binding in kidney, lung and adrenal cortex. In vivo experiments in rats demonstrate a high accumulation in kidney, liver and intestinal wall.
Combinatorial synthesis is extensively used in drug development and lead optimisation. However, this approach has rarely been used for positron emission tomography because of limitations in available technologies. [11C]Carbon monoxide is amenable to combinatorial synthesis in transition-metal-catalysed reactions because it can react with a wide variety of electrophiles and nucleophiles, which opens up the possibilities for combinatorial radiochemistry. Herein, we exemplify the combinatorial approach by 11C-labelling a library of epidermal growth factor receptor inhibitors. The selection of candidates was guided by molecular docking. Epidermal growth factor receptor is overexpressed in a variety of tumours, and it has become an important drug target. The 11C-labelling reactions were performed using four substituted vinyl iodides and three different 4-anilino-6-aminoquinazolines using a palladium-mediated reaction with [11C]carbon monoxide using a single set of reaction conditions. In total, 12 labelled acrylamide derivatives were radiolabelled and obtained in 24–61% decay-corrected radiochemical yield (from [11C]carbon monoxide). Starting from 5.6 GBq [11C]carbon monoxide, 0.85 GBq of formulated N-[4-(3-bromo-phenylamino)-quinazolin-6-yl]-acryl[11C]amide [11C]12da was obtained within 47 min from end of bombardment (specific activity of 60 GBq µmol−1). This strategy is an example of how [11C]carbon monoxide can be utilised in the labelling of libraries of drug candidates and positron emission tomography tracers for in vitro and in vivo testing.
Three C-11-radiolabelled high-affinity nonpeptide AT(2) receptor-selective ligands were synthesized and one of these was evaluated as positron emission tomography (PET) tracer. The labelling reaction was performed via palladium(0)-mediated aminocarbonylation of the aryl iodide substrate using [C-11] carbon monoxide as the labelled precursor. As an example, starting with 10.0 GBq [C-11] carbon monoxide, 1.10 GBq of the product N-butoxycarbonyl-3-[4-(N-benzyl-[C-11] carbamoyl)phenyl]-5-isobutylthiophene-2-sulphonamide [C-11]4d was obtained in 36% decay-corrected radiochemical yield (from [C-11] carbon monoxide), 42 min from end of bombardment with a specific activity of 110 GBq.mu mol(-1). The N-isopropyl-[C-11] carbamoyl-analogue [C-11]4c (radiochemical purity >95%) was studied employing autoradiography, organ distribution, and small animal PET. In vitro autoradiography showed specific binding in the pancreas and kidney. Organ distribution in six rats revealed a high uptake in the liver, intestine, kidney, and adrenals. Small animal PET showed rapid and reversible uptake in the kidneys followed by accumulation in the urinary bladder suggesting fast renal excretion of the tracer. In addition, high accumulation was also seen in the liver. For future studies, more metabolically stable tracers will need to be developed. To the best of our knowledge, this is the first attempt of the use of PET imaging for the detection of expressed, fully functional AT(2) receptors in living subjects.
PURPOSE: Positron emission tomography (PET) with 11C-5-hydroxytryptophan (5-HTP) as tracer is a promising imaging instrument in the management of patients with neuroendocrine tumours (NETs). However, high radioactivity concentrations in the urinary collecting system sometimes produce image reconstruction artefacts that can make detection of small NETs difficult. As a means to decrease urinary excretion of radioactivity and thereby improve image quality, we examined the effect of pretreatment with carbidopa (CD), a peripheral inhibitor of aromatic amino acid decarboxylase (AADC), which converts 5-HTP to serotonin (5-hydroxytryptamine, 5-HT). METHODS: Six patients with midgut carcinoid metastases were examined with 11C-5-HTP PET before and 1 h after oral administration of 100 or 200 mg of CD. RESULTS: There was a fourfold significant reduction of tracer uptake in the urinary collecting system after CD administration (p=0.0277, n=6), with a mean standard uptake value (SUV) of 155+/-195 before CD and 39+/-14 after CD. In tumour lesions there was a significant increase in SUV after CD administration (p<0. 0001, n=18), with a mean SUV of 11+/-3 before CD and 14+/-3 after CD. There was no difference between the doses (100 and 200 mg) of CD in this respect. In all patients, image interpretation and tumour detection were markedly improved after CD administration. CONCLUSION: We conclude that CD premedication improves 11C-5-HTP PET image quality and facilitates detection of NET lesions. Because of the similarity of metabolic pathways, this method could probably be applied to improve PET imaging using other tracers like 18F-DOPA and 11C-DOPA.