Search

Biocatalysis is emerging as a new approach to radiolabel biologically active molecules with short half-lived positron emitters for their use in positron emission tomography. Despite the golden era of biocatalysis in radiochemistry occurred in the 70s and 80s, advances in enzyme engineering during the last decade have significantly enhanced the toolbox of enzymes available for chemical reactions, which may find application in the context of radiochemistry. In the present review, we intend to give an overview of the biocatalytic approaches that have been reported during the last 4 decades for the synthesis of PET radiotracers using nitrogen-13, carbon-11, and fluorine-18., (Copyright © 2017 John Wiley & Sons, Ltd.)

Introduction: The use of ¹¹C-labeled radiotracers in routine positron emission tomography studies is dependent on the production capability of radiochemistry laboratories. Therefore, considerable efforts are being focused on the development of fast, efficient, and robust methods for the preparation of such radiotracers., Methods: The fully automated syntheses of [¹¹C]raclopride and [¹¹C]Pittsburgh compound-B (PIB) starting from cyclotron-produced [¹¹C]CH4 are reported. [¹¹C]methyl iodide and [¹¹C]methyl triflate were produced in the TRACERlab FXC Pro synthesis box. Methylation reactions and the final formulation were performed using the AutoLoop (captive solvent method) and the ReFORM-plus systems, respectively., Results: [¹¹C]raclopride (n=30) and [¹¹C]PIB (n=24) were synthesized by O-[¹¹C]-methylation and N-[¹¹C]-methylation of (S)-O-desmethylraclopride and 6-OH-BTA-0 using [¹¹C]methyl iodide and [¹¹C]methyl triflate, respectively. Good radiochemical yields (51.3 ± 11.2 and 32.9 ± 6.6%, referred to as [¹¹C]methyl iodide, decay corrected) and specific activities (109 ± 20 and 143 ± 26 GBq/µmol) were obtained for [¹¹C]raclopride and [¹¹C]PIB, respectively, in a fully automated process. Radiochemical purity was higher than 99% in all cases., Conclusion: The fast, robust and fully automated processes reported here allow [¹¹C]raclopride and [¹¹C]PIB synthesis with good radiochemical yields and high specific activities. Consecutive productions can be performed with minimal intervention on the synthesis modules and minimal exposure to radiation.

AuIII complexes with N‐heterocyclic carbene (NHC) ligands have shown remarkable potential as anticancer agents, yet their fate in vivo has not been thoroughly examined and understood. Reported herein is the synthesis of new AuIII‐NHC complexes by direct oxidation with radioactive [124I]I2 as a valuable strategy to monitor the in vivo biodistribution of this class of compounds using positron emission tomography (PET). While in vitro analyses provide direct evidence for the importance of AuIII‐to‐AuI reduction to achieve full anticancer activity, in vivo studies reveal that a fraction of the AuIII‐NHC prodrug is not immediately reduced after administration but able to reach the major organs before metabolic activation. [ABSTRACT FROM AUTHOR]

Nitrogen-13 can be efficiently produced in biomedical cyclotrons in different chemical forms, and its stable isotopes are present in the majority of biologically active molecules. Hence, it may constitute a convenient alternative to Fluorine-18 and Carbon-11 for the preparation of positron-emitter-labelled radiotracers; however, its short half-life demands for the development of simple, fast, and efficient synthetic processes. Herein, we report the one-pot, enzymatic and non-carrier-added synthesis of the 13N-labelled amino acids l-[13N]alanine, [13N]glycine, and l-[13N]serine by using l-alanine dehydrogenase from Bacillus subtilis, an enzyme that catalyses the reductive amination of α-keto acids by using nicotinamide adenine dinucleotide (NADH) as the redox cofactor and ammonia as the amine source. The integration of both l-alanine dehydrogenase and formate dehydrogenase from Candida boidinii in the same reaction vessel to facilitate the in situ regeneration of NADH during the radiochemical synthesis of the amino acids allowed a 50-fold decrease in the concentration of the cofactor without compromising reaction yields. After optimization of the experimental conditions, radiochemical yields were sufficient to carry out in vivo imaging studies in small rodents. [ABSTRACT FROM AUTHOR]

Positron emission tomography is an ultra-sensitive, in vivo molecular imaging technique that allows the determination of the spatiotemporal distribution of a positron emitter labeled radiotracer after administration into living organisms. Among all existing positron emitters, 18F has been by far the most widely used both in clinical diagnosis and in preclinical investigation, while the use of 11C significantly increased after the 1980s because of the widespread installation of biomedical cyclotrons. The use of other shorter-lived positron emitters such as 13N ( T1/2 = 9.97 min) has been historically more restricted. Paradoxically, its stable isotope (14N) is present in many biological active molecules; consequently, the development of strategies for the efficient incorporation of 13N into radiotracers would represent an interesting alternative to 11C- and 18F-labeling. In the current paper, the developments related to 13N chemistry are reviewed, including different production routes of primary precursors and their applications to the preparation of more complex 13N-labeled molecules. The current situation and future perspectives are also briefly discussed. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

l -Alanine dehydrogenase from Bacillus subtillis is a NADH-dependent enzyme that catalyzes the reversible reductive amination of pyruvate using ammonia as amine source. This enzyme has been intensively exploited in biocatalysis but its immobilized form is rarely used. In this work, we have immobilized this enzyme on agarose microbeads activated with glyoxyl groups (aliphatic aldehydes). 85% of the offered enzyme was immobilized on these microbeads and the enzyme recovered 45% of its initial reduction amination activity upon the immobilization. The resulting immobilized preparation was 13 times more thermostable than the soluble enzyme because the immobilization attenuated the negative conformational changes induced by the temperature. The optimally immobilized enzyme was also stabilized against acidic pH. Finally, we have applied this heterogeneous biocatalyst to the synthesis of L-[ 13 N]alanine using pyruvate and [ 13 N]NH 4 OH obtaining a radiochemical yield of >95% in 20 min. This immobilized enzyme was re-used for up to 5 cycles keeping the maximum yield. Herein, we have fabricated a highly stable and active heterogeneous biocatalyst for reductive aminations of α-ketoacids that has an enormous potential in biocatalysis. [ABSTRACT FROM AUTHOR]

Abstract: In the present paper, a fast and automated method for the synthesis of 13N-labelled azo compounds is reported for the first time. The labelling strategy is based on trapping [13N]NO2 − in an anion exchange resin. The reaction with primary aromatic amines in acidic media led to the formation of the corresponding diazonium salts, which were further reacted with aromatic amines and alcohols to yield the corresponding 13N-labelled azo derivatives with good radiochemical conversion (40.0–58.3%). Good radiochemical yields (20.4–47.2%, decay corrected) and radiochemical purities (>99.9%) were obtained after purification by HPLC. Such methodology can be easily applied to the preparation of a wide range of 13N-labelled azo derivatives by adequate selection of the non-radioactive precursors. [ABSTRACT FROM AUTHOR]

Abstract: In the present paper, the synthesis of 11C-labeled Kendine 91 (a HDAC inhibitor which has shown in vitro and in vivo activity in HCT 116 and MOLT 4 human cancer cell lines) is described for the first time. The radiosynthesis has been approached by reaction of the non-radioactive precursor 6-((3-(4-hydroxyphenyl)-5-phenyl-1H-pyrrole-2-carboxamide))hexanehydroxamic acid with [11C]CH3I in basic media. Despite the presence of more than one reactive site in the chemical structure of the precursor, acceptable radiochemical yield (8.2±2.1%, decay corrected to the end of bombardment), specific activity (28.2±9.4GBq/μmol) and radiochemical purity values (>95%) were obtained in reasonably short preparation times (∼40min). Despite the moderate radiochemical yield, final radioactivity and radioactivity concentration values (1.8±0.3GBq and 180MBq/ml, respectively) should be sufficient for putative in vivo studies in animals. [Copyright &y& Elsevier]

Abstract: In the present Letter, a fast, automated, and reproducible method for the synthesis of S-[13N]nitrosothiols is reported. The labeling strategy is based on trapping in an anion exchange resin. The reaction with thiols in acidic media led to the formation of the desired nitrosothiols in short reaction times (60s) with excellent radiochemical conversions (from 48.7% to 74.5%). Final radiotracers were purified by HPLC. Good radiochemical yields (from 33.8% to 60.6%, decay corrected) and radiochemical purities (>99%) were obtained in all cases. Stability of the labeled compounds was checked. [Copyright &y& Elsevier]