4) Synthesize and isolated the 11C-radiotracer.
5) Measure an exact volume of a solution containing the 11C-radiotracer and amount of radioactivity via dose calibrator and decay correct the activity at end of synthesis (EOS) (or at end of delivery - EOB). Calculate the activity in GBq in the HPLC injection volume (e.g. in 20 µL activityinj).
6) Inject a sample (20 µL) of a solution containing the 11C-radiotracer into the HPLC using the same method used to derive the calibration curve. Obtain the area from the UV-chromatograms.
7) Using the appropriate calibration curve, this area is converted to the number of moles of compound injected (amountinj, in µmol).
8) Divide the activityinj by amountinj to obtain the Am in GBq/µmol (and stated that it is corrected at EOS or EOB).
9) To obtain the As in GBq/µg, divide Am by the molecular weight of the compound.
Note: The calibration curve has to be repeated at anytime if any of the HPLC parameters are changed due to maintenance (e.g. change of UV lamp), or a new HPLC column is used, or after a determined time period (e.g. this is stated in your standard operating procedure).
Closing remarks and critical aspects
The time-dependency of the molar activity makes it a necessity to provide a time-cut-off for the applicability of a radiopharmaceutical to avoid side-effects for the last patient. Besides, also the maximal injectable volume in terms of potential unwanted pharmacodynamics or side effects should be stated in the release criteria.
In most cases, only the Am is stated, as the apparent Am is very difficult to determine as precise information on impurities affecting the target region must be available. Nevertheless, the apparent Am is the more critical one in terms of toxicology, interdependencies and signal-to-noise ratio. A starting point is to include the residual amount of precursor in the calculation of the maximal applicable dose and the time-cut-off. Interestingly, this is already standard procedure for peptides, proteins and antibodies as here the ratio of the activity per unlabelled compound is determined, as the separation of precursor and radiopharmaceutical is not possible. Therefore, the borders between the definitions of Am and apparent Am are fluid for these cases.
References
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Authors:
Salvatore has worked at King’s College London (KCL) since Apr 2014 under the supervision of Prof Antony Gee, originally as MRC postdoctoral fellow (Apr 2014 - Dec 2017) and from December 2017 as translational radiochemist in the Wellcome EPSRC Centre Medical Engineering KCL. His active research focuses on developing carbon-11 and fluorine-18 PET radiotracers. He received his Ph.D. in Physics and Chemistry of Biological Systems (2011) from the International School of Advanced Studies (SISSA/ISAS) Trieste. In November 2013, he completed a postdoctoral research fellowship at the Institute of Research in Biomedicine (IRB), Barcelona, Spain, cofunded by EU MarieCurie Actions.
Verena finished her Ph.D. in Bioinorganic Chemistry at the University of Vienna in 2013. Afterwards, she was research associate and lecturer at the Institute of Biomedical Engineering at the University of Applied Sciences in Vienna. Since August 2016, she works as postdoctoral fellow and PET production manager at the Medical University of Vienna in the working group of Prof. Marcus Hacker. Besides her research interest for carbon-11 and fluorine-18 radiochemistry, she establishes new methods for preclinical evaluation of PET tracers based on spheroid cultures.