Hydroxyl radical generating methods have been explored in search for a PET formulation screening method that can predict the qualities of formulations regarding radiochemical stability.
The idea of using the hydroxyl radical originates from work with polysorbate 80 screening methods, literature on the subject of stabilizing excipients in radiopharmaceuticals, and the process of radiolysis of water.
The Fenton reaction and UV irradiation of hydrogen peroxide have been used to generate the hydroxyl radical. A GE proprietary model substance (the API) where the PET radionuclide 18F has been replaced by the stable nuclide 19F, has been used to model the effects of the radical generating methods. A hydroxyl radical loading procedure as well as a procedure where the hydroxyl radical was generated in the presence of the 19F API, have been applied for both the hydroxyl radical generating methods. A qualitative test (the methylene blue dye test) was used to verify the presence of the hydroxyl radical.
The concentration of 19F API has been analyzed by HPLC with UV detection, and the effects of the hydroxyl radical generated from the mentioned methods estimated. The impurity profiles of these same samples have been compared to those of equivalent radioactive 18F PET drug product, to get indications of how representative the hydroxyl radical based screening method could be of the radiolytic processes.
As a part of the method development, alternative formulations to that already used with the model substance have been developed and manufactured.
In samples treated with the hydroxyl radical generating methods, the 19F API concentration was found to be reduced, and the presence of the hydroxyl radical verified in the applied procedures. The impurity profiles vary for different 19F API sample matrixes, as well as for the hydroxyl radical generating methods and procedures, but many degradation peaks with the same relative retention time as those from radioactive 18F drug product analyses were found. The methods were also able to detect some trends with regards to the effect of chosen formulation factors.
To conclude on the value of both the use of the hydroxyl radical in such a screening method and the radical generating methods employed, further analyses and methods to determine the mechanism behind the observed effects, the identity of degradation peaks, as well as quantifications of the hydroxyl radical must be implemented. In addition the method will need optimization to both reduce and estimate uncertainty, and to establish reproducibility.