Step-by-step optimisation of the radiosynthesis of the brain HDAC6 radioligand [18F]FSW-100 for clinical applications

Background

Histone deacetylase 6 (HDAC6) is an emerging target for the treatment and diagnosis of proteinopathies. [¹⁸F]FSW-100 was recently developed as a promising brain-penetrating radioligand for HDAC6 PET imaging and the process validation of [¹⁸F]FSW-100 radiosynthesis for clinical use is complete, but no detailed synthetic strategy nor process optimisation has been reported. Here, we describe the optimisation of several processes in [¹⁸F]FSW-100 radiosynthesis, including the ¹⁸F-fluorination reaction, semipurification of the ¹⁸F-intermediate, and purification of the product by high-performance liquid chromatography (HPLC), to achieve a radiochemical yield (RCY) adequate for clinical applications of the radioligand. Our findings will aid optimisation of radiosynthesis processes in general.

Results

In the ¹⁸F-fluorination reaction, the amount of copper reagent was reduced without reducing the nonisolated RCY of the intermediate (50%), thus reducing the risk of copper contamination in the product injection solution. Optimising the solid-phase extraction (SPE) conditions for semipurification of the intermediate improved its recovery efficiency. The addition of anti-radiolysis reagents to the mobile phase for the HPLC purification of [¹⁸F]FSW-100 increased its activity yield in radiosynthesis using a high [¹⁸F]fluoride radioactivity of approximately 50 GBq. The SPE-based formulation method and additives for the injection solution were optimised, and the resulting [¹⁸F]FSW-100 injection solution was stable for over 2 h with a radiochemical purity of greater than 95%.

Conclusions

Of all the reconsidered processes, we found that optimisation of the SPE-based semipurification of the intermediate and of the mobile phase for HPLC purification in particular improved the RCY of [¹⁸F]FSW-100, doubling it compared to that of the original protocol. The radioactivity of [¹⁸F]FSW-100 synthesized using the optimized protocol was sufficient for multiple doses for a clinical study.