[¹⁸F]-Radiolabelling and preliminary PET imaging of ischemia using fluorinated 5-pyrrolidinylsulfonyl isatins

Objectives Apoptosis can occur in cells that are damaged during an ischemic event. The apoptosis pathway culminates in the activation of the executioner caspases 3, 6 and 7 which destroy key cellular components and initiate cell death. [¹⁸F]-radiolabelled caspase inhibitors therefore have potential for diagnosing damage incurred after an ischemic event. Given this we designed a new class of [¹⁸F]-radiolabelled 5-pyrrolidinylsulfonyl isatins that were anticipated to have increased metabolic stability - relative to other isatin based inhibitors - and a functional handle for varying lipophilicty such that pharmacokinetic parameters including uptake and clearance and could be optimised.

Methods Synthesis of radiolabelling precursors was achieved through direct coupling of L-prolinol 1 with 5-chlorosulfonyl isatin 2 to give alcohol 3 which was immediately converted to the tosylate 4 and then alkylated with the appropriate benzylic halide to yield the precursors 5-7. [¹⁸F]-radiolabelling of precursors was performed by nucleophilic substitution, in the presence of (K₂₂₂)/[¹⁸F]KF complex, followed by treatment with HCl to yield the [¹⁸F]-radiolabelled compounds 8-10.

Results Radiolabeling of precursors was performed using the (K₂₂₂)/[¹⁸F]KF complex followed by treatment with HCl to close the isatin ring. Radiochemical yields ranging from 2-15% were obtained through optimisation of precursor mass, reaction time, temperature, solvent and work-up conditions. All radiotracers were obtained in >95% radiochemical purity [1].

Conclusions A new class of fluorinated 5-pyrrolidinylsulfonyl isatin compounds were synthesised that have significant potential for PET imaging of apoptosis. All compounds showed potent inhibitory activity against caspase-3 and caspase-7. Manual radiolabelling with [¹⁸F]-fluoride on the pyrrolidinylsulfonyl group was demonstrated for the first time and proceeded in good yield and purity. Initial PET imaging in a rat model of cerebral ischemia will be presented.