[¹¹C]Enzastaurin as a new PET cancer imaging agent for protein kinase C

Objectives Enzastaurin (LY317615) is a potent and selective inhibitor of protein kinase C (PKC) with IC₅₀ ~6 nM, developed for the treatment of cancer. PKC activation contributes to tumor cell survival and proliferation of human cancers, and is directly responsible for the most common and direct acting angiogenic factor VEGF (vascular endothelial growth factor) signaling. This study was designed to develop [¹¹C]Enzastaurin as a new PET cancer imaging agent for PKC.

Methods Enzastaurin {3-(1-methyl-1H-indol-3-yl)-4-[1-[1-(2-pyridinylmethyl)-4-piperidinyl]-1H-indol-3-yl]-1H-pyrrole-2,5-dione} and its precursor 3-(1H-indol-3-yl)-4-(1-(1-(pyridin-2-ylmethyl)piperidin-4-yl)-1H-indol-3-yl)furan-2,5-dione for radiolabeling were synthesized from indole-3-acetamide and 1-methyl-1H-indole; and indoline and indole-3-acetic acid in 5 and 6 steps, respectively. The precursor was labeled with [¹¹C]CH₃I to provide a radiolabeled intermediate 3-(1-[¹¹C]methyl-1H-indol-3-yl)-4-(1-(1-(pyridin-2-ylmethyl)piperidin-4-yl)-1H-indol-3-yl)furan-2,5-dione, which was then quickly reacted with hexamethyldisilazane in DMF/MeOH to give the target tracer [¹¹C]Enzastaurin. The two-step radiolabeling reaction was performed in a home-built automated ¹¹C-radiosynthesis module, and the target tracer was purified by HPLC method.

Results The overall chemical yields for the target compound and the precursor in 5 and 6 steps were 5% and 3%, respectively. The radiochemical yield for the target tracer was 20-25%, decay corrected to end of bombardment (EOB), based on [¹¹C]CO₂. The specific activity was 370-555 GBq/μmol at EOB.

Conclusions A new synthetic route to labeling precursor and Enzastaurin has been described, and a two-step radiosynthesis approach to [¹¹C]Enzastaurin has been well-developed, for the first time. The synthetic strategy was to prepare a carbon-11-labeled maleic anhydride intermediate followed by the conversion to maleimide.

Research Support The Breast Cancer Research Foundation