Synthesis and biological evaluation of a novel 99mTc cyclopentadienyl tricarbonyl complex ([(Cp-R)99mTc(CO)3]) for sigma-2 receptor tumor imaging

We report the design, synthesis and biological evaluation of a novel ^99mTc 4-(4-cyclohexylpiperazine-1-yl)-butan-1-one-1-cyclopentadienyltricarbonyl technetium (^99mTc]5) as a potential SPECT tracer for imaging of σ₂ receptors in tumors. ^99mTc]5 was prepared in 25 ± 5% isolated radiochemical yield with radiochemical purity of >99% via double-ligand transfer (DLT) reaction from the ferrocene precursor 2b (4-(4-cyclohexylpiperazine-1-yl)-1-ferrocenylbutan-1-one). The corresponding Re-complex 4 and the ferrocenyl complex 2b showed relatively high affinity towards σ₂ receptors in in vitro competition binding assay (K_i values of 4 and 2b were 64.4 ± 18.5 nM and 43.6 ± 21.3 nM, respectively) and moderate to high selectivity versus σ₁ receptors (K_iσ₁/K_iσ₂ ratios were 12.5 and 95.5, respectively). The log D value of ^99mTc]5 was determined to be 2.52 ± 0.33. Biodistribution studies in mice revealed comparably high initial brain uptake of ^99mTc]5 and slow washout. Administration of haloperidol 5 min prior to injection of ^99mTc]5 significantly reduced the radiotracer uptake in brain, heart, lung, and spleen by 40–50% at 2 h p.i.. Moreover, ^99mTc]5 showed high uptake in C6 glioma cell lines (8.6%) after incubation for 1 h. Blocking with haloperidol to compete with ^99mTc]5 significantly reduced the cell uptake. Preliminary blocking study in C6-brain-tumor bearing rats showed that ^99mTc]5 binds to σ receptors in the brain-tumor specifically. These results are encouraging for further exploration of ^99mTc-labeled probes for σ₂ receptor tumor imaging in vivo.