Radiosynthesis and Preliminary Evaluation of [¹¹C]SSI-4 for the Positron Emission Tomography Imaging of Stearoyl CoA Desaturase 1

Stearoyl CoA desaturase 1 (SCD1) is the rate-limiting enzyme for converting saturated fatty acids (SFAs) into monounsaturated fatty acids (MUFAs) and plays a key role in endogenous (de novo) fatty acid metabolism. Given that this pathway is broadly upregulated across many tumor types with an aggressive phenotype, SCD1 has emerged as a compelling target for cancer imaging and therapy. The ligand 2-(4-(2-chlorophenoxy)piperidine-1-carboxamido)-N-methylisonicotinamide (SSI-4) was identified as a potent and highly specific SCD1 inhibitor with a strong binding affinity for SCD1 at our laboratory. We herein report the radiosynthesis of [¹¹C]SSI-4 and the preliminary biological evaluation including in vivo PET imaging of SCD1 in a human tumor xenograft model. Radiotracer [¹¹C]SSI-4 was labeled at the carbamide position via the direct [¹¹C]CO₂ fixation on the Synthra MeIplus module in high molar activity and good radiochemical yield. In vitro cell uptake assays were performed with three hepatocellular carcinoma (HCC) cell lines and three renal cell carcinoma (RCC) cell lines. Additionally, in vivo small animal PET/CT imaging with [¹¹C]SSI-4 and the biodistribution were carried out in a mouse model bearing HCC xenografts. Radiotracer [¹¹C]SSI-4 afforded a 4.14 ± 0.44% (decay uncorrected, n = 10) radiochemical yield based on starting [¹¹]CO₂ radioactivity. The [¹¹C]SSI-4 radiosynthesis time including HPLC purification and SPE formulation was 25 min from the end of bombardment to the end of synthesis (EOS). The radiochemical purity of [¹¹C]SSI-4 was 98.45 ± 1.43% (n = 10) with a molar activity of 225.82 ± 33.54 GBq/μmol (6.10 ± 0.91 Ci/μmol) at the EOS. In vitro cell uptake study indicated all SSI-4 responsive HCC and RCC cell line uptakes demonstrate specific uptake and are blocked by standard compound SSI-4. Preliminary small animal PET/CT imaging study showed high specific uptake and block of [¹¹C]SSI-4 uptake with co-injection of cold SSI-4 in high SCD1-expressing organs including lacrimal gland, brown fat, liver, and tumor. In summary, novel radiotracer [¹¹C]SSI-4 was rapidly and automatedly radiosynthesized by direct [¹¹C]CO₂ fixation. Our preliminary biological evaluation results suggest [¹¹C]SSI-4 could be a promising radiotracer for PET imaging of SCD1 overexpressing tumor tissues.