Design, synthesis, and biological evaluation of a 99mTc-labeled small-molecule tracer for PD-L1 imaging

Enormous success has been achieved in antitumor therapy with PD-1/PD-L1 inhibitors; however, less than 3 out of 10 patients show clinical benefits. An interesting phenomenon of patients who experienced durable clinical responses to PD-L1 blockade usually being the patients with high PD-L1 expression was noticed. Accordingly, the evaluation of PD-L1 is of great benefit to predict the immunotherapy response against PD-1/PD-L1. A small-molecule single photon emission computed tomography (SPECT) tracer [Tc-99m]Tc-SG2C-CBM containing a biphenyl moiety was designed and synthesized for the measurement of PD-L1 expression. The radiolabeling yield (RLY) and radiochemical purity (RCP) of [Tc-99m]Tc-SG2C-CBM were detected to be over 93% under the optimized labeling conditions. [Tc-99m]Tc-SG2C-CBM was discovered to be equipped with not only good stability but also suitable lipophilicity. In the cellular assays, [Tc-99m]Tc-SG2C-CBM showed moderate affinity to PD-L1 and 6.87-fold higher uptake in A375-hPD-L1 cells than in A375 cells. Then, the in vivo biodistribution results of the A375-hPD-L1 tumor-bearing mice showed that the radioactivity uptake in tumors was about 2.96 +/- 0.76 and 2.31 +/- 0.26 fold that in muscles and bones at 2 h post-injection, respectively. SPECT imaging was consistent with the biological distribution of [Tc-99m]Tc SG2C-CBM, with the uptake being mainly in the liver and less pronounced in tumors. The results of these studies indicate that [Tc-99m]Tc-SG2C-CBM may be a probe for imaging PD-L1 expression, but further research is needed.