Novel technetium-99m-labelled ribociclib isocyanide derivatives for imaging cyclin-dependent kinase 4/6 (CDK4/6) expression in cancer.

Cyclin-dependent kinase 4/6 (CDK4/6) plays a crucial role in cell cycle regulation, is overexpressed in various cancers and is an important target in the development of radiotracers for tumour imaging. Despite the increasing recognition of CDK4/6 inhibitors in cancer therapy, their application is limited by the lack of suitable biomarkers. Herein, we developed a series of technetium-99m-labelled CDK4/6 radiotracers and utilized a linker optimization strategy to reduce their abdominal uptake and enhance their imaging properties. By introducing polyethylene glycol chains (PEGn, n = 2, 3, or 4) of different lengths, we successfully prepared the first technetium-99m-labelled ribociclib isocyanide derivatives via a one-step method. After rapid screening, we selected [99mTc]Tc-RIB-PEG4-CN (LogD7.4 = 0.01 ± 0.01) because of its superior uptake in the cell lines and suitable nontarget uptake in vivo. Additionally, it displayed nanomolar affinity (5.887 ± 0.3579 nM). In HCT116 xenograft models, the probe exhibited significant tumour uptake (2.44 ± 0.29 % ID/g at 4 h p.i.) while maintaining reduced abdominal uptake. Moreover, the probe showed specificity in HCT116 xenograft models, as evidenced by a 49.2 % decrease in the tumour-to-muscle ratio in the presence of excess ribociclib for blocking. Micro-SPECT/CT images of HCT116 and MCF-7 xenografts revealed the liver metabolism of [99mTc]Tc-RIB-PEG4-CN, with robust tumour retention and comparatively low abdominal uptake at 4 h p.i. This novel radiotracer enables the noninvasive evaluation of CDK4/6 expression, providing valuable insights for clinical treatment strategies and further mechanistic studies.