The cell cycle is a progression of 4 distinct phases (G1, S, G2, and M), with various cycle proteins being essential in regulating this process. We aimed to develop a radiolabeled cyclin-dependent kinase 4/6 (CDK4/6) inhibitor for breast cancer imaging. Our transfluorinated analog (¹⁸F-CDKi) was evaluated and validated as a novel PET imaging agent to quantify CDK4/6 expression in estrogen receptor (ER)–positive human epidermal growth factor receptor 2 (HER₂)–negative breast cancer. Methods: ¹⁸F-CDKi was synthesized and assayed against CDK4/6 kinases. ¹⁸F-CDKi was prepared with a 2-step automated synthetic strategy that yielded the final product with remarkable purity and molar activity. In vitro and in vivo biologic specificity was assessed in a MCF-7 cell line and in mice bearing MCF-7 breast tumors. Nonradioactive palbociclib was used as a blocking agent to investigate the binding specificity and selectivity of ¹⁸F-CDKi. Results: ¹⁸F-CDKi was obtained with an overall radiochemical uncorrected yield of 15% and radiochemical purity higher than 98%. The total time from the start of synthesis to the final injectable formulated tracer is 70 min. The retention time reported for ¹⁸F-CDKi and ¹⁹F-CDKi is 27.4 min as demonstrated by coinjection with ¹⁹F-CDKi in a high-pressure liquid chromatograph. In vivo blood half-life (weighted, 7.03 min) and octanol/water phase partition coefficient (1.91 ± 0.24) showed a mainly lipophilic behavior. ¹⁸F-CDKi is stable in vitro and in vivo (>98% at 4 h after injection) and maintained its potent targeting affinity to CDK4/6. Cellular uptake experiments performed on the MCF-7 breast cancer cell line (ER-positive and HER₂-negative) demonstrated specific uptake with a maximum intracellular concentration of about 65% as early as 10 min after incubation. The tracer uptake was reduced to less than 5% when cells were coincubated with a molar excess of palbociclib. In vivo imaging and ex vivo biodistribution of ER-positive, HER₂-negative MCF-7 breast cancer models showed a specific uptake of approximately 4% injected dose/g of tumor (reduced to ∼0.3% with a 50-fold excess of cold palbociclib). A comprehensive biodistribution analysis also revealed a significantly lower activation of CDK4/6 in nontargeting organs. Conclusion: ¹⁸F-CDKi represents the first ¹⁸F PET CDK4/6 imaging agent and a promising imaging agent for ER-positive, HER₂-negative breast cancer.

细胞周期是4个不同阶段（G1，S，G2和M）的进程，各种周期蛋白对于调节该过程至关重要。我们旨在为乳腺癌成像开发一种放射性标记的细胞周期蛋白依赖性激酶4/6（CDK4 / 6）抑制剂。我们的超氟化类似物（¹⁸ F-CDKi）被评估为一种新型PET显像剂，可量化雌激素受体（ER）阳性的人类表皮生长因子受体2（HER ₂）阴性的乳腺癌中CDK4 / 6的表达。方法： 合成¹⁸ F-CDKi并针对CDK4 / 6激酶进行测定。^18岁F-CDKi采用两步自动合成策略制备，可产生具有显着纯度和摩尔活性的最终产物。在MCF-7细胞系和患有MCF-7乳腺肿瘤的小鼠中评估了体外和体内生物学特异性。非放射性palbociclib被用作封闭剂来研究¹⁸ F-CDKi的结合特异性和选择性。结果：获得 ¹⁸ F-CDKi，总放射化学未校正产率为15％，放射化学纯度高于98％。从合成开始到最终可注射配制的示踪剂的总时间为70分钟。报道的保留时间¹⁸ F-CDKI和¹⁹ F-CDKI是27.4分钟所证明通过共注射与¹⁹高压液相色谱仪中的F-CDKi。体内血液半衰期（加权，7.03分钟）和辛醇/水相分配系数（1.91±0.24）显示出主要的亲脂行为。¹⁸ F-CDKi在体外和体内均稳定（注射后4 h> 98％），并保持其对CDK4 / 6的有效靶向亲和力。在MCF-7乳腺癌细胞系（ER阳性和HER ₂阴性）上进行的细胞摄取实验表明，特异性孵育的最早时间是孵育后10分钟，最大细胞内浓度约为65％。当将细胞与摩尔过量的palbociclib共孵育时，示踪剂的吸收降低到小于5％。ER阳性HER _2的体内成像和离体生物分布阴性的MCF-7乳腺癌模型显示约4％注射剂量/ g肿瘤的特定摄入量（冷的palbociclib过量50倍时降低至〜0.3％）。全面的生物分布分析还显示，非靶向器官中CDK4 / 6的活化显着降低。结论： ¹⁸ F-CDKi代表了首个¹⁸ F PET CDK4 / 6显像剂，也是有前景的ER阳性，HER ₂阴性乳腺癌的显像剂。