Preclinical evaluation of a ¹⁸F-labeled phosphatidylinositol 3-kinase inhibitor for breast cancer imaging

Objectives: Breast cancer is one of the most common malignancy in women, especially in middle-aged and elderly women. Phosphatidylinositol 3-kinase (PI3K/AKt/mTOR) pathway abnormal activation was found to be involved in breast cancers proliferations. Pictilisib (GDC-0941) is a potent inhibitor of PI3K with high affinity, which is on Phase 2 clinical trials. Noninvasive development of imaging probe would help to determine the mechanism of this pathway to diagnose and predict therapy.Therefore, based on the structure of Pictilisib, we designed a new fluorine-18 radiolabeled radiopharmaceutical to noninvasively evaluate PI3K/AKT/mTOR pathway abnormal activation.

Methods: To increase the water solubility, Pictilisib was modified with ethylene di(p-toluenesulfonate) to obtain TsO-PEG₃-GDC-0941 as the precursor for ¹⁸F labeling. A non-radiolabeled reference compound ¹⁹F-PEG₃-GDC-0941 was also prepared. The radiolabeling of precursor TsO-PEG₃-GDC-0941 was conducted using a GE FXn pro synthesizer. Breast cancer cell lines, MDA-MBA-231 and MCF7, were used as mild and high PI3K expression models, respectively. The IC50 of Pictilisib and cell uptake of ¹⁸F-PEG₃-GDC-0941 were investigated and compared. PET imaging and in vivo biodistribution of ¹⁸F-PEG₃-GDC-0941 in MDA-MBA-231 and MCF7 Xenograft models were also performed and the results were compared.

Results: The precursor compound and reference standard compound were successfully synthesized and identified using NMR and Mass. The ¹⁸F radiolabeling was achieved with high yield (61±1%) at high specific activity (2100±100 MBq/mg). The IC50 of Pictilisib were determined to be 19.57µM and 7.14µM in MDA-MB-231 and MCF7 cells, respectively. Cell uptake of ¹⁸F-PEG₃-GDC-0941 in MCF7 were also significantly higher than those in MDA-MB-231 at all times points. MicroPET images and biodistribution studies showed higher uptake of the tracer in MCF7 tumors than in MDA-MB-231 tumors (7.15±1.14 %ID/g vs 4.51±0.23 %ID/g, 0.5 h p.i.). Western blotting and immunohistochemial staining results confirmed the higher PI3K expression in MCF7 cells and tumors and lower expression in MDA-MB-231 cells and tumors. Additionally, the brightness and tumor uptake values of MCF7 tumors were significantly decreased when a blocking doses of ¹⁹F-PEG₃-GDC-0941 were coinjected, indicating high specificity. Liver was found to be the major excretory organ with 5.82±0.88 %ID/g at 0.5 h p.i for MCF7 group. Conclusions: We successfully developed and evaluated a ¹⁸F radiolabeled tracer ¹⁸F-PEG₃-GDC-0941 for noninvasively imaging PI3K expression levels in breast cancer. This tracer held great potential for diagnosis, patient screening and therapy prediction. Acknowledgement: This work was supported by the National Natural Science Foundation of China (No. 81630049 and 81801738).