Assessment of tumor proliferation with ⁶⁸Ga-EC-guanine by PET

Abstract

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Objectives: [¹⁸F]FDG has shown promising in the assessment of cell proliferation, however, it is also accumulated in the infectious lesions. Though [¹⁸F]FLT is involved in tumor proliferation, yet it is with low tumor uptake and complex chemistry with low yield. The purpose of this study was to develop an efficient ⁶⁸Ga- and ⁶⁴Cu-labeled guanine using ethylenedicysteine (EC) as a chelator and to evaluate their potential in the assessment of tumor proliferation by positron emission tomography (PET).

Methods: EC-Guanine was synthesized by reacting the amino analogue of guanine with EC and carbodiimide. ⁶⁴Cu-and ⁶⁸Ga- EC-Guan were prepared by adding ⁶⁴Cu-acetate and ⁶⁸Ga-chloride to EC-Guan. In vitro cellular uptakes of ⁶⁸Ga-EC-guanine were determined in head and neck, lung and breast tumor cells at 0.5–4 h. Tumor cells were separated using cell sorters then incubated with ⁶⁸Ga- and ⁶⁴Cu-EC-Guanine for cell cycle analysis. Confluency studies and cellular fraction uptake studies were also performed. Thymidine incorporation assays were performed in lung cancer cells incubated with EC-Guan at various doses (0.1-1 mg/well). The tissue distribution of ⁶⁸Ga-EC-guanine, ⁶⁴Cu-EC-Guanine ,and [¹⁸F]-FDG were conduced using mammary tumor-bearing rats. PET imaging studies were performed with ⁶⁸Ga-EC-guanine in mammary tumor-bearing rats with or without inflammation. Autoradiographic studies were performed with ⁶⁴Cu-EC-Guanine in glioma tumor-bearing mice.

Results: Radiochemical purity was >95%. In vitro cell culture assays indicated both ⁶⁸Ga- and ⁶⁴Cu-EC-Guan were incorporated into cell cycles. S-Phase showed higher uptake than other phases. Confluency studies showed that ⁶⁸Ga-EC-guanine had uptake plateaued at 250x103 cells/well. ⁶⁸Ga-EC-guanine had higher uptake than [¹⁸F]-FDG and ⁶⁸Ga-citrate. Cell nuclei fraction uptake was higher than cytosolic fraction. Biodistribution of ⁶⁸Ga-EC-guanine and [¹⁸F]-FDG in breast tumor bearing rats showed that increased tumor-to-blood and tumor-to-muscle count density ratios as a function of time. ⁶⁸Ga-EC-Guanine had slightly higher tumor uptake (%ID/g) than ¹⁸F-FDG (0.69-0.90 for [¹⁸F]-FDG vs 0.63-1.82 for ⁶⁸Ga-EC-guanine). MicroPET images confirmed that the tumor-to-muscle ratios of ⁶⁸Ga-EC-guanine ranged from 3.5 to 9 over a period of 2 h. ⁶⁸Ga-EC-guanine could differentiate inflammation from tumor. Autoradiography of ⁶⁴Cu-EC-Guan showed that glioma could be visualized at 2hrs post-administration.

Conclusions: ⁶⁸Ga- and ⁶⁴Cu-EC-guanine are useful to assess tumor proliferation by PET.