Synthesis of [18F]-fluoro-3'-deoxy-3'-L-fluorothymidine (FLT) in a fully automated modular system

Objectives FLT is a promising PET biomarker for assessing cell proliferation in tumor. The overall yield of FLT was previously reported to be 5-25% with kryptofix 2.2.2 (K222) in CH3CN or DMSO. The yield was increased substantially when tetrabutylammonium bicarbonate (TBAHCO3) and t-butanol were used instead. The multifunctional and fully automated Modular-Lab (Eckert & Ziegler; Berlin, Germany) consists of small individual operation modules which can be easily assembled, exchanged or expanded to fulfill the specific synthesis needs. Thus the aim of this study was to evaluate this system by synthesizing FLT with these two methods. The quality of FLT was further assessed in mice with microPET imaging.

Methods FLT was synthesized based on the methods published by Martin et al. (2002) using K222 and Lee et al. (2007) using TBAHCO3. 20 mg of precursor (3-N-Boc-5′-O-dimethoxytrityl-3′-O-nosyl-thymidine) was used in both methods. The overall yield was calculated by % (collected activity from HPLC)/(Initial activity) and the radiochemical yield by TLC. In vivo biodistribution of FLT was examined using microPET imaging in nude mice subcutaneously xenografted with the human lung cancer cell lines, A549 and HTB177 (n=5).

Results The overall yields and the radiochemical yields were 20±5% and 44±14% for the K222 method (n=9) and 16±4% and 35±15% for the TBAHCO3 method (n=6) respectively. The yields were not statistically different between these two methods (p>0.05). The total synthesis time was about 100 min for either method. MicroPET imaging in mice demonstrated predominant accumulation of FLT in tumors and distinct localization of FLT in other normal organs having rapid cell division rate such as the bone marrow, which is consistent with the literature.

Conclusions FLT was successfully and reproducibly synthesized with the fully automated Modular-Lab system using two different published methods with no significant difference in yields. Thus either method can be used to synthesize FLT for routine microPET imaging. Further refinement on the automation can possibly improve the yields