Improved Radiosynthesis and Biological Evaluations

Authors: Yangchun Xin Hancheng Cai

Publish Date: 2016/11/04

Volume: 19, Issue: 4, Pages: 589-598

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Abstract

Tryptophan metabolism via indoleamine 23dioxygenase IDOmediated kynurenine pathway plays a role in immunomodulation and has been emerging as a plausible target for cancer immunotherapy Imaging IDOmediated kynurenine pathway of tryptophan metabolism with positron emission tomography PET could provide valuable information for noninvasive assessment of cancer immunotherapy response In this work radiotracer 1218FfluoroethylLtryptophan 1L18FFETrp and its enantioisomer 1D18FFETrp were synthesized and evaluated for PET imaging of IDOmediated kynurenine pathway of tryptophan metabolismEnantiopure 1L18FFETrp and 1D18FFETrp were prepared by a nucleophilic reaction of Nboc12tosylethyl tryptophan tertbutyl ester with 18FFluoride followed by acid hydrolysis in a GE Tracerlab FXN module In vitro cell uptake assays were performed with a breast cancer cell line MDAMB231 Small animal PET/computed tomography CT imaging was carried out in a mouse model bearing MDAMB231 xenograftsAutomatic radiosynthesis of 1L18FFETrp and 1D18FFETrp was achieved by a onepot twostep approach in 190 ± 70 and 90 ± 30  n = 3 decaycorrected yield with radiochemical purity over 99  respectively In vitro cell uptake study indicated the uptake of 1D18FFETrp in MDAMB231 cells was 073 ± 007 /mg of protein at 60 min while the corresponding uptake of 1L18FFETrp was 660 ± 077 /mg Further mechanistic assays revealed that amino acid transport systems Ltpye amino acid transporter LAT and alanine serine and cysteinepreferring ASC and enzyme IDO expression were involved in cell uptake of 1L18FFETrp Small animal PET/CT imaging study showed the tumor uptake of 1L18FFETrp was 46 ± 04  ID/g while the tumor uptake of 1D18FFETrp was low to 10 ± 02  ID/g which were confirmed by ex vivo biodistribution studyWe have developed a practical method for the automatic radiosynthesis of 1L18FFETrp and 1D18FFETrp Our biological evaluation results suggest that 1L18FFETrp is a promising radiotracer for PET imaging of IDOmediated kynurenine pathway of tryptophan metabolism in cancerWe would like to thank Prof Xiankai Sun for providing insight that greatly assisted our work Robert Hallgren for producing 18F for radiochemistry study and Hussein Diab for acquiring and analyzing the microPET/CT imaging study We also thank Drs William Silvers Guiyang Hao and Aditi Mulgaonkar for their assistance in this study This work was supported by Cancer Prevention Research Institute of Texas CPRIT Grant RP110771 the Simmons Cancer Center Grant NIH 5P30 CA 142543 and American Cancer Society and the Simmons Cancer Center ACSIRG02196

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