Yangchun Xin1, Hancheng Cai2,3. 1. Department of Radiology, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA. 2. Department of Radiology, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA. hancheng.cai@utsouthwestern.edu. 3. Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA. hancheng.cai@utsouthwestern.edu.
Abstract
PURPOSE: Tryptophan metabolism via indoleamine 2,3-dioxygenase (IDO)-mediated kynurenine pathway plays a role in immunomodulation and has been emerging as a plausible target for cancer immunotherapy. Imaging IDO-mediated 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 1-(2-[18F]fluoroethyl)-L-tryptophan (1-L-[18F]FETrp) and its enantioisomer 1-D-[18F]FETrp were synthesized and evaluated for PET imaging of IDO-mediated kynurenine pathway of tryptophan metabolism. PROCEDURES: Enantiopure 1-L-[18F]FETrp and 1-D-[18F]FETrp were prepared by a nucleophilic reaction of N-boc-1-(2-tosylethyl) tryptophan tert-butyl ester with [18F]Fluoride, followed by acid hydrolysis in a GE Tracerlab FX-N module. In vitro cell uptake assays were performed with a breast cancer cell line MDA-MB-231. Small animal PET/computed tomography (CT) imaging was carried out in a mouse model bearing MDA-MB-231 xenografts. RESULTS: Automatic radiosynthesis of 1-L-[18F]FETrp and 1-D-[18F]FETrp was achieved by a one-pot two-step approach in 19.0 ± 7.0 and 9.0 ± 3.0 % (n = 3) decay-corrected yield with radiochemical purity over 99 %, respectively. In vitro cell uptake study indicated the uptake of 1-D-[18F]FETrp in MDA-MB-231 cells was 0.73 ± 0.07 %/mg of protein at 60 min, while, the corresponding uptake of 1-L-[18F]FETrp was 6.60 ± 0.77 %/mg. Further mechanistic assays revealed that amino acid transport systems L-tpye amino acid transporter (LAT) and alanine-, serine-, and cysteine-preferring (ASC), and enzyme IDO expression were involved in cell uptake of 1-L-[18F]FETrp. Small animal PET/CT imaging study showed the tumor uptake of 1-L-[18F]FETrp was 4.6 ± 0.4 % ID/g, while, the tumor uptake of 1-D-[18F]FETrp was low to 1.0 ± 0.2 % ID/g, which were confirmed by ex vivo biodistribution study. CONCLUSIONS: We have developed a practical method for the automatic radiosynthesis of 1-L-[18F]FETrp and 1-D-[18F]FETrp. Our biological evaluation results suggest that 1-L-[18F]FETrp is a promising radiotracer for PET imaging of IDO-mediated kynurenine pathway of tryptophan metabolism in cancer.
PURPOSE:Tryptophan metabolism via indoleamine 2,3-dioxygenase (IDO)-mediated kynurenine pathway plays a role in immunomodulation and has been emerging as a plausible target for cancer immunotherapy. Imaging IDO-mediated 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 1-(2-[18F]fluoroethyl)-L-tryptophan (1-L-[18F]FETrp) and its enantioisomer 1-D-[18F]FETrp were synthesized and evaluated for PET imaging of IDO-mediated kynurenine pathway of tryptophan metabolism. PROCEDURES: Enantiopure 1-L-[18F]FETrp and 1-D-[18F]FETrp were prepared by a nucleophilic reaction of N-boc-1-(2-tosylethyl) tryptophan tert-butyl ester with [18F]Fluoride, followed by acid hydrolysis in a GE Tracerlab FX-N module. In vitro cell uptake assays were performed with a breast cancer cell line MDA-MB-231. Small animal PET/computed tomography (CT) imaging was carried out in a mouse model bearing MDA-MB-231 xenografts. RESULTS: Automatic radiosynthesis of 1-L-[18F]FETrp and 1-D-[18F]FETrp was achieved by a one-pot two-step approach in 19.0 ± 7.0 and 9.0 ± 3.0 % (n = 3) decay-corrected yield with radiochemical purity over 99 %, respectively. In vitro cell uptake study indicated the uptake of 1-D-[18F]FETrp in MDA-MB-231 cells was 0.73 ± 0.07 %/mg of protein at 60 min, while, the corresponding uptake of 1-L-[18F]FETrp was 6.60 ± 0.77 %/mg. Further mechanistic assays revealed that amino acid transport systems L-tpye amino acid transporter (LAT) and alanine-, serine-, and cysteine-preferring (ASC), and enzyme IDO expression were involved in cell uptake of 1-L-[18F]FETrp. Small animal PET/CT imaging study showed the tumor uptake of 1-L-[18F]FETrp was 4.6 ± 0.4 % ID/g, while, the tumor uptake of 1-D-[18F]FETrp was low to 1.0 ± 0.2 % ID/g, which were confirmed by ex vivo biodistribution study. CONCLUSIONS: We have developed a practical method for the automatic radiosynthesis of 1-L-[18F]FETrp and 1-D-[18F]FETrp. Our biological evaluation results suggest that 1-L-[18F]FETrp is a promising radiotracer for PET imaging of IDO-mediated kynurenine pathway of tryptophan metabolism in cancer.
Entities:
Keywords:
Cancer; IDO; Kynurenine pathway; PET imaging; Radiosynthesis; Tryptophan metabolism
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