Milena Čolović1,2, Hua Yang1, Helen Merkens2, Nadine Colpo2, François Bénard3,4, Paul Schaffer5,6,7. 1. Life Sciences Division, TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, V6T 2A3, Canada. 2. Molecular Oncology, British Columbia Cancer Research Centre, 675 W 10th Ave, Vancouver, BC, V5Z 1L3, Canada. 3. Molecular Oncology, British Columbia Cancer Research Centre, 675 W 10th Ave, Vancouver, BC, V5Z 1L3, Canada. fbenard@bccrc.ca. 4. Department of Radiology, Faculty of Medicine, University of British Columbia, Vancouver, Canada. fbenard@bccrc.ca. 5. Life Sciences Division, TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, V6T 2A3, Canada. pschaffer@triumf.ca. 6. Department of Radiology, Faculty of Medicine, University of British Columbia, Vancouver, Canada. pschaffer@triumf.ca. 7. Department of Chemistry, Faculty of Science, Simon Fraser University, Vancouver, Canada. pschaffer@triumf.ca.
Abstract
PURPOSE: The cystine transporter, system xC-, plays a crucial role in sustaining redox homeostasis and is reported to be overexpressed in several cancer subtypes. 5-[18F]Fluoroaminosuberic acid ([18F]FASu) is a novel positron emission tomography (PET) tracer, which exhibits specific uptake via system xC-. [18F]FASu synthesis by the commonly used Kryptofix 2.2.2/K2CO3-facilitated fluorination method results in four diastereomers, as a result of 2 chiral centers at positions 2- and 5- of the tracer. We recently reported the synthesis of the optically pure 2S-[18F]FASu from chiral precursors. Our preliminary results indicated preferential uptake of the 2S-isomer by tumor cells compared to 2R-[18F]FASu. Few studies have investigated the biodistribution of chiral 18F-labeled amino acids. The aim of this study was to evaluate the imaging utility and biodistribution of the 5-position diastereomers as well as the racemic (2S,5R/S-) mixture in three different tumor models. PROCEDURES: In vitro tracer uptake experiments and Western blotting were performed in breast cancer (MDA-MB-231), glioblastoma (U-87), and prostate (PC-3) cancer cell lines. PET imaging and biodistribution studies were conducted in xenograft-bearing immunocompromised Rag2M female mice. RESULTS: All three tracer conformations allowed for the visualization of tumor xenografts at 1 h (for U-87 and PC-3 tumors) or 2 h (in the case of MDA-MB-231 xenografts) post-injection, with the racemate (2S,5R/S-) displaying similar image contrast as compared to the 5- position diastereomers and the 2S,5S-[18F]FASu conformation exhibiting relatively higher contrast for imaging U-87 and PC-3 xenografts. Tumor uptake of the isomers was blocked by an excess of the non-radioactive standard, aminosuberic acid (ASu), confirming target specificity. All three isomers were excreted via the renal pathway. Biodistribution analyses showed that PC-3 tumors had the highest tracer uptake, and the accumulation (%ID/g) of the 2S,5R/S-, 2S,5S-, and 2S,5R- isomers was 9.19 ± 1.14, 8.00 ± 1.41, and 7.16 ± 2.13 at 1 h post-injection, respectively. This gave corresponding tumor-to-muscle ratios of 33.68 ± 9.52, 31.42 ± 4.54, and 25.33 ± 4.97, respectively. CONCLUSION: Our data suggest that pure 2S-[18F]FASu can be used to noninvasively image system xC- in a variety of cancers, either as the racemic mixture (2S,5R/S-) or optically pure form. Furthermore, this work shows potential utility of [18F]FASu for detection of glioblastoma and prostate cancer.
PURPOSE: The cystine transporter, system xC-, plays a crucial role in sustaining redox homeostasis and is reported to be overexpressed in several cancer subtypes. 5-[18F]Fluoroaminosuberic acid ([18F]FASu) is a novel positron emission tomography (PET) tracer, which exhibits specific uptake via system xC-. [18F]FASu synthesis by the commonly used Kryptofix 2.2.2/K2CO3-facilitated fluorination method results in four diastereomers, as a result of 2 chiral centers at positions 2- and 5- of the tracer. We recently reported the synthesis of the optically pure 2S-[18F]FASu from chiral precursors. Our preliminary results indicated preferential uptake of the 2S-isomer by tumor cells compared to 2R-[18F]FASu. Few studies have investigated the biodistribution of chiral 18F-labeled amino acids. The aim of this study was to evaluate the imaging utility and biodistribution of the 5-position diastereomers as well as the racemic (2S,5R/S-) mixture in three different tumor models. PROCEDURES: In vitro tracer uptake experiments and Western blotting were performed in breast cancer (MDA-MB-231), glioblastoma (U-87), and prostate (PC-3) cancer cell lines. PET imaging and biodistribution studies were conducted in xenograft-bearing immunocompromised Rag2M female mice. RESULTS: All three tracer conformations allowed for the visualization of tumor xenografts at 1 h (for U-87 and PC-3tumors) or 2 h (in the case of MDA-MB-231 xenografts) post-injection, with the racemate (2S,5R/S-) displaying similar image contrast as compared to the 5- position diastereomers and the 2S,5S-[18F]FASu conformation exhibiting relatively higher contrast for imaging U-87 and PC-3 xenografts. Tumor uptake of the isomers was blocked by an excess of the non-radioactive standard, aminosuberic acid (ASu), confirming target specificity. All three isomers were excreted via the renal pathway. Biodistribution analyses showed that PC-3tumors had the highest tracer uptake, and the accumulation (%ID/g) of the 2S,5R/S-, 2S,5S-, and 2S,5R- isomers was 9.19 ± 1.14, 8.00 ± 1.41, and 7.16 ± 2.13 at 1 h post-injection, respectively. This gave corresponding tumor-to-muscle ratios of 33.68 ± 9.52, 31.42 ± 4.54, and 25.33 ± 4.97, respectively. CONCLUSION: Our data suggest that pure 2S-[18F]FASu can be used to noninvasively image system xC- in a variety of cancers, either as the racemic mixture (2S,5R/S-) or optically pure form. Furthermore, this work shows potential utility of [18F]FASu for detection of glioblastoma and prostate cancer.
Entities:
Keywords:
Cancer PET imaging; Chirality; Cystine transporter; FASu; Fluoroaminosuberic acid; Glioblastoma; Isomers; Oxidative stress imaging; Prostate cancer; TNBC
Authors: Hannah E Greenwood; Richard Edwards; Norman Koglin; Mathias Berndt; Friedrich Baark; Jana Kim; George Firth; Eman Khalil; Andre Mueller; Timothy H Witney Journal: Theranostics Date: 2022-01-24 Impact factor: 11.556