Abstract Cysteine is a sulfur-containing amino acid that can be biosynthesized in human (1). It is not stable in free solution and can be auto-oxidized to cystine. Cystine is more stable and can travel freely through the blood (2). After intracellular accumulation by different transporters, especially Xc-transporter (3, 4), cystine will be rapidly reduced to cysteine which can then be used to synthesis protein or glutathione (GSH) synthesis5. Cysteine may also release from the cells by Alanine-Serine-Cysteine transporter (3). The accumulation of cysteine can feed other cells with low expression or mutation of Xc-transporter. It has been reported that stromal cells can promote cancer cell survival by the control of cystine metabolism to cysteine(5). Cysteine concentration distribution now may affect cell survival and cell-cell signaling (2, 5). It is still hard to study the total cysteine concentration in different culture system or in vivo in a high-through put way. Traditional LC-MS methods are not only cumbersome and time consuming, it also hard to process different samples in parallel. Here we report a series of novel fluorescent sensors which have great selectivity and sensitivity towards cysteine. Using these sensor compounds, we can monitor cysteine level in situ in cell culture system. Upon screening different culture medium, we found cysteine concentrations are different with cell type and culture time. We envision our fluorescent probes can be used to elucidate cysteine's function in cell survival as well as signal transduction between cells. 1. Lehmann, A.; Hagberg, H.; Orwar, O.; Sandberg, M., Cysteine sulphinate and cysteate: mediators of cysteine toxicity in the neonatal rat brain? Eur J Neurosci 1993, 5, (10), 1398-412. 2. Li, L.; Lim, J.; Jacobs, M. D.; Kistler, J.; Donaldson, P. J., Regional differences in cystine accumulation point to a sutural delivery pathway to the lens core. Invest Ophthalmol Vis Sci 2007, 48, (3), 1253-60. 3. Scopelliti, A. J.; Ryan, R. M.; Vandenberg, R. J., Molecular determinants for functional differences between alanine-serine-cysteine transporter 1 and other glutamate transporter family members. J Biol Chem 288, (12), 8250-7. 4. Huang, Y.; Dai, Z.; Barbacioru, C.; Sadee, W., Cystine-glutamate transporter SLC7A11 in cancer chemosensitivity and chemoresistance. Cancer Res 2005, 65, (16), 7446-54. 5. Zhang, W.; Trachootham, D.; Liu, J.; Chen, G.; Pelicano, H.; Garcia-Prieto, C.; Lu, W.; Burger, J. A.; Croce, C. M.; Plunkett, W.; Keating, M. J.; Huang, P., Stromal control of cystine metabolism promotes cancer cell survival in chronic lymphocytic leukaemia. Nat Cell Biol 14, (3), 276-86. Citation Format: Hong Guan. High-throughput cysteine concentration determination by specific fluorescence sensors in cell culture medium. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5395. doi:10.1158/1538-7445.AM2014-5395