Synthesis of ¹³C‐labeled derivatives of cysteine for magnetic resonance imaging studies of drug uptake and conversion to glutathione in rat brain

Abstract

Effects of neurodegeneration have been linked to inefficient detoxification of free radicals due to lowered concentrations of antioxidants, especially glutathione, in the brain. In the biosynthesis of glutathione, cysteine concentration is generally the limiting factor. Glutathione and cysteine administrations are not effective treatments for neurodegeneration because glutathione inefficiently crosses cell membranes and cysteine is neurotoxic at high concentrations. Prodrugs of glutathione and cysteine may have more favorable uptake and/or toxicity profiles. Three such prodrugs were synthesized with a 13C‐label such that in vivo uptake of each and conversion to glutathione in the brain could be monitored by magnetic resonance imaging. L‐[3‐13C]‐Cysteine was treated with sodium acetate trihydrate and acetic anhydride to give 2(R)‐N‐acetyl‐[3‐13C]‐cysteine ([13C]‐NAC; 96%). Addition of triphosgene to L‐[3‐13C]‐cysteine provided 4(R)‐[5‐13C]‐2‐oxothiazolidine‐4‐carboxylic acid ([13C]‐OTZ; 65%). A four‐step pathway was used to synthesize ethyl γ‐L‐glutamyl‐[3‐13C]‐L‐cysteinate ([13C]‐GCEE). L‐[3‐13C]‐Cysteine was esterified (100% yield) and then cyclized with acetaldehyde to give ethyl 2(R,S)‐methyl‐[5‐13C]‐thiazolidine‐4(R)‐carboxylate (73%) as a mixture of two diastereomers (65:35). The thiazolidine was silylated (bis(trimethylsilyl)trifluoroacetamide) and reacted with N‐phthaloyl‐L‐glutamic anhydride. Treatment with hydrazine afforded ethyl N‐[γ‐4′(S)‐glutamyl]‐2(R,S)‐methyl‐[5‐13C]‐thiazolidine‐4(R)‐carboxylate (48%; 73:27 mixture of diastereomers). This was converted to the desired product, [13C]‐GCEE (49%), using mercury (II) acetate and hydrogen sulfide.

Real-Time insight into in vivo redox status utilizing hyperpolarized [1-13C] N-acetyl cysteine

“…Briefly, [1- 13 C] l -cysteine was reacted with acetic anhydride in the presence of sodium acetate as the base in deoxygenated tetrahydrofuran 28 . Isolating the resulting product by crystallization was not successful as previously reported 29 , however the product could be purified by HPLC to afford [1- 13 C] NAC as a white, hygroscopic powder in 64% yield. The use of either HCl gas or concentrated aqueous HCl to convert the sodium salt to the free acid gave similar yields.…”

Non-invasive Monitoring of L-2-Oxothiazolidine-4-Carboxylate Metabolism in the Rat Brain by In vivo 13C Magnetic Resonance Spectroscopy

“…[3- 13 C]-L-Cysteine was obtained from Cambridge Isotope Laboratories (Andover, MA). The L-2-oxo-[5- 13 C]-thiazolidine-4-carboxylate ( 13 C-OTZ) was prepared from [3- 13 C]-L-cysteine [15]. …”