Feasibility of labeled α-acetamido-aminoisobutyric acid as new tracer compound for kinetic labeling of neutral amino acid transport: Preparation of α-( N-[1- 11C]Acetyl)- and α-( N-[1- 14C]Acetyl)-aminoisobutyric acid

Abstract

The nonphysiological, nonracemic, branched-chain α-acetamido-aminoisobutyric acid was labeled with the carbon isotope 11C with the intention to use it in conjunction with positron emission tomography (PET) to measure the kinetics of amino acid transport in vivo. It was produced by the reaction of the novel 11C-precursor N-[1- 11C]acetylpyridinium chloride with α-aminoisobutyric acid. Typically, 2 GBq of α-( N-[1- 11C]acetyl)-aminoisobutyric acid were isolated with a specific activity of 12 to 20 GBq · μmol −1 at the time of application, and with a radiochemical purity of >98%. The chemical identity of α-( N-[1- 11C]acetyl)-aminoisobutyric acid was confirmed by comparison with α-( N-[1- 14C]acetyl)-aminoisobutyric acid that was independently prepared by a standard acetylation procedure of α-aminoisobutyric acid using [1- 14C]acetic anhydride. In vivo, both labeled substrates were not metabolized. In cell-culture experiments, 84% of the substrate entered the cells by the sodium-dependent amino acid transport system A, whereas 16% was taken up by the sodium-independent system. The uptake of the radiotracer was measured 20 min and 40 min postinjection in tumor-bearing male Copenhagen rats for assessment of its in vivo biodistribution.