Semi-automated preparation of a 11C-labelled antibiotic--[N-methyl-11C]erythromycin A lactobionate.

A fast semi-automated method is described for labeling the antibiotic, erythromycin A (1), with the short-lived positron-emitting radionuclide, 11C (t 1/2 = 20.4 min), in order to permit the non-invasive study of its tissue uptake in vivo. Labelling was achieved by the fast reductive methylation of N-demethylerythromycin A (2) with [11C]formaldehyde, itself prepared from cyclotron-produced [11C]-carbon dioxide. Rapid chemical and radiochemical purification of the [N-methyl-11C]erythromycin A (3) were achieved by HPLC and verified by TLC with autoradiography. The purified material was formulated for human i.v. injection as a sterile apyrogenic solution of the lactobionate salt. The preparation takes 42 min from the end of radionuclide production and from [11C]carbon dioxide produces [N-methyl-C11]erythromycin A lactobionate in 1-12% radiochemical yield, corrected for radioactive decay.