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Simplified radiolabeling and initial experimental studies of the in vivo kinetics of iodine-123 atrial natriuretic peptide.

Conventional methods regarding the iodination of atrial natriuretic factor (ANP) were associated with the use of high performance liquid chromatography in a linear elution technique. This chromatographic system is not standard in most radiochemical laboratories. The present study describes a method that allows labeling of ANP with 123I-iodide by using conventional isocratic HPLC equipment. First, the labeled protein was purified in a cascaded way. Then eight gradients with a ratio of acetonitrile/water of 5/95 up to 60/40 (v/v) and a time range of 5 min were applied. The iodinated ANP eluted at about 25 to 30% of acetonitrile. Centrifugation with a speedvac resulted in an injectable solution. Administration of about 20 MBq/kg [123I]iodo-ANP to a rabbit was performed in a pilot study to evaluate the in vivo kinetics of various organs by dynamic acquisition with a gamma camera. The kidneys showed an increasing uptake from 4.2 to 5.3% of total body activity after 75 min. An increasing tracer uptake was also observed in the skull region from 2.4% after 10 min to 3.6% at 75 min. The time activity curve of the liver showed a decrease from 6.6 to 5.1% and that of the heart from 3.7 to 2.9% at 75 min. Thus, labeling of ANP--and probably other peptides--is possible with a simplified cascade elution procedure without the use of gradient HPLC equipment.