Metabolism of guanido-labeled (C-14)arginine in rats, mice, and man

[6-¹⁴C]arginine, injected intraperitoneally into normal rats, was cleared from the plasma with biphasic decay kinetics. Urinary excretion was efficient (32% of the 25-μCi dose within the first 24 hr) with no preferential tissue retention. In mice, the effective duration of the radiotracer's availability for protein biosynthesis was less than 30 min. When the tracer was administered i.v. to patients with multiple myeloma, it was similarly cleared from the plasma with biphasic kinetics, and was excreted rapidly in the urine (22% of the dose within the first 24 hr). In patients, the guanido-tagged arginine labeled only tumor M component, and the labeling was most intense in patients who had far advanced disease. Estimated radiation dose to humans from a 100-μCi injection was 10 mrads. These studies demonstrate the feasibility of in vivo labeling with [6-¹⁴C]arginine, with minimal radiation hazard, thus providing a simple, sensitive, and specific method for monitoring the synthesis of the plasmacytoma M component in patients with multiple myeloma.