Metabolism of the Lanthanons in the Rat.

The metabolism of the lanthanons has been studied in the rat on a systematic basis using tracer technics. High-specific activity preparations were administered orally or intramuscularly as citrate complexes. 2. Data are presented for the fifteen lanthanons 1 and 4 days after administration and at intervals up to 8 months for those isotopes with sufficiently long half lives. In general, absorption from the parenteral injection site was relatively complete at four days. Gastrointestinal absorption of the 4 lanthanons administered orally was insignificant. 3. The light lanthanons (lanthanum through samarium). Deposition was primarily in the liver and skeleton, 50 and 25% of the administered dose respectively. Elimination from the liver (presumably by way of the bile duct to the gastrointestinal tract) was quite rapid with a half-time of about 15 days. Two months after injection the skeleton retained about two-thirds of its initial deposition; there was no further elimination from the skeleton during the subsequent 8 months. 4. The transition lanthanons (europium and gadolinium). Deposition was more nearly equal in liver and skeleton, 30 and 40% of the administered dose respectively. Excretion was both fecal and urinary. 5. The heavy lanthanons (terbium through lutetium). Deposition was mainly skeletal, 50 to 65% of the administered dose. Elimination from the skeleton was slow with a half-time of approximately 2.5 years. Excretion of extra-skeletal heavy lanthanon occurred within the first two weeks after injection and was almost entirely urinary.