Distribution and elimination characteristics of 111In-DTPA-D-Phe1-octreotide and 111In-DTPA-L-Phe1-octreotide in rats

The present study compares distribution and elimination characteristics of In-111-DTPA-D-Phe(1)-octreotide and In-111-DTPA-L-Phe(1)-octreotide in rats and evaluated the effect of the replacement of the terminal L-phenylalanine by D-phenylalanine on pharmacokinetic profiles of the radiolabelled peptides. Both agents exhibited rapid radioactivity clearance from the blood and most organs and tissues with no systematic and significant differences in activity accumulation. The long-term retention and high radioactivity concentrations for both compounds under study were found in the kidneys and organs with a high density of somatostatin receptors, such as the pancreas and adrenals. The residence times in these organs were longer for In-111-DTPA-D-Phe(1)-octreotide in comparison with In-111-DTPA-L-Phe(1)-octreotide. The major elimination pathway for both radiolabelled peptides was relatively rapid excretion into the urine. Analysis of the renal handling by an employment of the perfused rat kidney showed that both peptides were eliminated mainly by the mechanism of glomerular filtration. Rat liver perfusion experiments confirmed a very low value of bile clearance of radioactivity for both agents under study.