Intracellular metabolism of indium-111-DTPA-labeled receptor targeted proteins.

The mechanisms of hepatic and renal accumulation and retention of 111In-labeled proteins has been the subject of many investigations. Because the lysosome is a common intracellular destination for a variety of agents including antibodies and polypeptide hormones, we studied the in vitro and in vivo metabolism of 111In-DTPA-labeled polypeptides using a series of glycoproteins that were concentrated within the lysosome by receptor mediated endocytosis. Indium-111-DTPA-labeled glycoproteins targeted to the mannose, asialoglycoprotein and mannose 6-phosphate receptors were studied in vitro using cell lines known to express these receptors and in vivo using Sprague-Dawley rats. Once internalized, the 111In label was released slowly with 60%-90% (depending on the cell type) of the activity remaining cell associated at 24 hr. Subcellular fractionation using Percoll gradients indicated that the activity remained within the lysosome. Following internalization of the 111In-DTPA-labeled glycoproteins, the label was rapidly converted to a low molecular weight species (estimated molecular weight < or = 1000 daltons). This conversion was not seen with 111In-DTPA-alpha-galactosidase. As a lysosomal enzyme, alpha-galactosidase is relatively resistant to proteolysis within the lysosome. These results suggest that following internalization, 111In-DTPA-polypeptides are delivered to the lysosome where the polypeptide backbone can be degraded to yield 111In-DTPA-amino acid(s). These metabolites remain within the lysosome and are only slowly released from the cell. The model systems used in these studies can also be used to evaluate the intracellular metabolism of polypeptides labeled by other techniques.