Pharmacokinetic evaluation of biodistribution data obtained with radiolabeled proteins in mice

Radiolabeling of proteins is a widely used approach to study their in vitro disposition patterns. However, the obtained results may largely depend on the radiolabeling method used. The purpose of the present study was to inve stigate the effect of the radiolabeling method on the pharmacokinetic analy sis of liver targeted protein in mice. Galactosylated bovine serum albumin (Gal-BSA) was labeled with I-125 or In-111, using diethylenetriaminepentaac etic dianhydride (cDTPA) or 1-(4-isothiocyanobenzyl)ethylenediaminetetraace tic acid (SCN-Bz-EDTA) as bifunctional chelating agents. The Gal-BSA was th en injected in mice by a bolus intravenous injection. Samples of plasma, ur ine, liver, kidney, intestine and feces were collected at various time inte rvals and their radioactivity was measured, In none of the samples examined was there any significant difference in radioactivity distribution origina ting from the radiolabeling methods within 5 min after administration, Afte r this period, I-125 radioactivity in the liver started to decrease signifi cantly faster than that of In-111, which would indicate the intracellular d egradation of the protein. Consequently, the reappearance of trichloracetic acid (TCA) soluble I-125 radioactivity in the plasma occurred. But whereas the hepatic uptake clearance (CLliver) of [In-111]DTPA-Gal-BSA remained co nstant during 8 h postinjection, the CLliver of [I-125]Gal-BSA at 30 min re presented only one eighth of its initial values. The CLliver of [In-111]SCN -Bz-EDTA-Gal-BSA resembled that of [In-111]DTPA-Gal-BSA within 1 h of the e xperiment but it started to decline after this interval. The observed discr epancies most probably resulted from the formation of different radiolabele d metabolites in the hepatocytes and their different capability of crossing biological membranes. Our findings indicate that among the three methods e mployed, [In-111]DTPA radiolabeling of Gal-BSA is the most appropriate meth od to study its tissue disposition.