Clearance mechanism of prostate specific antigen and its complexes with alpha2-macroglobulin and alpha1-antichymotrypsin.

PURPOSE: To investigate the rate of elimination of prostate specific antigen (PSA) and its complexes with human alpha2-macroglobulin (alpha2-M) and alpha1-antichymotrypsin (ACT) and to elucidate the role of the alpha2-macroglobulin-receptor/low density lipoprotein receptor-related protein (alpha2-M-R/LRP) in the clearance mechanism. MATERIALS AND METHODS: PSA and complexes of PSA with alpha2-M and ACT were prepared and radiolabeled with [125I]Na (Amersham, Braunschweig, Germany). Radiolabeled proteins were injected into rats and the elimination of radioactivity from circulation was measured by gamma-counting of 20 microL aliquots over time. After 30 minutes different organs were removed and the total radioactivity was counted. The elimination rate and distribution of PSA and PSA-complexes was studied in the absence and presence of an excess of transformed alpha2-M. RESULTS: Radiolabeled PSA is rapidly eliminated from circulation with an initial half-life of 6.4+/-2.1 minutes mainly due to extraction by the liver and kidney. The clearance is slightly inhibited by transformed alpha2-M. PSA-alpha2-M is solely eliminated by the liver with a half-life of 6.7+/-1 minutes. Uptake by the liver is competitively inhibited by transformed alpha2-M. PSA-ACT is eliminated by the liver and kidney with an initial half-life of 3.51+/-1.1 minutes. Transformed alpha2-M failed to inhibit the clearance of PSA-ACT. CONCLUSIONS: Free PSA and PSA-inhibitor complexes are removed from the circulation by different clearance mechanisms. The sites of metabolism of the different forms of PSA are different but include liver and kidney as main organs for uptake. There are indications that alpha2-M-R/LRP is involved in PSA elimination. Thus, factors which modulate the receptor function and expression as well as the concentration of its natural ligands may interfere with the steady state concentrations of different PSA forms in blood.

Biodistribution and pharmacokinetics of 131I-labeled LEX 032, a recombinant hybrid of antichymotrypsin.

Pharmacokinetic and biodistribution studies were conducted in rats on a novel serine protease inhibitor, LEX 032, that was radiolabeled with 131I by the Bolton-Hunter reagent. LEX 032, a genetically engineered recombinant human nonglycosylated serpin, has been shown to have antiinflammatory properties in a number of animal models of inflammation and reperfusion injury. When 131I-LEX 032 was injected intravenously, a rapid whole body clearance of radioactivity was seen. Blood clearance followed a similar pattern. Forty-eight hours postinjection, 2.00 +/- 0.65 of the administered dose remained in the body. Greater than 59% of the radio-activity was excreted in the urine within the first 24 hr. Little radioactivity was found in the feces. With the exception of the thyroid, no significant organ-related uptake was noted. Radioactivity in the liver peaked at 20 min postinjection, with 1.00 +/- 0.13% administered dose/g and approximately 10% administered dose in the whole liver. At 1 hr, uptake in the kidney (9.30 +/- 1.52% administered dose/g) was the highest among all tissues, except for the thyroid. Gamma camera images were consistent with the biodistribution pattern. Pharmacokinetics and biodistribution were not affected by the dose of LEX 032 and were quite different from those of glycosylated wild type antichymotrypsin. These data indicate that LEX 032 exhibits the pharmacokinetics expected of a nonglycosylated 45 kDa protein.

In vivo catabolism of alpha 1-antichymotrypsin is mediated by the Serpin receptor which binds alpha 1-proteinase inhibitor, antithrombin III and heparin cofactor II.

The in vivo catabolism of 125I-labeled alpha 1-antichymotrypsin was studied in our previously described mouse model. Native alpha 1-antichymotrypsin cleared with an apparent t1/2 of 85 min, but alpha 1-antichymotrypsin in complex with chymotrypsin or cathepsin G cleared with a t1/2 of 12 min. Clearance of the complex was blocked by a large molar excess of unlabeled complexes of proteinases with either alpha 1-antichymotrypsin or alpha 1-proteinase inhibitor. These studies indicate that the clearance of alpha 1-antichymotrypsin-proteinase complexes utilizes the same pathway as complexes with the homologous inhibitor alpha 1-proteinase inhibitor. Previous studies have demonstrated that this pathway is also responsible for the catabolism of two other serine proteinase inhibitors, antithrombin III and heparin cofactor II. This pathway is thus responsible for removing several proteinases involved in coagulation and inflammation from the circulation, thereby decreasing the likelihood of adventitious proteolysis.