title: Different radiolabelling methods alter the pharmacokinetic and biodistribution properties of plasminogen activator inhibitor type 2 (PAI-2) forms.
abstract: BACKGROUND
  Tumour-associated urokinase plasminogen activator (uPA) is a critical marker of invasion and metastasis, and it is recognised as having strong prognostic relevance as well as being a therapeutic target. The specific uPA inhibitor plasminogen activator inhibitor type-2 (PAI-2, SerpinB2) specifically targets cell bound uPA and is internalised. Furthermore, preclinical studies have established the "proof-of-principle" of uPA-targeting by PAI-2-cytotoxin conjugates in human carcinoma models. However, these studies also suggest that PAI-2 is rapidly cleared via the renal system with low total dose reaching the tumour. In this study, a comparative single photon emission computed tomography (SPECT) and biodistribution (BD) analysis of different forms of PAI-2 labelled with the radioisotopes iodine-123 ((123)I) and technetium-99m ((99m)Tc) was undertaken.
  
  
  METHODS
  The pharmacokinetic (PK) properties and BD of wild-type, ΔCD-loop and PEGylated ΔCD-loop PAI-2 labelled with the commonly used diagnostic SPECT radioisotopes (99m)Tc or (123)I were compared in mouse models of human prostate carcinoma. Whole body SPECT imaging was also performed.
  
  
  RESULTS
  Both wild-type and the shorter but active ΔCD-loop form of PAI-2 (123)I-labelled indirectly via conjugation to free amine groups (termed (123)I-Bn-PAI-2) exhibited low tumour uptake, rapid excretion and similar PK profiles. Preliminary studies with a short branched-chain PEGylated (123)I-Bn-PAI-2 ΔCD-loop indicated an increase in blood retention time and tumour uptake. All (123)I-Bn-labelled radiotracers were largely excreted through the kidneys. By comparison, both wild-type (123)I-PAI-2 (labelled directly via tyrosine residues) and (99m)Tc-PAI-2 displayed different PK/BD patterns compared to (123)I-Bn-PAI-2, suggesting greater liver based catabolism and thus slower elimination. SPECT imaging mimicked the BD results of all radiotracers.
  
  
  CONCLUSIONS
  The different labelling methods gave distinct PAI-2 BD and tumour uptake profiles, with radioiodination resulting in the best non-tumour organ clearance profiles. Preliminary analyses with short branched-chain PEGylated (123)I-Bn-PAI-2 ΔCD-loop suggest that further investigations with other PEGylation reagents are required to optimise this approach for tumour imaging. These findings impact on the use of PAI-2 for drug delivery and/or diagnostic development.
authors: School of Biological Sciences, University of Wollongong, Wollongong, New South Wales 2522, Australia. mranson@uow.edu.au
authors: Marie
authors: M
authors: Ranson
authors: Paula
authors: P
authors: Berghofer
authors: Kara L
authors: KL
authors: Vine
authors: Ivan
authors: I
authors: Greguric
authors: Rachael
authors: R
authors: Shepherd
authors: Andrew
authors: A
authors: Katsifis
mesh_terms: D000818:Animals; D001692:Biological Transport; D045744:Cell Line, Tumor; D006801:Humans; D007457:Iodine Radioisotopes; D007553:Isotope Labeling / Q000379:methods*; D008297:Male; D051379:Mice; D015609:Organotechnetium Compounds; D017396:Plasminogen Activator Inhibitor 2 / Q000378:metabolism / Q000493:pharmacokinetics*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism / Q000473:pathology; D019275:Radiopharmaceuticals / Q000378:metabolism / Q000493:pharmacokinetics*; D015899:Tomography, Emission-Computed, Single-Photon
chemical_list: D007457:Iodine Radioisotopes; D015609:Organotechnetium Compounds; D017396:Plasminogen Activator Inhibitor 2; D019275:Radiopharmaceuticals