title: Radiolabelling of glucose-Tyr3-octreotate with 125I and analysis of its metabolism in rats: comparison with radiolabelled DOTA-Tyr3-octreotate.
abstract: BACKGROUND
  Somatostatin analogues labelled with radiometals or radiohalogens are useful for the imaging and treatment of somatostatin receptor-containing tumours. In this study, the procedures for the radioiodination of glucose-Tyr3-octreotate (gluc-Tyr3-tate) and radiolabelling of DOTA-Tyr3-octreotate (DOTA-Tyr3-tate) with 111In, 177Lu and 125I were compared and their metabolism in rats was analyzed. The usefulness of high performance liquid chromatography (HPLC) analysis and instant thin-layer chromatography on silica gel (ITLC-SG) for both radiochemical purity determination and analysis of metabolism in urine was investigated.
  
  
  METHODS
  For labelling with radiometals, the formation of a complex with the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) functionality of the peptide was employed. Radioiodination was performed by the chloramime-T method. The radiochemical purity of radiolabelled peptides and the analyses of rat urine were determined by HPLC and/or ITLC-SG methods. Male Wistar rats were used in the elimination studies.
  
  
  RESULTS
  DOTA-Tyr3-tate was simply radiolabelled with radiometals with high yield and high radiochemical purity. Stopping of the reaction was a critical step for radioiodination, therefore labelling of gluc-Tyr3-tate and DOTA-Tyr3-tate with 125I was not so simple and the reaction product had to be purified by preparative HPLC analysis. Whereas 111In-DOTA-Tyr3-tate and 177Lu-DOTA-Tyr3-tate were eliminated in rat urine in a practically unchanged form, a significant proportion of metabolites was observed with radioiodinated peptides, particularly at longer time intervals.
  
  
  CONCLUSIONS
  Labelling of DOTA-Tyr3-tate with radiometals is simple and the radiochemical purity of prepared compounds is very high, while iodination of the peptides demands purification of the product by preparative HPLC. The analysis of rat urine showed that excretion of radioiodinated peptides included a significant proportion of metabolites.
authors.affiliation: Department of Biophysics and Physical Chemistry, Faculty of Pharmacy, Charles University, Heyrovskeho 1203, 500 05 Hradec Kralove, Czech Republic.
authors.forename: Milos
authors.initials: M
authors.lastname: Petrik
authors.forename: Alice
authors.initials: A
authors.lastname: Laznickova
authors.forename: Milan
authors.initials: M
authors.lastname: Laznicek
authors.forename: Michael R
authors.initials: MR
authors.lastname: Zalutsky
mesh_terms: D000818:Animals; D001798:Blood Proteins / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D005947:Glucose / Q000031:analogs & derivatives* / Q000378:metabolism; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry / Q000378:metabolism; D007205:Indium Radioisotopes / Q000737:chemistry; D007457:Iodine Radioisotopes / Q000737:chemistry*; D007553:Isotope Labeling; D008187:Lutetium / Q000737:chemistry; D008297:Male; D010456:Peptides, Cyclic / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D011868:Radioisotopes / Q000737:chemistry; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
chemical_list: D001798:Blood Proteins; D006573:Heterocyclic Compounds, 1-Ring; D007205:Indium Radioisotopes; D007457:Iodine Radioisotopes; D010456:Peptides, Cyclic; D011868:Radioisotopes; D019275:Radiopharmaceuticals; C470036:octreotate, Tyr(3)-; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; D008187:Lutetium; D005947:Glucose