title: (67/68)Ga-labeling agent that liberates (67/68)Ga-NOTA-methionine by lysosomal proteolysis of parental low molecular weight polypeptides to reduce renal radioactivity levels.
abstract: The renal localization of gallium-67 or gallium-68 ((67/68)Ga)-labeled low molecular weight (LMW) probes such as peptides and antibody fragments constitutes a problem in targeted imaging. Wu et al. previously showed that (67)Ga-labeled S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (SCN-Bz-NOTA)-conjugated methionine ((67)Ga-NOTA-Met) was rapidly excreted from the kidney in urine following lysosomal proteolysis of the parental (67)Ga-NOTA-Bz-SCN-disulfide-stabilized Fv fragment (Bioconjugate Chem., (1997) 8, 365-369). In the present study, a new (67/68)Ga-labeling reagent for LMW probes that liberates (67/68)Ga-NOTA-Met was designed, synthesized, and evaluated using longer-lived (67)Ga in order to reduce renal radioactivity levels. We employed a methionine-isoleucine (MI) dipeptide bond as the cleavable linkage. The amine residue of MI was coupled with SCN-Bz-NOTA for (67)Ga-labeling, while the carboxylic acid residue of MI was derivatized to maleimide for antibody conjugation in order to synthesize NOTA-MI-Mal. A Fab fragment of the anti-Her2 antibody was thiolated with iminothiolane, and NOTA-MI-Mal was conjugated with the antibody fragment by maleimide-thiol chemistry. The Fab fragment was also conjugated with SCN-Bz-NOTA (NOTA-Fab) for comparison. (67)Ga-NOTA-MI-Fab was obtained at radiochemical yields of over 95% and was stable in murine serum for 24 h. In the biodistribution study using normal mice, (67)Ga-NOTA-MI-Fab registered significantly lower renal radioactivity levels from 1 to 6 h postinjection than those of (67)Ga-NOTA-Fab. An analysis of urine samples obtained 6 h after the injection of (67)Ga-NOTA-MI-Fab showed that the majority of radioactivity was excreted as (67)Ga-NOTA-Met. In the biodistribution study using tumor-bearing mice, the tumor to kidney ratios of (67)Ga-NOTA-MI-Fab were 4 times higher (6 h postinjection) than those of (67)Ga-NOTA-Fab. Although further studies including the structure of radiometabolites and/or cleavable linkages are required, the results of the present study indicate that the current chemical design is applicable to the development of (67)Ga-labeled Fabs for low renal radioactivity levels.
authors.affiliation: Graduate School of Pharmaceutical Sciences, Chiba University , 1-8-1 Inohana, Chuo-ku, Chiba 263-8675, Japan.
authors.forename: Tomoya
authors.initials: T
authors.lastname: Uehara
authors.forename: Takemi
authors.initials: T
authors.lastname: Rokugawa
authors.forename: Mai
authors.initials: M
authors.lastname: Kinoshita
authors.forename: Souki
authors.initials: S
authors.lastname: Nemoto
authors.forename: Guerra Gomez
authors.initials: GG
authors.lastname: Fransisco Lazaro
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Hanaoka
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Arano
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D004355:Drug Stability; D005710:Gallium Radioisotopes; D006571:Heterocyclic Compounds / Q000737:chemistry / Q000378:metabolism* / Q000493:pharmacokinetics / Q000652:urine; D006573:Heterocyclic Compounds, 1-Ring; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry; D007553:Isotope Labeling; D007668:Kidney / Q000378:metabolism / Q000528:radiation effects*; D008247:Lysosomes / Q000378:metabolism*; D008297:Male; D008715:Methionine / Q000737:chemistry*; D051379:Mice; D008970:Molecular Weight; D010455:Peptides / Q000737:chemistry* / Q000378:metabolism*; D059748:Proteolysis; D011851:Radioactivity; D013329:Structure-Activity Relationship
chemical_list: D005710:Gallium Radioisotopes; D006571:Heterocyclic Compounds; D006573:Heterocyclic Compounds, 1-Ring; D007140:Immunoglobulin Fab Fragments; D010455:Peptides; C048993:1,4,7-triazacyclononane-N,N',N''-triacetic acid; D008715:Methionine