title: Rapid kit-based (68)Ga-labelling and PET imaging with THP-Tyr(3)-octreotate: a preliminary comparison with DOTA-Tyr(3)-octreotate.
abstract: BACKGROUND
  Ge/(68)Ga generators provide an inexpensive source of a PET isotope to hospitals without cyclotron facilities. The development of new (68)Ga-based molecular imaging agents and subsequent clinical translation would be greatly facilitated by simplification of radiochemical syntheses. We report the properties of a tris(hydroxypyridinone) conjugate of the SSTR2-targeted peptide, Tyr(3)-octreotate (TATE), and compare the (68)Ga-labelling and biodistribution of [(68)Ga(THP-TATE)] with the clinical radiopharmaceutical [(68)Ga(DOTATATE)].
  
  
  METHODS
  A tris(hydroxypyridinone) with a pendant isothiocyanate group was conjugated to the primary amine terminus of H2N-PEG2-Lys(iv-Dde)(5)-TATE, and the resulting conjugate was deprotected to provide THP-TATE. THP-TATE was radiolabelled with (68)Ga(3+) from a (68)Ge/(68)Ga generator. In vitro uptake was assessed in SSTR2-positive 427-7 cells and SSTR2-negative 427 (parental) cells. Biodistribution of [(68)Ga(THP-TATE)] was compared with that of [(68)Ga(DOTATATE)] in Balb/c nude mice bearing SSTR2-positive AR42J tumours. PET scans were obtained 1 h post-injection, after which animals were euthanised and tissues/organs harvested and counted.
  
  
  RESULTS
  [(68)Ga(THP-TATE)] was radiolabelled and formulated rapidly in <2 min, in ≥95 % radiochemical yield at pH 5-6.5 and specific activities of 60-80 MBq nmol(-1) at ambient temperature. [(68)Ga(THP-TATE)] was rapidly internalised into SSTR2-positive cells, but not SSTR2-negative cells, and receptor binding and internalisation were specific. Animals administered [(68)Ga(THP-TATE)] demonstrated comparable SSTR2-positive tumour activity (11.5 ± 0.6 %ID g(-1)) compared to animals administered [(68)Ga(DOTATATE)] (14.4 ± 0.8 %ID g(-1)). Co-administration of unconjugated Tyr(3)-octreotate effectively blocked tumour accumulation of [(68)Ga(THP-TATE)] (2.7 ± 0.6 %ID g(-1)). Blood clearance of [(68)Ga(THP-TATE)] was rapid and excretion was predominantly renal, although compared to [(68)Ga(DOTATATE)], [(68)Ga(THP-TATE)] exhibited comparatively longer kidney retention.
  
  
  CONCLUSIONS
  Radiochemical synthesis of [(68)Ga(THP-TATE)] is significantly faster, proceeds under milder conditions, and requires less manipulation than that of [(68)Ga(DOTATATE)]. A (68)Ga-labelled tris(hydroxypyridinone) conjugate of Tyr(3)-octreotate demonstrates specificity and targeting affinity for SSTR2 receptors, with comparable in vivo targeting affinity to the clinical PET tracer, [(68)Ga(DOTATATE)]. Thus, peptide conjugates based on tris(hydroxypyridinones) are conducive to translation to kit-based preparation of PET tracers, enabling the expansion and adoption of (68)Ga PET in hospitals and imaging centres without the need for costly automated synthesis modules.
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, 4th Floor Lambeth Wing, St Thomas' Hospital, London, SE1 7EH, UK. michelle.ma@kcl.ac.uk.
authors.forename: Michelle T
authors.initials: MT
authors.lastname: Ma
authors.affiliation: Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia.|Sir Peter MacCallum Department of Oncology, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Carleen
authors.initials: C
authors.lastname: Cullinane
authors.affiliation: Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia.
authors.forename: Kelly
authors.initials: K
authors.lastname: Waldeck
authors.affiliation: Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia.
authors.forename: Peter
authors.initials: P
authors.lastname: Roselt
authors.affiliation: Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia.|Sir Peter MacCallum Department of Oncology, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Rodney J
authors.initials: RJ
authors.lastname: Hicks
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, 4th Floor Lambeth Wing, St Thomas' Hospital, London, SE1 7EH, UK.
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Blower
keywords: Bifunctional chelator; Gallium-68; Hydroxypyridinone; Molecular imaging; Octreotate; Peptide receptor; Somatostatin