Rapid kit-based 68Ga-labelling and PET imaging with THP-Tyr3-octreotate: a preliminary comparison with DOTA-Tyr3-octreotate

Abstract

Background: Ge/68Ga generators provide an inexpensive source of a PET isotope to hospitals without cyclotron facilities. The development of new 68Ga-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, Tyr3-octreotate (TATE), and compare the 68Ga-labelling and biodistribution of [68Ga(THP-TATE)] with the clinical radiopharmaceutical [68Ga(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 68Ga3+ from a 68Ge/68Ga generator. In vitro uptake was assessed in SSTR2-positive 427-7 cells and SSTR2-negative 427 (parental) cells. Biodistribution of [68Ga(THP-TATE)] was compared with that of [68Ga(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: [68Ga(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. [68Ga(THP-TATE)] was rapidly internalised into SSTR2-positive cells, but not SSTR2-negative cells, and receptor binding and internalisation were specific. Animals administered [68Ga(THP-TATE)] demonstrated comparable SSTR2-positive tumour activity (11.5 ± 0.6 %ID g−1) compared to animals administered [68Ga(DOTATATE)] (14.4 ± 0.8 %ID g−1). Co-administration of unconjugated Tyr3-octreotate effectively blocked tumour accumulation of [68Ga(THP-TATE)] (2.7 ± 0.6 %ID g−1). Blood clearance of [68Ga(THP-TATE)] was rapid and excretion was predominantly renal, although compared to [68Ga(DOTATATE)], [68Ga(THP-TATE)] exhibited comparatively longer kidney retention. Conclusions: Radiochemical synthesis of [68Ga(THP-TATE)] is significantly faster, proceeds under milder conditions, and requires less manipulation than that of [68Ga(DOTATATE)]. A 68Ga-labelled tris(hydroxypyridinone) conjugate of Tyr3-octreotate demonstrates specificity and targeting affinity for SSTR2 receptors, with comparable in vivo targeting affinity to the clinical PET tracer, [68Ga(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 68Ga PET in hospitals and imaging centres without the need for costly automated synthesis modules.