Twins in spirit - episode I: comparative preclinical evaluation of [68Ga]DOTATATE and [68Ga]HA-DOTATATE

Abstract

Background: Recently, an intra-patient comparison demonstrated that the somatostatin (sst) ligand [68Ga]HA-DOTATATE ([68Ga]DOTA-3-iodo-Tyr3-octreotate) provides PET images comparable to or superior to those obtained with [68Ga]DOTATATE. To provide a comprehensive basis for nevertheless observed slight differences in tracer biodistribution and dosimetry, the characteristics of [68Ga]HA-DOTATATE were investigated in a detailed preclinical study. Methods: Affinities of natGa-HA-DOTATATE and natGa-DOTATATE to sst1–5 were determined using membrane preparations and [125I]SST-28 as radioligand. Internalization into AR42J cells was studied in dual-tracer studies with [125I]TOC as internal reference. Biodistribution was investigated using AR42J tumor-bearing CD1 mice, and specificity of tracer uptake was confirmed in competition studies by coinjection of 0.8 mg TOC/kg. Results: Sst2 affinities (IC50) of [natGa]HA-DOTATATE (1.4 ± 0.8 nM, logP: −3.16) and [natGa]DOTATATE (1.2 ± 0.6 nM, logP: −3.69) were nearly identical. Both compounds displayed IC50 > 1 μM for sst1,3,4, while sst5 affinity was markedly increased for natGa-HA-DOTATATE (102 ± 65 nM vs >1 μM for natGa-DOTATATE). [natLu]HA-DOTATATE and [natLu]DOTATATE showed slightly lower, identical sst2 affinities (2.0 ± 1.6 and 2.0 ± 0.8 nM, respectively) and sst3 affinities of 93 ± 1 and 162 ± 16 nM. Internalization of [68Ga]HA-DOTATATE was tenfold higher than that of [125I]TOC but only sixfold higher for [68Ga]DOTATATE and [177Lu]HA-DOTATATE. While [68Ga]HA-DOTATATE and [68Ga]DOTATATE had shown similar target- and non-target uptake in patients, biodistribution studies in mice at 1 h post injection (n = 5) revealed slightly increased non-specific uptake of [68Ga]HA-DOTATATE in the blood, liver, and intestines (0.7 ± 0.3, 1.0 ± 0.2, and 4.0 ± 0.7 %iD/g vs 0.3 ± 0.1, 0.5 ± 0.1, and 2.7 ± 0.8 %iD/g for [68Ga]DOTATATE). However, sst-mediated accumulation of [68Ga]HA-DOTATATE in the pancreas, adrenals, and tumor was significantly enhanced (36.6 ± 4.3, 10.8 ± 3.2, and 33.6 ± 10.9 %iD/g vs 26.1 ± 5.0, 5.1 ± 1.4, and 24.1 ± 4.9 %iD/g, respectively). Consequently, tumor/background ratios for [68Ga]HA-DOTATATE in the AR42J model are comparable or slightly increased compared to [68Ga]DOTATATE. Conclusions: The present preclinical data fully confirm the general biodistribution pattern and excellent in vivo sst-targeting characteristics previously observed for [68Ga]HA-DOTATATE in patients. The effect of slightly enhanced lipophilicity on background accumulation and normal organ dose is compensated by the high uptake of [68Ga]HA-DOTATATE in tumor. Thus, [68Ga]HA-DOTATATE represents a fully adequate, freely available substitute for [68Ga]DOTATATE and, given the superb sst-targeting characteristics of [177Lu]HA-DOTATATE in vitro, potential applicability for sst-targeted PRRT.