Uptake kinetics of the somatostatin receptor ligand [ 86 Y]DOTA- d Phe 1 -Tyr 3 -octreotide ([ 86 Y]SMT487) using positron emission tomography in non-human primates and calculation of radiation doses of the 90 Y-labelled analogue

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Abstract
[90Y]DOTA-dPhe1-Tyr3-octreotide ([90Y]-SMT487) has been suggested as a promising radiotherapeutic agent for somatostatin receptor-expressing tumours. In order to quantify the in vivo parameters of this compound and the radiation doses delivered to healthy organs, the analogue [86Y]DOTA-dPhe1-Tyr3-octreotide was synthesised and its uptake measured in baboons using positron emission tomography (PET). [86Y]DOTA-dPhe1-Tyr3-octreotide was administered at two different peptide concentrations, namely 2 and 100 µg peptide per m2 body surface. The latter concentration corresponded to a radiotherapeutic dose. In a third protocol [86Y]DOTA-dPhe1-Tyr3-octreotide was injected in conjunction with a simultaneous infusion of an amino acid solution that was high in l-lysine in order to lower the renal uptake of radioyttrium. Quantitative whole-body PET scans were recorded to measure the uptake kinetics for kidneys, liver, lung and bone. The individual absolute uptake kinetics were used to calculate the radiation doses for [90Y]DOTA-dPhe1-Tyr3-octreotide according to the MIRD recommendations extrapolated to a 70-kg human. The highest radiation dose was received by the kidneys, with 2.1–3.3 mGy per MBq [90Y]DOTA-dPhe1-Tyr3-octreotide injected. For the 100 µg/m2 SMT487 protocol with amino acid co-infusion this dose was about 20%–40% lower than for the other two treatment protocols. The liver and the red bone marrow received doses ranging from 0.32 to 0.53 mGy and 0.03 to 0.07 mGy per MBq [90Y]DOTA-dPhe1-Tyr3-octreotide, respectively. The average effective dose equivalent amounted to 0.23–0.32 mSv/MBq. The comparatively low estimated radiation doses to normal organs support the initiation of clinical phase I trials with [90Y]DOTA-dPhe1-Tyr3-octreotide in patients with somatostatin receptor-expressing tumours.

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