The addition of DTPA to [177Lu-DOTA0,Tyr3]octreotate prior to administration reduces rat skeleton uptake of radioactivity

Peptide receptor-targeted radionuclide therapy is nowadays also being performed with DOTA-conjugated peptides, such as [DOTA(0),Tyr(3)]octreotate, labelled with radionuclides like (177)Lu. The incorporation of (177)Lu is typically >/=99.5%; however, since a total patient dose can be as high as 800 mCi, the amount of free (177)Lu(3+) (= non-DOTA-incorporated) can be substantial. Free (177)Lu(3+) accumulates in bone with unwanted irradiation of bone marrow as a consequence. (177)Lu-DTPA is reported to be stable in serum in vitro, and in vivo it has rapid renal excretion. Transforming free Lu(3+) to Lu-DTPA might reroute this fraction from accumulation in bone to renal clearance. We therefore investigated: (a) the biodistribution in rats of (177)LuCl(3), [(177)Lu-DOTA(0),Tyr(3)]octreotate and (177)Lu-DTPA; (b) the possibilities of complexing the free (177)Lu(3+) in [(177)Lu-DOTA(0),Tyr(3)]octreotate to (177)Lu-DTPA prior to intravenous injection; and (c) the effects of free (177)Lu(3+) in [(177)Lu-DOTA(0),Tyr(3)]octreotate, in the presence and absence of DTPA, on the biodistribution in rats. (177)LuCl(3) had high skeletal uptake (i.e. 5% ID per gram femur, with localization mainly in the epiphyseal plates) and a 24-h total body retention of 80% injected dose (ID). [(177)Lu-DOTA(0),Tyr(3)]octreotate had high and specific uptake in somatostatin receptor-positive tissues, and 24-h total body retention of 19% ID. (177)Lu-DTPA had rapid renal clearance, and 24-h total body retention of 4% ID. Free (177)Lu(3+) in [(177)Lu-DOTA(0),Tyr(3)]octreotate could be complexed to (177)Lu-DTPA. Accumulation of (177)Lu in femur, blood, liver and spleen showed a dose relation to the amount of free (177)Lu(3+), while these accumulations could be normalized by the addition of DTPA. After labelling [DOTA(0),Tyr(3)]octreotate with (177)Lu the addition of DTPA prior to intravenous administration of [(177)Lu-DOTA(0),Tyr(3)]octreotate is strongly recommended.