NODAGATOC, a new chelator-coupled somatostatin analogue labeled with [^67/68Ga] and [¹¹¹In] for SPECT, PET, and targeted therapeutic applications of somatostatin receptor (hsst2) expressing tumors

A monoreactive NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) derived prochelator (1-(1-carboxy 3-carbo-tert-butoxypropyl)-4,7-(carbo-tert-butoxymethyl)-1,4,7- triazacyclononane (NODAGA(tBu)₃)) was synthesized in five steps with an overall yield of 21%. It is useful for the coupling to the N-terminus of peptides on solid phase and in solution; it was coupled to [Tyr³]-octreotide (TOC) on solid phase, and the resulting peptide, NODAGA-Tyr³-octreotide (NODAGATOC), was labeled with the radiometals rain and ⁶⁷Ga in high yields and good specific activities. [⁶⁷Ga] - and [¹¹¹In] - NODAGA-Tyr³-octreotide appear to be useful to visualize primary tumors and metastases which express somatostatin receptors subtype 2 (sstr2), such as neuroendocrine tumors, because of their high affinity to this receptor subtype with IC₅₀ = 3.5 ± 1.6 nM and 1.7 ± 0.2 nM, respectively. NODAGATOC could be used as a SPECT and PET tracer, when labeled with ¹¹¹In, ⁶⁷Ga, or ⁶⁸Ga, and even for therapeutic applications. Surprisingly, [¹¹¹In] - NODAGATOC shows 2 times higher binding affinity to sstr2, but also a factor of 4 higher affinity to sstr5 compared to [⁶⁷Ga] - NODAGATOC. [⁶⁷Ga] - NODAGATOC is very stable in serum and rat liver homogenate. There is no difference in the rate of internalization into AR4-2J rat pancreatic tumor cells; both radioligands are highly internalized, at 4 h a 3 times higher uptake compared to [¹¹¹In] - DOTA-Tyr³-octreotide ([¹¹¹In] - DOTATOC) was found. The biodistribution of [⁶⁷Ga] - NODAGATOC in AR4-2J tumor bearing nude mice is very favorable at short times after injection; there is fast excretion from all nontarget organs except the kidneys and high uptake in sst receptor rich organs and in the AR4-2J tumor. Again it is superior to [¹¹¹In] - DOTATOC in this respect. The results indicate an improved biological behavior which is likely due to the fact that an additional spacer group separates the chelate from the pharmacophoric part of the somatostatin analogue.