Novel DOTA-Neurotensin Analogues for ¹¹¹In Scintigraphy and ⁶⁸Ga PET Imaging of Neurotensin Receptor-Positive Tumors

Abstract

Overexpression of the high affinity neurotensin receptor 1 (NTSR1), demonstrated in several human cancers, has been proposed as a new marker for human ductal pancreatic carcinoma and as an independent factor for poor prognosis for ductal breast cancer, head and neck squamous cell carcinoma, and non-small cell lung cancer. The aim of the present study was to develop new DOTA-neurotensin analogues for positron emission tomography (PET) imaging with (68)Ga and for targeted radiotherapy with (90)Y or (177)Lu. We synthesized a DOTA-neurotensin analogue series. Two of these peptides bear two sequence modifications for metabolic stability: DOTA-NT-20.3 shares the same peptide sequence as the previously described DTPA-NT-20.3. In the sequence of DOTA-NT-20.4, the Arg(8)-Arg(9) bond was N-methylated instead of the Pro(7)-Arg(8) bond in DOTA-NT-20.3. An additional sequence modification was introduced in DOTA-LB119 to increase stability. A spacer was added between DOTA and the peptide sequence to increase affinity. Binding to HT29 cells, which express NTSR1, in vivo stability, and biodistribution of the various analogues were compared, and the best candidate was used to image tumors of various sizes with the microPET in mice. (111)In-DOTA-NT-20.3, in spite of a relatively high uptake in kidneys, showed specific tumor uptake and elevated tumor to other organ uptake ratios. High contrast images were obtained at early time points after injection that allowed tumor detection at a time interval postinjection appropriate for imaging with the short-lived radionuclide (68)Ga. (111)In-DOTA-NT-20.4 displayed inferior binding to HT29 cells and reduced tumor uptake. (111)In-DOTA-LB119 displayed at early time points a significantly lower renal uptake but also a lower tumor uptake than (111)In-DOTA-NT-20.3, although binding to HT29 cells was similar. (68)Ga-DOTA-NT-20.3 displayed higher tumor uptake than (68)Ga-DOTA-LB119 and allowed the detection of very small tumors by PET. In conclusion, DOTA-NT-20.3 is a promising candidate for (68)Ga-PET imaging of neurotensin receptor-positive tumors. DOTA-NT-20.3 may also be considered for therapy, as the yttrium-labeled peptide has higher affinity than that of the indium-labeled one. A prerequisite for therapeutic application of this neurotensin analogue would be to lower kidney uptake, for example, by infusion of basic amino acids, gelofusin, or albumin fragments, to prevent nephrotoxicity, as with radiolabeled somatostatin analogues.

New neurotensin analogue radiolabeled by 99m‐technetium as a potential agent for tumor identification

“…It seems that the exchange reaction of tricine with plasma proteins is a major reason of protein binding, and it is not related to structural conformational changes caused by the BFCAs. [24] The radioconjugate showed nanomolar affinity toward the NTR receptor (32.66 ± 4.01 nm), but this affinity is worse than affinities reported for this pharmacophore by Alshoukr et al [19] This deficiency probably is due to the addition of β-alanine in the structure of peptide and not related to changes performed in radiometal, BFCA, and coligands. Alshoukr et al have reported similar affinities for this pharmacophore through labeling by different radioisotopes and BFCA (16 ± 2 nm for 111 In-DTPA-NT-20.3a, 15 ± 1 nm for 111 In-DOTA-NT-20.3 and 14 ± 2 nm (68) Ga-DOTA-NT-20.3).…”

“…Based on the research of Alshoukr et al, the stability and pharmacokinetic of NT analogues were improved by substitution of Ile residue with Tle (on the cleavage bond 11-12). [2,19] In this research,…”

“…But the affinity reported for (90)Y-DOTA-NT-20.3 analogue (5.6 ± 0.7 nm) challenges this hypothesis. [19] Biodistribution studies exhibited fast blood clearance mainly through the urinary system. Except the kidneys, there was rapid clearance of radiopeptide from NT receptor-negative organs/tissues.…”

“…[2] NT analogues are extensively labeled via various radionuclides like 18 F, 64 Cu, 68 Ga, 99 m Tc, and 111 In, and they are evaluated for the imaging of NT receptors' overexpression in different tumor types. [9,[16][17][18][19] Among these radionuclides, labeling through 99m Tc due to favorite physical properties of radionuclide and abundant SPECT apparatus is preferred. So that NT analogue preserves its affinity to the receptor during the radiolabeling reaction, labeling should be performed through bifunctional chelating agents (BFCAs).…”

Neuropeptide Receptors in Pain Circuitries: Useful Targets for CNS Imaging with Non-Peptide Ligands Suitable for PET?

“…The 13 a.a.r. peptide neurotensin (ELYENKPRRPYIL) is a neurotransmitter as well as neuromodulator in the brain and a paracrine or circulating hormone in the periphery [110] [111]. Neurotensin is facilitating pain if applied in low doses (0.03 pM -0.03 nM; [119] [120]) to the rostral ventromedial medulla (RVM) [121] but revealed an analgesic effect at higher dosage [10].…”