Development and Preclinical Evaluation of 99mTc- and 186Re-Labeled NOTA and NODAGA Bioconjugates Demonstrating Matched Pair Targeting of GRPR-Expressing Tumors

NOTA and NODAGA Radionuclide Complexing Agents: Versatile Approaches for Advancements in Radiochemistry

Effective molecular imaging and targeted cancer therapy rely on receptor-specific targeted delivery systems that are both metabolically stable and kinetically inert for optimal in vivo performance. Until now, no single metal complexing agent has demonstrated the versatility to coordinate metals across the periodic table while maintaining the kinetic inertness required for clinical theranostic applications. Therefore, enhancing the in vivo kinetic stability of radiolabeled, cell-targeting, biologically active compounds remains a critical goal to minimize unintended accumulation of radioactivity in collateral tissues. This review describes the usage of NOTA [NOTA = 1,4,7-triazacyclononane-1,4,7-triacetic acid] and derivatives of NOTA, a metal complexing agent that has been found to have the ability to effectively coordinate with a wide range of radiometals, including metal-radiohalogens, to form stable complexes. This enables the development of new cell-targeting small molecule and peptide conjugates with the potential to resist demetallation in vivo, thereby reducing radionuclide uptake in non-target tissues. Herein, we discuss the design and development of NOTA-based, cell-targeting, small molecules having very high affinity and selectivity for the GRPR (Gastrin-Releasing Peptide Receptor), the SSTR2 (Somatostatin Receptor Subtype 2), and the MC1R (Melanocortin-1) receptors that are present on the surfaces of numerous solid primary human tumors and their metastatic counterparts.

[186Re]Re- and [99mTc]Tc-Tricarbonyl Metal Complexes with 1,4,7-Triazacyclononane-Based Chelators Bearing Amide, Alcohol, or Ketone Pendent Groups

1,4,7-Triazacyclononane (TACN)-based chelators, such as NOTA and NODAGA, have shown great promise as bifunctional chelators for [M(CO)3]+ cores (M = 99mTc and 186Re) in radiopharmaceutical development. Previous investigations of TACN-based chelators bearing pendent acid and ester arms demonstrated the important role the pendent arms have in successful coordination of the [M(CO)3]+ core with the TACN backbone nitrogens. In this work, we introduce three TACN-based bifunctional chelators bearing amide, alcohol, and ketone pendent arms and evaluate their (radio)labeling efficiency with the [M(CO)3]+ core as well as the in vitro stability and hydrophilicity of the resulting radiometal complexes. Following their synthesis and characterization, the amide (2) and alcohol (3) chelators were successfully labeled with the [M(CO)3]+ cores (M = natRe, 99mTc, and 186Re), while the ketone (4) was not successfully labeled. Radiometal complexes M-2 and M-3 demonstrated hydrophilic character in logD7.4 studies as well as excellent stability in phosphate-buffered saline (pH 7.4), l-histidine, l-cysteine, and rat serum at 37 °C through 24 h. While the hydrophilicity and stability of these radiocomplexes are attractive, future TACN chelator design modifications to increase radiolabeling yields under milder reaction conditions would improve their potential for use in development of [M(CO)3]+ radiopharmaceuticals.

1,4,7-Triazacyclononane-Based Chelators for the Complexation of [186Re]Re- and [99mTc]Tc-Tricarbonyl Cores

Metal complexes with the general formula [MI(CO)3(k3-L)]+, where M = Re, 186Re, or 99mTc and L = 1,4,7-triazacyclononane (TACN), NOTA, or NODAGA chelators, have previously been conjugated to peptide-based biological targeting vectors and investigated as potential theranostic radiopharmaceuticals. The promising results demonstrated by these bioconjugate complexes prompted our exploration of other TACN-based chelators for suitability for (radio)labeling with the [M(CO)3]+ core. In this work, we investigated the role of the TACN pendant arms in complexation of the [M(CO)3]+ core through (radio)labeling of TACN chelators bearing acid, ester, mixed acid-ester, or no pendant functional groups. The chelators were synthesized from TACN, characterized, and (radio)labeled with nonradioactive Re-, [186Re]Re-, and [99mTc]Tc-tricarbonyl cores. The nonfunctionalized (3), diacid (4), and monoacid monoester (7 and 8) chelators underwent direct labeling, while the diester (M-5 and M-6) complexes required indirect synthesis from M-4. All six chelators demonstrated stable radiometal coordination. The ester-bearing derivatives, which exhibited more lipophilic character than their acid-bearing counterparts, were prone to ester hydrolysis over time, making them less suitable for radiopharmaceutical development. These studies confirmed that the TACN pendant functional groups were key to efficient labeling with the [M(CO)3]+ core, with ionizable pendant arms favored over nonionizable pendant arms.

Optimization of the Pharmacokinetic Profile of [99mTc]Tc-N4-Bombesin Derivatives by Modification of the Pharmacophoric Gln-Trp Sequence

Current radiolabeled gastrin-releasing peptide receptor (GRPR) ligands usually suffer from high accumulation in GRPR-positive organs (pancreas, stomach), limiting tumor-to-background contrast in the abdomen. In novel N4-bombesin derivatives this was addressed by substitutions at the Gln7-Trp8 site within the MJ9 peptide (H-Pip5-phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Sta13-Leu14-NH2) either by homoserine (Hse7), β-(3-benzothienyl) alanine (Bta8) or α-methyl tryptophan (α-Me-Trp8), with the aim of optimizing pharmacokinetics. We prepared and characterized the peptide conjugates 6-carboxy-1,4,8,11-tetraazaundecane (N4)-asp-MJ9, N4-asp-[Bta8]MJ9, N4-[Hse7]MJ9 and N4-[α-Me-Trp8]MJ9, and evaluated these compounds in vitro (GRPR affinity via IC50,inverse; internalization; lipophilicity via logD7.4) and in vivo (biodistribution and μSPECT/CT studies at 1 h post injection (p.i.) in PC-3 tumor-bearing CB17-SCID mice). 99mTc-labeling resulted in radiochemical yields (RCYs) > 95%. All 99mTc-labeled MJ9 analogues showed comparable or higher GRPR affinity than the external reference [99mTc]Tc-Demobesin 4. Receptor-bound fractions were noticeably higher than that of the reference. Despite a slightly enhanced lipophilicity, all novel MJ9 derivatives revealed improved in vivo pharmacokinetics compared to the reference. The Bta8-modified ligand revealed the most favorable tumor-to-abdomen contrast at 1 h p.i. Substitutions at the Gln7-Trp8 site within GRPR ligands hold great potential to modify pharmacokinetics for improved imaging.

Calix[4]arene-Analogous Technetium Supramolecules

Calix[4]arene-analogous technetium supramolecules (1 and 2) were assembled using (NBu₄)[Tc₂(μ-Cl)₃(CO)₆] and neutral flexible bidentate nitrogen-donor ligands (L¹ and L²) consisting of four arene units covalently joined via methylene units. The neutral homoleptic technetium macrocycles adopt a partial cone/cone-shaped conformation in the solid state. These supramolecules are the first example of fac-[Tc(CO)₃]⁺ core-based metallocalix[4]arenes and second example of fac-[Tc(CO)₃]⁺ core-based metallomacrocycles. Structurally similar fac-[Re(CO)₃]⁺ core-based macrocycles (3 and 4) were also prepared using [Re(CO)₅X] (where X = Cl or Br) and L¹ or L². The products were characterized spectroscopically and by X-ray analysis.

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

Synthesis and evaluation of new mixed "2 + 1" Re, 99mTc and 186Re tricarbonyl dithiocarbamate complexes with different monodentate ligands

A series of "2 + 1" mixed ligand tricarbonyl complexes of the general formula fac-[Re/^99mTc/¹⁸⁶Re(CO)₃(DDTC)(L)] containing diethyldithiocarbamate (DDTC) as a monoanionic bidentate ligand and a series of monodentate ligands L was synthesized, characterized and evaluated. The impact of ligand L on the radiochemical yield (RCY) and biodistribution of the final compounds was also investigated. DDTC and the appropriate L ligand [cyclohexyl isocyanide (cisc), tert-butyl isocyanide (tbi), triphenylphosphine (PPh₃), methyldiphenylphosphine (PPh₂Me), triphenylarsine (AsPh₃), imidazole (im), and 4-aminopyridine (4AP)] readily reacted in equimolar amounts with the [Et₄N]₂[Re(CO)₃Br₃] precursor to afford fac-[Re(CO)₃(DDTC)(cisc)], Re1, fac-[Re(CO)₃(DDTC)(tbi)], Re2, fac-[Re(CO)₃(DDTC)(PPh₃)], Re3, fac-[Re(CO)₃(DDTC)(PPh₂Me)], Re4, fac-[Re(CO)₃(DDTC)(AsPh₃)], Re5, fac-[Re(CO)₃(DDTC)(im)], Re6 and fac-[Re(CO)₃(DDTC)(4AP)], Re7, complexes in high yields (>80%). All Re complexes were fully characterized by IR, NMR, and in addition Re4, Re5, and Re7 with X-ray crystallography. Analogous reactions as performed with Re were subsequently explored on the ^99mTc and ¹⁸⁶Re-tracer levels using the corresponding fac-[^99mTc/¹⁸⁶Re(CO)₃(H₂O)₃]⁺ precursor. Complexes ^99mTc1 - ^99mTc5, ¹⁸⁶Re1 and ¹⁸⁶Re3 were obtained in high radiochemical yield (>91%), while the complexes ^99mTc6, ^99mTc7 and ¹⁸⁶Re7 formed with radiochemical yields of 55%, 28%, and 75%, respectively. The ^99mTc and ¹⁸⁶Re-complexes were characterized by comparative HPLC analysis using the analogous Re complexes. During histidine and cysteine challenge experiments at 37 °C through 6 h, complexes ^99mTc1 - ^99mTc5 remained > 92% stable, while complexes ^99mTc6 and ^99mTc7 remained only 8% stable through 3 h. Similar studies for ¹⁸⁶Re-complexes showed that ¹⁸⁶Re1 and ¹⁸⁶Re3 remained > 95% stable for up to 48 h, while ¹⁸⁶Re7 had decreased to 7% after 3 h. LogD_7.4 data of ^99mTc1 - ^99mTc5, ¹⁸⁶Re1, and ¹⁸⁶Re3 complexes, which ranged from 2.59 to 3.39, suggested high lipophilicity. Biodistribution studies in healthy Swiss albino mice showed hepatobiliary excretion for ^99mTc1, ^99mTc2, and ^99mTc4, fast blood clearance for ^99mTc4, while high liver uptake and retention for ^99mTc3 and ^99mTc5 were measured. Moreover, ^99mTc2 showed high accumulation in the lungs with sustained retention (52.80% ID/g at 4 h p.i.) and significant brain uptake at 2 min p.i. (1.89% ID/g). The study showed the great influence of monodentate ligand in the synthesis and biodistribution of the mixed ligand complexes.

A New, Second Generation Trithiol Bifunctional Chelate for 72,77As: Trithiol(b)-(Ser)2-RM2

Trithiol chelates are suitable for labeling radioarsenic (72As: 2.49 MeV β+, 26 h; 77As: 0.683 MeV β-, 38.8 h) to form potential theranostic radiopharmaceuticals for positron emission tomography (PET) imaging and therapy. A trithiol(b)-(Ser)2-RM2 bioconjugate and its arsenic complex were synthesized and characterized. The trithiol(b)-(Ser)2-RM2 bioconjugate was radiolabeled with no-carrier-added 77As in over 95% radiochemical yield and was stable for over 48 h, and in vitro IC50 cell binding studies of [77As]As-trithiol(b)-(Ser)2-RM2 in PC-3 cells demonstrated high affinity for the gastrin-releasing peptide (GRP) receptor (low nanomolar range). Limited biodistribution studies in normal mice were performed with HPLC purified 77As-trithiol(b)-(Ser)2-RM2 demonstrating both pancreatic uptake and hepatobiliary clearance.

Preclinical Evaluation of Novel 64Cu-Labeled Gastrin-Releasing Peptide Receptor Bioconjugates for PET Imaging of Prostate Cancer

We report herein the preclinical evaluation of new [⁶⁴Cu]Cu-gastrin-releasing peptide receptor (GRPR)-targeting tracers, employing the potent peptide antagonist _DPhe-Gln-Trp-Ala-VaI-Gly-His-Sta-Leu-NH₂ conjugated to NOTA (in 1) or NODAGA (in 2) chelators via a 6-aminohexanoic acid linker. The Cu-1/2 metalated peptides were synthesized by reacting 1/2 with CuCl₂ and were characterized by LC-ESI-MS and HR-ESI-MS. Cu-1/2 exhibited high GRPR-binding affinities with IC₅₀ values <3 nM, as measured in a competition assay using the GRPR-expressing human PC-3 prostate cancer cell line and [¹²⁵I]I-Tyr⁴-BBN as the competing ligand. Tracers [⁶⁴Cu]Cu-1/2 were prepared in quantitative radiochemical yield (by radio-HPLC), and their identities were confirmed by coelution with their Cu-1/2 standards via comparative HPLC studies. Lipophilicity was measured in 1-octanol/PBS (pH 7.4), and the negative log D_7.4 values (≤-1) confirmed the anticipated hydrophilic character for [⁶⁴Cu]Cu-1/2. Both tracers demonstrated excellent in vitro stability, with ≥98% remaining intact through 24 h at physiological conditions (PBS, pH 7.4, 37 °C). Biodistribution in PC-3 tumor-bearing mice demonstrated good tumor uptake (%ID/g at 4 h: 4.34 ± 0.71 for [⁶⁴Cu]Cu-1, 3.92 ± 1.03 for [⁶⁴Cu]Cu-2) and rapid renal clearance (≥87% ID at 4 h). Tumor uptake was receptor-mediated, as verified by parallel GRPR-blocking studies. Small-animal PET/CT imaging studies validated the biodistribution data. These preclinical data support that the [⁶⁴Cu]Cu-1/2 tracers show promise for further development as diagnostic PET imaging agents of GRPR-expressing tumors.