Synthesis, Radiolabeling and In vitro GRP Receptor Targeting Studies of 99mTc‐Triaza‐X‐BBN[7‐14]NH2 (X=Serylserylserine, Glycylglycylglycine, Glycylserylglycine, or Beta Alanine)

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.

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

PURPOSE

The goal of this work was to develop hydrophilic gastrin-releasing peptide receptor (GRPR)-targeting complexes of the general formula fac-[M(CO)3(L)]+ [M = natRe, 99mTc, 186Re; L: NOTA for 1, NODAGA for 2] conjugated to a powerful GRPR peptide antagonist (DPhe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2) via a 6-aminohexanoic acid linker.

PROCEDURES

Metallated-peptides were prepared employing the [M(OH2)3(CO)3]+ [M = Re, 99mTc, 186Re] precursors. Re-1/2 complexes were characterized with HR-MS. IC50 studies were performed for peptides 1/2 and their respective Re-1/2 complexes in a binding assay utilizing GRPR-expressing human PC-3 prostate cancer cells and [125I]I-Tyr4-BBN as the competing ligand. The 99mTc/186Re-complexes were identified by HPLC co-injection with their Re-analogues. All tracers were challenged in vitro at 37 °C against cysteine/histidine (phosphate-buffered saline 10 mM, pH 7.4) and rat serum. Biodistribution and micro-SPECT/CT imaging of [99mTc]Tc-1/2 and [186Re]Re-2 were performed in PC-3 tumor-bearing ICR SCID mice.

RESULTS

High in vitro receptor affinity (IC50 2-3 nM) was demonstrated for all compounds. The 99mTc/186Re-tracers were found to be hydrophilic (log D7.4 ≤ - 1.35) and highly stable. Biodistribution in PC-3 xenografted mice revealed good tumor uptake (%ID/g at 1 h: 4.3 ± 0.7 for [99mTc]Tc-1, 8.3 ± 0.9 for [99mTc]Tc-2 and 4.2 ± 0.8 for [186Re]Re-2) with moderate retention over 24 h. Rapid renal clearance was observed for [99mTc]Tc-2 and [186Re]Re-2 (> 84 % at 4 h), indicating favorable pharmacokinetics. Micro-SPECT/CT images for the 99mTc-tracers clearly visualized PC-3 tumors in agreement with the biodistribution data and with superior imaging properties found for [99mTc]Tc-2.

CONCLUSIONS

[99mTc]Tc-2 shows promise for further development as a GRPR-imaging agent. [186Re]Re-2 demonstrated very similar in vivo behavior to [99mTc]Tc-2, and further studies are therefore justified to explore the theranostic potential of our approach for targeting of GRPR-positive cancers.

Novel [99mTc]-Tricarbonyl-NOTA-Conjugated Lactam-Cyclized Alpha-MSH Peptide with Enhanced Melanoma Uptake and Reduced Renal Uptake

The purpose of this study was to examine the melanoma targeting and imaging properties of ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex {1,4,7-triazacyclononane-1,4,7-triyl-triacetic acid-GlyGlyNle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH₂} and ^99mTc(CO)₃-NODAGA-GGNle-CycMSH_hex {1,4,7-triazacyclononane,1-gluteric acid-4,7-acetic acid-GlyGlyNle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH₂} on B16/F10 melanoma-bearing C57 mice to demonstrate the feasibility of NOTA/NODAGA as metal chelators for ^99mTc(CO)₃⁺ radiolabeling. NOTA/NODAGA-GGNle-CycMSH_hex were synthesized using fluorenylmethoxycarbonyl (Fmoc) chemistry. The melanocortin-1 (MC1) receptor binding affinities of the peptides were determined on B16/F10 melanoma cells. The biodistribution of ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex and ^99mTc(CO)₃-NODAGA-GGNle-CycMSH_hex were determined on B16/F10 melanoma-bearing C57 mice at 2 h postinjection to select a lead peptide for further evaluation. The melanoma targeting and imaging properties of ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex and ^99mTc(CO)₃-NODAGA-GGNle-CycMSH_hex were determined on B16/F10 melanoma-bearing C57 mice. The IC₅₀ values of NOTA/NODAGA-GGNle-CycMSH_hex were 0.8 ± 0.1 and 0.9 ± 0.1 nM on B16/F10 cells. ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex and ^99mTc(CO)₃-NODAGA-GGNle-CycMSH_hex were readily prepared via the [^99mTc(CO)₃(OH₂)₃]⁺ intermediate and displayed MC1R-specific binding on B16/F10 cells. ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex was further evaluated as a lead peptide because of its higher tumor uptake (19.76 ± 3.62% ID/g) and lower kidney uptake (1.59 ± 0.52% ID/g) at 2 h postinjection than ^99mTc(CO)₃-NODAGA-GGNle-CycMSH_hex. The B16/F10 melanoma uptake of ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex was 16.07 ± 4.47, 19.76 ± 3.62, 11.30 ± 2.81, and 3.16 ± 2.28% ID/g at 0.5, 2, 4, and 24 h postinjection, respectively. ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex showed high tumor to normal organ uptake ratios after 2 h postinjection. The B16/F10 melanoma lesions were clearly visualized by SPECT/CT using ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex as an imaging probe at 2 h postinjection. High tumor uptake, low kidney uptake, and fast urinary clearance of ^99mTc(CO)₃-NOTA-GGNle-CycMSH_hex highlighted its potential for melanoma imaging and facilitated the evaluation of ¹⁸⁸Re(CO)₃-NOTA-GGNle-CycMSH_hex for melanoma therapy.

NOTA and NODAGA [99mTc]Tc- and [186Re]Re-Tricarbonyl Complexes: Radiochemistry and First Example of a [99mTc]Tc-NODAGA Somatostatin Receptor-Targeting Bioconjugate

With the long-term goal of developing theranostic agents for applications in nuclear medicine, in this work we evaluated the well-known NOTA and NODAGA chelators as bifunctional chelators (BFCs) for the [^99mTc/¹⁸⁶Re]Tc/Re-tricarbonyl core. In particular, we report model complexes of the general formula fac-[M(L)(CO)₃]⁺ (M = Re, ^99mTc, ¹⁸⁶Re) where L denotes NOTA-Pyr (1) or NODAGA-Pyr (2), which are derived from conjugation of NOTA/NODAGA with pyrrolidine (Pyr). Further, as proof-of-principle, we synthesized the peptide bioconjugate NODAGA-sst₂-ANT (3) and explored its complexation with the fac-[Re(CO)₃]⁺ and fac-[^99mTc][Tc(CO)₃]⁺ cores; sst₂-ANT denotes the somatostatin receptor (SSTR) antagonist 4-NO₂-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH₂. Rhenium complexes Re-1 through Re-3 were synthesized and characterized spectroscopically, and receptor binding affinity was demonstrated for Re-3 in SSTR-expressing cells (AR42J, IC₅₀ = 91 nM). Radiolabeled complexes [^99mTc]Tc/[¹⁸⁶Re]Re-1/2 and [^99mTc]Tc-3 were prepared in high radiochemical yield (>90%, determined by radio-HPLC) by reacting [^99mTc]/[¹⁸⁶Re][Tc/Re(OH₂)₃(CO)₃]⁺ with 1-3 and correlated well with the respective Re-1 through Re-3 standards in comparative HPLC studies. All radiotracers remained intact through 24 h (^99mTc-labeled complexes) or 48 h (¹⁸⁶Re-labeled complexes) against 1 mM l-histidine and 1 mM l-cysteine (pH 7.4, 37 °C). Similarly, rat serum stability studies displayed no decomposition and low nonspecific binding of 9-24% through 4 h. Biodistribution of [^99mTc]Tc-3 in healthy CF-1 mice demonstrated a favorable pharmacokinetic profile. Rapid clearance was observed within 1 h post-injection, predominantly via the renal system (82% of the injected dose was excreted in urine by 1 h), with low kidney retention (% ID/g: 11 at 1 h, 5 at 4 h, and 1 at 24 h) and low nonspecific uptake in other organs/tissues. Our findings establish NOTA and NODAGA as outstanding BFCs for the fac-[M(CO)₃]⁺ core in the design and development of organometallic radiopharmaceuticals. Future in vivo studies of [^99mTc]Tc- and [¹⁸⁶Re]Re-tricarbonyl complexes of NODAGA/NOTA-biomolecule conjugates will further probe the potential of these chelates for nuclear medicine applications in diagnostic imaging and targeted radiotherapy, respectively.

APPLICATION OF TECHNETIUM AND RHENIUM IN NUCLEAR MEDICINE

Technetium and Rhenium are the two lower elements in the manganese triad. Whereas rhenium is known as an important part of high resistance alloys, technetium is mostly known as a cumbersome product of nuclear fission. It is less known that its metastable isotope 99mTc is of utmost importance in nuclear medicine diagnosis. The technical application of elemental rhenium is currently complemented by investigations of its isotope 188Re , which could play a central role in the future for internal, targeted radiotherapy. This article will briefly describe the basic principles behind diagnostic methods with radionuclides for molecular imaging, review the 99mTc -based radiopharmaceuticals currently in clinical routine and focus on the chemical challenges and current developments towards improved, radiolabeled compounds for diagnosis and therapy in nuclear medicine.

Structural modifications of ⁹⁹mTc-labelled bombesin-like peptides for optimizing pharmacokinetics in prostate tumor targeting

PURPOSE

The main goal of the present study was to investigate the importance of the addition of a positively charged aa in the naturally occurring bombesin (BN) peptide for its utilization as radiodiagnostic agent, taking into consideration the biodistribution profile, the pharmacokinetic characteristics and the tumor targeting ability.

METHODS

Two BN-derivatives of the general structure [M-chelator]-(spacer)-BN(2-14)-NH(2), where M: (99m)Tc or (185/187)Re, chelator: Gly-Gly-Cys-, spacer: -(arginine)(3)-, M-BN-A; spacer: -(ornithine)(3)-, M-BN-O; have been prepared and evaluated as tumor imaging agents.

RESULTS

The peptides under study presented high radiolabelling efficiency (>98%), significant stability in human plasma (>60% intact radiolabelled peptide after 1h incubation) and comparable receptor binding affinity with the standard [(125)I-Tyr(4)]-BN. Their internalization rates in the prostate cancer PC-3 cells differed, although the amount of internalized peptide was the same. The biodistribution and the dynamic γ-camera imaging studies in normal and PC-3 tumor-bearing SCID mice have shown significant tumor uptake, combined with fast blood clearance, through the urinary pathway.

CONCLUSION

The addition of the charged aa spacer in the BN structure was advantageous for biodistribution, pharmacokinetics and tumor targeting ability, because it reduced the upper abdominal radioactivity levels and increased tumor/normal tissue contrast ratios.

Spacer site modifications for the improvement of the in vitro and in vivo binding properties of (99m)Tc-N(3)S-X-bombesin[2-14] derivatives

It has been shown that gastrin releasing peptide receptors (GRPRs) are overexpressed in various types of cancer cells. Bombesin is an analogue of the mammalian GRP that binds with high specificity and affinity to GRPRs. Significant research efforts have been lately devoted to the design of radiolabeled 8 or 14 aminoacid bombesin (BN) peptides for the detection (either with gamma or positron emitting radionuclides) and therapy (with beta(-) emitting radionuclides) of cancer. The specific aim of the present study was to further investigate the radiolabeled peptide structure and to determine whether the total absence of a linker or the use of a basic diverse amino acid linker could influence the biodistribution profile of the new compounds for specific targeting of human prostate cancer. Thus, two new derivatives with the structure Gly-Gly-Cys-X-BN[2-14], where linker X is either zero (I) or Orn-Orn-Orn (Orn: ornithine) (II) were designed and synthesized. The corresponding (99m)Tc-BN derivatives were obtained with high radiochemical yield (>98%) and had almost identical retention times in RP-HPLC with the (185/187)Re complexes, which were also characterized by ESI-MS. Metabolic stability was found to be high in human plasma, moderate in PC-3 cells, and rather low in mouse liver and kidney homogenates for both BN derivatives studied. The BN derivative without the spacer was less stable in cell culture and liver homogenates. A satisfactory binding affinity to GRPRs, in the nanomolar range, was obtained for both BN derivatives as well as for their Re complexes, with BN (II) demonstrating the highest one. In vitro internalization/externalization assays indicated that approximately 6% of BN (I) and approximately 25% of BN (II) were internalized into PC-3 cells. In vivo evaluation in normal Swiss mice and in tumor bearing SCID mice showed that BN (II) presented higher tumor and pancreas uptake than BN (I). Small animal SPECT dynamic imaging, carried out after an injection of BN (II) in mice bearing PC-3 tumors, resulted in PC-3 tumor delineation with low background activity. Overall, this study performed for two new N(3)S-X-BN[2-14] derivatives indicated that hydrophilicity and charge strongly affected the in vitro and in vivo binding properties and the biodistribution pattern. This finding is confirmed by SPECT imaging of BN (II), which is under further in vivo evaluation for detecting cancer-positive GRPRs.

99mTc(CO)3-DTMA bombesin conjugates having high affinity for the GRP receptor

INTRODUCTION

Targeted diagnosis of specific human cancer types continues to be of significant interest in nuclear medicine. 99mTc is ideally suited as a diagnostic radiometal for in vivo tumor targeting due to its ideal physical characteristics and diverse labeling chemistries in numerous oxidation states.

METHODS

In this study, we report a synthetic approach toward design of a new tridentate amine ligand for the organometallic aqua-ion [99mTc(H2O)3(CO)3]+. The new chelating ligand framework, 2-(N,N'-Bis(tert-butoxycarbonyl)diethylenetriamine) acetic acid (DTMA), was synthesized from a diethylenetriamine precursor and fully characterized by mass spectrometry and nuclear magnetic resonance spectroscopy (1H and 13C). DTMA was conjugated to H2N-(X)-BBN(7-14)NH2, where X=an amino acid or aliphatic pharmacokinetic modifier and BBN=bombesin peptide, by means of solid phase peptide synthesis. DTMA-(X)-BBN(7-14)NH2 conjugates were purified by reversed-phase high-performance chromatography and characterized by electrospray-ionization mass spectrometry.

RESULTS

The new conjugates were radiolabeled with [99mTc(H2O)3(CO)3]+ produced via Isolink radiolabeling kits to produce [99mTc(CO)3-DTMA-(X)-BBN(7-14)NH2]. Radiolabeled conjugates were purified by reversed-phase high-performance chromatography. Effective receptor binding behavior was evaluated in vitro and in vivo.

CONCLUSIONS

[99mTc(CO)3-DTMA-(X)-BBN(7-14)NH2] conjugates displayed very high affinity for the gastrin releasing peptide receptor in vitro and in vivo. Therefore, these conjugates hold some propensity to be investigated as molecular imaging agents that specifically target human cancers uniquely expressing the gastrin releasing peptide receptor subtypes.