MO tripeptide diastereomers (M=99/99mTc, Re): models to identify the structure of 99mTc peptide targeted 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.

Evaluation of Re/99mTc-labeled somatostatin receptor-targeting peptide complexes synthesized via direct metal cyclization

With interest in the development of somatostatin receptor (SSTR) targeting agents for potential application in diagnostic SPECT imaging (99mTc) or Peptide Radionuclide Receptor Therapy (PRRT, 186Re or 188Re) of neuroendocrine tumors, we present herein 99mTc/Re (radio)complexes synthesized by the integrated (radio)labeling approach of peptide cyclization via metal complexation. In particular, we utilized the potent SSTR2 peptide antagonist sequence DOTA-4-NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH2 (DOTA-sst2-ANT) and report the syntheses and in vitro evaluations of its respective [99mTc]Tc/Re-cyclized peptides ([99mTc]Tc/Re-cyc-DOTA-sst2-ANT). The Re-cyc-DOTA-sst2-ANT complex was synthesized via an on-resin Re(V)-cyclization reaction using the ReOCl3(PPh3)2 precursor and consisted of three isomers characterized by LC–ESI-MS. The [99mTc]Tc-cyclized analogue was prepared via a ligand exchange reaction of the [99mTc][TcO]3+ core through a [99mTc]Tc-glucoheptonate intermediate with linear DOTA-sst2-ANT and was characterized by comparative HPLC studies against Re-cyc-DOTA-sst2-ANT. Good in vitro binding affinity was demonstrated in SSTR-expressing cells (AR42J) by the Re-cyc-DOTA-sst2-ANT major isomer, similar to the potent binder Lu-DOTA-sst2-ANT, in which the Lu metal was complexed by the bifunctional chelator DOTA versus via peptide cyclization. [99mTc]Tc-cyc-DOTA-sst2-ANT was obtained in high radiochemical yield, also with an elution pattern of three isomers observed by HPLC analysis, which were comparable yet not identical to those of Re-cyc-DOTA-sst2-ANT. The [99mTc]Tc-tracer complex was shown to be hydrophilic, and stability studies at 4 h demonstrated that it remained intact in both PBS and in rat serum, with low non-specific rat serum protein binding, while exhibiting more moderate stability in 1 mM cysteine. These findings demonstrate that direct Re/[99mTc]Tc-cyclization of DOTA-sst2-ANT is feasible and may be used as an alternative approach to the bifunctional chelate labeling strategy. However, given that the non-radioactive (Re) and radiotracer (99mTc) analogues are not identical and both form isomeric products in equilibrium, additional design modifications will be necessary prior to in vivo application of [99mTc]Tc/Re-cyc-DOTA-sst2-ANT.

Synthesis, characterization and biological studies of rhenium, technetium-99m and rhenium-188 pentapeptides

A pentapeptide macrocyclic ligand, KYCAR (lysyl-tyrosyl-cystyl-alanyl-arginine), has been designed as a potential chelating ligand for SPECT imaging and therapeutic in vivo agents. This study shows the synthesis and characterization of KYCAR complexes containing nonradioactive rhenium, ^99mTc, or ¹⁸⁸Re. The metal complexes were also biologically evaluated to determine in vivo distribution in healthy mice. The overall goals of this project were (1) to synthesize the Tc/Re pentapeptide complexes, (2) to identify spectroscopic methods for characterization of syn versus anti rhenium peptide complexes, (3) to analyze the ex vivo stability, and (4) to assess the biological properties of the [^99mTc]TcO-KYCAR and [¹⁸⁸Re]ReO-KYCAR complexes in vivo. Details on these efforts are provided below.

METHODS

^NatRe/^99mTc/¹⁸⁸ReO-KYCAR complexes were synthesized, and macroscopic species were characterized via HPLC, IR, NMR, and CD. These characterization data were compared to the crystallographic data of ReO-KYC to assist in the assignment of diastereomers and to aid in the determination of the structure of the complex.

RESULTS

The radiometal complexes were synthesized with high purity (>95%). HPLC, IR, NMR and CD data on the macroscopic ^natReO-KYCAR complexes confirm the successful complexation as well as the presence of two diastereomers in syn and anticonformations. Tracer level complexes show favorable stabilities ex vivo for 2+ h.

CONCLUSION

Macroscopic metal complexes form diastereomers with the KYCAR ligand; however, this phenomenon is not readily observed on the tracer level due to the rapid interconversion. It was determined through pK_a measurements that the macroscopic ^natReO-KYCAR complex is 0 at physiological pH. The [^99mTc]TcO-KYCAR is stable in vitro while the [¹⁸⁸Re]ReO-KYCAR shows 50% decomposition in PBS and serum. Biologically, the tracer level complexes clear through the hepatobiliary pathway. Some decomposition of both tracers is evident by uptake in the thyroid and stomach.

Technetium-99m radiochemistry for pharmaceutical applications

Technetium-99m (^99m Tc) is a widely used radionuclide, and the development of ^99m Tc imaging agents continues to be in demand. This overview discusses basic principles of ^99m Tc radiopharmaceutical preparation and design and focuses on the ^99m Tc radiochemistry relevant to its pharmaceutical applications. The ^99m Tc complexes are described based on the most typical examples in each category, keeping up with the state-of-the-art in the field. In addition, the main current strategies to develop targeted ^99m Tc radiopharmaceuticals are summarized.

Synthesis, Characterization, and In Vitro Evaluation of New (99m)Tc/Re(V)-Cyclized Octreotide Analogues: An Experimental and Computational Approach

Radiolabeled proteolytic degradation-resistant somatostatin analogues have been of long-standing interest as cancer imaging and radiotherapy agents for targeting somatostatin receptor-positive tumors. Our interest in developing (186)Re- and (188)Re-based therapeutic radiopharmaceuticals led to investigation of a new Re(V)-cyclized octreotide analogue, Re(V)-cyclized, thiolated-DPhe(1)-Cys(2)-Tyr(3)-DTrp(4)-Lys(5)-Thr(6)-Cys(7)-Thr(OH)(8) (Re-SDPhe-TATE) using both experimental and quantum chemical methods. The metal is directly coordinated to SDPhe-TATE through cyclization of the peptide around the [ReO](3+) core. Upon complexation, four isomers were observed; the isolated/semi-isolated isomers exhibited different somatostatin receptor (sstr) binding affinities, 0.13 to 1.5 μM, in rat pancreatic tumor cells. Two-dimensional NMR experiments and electronic structure calculations were employed to elucidate the structural differences among the different isomers. According to NMR studies, the metal is coordinated to three thiolates and the backbone amide of Cys(2) in isomers 1 and 4, whereas the metal is coordinated to three thiolates and the backbone amide of Tyr(3) in isomer 2. Quantum chemical methods clarified the stereochemistry of Re-SDPhe-TATE and the possible peptide arrangements around the [ReO](3+) core. The re-cyclization reaction was translated to the (99m)Tc radiotracer level with four isomers observed on complexation with comparable HPLC retention times as the Re-SDPhe-TATE isomers. About 85% total (99m)Tc labeling yield was achieved by ligand exchange from (99m)Tc-glucoheptonate at 60 °C for an hour. About 100% and 51% of (99m)Tc(V)-cyclized SDPhe-TATE remained intact in phosphate buffered saline and 1 mM cysteine solution under physiological conditions at 6 h, respectively.

Triamidetriamine bearing macrobicyclic and macrotricyclic ligands: potential applications in the development of copper-64 radiopharmaceuticals

A versatile and straightforward synthetic approach is described for the preparation of triamide bearing analogues of sarcophagine hexaazamacrobicyclic cage ligands without the need for a templating metal ion. Reaction of 1,1,1-tris(aminoethyl)ethane (tame) with 3 equiv of 2-chloroacetyl chloride, yields the tris(α-chloroamide) synthetic intermediate 6, which when treated with either 1,1,1-tris(aminoethyl)ethane or 1,4,7-triazacyclononane furnished two novel triamidetriamine cryptand ligands (7 and 8 respectively). The Co(III) and Cu(II) complexes of cryptand 7 were prepared; however, cryptand 8 could not be metalated. The cryptands and the Co(III) complex 9 have been characterized by elemental analysis, (1)H and (13)C NMR spectroscopy, and X-ray crystallography. These studies confirm that the Co(III) complex 9 adopts an octahedral geometry with three facial deprotonated amido-donors and three facial amine donor groups. The Cu(II) complex 10 was characterized by elemental analysis, single crystal X-ray crystallography, cyclic voltammetry, and UV-visible absorption spectroscopy. In contrast to the Co(III) complex (9), the Cu(II) center adopts a square planar coordination geometry, with two amine and two deprotonated amido donor groups. Compound 10 exhibited a quasi-reversible, one-electron oxidation, which is assigned to the Cu(2+/3+) redox couple. These cryptands represent interesting ligands for radiopharmaceutical applications, and 7 has been labeled with (64)Cu to give (64)Cu-10. This complex showed good stability when subjected to L-cysteine challenge whereas low levels of decomplexation were evident in the presence of L-histidine.

Inorganic chemistry in nuclear imaging and radiotherapy: current and future directions

Radiometals play an important role in diagnostic and therapeutic radiopharmaceuticals. This field of radiochemistry is multidisciplinary, involving radiometal production, separation of the radiometal from its target, chelate design for complexing the radiometal in a biologically stable environment, specific targeting of the radiometal to its in vivo site, and nuclear imaging and/or radiotherapy applications of the resultant radiopharmaceutical. The critical importance of inorganic chemistry in the design and application of radiometal-containing imaging and therapy agents is described from a historical perspective to future directions.

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.

Synthesis and evaluation of novel Tc-99m labeled probestin conjugates for imaging APN/CD13 expression in vivo

The enzyme aminopeptidase N (APN, also known as CD13) is known to play an important role in tumor proliferation, attachment, angiogenesis, and tumor invasion. In this study, we hypothesized that a radiolabeled high affinity APN inhibitor could be potentially useful for imaging APN expression in vivo. Here, we report synthesis, radiolabeling, and biological evaluation of new probestin conjugates containing a tripeptide, N,N-dimethylglycyl-l-lysinyl-l-cysteinylamide (N(3)S), chelator. New probestin conjugates were synthesized by solid-phase peptide synthesis method, purified by reversed-phase HPLC, and characterized by electrospray mass spectrometry. The conjugates were complexed with Re(V) and (99m)Tc(V) by transmetalation using corresponding Re(V) or (99m)Tc(V) gluconate synthon. The mass spectral analyses of ReO-N(3)S-Probestin conjugates were consistent with the formation of neutral Re(V)O-N(3)S complexes. Initial biological activity of ReO-N(3)S-Probestin conjugates determined by performing an in vitro APN enzyme assay using intact HT-1080 cells demonstrated higher inhibition of APN enzyme activity than bestatin. In vivo biodistribution and whole body planar imaging studies of (99m)TcO-N(3)S-PEG(2)-Probestin performed in nude mice xenografted with human fibrosarcoma tumors derived from HT-1080 cells demonstrated a tumor uptake value of 2.88 ± 0.64%ID/g with tumor-to-blood and tumor-to-muscle ratios of 4.8 and 5.3, respectively, at 1 h postinjection (p.i.). Tumors were clearly visible in whole body planar image obtained at 1 h p.i., but not when the APN was competitively blocked with a coinjection of excess nonradioactive ReO-N(3)S-PEG(2)-Probestin conjugate. These results demonstrate the feasibility of using high affinity APN inhibitor conjugates as targeting vectors for in vivo targeting of APN.

Binding of ReO4(-) with an engineered MoO4(2-)-binding protein: towards a new approach in radiopharmaceutical applications

Radiolabeled biomolecules are routinely used for clinical diagnostics. (99m)Tc is the most commonly used radioactive tracer in radiopharmaceuticals. (188)Re and (186)Re are also commonly used as radioactive tracers in medicine. However, currently available methods for radiolabeling are lengthy and involve several steps in bioconjugation processes. In this work we present a strategy to engineer proteins that may selectively recognize the perrhenate (ReO(4)(-)) ion as a new way to label proteins. We found that a molybdate (MoO(4)(2-))-binding protein (ModA) from Escherichia coli can bind perrhenate with high affinity. Using fluorescence and isothermal titration calorimetry measurements, we determined the dissociation constant of ModA for ReO(4)(-) to be 541 nM and we solved a crystal structure of ModA with a bound ReO(4)(-). On the basis of the structure we created a mutant protein containing a disulfide linkage, which exhibited increased affinity for perrhenate (K(d) = 104 nM). High-resolution crystal structures of ModA (1.7 Å) and A11C/R153C mutant (2.0 Å) were solved with bound perrhenate. Both structures show that a perrhenate ion occupies the molybdate binding site using the same amino acid residues that are involved in molybdate binding. The overall structure of the perrhenate-bound ModA is unchanged compared with that of the molybdate-bound form. In the mutant protein, the bound perrhenate is further stabilized by the engineered disulfide bond.

Synthesis, radiolabeling and biological evaluation of a neutral tripeptide and its derivatives for potential nuclear medicine applications

Peptides are important regulators of growth and cellular functions not only in normal tissue but also in tumors. So they are becoming radioligands of increasing interest in nuclear oncology for targeted tumor diagnosis and therapy. So development of new peptide radiopharmaceuticals is becoming one of the most important areas in nuclear medicine research. A small tripeptide derivative NH(2)PhePheCys was synthesized by Fmoc solid phase peptide synthesis using an automated synthesizer. The oxidized form, i.e. NH(2)PhePheCysCysPhePheNH(2,) was also prepared by iodine oxidation method from NH(2)PhePheCys(ACM). The ligands were analyzed by HPLC and mass spectroscopy. They were radiolabeled with (99m)Tc using SnCl(2). In vitro analytical studies and biological characterizations were performed using the peptide radiopharmaceuticals. Images taken under gamma camera showed very high uptake in the liver, lung and spleen. Significant uptake was also observed in bone marrow and brain for (99m)Tc-NH(2)PhePheCys. Metabolites were produced in vivo when the radiopharmaceuticals were injected intravenously and were identified from rat brain and liver homogenate studies. Clearance through kidney did not show any evidence of breaking of the labeled compounds and formation of free (99m)Tc. Radiopharmaceuticals prepared using tripeptide and hexapeptide ligands were transported into the brain through blood brain barrier depending on the size and sequence characteristics. Using this property of peptide new derivatives can be prepared to develop (99m)Tc radiopharmaceuticals for imaging normal brain tissues as well as for diagnosing various brain disorders.

99m-Technetium carbohydrate conjugates as potential agents in molecular imaging

This feature article covers recent reports of work towards the development of (99m)Tc-carbohydrate based agents for use in SPECT imaging, particularly of cancerous tissue. An outline of some of the key biological functions and coordination chemistry of carbohydrates is given, along with an introduction to bioconjugation and molecular imaging. Technetium coordination chemistry and the subset of this involving bioconjugates are discussed before moving into the focus of the article: glycoconjugates of (99m)Tc(v) and the more recently developed [(99m)Tc(I)(CO)(3)](+). Significant work in the last decade has featured the very attractive [(99m)Tc(CO)(3)](+) core, and the ligand sets designed for this core are discussed in detail.

Isolation, characterization, and biological evaluation of syn and anti diastereomers of [(99m)Tc]technetium depreotide: a somatostatin receptor binding tumor imaging agent

The early and later eluting [(99m)TcO]depreotide products on RP-HPLC were confirmed to be the anti and syn diastereomers, respectively, based on proton NMR and circular dichroism spectroscopy. NMR provided evidence of a folded, conformationally constrained structure for the syn diastereomer. The syn diastereomer is predominant (anti/syn approximately 10:90) in the [(99m)TcO]depreotide preparation and shows a slightly higher affinity (IC50 = 0.15 nM) for the somatostatin receptor than the anti diastereomer (IC50 = 0.89 nM). Both diastereomers showed higher binding affinities than the free peptide (IC(50) = 7.4 nM). Biodistribution studies in AR42J tumor xenograft nude mice also showed higher tumor uptake for syn [(99m)TcO]depreotide (6.58% ID/g) than for the anti [(99m)TcO]depreotide (3.38% ID/g). Despite the differences in biological efficacy, the favorable binding affinity, tumor uptake, and tumor-to-background ratio results for both diastereomeric species predict that both are effective for imaging somatostatin receptor-positive tumors in vivo.