Mixed-ligand rhenium-188 complexes with tetradentate/monodentate NS3/P ('4 + 1') coordination: relation of structure with antioxidation stability

Synthesis and preclinical evaluation of rhenium and technetium-99m "4 + 1" mixed-ligand complexes bearing quinazoline derivatives as potential EGFR imaging agents

Epidermal growth factor receptors (EGFR) of tyrosine kinase (TK) have shown high expression levels in most cancers and are considered a promising target for cancer diagnosis and therapy. Expanding the investigation for novel targeted radiopharmaceuticals, an EGFR inhibitor such as 4-aminoquinazoline derivatives along with a radionuclide such as technetium-99m (^99mTc) could be ideal. Thus, we report herein the synthesis, characterization, and biological evaluation of new "4 + 1" mixed-ligand Re^III- and ^99mTc^III-complexes of the general formula [^99mTc][Tc(NS₃)(CN-R)] bearing tris(2-mercaptoethyl)-amine (NS₃) as the tetradentate tripodal ligand and a series of isocyanide derivatives (CN-R) of tyrosine kinase inhibitor (3-bromophenyl)quinazoline-4,6-diamine as the monodentate ligand. The quinazoline isocyanide derivatives 4a-d were prepared in two steps and reacted with the [Re(NS₃)PMe₂Ph] precursor leading to the final complexes 5a-d in high yield. All compounds were characterized by elemental analysis, IR, and NMR spectroscopies. In vitro studies, for their potency to inhibit the cell growth, using intact A431 cells indicate that the quinazoline derivatives 4a-d and the Re complexes 5a-d significantly inhibit the A431 cell growth. In addition, the EGFR autophosphorylation study of complex 5b shows an IC₅₀ value in the nanomolar range. The corresponding "4 + 1" ^99mTc-complexes 6a-d were prepared by employing the [^99mTc]TcEDTA intermediate and the appropriate monodentate 4a-d in a two-step synthetic procedure with a radiochemical yield (RCY) from 63 to 77 % and a radiochemical purity (RCP) > 99 % after HPLC purification. Their structures have been established by HPLC comparative studies using the well-characterized Re-complexes 5a-d as reference. All ^99mTc-complexes remain stable for at least 6 h, and their logD_7.4 values confirmed their anticipated lipophilic character. Biodistribution studies in healthy Swiss albino mice of ^99mTc-complexes showed hepatobiliary excretion and initial fast blood clearance. Complex 6b was also tested in Albino SCID mice bearing A431 tumors and showed rapid tumor uptake at 5 min (2.80 % ID/g) with a moderate tumor/muscle ratio (2.06) at 4 h p.i. The results encourage further investigation for this type of ^99mTc-complexes as single-photon emission computed tomography (SPECT) radio agents for imaging tumors overexpressing EGFR.

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.

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

Synthesis and characterization of rhenium(iii) complexes with (Ph2PCH2CH2)2NR diphosphinoamine ligands

The synthesis and characterization of a new series of neutral, six-coordinated compounds [Re^IIIX₃(PNPR)], where X is Cl or Br and PNPR is a diphosphinoamine having the general formula (Ph₂PCH₂CH₂)₂NR (R = H, CH₃, CH₂CH₃, CH₂CH₂CH₃, CH₂CH₂CH₂CH₃ and CH₂CH₂OCH₃) are reported. Stable [Re^IIIX₃(PNPR)] complexes were synthesized, in variable yields, starting from precursors where the metal was in different oxidation states (iii and v), by ligand-exchange and/or redox-substitution reactions. The compounds were characterized by elemental analysis, proton NMR spectroscopy, cyclic voltammetry, UV/vis spectroscopy, positive-ion electrospray ionization mass spectrometry (ESI(+)-MS) and X-ray diffraction analysis. Although the formulation of the complexes allows either meridional or facial isomers, the latter arrangement was prevalent both in the solid and solution states. Only [ReCl₃(PNPH)] showed a meridional configuration both in solution and in the crystalline state. [ReBr₃(PNPme)] prefers the meridional configuration in the crystalline state and the facial one in solution. While ESI(+)-MS and voltammetric data seem to indicate some dependency from the nature of the alkyl substituent at the nitrogen, the available structural data of the complexes show only slight differences both for angles and bond lengths upon change of the alkyl chain tethered to the nitrogen.

Synthesis, physicochemical and biological studies of technetium-99m labeled tacrine derivative as a diagnostic tool for evaluation of cholinesterase level

In the present work the synthesis and physicochemical investigations of new tacrine analogues labeled with technetium-99m are reported. All obtained novel radioconjugates showed high stability in the presence of an excess of standard amino acids cysteine or histidine, as well as in human serum. Lipophilicity (LogD values) of these compounds is within the range from 0.92 to 1.56. For the selected radioconjugate ^99mTc(NS₃)(CN-NH(CH₂)₇Tac) (LogD=1.56) the biological activity studies in the course of inhibition of acetylcholinesterase action have been performed (IC₅₀=45.0nM, estimated by means of Ellman's method). Biodistribution studies of this compound showed its uptake in brain on the level of 0.07%ID/g and its clearance through the hepatic and renal route in comparable degree. The ascertained presence of the radioconjugate in brain indicates its possibility to cross the blood-brain barrier. Molecular modeling of ^99mTc(NS₃)(CN-NH(CH₂)₇Tac) radioconjugate showed that the main structural fragment is tacrine moiety which is responsible for most interactions within catalytic and peripheral active sites and provides the anti-acetylcholinesterase activity. The ^99mTc(NS₃)(CN-NH(CH₂)₇Tac) radioconjugate may be considered to be a diagnostic tool for patients suffering from Alzheimer's disease as well as a marker to determine the physiological condition of liver and intestines.

Synthesis, physicochemical and biological evaluation of technetium-99m labeled lapatinib as a novel potential tumor imaging agent of Her-2 positive breast cancer

Tumors that are Her-2-positive tend to grow and spread more quickly than other types of breast cancer. Overexpression of Her-2 can be a predictive biomarker for stratification of patients for therapy with Herceptin (containing humanized IgG1 monoclonal antibody trastuzumab) or Tykerb (containing lapatinib di-p-toluenesulfonate) drug. Usually, Her-2 status is determined by immunohistochemical (IHC) as well as fluorescent or chromogenic in situ hybridisation (FISH or CISH) analysis of biopsy material. The objective of the present work was to standardize the conjugation of anti-cancer drug lapatinib (which recognizes selectively the Her-2 extracellular domain) with technetium-99m complex, of type '4+1', to obtain (99m)Tc(NS3)(CN-lapatinib) conjugate for use as in vivo tracer of the Her-2 expression in breast cancer. The conjugate (99m)Tc(NS3)(CN-lapatinib) was formed with high yield, high radiochemical purity and specific activity within the range 25-30 GBq/μmol. The biological in vitro and in vivo studies of the conjugate showed its high affinity to Her-2 receptor (Kd = 3.5 ± 0.4 nM, Ki = 2.9 ± 0.5 nM, Bmax = 2.4 ± 0.3 nM, approximate number of 2.4 × 10(6) binding sites per cell, IC50 = 41.2 ± 0.4 nM) and also pointed out to the clearance through the hepatic and renal route in comparable degree. Basing on these results one can conclude that (99m)Tc(NS3)(CN-lapatinib) conjugate could be a promising radiopharmaceutical for in vivo diagnosis of the Her-2 status in breast with impact on treatment planning.

Synthesis and in vitro/in vivo evaluation of novel mono- and trivalent technetium-99m labeled ghrelin peptide complexes as potential diagnostic radiopharmaceuticals

INTRODUCTION

Ghrelin is an endogenous hormone present in blood. It is released from the oxyntic cells (X/A-like cells) of the stomach and fundus and can exist in two forms: as an acylated and des-acylated ghrelin. Ghrelin is an endogenous ligand of the growth hormone receptor (growth hormone secretagogue receptor, GHS-R). Overexpression of GHS-R1a receptor was identified in cells of different types of tumors (e.g. pituitary adenoma, neuroendocrine tumors of the thyroid, lung, breast, gonads, prostate, stomach, colorectal, endocrine and non-endocrine pancreatic tumors). This fact suggests that gamma radionuclide labeled ghrelin peptide may be considered as a potential diagnostic radiopharmaceutical.

METHODS

Ghrelin peptide labeled with mono- and trivalent technetium-99m complexes, (99m)Tc-Lys-GHR, has been prepared on the n.c.a. scale. The physicochemical (stability, charge, shape, lipophilicity) and biological (receptor affinity, biodistribution) properties of the conjugates have been studied relevant to use the conjugates as receptor-based diagnostic radiopharmaceuticals.

RESULTS

The obtained conjugates [(99m)Tc(CO)3LN,O(CN-Lys-GHR)](+), (99m)Tc(CO)3LS,O(CN-Lys-GHR) and (99m)Tc(NS3)(CN-Lys-GHR) show different shape, charge, lipophilicity and two of them, (99m)Tc(CO)3LS,O(CN-Lys-GHR) and (99m)Tc(NS3)(CN-Lys-GHR), high stability in neutral aqueous solutions, even in the presence of excess concentration of histidine/cysteine competitive standard ligands or human serum. The in vitro binding affinity of (99m)Tc-Lys-GHR conjugates with respect to growth hormone secretagogue receptor (GHS-R1a) present on DU-145 cells was in the range of IC50 from 45 to 54 nM. The conjugate (99m)Tc(CO)3LS,O(CN-Lys-GHR) exhibited excretion route by the liver and kidney in comparable degree, while the more lipophilic conjugate (99m)Tc(NS3)(CN-Lys-GHR)-mainly by the liver.

CONCLUSIONS

Basing on the results concerning physicochemical and biochemical properties, the conjugates (99m)Tc(CO)3LS,O(CN-Lys-GHR) and (99m)Tc(NS3)(CN-Lys-GHR) might be considered to be promising models for diagnostic radiopharmaceutical.

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.

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.

Design and synthesis of a bombesin peptide-conjugated tripodal phosphino dithioether ligand topology for the stabilization of the fac-[M(CO)3]+ core (M=(99 m)Tc or Re)

A new tumor-seeking tridentate topology consisting of a phosphino dithioether ((HOCH(2))(2)PCH(2)CH(2)S(CH(2))(n)CH(2)SR; PS(2)) ligand framework for the production of kinetically inert and in vivo stable facial [(99m)Tc(CO)(3)(PS(2))](+) or [Re(CO)(3)(PS(2))](+) is described. The X-ray crystal structure of fac-Re(CO)(3)(PS(2))PF(6) is reported. The bioconjugation strategies for incorporating bombesin (BBN) peptides on to the PS(2) tripodal framework and, thereby, de novo designing of GRP receptor-seeking Tc(PS(2)-BBN)(CO)(3) are developed.

Organometallic 99mTc(III) ‘4 + 1' Bombesin(7−14) Conjugates: Synthesis, Radiolabeling, and in Vitro/in Vivo Studies

Bombesin (BBN) peptide exhibits high selectivity and affinity for the gastrin-releasing peptide receptor (GRPr). The GRPr is overexpressed on many human cancer cell types, thus making BBN a potent delivery vehicle for radionuclide targeting. In this study, the biologically active minimal sequence BBN(7-14) was labeled using the novel Tc '4 + 1' mixed-ligand system, [Tc(NS3)(CN-R)], in which Tc(III) is coordinated by a monodentate isocyanide linker bearing the peptide and the tetradentate, tripodal chelator, 2,2',2''-nitrilotriethanethiol (NS3). BBN(7-14) was N-terminally modified with Gly-Gly-Gly, betaAla, and Ser-Ser-Ser spacer groups (X) and functionalized with 4-(isocyanomethyl)benzoic acid (L1) or 4-isocyanobutanoic acid (L2), resulting in a series of [M(NS3)(L-X-BBN(7-14))] conjugates (M = 99mTc, Re). The isocyanide ligand frameworks were introduced using novel bifunctional coupling agents. The spacer groups (X), the monodentate isocyanide units, and a tetradentate NS3 chelator bearing a pendant carboxylic acid (NS3COOH) were proposed as pharmacological modifiers. 99mTc-labeling was performed in a two-step procedure by first preparing 99mTc-EDTA/mannitol followed by reactions with the isocyanides and NS3 or NS3COOH ligand frameworks. The 99mTc complexes were obtained with a radiochemical yield of 30-80% depending on the amount of the isocyanide (20-100 nmol) used. These new conjugates were purified by reversed-phased high-performance liquid chromatography (RP-HPLC) to give a radiochemical purity of >or=95%. The 99mTc conjugates exhibited high in vitro stability (>90%, 24 h). Analogous nonradioactive Re conjugates were synthesized and characterized by electrospray ionization mass spectrometry (ESI-MS). RP-HPLC analyses of the Re conjugates indicated that they exhibited identical retention times to the corresponding 99mTc conjugates under identical HPLC conditions, demonstrating structural similarity between the two metalated species. The [Re(NS3)(L-X-BBN(7-14))] conjugates exhibited GRPr affinity in the nanomolar range as demonstrated by in vitro competitive binding assays using PC-3 human prostate cancer cells. In vitro internalization/externalization assays indicated that approximately 65% of [99mTc(NS3)(L2-betaAla-BBN(7-14))] conjugate was either surface-bound or internalized in PC-3 cells. Cell-associated activity for all other 99mTc conjugates was below 20%. Biodistribution studies of [99mTc(NS3)(L-betaAla-BBN(7-14))], L = L1 or L2, in normal, CF-1 mice showed minimal accumulation in normal pancreas (a tissue expressing the GRPr in high density in rodent models) and rapid hepatobiliary elimination. Introduction of a carboxyl group onto the NS3 ligand framework had only minimal effects to increase renal excretion. Activity distribution and accumulation was highly dominated by the relatively lipophilic '4 + 1' complex unit.

Synthesis and Biological Evaluation of a New Type of 99mTechnetium-Labeled Fatty Acid for Myocardial Metabolism Imaging

Technetium-labeled fatty acids intended for myocardial metabolism imaging and the respective rhenium model complexes were synthesized according to the "4 + 1" mixed-ligand approach and investigated in vitro and in vivo. The non-radioactive rhenium model complexes were characterized by NMR, IR, and EA, and the geometrical impact of the chelate unit on the integrity of the fatty acid head structure was determined by single-crystal X-ray analyses. To estimate the diagnostic value of the 99mTc-labeled fatty acids, the compounds were investigated in experiments in vitro and in biodistribution studies using male Wistar rats. The new fatty acid tracers contain the metal core in the oxidation states +3, well-wrapped in a trigonal-bipyramidal coordination moiety, which is attached at the omega-position of a fatty acid chain. This structural feature is considered to be a good imitation of the well-established iodinated phenyl fatty acids. High heart extraction in perfused heart studies (up to 26% injected dose (ID)) and noticeable heart uptake of the 99mTc tracers in vivo being in the order of 2% ID/g at 5 min (postinjection, pi.), accompanied by a good heart to blood ratio of 8, confirms that the new Tc compounds are suitable as fatty acid tracers.

Preparation and Biological Characterization of Isomeric 188Re(V) Oxocomplexes with Tetradentate S4 Ligands Derived from meso-Dimercaptosuccinic Acid for Labeling of Biomolecules

A new type of tetradentate S4 ligand has been synthesized by bridging two molecules of meso-2,3-dimercaptosuccinic acid for stable binding and easy conjugation of rhenium-188 to tumor targeting structures. The stereoisomeric tetrathiolato S4 ligands form very robust anionic five-coordinated oxorhenium(V) and oxotechnetium(V) complexes. Two routes for the preparation of the (188)Re(V) oxocomplexes with (iBu)2N(O)C-C(SH)C(SH)C(O)NH(CH2)3NH(CH2)3NHC(O)C(SH)C(SH)C(O)N(iBu)2 (ligand 1) and its hydrophilic crown ether derivative (ligand 2) were tested and optimized. Several isomers were separated by HPLC from the preparation solutions and characterized in vitro and in vivo. The identity of the species obtained was determined by comparison with the HPLC profiles of reference (185/187)Re analogues and (99/99m)Tc complexes which were characterized by ESI-MS. All of them were absolutely stable in rat and human plasma solutions. Challenge experiments with cysteine corroborated the high inertness of the isomers toward ligand exchange reactions. Various in vivo samples, taken off at different times from blood, intestine, and urine of rats, confirmed the high in vivo stability of the (188)Re-S4 complexes. Biodistribution studies using male Wistar rats were performed and exhibited a high uptake and fast clearance from the liver of the more lipophilic cis and trans isomers of complex I (log P(o/w) between 1.5 and 1.7), whereas the isomers of the hydrophilic complex II (log P(o/w) about -1.75) were rapidly excreted via the renal and the hepatobiliary pathway. The low level of activity in the stomach confirms good in vivo stability. Thus, these new (188)Re-S4 complexes fulfill the requirements for a stable and high specific activity labeling of biomolecules with rhenium-188.

The 188Re(III)–EDTA complex—a multipurpose starting material for the preparation of relevant 188Re complexes under mild conditions

An easy and gentle method for the preparation of 188Re(V) complexes with bidentate and tetradentate ligands is described starting from the precursor complex 188Re(III)-EDTA. That complex is prepared at room temperature in acidic solution and reacts by a combined re-oxidation/ligand exchange reaction with appropriate ligands like DMSA or ECD (DMSA = dimercapto succinic acid, ECD = L,L-ethylene dicysteine diethyl ester) or en, tau, and cyclam (en = ethylene diamine, tau = 1,4,8,11-tetraazaundecane, cyclam = 1,4,8,11-tetraazacyclo-tetradecane) to the 188Re(V)-oxo- and dioxocomplexes, respectively. The chelates were unambiguously identified by chromatographic comparison with spectroscopically characterised samples or known 99mTc-kit reconstitutions. The reaction succeeds under mild conditions (room temperature, short time, neutral or weak basic solutions) with high yields and has potential for labelling of sensitive biomolecules with 188Re.