Technetium-99m-labeled medium-chain fatty acid analogues metabolized by beta-oxidation: radiopharmaceutical for assessing liver function

Zn Complex of Diaminedithiol Tetradentate Ligand as a Stable Precursor for 99mTc-Labeled Compounds

The diaminedithiol (N₂S₂) tetradentate ligand constitutes a useful chelating molecule for preparing ^99mTc-labeled compounds of high in vivo stability in high radiochemical yields. However, since the thiol groups in the N₂S₂ ligand are easy to be oxidized to disulfide bonds, they need to be protected with an appropriate protecting group, which hinders the broad applications of the N₂S₂ ligand for radiopharmaceuticals. In this study, a Zn chelate of N₂S₂ was evaluated as a precursor for purification-free ^99mTc-labeled N₂S₂ under the mild and simple procedure. Zn-N₂S₂ was prepared by reacting Zn acetate with N₂S₂, and the Zn-N₂S₂ remained stable under aerobic conditions at room temperature. ^99mTc-N₂S₂ was obtained over 90% radiochemical yields at room temperature by a one-pot reaction, consisting of Zn-N₂S₂ (10⁻⁵ M), ^99mTcO₄⁻, ethylenediaminetetraacetic acid (EDTA), and a reducing agent (Sn²⁺) at pH = 5.5 to 7.5. ^99mTc-N₂S₂ was also obtained over 90% radiochemical yields when the reaction was conducted in the presence of an equimolar amount of IgG antibody. These findings indicate the Zn complex of N₂S₂ ligand constitutes a stable and useful precursor to prepare ^99mTc-labeled N₂S₂ compounds in high yields under the mild and simple procedure.

Cysteine Derivatized 99mTc-Labelled Fatty Acids as β-Oxidation Markers

With the aim of developing 99mTc-labeled fatty acids intended for myocardial metabolism imaging we report herein the synthesis and characterization of two novel derivatives of undecanonoic and hexadecanonoic acid that have been functionalized at the ω-site by cysteine through the formation of a thioether bond (Cys–FA11 and Cys–FA16). Equimolar amounts of each ligand and the [NEt4]2[Re(CO)3Br3] precursor generated the respective hexacoordinated neutral complexes in which the ligand coordinated to the metal through the SNO donor system of cysteine. The rhenium complexes were characterized by elemental analysis, IR and NMR spectroscopies. The analogous technetium-99m complexes, 99mTc–Cys–FA11 and 99mTc–Cys–FA16 were prepared by incubation of the ligand with the precursor [99mTc(CO)3(H2O)3]+ (radiochemical yield ≥98%). Their structure was established by comparative HPLC techniques. In vivo studies in mice showed high initial heart uptake for both 99mTc complexes (7.4 ± 0.53 and 7.07 ± 0.73 percentage of injected dose (%ID)/g at 1 min post injection. Rapid clearance (0.60 ± 0.02 %ID/g) was observed for 99mTc–Cys–FA11 while the clearance of the longer fatty acid 99mTc–Cys–FA16 was slower (2.31 ± 0.09 %ID/g at 15 min p.i.). Metabolite analysis study indicated that complexes were catabolized through the β-oxidation process.

Monooxorhenium(V) complexes with 222-N2S2 MAMA ligands for bifunctional chelator agents: Syntheses and preliminary in vivo evaluation

INTRODUCTION

Targeted radiotherapy using the bifunctional chelate approach with ^186/188Re(V) is challenging because of the susceptibility of monooxorhenium(V)-based complexes to oxidize in vivo at high dilution. A monoamine-monoamide dithiol (MAMA)-based bifunctional chelating agent was evaluated with both rhenium and technetium to determine its utility for in vivo applications.

METHODS

A 222-MAMA chelator, 222-MAMA(N-6-Ahx-OEt) bifunctional chelator, and 222-MAMA(N-6-Ahx-BBN(7-14)NH₂) were synthesized, complexed with rhenium, radiolabeled with ^99mTc and ¹⁸⁶Re (carrier added and no carrier added), and evaluated in initial biological distribution studies.

RESULTS

An IC₅₀ value of 2.0±0.7nM for ^natReO-222-MAMA(N-6-Ahx-BBN(7-14)NH₂) compared to [¹²⁵I]-Tyr⁴-BBN(NH₂) was determined through competitive cell binding assays with PC-3 tumor cells. In vivo evaluation of the no-carrier added ^99mTc-222-N₂S₂(N-6-Ahx-BBN(7-14)NH₂) complex showed little gastric uptake and blockable pancreatic uptake in normal mice.

CONCLUSIONS

The ¹⁸⁶ReO-222-N₂S₂(N-6-Ahx-BBN(7-14)NH₂) complex showed stability in biological media, which indicates that the 222-N₂S₂ chelator is appropriate for chelating ^186/188Re in radiopharmaceuticals involving peptides. Additionally, the in vitro cell studies showed that the ReO-222-N₂S₂(N-6-Ahx-BBN(7-14)NH₂) complex (macroscopically) bound to PC3-tumor cell surface receptors with high affinity. The ^99mTc analog was stable in vivo and exhibited pancreatic uptake in mice that was blockable, indicating BB2r targeting.

Development and evaluation of a novel (99m)tc-labeled annexin A5 for early detection of response to chemotherapy

^99mTc-HYNIC-annexin A5 can be considered as a benchmark in the field of apoptosis imaging. However, ^99mTc-HYNIC-annexin A5 has characteristics of high uptake and long retention in non-target tissues such as kidney and liver. To minimize this problem, we developed a novel ^99mTc-labeled annexin A5 using a bis(hydroxamamide) derivative [C₃(BHam)₂] as a bifunctional chelating agent, and evaluated its usefulness as an imaging agent for detecting apoptosis. The amino group of C₃(BHam)₂ was converted to a maleimide group, and was coupled to thiol groups of annexin A5 pretreated with 2-iminothiolane. ^99mTc labeling was performed by a ligand exchange reaction with ^99mTc-glucoheptonate. Biodistribution experiments for both ^99mTc-C₃(BHam)₂-annexin A5 and ^99mTc-HYNIC-annexin A5 were performed in normal mice. In addition, in tumor-bearing mice, the relationship between the therapeutic effects of chemotherapy (5-FU) and the tumor accumulation of ^99mTc-C₃(BHam)₂-annexin A5 just after the first treatment of 5-FU was evaluated. ^99mTc-C₃(BHam)₂-annexin A5 was prepared with a radiochemical purity of over 95%. In biodistribution experiments, ^99mTc-C₃(BHam)₂-annexin A5 had a much lower kidney accumulation of radioactivity than ^99mTc-HYNIC-annexin A5. In the organs for metabolism, such as liver and kidney, radioactivity after the injection of ^99mTc-HYNIC-annexin A5 was residual for a long time. On the other hand, radioactivity after the injection of ^99mTc-C₃(BHam)₂-annexin A5 gradually decreased. In therapeutic experiments, tumor growth in the mice treated with 5-FU was significantly inhibited. Accumulation of ^99mTc-C₃(BHam)₂-annexin A5 in tumors significantly increased after 5-FU treatment. The accumulation of radioactivity in tumor correlated positively with the counts of TUNEL-positive cells. These findings suggest that ^99mTc-C₃(BHam)₂-annexin A5 may contribute to the efficient detection of apoptotic tumor response after chemotherapy.

Technetium-99m-Labeled Long Chain Fatty Acid Analogues Metabolized by β-Oxidation in the Heart

The development of 99mTc-labeled fatty acid analogues metabolized by beta-oxidation in the myocardium constitutes an unsolved challenge. On the basis of our recent findings that [188Re]tricarbonyl(cyclopentadienylcarbonate)rhenium ([188Re]CpTR-COOH) was recognized as an aromatic compound and was metabolized as such in the body, [99mTc]cyclopentadienyltricarbonyltechnetium ([99mTc]CpTT) was conjugated at the omega-position of pentadecanoic acid to prepare [99mTc]CpTT-PA. When injected into rats, [99mTc]CpTT-PA exhibited the maximum myocardial accumulation and heart-to-blood ratio of 3.85 %ID/g at 1 min and 4.60 at 10 min postinjection, respectively. The metabolic study using isolated Langendorff perfused rat hearts demonstrated that approximately 67% of perfused [99mTc]CpTT-PA was incorporated and [99mTc]CpTT-propionic acid, the metabolite after six cycles of beta-oxidation of [99mTc]CpTT-PA, was detected as the major radiometabolite in the perfusate and myocardium. These findings indicate that [99mTc]CpTT-PA was recognized, transported, and metabolized as a long chain fatty acid analogue for energy production in the myocardium.

Synthesis and biodistribution of (99m)Tc-carbonyltechnetium-labeled fatty acids

In order to image myocardial metabolism, (99m)Tc(CO)(3)(+) complexes containing fatty acids were evaluated. At first, 11-(1-imidazolyl)-undecanoic acid (IUA) and 11-(2-(4-bromo-phenylazo)-1-imidazolyl)-undecanoic acid (BPIUA) were synthesized and the corresponding carbonyltechnetium-labeled complexes were prepared. However, unfortunately similar to those of other technetium-labeled fatty acid analogs, the biodistribution studies of (99m)Tc(CO)(3)-IUA and (99m)Tc(CO)(3)-BPIUA in mice demonstrated poor heart-to-blood ratios, thus these complexes cannot be used as heart-imaging reagents. In addition, Tween-80 can improve the biodistribution of the complexes for myocardial imaging.

Synthesis and characterization of novel 99gTc(V) and Re(V) complexes with water-soluble tetraaza diamido dipyridino ligands: single-crystal X-ray structural investigations of mono- and dinuclear complexes

Rhenium and technetium are known for their useful applications in nuclear medicine with similar properties. In this study, new diamido dipyridino (N(4)) water-soluble ligands (2-C(5)H(4)NCH(2)NHCO)(2)CH(2), 1 (L(1)H2), (2-C(5)H(4)NNHNHCO)(2)CH(2), 2, and [2-C(5)H(4)N(+)(O)(-)CH(2)NHCO](2)CH(2), 3, were synthesized. Reaction of L(1)H2 with ReOCl(3)(PPh(3))(2) resulted in the novel six-coordinated rhenium(V) complex, trans-ReO(L(1))(OEt), 4. The complex was characterized by spectroscopic methods, and its X-ray crystallographic analysis revealed that rhenium is coordinated to four nitrogen atoms of the ligand and to two oxygen atoms from the deprotonated ethanol and the oxo group respectively in a distorted octahedral geometry. In solution, complex 4 was transformed to a new complex 5, which was proved to be the dinuclear complex mu-oxo [ReO(L(1))](2)O. Reaction of 1 with [n-Bu(4)N][ReOCl(4)] resulted in the neutral complex 6, trans-[ReO(L(1))]Cl. Similarly, when ligand 1 was reacted with [n-Bu(4)N][(99g)TcOCl(4)], the neutral trans-[(99)TcO(L(1))]Cl complex 7 was formed, which upon dissolution transformed into a cationic complex 8, trans-[(99)TcO(L(1))(OH(2))](+)Cl(-). The single-crystal X-ray structure of 8 reveals that the coordination sphere about technetium is a distorted octahedron with four nitrogen atoms in the equitorial plane, while doubly bonded oxygen and coordinated water occupy the apical positions. Further dissolution of 8 resulted in the formation of dinuclear mu-oxo [TcO(L(1))](2)O, 9. This study shows that Tc and Re have similar metal core structures in solution for diamido dipyridino systems, besides similarity in geometrical structure, proved by the X-ray structures on the same ligands.

Chemistry of bifunctional photoprobes. 6. Synthesis and characterization of high specific activity metalated photochemical probes: development of novel rhenium photoconjugates of human serum albumin and fab fragments

Functionalization of perfluoro aryl azides by bifunctional chelating agents (BFCAs) capable of forming high specific activity complexes with (99m)Tc (for gamma-imaging) and (188)Re (for radiotherapy) is described. The synthesis of multidonor BFCAs containing N(2)S(2), N(4), and N(3)S donor groups containing imidazole, pyridine, and pyrazine functionalities that may be important for tuning the pharmacokinetic parameters is also described. Functionalization of perfluoro aryl azides at various sites on BFCAs yields novel bifunctional photolabile chelating agents (BFPCAs) that are useful for covalent attachment to biomolecules. A representative Re-BFPCA 8a in a model solvent, diethylamine, proceeded to give a high yield of intermolecular NH insertion product without the decomplexation of the metal ion from 8a. All products originated from the photolysis of 8a in diethylamine are characterized by analytical techniques, and a plausible mechanism of formation of different photolytic products is suggested. The high yield of intermolecular NH insertion of Re-BFPCA 8a is extended to labeling of human serum albumin (HSA) and Fab fragments under aqueous conditions. The photolabeling technology developed here offers a new way to attach diagnostically and therapeutically useful radiotracers (e.g., (99m)Tc, (188)Re) to Fab fragments for potential noninvasive imaging and therapy of cancer.

Recent advances in 99mTc radiopharmaceuticals

99mTc radiopharmaceuticals play an important role in widespread applications of nuclear medicine. When 99mTc radiopharmaceuticals first came into use, major efforts were directed toward the development of 99mTc radiopharmaceuticals for bone imaging and for the excretory functions of the liver and kidneys. In the past 20 years, a significant advance has been made in technetium chemistry, which provided 99mTc radiopharmaceuticals for assessment of regional cerebral and myocardial blood flow. Recent efforts have been directed toward the design of 99mTc-labeled compounds for estimating receptor or transporter functions. A number of bifunctional chelating agents that provide 99mTc labeled proteins and peptides of high in vivo stability with high radiochemical yields have also been developed. More recently, organometallic technetium and rhenium compounds have been introduced as another class of 99mTc radiopharmaceutical design. In this manuscript, recent progress in 99mTc radiopharmaceuticals is reviewed with the major emphasis laid on key innovations in this field to provide the 99mTc radiopharmaceuticals available today.