Synthesis and evaluation of hydroxamamide-based tetradentate ligands as a new class of thiol-free chelating molecules for 99mTc radiopharmaceuticals

Development of Technetium-99m-Labeled BODIPY-Based Probes Targeting Lipid Droplets Toward the Diagnosis of Hyperlipidemia-Related Diseases

Hyperlipidemia causes systemic lipid disorder, which leads to hepatic steatosis and atherosclerosis. Thus, it is necessary to detect these syndromes early and precisely to improve prognosis. In the affected regions, abnormal formation and growth of lipid droplets is observed; therefore, lipid droplets may be a suitable target for the diagnosis of hyperlipidemia-related syndromes. In this study, we designed and synthesized [^99mTc]Tc-BOD and [^99mTc]Tc-MBOD composed of one technetium-99m and two BODIPY scaffolds with hydroxamamide (Ham) or N-methylated hydroxamamide (MHam) in radiochemical yields of 54 and 35%, respectively, with a radiochemical purity of over 95%. [^99mTc]Tc-BOD showed significantly higher accumulation levels in foam cells than in non-foam cells (foam cells: 213.8 ± 64.8, non-foam cell: 126.2 ± 26.9 %dose/mg protein, p < 0.05) 2 h after incubation. In contrast, [^99mTc]Tc-MBOD showed similar accumulation levels in foam cells and non-foam cells (foam cells: 92.2 ± 23.3, non-foam cell: 83.8 ± 19.8 %dose/mg protein). In normal mice, [^99mTc]Tc-BOD exhibited gradual blood clearance (0.5 h: 4.98 ± 0.35, 6 h: 1.94 ± 0.12 %ID/g) and relatively high accumulation in the liver 6 h after administration (15.22 ± 1.72 %ID/g). Therefore, [^99mTc]Tc-BOD may have potential as an imaging probe for detecting lipid droplets in disease lesions of hyperlipidemia.

Development of the 99mTc-Hydroxamamide Complex as a Probe Targeting Carbonic Anhydrase IX

Carbonic anhydrase IX (CA-IX) is regarded as a favorable target for in vivo imaging because of its specific expression in hypoxic regions of tumors. Hypoxia assists tumor propagation and growth and is resistant to chemotherapy and radiotherapy. Here, we designed and synthesized [^99mTc]hydroxamamide ([^99mTc]Ham) and [^99mTc]methyl-substituted-hydroxamamide ([^99mTc]MHam) complexes including a bivalent CA-IX ligand, sulfonamide (SA), and ureidosulfonamide (UR). In a cell binding assay, [^99mTc]Ham complexes with bivalent SA ([^99mTc]SAB2A and [^99mTc]SAB2B) and UR ([^99mTc]URB2A and [^99mTc]URB2B) showed significantly greater uptake into CA-IX high-expressing (HT-29) cells than that into CA-IX low-expressing cells. Since the binding affinity of [^99mTc]URB2A and [^99mTc]URB2B for CA-IX was significantly higher than that of [^99mTc]SAB2A and [^99mTc]SAB2B, we additionally synthesized [^99mTc]MURB2 (a [^99mTc]MHam complex with bivalent UR) and evaluated the CA-IX-specific binding affinity of [^99mTc]URB2A, [^99mTc]URB2B, and [^99mTc]MURB2. Their uptake into HT-29 cells was reduced by the addition of a CA inhibitor, acetazolamide, suggesting their CA-IX-specific binding affinity. A biodistribution study in HT-29 tumor-bearing mice was carried out using [^99mTc]URB2A and [^99mTc]MURB2 with the highest specificity for HT-29 cells. [^99mTc]URB2A showed moderate tumor uptake and reduction by coinjection with acetazolamide; however, the tumor/blood ratio was insufficient for in vivo imaging. These results provided key information for the design of novel Ham-based imaging probes targeting CA-IX.

Coordination-Mediated Synthesis of Purification-Free Bivalent 99mTc-Labeled Probes for in Vivo Imaging of Saturable System

In the synthesis of technetium-99m (^99mTc) labeled target-specific ligands, the presence of a large excess of unlabeled ligands over ^99mTc in the injectate hinders target accumulation of ^99mTc-labeled ligands by competing for target molecules. To circumvent the problem, we recently developed a concept of the metal coordination-mediated multivalency, and proved the concept with a ^99mTc-labeled trivalent compound [^99mTc(CO)₃(CN-RGD)₃]⁺. In this study, D-penicillamine (Pen) was selected as a chelating molecule and a cyclic RGDfK peptide was conjugated to Pen via a hexanoic linkage (Pen-Ahx-c(RGDfK)). ^99mTc complexation reaction, and the stability, integrin α_vβ₃ binding affinity, and biodistribution of the ^99mTc-labeled probe were investigated to evaluate the applicability of the concept to bivalent probes. ^99mTc-[Pen-Ahx-c(RGDfK)]₂ was obtained over 95% radiochemical yields under low Pen-Ahx-c(RGDfK) concentration (50 μM). ^99mTc-[Pen-Ahx-c(RGDfK)]₂ showed approximately 10-times higher integrin α_vβ₃ binding affinity than the monovalent compounds, Pen-Ahx-c(RGDfK) and c(RGDyV). In biodistribution studies, the tumor accumulation of ^99mTc-[Pen-Ahx-c(RGDfK)]₂ was decreased to 77% and 43% of HPLC-purified (Pen-Ahx-c(RGDfK)-free) ^99mTc-[Pen-Ahx-c(RGDfK)]₂ by the presence of 5 nmol of unlabeled Pen-Ahx-c(RGDfK) and Re-[Pen-Ahx-c(RGDfK)]₂, respectively. ^99mTc-[Pen-Ahx-c(RGDfK)]₂ provided tumor image without removing unlabeled ligand, while a ^99mTc-labeled monovalent probe prepared from a monovalent ligand could not. These findings indicate the availability of the design concept to prepare ^99mTc-labeled bivalent probes with a variety of ^99mTc core and other metallic radionuclides of clinical relevance.

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.

Bisamidoximes: Synthesis and Complexation with Iron(III)

Bisamidoximes have been synthesized by the reaction of 4-methylbenzohydroximoyl chloride with 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, and 3,3′-diamino-N-methyl-dipropylamine. A monoamidoxime and a trisamidoxime were also prepared in the present work by the reaction of 4-methylbenzohydroximoyl chloride with N,N-dimethylethylenediamine and tris(2-aminoethyl)amine. Single crystal X-ray structures of three of the bisamidoximes have shown that the two amidoxime moieties have the Z configuration in all three compounds. Job’s method of continuous variations showed that three of the bisamidoximes prepared in this work form 1:1 complexes with iron(iii) and therefore are acting as tetradentate ligands.

Facile synthesis of bis(hydroxamamide)-based tetradentate ligands for 99mTc-radiopharmaceutical

A facile, two-step synthesis of the bis(hydroxamamide)-based tetradentate ligands for 99mTc-radiopharmaceuticals is described. Firstly, the hydroxamamide was converted to hydroximic acid chloride by reaction with sodium nitrite in hydrochloric acid at 0 degrees C. Secondly, treating the halide with the ethylenediamine or 1,3-propylenediamine in absolute ethanol formed the desired products, N,N'-ethylene bis(1-(4-nitroimidazole-1-yl)-propan-hydroxyiminoamide) (I) and N,N'-propylene bis(1-(4-nitroimidazole-1-yl)-propanhydroxyiminoamide) (II). The corresponding 99mTc complexes showed high yields and were found by paper electrophoresis to be electrically neutral under physiological conditions. The partition coefficients indicated a distinct difference between the two complexes.

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.

The integrity of the disulfide bond in a cyclic somatostatin analog during 99mTc complexation reactions

Recent development of a variety of thiol-free chelating agents has facilitated the design of 99mTc-labeled somatostatin analogs suitable for receptor imaging of somatostatin-positive tumors. However, it remains ambiguous whether the disulfide bonds in cyclic peptides are stable during 99mTc complexation reactions, and contradictory results have been reported regarding the integrity of disulfide bonds in cyclic somatostatin analogs. To estimate the stability of the disulfide bond in a synthetic somatostatin analog at low peptide concentrations, [125I]I-RC-160, in which radioiodine was incorporated into the 3-Tyr residue, was synthesized and the integrity of the disulfide bond of the peptide was investigated in the presence of reducing agents such as ascorbic acid, dithionite, and stannous ions. The disulfide bond in [125I]I-RC-160 remained stable in the presence of ascorbic acid in boiling water. The disulfide bond was also stable when treated with stannous ions at concentrations sufficient to reduce 99mTc for complexation with a thiol-free chelating agent, bis(hydroxamamide) analog when the 99mTc complexation reaction was performed at room temperature. However, the disulfide bond of [125I]I-RC-160 was slightly cleaved in the presence of a small amount of stannous ions when the reaction was performed in boiling water. Treatment of [125I]I-RC-160 with dithionite in boiling water markedly reduced the disulfide bond of the parental peptide. These findings indicated that synthetic somatostatin analogs may be labeled with 99mTc with stannous ions as the reducing agent without impairing their structure after conjugation of thiol-free chelating agents that provide 99mTc chelates under mild reaction conditions.