Yttrium-90 chelation properties of tetraazatetraacetic acid macrocycles, diethylenetriaminepentaacetic acid analogues, and a novel terpyridine acyclic chelator

Synthesis and Evaluation of Novel 111In-Labeled Picolinic Acid-Based Radioligands Containing an Albumin Binder for Development of a Radiotheranostic Platform

Picolinic acid-based metallic chelators, e.g., neunpa and octapa, have attracted much attention as promising scaffolds for radiotheranostic agents, particularly those containing larger α-emitting radiometals. Furthermore, albumin binder (ALB) moieties, which noncovalently bind to albumin, have been utilized to improve the pharmacokinetics of radioligands targeting various biomolecules. In this study, we designed and synthesized novel neunpa and octapa derivatives (Neunpa-2 and Octapa-2, respectively), which contained a prostate-specific membrane antigen (PSMA)-binding moiety (model targeting vector) and an ALB moiety. We evaluated the fundamental properties of these derivatives as radiotheranostic agents using ¹¹¹In. In a cell-binding assay using LNCaP (PSMA-positive) cells, [¹¹¹In]In-Neunpa-2 and [¹¹¹In]In-Octapa-2 specifically bound to the LNCaP cells. In addition, a human serum albumin (HSA)-binding assay revealed that [¹¹¹In]In-Neunpa-2 and [¹¹¹In]In-Octapa-2 exhibited greater binding to HSA than their non-ALB-conjugated counterparts ([¹¹¹In]In-Neunpa-1 and [¹¹¹In]In-Octapa-1, respectively). A biodistribution assay conducted in LNCaP tumor-bearing mice showed that the introduction of the ALB moiety into the ¹¹¹In-labeled neunpa and octapa derivatives resulted in markedly enhanced tumor uptake and retention of the radioligands. Furthermore, single-photon emission computed tomography imaging of LNCaP tumor-bearing mice with [¹¹¹In]In-Octapa-2 produced tumor images. These results indicate that [¹¹¹In]In-Octapa-2 may be a useful PSMA imaging probe and that picolinic acid-based ALB-conjugated radiometallic complexes may be promising candidates as radiotheranostic agents.

AAZTA5-squaramide ester competing with DOTA-, DTPA- and CHX-A″-DTPA-analogues: Promising tool for 177Lu-labeling of monoclonal antibodies under mild conditions

BACKGROUND

Combining the advantages of both cyclic and acyclic chelator systems, AAZTA (1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-methylperhydro-1,4-diazepine) is well suited for complexation of various diagnostic and therapeutic radiometals such as gallium-68, scandium-44 and lutetium-177 under mild conditions. Due to its specificity for primary amines and pH dependent binding properties, squaric acid (SA) represents an excellent tool for selective coupling of the appropriate chelator to different target vectors. Therefore, the aim of this study was to evaluate radiolabeling properties of the novel bifunctional AAZTA⁵-SA being coupled to a model antibody (bevacizumab) in comparison to DOTA-SA, DTPA-p-Bn-SA and CHX-A″-DTPA-p-Bn-SA using the therapeutic nuclide lutetium-177.

METHODS AND RESULTS

As proof-of-concept, bevacizumab was first functionalized with AAZTA⁵-SA, DOTA-SA, DTPA-p-Bn-SA or CHX-A″-DTPA-p-Bn-SA. After purification via fractionated size exclusion chromatography (SEC), the corresponding immunoconjugates were subsequently radiolabeled with lutetium-177 at pH 7 and room temperature (RT) as well as 37 °C. After 90 min, labeling of AAZTA⁵-SA-mAb resulted in almost quantitative radiochemical yields (RCY) of >98% and >99%, respectively. Formation of [¹⁷⁷Lu]Lu-DTPA-p-Bn-SA-mAb indicated rapid labeling kinetics reaching similar yields at RT already after 30 min. Fast but incomplete radiolabeling of the CHX-A″-analogue could be observed with a yield of 74% after 10 min and no further significant increase. In contrast, ¹⁷⁷Lu-labeling of DOTA-SA-mAb showed negligible radiochemical yields of <2% both at room temperature and 37 °C. In vitro complex stability measurements of [¹⁷⁷Lu]Lu-AAZTA⁵-SA-mAb at 37 °C indicated >94% protein bound activity in human serum and >92% in phosphate buffered saline (PBS), respectively within 15 days. [¹⁷⁷Lu]Lu-DTPA-p-Bn-SA-mAb and [¹⁷⁷Lu]Lu-CHX-A″-DTPA-p-Bn-SA-mAb revealed similar to even slightly higher in vitro stability in both media.

CONCLUSION

Coupling of AAZTA⁵-SA to the monoclonal antibody bevacizumab allowed for ¹⁷⁷Lu-labeling with almost quantitative radiochemical yields both at room temperature and 37 °C. Within 15 days, the resulting radioconjugate indicated very high in vitro complex stability both in human serum and PBS. Therefore, AAZTA⁵-SA is a promising tool for ¹⁷⁷Lu-labeling of sensitive biomolecules such as antibodies for theranostic applications.

Cationic radionuclides and ligands for targeted therapeutic radiopharmaceuticals

This review considers the already used and potential α- and β-emitting cationic radionuclides for targeted radionuclide therapy. Recent results of laboratory, preclinical and clinical applications of these radionuclides are discussed. As opposed to β-emitters, which are already used in nuclear medicine, α-emitters involved in targeted radiopharmaceuticals were subjected to clinical trials only recently and were found to be therapeutically effective. The review summarizes recent trends in the development of ligands as components of radiopharmaceuticals addressing specific features of short-lived cationic radionuclides applied in medicine. Despite a steadily growing number of chelating ligands, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and diethylenetriaminepentaacetic acid (DTPA) remain the most widely used agents in nuclear medicine. The drawbacks of these compounds restrict the application of radionuclides in medicine. Variations in the macrocycle size, the introduction and modification of substituents can significantly improve the chelating ability of ligands, enhance stability of radionuclide complexes with these ligands and eliminate the influence of ligands on the affinity of biological targeting vectors.

The bibliography includes 189 references.

Chemical aspects of metal ion chelation in the synthesis and application antibody-based radiotracers

Radiometals are becoming increasingly accessible and are utilized frequently in the design of radiotracers for imaging and therapy. Nuclear properties ranging from the emission of γ-rays and β+ -particles (imaging) to Auger electron and β- and α-particles (therapy) in combination with long half-lives are ideally matched with the relatively long biological half-life of monoclonal antibodies in vivo. Radiometal labeling of antibodies requires the incorporation of a metal chelate onto the monoclonal antibody. This chelate must coordinate the metal under mild conditions required for the handling of antibodies, as well as provide high kinetic, thermodynamic, and metabolic stability once the metal ion is coordinated to prevent release of the radionuclide before the target site is reached in vivo. Herein, we review the role of different radiometals that have found applications of the design of radiolabeled antibodies for imaging and radioimmunotherapy. Each radionuclide is described regarding its nuclear synthesis, coordinative preference, and radiolabeling properties with commonly used and novel chelates, as well as examples of their preclinical and clinical applications. An overview of recent trends in antibody-based radiopharmaceuticals is provided to spur continued development of the chemistry and application of radiometals for imaging and therapy.

Bispidines for radiopharmaceuticals

Radiometal based radiopharmaceuticals for imaging and therapy require selective ligands (bifunctional chelators, BFCs) that form metal complexes, which are inert against trans-chelation under physiological conditions, linked to a biological vector, directing them to the targeted tissue. Bispidine ligands with a very rigid backbone and widely variable donor sets are reviewed as an ideal class of BFCs, and recent applications are discussed.

Comparative preclinical evaluation of 68Ga-NODAGA and 68Ga-HBED-CC conjugated procainamide in melanoma imaging

Malignant melanoma is the most aggressive form of skin cancer. The early detection of primary melanoma tumors and metastases using non-invasive PET imaging determines the outcome of this disease. Previous studies have shown that benzamide derivatives (e.g. procainamide) conjugated with PET radionuclides specifically bind to melanin pigment of melanoma tumors. ⁶⁸Ga chelating agents can have high influence on physiological properties of ⁶⁸Ga labeled bioactive molecules, as was experienced during the application of HBED-CC on PSMA ligand. The aim of this study was to assess this concept in the case of the melanin specific procaindamide (PCA) and to compare the melanin specificity of ⁶⁸Ga-labeled PCA using HBED-CC and NODAGA chelators under in vitro and in vivo conditions. Procainamide (PCA) was conjugated with HBED-CC and NODAGA chelators and was labeled with Ga-68. The melanin specificity of ⁶⁸Ga-HBED-CC-PCA and ⁶⁸Ga-NODAGA-PCA was investigated in vitro and in vivo using amelanotic (MELUR and A375) and melanin containing (B16-F10) melanoma cell lines. Tumor-bearing mice were prepared by subcutaneous injection of B16-F10, MELUR and A375 melanoma cells into C57BL/6 and SCID mice. 21±2days after tumor cell inoculation and 90min after intravenous injection of the ⁶⁸Ga-labelledlabeled radiopharmacons whole body PET/MRI scans were performed. ⁶⁸Ga-NODAGA-PCA and ⁶⁸Ga-HBED-CC-PCA were produced with excellent radiochemical purity (98%). In vitro experiments demonstrated that after 30 and 90min incubation time ⁶⁸Ga-NODAGA-PCA uptake of B16-F10 cells was significantly (p≤0.01) higher than the ⁶⁸Ga-HBED-CC-conjugated PCA accumulation in the same cell line. Furthermore, significant difference (p≤0.01 and 0.05) was found between the uptake of melanin negative and positive cell lines using ⁶⁸Ga-NODAGA-PCA and ⁶⁸Ga-HBED-CC-PCA. In vivo PET/MRI studies using tumor models revealed significantly (p≤0.01) higher ⁶⁸Ga-NODAGA-PCA uptake (SUVmean: 0.46±0.05, SUVmax: 1.96±0.25,T/M ratio: 40.7±4.23) in B16-F10 tumors in contrast to ⁶⁸Ga-HBED-CC-PCA where the SUVmean, SUVmax and T/M ratio were 0.13±0.01, 0.56±0.11 and 11.43±1.24, respectively. Melanin specific PCA conjugated with NODAGA chelator showed higher specific binding properties than conjugated with HBED-CC. The chemical properties of the bifunctional chelators used for ⁶⁸Ga-labeling of PCA determine the biological behaviour of the probes. Due to the high specificity and sensitivity ⁶⁸Ga-labeled PCA molecules are promising radiotracers in melanoma imaging.

Picolinic acid based acyclic bifunctional chelating agent and its methionine conjugate as potential SPECT imaging agents: syntheses and preclinical evaluation

Syntheses and preclinical evaluation of picolinic acid based acyclic bifunctional chelating agent and its methionine conjugate as SPECT imaging agents.

What a difference a carbon makes: H₄octapa vs H₄C3octapa, ligands for In-111 and Lu-177 radiochemistry

The acyclic ligands H₄C3octapa and p-SCN-Bn-H₄C3octapa were synthesized for the first time, using nosyl protection chemistry. These new ligands were compared to the previously studied ligands H₄octapa and p-SCN-Bn-H₄octapa to determine the extent to which the addition of a single carbon atom to the backbone of the ligand would affect metal coordination, complex stability, and, ultimately, utility for in vivo radiopharmaceutical applications. Although only a single carbon atom was added to H₄C3octapa and the metal donor atoms and denticity were not changed, the solution chemistry and radiochemistry properties were drastically altered, highlighting the importance of careful ligand design and radiometal–ligand matching. It was found that [In(C3octapa)]⁻ and [Lu(C3octapa)]⁻ were substantially different from the analogous H₄octapa complexes, exhibiting fluxional isomerization and a higher number of isomers, as observed by ¹H NMR, VT-NMR, and 2D COSY/HSQC-NMR experiments. Past evaluation of the DFT structures of [In(octapa)]⁻ and [Lu(octapa)]⁻ revealed very symmetric complexes; in contrast, the [In(C3octapa)]⁻ and [Lu(C3octapa)]⁻ complexes were much less symmetric, suggesting lower symmetry and less rigidity than that of the analogous H₄octapa complexes. Potentiometric titrations revealed the formation constants (log K_ML, pM) were ∼2 units lower for the In³⁺ and Lu³⁺ complexes of H₄C3octapa when compared to that of the more favorable H₄octapa ligand (∼2 orders of magnitude less thermodynamically stable). The bifunctional ligands p-SCN-Bn-H₄C3octapa and p-SCN-Bn-H₄octapa were conjugated to the antibody trastuzumab and radiolabeled with ¹¹¹In and ¹⁷⁷Lu. Over a 5 day stability challenge experiment in blood serum, ¹¹¹In-octapa– and ¹¹¹In-C3octapa–trastuzumab immunoconjugates were determined to be ∼91 and ∼24% stable, respectively, and ¹⁷⁷Lu-octapa– and ¹⁷⁷Lu-C3octapa–trastuzumab, ∼89% and ∼4% stable, respectively. This work suggests that 5-membered chelate rings are superior to 6-membered chelate rings for large metal ions like In³⁺ and Lu³⁺, which is a crucial consideration for the design of bifunctional chelates for bioconjugation to targeting vectors for in vivo work.

New ligands H₄C3octapa and p-SCN-Bn-H₄C3octapa were synthesized and compared to the previously studied ligands H₄octapa and p-SCN-Bn-H₄octapa to determine the extent to which the addition of a single carbon atom to the backbone of the ligand would affect metal coordination, complex stability, and, ultimately, utility for in vivo radiopharmaceutical applications. It was found that [In(C3octapa)]⁻ and [Lu(C3octapa)]⁻ were substantially different from the analogous H₄octapa complexes.

High-denticity ligands based on picolinic acid for 111In radiochemistry

Four new acyclic ligands, Bn-H3nonapa (3), H3nonapa (4), p-NO2-Bn-H3nonapa (10), and Bn-H3trenpa (7), were synthesized and studied with nonradioactive In3+ and with radioactive 111In3+. The coordination of these ligands to In3+ was confirmed by high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy. Radiolabeling experiments were performed with 111In3+; these demonstrated H3nonapa (4) to be the best indium ligand of those studied herein, achieving radiochemical yields of ∼97% in 10 min at ambient temperature, and stability to transchelation in mouse serum of 44.5% ± 25.9% after 24 h. Although the radiolabeling kinetics of H3nonapa (4) were excellent, serum stability results were inferior to the previously studied ligands DOTA, DTPA, and H4octapa, suggesting that the presented ligands may find their optimum radiopharmaceutical applications with isotopes other than 111In. Owing to the high denticity of these ligands (9–10 coordinate), they may realize their potential with large ion isotopes such as 177Lu, 86/90Y, and 225Ac.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.

H(4)octapa-trastuzumab: versatile acyclic chelate system for 111In and 177Lu imaging and therapy

A bifunctional derivative of the versatile acyclic chelator H4octapa, p-SCN-Bn-H4octapa, has been synthesized for the first time. The chelator was conjugated to the HER2/neu-targeting antibody trastuzumab and labeled in high radiochemical purity and specific activity with the radioisotopes (111)In and (177)Lu. The in vivo behavior of the resulting radioimmunoconjugates was investigated in mice bearing ovarian cancer xenografts and compared to analogous radioimmunoconjugates employing the ubiquitous chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). The H4octapa-trastuzumab conjugates displayed faster radiolabeling kinetics with more reproducible yields under milder conditions (15 min, RT, ~94-95%) than those based on DOTA-trastuzumab (60 min, 37 °C, ~50-88%). Further, antibody integrity was better preserved in the (111)In- and (177)Lu-octapa-trastuzumab constructs, with immunoreactive fractions of 0.99 for each compared to 0.93-0.95 for (111)In- and (177)Lu-DOTA-trastuzumab. These results translated to improved in vivo biodistribution profiles and SPECT imaging results for (111)In- and (177)Lu-octapa-trastuzumab compared to (111)In- and (177)Lu-DOTA-trastuzumab, with increased tumor uptake and higher tumor-to-tissue activity ratios.

Radiosynthesis of 68Ga-labelled DOTA-biocytin (68Ga-r-BHD) and assessment of its pharmaceutical quality for clinical use

OBJECTIVES

Biocytin analogues labelled with indium-111, yttrium-90 and lutetium-177 have shown their effectiveness in the imaging of infections/inflammation in patients with osteomyelitis and function as efficient tools in pretargeted antibody-guided radioimmunotherapy. In this study, the labelling of a biocytin analogue coupled with DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), namely, r-BHD, with gallium-68 (68Ga) was optimized, and the quality and stability of the preparations were assessed for clinical use.

MATERIALS AND METHODS

Synthesis of 68Ga-r-BHD was carried out by heating a fraction of the 68Ge/68Ga eluate in a reactor containing the biocytin analogue with the appropriate buffer. The influence of the precursor amount (from 2.5 to 140 nmol), the pH of the reaction (from 2 to 5.5) and the buffer species (1.5 mol/l sodium acetate, 1.5 mol/l sodium formate, 4.5 mol/l HEPES) on radiochemical yield and radiochemical purity was assessed. Studies on stability and binding to avidin (Av) were also conducted in different media.

RESULTS

Under the best labelling condition (56 nmol of precursor, 3.8 pH, sodium formate buffer) synthesis of 68Ga-r-BHD resulted in a yield of 64 ± 3% (not decay corrected). Radiochemical purity was around 95% because a 68Ga-coordinated sulfoxide form of the ligand was detected as a by-product of the reaction (68Ga-r-SBHD). The by-product was identified and characterized by liquid chromatography-electrospray ionization tandem mass spectrometry. At the natural 1 : 4 Av/68Ga-r-BHD molar ratio, affinity results were 62 ± 2 and 80 ± 2% in saline and human serum, respectively. Stability of 68Ga-r-BHD and of the radiotracer/Av complex remains almost constant over 180 min. 68Ga-r-BHD appears to be a good candidate for clinical applications.

(68)Ga-labeled bombesin analogs for receptor-mediated imaging

Targeted receptor-mediated imaging techniques have become crucial tools in present targeted diagnosis and radiotherapy as they provide accurate and specific diagnosis of disease information. Peptide-based pharmaceuticals are gaining popularity, and there has been vast interest in developing (68)Ga-labeled bombesin (Bn) analogs. The gastrin-releasing peptide (GRP) family and its Bn analog have been implicated in the biology of several human cancers. The three bombesin receptors GRP, NMB, and BRS-3 receptor are most frequently ectopically expressed by common, important malignancies. The low expression of Bn/GRP receptors in normal tissue and relatively high expression in a variety of human tumors can be of biological importance and form a molecular basis for Bn/GRP receptor-mediated imaging. To develop a Bn-like peptide with favorable tumor targeting and pharmacokinetic characteristics for possible clinical use, several modifications in the Bn-like peptides, such as the use of a variety of chelating agents, i.e., acyclic and macrocyclic agents with different spacer groups and with different metal ions (gallium), have been performed in recent years without significant disturbance of the vital binding scaffold. The favorable physical properties of (68)Ga, i.e., short half-life, and the fast localization of small peptides make this an ideal combination to study receptor-mediated imaging in patients.

Synthesis and preclinical evaluation of bifunctional ligands for improved chelation chemistry of 90Y and 177Lu for targeted radioimmunotherapy

We report a practical and high-yield synthesis of a bimodal bifunctional ligand 3p-C-NETA-NCS containing the isothiocyanate group for conjugation to a tumor targeting antibody. 3p-C-NETA-NCS was conjugated to a tumor-targeting antibody, trastuzumab, and the corresponding 3p-C-NETA-trastuzumab conjugate was evaluated and compared to trastuzumab conjugates of the known bifunctional ligands C-DOTA, C-DTPA, and 3p-C-DEPA for radiolabeling kinetics with (90)Y and (177)Lu. 3p-C-NETA-trastuzumab conjugate exhibited extremely rapid complexation kinetics with (90)Y and (177)Lu. (90)Y-3p-C-NETA-trastuzumab and (177)Lu-3p-C-NETA-trastuzumab conjugates were stable in human serum for 2 weeks. A pilot biodistribution study was conducted to evaluate in vivo stability and tumor targeting of (177)Lu-radiolabeled trastuzumab conjugate using nude mice bearing ZR-75-1 human breast cancer. (177)Lu-3p-C-NETA-trastuzumab conjugate displayed low radioactivity level at blood (1.6%), low organ uptake (<2.2%), and high tumor-to-blood ratio (6.4) at 120 h. 3p-C-NETA possesses favorable in vitro and in vivo profiles and is an excellent bifunctional chelator that can be used for targeted RIT applications using (90)Y and (177)Lu and has the potential to replace DOTA and DTPA analogues in current clinical use.

H4octapa: an acyclic chelator for 111In radiopharmaceuticals

This preliminary investigation of the octadentate acyclic chelator H(4)octapa (N(4)O(4)) with (111)In/(115)In(3+) has demonstrated it to be an improvement on the shortcomings of the current industry "gold standards" DOTA (N(4)O(4)) and DTPA (N(3)O(5)). The ability of H(4)octapa to radiolabel quantitatively (111)InCl(3) at ambient temperature in 10 min with specific activities as high as 2.3 mCi/nmol (97.5% radiochemical yield) is presented. In vitro mouse serum stability assays have demonstrated the (111)In complex of H(4)octapa to have improved stability when compared to DOTA and DTPA over 24 h. Mouse biodistribution studies have shown that the radiometal complex [(111)In(octapa)](-) has exceptionally high in vivo stability over 24 h with improved clearance and stability compared to [(111)In(DOTA)](-), demonstrated by lower uptake in the kidneys, liver, and spleen at 24 h. (1)H/(13)C NMR studies of the [In(octapa)](-) complex revealed a 7-coordinate solution structure, which forms a single isomer and exhibits no observable fluxional behavior at ambient temperature, an improvement to the multiple isomers formed by [In(DTPA)](2-) and [In(DOTA)](-) under the same conditions. Potentiometric titrations have determined the thermodynamic formation constant of the [In(octapa)](-) complex to be log K(ML) = 26.8(1). Through the same set of analyses, the [(111/115)In(decapa)](2-) complex was found to have nonoptimal stability, with H(5)decapa (N(5)O(5)) being more suitable for larger metal ions due to its higher potential denticity (e.g., lanthanides and actinides). Our initial investigations have revealed the acyclic chelator H(4)octapa to be a valuable alternative to the macrocycle DOTA for use with (111)In, and a significant improvement to the acyclic chelator DTPA.

Influence of cations on the complexation yield of DOTATATE with yttrium and lutetium: a perspective study for enhancing the 90Y and 177Lu labeling conditions

The DOTA macrocyclic ligand can form stable complexes with many cations besides yttrium and lutetium. For this reason, the presence of competing cationic metals in yttrium-90 and lutetium-177 chloride solutions can dramatically influence the radiolabeling yield. The aim of this study was to evaluate the coordination yield of yttrium- and lutetium-DOTATATE complexes when the reaction is performed in the presence of varying amounts of competing cationic impurities. In the first set of experiments, the preparation of the samples was performed by using natural yttrium and lutetium (20.4 nmol). The molar ratio between DOTATATE and these metals was 1 to 1. Metal competitors (Pb(2+), Zn(2+), Cu(2+), Fe(3+), Al(3+), Ni(2+), Co(2+), Cr(3+)) were added separately to obtain samples with varying molar ratio with respect to yttrium or lutetium (0.1, 0.5, 1, 2 and 10). The final solutions were analyzed through ultra high-performance liquid chromatography with an UV detector. In the second set of experiments, an amount of (90)Y or (177)Lu chloride (6 MBq corresponding to 3.3 and 45 pmol, respectively) was added to the samples, and a radio-thin layer chromatography analysis was carried out. The coordination of Y(3+) and Lu(3+) was dramatically influenced by low levels of Zn(2+), Cu(2+) and Co(2+). Pb(2+) and Ni(2+) were also shown to be strong competitors at higher concentrations. Fe(3+) was expected to be a strong competitor, but the effect on the incorporation was only partly dependent on its concentration. Al(3+) and Cr(3+) did not compete with Y(3+) and Lu(3+) in the formation of DOTATATE complexes.

A highly effective bifunctional ligand for radioimmunotherapy applications

A novel bifunctional ligand (3p-C-NETA) for antibody-targeted radioimmunotherapy (RIT) of β-emitting radioisotopes (90)Y and (177)Lu was efficiently synthesized via an unexpected regiospecific ring opening of an aziridinium ion. 3p-C-NETA instantly formed a very stable complex with (90)Y or (177)Lu. 3p-C-NETA is an excellent bifunctional ligand for RIT.

An adapted purification procedure to improve the quality of 90Y for clinical use

There is an increasing interest for 90Y for radionuclide therapy. However, radioimmunotherapy, one of the most important applications for 90Y, demands a very high purity product. Obtaining a high quality 90Y is difficult not only because of the complex and time consuming production schemes but also because of the quality control which has challenging tasks like the determination of 90Sr at very low concentrations. The present paper investigates a reported purification procedure for the removal of stable metal trace contaminants from an 90Y solution, seeking for its potential use in the elimination of the radioactive contaminant 90Sr and its fast determination. For this purpose a washing step with HNO3 acid is introduced to elute 90Sr, the order of each acid solution is rearranged to reduce the potential contaminants present in acids and the size of the column is reduced to further optimize the procedure. As a result, an improved purification method is obtained, which allows the removal of both trace metal contaminants and 90Sr from an 90Y solution and the measurement of 90Sr/90Y ratios of the order of 10-7, which are well below the established pharmacopeia limit of 2×10-5.

177Lu-labeled-VG76e monoclonal antibody in tumor angiogenesis: A comparative study using DOTA and DTPA chelating systems

Vascular Endothelial Growth Factor (VEGF) is one of the molecules which regulate angiogenesis, a phenomenon observed in many diseases, including cancer. VG76e, an anti-VEGF monoclonal antibody, was labeled with 177Lu via p-SCN-Bz-DOTA and CHX-A″-DTPA chelating systems, in order to investigate its possible therapeutic use. Labeling was performed by a 30 min incubation of 177LuCl3 and each immunoconjugate, at 37 °C. Radiochemical analysis showed the formation of a single radioactive species, at a yield higher than 98%, for both immunoconjugates. Kits have been formulated for both VG76e-DOTA and VG76e-DTPA. Stability studies, in the presence of a competitor excess, showed that both radiolabeled species remained sufficiently stable (95%) for at least 48 h. Biodistribution results in normal mice were similar for both radioimmunoconjugates, with no significant bone uptake. Gamma camera images of tumor-bearing mice showed satisfactory visualization of the tumor 24 h p.i., while a higher uptake was observed at 48 h p.i. Our findings indicate that both the bifunctional chelating agents p-SCN-Bz-DOTA and CHX-A″-DTPA can be used for the labeling of VG76e with 177Lu, with high labeling yield and stability. Their in vivo behaviour in normal and tumor-bearing mice looks promising and they can be successfully used for tumor imaging studies.

Melanoma imaging using (111)In-, (86)Y- and (68)Ga-labeled CHX-A''-Re(Arg11)CCMSH

INTRODUCTION

A novel alpha-melanocyte-stimulating hormone peptide analog CHX-A''-Re(Arg(11))CCMSH, which targeted the melanocortin-1 receptor (MC1-R) overexpressed on melanoma cells, was investigated for its biodistribution and tumor imaging properties.

METHODS

The metal bifunctional chelator CHX-A'' was conjugated to the melanoma targeting peptide (Arg(11))CCMSH and cyclized by Re incorporation to yield CHX-A''-Re(Arg(11))CCMSH. CHX-A''-Re(Arg(11))CCMSH was labeled with (111)In, (86)Y and (68)Ga, and the radiolabeled peptides were examined in B16/F1 melanoma-bearing mice for their pharmacokinetic as well as their tumor targeting properties using small animal SPECT and PET.

RESULTS

The radiolabeling efficiencies of the (111)In-, (86)Y- and (68)Ga-labeled CHX-A''-Re(Arg(11))CCMSH peptides were >95%, resulting in specific activities of 4.44, 3.7 and 1.85 MBq/microg, respectively. Tumor uptake of the (111)In-, (86)Y- and (68)Ga-labeled peptides was rapid with 4.17+/-0.94, 4.68+/-1.02 and 2.68+/-0.69 %ID/g present in the tumors 2 h postinjection, respectively. Disappearance of radioactivity from the normal organs and tissues was rapid with the exception of the kidneys. Melanoma tumors were imaged with all three radiolabeled peptides 2 h postinjection. MC1-R-specific uptake was confirmed by competitive receptor blocking studies.

CONCLUSIONS

Melanoma tumor uptake and imaging was exhibited by the (111)In-, (86)Y- and (68)Ga-labeled Re(Arg(11))CCMSH peptides, although the tumor uptake was moderated by low specific activity. The facile radiolabeling properties of CHX-A''-Re(Arg(11))CCMSH allow it to be employed as a melanoma imaging agent with little or no purification after (111)In, (86)Y and (68)Ga labeling.

Bifunctional coupling agents for radiolabeling of biomolecules and target-specific delivery of metallic radionuclides

Receptor-based radiopharmaceuticals are of great current interest in molecular imaging and radiotherapy of cancers, and provide a unique tool for target-specific delivery of radionuclides to the diseased tissues. In general, a target-specific radiopharmaceutical can be divided into four parts: targeting biomolecule (BM), pharmacokinetic modifying (PKM) linker, bifunctional coupling or chelating agent (BFC), and radionuclide. The targeting biomolecule serves as a "carrier" for specific delivery of the radionuclide. PKM linkers are used to modify radiotracer excretion kinetics. BFC is needed for radiolabeling of biomolecules with a metallic radionuclide. Different radiometals have significant difference in their coordination chemistry, and require BFCs with different donor atoms and chelator frameworks. Since the radiometal chelate can have a significant impact on physical and biological properties of the target-specific radiopharmaceutical, its excretion kinetics can be altered by modifying the coordination environment with various chelators or coligand, if needed. This review will focus on the design of BFCs and their coordination chemistry with technetium, copper, gallium, indium, yttrium and lanthanide radiometals.

Preparation of high specific activity (86)Y using a small biomedical cyclotron

86Y is an attractive PET radionuclide due to its intermediate half-life. (86)Y was produced via the 86Sr(p,n)86Y nuclear reaction. Enriched SrCO3 or SrO was irradiated with 2-6 microA of beam current for <4 h on a CS-15 cyclotron. It was shown that the SrO target could withstand at least 6 microA of beam current, a significant improvement over a maximum of 2 microA on the SrCO3 target. Average yields of 4.5 mCi/microA.h were achieved with SrO, which represent 71% of the theoretical yield, compared to 2.3 mCi/microA.h with SrCO3. The radioisotopic contaminants were (86m)Y (220%), 87Y (0.27%), (87m)Y (0.43%) and 88Y (0.024%). 86Y was isolated in an electrochemical cell consisting of three Pt electrodes. The solution was electrolyzed at 2000 mA (40 min) using two Pt plate electrodes. A second electrolysis (230 mA for 20 min) was performed using one Pt plate and a Pt wire. On average, 97.1% of the 86Y was recollected on the Pt wire after a second electrolysis. The (86)Y was collected from the Pt wire using 2.8 M HNO3/EtOH (3:1). After evaporation, 86Y was reconstituted in 100 microl of 0.1 M HCl. Target materials were recovered as SrCO3 and then converted to SrO by thermal decomposition at 1150 degrees C. Specific activity of 86Y was determined to be 29+/-19 mCi/microg via titration of 86Y(OAc)3 with DOTA or DTPA. We have established techniques for the routine, economical production of high purity, high specific activity 86Y on a small biomedical cyclotron that are translatable to other institutions.

90Y Labeling of monoclonal antibody MOv18 and preclinical validation for radioimmunotherapy of human ovarian carcinomas

The monoclonal antibody (mAb) MOv18 binds the membrane alpha isoform of the folate receptor (FR) which is overexpressed in human ovarian carcinoma cells. Exploiting the targeting capacity of this mAb, we developed and preclinically validated a protocol for the stable labeling of the mAb with 90Y, an isotope which has shown promise in cancer radioimmunotherapy. MOv18 was derivatized with the stable macrocyclic ligand p-isothiocyanatobenzyl-1,4,7,10-tetraazacyclododecane-1,4,7,10- tetraacetic acid (Bz-DOTA). MOv18-Bz-DOTA conjugates were labeled with 90Y or 111In under metal-free and good laboratory practice conditions. At the optimal Bz-DOTA/mAb derivatization ratio of 4-5, conjugates maintained binding activity up to 6 months, were efficiently labeled with 90Y or 111In (mean labeling yield 85 and 64%, associated to a final mean specific activity of 74 and 37 MBq/mg) and displayed a mean immunoreactivity of 60 and 58%, respectively. The radiolabeled preparations were stable in human serum, with >97% radioactivity associated to mAb at 48 h after labeling. The ability of 90Y- and 111In-MOv18 to localize FR on tumors in vivo was analyzed in nude mice bearing tumors induced by isogenic cell lines differing only in the presence or absence of the relevant antigen [A431FR (FR-positive) and A431tMock (FR-negative)]. In vivo biodistribution in organs other than tumor was comparable in non-tumor-, A431tMock- and A431FR-bearing mice, whereas the median tumor uptake of the radiolabeled reagents, expressed as area under the curve (AUC) and maximum uptake (Umax), was significantly higher (sixfold to sevenfold) in A431FR than in A431tMock tumors (P=0.0465 and P=0.0332, respectively). Mean maximum uptake (% ID/g) for 90Y-MOv18 was 53.7 and 7.4 in A431FR and A431tMock respectively; corresponding values for 111In-Mov18 were 45.0 and 11.3. These data demonstrate the feasibility of 90Y-labeling of MOv18 without compromising antibody binding ability and the immunoreagent-specific localization in vivo on FR-expressing tumors, suggesting the suitability of 90Y-MOv18 for clinical studies.

Pharmacokinetics, Biodistribution, and Radioimmunotherapy with Monoclonal Antibody 776.1 in a Murine Model of Human Ovarian Cancer

776.1 is a murine IgG1 monoclonal antibody to the human ovarian cancer antigen CA 125 that has the unique property of having a clear preference for binding to the cell-associated form of the antigen. We have examined the tumor localization properties and efficacy of 776.1 in a subcutaneous OVCAR-3 xenograft mouse model of human ovarian cancer. Biodistribution experiments using (125)I-labeled 776.1 demonstrated a peak uptake in tumors at 72 hours postinjection, with an average of 17.7% of injected dose per gram localized to the tumor. Little uptake in other organs was observed. Further experiments using CA 125-transfected syngeneic tumors, as well as an immunoprecipitation assay using human chimeric 776.1, both clearly demonstrated that 776.1 localizes to the tumor in a CA 125-dependent manner. DOTA-776.1 (1,4,7,10-tetraazacyclododecane-N,N',N",N'" tetraacetic acid-conjugated 776.1) was labeled with (90)Y and used in efficacy studies. [(90)Y-DOTA]776.1 at a single dose of 150 microCi was able to mediate efficient reduction of tumor growth, with regression observed in a subset of animals for a period ranging from 3 to 48 days, equivalent to 3 weekly administrations of cisplatin at 6 mg/kg. No significant regression was observed in groups receiving [(90)Y-DOTA]MOPC-21 control antibody at any dose. These results suggest that 776.1 may be a promising radioimmunotherapeutic agent for the treatment of human ovarian cancer.

Influence of trans-1,2-diaminocyclohexane structure and mixed carboxylic/phosphonic group combinations on samarium-153 chelation capacity and stability

A simple procedure was developed to compare chelating agents for (153)Sm complexes as a preliminary step to synthesise bifunctional analogues. Several variables affecting the efficiency of complex stability were investigated, such as the pre-organisation concept, cavity size, and the nature of coordination sites. Four semi-rigid agents incorporating carboxylic and/or phosphonic groupings fixed at trans-1,2-diaminocyclohexane were evaluated for their (153)Sm chelation properties, and competition studies were performed. Data on the stability of the best chelating agent compound 3: trans-cyclohexane-1,2-bis(aminomethylphosphonic)-N,N'-bis(ethyl-2-iminodiacetic acid) in human serum are presented.

Isotachophoretic determination of stability constants of Ho and Y complexes with diethylenetriaminepentaacetic acid and 1,4,7,10-tetraazadodecane-N,N',N'',N'''-tetraacetic acid

The method of capillary isotachophoresis with conductivity detection was applied for the determination of the physico-chemical characteristics (conditional stability constants log beta') of holmium and yttrium complexes with DTPA (diethylenetriaminepentaacetic acid) and DOTA (1,4,7,10-tetraazadodecane-N,N',N'',N'''-tetraacetic acid). The log beta' determination is based on the linear relation between the stability constants of lanthanide-DTPA (lanthanide-DOTA) complexes and the reduction of the zone of the complex owing to the bleeding phenomena (liberating free metal ion). The stability constants calculated using this relationship are comparable with the literary data obtained by other methods for both holmium (log beta'(Ho-DTPA)=21.9, log beta'(Ho-DOTA)=24.5) and yttrium complexes (log beta'(Y-DTPA)=21.2, log beta'(Y-DOTA)=24.4). Capillary isotachophoresis was applied for the determination of the optimum composition of the reaction mixture (metal:ligand ratio) as well.

Radiolabeling morpholinos with 90Y, 111In, 188Re and 99mTc

This laboratory is investigating morpholinos (MORF), a DNA analogue, for radiopharmaceutical applications. While we routinely radiolabel with (99m)Tc, we have now labeled MORFs with (111)In, (188)Re and (90)Y in anticipation of therapeutic studies.

METHODS

A 25 mer MORF with a primary amine on the 3' equivalent end attached via a 10 member linker was conjugated with an isothiocyanate backbone derivative of DOTA (for labeling with (111)In and (90)Y) and with NHS-MAG(3) (for labeling with (188)Re and (99m)Tc). The in vitro stability of labeled MORFs were investigated and biodistribution was carried out in normal mice.

RESULTS

As evident by size exclusion HPLC, ITLC and Sep-Pak analysis, all four radiolabeled MORFs were successfully radiolabeled. In each case, the labeled MORFs showed one sharp peak in HPLC that shifted completely to earlier retention times following addition of a polymer conjugated with the complementary MORF. In saline at room temperature and in 37 degrees C serum, the radioactivity profile of (111)In, (188)Re and (99m)Tc was unchanged over 48 h while over the same period, the (90)Y profile showed a pronounced lower molecular weight peak which did not shift and was shown to be most probably due to (90)Y-DOTA resulting from radiolysis. In addition, the recovery of (188)Re on HPLC decreased as samples aged probably due to oxidation to perrhenate which was retained by the HPLC column. The biodistributions at 1, 3 and 6 h in normal mice showed no important differences among all four labels with the exception that levels of radioactivity in stomach and thyroid were higher in the case of (188)Re due to in vivo oxidation of the radiolabel to perrhenate.

CONCLUSIONS

When radiolabeled with DOTA, (90)Y-labeled MORF showed increased instabilities relative to that of (111)In and when radiolabeled with MAG(3), (188)Re showed in vitro and in vivo instabilities compared to (99m)Tc, but all labels were still largely intact after 48 h in saline or serum. Possibly because of the rapid clearance of MORFs, no important differences in biodistribution among (90)Y, (111)In and (99m)Tc labels were evident in normal mice. These strategies for labeling MORF with (90)Y and (188)Re therefore appear to be suitable for therapeutic applications although both show some evidence of instabilities.

New modular delivery system for diagnostic and therapeutic pre-targeting using tautomer-specific monoclonal antibody EM-6-47 and 3-substituted adenines

We have developed a new modular affinity system for the 2-step delivery of functional molecules to target cells. The system is based on the tautomer-specific monoclonal antibody (MAb) EM-6-47, which binds to 3- and 3,8-substituted adenines with high affinity (Ka > 10(9) l/mol) without cross-reacting with naturally occurring purine derivatives. This MAb serves as the hapten-specific fusion partner to produce bispecific MAbs (bs-MAbs) recognizing a target cell antigen and a low-m.w. hapten as carrier molecule for, e.g., radionuclides. Either the C-8 or the N-3 position of adenines can be used for conjugation with effector molecules; the remaining position may be substituted with different moieties to modulate the pharmacokinetics of the haptens. Different 3- and 3,8-substituted adenines conjugated to the chelates DOTA and DTPA or to the drug daunomycin were synthesized. Adenine-chelate derivatives were efficiently labeled with (111)In and 90Y, while high-affinity binding of 3-substituted adenines to MAb EM-6-47 remained almost unaffected by the conjugation to radiochelates. To confirm the validity of the delivery system, a prototype bs-MAb, EM-168-47, was generated by somatic cell fusion of MAb EM-6-47 and MAb EM-168-2, the latter recognizing a surface antigen on canine hematopoietic cells. Two-step targeting assays in vitro verified the bs-MAb-mediated, dose-dependent delivery of (111)In-labeled adenine-chelate derivatives to myeloid cells. This system represents a powerful tool for new pre-targeting approaches relying on bs-MAbs and low-m.w. haptens. Suitable cellular antigens can be targeted by fusing the appropriate MAbs with hapten-specific MAb EM-6-47, and tailor-made 3-substituted adenines may be labeled with diagnostic or therapeutic radionuclides, cytotoxic drugs or other functional molecules.

Samarium-153 and lutetium-177 chelation properties of selected macrocyclic and acyclic ligands

We describe a simple in vitro characterization of chelation that is useful when choosing an appropriate ligand-metal combination for clinical applications. These properties include the effect of concentration on chelation efficiency, time to maximum chelation, and stability in acidic and serum environments. The macrocyclic ligands nitro-DOTA and nitro-PADOTA, the acyclic ligands nitro-CHX-A-DTPA, nitro-MX-DTPA, DTPA, and a novel terpyridine ligand, TMT-amine, were evaluated as chelate complexes of both intermediate energy beta-emitting lanthanides lutetium-177 and samarium-153. The data were compared to results obtained in a previously published study with yttrium-90. Acid lability, time to achieve maximum chelation, and stability in human serum are properties unique to each ligand-metal combination and should be evaluated prior to choosing an appropriate combination for therapeutic applications. Concentration dependence and duration of chelation are general properties of lanthanide and yttrium chelation that can be applied to an appropriate ligand-metal combination to achieve optimum chelation efficiencies.