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. 2015 Oct 20;6(11):1162–1166. doi: 10.1021/acsmedchemlett.5b00362

Phosphonate Pendant Armed Propylene Cross-Bridged Cyclam: Synthesis and Evaluation as a Chelator for Cu-64

Nikunj Bhatt †, Nisarg Soni †, Yeong Su Ha †, Woonghee Lee †, Darpan N Pandya †, Swarbhanu Sarkar †, Jung Young Kim ‡, Hochun Lee §, Sun Hee Kim ∥ , Gwang Il An ‡, Jeongsoo Yoo †,*
PMCID: PMC4645244  PMID: 26617972

Abstract

graphic file with name ml-2015-00362n_0009.jpg

A propylene cross-bridged macrocyclic chelator with two phosphonate pendant arms (PCB-TE2P) was synthesized from cyclam. Various properties of the synthesized chelator, including Cu-complexation, Cu-complex stability, 64Cu-radiolabeling, and in vivo behavior, were studied and compared with those of a previously reported propylene cross-bridged chelator (PCB-TE2A).

Keywords: Bifunctional chelator, propylene cross bridge, phosphonate pendant arms, radiopharmaceuticals

The development of target-specific radiopharmaceuticals using Cu(II) ions requires the use of bifunctional chelators (BFCs). A wide range of acyclic and macrocyclic chelators have been utilized to coordinate Cu(II) ion.1−8

Our group has introduced various types of azamacrocyclic chelators used to form stable Cu(II) complexes, including TE2A,9 TE2A-Bn-NCS,10 MM/DM-TE2A,11 PCB-TE2A,12 and PCB-TE2A-NCS.13 Among these azamacrocyclic chelators, PCB-TE2A was found to form the more stable Cu-complex, which was better than that formed by the chelator CB-TE2A. However, rapid radiolabeling of BFCs is recommended to prevent or minimize radiolytic damage of targeting vector in conjugates,14 and the harsh radiolabeling conditions and slow labeling kinetics of the azamacrocyclic chelators restrict the use of such BFCs in diagnostic and therapeutic applications.

The ability of BFCs to coordinate Cu(II) ions is improved by replacing the acetic acid pendant arm with a phosphonate pendant arm, resulting in copper complexes with improved physical properties.15−19 To take advantage of fast metal binding kinetics, cross-bridged BFCs having a phosphonate pendant arm (CB-TE2P and CB-TE1A1P) have been introduced.20,21

Chelators CB-TE2P and CB-TE1A1P were radiolabeled with 64Cu at room temperature. Unfortunately, the advantages of the changed pendant arm were overshadowed by decreased Cu-complex stability. Although CB-TE2P and CB-TE1A1P form Cu(II) complexes within 5 min at ambient temperature, the stability of the Cu-CB-TE2P and Cu-CB-TE1A1P complexes decreased dramatically.20−22

In comparison with Cu complexes formed using CB-TE2A, Cu-PCB-TE2A complexes had better Cu-complex stability in the acid decomplexation assay. Moreover, in comparison with CB-TE2A, PCB-TE2A has the advantages of milder radiolabeling conditions and easy modification for bioconjugation (Figure 1).

Figure 1.

Figure 1

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Various cross-bridged Cu-chelators described in this letter.

Modification of PCB-TE2A into PCB-TE2P by replacing its acetic acid pendant arms with phosphonate would be expected to retain higher Cu-complex stability in comparison with those formed using CB-TE2A. The propylene cross-bridge of PCB-TE2P confers high stability on Cu-complexes, whereas the phosphonate pendant arm may allow radiolabeling under milder conditions and with faster kinetics in comparison with those associated with PCB-TE2A radiolabeling, as was observed in the case of CB-TE2P/TE1A1P.

To study our hypothesis, we synthesized a chelator with a propylene cross-bridge and phosphonate pendant arm, which was designated as PCB-TE2P.

Overall, PCB-TE2P was synthesized from cyclam in six steps. To prepare PCB-TE2P (2) from PCB-cyclam (1), a single-step process was used in which phosphonate pendant arms were introduced directly onto the secondary amines (Scheme 1).23 This single-step process was utilized instead of the typical two-step process in which a phosphonate ester was prepared initially, followed by acid hydrolysis to obtain a phosphonate group.24,25 In the process reported here, PCB-cyclam was directly treated with paraformaldehyde and H3PO3 in 6 M HCl as a solvent. To minimize side product formation (N-methylation), H3PO3 was used in high excess, while paraformaldehyde was added in small amounts over a period of 2 h. We were unable to completely eliminate side product formation, and purification of the reaction mixture by ion exchange chromatography (strong cation exchange resin (Dowex 50WX8) followed by weakly acidic cation exchange resin (Amberlite CG50, H+ form)) was required to isolate pure PCB-TE2P. The isolation yield of pure chelator was 36%.

Scheme 1. Synthesis of PCB-TE2P from PCB-cyclam.

Scheme 1

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To check the ability of PCB-TE2P to form a stable Cu-complex, a cold Cu-complex of 2 was prepared. To obtain Cu-PCB-TE2P, 2 was heated at 90 °C with CuCl2 in water (pH adjusted to 8.0 by addition of 1 M NaOH). Using this method, Cu-PCB-TE2P was synthesized within 30 min with a 90% yield (Scheme 2). At room temperature, the reaction did not proceed at all. The recrystallized pure Cu-PCB-TE2P complex was evaluated in EPR study, acidic decomplexation and cyclic voltammetry assays.

Scheme 2. Synthesis of Cu-PCB-TE2P.

Scheme 2

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The EPR spectrum of Cu-PCB-TE2P showed a characteristic axial EPR spectrum with the g∥ = 2.25, g⊥ = 2.05 (Figure 2). The four line hyperfine splittings, arisen from the interaction between the electron spin of Cu2+ (d9, S = 1/2) and the nuclear spin of 63,65Cu(I = 3/2), are well resolved with the hyperfine value of A∥ = 166 G. The continuous wave electron paramagnetic resonance (CW-EPR) of Cu-PCB-TE2P indicates that the Cu-PCB-TE2P has one paramagnetic copper species of octahedral geometry with nitrogen and oxygen atoms coordinated with the copper ion.5,26 This EPR spectrum complies with the expected structure of Cu-PCB-TE2P.

Figure 2.

Figure 2

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Experimental (solid) and simulated (dashed) X-band EPR spectrum of the copper(II) complex of PCB-TE2P recorded in frozen aqueous solution.

Although acid decomplexation studies of Cu-complexes are not useful for predicting in vivo stability, this type of study provides useful information on the relative strength of the Cu-complex, as it can be performed in various strengths of acid and at various temperatures.9,12

First, the synthesized Cu-complex was evaluated in the acid-catalyzed half-life determination assay by using a spectrophotometer. After acid decomplexation of Cu-PCB-TE2P in 5 M HCl at 90 °C, it did not show any change in absorption, even after 48 h, which strongly indicated that Cu-PCB-TE2P was highly stable under these conditions and suggested that harsher conditions were required for Cu-decomplexation. In comparison with the propylene cross-bridged chelators, ethylene cross-bridged chelators showed rapid degradation under the same conditions (5 M HCl at 90 °C), and the half-lives of CB-TE2P and CB-TE1A1P were 3.8 and 6.8 h, respectively21 (Supporting Information Table S1).

To carry out Cu-decomplexation and compare the relative strength of Cu-complexes synthesized with Cu-PCB-TE2P, acid decomplexation was performed in 12 M HCl at 90 °C, and degradation of the Cu-complexes was measured quantitatively by HPLC analysis of the reaction mixture. The conditions used were standard conditions for Cu-decomplexation of propylene cross-bridged chelators.12 The results of the HPLC analysis of the reaction mixture are shown in Figure 3a. Although Cu-PCB-TE2P showed degradation under the relatively harsh conditions of the test, the rate of degradation was slow. After 2 d, 73% of the intact Cu-PCB-TE2P was observed in the reaction mixture, and this proportion decreased to <10% after 8 days (Supporting Information Table S2). The observed rate of Cu-decomplexation was very high in comparison with that of Cu-PCB-TE2A complexes, which did not show decomplexation after 7 d under the same conditions. In contrast, in comparison with Cu-CB-TE2A complexes, the Cu-PCB-TE2P complexes showed exceptionally high stability, as less than 3% of Cu-CB-TE2A complexes remained intact under the same conditions only after 8 h heating. Because CB-TE2A contains two acetic acid pendant arms, and replacing the acetic acid pendant arm with a phosphonate pendant arm usually decreases Cu-complex stability; the higher stability of Cu-PCB-TE2P in comparison with that of Cu-CB-TE2A should be attributed to the presence of the propylene cross-bridge in PCB-TE2P, suggesting that the influence of propylene cross-bridge is more crucial in the determination of copper complex stability than that of phosphonate pendant arms.

Figure 3.

Figure 3

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Time-dependent UV-HPLC chromatograms of Cu-PCB-TE2P in 12 M HCl at 90 °C (a) and cyclic voltammogram of Cu-PCB-TE2P (scan rate 100 mV/s, 0.2 M phosphate buffer, pH 7) (b).

After studying Cu-decomplexation mediated by a strong acid, Cu-PCB-TE2P was evaluated via cyclic voltammetry for Cu-decomplexation mediated by reduction of Cu(II) to Cu(I). The cyclic voltammogram for Cu-PCB-TE2P is shown in Figure 3b. As expected, Cu-PCB-TE2P showed a reversible cyclic voltammogram at a reduction potential value of −0.67, which closely matched the pattern observed for Cu-PCB-TE2A.12

After confirming the high in vitro stability of the synthesized Cu-PCB-TE2P complexes, the efficiency of radiolabeling PCB-TE2P with 64Cu was determined. PCB-TE2P was quantitatively radiolabeled with 64Cu within 10 min of incubation at 90 °C in 0.1 M NaOAc buffer solution (Figure 4). It has been reported that replacing one or two acetic acid pendant arms of CB-TE2A with a phosphonate pendant arm allowed easy radiolabeling of chelators at room temperature. However, although we tried to radiolabel PCB-TE2P at 60 °C, we could not achieve quantitative radiolabeling. HPLC analysis of this radiolabeling reaction mixture showed intermediate product formation at retention time (RT) of 2.5 to 3 min in the chromatogram, which is neither free 64Cu (RT 2.0 min) nor 64Cu-PCB-TE2P (RT 4.9 min).

Figure 4.

Figure 4

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UV-HPLC chromatogram of Cu-PCB-TE2P (black) and radio-HPLC chromatogram of 64Cu-PCB-TE2P (red).

After radiolabeling of PCB-TE2P, its stability was tested in serum and in vivo. As expected, 64Cu-PCB-TE2P did not show any decomplexation in serum after up to 24 h at 37 °C.

As serum stability data are insufficient for prediction of the in vivo stability of 64Cu-labeled chelators, a radiotracer was directly evaluated for in vivo stability at 1 h postinjection (PI) in Sprague–Dawley (SD) rats. For comparison, the in vivo stability of 64Cu-labeled PCB-TE2A was also evaluated under the same conditions. As shown in Figure 5, only two peaks, which corresponded to free (demetalated) 64Cu ions and the intact tracer, were observed in the HPLC chromatogram of the blood sample. In the case of 64Cu-PCB-TE2A, approximately 7.5% demetalated 64Cu was observed out of total activity in the blood at 1 h PI, whereas the percentage of demetalated 64Cu observed in the blood after 64Cu-PCB-TE2P treatment was approximately 13%. The results of this in vivo stability study corroborated the results of the acid-catalyzed Cu-decomplexation study, in which Cu-PCB-TE2P was less stable than Cu-PCB-TE2A.

Figure 5.

Figure 5

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In vivo stability in blood at 1 h postinjection of (a) free 64Cu ions, (b) 64Cu-PCB-TE2P, and (c) 64Cu-PCB-TE2A.

After confirming the high stability of Cu-PCB-TE2P complex in vitro and in vivo, the biodistribution and clearance pattern of 64Cu-PCB-TE2P in SD rats were studied. 64Cu-PCB-TE2P showed rapid clearance from the blood. At 1 h PI, PCB-TE2P showed 0.18 ± 0.04%ID/g uptake, which decreased rapidly to 0.05 ± 0.01%ID/g at 4 h and remained constant until 24 h.

Rapid clearance of 64Cu-PCB-TE2P was observed via the renal and hepatobiliary routes. In the kidney, PCB-TE2P showed 1.72 ± 0.52%ID/g uptake at 1 h, which was reduced to 52% and 25% of the initial activity at 4 and 24 h, respectively (0.89 ± 0.13% ID/g at 4 h and 0.43 ± 0.05% ID/g at 24 h). In comparison, kidney uptake of PCB-TE2A at 30 min PI was 1.32 ± 0.35%ID/g, which was reduced to 15% and 3% of the initial activity at 4 and 24 h PI, respectively (0.21 ± 0.03%ID/g at 4 h PI and 0.04 ± 0.006%ID/g at 24 h PI).12 The slow kidney clearance of 64Cu-PCB-TE2P in comparison with that of 64Cu-PCB-TE2A might be due to the difference in the overall charge of each Cu-complex. PCB-TE2P has −4 charge, which becomes −2 after complexation with Cu2+ ions, whereas PCB-TE2A has −2 charge and becomes neutral after complexation with Cu2+. This result was well matched with the results reported earlier.18 In the case of 64Cu-complexes of DOTP, DO3P, and DO2P, it was observed that 64Cu-DOTP shows the highest kidney uptake at 24 h PI followed by 64Cu-DO3P and 64Cu-DO2P.

Figure 6.

Figure 6

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Biodistribution data of 64Cu-PCB-TE2P at 1, 4, and 24 h (n = 5).

In the liver, PCB-TE2P showed 0.55 ± 0.04%ID/g uptake at 1 h PI, which was reduced to 60% of the initial value at 4 h (0.33 ± 0.07%ID/g). At 24 h, uptake was 0.09 ± 0.01%ID/g, which was 18% of the initial value. The observed clearance pattern was very similar to that of PCB-TE2A.

For all other nonclearance organs, PCB-TE2P showed less than 0.05%ID/g uptake at 24 h PI, suggesting that 64Cu-PCB-TE2P was stable in vivo. This biodistribution study confirms the high in vivo stability of 64Cu-PCB-TE2P, which was demonstrated in vitro.

In conclusion, the phosphonate pendant-armed propylene cross-bridged chelator PCB-TE2P was easily synthesized. PCB-TE2P shows very high Cu-complex stability, which was confirmed by acid decomplexation and in vivo stability studies. 64Cu-PCB-TE2P showed very rapid body clearance with negligible uptake in nonclearance organs. Although we could not succeed in radiolabeling PCB-TE2P at room temperature and it required high temperature (90 °C) to achieve quantitative 64Cu labeling,20 it showed 100% labeling within 10 min, which was rapider than CB-TE2A and PCB-TE2A radiolabeling. Even though Cu-PCB-TE2P is not as stable as Cu-PCB-TE2A it showed comparable stability as Cu-CB-TE2A and was radiolabeled under milder conditions than those required for CB-TE2A. PCB-TE2P can be used as a bifunctional chelator by introducing an extra functionality to the propylene cross-bridge backbone as demonstrated earlier.13

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmedchemlett.5b00362.

  • Experimental procedures and detailed characterization for synthesis of PCB-TE2P, Cu(II) complexation, acidic decomplexation, cyclic voltammetry, EPR, 64Cu radiolabeling, in vitro serum stability experiments, in vivo stability of 64Cu-PCB-TE2P, and comparative biodistribution experiments of 64Cu-PCB-TE2P (PDF)

This work was supported by an R&D program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (No. 2013R1A2A2A01012250 and 1711026888). The Korea Basic Science Institute (Daegu) is acknowledged for providing assistance with the NMR and MS measurements.

The authors declare no competing financial interest.

Supplementary Material

ml5b00362_si_001.pdf (685.2KB, pdf)

References

  1. Wadas T. J.; Wong E. H.; Weisman G. R.; Anderson C. J. Coordinating Radiometals of Copper, Gallium, Indium, Yttrium, and Zirconium for PET and SPECT Imaging of Disease. Chem. Rev. 2010, 110, 2858. 10.1021/cr900325h. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Donnelly P. S. The role of coordination chemistry in the development of copper and rhenium radiopharmaceuticals. Dalton Trans. 2011, 40, 999. 10.1039/c0dt01075h. [DOI] [PubMed] [Google Scholar]
  3. Mewis R. E.; Archibald S. J. Biomedical applications of macrocyclic ligand complexes. Coord. Chem. Rev. 2010, 254, 1686. 10.1016/j.ccr.2010.02.025. [DOI] [Google Scholar]
  4. Shokeen M.; Anderson C. J. Molecular Imaging of Cancer with Copper-64 Radiopharmaceuticals and Positron Emission Tomography (PET). Acc. Chem. Res. 2009, 42, 832. 10.1021/ar800255q. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Esteves C. V.; Lamosa P.; Delgado R.; Costa J.; Désogère P.; Rousselin Y.; Goze C.; Denat F. Remarkable Inertness of Copper(II) Chelates of Cyclen-Based Macrobicycles with Two trans-N-Acetate Arms. Inorg. Chem. 2013, 52, 5138. 10.1021/ic400015v. [DOI] [PubMed] [Google Scholar]
  6. Boros E.; Rybak-Akimova E.; Holland J. P.; Rietz T.; Rotile N.; Blasi F.; Day H.; Latifi R. Caravan, P. Pycup—A Bifunctional, Cage-like Ligand for 64Cu Radiolabeling. Mol. Pharmaceutics 2014, 11, 617. 10.1021/mp400686z. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Cai H.; Fissekis J.; Conti P. S. Synthesis of a novel bifunctional chelator AmBaSar based on sarcophagine for peptide conjugation and 64Cu radiolabelling. Dalton Trans. 2009, 5395. 10.1039/b902210d. [DOI] [PubMed] [Google Scholar]
  8. Boros E.; Cawthray J. F.; Ferreira C. L.; Patrick B. O.; Adam M. J.; Orvig C. Evaluation of the H2dedpa Scaffold and its cRGDyK Conjugates for Labeling with 64Cu. Inorg. Chem. 2012, 51, 6279. 10.1021/ic300482x. [DOI] [PubMed] [Google Scholar]
  9. Pandya D. N.; Kim J. Y.; Park J. C.; Lee H.; Phapale P. B.; Kwak W.; Choi T. H.; Cheon G. J.; Yoon Y.-R.; Yoo J. Revival of TE2A; a better chelate for Cu(II) ions than TETA?. Chem. Commun. 2010, 46, 3517. 10.1039/b925703a. [DOI] [PubMed] [Google Scholar]
  10. Pandya D. N.; Bhatt N.; Dale A. V.; Kim J. Y.; Lee H.; Ha Y. S.; Lee J.-E.; An G. I.; Yoo J. New Bifunctional Chelator for 64Cu-Immuno-Positron Emission Tomography. Bioconjugate Chem. 2013, 24, 1356. 10.1021/bc400192a. [DOI] [PubMed] [Google Scholar]
  11. Dale A. V.; Pandya D. N.; Kim J. Y.; Lee H.; Ha Y. S.; Bhatt N.; Kim J.; Seo J. J.; Lee W.; Kim S. H.; Yoon Y.-R.; An G. I.; Yoo J. Non-Cross-Bridged Tetraazamacrocyclic Chelator for Stable 64Cu-Based Radiopharmaceuticals. ACS Med. Chem. Lett. 2013, 4, 927. 10.1021/ml400142s. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Pandya D. N.; Dale A. V.; Kim J. Y.; Lee H.; Ha Y. S.; An G. I.; Yoo J. New Macrobicyclic Chelator for the Development of Ultrastable 64Cu-Radiolabeled Bioconjugate. Bioconjugate Chem. 2012, 23, 330. 10.1021/bc200539t. [DOI] [PubMed] [Google Scholar]
  13. Pandya D. N.; Bhatt N.; An G. I.; Ha Y. S.; Soni N.; Lee H.; Lee Y. J.; Kim J. Y.; Lee W.; Ahn H.; Yoo J. Propylene Cross-Bridged Macrocyclic Bifunctional Chelator: A New Design for Facile Bioconjugation and Robust 64Cu Complex Stability. J. Med. Chem. 2014, 57, 7234. 10.1021/jm500348z. [DOI] [PubMed] [Google Scholar]
  14. Chakrabarti M. C.; Le N.; Paik C. H.; De Graff W. G.; Carrasquillo J. A. Prevention of Radiolysis of Monoclonal Antibody during Labeling. J. Nucl. Med. 1996, 37, 1384. [PubMed] [Google Scholar]
  15. Lukeš I.; Kotek J.; Vojtíšek P.; Hermann P. Complexes of tetraazacycles bearing methylphosphinic/phosphonic acid pendant arms with copper(II), zinc(II) and lanthanides(III). A comparison with their acetic acid analogues. Coord. Chem. Rev. 2001, 216–217, 287. 10.1016/S0010-8545(01)00336-8. [DOI] [Google Scholar]
  16. Fuzerova S.; Kotek J.; Cisarova I.; Hermann P.; Binnemans K.; Lukes I. Cyclam (1,4,8,11-tetraazacyclotetradecane) with one methylphosphonate pendant arm: a new ligand for selective copper(II) binding. Dalton Trans. 2005, 2908. 10.1039/b507062g. [DOI] [PubMed] [Google Scholar]
  17. Kotek J.; Lubal P.; Hermann P.; Císařová I.; Lukeš I.; Godula T.; Svobodová I.; Táborský P.; Havel J. High Thermodynamic Stability and Extraordinary Kinetic Inertness of Copper(II) Complexes with 1,4,8,11-Tetraazacyclotetradecane-1,8-bis(methylphosphonic acid): Example of a Rare Isomerism between Kinetically Inert Penta- and Hexacoordinated Copper(II) Complexes. Chem. - Eur. J. 2003, 9, 233. 10.1002/chem.200390017. [DOI] [PubMed] [Google Scholar]
  18. Sun X.; Wuest M.; Kovacs Z.; Sherry D.; Motekaitis R.; Wang Z.; Martell A.; Welch M.; Anderson C. In vivo behavior of copper-64-labeled methanephosphonate tetraaza macrocyclic ligands. JBIC, J. Biol. Inorg. Chem. 2003, 8, 217. 10.1007/s00775-002-0408-5. [DOI] [PubMed] [Google Scholar]
  19. Svobodová I.; Havlíčková J.; Plutnar J.; Lubal P.; Kotek J.; Hermann P. Metal Complexes of 4,11-Dimethyl-1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid) – Thermodynamic and Formation/Decomplexation Kinetic Studies. Eur. J. Inorg. Chem. 2009, 2009, 3577. 10.1002/ejic.200900358. [DOI] [Google Scholar]
  20. Stigers D. J.; Ferdani R.; Weisman G. R.; Wong E. H.; Anderson C. J.; Golen J. A.; Moore C.; Rheingold A. L. A new phosphonate pendant-armed cross-bridged tetraamine chelator accelerates copper(II) binding for radiopharmaceutical applications. Dalton Trans. 2010, 39, 1699. 10.1039/B920871B. [DOI] [PubMed] [Google Scholar]
  21. Ferdani R.; Stigers D. J.; Fiamengo A. L.; Wei L.; Li B. T. Y.; Golen J. A.; Rheingold A. L.; Weisman G. R.; Wong E. H.; Anderson C. J. Synthesis, Cu(II) complexation, 64Cu-labeling and biological evaluation of cross-bridged cyclam chelators with phosphonate pendant arms. Dalton Trans. 2012, 41, 1938. 10.1039/C1DT11743B. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Sprague J. E.; Peng Y.; Sun X.; Weisman G. R.; Wong E. H.; Achilefu S.; Anderson C. J. Preparation and Biological Evaluation of Copper-64–Labeled Tyr3-Octreotate Using a Cross-Bridged Macrocyclic Chelator. Clin. Cancer Res. 2004, 10, 8674. 10.1158/1078-0432.CCR-04-1084. [DOI] [PubMed] [Google Scholar]
  23. Lazar I.; Hrncir D. C.; Kim W. D.; Kiefer G. E.; Sherry A. D. Optimized synthesis, structure, and solution dynamics of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylenephosphonic acid) (H8DOTP). Inorg. Chem. 1992, 31, 4422. 10.1021/ic00047a034. [DOI] [Google Scholar]
  24. Syamala M. A DECADE OF ADVANCES IN THREE-COMPONENT REACTIONS. Org. Prep. Proced. Int. 2005, 37, 103. 10.1080/00304940509354882. [DOI] [Google Scholar]
  25. Bhagat S.; Chakraborti A. K. An Extremely Efficient Three-Component Reaction of Aldehydes/Ketones, Amines, and Phosphites (Kabachnik–Fields Reaction) for the Synthesis of α-Aminophosphonates Catalyzed by Magnesium Perchlorate. J. Org. Chem. 2007, 72, 3158. 10.1021/jo0703348. [DOI] [PubMed] [Google Scholar]
  26. Hubin T. J.; McCormick J. M.; Alcock N. W.; Clase H. J.; Busch D. H. Crystallographic Characterization of Stepwise Changes in Ligand Conformations as Their Internal Topology Changes and Two Novel Cross-Bridged Tetraazamacrocyclic Copper(II) Complexes. Inorg. Chem. 1999, 38, 4435. 10.1021/ic990491s. [DOI] [PubMed] [Google Scholar]

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Supplementary Materials

ml5b00362_si_001.pdf (685.2KB, pdf)

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