Background

[PubMed]

^99mTc-Labeled acetylated, 2,3,5-triiodobenzoic acid (TIBA)- and diethylenetriamine pentaacetic acid (DTPA)-conjugated, and PEGylated ethylenediamine-core generation 4 polyamidoamine dendrimers (G4 PAMAM), abbreviated as ^99mTc-G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂], is a blood pool multimodal agent synthesized by Criscione et al. for single-photon emission computed tomography (SPECT)/computed tomography (2) (1).

In the recent years, the fact that imaging modalities with high sensitivity have relatively poor resolution, while those with high resolution have relatively poor sensitivity, has triggered the integration of multiple modalities and the use of hybrid instrument technology in imaging (3, 4). This has also boosted the development of multimodal imaging agents, which further enhances the benefits of hybrid technology, allowing better characterization of diseases and disease processes (5-7). However, it is still challenging to incorporate enough labels for detection by the relatively low-sensitive modalities and to select an appropriate radionuclide in a radiotracer-based imaging approach in the development of multimodal agents (1).

Criscione et al. synthesized a radiolabeled, multimodal contrast agent, ^99mTc-G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂], for preclinical microSPECT/CT imaging (1). The design of this agent is based on the G4 PAMAM dendrimers, which were used as the core structure of the multimodal agent. Dendrimers have been selected for two main reasons. First, their well-defined, multifunctional surface (64 primary amines) offers the ability to conjugate several different moieties with negligible steric hindrance. Second, dendrimer-based contrast agents have already been successfully designed and examined with different imaging modalities (1, 8, 9). However, PAMAM dendrimers have a highly cationic surface, which limits their water solubility and can cause hemolysis in humans (1). Dendrimers also have a relatively short circulation time when they are used for developing blood pool imaging agents. To overcome these problems, Criscione et al. partially acetylated the dendrimer surface to reduce the positive surface charge and PEGylated the remaining surface amines to enhance the circulation time and limit the clearance by the reticuloendothelial system (1). These modifications provided dendrimers the desired water solubility, long intravascular residence time, and predominant renal clearance (1). To induce the dendrimer construct's multimodal capability, Criscione et al. selected TIBA and ^99mTc-DTPA for X-ray attenuation and radioactive signal, respectively (1). TIBA possesses X-ray attenuation properties similar to those of the clinically used, small-molecule triiodobenzoic acid derivative Omnipaque 350. The studies by Criscione et al. have also shown that sufficient iodine weight percent for effective X-ray attenuation without sacrificing aqueous solubility can be achieved by conjugating TIBA to dendrimers (1). The acyclic chelator DTPA was used to chelate tin(II)-reduced ^99mTc because of its established chemistry and stability. In vitro and animal studies have confirmed the potential usefulness of this multimodal agent in the quantification of intramyocardial blood volume and blood flow (1).

Synthesis

[PubMed]

Criscione et al. described the synthesis of ^99mTc-G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂] in detail (1). The synthesis could be roughly divided into five steps. The G4 dendrimers were partially acetylated with sulfosuccinimidyl acetate to generate acetylated dendrimers (G4-[Ac]). Then TIBA was covalently conjugated to G4-[Ac] through the surface amines to generate G4-[[Ac]-TIBA]. To chelate the nuclear probe ^99mTc, 1-(4-isothiocyanatobenzyl)diethylenetriamine pentaacetic acid (p-SCN-Bn-DTPA) was coupled to the primary surface amines of the dendrimer via isothiocyanate linkage. This step resulted in the product G4-[[[Ac]-TIBA]-DTPA]. G4-[[[Ac]-TIBA]-DTPA] was subsequently PEGylated with succinimidyl-(N-methyl-dodecaethylene glycol)ester (mPEG₁₂) to generate the G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂]. The chemical yields of the intermediate products were 97% for G4-[Ac], 82% for G4-[[Ac]-TIBA], 89% for G4-[[[Ac]-TIBA]-DTPA], and 83% for G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂]. Finally, ^99mTc-labeling was completed with the reaction of G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂] (50 mg for imaging purposes and 20 mg for biodistribution studies) and ^99mTcO₄^– (18.5 MBq (500 μCi)) for 2 min (for imaging purposes) or for 10 min (for biodistribution studies) at 25ºC. The reaction mixture was directly used for studies without further purification. The radiochemical purity was >80%.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Criscione et al. characterized the intermediate products with ¹H nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering (1). The results showed that G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂] had 3 remaining primary amines, 35 coupled acetyl groups, ~15 TIBA moieties, 4 DTPA, and 7 mPEG₁₂. The hydrodynamic diameter and molecular weight of G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂] were 12.37 ± 6.09 nm and 19,762, respectively.

The X-ray attenuation property of G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂] was analyzed with serially diluted G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂] in deionized water using microCT imaging (1). A linear attenuation versus the iodine concentration curve, an essential criterion for X-ray contrast agents, was obtained for G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂]. The extrapolated iodine concentrations in G4-[[[[Ac]-TIBA]-DTPA]-mPEG₁₂] were consistent with the estimated iodine weight percent (~19% wt) determined with ¹H NMR.

Animal Studies

Human Studies

[PubMed]

No references are currently available.

References

1.

Criscione J.M. et al. Development and application of a multimodal contrast agent for SPECT/CT hybrid imaging. Bioconjug Chem. 2011;22(9):1784–92. [PMC free article: PMC3204385] [PubMed: 21851119]

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Cherry S.R. Multimodality in vivo imaging systems: twice the power or double the trouble? Annu Rev Biomed Eng. 2006;8:35–62. [PubMed: 16834551]

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Louie A. Multimodality imaging probes: design and challenges. Chem Rev. 2010;110(5):3146–95. [PMC free article: PMC2878382] [PubMed: 20225900]

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Jennings L.E., Long N.J. 'Two is better than one'--probes for dual-modality molecular imaging. Chem Commun (Camb). 2009;(24):3511–24. [PubMed: 19521594]

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Mintzer M.A., Grinstaff M.W. Biomedical applications of dendrimers: a tutorial. Chem Soc Rev. 2011;40(1):173–90. [PubMed: 20877875]

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Cheng Y. et al. Design of biocompatible dendrimers for cancer diagnosis and therapy: current status and future perspectives. Chem Soc Rev. 2011;40(5):2673–703. [PubMed: 21286593]