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WIREs Nanomed Nanobiotechnol
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Fluorinated dendrimers as imaging agents for 19F MRI

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19F is the second most sensitive stable nucleus for magnetic resonance imaging (MRI). Because there is no endogenous 19F signal, 19F MRI is much more suited for quantification and tracking than 1H MRI. However, 19F MRI is not in clinical use because in spite of more than three decades of research, there are no approved 19F imaging agents. New approaches and new methodologies are needed to move the field forward. Water‐soluble fluorinated dendrimers present a promising alternative to conventional perfluorocarbon emulsions. This article outlines recent development of fluorinated dendrimers as 19F imaging agents. This is not meant to be a comprehensive review of 19F imaging agents, for which there is an excellent recent review by Knight et al. Rather, the article aims to give an insider's account on research efforts in this exciting and challenging field. WIREs Nanomed Nanobiotechnol 2013, 5:646–661. doi: 10.1002/wnan.1227 This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > In Vivo Nanodiagnostics and Imaging

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Water relaxation rate constants R1 (a) and R2 (b) versus gadolinium concentration [Gd]. In the solution state, R1 and R2 both exhibit linear dependency on [Gd]. In the hydrogel state, only R1 exhibits linear dependency on [Gd], whereas R2 exhibits nonlinear dependency on [Gd] in the sub‐millimolar range. The cause of this nonlinear dependency is owing to the clustering of gadolinium in the gelled state, which leads to high local concentration. (Reprinted with permission from Ref . Copyright 2011 Wiley‐Blackwell)
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19F NMR spectra of three representative fluorinated metal complexes. Of the three, Tb3+ and Gd3+ are paramagnetic while Y3+ is diamagnetic. The free fluorinated chelator serves as a control. (Reprinted with permission from Ref . Copyright 2011 The Royal Society of Chemistry)
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Structure of first generation of fluorinated metal complexes. Mn+ is a metal ion. (Reprinted with permission from Ref . Copyright 2011 The Royal Society of Chemistry)
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Structures of fluorocarbon dendron 19F‐243. Orange: core (0th shell); black: 1st shell; purple: 2nd shell; blue: 3rd shell; red: 4th shell; green: periphery (leaves). The number of bonds between the 4th and 3rd shells is 1 (l4 = 1); that between the 3rd and 2nd shells is 3 (l3 = 3); that between the 2nd and 1st shells is 8 (l2 = 8); and that between the 1st and 0th shells is 19 (l3 = 19). Branch multiplicity a = 3; length multiplier b = 2.5; proportionality constant c = 75%. (Reprinted with permission from Ref . Copyright 2012 American Chemical Society)
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Three levels of proportionate branching for dendrimer growth. Conventional dendrimer growth is a special case of proportionate branching with c = 0%. mn refers to the number of branching points in the nth layer and it grows exponentially as an; ln refers to the number of covalent bonds between the nth layer and the (n − 1)th layer and it grows exponentially as bn. l1, l2, and l3 are highlighted for 100% proportionation branching with purple brackets. (Reprinted with permission from Ref . Copyright 2012 American Chemical Society)
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Two‐dimensional separation of fluorinated compounds using fluorous HPLC columns. When the hydrocarbons are similar in size (e.g., stereoisomers), fluorocarbons of different sizes are needed to enable separation (blue axis). When the hydrocarbons are very different in size (e.g., different dendrimer generations), the same fluorocarbon can be used to achieve separation (green axis). (Reprinted with permission from Ref . Copyright 2010 American Chemical Society)
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Elution profile of a mixture of four generations of fluorinated bifunctional dendrimers using a FluoroFlash® column, whose stationary phase contains a fluoroalkyl chain ‐(CF2)7CF3. In spite of having the same number of fluorine atoms (27), these dendrimers differ in their fluorine content (F%). Additionally, these molecules also differ in molar volumes and polarity, which might also contribute to their chromatographic separation. Dendrimers with higher F% have higher retention time, hence separation. (Reprinted with permission from Ref . Copyright 2010 American Chemical Society)
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Structures of four generations of fluorinated bifunctional dendrimers. They have the same F‐dendron (19F‐27), which contains 27 chemically identical fluorine atoms. But the H‐dendrons grow from one branch in G0 to eight branches in G3. G2 is 19FIT‐27 as shown in Figure .
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19F MRI via 19FIT‐27. (a) 2.22 mmol kg−1 of 19FIT‐27 (equivalent to 60 mmol kg−1 of 19F). (b) 1.11 mmol kg−1 of 19FIT‐27 (equivalent to 30 mmol kg−1 of 19F). Each image is the superimposition of 1H signals (white), which gives the anatomy, and 19F signals (red), which gives the location of 19FIT‐27. (Reprinted with permission from Ref . Copyright 2009 Wiley‐Blackwell)
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Solubility comparison of 19FIT‐27 with PFC. Vial 1: Phosphate‐buffered saline (PBS). Vial 2: 150 mM 19FIT‐27 (4.05 M 19F) in PBS. There is no phase separation. Vial 3: 202.5 mM perfluoro‐15‐crown‐5‐ether (4.05 M 19F) in PBS. Phase separation is apparent in this case. See Ref for details.
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Fluorinated bifunctional dendrimers as 19F imaging agents. (a) Schematic of a bifunctional dendrimer comprised of a F‐dendron (19F signal emitter) and a H‐dendron (solubility enhancer). (b) Structure of 19FIT‐27, first generation of fluorinated bifunctional dendrimer made by our group.
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(a) Proportionality between 19F signal intensity, I(19F), and 19F concentration, [19F], in phantom experiment. (b) Proportionality between 19F signal intensity, I(19F), and 19F dose level in mice. Black line: 19F dose = 60 mmol kg−1; purple line: 19F dose = 30 mmol kg−1. The signal intensity ratio is 2.08:1; the dose ratio is 2:1. (Reprinted with permission from Ref . Copyright 2009 Wiley‐Blackwell)
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