Emergent Fluorous Molecules and Their Uses in Molecular Imaging

Abstract

Metrics & More Article RecommendationsCONSPECTUS: This Account summarizes recent advances in the chemistry of fluorocarbon nanoemulsion (FC NE) functionalization. We describe new families of fluorous molecules, such as chelators, fluorophores, and peptides, that are soluble in FC oils. These materials have helped transform the field of in vivo molecular imaging by enabling sensitive and cell-specific imaging using magnetic resonance imaging (MRI), positron emission tomography (PET), and fluorescence detection. FC emulsions, historically considered for artificial blood substitutes, are routinely used for ultrasound imaging in clinic and have a proven safety profile and a well-characterized biodistribution and pharmacokinetics. The inertness of fluorocarbons contributes to their low toxicity but makes functionalization difficult. The high electronegativity of fluorine imparts very low cohesive energy density and Lewis basicity to heavily fluorinated compounds, making dissolution of metal ions and organic molecules challenging. Functionalization is further complicated by colloidal instability toward heat and pH, as well as limited availability of biocompatible surfactants.We have devised new fluorous chelators that overcome solubility barriers and are able to bind a range of metal ions with high thermodynamic stability and biocompatibility. NE harboring chelators in the fluorous phase are a powerful platform for the development of multimodal imaging agents. These compositions rapidly capture metal ions added to the aqueous phase, thereby functionalizing NEs in useful ways. For example, Fe 3+ encapsulation imparts a strong paramagnetic relaxation effect on 19 F T 1 that dramatically accelerates 19 F MRI data acquisition times and hence sensitivity in cell tracking applications. Alternatively, 89 Zr encapsulation creates a sensitive and versatile PET probe for inflammatory macrophage detection. Adding lanthanides, such as Eu 3+ , renders NE luminescent. Beyond chelators, this Account further covers our progress in formulating NEs with fluorophores, such as cyanine or BODIPY dyes, with their utility demonstrated in fluorescence imaging, biosensing, flow cytometry and histology. Fluorous dyes soluble in FC oils are also key enablers for nascent whole-body imaging technologies such as cryo-fluorescence tomography (CFT). Additionally, fluorous cell-penetrating peptides inserted on the NE surface increase the uptake of NE by ∼8fold in weakly phagocytic stem cells and lymphocytes used in immunotherapy, resulting in significant leaps in detection sensitivity in vivo.

19F Barcoding Enables Multiplex Detection of Biomarkers Associated with Organ Injury and Cancer

“…19 F is of excellent magnetic resonance (MR) sensitivity (94 % of that of 1 H), 100 % natural abundance, negligible biological background, and quantitative MR signal output, which is an appealing signal source of the probes for various biomarkers in complex biological environments. [17][18][19][20][21][22][23] Besides, the chemical shift of 19 F is very sensitive to its chemical environment, which could be used as barcoding information for constructing responsive probes that could reduce detecting background interferences. [24][25][26][27][28] More importantly, a chemical shift difference of 0.03 ppm is usually sufficient to achieve baseline separation of two singlet 19 F resonances by using a common 400 MHz NMR spectrometer, [29][30][31][32] indicating 19 F probes' excellent potentiality for responding sensitivity and detecting multiplexity.…”

“…19 F is of excellent magnetic resonance (MR) sensitivity (94 % of that of 1 H), 100 % natural abundance, negligible biological background, and quantitative MR signal output, which is an appealing signal source of the probes for various biomarkers in complex biological environments [17–23] . Besides, the chemical shift of 19 F is very sensitive to its chemical environment, which could be used as barcoding information for constructing responsive probes that could reduce detecting background interferences [24–28] .…”

An Activatable 19F MRI Molecular Probe for Sensing and Imaging of Norepinephrine

“…Most importantly, [26][27] the 19 F content in the human body is extremely low (< 10 À 6 m), [28][29] which allows 19 F MRI to detect low concentration species with negligible background and render their distributions as "hot-spot" images. [30][31][32][33][34][35][36] Therefore, 19 F MRI is gaining momentum in the field of sensing and imaging low concentration bioactive species. [37][38][39][40][41][42][43][44][45][46][47] In light of these considerations, we envision that detecting NE with 19 F MRI might offer a potential means to circumvent the obstacles encountered by fluorescent approaches.…”

“…19 F, which is of 100 % natural abundance, a wide range of chemical shifts, a gyromagnetic ratio close to 1 H (94 % relative to 1 H), [25] is regarded as an ideal type of nuclei complementary to 1 H for MRI. Most importantly, [26–27] the 19 F content in the human body is extremely low (<10 −6 m ), [28–29] which allows 19 F MRI to detect low concentration species with negligible background and render their distributions as “hot‐spot” images [30–36] . Therefore, 19 F MRI is gaining momentum in the field of sensing and imaging low concentration bioactive species [37–47] …”

Small Molecule Probes for 19F Magnetic Resonance Imaging

“…The search for alternative magnetic nuclei for imaging to compensate for the drawbacks of 1 H MRI has been conducted for decades. 19 F has been gradually realized and accepted as a promising complement to 1 H for MRI owing to its favorable properties, such as excellent sensitivity (∼90% of 1 H), high natural abundance (100%), broad range of chemical shifts (>350 ppm), and low biological distribution (<10 –6 M and only in bones and teeth). − These advantages allow for in vivo multiplex detection of low-concentration targets by 19 F MRI and rendering them quantitatively in “hot-spot” images with negligible background. − Furthermore, some substantial progress has been made in challenging pulmonary imaging with 19 F MRI. − It is noteworthy that some fluorinated compounds, such as perflubron (perfluorooctyl bromide, PFOB), perflexane (tetradecafluorohexane, C 6 F 14 ), and perflutren (octafluoropropane, C 3 F 8 ), have been approved for clinical use, which facilitates the popularization of 19 F MRI …”