Fluorine polymer probes for magnetic resonance imaging: quo vadis?

Over the last few years, the development and relevance of 19F magnetic resonance imaging (MRI) for use in clinical practice has emerged. MRI using fluorinated probes enables the achievement of a specific signal with high contrast in MRI images. However, to ensure sufficient sensitivity of 19F MRI, fluorine probes with a high content of chemically equivalent fluorine atoms are required. The majority of 19F MRI agents are perfluorocarbon emulsions, which have a broad range of applications in molecular imaging, although the content of fluorine atoms in these molecules is limited. In this review, we focus mainly on polymer probes that allow higher fluorine content and represent versatile platforms with properties tailorable to a plethora of biomedical in vivo applications. We discuss the chemical development, up to the first imaging applications, of these promising fluorine probes, including injectable polymers that form depots that are intended for possible use in cancer therapy.

Self-Assembled Thermoresponsive Polymeric Nanogels for 19F MR Imaging.

Magnetic resonance imaging using fluorinated contrast agents (19F MRI) enables to achive highcontrast in images due to the negligible fluorine background in living tissues. In this pilot study, we developed new biocompatible, temperature-responsive, and easily synthesized polymeric nanogels containing a sufficient concentration of magnetically equivalent fluorine atoms for 19F MRI purposes. The structure of the nanogels is based on amphiphilic copolymers containing two blocks, a hydrophilic poly[ N-(2-hydroxypropyl)methacrylamide] (PHPMA) or poly(2-methyl-2-oxazoline) (PMeOx) block, and a thermoresponsive poly[ N(2,2difluoroethyl)acrylamide] (PDFEA) block. The thermoresponsive properties of the PDFEA block allow us to control the process of nanogel self-assembly upon its heating in an aqueous solution. Particle size depends on the copolymer composition, and the most promising copolymers with longer thermoresponsive blocks form nanogels of suitable size for angiogenesis imaging or the labeling of cells (approximately 120 nm). The in vitro 19F MRI experiments reveal good sensitivity of the copolymer contrast agents, while the nanogels were proven to be noncytotoxic for several cell lines.

H+ translocation by weak acid uncouplers is independent of H+ electrochemical gradient.

According to the common view, weak acid uncouplers increase proton conductance of biological (and phospholipid bilayer) membranes, thus effecting H+ fluxes driven by their electrochemical gradients. Under certain conditions, however, uncouplers can induce unexpected effects opposite to the dissipation of H+ gradients. Results are presented here demonstrating CCCP-induced proton influx into Saccharomyces cerevisiae cytosol driven by the electrochemical potentials of CCCP and its CCCP- anions, independent of electrochemical H+-gradient. Another view of week acid uncouplers' action is proposed that is logically consistent with these observations.

A novel method for assessment of local pH in periplasmic space and of cell surface potential in yeast.

Yeast cells exhibit a negative surface potential due to negative charges at the cell membrane surface. Consequently, local concentrations of cations at the periplasmic membrane surface may be significantly increased compared to their bulk environment. However, in cell suspensions only bulk concentrations of cations can be measured directly. Here we present a novel method enabling the assessment of local pH at the periplasmic membrane surface which can be directly related to the underlying cell surface potential. In this proof of concept study using Saccharomyces cerevisiae cells with episomally expressed pH reporter, pHluorin, intracellular acidification induced by the addition of the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) was measured using synchronously scanned fluorescence spectroscopy (SSF). The analysis of titration curves revealed that the pH at the periplasmic surface of S. cerevisiae cells was about two units lower than the pH of bulk medium. This pH difference was significantly decreased by increasing the ionic strength of the bulk medium. The cell surface potential was estimated to amount to -130 mV. Comparable results were obtained also with another protonophore, pentachlorophenol (PCP).

Internal Structure of Thermoresponsive Physically Crosslinked Nanogel of Poly[N-(2-hydroxypropyl)methacrylamide]-Block-Poly[N-(2,2-difluoroethyl)acrylamide], Prominent 19F MRI Tracer.

Fluorine-19 MRI is a promising noninvasive diagnostic method. However, the absence of a nontoxic fluorine-19 MRI tracer that does not suffer from poor biodistribution as a result of its strong fluorophilicity is a constant hurdle in the widespread applicability of this otherwise versatile diagnostic technique. The poly[N-(2-hydroxypropyl)methacrylamide]-block-poly[N-(2,2-difluoroethyl)acrylamide] thermoresponsive copolymer was proposed as an alternative fluorine-19 MRI tracer capable of overcoming such shortcomings. In this paper, the internal structure of self-assembled particles of this copolymer was investigated by various methods including 1D and 2D NMR, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). The elucidated structure appears to be that of a nanogel with greatly swollen hydrophilic chains and tightly packed thermoresponsive chains forming a network within the nanogel particles, which become more hydrophobic with increasing temperature. Its capacity to provide a measurable fluorine-19 NMR signal in its aggregated state at human body temperature was also investigated and confirmed. This capacity stems from the different fluorine-19 nuclei relaxation properties compared to those of hydrogen-1 nuclei.