The Synthesis and Evaluation of Non-Targeted Near-Infrared Heptamethine Molecular Probes

Abstract

Detecting and quantifying biomolecules is an important tool in biological research. Ideal dyes have to have a number of characteristics i.e. low toxicity and limited autofluorescence. The properties of near infrared dyes make these ideal for cell and tissue imaging.

The work within this thesis focuses on the development of three families of NIR dyes (linear, rigid and substituted polymethine) and these are actively compared against the clinical standard Indocyanine Green (ICG) and its structural derivative New Indocyanine Green (IR-820). The ultimate aim being to identify dyes which can be used alongside the clinical standards.

The compounds developed within this thesis are structurally based on the NIR heptamethine cyanine (Cy7) dyes. To expand, variations fall into three categories, linear, rigid and polymethine substituted, each being synthesised using either existing methodology or through the development of a novel cascade reaction. The photophysical properties of each dye have been evaluated experimentally, focusing on absorption and emission wavelengths, fluorescence quantum yields and Stokes shifts. It is noted that most dyes synthesised within this thesis, show comparable Stokes shift but increased fluorescence quantum yields when compared against ICG and IR-820. All dyes absorb and emit within the NIR region based on excitation at 785 nm.

The growth inhibition characteristic is an important criterion which determines the practical use of the dyes in living cells. Most of the dyes which showed no growth inhibition were the dyes bearing the sulfonic acid group, suggesting the sulfonic acid limited cellular uptake as a result of decreased membrane permeability. Dyes possessing growth inhibitory characteristics all contain linear N-alkyl moieties. It is thus postulated that the increased lipophilicity of these molecules result in increased lipid distortion and subsequent toxicity.