Structural Optimization of Heptamethine Cyanine Dyes

The near-infrared window of fluorescent heptamethine cyanine dyes greatly facilitates biological imaging because there is deep penetration of the light and negligible background fluorescence. However, dye solubility, aggregation, poor pharmacokinetics and biodistribution, quenching in water, and instability are current drawbacks that limit performance and the scope of possible applications as summarized in Chapter 1. This thesis describes a series of chemical efforts to solve these heptamethine cyanine problems.

Chapters 2, 3, and 4 report molecular design strategies that produce new families of sterically shielded heptamethine dyes which overcome the limitations mentioned above. The key design feature is a meso-Oaryl or a meso-aryl group that simultaneously projects two shielding arms directly over each face of a linear heptamethine polyene. Fluorescence imaging experiments in cells and animals compared the shielded heptamethine dyes (and several peptide and antibody bioconjugates) to benchmark heptamethine dyes in Chapter 5, and found that the shielded systems possess an unsurpassed combination of photophysical, physiochemical and biodistribution properties that greatly enhance bioimaging performance.

The study in Chapter 6 overcomes the instability problem of FDA-approved heptamethine dye, indocyanine green (ICG) by making a suitable designed deuterated derivative. The deuterated dyes bear deuterium atoms on the polymethine chain, and the spectral, physiochemical, and photostability properties were quantified. A notable mechanistic finding is that self-aggregation of ICG in water strongly favors dye degradation by a photochemical oxidative dimerization reaction that gives a non-fluorescent product. Storage stability studies showed that replacement of C-H with C-D slowed the dimerization rate constant by a factor of 3.1, and it is likely that many medical and pre-clinical procedures will benefit from the longer shelf-lives of these two deuterated ICG dyes.

The study in Chapter 7 shows how supramolecular encapsulation of a newly designed series of cyanine dyes by cucurbit[7]uril (CB7) can alter the π-electron distribution within the cyanine chromophore alter fluorescent properties. For two sets of dyes, the most stable co-conformation for the supramolecular complex locates CB7 around the center of the dye chromophore, and the results are nonpolar, symmetric π-electron, ground states that produce sharpened absorption bands with enhanced fluorescence brightness. From the perspective of enhanced near-infrared bioimaging and sensing in water, the results show how the principles of host/guest chemistry can be employed to solve the “cyanine limit” problem.