New fluorescent labels needed
The next frontier for in vivo imaging is being defined by new fluorescent contrast agents and probes. The advent of both long wavelength imaging probes and whole-body and endoscopic imaging technologies have contributed to a vibrant and rapidly growing field in medical imaging. New targeting moieties including peptides and antibodies are appearing weekly with the promise to tag the rarest of subcellular structures. But they must be seen to realize their potential, so improved fluorescent labels are needed.

Current fluorescent contrast agents include indocyanine green and fluorescein dyes for visualization in the body. These free dyes are FDA-approved to image tissues in the body, but have limitations. Injected dyes lack specificity and are prone to staining large portions of the body as they leak from the vasculature. An ideal in vivo imaging agent needs to simultaneously address the challenges of high contrast, biocompatibility and clearance.

Contrast - Traditional visual wavelength fluorophores are ineffective for generating contrast in tissue, due to the strong visual wavelength absorptions of blood, collagen and scattering from tissue structures. In the near infrared (NIR) region of the spectrum tissue absorption is minimized and is the ideal spectral region for in vivo probe development. HST has developed a range of probes incorporating dyes with emissions from 670 nm to 800 nm. Even in C•spec® particles with the smaller sizes required for certain in vivo applications, silica matrix-dye interactions still lead to enhancement in both brightness and photostability.

Biocompatibility - A fundamental tenet of medicine is “first, do no harm”. The hybrid silica matrix of C•spec® particles is well suited to in vivo use, because it is essentially inert and has low toxicity. Covalent dye attachment in the core of the C•spec® particles prevents dye molecules from leaching into the bloodstream.

Clearance - One of the major challenges for any contrast agent is balancing circulation time within the body with the ability to escape from the body once the imaging is complete. We have designed C•spec® particles containing long-wavelength emission dyes that are capable of fast and efficient renal excretion. Such clearance is made possible by tailoring the particle size below 7 nm diameter and passivating the particle surface with polymers to avoid binding by serum proteins (opsonization).

Working with collaborators at Cornell and Memorial Sloan-Kettering Cancer center, we have demonstrated efficient renal clearance of 3 and 6 nm diameter polymer-coated C•spec® particles with emission in the NIR as recently published in Nano Letters.

A NIR fluorescence image (Red-to Orange) overlaid on a brightfield image of a mouse showing intense NIR fluorescence of 6 nm diameter polymer-coated C•s through the skin as the particles accumulate in the bladder forty minutes after intravenous injection.