Near-infrared (NIR) fluorescence has the potential to revolutionize image-guided surgery. However, ideal fluorophores for in vivo, and eventually clinical, use have not yet been described. Under an NIH Bioengineering Research Partnership (BRP) grant, the PI's laboratory has developed a surgical imaging system that simultaneously, and in real-time, acquires two independent wavelengths of NIR fluorescence emission images along with color video images. The imaging system has already been translated to the clinic, and is being formally evaluated in three NIH-funded clinical trials. Nevertheless, the fundamental limitation to the future success of this technology is the development of NIR fluorophores that perform optimally in the body, and which can be made widely available to other academic researchers. To be clinically viable, the ideal NIR fluorophore requires certain optical properties, including excitation and emission H800 nm, and both a high extinction coefficient (5) and quantum yield (QY) in serum. However, the reason why existing NIR fluorophores perform so poorly in vivo has more to do with biodistribution and clearance. After IV injection, the ideal NIR fluorophore would rapidly equilibrate between the intravascular and extra vascular spaces and would be cleared efficiently via renal filtration. To date, every NIR fluorophore described in the literature suffers from two fundamental flaws: 1) hepatic clearance, which results in NIR fluorescence signal throughout the GI tract that persists for hours, and/or 2) non-specific background uptake in normal tissues, which typically persists for hours and results in a low signal-to-background ratio (SBR). This grant builds upon an observation we made two years ago using NIR fluorescent quantum dots (Choi et al., Nature Biotechnol. 2007;25: 1165-70). Unexpectedly, and for reasons only partially understood, zwitterionic organic coatings resulted in extremely low non-specific tissue uptake, rapid renal clearance, and no serum protein binding. However, purely anionic or cationic coatings gave the opposite results. Based on these data, we began collaborating with Drs. Patonay and Strekowski at Georgia State University, leaders in the field of NIR fluorophore chemistry, to synthesize zwitterionic heptamethine indocyanine NIR fluorophores. The preliminary results, described herein, demonstrate that both non-targeted and tumor-targeted zwitterionic NIR fluorophores have remarkable optical and in vivo properties, including 800 nm fluorescence, high 5 and QY, rapid renal clearance, absence of protein binding, and ultra-low non-specific tissue uptake (i.e., background).
The specific aims of this grant are focused on the synthesis of optimized zwitterionic NIR fluorophores for in vivo and surgical imaging, on validating their use as targeted diagnostic agents for prostate cancer, and for scale-up from analytical to preparative production. Completion of these aims will lay the foundation for future clinical testing during image-guided surgery. Importantly, we also present an intellectual property strategy that will permit free sharing of optimized NIR fluorophores within the academic community.
Near-infrared light is invisible to the human eye, but penetrates relatively deeply into living tissue. It is therefore ideal for image-guided surgery, because it provides surgeons with high- sensitivity, high-resolution detection of diseases, such as cancer, without changing the look of the surgical field. Although hardware systems that use near-infrared fluorescent light for image-guided surgery are already available, optimized fluorophores, or "light bulbs" are not. The goal of this grant is to develop a new class of ideal near-infrared fluorophores that can be injected into the bloodstream. These fluorophores would "stick" to tumors and other diseased tissue, but not to normal tissue.
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