A genetic engineering strategy for editing near-infrared-II fluorophores

It is currently difficult to synthesise NIR-II probes with good quantum yields, biocompatibility and pharmacokinetics. Here the authors report a strategy to alter these properties by modifying the protein coatings with biofunctional molecules, and generate long-wavelength fluorophores for in vivo imaging. The second near-infrared (NIR-II) window is a fundamental modality for deep-tissue in vivo imaging. However, it is challenging to synthesize NIR-II probes with high quantum yields (QYs), good biocompatibility, satisfactory pharmacokinetics, and tunable biological properties. Conventional long-wavelength probes, such as inorganic probes (which often contain heavy metal atoms in their scaffolds) and organic dyes (which contain large pi-conjugated groups), exhibit poor biosafety, low QYs, and/or uncontrollable pharmacokinetic properties. Herein, we present a bioengineering strategy that can replace the conventional chemical synthesis methods for generating NIR-II contrast agents. We use a genetic engineering technique to obtain a series of albumin fragments and recombinant proteins containing one or multiple domains that form covalent bonds with chloro-containing cyanine dyes. These albumin variants protect the inserted dyes and remarkably enhance their brightness. The albumin variants can also be genetically edited to develop size-tunable complexes with precisely tailored pharmacokinetics. The proteins can also be conjugated to biofunctional molecules without impacting the complexed dyes. This combination of albumin mutants and clinically-used cyanine dyes can help widen the clinical application prospects of NIR-II fluorophores.

Automated summary

The authors propose a strategy to modify protein coatings with biofunctional molecules, resulting in long-wavelength fluorophores for in vivo imaging. These modified albumin variants protect inserted dyes, enhance their brightness, and can be genetically edited to develop size-tunable complexes with precise pharmacokinetics. This approach widens the clinical application prospects for NIR-II fluorophores.