Enzyme-Responsive Theranostic FRET Probe Based on l-Aspartic Amphiphilic Polyester Nanoassemblies for Intracellular Bioimaging in Cancer Cells.

The present study reports one of the first attempts on the design and development of an enzymatic-biodegradable theranostic fluorescence resonance energy transfer (FRET) probe constructed on l-amino acid polymer nanoassemblies and demonstrates the proof-of-concept in live cell bioimaging. l-Aspartic acid was converted into amide or carbamate pendants containing bis-carboxylic acid ester monomers, and they were subjected to melt polymerization along with commercial diols to produce amphiphilic aliphatic polyesters. Nanoparticles of size <200 nm were obtained because of self-assembly of these amphiphilic polyesters in an aqueous medium. These nanoparticles exhibited excellent encapsulation capability for green-fluorescent anti-inflammatory drug curcumin (CUR) and highly luminescent red-fluorophore Nile red (NR) to yield a CUR-NR theranostic FRET probe. Detailed photophysical studies were carried out to demonstrate photoexcitation energy transfer from CUR to NR for the occurrence of the FRET phenomena. The theranostic FRET probe was found to be very stable at extracellular environment and underwent biodegradation at the intracellular regions for delivery of the loaded cargoes. As a result, the theranostic FRET probe functioned as turn-on at the extracellular level and became turn-off at the intracellular level under lysosomal enzyme-responsiveness. The polymer nanoparticle was nontoxic to cells, whereas its CUR encapsulated nanoparticle showed relatively good cytotoxicity in breast cancer cell lines. Live cell confocal microscopy studies using lysotracker staining confirmed the colocalization of CUR as well as NR within the polymer nanoparticles in the lysosomes for enzymatic-biodegradation. Selective photoexcitation experiments in the confocal microscope were carried out to study the FRET probe action in cancer cells. Time-dependent FRET imaging directly supported the occurrence of FRET at the intracellular level and enabled the real-time drug release studies. The present approach opens natural resource-based biodegradable theranostic FRET probes for bioimaging application.