4 Targeted Fluorescent Probes for β-Galactosidase
Detection
Targeted imaging by NIR fluorescent probes has been developed as an efficient
alternative to the conventional methods for improving the diagnosis of a particular
disease and therapeutic response of applicable drugs. A variety of targeting moieties,
including antibodies, peptides, and low molecular weight non-peptide ligands, have
been used in various formulations to selectively target the tumor site for the delivery
of diagnostic agents and/or drugs.
Kim et al. [48] developed a ratiometric fluorescent probe (DCDHF-β-gal, 13) for
β-gal visualization in vivo. Probe 13 possessed β-D-galactopyranoside unit which not
only behaved as a substrate of β-gal but acted as a ligand of asialoglycoprotein
receptor (ASGPR). The β-D-galactopyranoside bond was cleaved by intracellular
β-gal, thereby releasing NIR fluorophore and exhibiting ratiometric optical response.
Initial fluorescence emission at 615 nm of probe 13 would red-shifted to 665 nm
upon the incubation with the enzyme. Cellular studies further indicated that
DCDHF-β-gal had great capability to target hepatocytes via ASGPRs without
significant cytotoxicity. Furthermore, as a ratiometric fluorescent probe, the fluorescent spectra changes of 13 could be measured at two different emission bands, which
renders relatively higher sensitivity and accuracy with concentration independence
of probes or environmental conditions. Taken together, the novel ratiometric
fluorogenic DCDHF-β-gal with NIR emission wavelength could be selectively
delivered into ASGPR-positive cells, where it allowed effective noninvasive visualization of the β-gal activity (Fig. 7).
Prost and Hasserodt [49] reported a new kind of activatable imaging pre-profluorescent probe applying the concept of “AND-type logic gate,” which meant
it emitted fluorescent signal only after being consecutively converted by two
different enzymes. A β-gal unit was linked via an eliminating parahydroxybenzyloxycarbonyl spacer to the leucine unit, which in return was linked
to the silenced fluorophore through aminomethylpiperidine cyclizing spacer
[50]. The fluorescent signal was observed only at the presence of both enzymes.
The probe’s simplicity indicated that there was a possible extension to triple-gating
probes but still much remains to be explored.
5 Conclusion and Prospective
In general, it has been demonstrated that β-gal is a significant biomarker for cell
senescence and primary ovarian cancers. Many efforts have been devoted to treating
β-gal as an enzymatic target and visualizing its activity in preclinical diagnosis with
fluorescent probes. Molecular bioimaging of enzyme activity in vivo is rapidly
emerging as a powerful strategy for accurate disease diagnostics. The development
of “smart” noninvasive imaging reagents for the determination of specific enzyme
Fluorescent Probes for Diagnostics of β-Galactosidase: From Micro to Macro
197
Detection
Targeted imaging by NIR fluorescent probes has been developed as an efficient
alternative to the conventional methods for improving the diagnosis of a particular
disease and therapeutic response of applicable drugs. A variety of targeting moieties,
including antibodies, peptides, and low molecular weight non-peptide ligands, have
been used in various formulations to selectively target the tumor site for the delivery
of diagnostic agents and/or drugs.
Kim et al. [48] developed a ratiometric fluorescent probe (DCDHF-β-gal, 13) for
β-gal visualization in vivo. Probe 13 possessed β-D-galactopyranoside unit which not
only behaved as a substrate of β-gal but acted as a ligand of asialoglycoprotein
receptor (ASGPR). The β-D-galactopyranoside bond was cleaved by intracellular
β-gal, thereby releasing NIR fluorophore and exhibiting ratiometric optical response.
Initial fluorescence emission at 615 nm of probe 13 would red-shifted to 665 nm
upon the incubation with the enzyme. Cellular studies further indicated that
DCDHF-β-gal had great capability to target hepatocytes via ASGPRs without
significant cytotoxicity. Furthermore, as a ratiometric fluorescent probe, the fluorescent spectra changes of 13 could be measured at two different emission bands, which
renders relatively higher sensitivity and accuracy with concentration independence
of probes or environmental conditions. Taken together, the novel ratiometric
fluorogenic DCDHF-β-gal with NIR emission wavelength could be selectively
delivered into ASGPR-positive cells, where it allowed effective noninvasive visualization of the β-gal activity (Fig. 7).
Prost and Hasserodt [49] reported a new kind of activatable imaging pre-profluorescent probe applying the concept of “AND-type logic gate,” which meant
it emitted fluorescent signal only after being consecutively converted by two
different enzymes. A β-gal unit was linked via an eliminating parahydroxybenzyloxycarbonyl spacer to the leucine unit, which in return was linked
to the silenced fluorophore through aminomethylpiperidine cyclizing spacer
[50]. The fluorescent signal was observed only at the presence of both enzymes.
The probe’s simplicity indicated that there was a possible extension to triple-gating
probes but still much remains to be explored.
5 Conclusion and Prospective
In general, it has been demonstrated that β-gal is a significant biomarker for cell
senescence and primary ovarian cancers. Many efforts have been devoted to treating
β-gal as an enzymatic target and visualizing its activity in preclinical diagnosis with
fluorescent probes. Molecular bioimaging of enzyme activity in vivo is rapidly
emerging as a powerful strategy for accurate disease diagnostics. The development
of “smart” noninvasive imaging reagents for the determination of specific enzyme
Fluorescent Probes for Diagnostics of β-Galactosidase: From Micro to Macro
197