Development of a fluorescent probe library enabling efficient screening of tumour-imaging probes based on discovery of biomarker enzymatic activities

Fluorescent probes that can selectively detect tumour lesions have great potential for fluorescence imaging-guided surgery. Here, we established a library-based approach for efficient screening of probes for tumour-selective imaging based on discovery of biomarker enzymes. We constructed a combinatorial fluorescent probe library for aminopeptidases and proteases, which is composed of 380 probes with various substrate moieties. Using this probe library, we performed lysate-based in vitro screening and/or direct imaging-based ex vivo screening of freshly resected clinical specimens from lung or gastric cancer patients, and found promising probes for tumour-selective visualization. Further, we identified two target enzymes as novel biomarker enzymes for discriminating between tumour and non-tumour tissues. This library-based approach is expected to be an efficient tool to develop tumour-imaging probes and to discover new biomarker enzyme activities for various tumours and other diseases.

Efficient methodology to develop tumor-imaging fluorescent probes based on screening with our newly constructed probe library for aminopeptidase/protease (380 probes) and clinical samples has been established.     

Design and Synthesis of a Highly Sensitive Off–On Fluorescent Chemosensor for Zinc Ions Utilizing Internal Charge Transfer

Fluorescence imaging is a powerful tool for the visualization of biological molecules in living cells, tissue slices, and whole bodies, and is important for elucidating biological phenomena. Furthermore, zinc (Zn²⁺) is the second most abundant heavy metal ion in the human body after iron, and detection of chelatable Zn²⁺ in biological studies has attracted much attention. Herein, we present a novel, highly sensitive off–on fluorescent chemosensor for Zn²⁺ by using the internal charge transfer (ICT) mechanism. The rationale of our approach to highly sensitive sensor molecules is as follows. If fluorescence can be completely quenched in the absence of Zn²⁺, chemosensors would offer a better signal‐to‐noise ratio. However, it is difficult to quench the fluorescence completely before Zn²⁺ binding, and most sensor molecules still show very weak fluorescence in the absence of Zn²⁺. But even though the sensor shows a weak fluorescence in the absence of Zn²⁺, this fluorescence can be further suppressed by selecting an excitation wavelength that is barely absorbed by the Zn²⁺‐free sensor molecule. Focusing on careful control of ICT within the 4‐amino‐1,8‐naphthalimide dye platform, we designed and synthesized a new chemosensor ( 1 ) that shows a pronounced fluorescence enhancement with a blueshift in the absorption spectrum upon addition of Zn²⁺. The usefulness of 1 for monitoring Zn²⁺ changes was confirmed in living HeLa cells. There have been several reports on 4‐amino‐1,8‐naphthalimide‐based fluorescent sensor molecules. However, 1 is the first Zn²⁺‐sensitive off–on fluorescent sensor molecule that employs the ICT mechanism; most off–on sensor molecules for Zn²⁺ employ the photoinduced electron transfer (PeT) mechanism.     

A time-resolved fluorescence probe for dipeptidyl peptidase 4 and its application in inhibitor screening

The prevalence of type 2 diabetes is increasing dramatically throughout the world. Recently, dipeptidyl peptidase 4 (DPP4) was identified as a potential antidiabetes target. Many DPP4 inhibitors, such as sitagliptin and vildagliptin, have been developed and marketed, but superior therapeutic agents are still required. Therefore, we have developed new methodology for screening of DPP4 inhibitors. Absorption-based measurements with para-nitroaniline or fluorescence-based measurements with the coumarin derivative 7-amino-4-methylcoumarin are often used for the screening of protease inhibitors, including DPP4 inhibitors, but these strategies are not sufficiently sensitive because of interfering background absorption and fluorescence, thus giving rise to many false-positive and false-negative results. Therefore, we have designed and synthesised a novel DPP4 probe (Gly-Pro-BCD-Tb; Gly=glycine, Pro=proline, andBCD defines the backbone of the probe comprising an aniline derivative as on/off switch, a 7-amino-4-methyl-2(1H)-quinolinone (cs-124) as antenna moiety, and a diethylenetriamine-N,N,N',N',N'-pentaacetic acid (DTPA) as chelator moiety, Tb=terbium) for time-resolved fluorescence (TRF) measurements. TRF measurements with Gly-Pro-BCD-Tb showed high sensitivity and reliability in the inhibitory assay relative to Gly-Pro-MCA (MCA=4-methylcoumarin-7-amide), a conventional fluorescence probe for DPP4. Further, we employed our probe for high-throughput DPP4 inhibitor screening with 3841 randomly selected compounds and found that epibestatin, an epimer of bestatin (a well-known anticancer drug and general aminopeptidase inhibitor), showed dose-dependent DPP4 inhibitory activity. Interestingly, bestatin did not exhibit DPP4 inhibitory activity. We believe that this screening system will be useful for the discovery of DPP4 inhibitors with novel structural scaffolds.     

Development of responsive lanthanide-based magnetic resonance imaging and luminescent probes for biological applications

Lanthanide complexes have unique chemical characteristics compared with typical organic complexes, and have recently attracted much interest because of the expanding need for new bioanalytical sensors. For example, magnetic resonance imaging (MRI) permits noninvasive three-dimensional imaging inside opaque organisms, and gadolinium ion (Gd(3+)) complexes have become important tools as MRI contrast agents. However, most of them are nonspecific, and report solely on anatomy. Therefore, responsive MRI contrast agents, so-called "smart" MRI contrast agents whose ability to relax water protons is greatly enhanced by recognition of a particular biomolecule, have great potential for elucidating biological phenomena. On the other hand, lanthanide complexes such as europium (Eu(3+)) and terbium (Tb(3+)) complexes have excellent luminescence properties for biological applications, i.e., long luminescence lifetime of the order of milliseconds and a large Stoke's shift of >200 nm. Their long-lived luminescence is especially suitable for time-resolved measurements, because the interference from short-lived background fluorescence and scattered light rapidly decays to a negligible level after a pulse of excitation light is applied, and the emitted light can be collected after an appropriate delay time. These luminescent lanthanide complexes have already found commercial use as highly sensitive luminescent probes in heterogeneous and homogeneous assays. This paper reviews our research on the design and synthesis of responsive lanthanide-based MRI and luminescent probes for advanced bioimaging.     

Asymmetric anionic polymerization of optically active N-1-cyclohexylethylmaleimide

Chiral (S)-(−)-N-1-cyclohexylethylmaleimide [(S)-CEMI] and (R)-(+)-N-1-cyclohexylethylmaleimide [(R)-CEMI] were synthesized successfully and then polymerized with chiral complexes of (−)-sparteine or (S,S)-(1-ethylpropylidene)bis(4-benzyl-2-oxazoline) [(S,S)-Bnbox] and organometal as initiators in toluene or tetrahydrofuran to obtain optically active polymers. The effects of the polymerization conditions on the optical activity and structure of poly(N-1-cyclohexylethylmaleimide)s were investigated with gel permeation chromatography, circular dichroism, specific rotation, and ¹³C NMR measurements. Poly[(R)-CEMI] obtained with dimethylzinc (Me₂Zn)/(S,S)-Bnbox had the highest specific rotation ([α]₄₃₅ = +323.7°). Complexes of Bnbox and diethylzinc or Me₂Zn were used very effectively as chiral initiators for the asymmetric anionic polymerization of (S)-CEMI and (R)-CEMI. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4682–4692, 2004     

Development of a nuclear spin polarizer with the optical pumping method

We have developed an optical pumping NMR system for semiconductors as an effective nuclear spin polarizer for a solid-state NMR quantum computer. The system was successfully applied to enhance ³¹P nuclear spin polarization in InP doped with Fe, and the enhancement of the ³¹P NMR signal by more than two orders of magnitude was achieved. We also observed the strong dependences of the enhancement on the helicity and the photon energy of incident light. The most effective enhancement was achieved with the helicity σ⁺ and photon energy smaller than the bandgap by 1015 meV.     

Syntheses and polymerizations of novel chiral poly(acrylamide) macromonomers and their chiral recognition abilities

Chiral poly(acrylamide) macromonomers (PMB‐1, PMB‐2, PPAE‐1, and PPAE‐2) were synthesized from 2‐methacryloyloxyethyl isocyanate and prepolymers, that is, poly[(S)‐methylbenzyl acrylamide] or poly(L ‐phenylalanine ethylester acrylamide with a terminal carboxylic acid or hydroxy group. Radical homopolymerizations of poly(acrylamide) macromonomers were carried out under several conditions to obtain the corresponding optically active polymers. A strong temperature dependence on the specific optical rotation was observed for poly(PPAE‐2) in comparison with that for the corresponding prepolymer. This might have resulted from a change in the conformation caused by hydrogen bonds between polymer‐graft branches in the polymacromonomer. Radical copolymerizations of poly(acrylamide) macromonomers with styrene and methyl methacrylate were performed with azobisisobutyronitrile in tetrahydrofuran at 60 °C. Chiroptical properties of the copolymers were slightly influenced by comonomer units. Chiral stationary phases were prepared by the radical polymerization of poly(acrylamide) macromonomers in the presence of silica gel containing vinyl groups on the surface. Some racemic compounds such as menthol and mandelic acid were resolved on the chiral stationary phases for high‐performance liquid chromatography. The conformation based on hydrogen bonds between polymer‐graft branches in the polymacromonomers may play an important role in chiral discrimination. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1726–1741, 2002     

Instantaneous structure and statistical feature of unsteady flow in a channel obstructed by a square rod

A two-dimensional numerical computation has been made for an unsteady flow in a channel obstructed by an inserted square rod. The results of the computation made for the flow with a parabolic inlet velocity profile at a specific value of channel Reynolds number are analyzed in detail. The obtained results reveal that momentum transfer is enhanced due to the apparent shear stress resulting from the nonzero value of cross-correlation between the streamwise and normal components of fluctuating velocity, , just as in turbulent shear flows, although the studied flow is quite different from turbulent flows in the sense that it is highly periodical and therefore free from randomness. This periodicity leads to a quick recovery of the velocity defect in some region of the wake of the rod. Special attention is paid to the time variation of flow structure. The crisscross motion of the Karman vortex previously found to occur is discussed again, and how it appears is explained in terms of the interaction between the Karman vortex and the disturbed wall shear layer. In the discussion of this relationship, wavering motion of the separation vorticity layers formed on both sides of the rod and the periodic formation of an isolated vortex island from the lifted tip of the wall vorticity layer are analyzed. The vortex island is found to play an important role not only for the occurrence of the crisscross motion of Karman vortex but also for the generation of the nonzero value of .     

Synthesis and fluorescent properties of conjugated copolymers containing maleimide and fluorene units at the main chain

Yamamoto or Suzuki–Miyaura coupling polymerizations of 2,3‐diiodo‐N‐cyclohexylmaleimide with fluorene derivatives (2,7‐dibromo‐9,9′‐dihexylfluorene and 9,9′‐dihexylfluorene‐2,7‐diboronic acid) were carried out. The number‐average molecular weights (M_n) of the resulting copolymers were 2600–3500 by gel permeation chromatography analysis. The fluorescence emission of the alternating copolymer showed the emission maxima at 551 nm in THF. On the other hand, the random copolymers showed the bimodal emission peaks at 418–420 and 555–557 nm region, respectively. The fluorescence peaks of the random copolymers on the long wavelength region (555–557 nm) were attributed to the conjugated neighboring N‐cyclohexylmaleimide‐9,9′‐dihexylfluorene units in the polymer main chain. Furthermore, the copolymers exhibited the fluorescence solvatochromism by the difference of the polarity of solvents. The alternating and random copolymers showed the different fluorescence solvatochromism, and the emission colors are distinguishable by the naked eye, respectively. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4945–4956     

Synthesis and polymerization of chiral methacrylates bearing a cholesteryl or menthyl group

Chiral methacrylates, that is, cholesteryl (ChMOC) and l‐menthyl (MnMOC) N‐(2‐methacryloyloxyethyl)carbamates, were synthesized from 2‐methacryloyloxyethyl isocyanate and cholesterol and l‐menthol, respectively. Radical polymerizations of ChMOC and MnMOC gave number‐average molecular weights for poly(ChMOC) and poly(MnMOC) of up to 3.74 × 10⁴ and 9.39 × 10⁴, respectively, and the specific rotations ([α]) were −43.1° to −47.7° and −87.6° to −89.0°, respectively. Temperature dependence of the specific optical rotation was observed for poly(ChMOC) but not for poly(MnMOC). The hydrogen bonds based on urethane segments for poly(ChMOC) were stronger than those for poly(MnMOC) according to IR spectra. In addition, the chiroptical properties of poly(ChMOC) were slightly affected by temperature in the presence of trifluoroacetic acid acting as an inhibitor for the formation of hydrogen bonds. Therefore, poly(ChMOC) may have a regular conformation due to hydrogen bonds and interaction between cholesteryl groups. Radical copolymerizations of ChMOC with styrene, methyl methacrylate, N‐cyclohexylmaleimide, and N‐phenylmaleimide were performed with 2,2′‐azobisisobutyronitrile in tetrahydrofuran at 60 °C. Monomer reactivity ratios and Alfrey–Price Q–e were determined. Chiroptical properties of the copolymers were influenced by co‐units. Thermal and X‐ray diffraction analyses were performed for the homopolymers and copolymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4315–4325, 2000