Thiophene bridged aldehydes (TBAs) image ALDH activity in cells via modulation of intramolecular charge transfer

Near-Infrared Fluorescence Probe with a New Recognition Moiety for Specific Detection and Imaging of Aldehyde Dehydrogenase Expecting the Identification and Isolation of Cancer Stem Cells

Aldehyde dehydrogenase (ALDH) is a vital enzyme that converts aldehyde to acetic acid during alcohol metabolism. ALDH is also a cellular marker of cancer stem cells (CSCs), which plays an important role in cancer diagnosis and prognosis assessment. Therefore, there is a need to explore convenient, selective, and sensitive methods for the detection and imaging of ALDH. Because of the low background fluorescence and high penetration, near-infrared (NIR) fluorescent probes are powerful tools for the detection of ALDH. Until now, only one NIR fluorescent probe has been reported for detecting ALDH. Hence, we synthesized a novel NIR fluorescent probe, Probe-ALDH, by linking the new specific recognition moiety 4-hydroxymethyl benzaldehyde with NIR fluorophore AXPI. Compared with the existing ALDH fluorescent probes, Probe-ALDH has excellent properties, such as a new specific recognition moiety without the substitution of benzaldehyde, a simple synthesis method, emission wavelength in the NIR region, reaction time of only 30 min, and a detection limit as low as 0.03 U·mL^-1, which is better than those of the previously reported probes. The probe effectively eliminates the interference from reactive oxygen species (ROS), amino acids, and amines. More importantly, the flow cytometry results showed that Probe-ALDH has great potential applications in the identification and isolation of CSCs. Ultimately, it was successfully applied to the imaging analysis of endogenous ALDH in HepG2 cells by the addition of inhibitor disulfiram. The excellent performance of Probe-ALDH makes it a promising candidate for drug discovery, cancer diagnosis, and so forth.

A prodrug strategy for the in vivo imaging of aldehyde dehydrogenase activity

Therapy resistance is one of the biggest challenges facing clinical oncology. Despite a revolution in new anti-cancer drugs targeting multiple components of the tumour microenvironment, acquired or innate resistance frequently blunts the efficacy of these treatments. Non-invasive identification of drug-resistant tumours will enable modification of the patient treatment pathway through the selection of appropriate second-line treatments. Here, we have designed a prodrug radiotracer for the non-invasive imaging of aldehyde dehydrogenase 1A1 (ALDH1A1) activity. Elevated ALDH1A1 activity is a marker of drug-resistant cancer cells, modelled here with matched cisplatin-sensitive and -resistant human SKOV3 ovarian cancer cells. The aromatic aldehyde of our prodrug radiotracer was intracellularly liberated by esterase cleavage of the geminal diacetate and specifically trapped by ALDH through its conversion to the charged carboxylic acid. Through this mechanism of action, ALDH-specific retention of our prodrug radiotracer in the drug-resistant tumour cells was twice as high as the drug-sensitive cells. Acylal masking of the aldehyde afforded a modest protection from oxidation in the blood, which was substantially improved in carrier-added experiments. In vivo positron emission tomography imaging of tumour-bearing mice produced high tumour-to-background images and radiotracer uptake in high ALDH-expressing organs but was unable to differentiate between drug-sensitive and drug-resistant tumours. Alternative strategies to protect the labile aldehyde are currently under investigation.

Multifunctional fluorescent probe for effective visualization, inhibition, and detoxification of β-amyloid aggregation via covalent binding

A multifunctional reactive fluorescent probe DTB was constructed for biosensing, aggregation inhibition, and toxicity alleviation of β-amyloid. The synergistic effect of hydrophobic interaction and covalent interaction makes DTB have more stable binding and better selectivity to Aβ. The detoxification effect of DTB on Aβ aggregates was also verified in live nerve cells and microglia cells. Furthermore, DTB exhibits an excellent staining of Aβ plaques.

Deep-Red/Near-Infrared Turn-On Fluorescence Probes for Aldehyde Dehydrogenase 1A1 in Cancer Stem Cells

Accumulating evidence supports that cancer stem cells (CSCs) are responsible for cancer proliferation, metastasis, and therapy resistance; therefore, an effective strategy to identify and isolate CSCs is required urgently. Because of their low invasiveness and high signal/noise ratio, "turn-on" fluorescence probes working in the deep-red/near-infrared (DR/NIR) region are one of the most attractive yet undeveloped tools for CSC detection. Herein, we report DR/NIR turn-on fluorescence probes, CS5-A and CS7-A, targeted to aldehyde dehydrogenase 1A1 as an intracellular CSC marker. In contrast to the conventional "always-on" green-fluorescent ALDEFLUOR, we succeeded in generating high-contrast (signal/noise ratio > 8.3) and wash-free in vitro CSC imaging with the DR probe C5S-A. This probe can facilitate CSC isolation with minimal contamination by autofluorescence from other tissues through fluorescence-activated cell sorting. Furthermore, the NIR absorbance/emission and turn-on properties of C7S-A allow simple and rapid CSC detection in vivo within 15 min.

Tailored Biosensors for Drug Screening, Efficacy Assessment, and Toxicity Evaluation

Biosensors have been flourishing in the field of drug discovery with pronounced developments in the past few years. They facilitate the screening and discovery of innovative drugs. However, there is still a lack of critical reviews that compare the merits and shortcomings of these biosensors from a pharmaceutical point of view. This contribution presents a critical and up-to-date overview on the recent progress of tailored biosensors, including surface plasmon resonance, fluorescent, photoelectrochemical, and electrochemical systems with emphasis on their mechanisms and applications in drug screening, efficacy assessment, and toxicity evaluation. Multiple functional nanomaterials have also been incorporated into the biosensors. Representative examples of each type of biosensors are discussed in terms of design strategy, response mechanism, and potential applications. In the end, we also compare the results and summarize the major insights gained from the works, demonstrating the challenges and prospects of biosensors-assisted drug discovery.

Amino BODIPY-Based Blue Fluorescent Probes for Aldehyde Dehydrogenase 1-Expressing Cells

Aldehyde dehydrogenase 1 (ALDH1) plays an important role as a stem cell marker. In the field of stem cell biology, a green fluorescent ALDH1 probe has been principally used, but there is a need for more options in probe color. We designed and synthesized two blue fluorescent ALDH1 probes using 8-amino BODIPY and aminomethylbenzaldehyde. These probes can be simultaneously used with other color probes. Here, we demonstrate successful examples of the simultaneous use of these probes with green fluorescent protein.

Mapping Aldehyde Dehydrogenase 1A1 Activity using an [18 F]Substrate-Based Approach

Aldehyde dehydrogenases (ALDHs) catalyze the oxidation of aldehydes to carboxylic acids. Elevated ALDH expression in human cancers is linked to metastases and poor overall survival. Despite ALDH being a poor prognostic factor, the non‐invasive assessment of ALDH activity in vivo has not been possible due to a lack of sensitive and translational imaging agents. Presented in this report are the synthesis and biological evaluation of ALDH1A1‐selective chemical probes composed of an aromatic aldehyde derived from N,N‐diethylamino benzaldehyde (DEAB) linked to a fluorinated pyridine ring either via an amide or amine linkage. Of the focused library of compounds evaluated, N‐ethyl‐6‐(fluoro)‐N‐(4‐formylbenzyl)nicotinamide 4 b was found to have excellent affinity and isozyme selectivity for ALDH1A1 in vitro. Following ¹⁸F‐fluorination, [¹⁸F]4 b was taken up by colorectal tumor cells and trapped through the conversion to its ¹⁸F‐labeled carboxylate product under the action of ALDH. In vivo positron emission tomography revealed high uptake of [¹⁸F]4 b in the lungs and liver, with radioactivity cleared through the urinary tract. Oxidation of [¹⁸F]4 b, however, was observed in vivo, which may limit the tissue penetration of this first‐in‐class radiotracer.