Mammalian carboxylesterases (CEs) are key enzymes from the serine hydrolase superfamily. In the human body, two predominant carboxylesterases (CES1 and CES2) have been identified and extensively studied over the past decade. These two enzymes play crucial roles in the metabolism of a wide variety of endogenous esters, ester-containing drugs and environmental toxicants. The key roles of CES in both human health and xenobiotic metabolism arouse great interest in the discovery of potent CES modulators to regulate endobiotic metabolism or to improve the efficacy of ester drugs. This review covers the structural and catalytic features of CES, tissue distributions, biological functions, genetic polymorphisms, substrate specificities and inhibitor properties of CES1 and CES2, as well as the significance and recent progress on the discovery of CES modulators. The information presented here will help pharmacologists explore the relevance of CES to human diseases or to assign the contribution of certain CES in xenobiotic metabolism. It will also facilitate medicinal chemistry efforts to design prodrugs activated by a given CES isoform, or to develop potent and selective modulators of CES for potential biomedical applications.

A highly selective and sensitive bioluminescent sensor (DME) for human carboxylesterase 1 (hCE1) has been developed and well characterized. DME could be used for real-time monitoring of hCE1 activities in complex biological samples and for bio-imaging of endogenous hCE1 in living cells.

In this study, a two-photon ratiometric fluorescent probe NCEN has been designed and developed for highly selective and sensitive sensing of human carboxylesterase 2 (hCE2) based on the catalytic properties and substrate preference of hCE2. Upon addition of hCE2, the probe could be readily hydrolyzed to release 4-amino-1,8-naphthalimide (NAH), which brings remarkable red-shift in fluorescence (90 nm) spectrum. The newly developed probe exhibits good specificity, ultrahigh sensitivity, and has been successfully applied to determine the real activities of hCE2 in complex biological samples such as cell and tissue preparations. NCEN has also been used for two-photon imaging of intracellular hCE2 in living cells as well as in deep-tissues for the first time, and the results showed that the probe exhibited high ratiometric imaging resolution and deep-tissue imaging depth. All these findings suggested that this probe holds great promise for applications in bioimaging of endogenous hCE2 in living cells and in exploring the biological functions of hCE2 in complex biological systems.

Mammalian carboxylesterases are key serine hydrolases that catalyze the hydrolysis of a wide variety of ester compounds in the corresponding carboxylic acids and alcohols. In human, two major carboxylesterases, CES1 and CES2, have been identified and well-studied over the past decade. CES1 inhibitors have potential applications in the treatment of hypertriglyceridaemia, obesity and type 2 diabetes, owing to that this enzyme plays prominent role in the metabolism of cholesteryl esters. CES2 plays crucial roles in the metabolic activation of many prodrugs including anticancer agents capecitabine and CPT-11. Co-administration with CES2 inhibitors may ameliorate CPT-11 associated lifethreatening diarrhea or improve the half-lives of CES2-substrate drugs. The important roles of carboxylesterases in both endogenous and xenobiotic metabolism arouse great interest in the discovery and development of potent and selective inhibitors against these enzymes. This review is focused on the application potentials and recent advances in the discovery and development of carboxylesterases inhibitors. The inhibitory capacities and inhibition mechanism of a variety of carboxylesterases inhibitors including synthetic, semi-synthetic and natural compounds are comprehensively summarized. Furthermore, the key structural features and structure-activity relationships (SARs) of different classes of CES1 and CES2 inhibitors are discussed. All information and knowledge summarized in this review will be very helpful for the medicinal chemists to design and develop more potent and highly selective carboxylesterases inhibitors for potential biomedical applications.

In this study, a highly specific ratiometric two-photon fluorescent probe GP-BAN was developed and well-characterized to monitor dipeptidyl peptidase IV in plasma and living systems. GP-BAN was designed on the basis of the catalytic properties and substrate preference of DPP-IV, and it could be readily hydrolyzed upon addition of DPP-IV under physiological conditions. Both reaction phenotyping and inhibition assays demonstrated that GP-BAN displayed good reactivity and high selectivity towards DPP-IV over other human serine hydrolases including FAP, DPP-VIII, and DPP-IX. The probe was successfully used to monitor the real activities of DPP-IV in complex biological systems including diluted plasma, while it could be used for high throughput screening of DPP-IV inhibitors by using human plasma or tissue preparations as enzyme sources. As a two-photon fluorescent probe, GP-BAN was also successfully used for two-photon imaging of endogenous DPP-IV in living cells and tissues, and showed high ratiometric imaging resolution and deep-tissue penetration ability. Taken together, a ratiometric two-photon fluorescent probe GP-BAN was developed and well-characterized for highly selective and sensitive detection of DPP-IV in complex biological systems, which could serve as a promising imaging tool to explore the biological functions and physiological roles of this key enzyme in living systems.

A novel library of chiral guanidines featuring a tartaric acid skeleton was developed from diethyl l-tartrate. These guanidines are easily accessed with tunable steric and electronic properties. The utilities of the guanidines were highlighted by their ability to catalyze the α-hydroxylation of β-ketoesters and β-diketones with remarkable efficiency and excellent enantioselectivity.

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