Click chemistry and drug delivery: A bird's-eye view.

Click chemistry has been proven to be very useful in drug delivery. Due to the availability of a large number of click reactions with a various characteristics, selection of appropriate chemistry for a given application is often not a trivial task. This review is written for pharmaceutical researchers who are interested in click chemistry applications and yet may not be click chemistry experts. For this, the review gives an overview of available click reactions organized by application types. Further, the general understanding of click reactions being fast and high yielding sometimes overshadows the need to analyze reaction kinetics in assessing suitability of a given reaction for certain applications. For this, we highlight the need to analyze the relationship among reaction kinetics, concentration effects, and reaction time scales, knowing that lack of such analysis could easily lead to failures. Further, possible issues such as chemical stability with various click reagents are also discussed to aid experimental designs. Recent examples and extensive references are also provided to aid in-depth understanding of technical details. We hope this review will help those interested in using click chemistry in drug delivery to select the appropriate reactions/reagents and minimize the number of pitfalls.

De Novo Construction of Fluorophores via CO Insertion-Initiated Lactamization: A Chemical Strategy toward Highly Sensitive and Highly Selective Turn-On Fluorescent Probes for Carbon Monoxide.

Extensive studies in the last few decades have led to the establishment of CO as an endogenous signaling molecule and subsequently to the exploration of CO's therapeutic roles. In the current state, there is a critical conundrum in CO-related research: the extensive knowledge of CO's biological effects and yet an insufficient understanding of the quantitative correlations between the CO concentration and biological responses of various natures. This conundrum is partially due to the difficulty in examining precise concentration-response relationships of a gaseous molecule. Another reason is the need for appropriate tools for the sensitive detection and concentration determination of CO in the biological system. We herein report a new chemical approach to the design of fluorescent CO probes through de novo construction of fluorophores by a CO insertion-initiated lactamization reaction, which allows for ultra-low background and exclusivity in CO detection. Two series of CO detection probes have been designed and synthesized using this strategy. Using these probes, we have extensively demonstrated their utility in quantifying CO in blood, tissue, and cell culture and in cellular imaging of CO from exogenous and endogenous sources. The probes described will enable many biology and chemistry labs to study CO's functions in a concentration-dependent fashion with very high sensitivity and selectivity. The chemical and design principles described will also be applicable in designing fluorescent probes for other small molecules.