Bio-informatics and in Vitro Experiments Reveal the Mechanism of Schisandrin A Against MDA-MB-231 cells.

Schisandrin A (SchA) has been reported to have good anti-cancer effects. However, its anti-cancer mechanism in breast cancer remains unknown. This study aimed to explore the mechanism of SchA in breast cancer treatment using bio-informatics analysis and in vitro experiments. The Cancer Genome Atlas (TCGA), Genotype-Tissue Expression (GTEx), Gene Cards, and PharmMapper databases were used to screen the candidate targets of SchA against MDA-MB-231 cells selected as the tested cell line through MTT analysis. The functions and pathways of the targets were identified using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis and further analyzed using DAVID 6.8.1 database. Network pharmacology analysis revealed 77 candidate targets, 31 signal pathways, and 208 GO entries (P < 0.05). The targets regulated serine-type endopeptidase and protein tyrosine kinase activities, thereby promoting the migration and inhibiting the apoptosis of MDA-MB-231 cells. Comprehensive analysis of the 'Protein-Protein Interaction' (PPI) and 'Component-Targets-Pathways' (C-T-P) networks constructed using Cytoscape 3.7.1 software revealed four core targets: EGFR, PIK3R1, MMP9 and Caspase 3. Their docking scores with SchA were subsequently investigated through molecular docking. The wound healing, Hoechst 33342/PI, and western blot assays confirmed that SchA significantly down-regulated EGFR, PIK3R1, and MMP9, but up-regulated cleaved-caspase 3, thus inhibiting the migration and promoting the apoptosis of MDA-MB-231 cells. Reckoning the findings of the study, SchA is a potential adjuvant treatment for breast cancer.

Single-Cell Profiling of Fatty Acid Uptake Using Surface-Immobilized Dendrimers.

We present a chemical approach to profile fatty acid uptake in single cells. We use azide-modified analogues to probe the fatty acid influx and surface-immobilized dendrimers with dibenzocyclooctyne (DBCO) groups for detection. A competition between the fatty acid probes and BHQ2-azide quencher molecules generates fluorescence signals in a concentration-dependent manner. By integrating this method onto a microfluidics-based multiplex protein analysis platform, we resolved the relationships between fatty acid influx, oncogenic signaling activities, and cell proliferation in single glioblastoma cells. We found that p70S6K and 4EBP1 differentially correlated with fatty acid uptake. We validated that cotargeting p70S6K and fatty acid metabolism synergistically inhibited cell proliferation. Our work provided the first example of studying fatty acid metabolism in the context of protein signaling at single-cell resolution and generated new insights into cancer biology.

Understanding selectivity of metabolic labelling and click-targeting in multicellular environments as a route to tissue selective drug delivery.

Cancer cells generally exhibit higher metabolic demands relative to that of normal tissue cells. This offers great possibilities to exploit metabolic glycoengineering in combination with bio-orthogonal chemistry reactions to achieve tumour site-targeted therapeutic delivery. This work addresses the selectivity of metabolic glycan labelling in diseased (i.e., cancer) versus normal cells grown in a multicellular environment. Dibenzocylooctyne (DBCO)-bearing acetylated-d-mannosamine (Ac4ManNDBCO) was synthesised to metabolically label three different types of cell lines originating from the human lung tissues: A549 adenocarcinomic alveolar basal epithelial cells, MeT5A non-cancerous mesothelial cells, and MRC5 non-cancerous fibroblasts. These cell lines displayed different labelling sensitivity, which trended with their doubling time in the following order: A549 ≈ MeT5A > MRC5. The higher metabolic labelling efficiency inherently led to a higher extent of specific binding and accumulation of the clickable N3-conjugated gold nanoparticles (N3-AuNps, core diameter = 30 nm) in the DBCO-glycan modified A549 and MeT5A cells, but to a less prominent effect in MRC5 cells. These findings demonstrate that relative rates of cell metabolism can be exploited using metabolic labelling to recruit nanotherapeutics whilst minimising non-specific targeting of surrounding tissues.

Enzyme-Mediated In Situ Self-Assembly Promotes In Vivo Bioorthogonal Reaction for Pretargeted Multimodality Imaging.

Pretargeted imaging has emerged as a promising approach to advance nuclear imaging of malignant tumors. Herein, we combine the enzyme-mediated fluorogenic reaction and in situ self-assembly with the inverse electron demand Diels-Alder (IEDDA) reaction to develop an activatable pretargeted strategy for multimodality imaging. The trans-cyclooctene (TCO) bearing small-molecule probe, P-FFGd-TCO, can be activated by alkaline phosphatase and in situ self-assembles into nanoaggregates (FMNPs-TCO) retained on the membranes, permitting to (1) amplify near-infrared (NIR) fluorescence (FL) and magnetic resonance imaging (MRI) signals, and (2) enrich TCOs to promote IEDDA ligation. The Gallium-68 (68 Ga) labeled tetrazine can readily conjugate the tumor-retained FMNPs-TCO to enhance radioactivity uptake in tumors. Strong NIR FL, MRI, and positron emission tomography (PET) signals are concomitantly achieved, allowing for pretargeted multimodality imaging of ALP activity in HeLa tumor-bearing mice.

Covalent Cell Surface Conjugation of Nanoparticles by a Combination of Metabolic Labeling and Click Chemistry.

Conjugation of nanoparticles (NP) to the surface of living cells is of interest in the context of exploiting the tissue homing properties of ex vivo engineered T cells for tumor-targeted delivery of drugs loaded into NP. Cell surface conjugation requires either a covalent or non-covalent reaction. Non-covalent conjugation with ligand-decorated NP (LNP) is challenging and involves a dynamic equilibrium between the bound and unbound state. Covalent NP conjugation results in a permanently bound state of NP, but the current routes for cell surface conjugation face slow reaction kinetics and random conjugation to proteins in the glycocalyx. To address the unmet need for alternative bioorthogonal strategies that allow for efficient covalent cell surface conjugation, we developed a 2-step click conjugation sequence in which cells are first metabolically labeled with azides followed by reaction with sulfo-6-methyl-tetrazine-dibenzyl cyclooctyne (Tz-DBCO) by SPAAC, and subsequent IEDDA with trans-cyclooctene (TCO) functionalized NP. In contrast to using only metabolic azide labeling and subsequent conjugation of DBCO-NP, our 2-step method yields a highly specific cell surface conjugation of LNP, with very low non-specific background binding.

Directed evolution of dibenzocyclooctyne-reactive peptide tags for protein labeling.

Protein labeling with a functional molecule is a technique widely used for protein research. The covalent reaction of self-labeling peptide tags with synthetic probe-modified small molecules enables tag-fused protein labeling with chemically diverse molecules, including fluorescent probes. We report the discovery, by in vitro directed evolution, of a novel 23-mer dibenzocyclooctyne (DBCO)-reactive peptide (DRP) tag using Systematic Evolution of Ligands by EXponential enrichment (SELEX) with a combination of a reconstituted cell-free translation system (PURE system) and cDNA display. The N- and C-terminal DRP truncations created a shorter 16-mer DBCO-reactive peptide (sDRP) tag without significant reactivity reduction. By fusing the sDRP tag to a model protein, we showed the chemical labeling and in-gel fluorescence imaging of the sDRP-fused protein using a fluorescent DBCO probe. Results showed that sDRP tag-mediated protein labeling has potential for use as a basic molecular tool in a variety of applications for protein research.

Schizandrin A exhibits potent anticancer activity in colorectal cancer cells by inhibiting heat shock factor 1.

Heat shock factor 1 (HSF1) is a powerful multifaceted oncogenic modifier that plays a role in maintaining the protein balance of cancer cells under various stresses. In recent studies, there have been reports of increased expression of HSF1 in colorectal cancer (CRC) cells, and the depletion of the HSF1 gene knockdown has inhibited colon cancer growth both in vivo and in vitro. Therefore, HSF1 is a promising target for colon cancer treatment and chemoprevention. In the present study, we found that Schizandrin A (Sch A) significantly inhibited the growth of CRC cell lines by inducing cell cycle arrest, apoptosis and death. Through HSE luciferase reporter assay and quantitative PCR (qPCR), we identified Sch A as a novel HSF1 inhibitor. In addition, Sch A could effectively inhibit the induction of HSF1 target proteins such as heat-shock protein (HSP) 70 (HSP70) and HSP27, whether in heat shock or normal temperature culture. In the Surface Plasmon Resonance (SPR) experiment, Sch A showed moderate affinity with HSF1, further confirming that Sch A might be a direct HSF1 inhibitor. The molecular docking and molecular dynamic simulation results of HSF1/Sch A suggested that Sch A formed key hydrogen bond and hydrophobic interactions with HSF1, which may contribute to its potent HSF1 inhibition. These findings provide clues for the design of novel HSF1 inhibitors and drug candidates for colon cancer treatment.

Interaction of deoxyschizandrin and schizandrin B with liver uptake transporters OATP1B1 and OATP1B3.

1. Deoxyschizandrin and schizandrin B have diverse pharmacological effects, including hepatoprotective activity. We aim to study their hepatic uptake and their effects on the hepatic uptake of other clinical drugs mediated by OATP1B1 and OATP1B3. 2. Deoxyschizandrin exhibited a high affinity for OATP1B1 with Km of 17.61 ± 0.43 µM but a low affinity for OATP1B3. Similarly, schizandrin B also showed a strong affinity for OATP1B1 with Km of 18.45 ± 1.23 µM but a weak affinity for OATP1B3. 3. Atorvastatin and rifampicin could inhibit the uptake of deoxyschizandrin and schizandrin B mediated by OATP1B1. 4. Intriguingly, both deoxyschizandrin and schizandrin B significantly promoted the uptake of atorvastatin (with EC50 of 50.58 ± 8.08 and 24.70 ± 5.82 µM, respectively) and rosuvastatin (with EC50 of 13.46 ± 2.70 and 8.99 ± 4.73 µM, respectively) mediated by OATP1B1. Deoxyschizandrin could markedly promote the uptake of fluvastatin but inhibit the uptake of sodium taurocholate (TCNa) mediated by OATP1B1. 5. The promotion on hepatic uptake of statins mediated by OATP1B1 might lead to enhanced efficacy of cholesterol lowering and reduced risk of myopathy for hyperlipidemia patients when given statins together with deoxyschizandrin or schizandrin B.

Schisandrin B elicits the Keap1-Nrf2 defense system via carbene reactive metabolite which is less harmful to mice liver.

BACKGROUND: Schisandrin B (Sch B) a main active component of Schisandra chinensis, has been shown to act as a liver protectant via activation of the Nrf2 pathway. Nevertheless, it remains unclear whether its reactive metabolite is responsible for Nrf2 activation; also, the effects of its reactive metabolite on liver function are still unknown. METHODS: The present study determined and identifed the carbene reactive metabolite of Sch B in human and mice liver microsomes. Its roles in activating Nrf2 pathway and modifying macromolecules were further explored in human liver microsomes. Moreover the potential cytotoxicity and hepatoxicity of carbene on HepG-2 and mice were also investigated. RESULTS: In the present study, cytochromes P450 (CYP450s) metabolized Sch B to carbene reactive metabolite, which, with the potential to modify peptides, were identifed and observed in human and mice liver microsomes. Moreover, the relevance of carbene in Nrf2 activation was verifed by co-incubation in the presence of CYP450 inhibitors in HepG-2 cells, as well as by molecular docking study of carbene and Keap1. Additionally, the cytotoxicity of Sch B on HepG-2 cells was signifcantly aggravated by CYP450 inducer (with LD50 decreasing from 63 to 21 µM) and signifcantly alleviated by CYP450 inhibitor and glutathione (with LD50 increasing from 63 µM to 200 µM). Besides, after oral administration of mice with Sch B (25-100 mg/kg) for 21 days, only the highest dose induced mild hepatotoxicity, which was accompanied by increasing the aminotransferase activity and centrilobular hepatocellular infltration of lymphocytes. In addition, upregulation of CYP450 activity; Nrf2, NQO-1, and GST expression; and glutathione level was observed in Sch B treatment groups. CONCLUSION: The present study revealed that CYP450s mediate the conversion of Sch B to carbene, which subsequently binds to Keap1 and elicits Nrf2 pathway, which could further increase the elimination of carbene and thus exhibit a less harmful effect on mice liver.

The efficiency of 18F labelling of a prostate specific membrane antigen ligand via strain-promoted azide-alkyne reaction: reaction speed versus hydrophilicity.

Here we report the 18F labeling of a prostate specific membrane antigen (PSMA) ligand via a strain promoted oxa-dibenzocyclooctyne (ODIBO)- or bicyclo[6.1.0]nonyne (BCN)-azide reaction. Although ODIBO reacts with azide 20 fold faster than BCN, in vivo PET imaging suggests that 18F-BCN-azide-PSMA demonstrated much higher tumor uptake and a much higher tumor to background contrast.

Click-to-Release from trans-Cyclooctenes: Mechanistic Insights and Expansion of Scope from Established Carbamate to Remarkable Ether Cleavage.

The bioorthogonal cleavage of allylic carbamates from trans-cyclooctene (TCO) upon reaction with tetrazine is widely used to release amines. We disclose herein that this reaction can also cleave TCO esters, carbonates, and surprisingly, ethers. Mechanistic studies demonstrated that the elimination is mainly governed by the formation of the rapidly eliminating 1,4-dihydropyridazine tautomer, and less by the nature of the leaving group. In contrast to the widely used p-aminobenzyloxy linker, which affords cleavage of aromatic but not of aliphatic ethers, the aromatic, benzylic, and aliphatic TCO ethers were cleaved as efficiently as the carbamate, carbonate, and esters. Bioorthogonal ether release was demonstrated by the rapid uncaging of TCO-masked tyrosine in serum, followed by oxidation by tyrosinase. Finally, tyrosine uncaging was used to chemically control cell growth in tyrosine-free medium.

Activators of Sirtuin-1 and their Involvement in Cardioprotection.

SIRT1 is a nicotinamide adenosine dinucleotide (NAD+)-dependent deacetylase, which removes acetyl groups from many target proteins, such as histone proteins, transcription factors and cofactors. SIRT1-catalyzed deacetylation of these factors modulates the activity of downstream proteins, thus influencing many biological processes. SIRT1 is involved in the regulation of metabolism, inflammation, and tumor growth. The activity of this enzyme is related to the beneficial health effects of calorie restriction, such as lifespan extension and, in particular, the activation of SIRT1 has a positive impact on the cardiovascular system. Therefore, SIRT1 is considered as an attractive drug target and modulation of SIRT1 may represent a new therapeutic strategy against cardiovascular diseases, as small molecules able to activate SIRT1 can be considered as cardioprotective agents. In this review, we summarize both natural and synthetic compounds developed as SIRT1 activators, with a focus on their promising therapeutic applications in cardiovascular pathologies.

Mechanisms of chromosomal aberrations induced by sesamin metabolites in Chinese hamster lung cells.

Sesamin is a major lignan in sesame seeds and oil. We previously demonstrated that sesamin induces chromosomal aberrations (CA) in Chinese hamster lung (CHL/IU) cells in the presence of a metabolic activation system (S9 mix), although no genotoxicity was detected in vivo. To clarify the mechanism of CA induction by sesamin, we identified its principal active metabolite. A mono-catechol derivative, [2-(3,4-methylenedioxyphenyl)-6-(3,4-dihydroxyphenyl)-3,7-dioxabi-cyclo[3.3.0]octane (SC-1)], was previously identified in culture medium when sesamin was incubated with S9 mix. In the present study, we show that SC-1 induces CA in CHL/IU cells but not in human hepatoblastoma (HepG2) cells. SC-1 was unstable in culture medium. Addition of glutathione (GSH) to the incubation mixture decreased the rate of decomposition and also suppressed induction of CA in CHL/IU cells. These results indicate that SC-1 itself may not contribute to the induction of CA. Two GSH adducts of SC-1 were identified when SC-1 was incubated with GSH, suggesting that SC-1 was converted to the semiquinone/quinone form and then conjugated with GSH in the culture medium. Sodium sulfite (a quinone-responsive compound) also suppressed CA induction by SC-1. These findings strongly suggest that SC-1 is oxidized to semiquinone/quinone derivatives extracellularly in culture medium, that these derivatives are responsible for the induction of CA in CHL/IU cells, and therefore that the positive results obtained with sesamin in in vitro CA tests using CHL/IU cells may not be relevant to the assessment of in vivo activity.

In Vivo Targeting of Metabolically Labeled Cancers with Ultra-Small Silica Nanoconjugates.

Unnatural sugar-mediated metabolic labeling of cancer cells, coupled with efficient Click chemistry, has shown great potential for in vivo imaging and cancer targeting. Thus far, chemical labeling of cancer cells has been limited to the small-sized azido groups, with the large-sized and highly hydrophobic dibenzocyclooctyne (DBCO) being correspondingly used as the targeting ligand. However, surface modification of nanomedicines with DBCO groups often suffers from low ligand density, difficult functionalization, and impaired physiochemical properties. Here we report the development of DBCO-bearing unnatural sugars that could directly label LS174T colon cancer cells with DBCO groups and subsequently mediate cancer-targeted delivery of azido-modified silica nanoconjugates with easy functionalization and high azido density in vitro and in vivo. This study, for the first time, demonstrates the feasibility of metabolic labeling of cancer cells with large-sized DBCO groups for subsequent, efficient targeting of azido-modified nanomedicines.

Controlled Multi-functionalization Facilitates Targeted Delivery of Nanoparticles to Cancer Cells.

A major objective of nanomedicine is to combine in a controlled manner multiple functional entities into a single nanoscale device to target particles with great spatial precision, thereby increasing the selectivity and potency of therapeutic drugs. A multifunctional nanoparticle is described for controlled conjugation of a cytotoxic drug, a cancer cell targeting ligand, and an imaging moiety. The approach is based on the chemical synthesis of polyethylene glycol that at one end is modified by a thioctic acid for controlled attachment to a gold core. The other end of the PEG polymers is modified by a hydrazine, amine, or dibenzocyclooctynol moiety for conjugation with functional entities having a ketone, activated ester, or azide moiety, respectively. The conjugation approach allowed the controlled attachment of doxorubicin through an acid-labile hydrazone linkage, an Alexa Fluor dye through an amide bond, and a glycan-based ligand for the cell surface receptor CD22 of B-cells using strain promoted azide-alkyne cycloaddition. The incorporation of the ligand for CD22 led to rapid entry of the nanoparticle by receptor-mediated endocytosis. Covalent attachment of doxorubicin via hydrazone linkage caused pH-responsive intracellular release of doxorubicin and significantly enhanced the cytotoxicity of nanoparticles. A remarkable 60-fold enhancement in cytotoxicity of CD22 (+) lymphoma cells was observed compared to non- targeted nanoparticles.

Drug-drug interation prediction between ketoconazole and anti-liver cancer drug Gomisin G.

BACKGROUND: Gomisin G, isolated from herb Schisandra chinensis, exhibits anti-tumor activities. Therefore, Gomisin G is a drug candidate for anti-liver cancer therapy. AIMS: To predict the metabolic behavior and metabolism-based drug-drug interaction of gomisin G. METHODS: Molecular docking method was used. The crystal structure of CYP3A4 with the ligand ketoconazole was chosen from protein data bank (http://www.rcsb.org/pdb). Chemdraw software was used to draw the two-dimensional structure of gomisin G with standard bond lengths and angles. RESULTS: Gomisin G can be well docked into the activity site of CYP3A4, and distance between gomisin G the heme active site was 2.75 Å. To evaluate whether the inhibitors of CYP3A4 can affect the metabolism of gomisin G, co-docking of gomisin G and ketoconazole was further performed. The distance between ketoconazole and activity center (2.10 Å) is closer than the distance between gomisin G and activity center of CYP3A4, indicating the easy influence of CYP3A4's strong inhibitor towards the metabolism of gomisin G. CONCLUSION: Gomisin G is a good substrate of CYP3A4, and CYP3A4 inhibitors easily affect the metabolism of Gomisin G.

Improved metabolic stability for 18F PET probes rapidly constructed via tetrazine trans-cyclooctene ligation.

The fast kinetics and bioorthogonal nature of the tetrazine trans-cyclooctene (TCO) ligation makes it a unique tool for PET probe construction. In this study, we report the development of an (18)F-labeling system based on a CF3-substituted diphenyl-s-tetrazine derivative with the aim of maintaining high reactivity while increasing in vivo stability. c(RGDyK) was tagged by a CF3-substituted diphenyl-s-tetrazine derivative via EDC-mediated coupling. The resulting tetrazine-RGD conjugate was combined with a (19)F-labeled TCO derivative to give HPLC standards. The analogous (18)F-labeled TCO derivative was combined with the diphenyl-s-tetrazine-RGD at µM concentration. The resulting tracer was subjected to in vivo metabolic stability assessment, and microPET studies in murine U87MG xenograft models. The diphenyl-s-tetrazine-RGD combines with an (18)F-labeled TCO in high yields (>97% decay-corrected on the basis of TCO) using only 4 equiv of tetrazine-RGD relative to the (18)F-labeled TCO (concentration calculated based on product's specific activity). The radiochemical purity of the (18)F-RGD peptides was >95% and the specific activity was 111 GBq/µmol. Noninvasive microPET experiments demonstrated that (18)F-RGD had integrin-specific tumor uptake in subcutaneous U87MG glioma. In vivo metabolic stability of (18)F-RGD in blood, urine, and major organs showed two major peaks: one corresponded to the Diels-Alder conjugate and the other was identified as the aromatized analog. A CF3-substituted diphenyl-s-tetrazine displays excellent speed and efficiency in (18)F-PET probe construction, providing nearly quantitative (18)F labeling within minutes at low micromolar concentrations. The resulting conjugates display improved in vivo metabolic stability relative to our previously described system.

Site-specific labeling of proteins and peptides with trans-cyclooctene containing handles capable of tetrazine ligation.

There is a growing library of functionalized non-natural substrates for the enzyme protein farnesyltransferase (PFTase). PFTase covalently attaches these functionalized non-natural substrates to proteins ending in the sequence CAAX, where C is a cysteine that becomes alkylated, A represents an aliphatic amino acid, and X is Ser, Met, Ala, or Gln. Reported substrates include a variety of functionalities that allow modified proteins to undergo subsequent bioconjugation reactions. To date the most common strategy used in this approach has been copper catalyzed azide-alkyne cycloaddition (CuAAC). While being fast and bioorthogonal CuAAC has limited use in live cell experiments due to copper's toxicity.(1) Here, we report the synthesis of trans-cyclooctene geranyl diphosphate. This substrate can be synthesized from geraniol in six steps and be enzymatically transferred to peptides and proteins that end in a CAAX sequence. Proteins and peptides site-specially modified with trans-cyclooctene geranyl diphosphate were subsequently targeted for further modification via tetrazine ligation. As tetrazine ligation is bioorthogonal, fast, and is contingent on ring strain rather than the addition of a copper catalyst, this labeling strategy should prove useful for labeling proteins where the presence of copper may hinder solubility or biological reactivity.

Identification of metabolites of deoxyschizandrin in rats by UPLC-Q-TOF-MS/MS based on multiple mass defect filter data acquisition and multiple data processing techniques.

Deoxyschizandrin is an active lignin ingredient originating from Schisandra chinensis (Turcz.) Baill or Schisandrae Sphenantherae Fructus. In the present study, a novel and efficient strategy was developed for the in vivo screening and identification of deoxyschizandrin metabolites using ultra high performance liquid chromatography combined with triple TOF mass spectrometry (UPLC-TOF/MS/MS). This strategy was characterized by the following: a novel and unique multiple mass defect filter (MMDF) combined with an on-line data acquisition method that is dependent on dynamic background subtraction (DBS) was developed to trace all of the probable metabolites of deoxyschizandrin. The MMDF and DBS methods could trigger an IDA scan for the low-level metabolites that are masked by background noise and endogenous components. A combination of data processing methods including extracted ion chromatography (XIC), mass defect filtering (MDF), product ion filtering (PIF) and neutral loss filtering (NLF) were employed to identify the metabolites of deoxyschizandrin. Next, the structures of the metabolites were elucidated based on an accurate mass measurement, the fragmentation patterns of the parent drug and relevant drug bio-transformation knowledge. Finally, an important parameter ClogP was used to estimate the retention time of isomers. Based on the proposed strategy, 51 metabolites (including 49 phase I and 2 phase II metabolites) were identified in rats after the oral administration of deoxyschizandrin. Among these metabolites, 41 metabolites were characterized in the rat urine, and 28 metabolites were identified in the rat bile. The results indicated that oxidization was the main metabolic pathway and that the methoxy group and the biphenyl cyclooctene were the metabolic sites. Conjugation with sulfate and cysteine groups produced two phase-II metabolites. This study firstly reported the description of deoxyschizandrin metabolism in vivo. This study provided a practical strategy for rapidly screening and identifying metabolites, and this methodology can be widely applied for the structural characterization of the metabolites of other compounds.

Direct light-up of cAMP derivatives in living cells by click reactions.

8-Azidoadenosine 3',5'-cyclic monophosphate (8-azido cAMP) was directly detected in living cells, by applying Cu-free azide-alkyne cycloaddition to probe cAMP derivatives by fluorescence light-up. Fluorescence emission was generated by two non-fluorescent molecules, 8-azido cAMP as a model target and difluorinated cyclooctyne (DIFO) reagent as a probe. The azide-alkyne cycloaddition reaction between 8-azido cAMP and DIFO induces fluorescence in 8-azido cAMP. The fluorescence emission serves as a way to probe 8-azido cAMP in cells.