Recent Developments On Activatable Turn-On Fluorogenic Donors of Hydrogen Sulfide (H2S).

Hydrogen sulfide (H2S) is considered the third member of the gasotransmitter family, along with nitric oxide (NO) and carbon monoxide (CO). Besides its role in physiological and pathophysiological conditions, the promising therapeutic potential of this small-molecule makes it advantageous for various pharmaceutical applications. The endogenous production of H2S at a lower concentration is crucial in maintaining redox balance and cellular homeostasis, and the dysregulation leads to various disease states. In the event of H2S deficiency, the exogenous donation of H2S could help maintain the optimal cellular concentration of H2S and cellular homeostasis. Over the last several years, researchers have developed numerous small-molecule non-fluorogenic organosulfur compounds as H2S donors and investigated their pharmacological potentials. However, reports on stimuli-responsive turn-on fluorogenic donors of H2S have appeared recently. Interestingly, the fluorogenic H2S donors offer additional advantages with the non-invasive real-time monitoring of the H2S release utilizing the simultaneous turn-on fluorogenic processes. The review summarizes the recent developments in turn-on fluorogenic donors of H2S and the potential biological applications that have developed over the years.

Inflammatory-stimuli-responsive turn-on NIR fluorogenic theranostic prodrug: adjuvant delivery of diclofenac and hydrogen sulfide attenuates acute inflammatory disorders.

Prolonged use of very commonly prescribed non-steroidal anti-inflammatory drugs (NSAIDs) is often associated with undesired side effects, including gastrointestinal ulcers due to the non-selective inhibition of cyclooxygenases. We describe the development of an inflammatory-stimuli-responsive turn-on fluorogenic theranostic prodrug DCF-HS for adjuvant drug delivery. Upon activation by reactive oxygen species (ROS), the prodrug releases diclofenac DCF (active drug) and the NIR fluorophore DCI-NH2 along with carbonyl sulfide (COS). The second activation of COS by the enzyme carbonic anhydrase (CA) generates hydrogen sulfide (H2S). The prodrug was conveniently synthesized using multi-step organic synthesis. The UV-Vis and fluorescence studies revealed the selective reactivity of DCF-HS towards ROS such as H2O2 in the aqueous phase and the desired uncaging of the drug DCF with turn-on NIR fluorescent reporter under physiological conditions. Furthermore, the release of fluorophore DCI-NH2 and drug DCF was confirmed using the reverse phase HPLC method. Compatibility of prodrug activation was studied next in the cellular medium. The prodrug DCF-HS was non-toxic in a representative cancer cell line (HeLa) and a macrophage cell line (RAW 264.7) up to 100 µM concentration, indicating its biocompatibility. The intracellular ROS-mediated activation of the prodrug with the release of NIR dye DCI-NH2 and H2S was investigated in HeLa cells using the H2S-selective probe WSP2. The anti-inflammatory activity of the active drug DCF from the prodrug DCF-HS was studied in the lipopolysaccharide (LPS)-induced macrophage cell line and compared to that of the parent drug DCF using western blot analysis and it was found that the active drug resulted in pronounced inhibition of COX-2 in a dose-dependent manner. Finally, the anti-inflammatory potential of the prodrug and the turn-on fluorescence were validated in the inflammation-induced Wister rat models.

Stimuli-responsive biotin-anchored prodrug for the targeted delivery of anti-cancer agent NBDHEX with turn-on NIR fluorescence.

Biothiol-activatable prodrug RK-296 was designed for the delivery of potent anti-cancer agent NBDHEX with concomitant turn-on near infrared (NIR) fluorescence. NBDHEX exhibits anti-cancer activity by selectively inhibiting glutathione-S-transferase pi (GSTP1), which is overexpressed in cancer cells and responsible for the inactivation of chemotherapeutic drugs. The sustained release of NBDHEX from the prodrug would be useful for ameliorating the off-target side-effects of NBDHEX.

Dual-Stimuli-Activatable Hybrid Prodrug for the Self-Immolative Delivery of an Anticancer Agent and Hydrogen Sulfide with Turn-On Fluorescence.

Stimuli-responsive fluorogenic prodrugs are advantageous for the targeted drug delivery enabling real-time non-invasive monitoring with turn-on fluorescence. We report herein the dual-stimuli (ROS and CA)-responsive thiocarbamate-based prodrug (AM-TCB) for the turn-on fluorogenic delivery of the naphthalimide-based anticancer agent amonafide along with the gasotransmitter hydrogen sulfide (H2 S). A carbamate-based prodrug AM-CB was also designed, capable of releasing the anticancer agent amonafide without any H2 S. The prodrugs were synthesized using multi-step organic synthesis. UV-Vis and fluorescence spectroscopic studies revealed selective reactivity of the boronate ester group of prodrugs towards ROS (primarily H2 O2 ) with the release of amonafide and COS/CO2 via self-immolative processes. Hydrolysis of the generated COS by carbonic anhydrase (CA) produces H2 S. While the prodrug AM-TCB retained the anticancer activity of free amonafide in cancer cells (MDA-MB-231 and HeLa), unlike amonafide, it enhanced the cellular viability of the non-malignant cells (HEK-293). Fluorescence imaging in HeLa cells revealed the simultaneous delivery of the anticancer agent and H2 S from AM-TCB with turn-on fluorescence. Western blot studies further revealed the cytoprotective effects of the released H2 S from AM-TCB. The present adjuvant strategy therefore would be helpful in future for ameliorating the anticancer drug-induced side-effects.

Small-molecule organoselenocyanates: Recent developments toward synthesis, anticancer, and antioxidant activities.

Cellular redox homeostasis is very important for the overall cellular development, function, and oxidative stress often disrupts the process. Small-molecule organoselenium compounds exert key roles in maintaining the redox homeostasis during oxidative stress and cancer owing to their notable antioxidant activities. Among different organoselenium compounds, small-molecule organoselenocyanates have attracted much research attention due to their synthetic utilities and therapeutic potentials. Therefore, the development of convenient synthetic methodologies to different classes of organoselenocyanates from various precursors was explored over the years as useful synthetic building blocks. Additionally, considering their inherent redox and antioxidant properties, the development of biologically relevant organoselenocyanates upon their conjugation with the existing drugs and natural products has been chosen for enhancing the drug potencies and in ameliorating the drug-induced side-effects. In the present report, we have discussed some of the very recent and relevant developments on these aspects in a very concise manner.

Cysteine-responsive prodrug of the anti-cancer drug amonafide: fluorogenic adjuvant drug delivery with hydrogen sulfide (H2S).

L-Cysteine (Cys)-responsive turn-on fluorogenic prodrug AM-ITC was developed for the adjuvant delivery of the anti-cancer drug amonafide and the gasotransmitter hydrogen sulfide (H2S) in aqueous and cellular media. Considering the cytoprotective roles of H2S, the present adjuvant strategy would be helpful in minimizing the anti-cancer drug-induced side-effects.

Targeting Wnt/ß-catenin signaling pathway in triple-negative breast cancer by benzylic organotrisulfides: Contribution of the released hydrogen sulfide towards potent anti-cancer activity.

The potent anti-cancer activity of naturally occurring organopolysulfides has attracted wide research attention over the last two decades. Sustained donation of hydrogen sulfide (H2S) from organopolysulfides is found to be beneficial for the treatment of several organ-specific cancers. In the present study, for the first time, the mechanism of action for the potent anti-cancer activity of bis(3,5-dimethoxybenzyl) trisulfide 4 against highly aggressive triple-negative breast cancer cells (MDA-MB-231) is described. Preliminary in vitro studies revealed potent anti-proliferative activity of the trisulfide 4 against triple-negative breast cancer cells with an IC50 value of 1.0 µM. Mechanistic studies reveal that the compound exhibited anti-cancer activity, primarily by targeting and suppressing the Wnt/ß-catenin signaling pathway. The inactivation of the ß-catenin level was associated with the cell cycle arrest in the G2/M phase and the significant down-regulation of downstream signaling genes such as Cyclin D1 and c-Myc expression. Several control experiments with analogous organosulfur compounds and the key enzyme inhibitors reveal that the presence of a trisulfide unit in the compound is crucial for the desired inactivation of ß-catenin expression, which is promoted by GSK-3ß-induced phosphorylation of ß-catenin and its proteasomal degradation. Moreover, the trisulfide unit or the released H2S induced down-regulation of the p53 expression with the possible S-sulfhydration process led to p53-independent up-regulation of p21 expression. Therefore, the key results of this study highlighting the potency of synthetic benzylic organotrisulfide and the released H2S towards the growth inhibition of triple-negative breast cancer via Wnt/ß-catenin signaling pathway would certainly be helpful for further studies and developing small-molecule anti-cancer therapeutics in future.

Stimuli-responsive prodrug of non-steroidal anti-inflammatory drug diclofenac: self-immolative drug release with turn-on near-infrared fluorescence.

Reactive oxygen species (ROS)-responsive near infrared (NIR) fluorogenic prodrug DCI-ROS is developed for the self-immolative release of diclofenac (DCF) with turn-on fluorescence. The non-toxic prodrug exhibited turn-on red fluorescence with endogenous ROS in cancer cells and inhibited COX-2 expression in the inflammation-induced macrophage cells. The prodrug strategy thus would be helpful for the controlled fluorogenic delivery of DCF for inflammatory diseases.

Thioredoxin reductase-triggered fluorogenic donor of hydrogen sulfide: a model study with a symmetrical organopolysulfide probe with turn-on near-infrared fluorescent emission.

We describe herein the rational development of an organopolysulfide-based fluorogenic donor of hydrogen sulfide (H2S) DCI-PS, which can be activated by the antioxidant selenoenzyme thioredoxin reductase (TrxR) with concomitant release of the dicyanoisophorone-based near-infrared (NIR) fluorophore. Along with the polysulfide probe DCI-PS capable of releasing the NIR fluorophore and H2S, the corresponding disulfide-probe DCI-DS was also rationally designed and synthesized, which releases the fluorophore without donating H2S. Detailed spectroscopic and kinetic studies in an aqueous medium revealed significantly higher reactivity of the probes towards DTT (for TrxR activity) over the well-known cellular abundant biothiol GSH. Mechanistically, the nucleophilic attack at the disulfide/polysulfide linkage by the thiol/selenol group of the bio-analytes leads to the self-immolative cyclization process with the release of the turn-on fluorophore with/without H2S. Considering the overexpression of mammalian TrxR in cancer cells, the turn-on fluorogenic H2S donation process from the cellular non-toxic DCI-PS was validated in a representative breast cancer cell line (MDA-MB-231) for the sustained donation of H2S with concomitant release of the red-emitting NIR fluorophore. The TrxR-triggered fluorescence turn-on process in DCI-PS was further supported by the significant inhibition of the fluorogenic process in the presence of TrxR-selective small-molecule inhibitors and by the significant binding affinity predicted by the protein-ligand docking study. Results with the antioxidant enzyme-triggered intracellular sustained donation of H2S with concomitant fluorescence turn-on will certainly find wider biomedical applications in the near future, particularly in H2S-mediated therapeutics in disease states.

The biothiol-triggered organotrisulfide-based self-immolative fluorogenic donors of hydrogen sulfide enable lysosomal trafficking.

Biothiol-reactive organotrisulfide-based self-immolative fluorogenic donors of H2S are rationally designed for the efficient monitoring of intracellular and lysosomal trafficking of H2S with a concomitant turn-on fluorescence. The non-toxic nature of the donors with a sustained release of H2S will certainly be helpful for their biomedical applications in the future.

Trisulfides over disulfides: highly selective synthetic strategies, anti-proliferative activities and sustained H2S release profiles.

Temperature- and solvent-induced selective synthesis of trisulfides and disulfides is demonstrated. A remarkable selectivity was achieved using Na2S as a sulfur-transfer agent under mild, greener, catalyst-free and additive-free conditions. This study reveals trisulfides as a better model than disulfides in general for a sustained release of H2S and potent anti-cancer activities.

Potent anti-proliferative activities of organochalcogenocyanates towards breast cancer.

The pharmacological importance, particularly the anti-cancer and chemopreventive potentials, of organochalcogen compounds has attracted wide research attention recently. Herein we describe the synthesis of a series of organochalcogenocyanates that have one or more selenocyanate or thiocyanate units in a single molecule. The anti-proliferative activity of these organochalcogenocyanates in different breast cancer cells shows that selenocyanates exhibit much higher anti-proliferative activities than thiocyanates in general. Our study reveals that the activity of benzyl selenocyanate (1, BSC) could be significantly enhanced by 4-nitro substitution (12), which was more selective towards triple-negative breast cancer cells (MDA-MB-231) over other ER+ breast cancer cells (MCF-7 and T-47D). Furthermore, to the best of our knowledge, this is the first report on the synthesis of compounds having more than two selenocyanate units with promising anti-proliferative activities. Our studies further indicate that the apoptotic activities of selenocyanates are associated with modulation of cellular morphology and cell cycle arrest at S-phase. Selenocyanates also inhibited cellular migration and exhibited weak antioxidant activities. An effective binding interaction of compound 12 with serum albumin indicates its feasible transport in the bloodstream for its enhanced anti-cancer properties. Mechanistic studies by western blot analysis demonstrate that benzylic selenocyanates exhibit anti-proliferative activities by modulating key cellular proteins such as Survivin, Bcl-2 and COX-2; this was further supported by molecular docking studies. The results of this study would be helpful in designing suitable chemotherapeutic and chemopreventive drugs in the future.

Development of a novel FRET probe for the real-time determination of ceramidase activity.

Fretful novelty: We developed two novel doubly labelled fluorescent ceramide analogues that exhibit significant FRET and undergo hydrolysis by ceramidases. We present a fluorescent sphingolipid FRET probe that allows homogeneous ratiometric determination of enzyme activity in real-time.

Novel drugs targeting sphingolipid metabolism.

While the evidence for an involvement of sphingolipids (SLs) in a variety of diseases is rapidly increasing, the development of sphingolipid-related drugs is still in its infancy. In fact, the recently FDA-approved fingolimod or FTY-720 (see chapter by J. Pfeilschifter for more information) is the first drug on the market to interfere with sphingolipid signaling. The reasons for this lagging are manifold and within this chapter we try to name some of them. Ceramide is in the center of sphingolipid metabolism. We describe the most important and most recent inhibitors for enzymes controlling cellular ceramide levels.

Effective inhibition of acid and neutral ceramidases by novel B-13 and LCL-464 analogues.

Induction of apoptosis mediated by the inhibition of ceramidases has been shown to enhance the efficacy of conventional chemotherapy in several cancer models. Among the inhibitors of ceramidases reported in the literature, B-13 is considered as a lead compound having good in vitro potency towards acid ceramidase. Furthermore, owing to the poor activity of B-13 on lysosoamal acid ceramidase in living cells, LCL-464 a modified derivative of B-13 containing a basic ω-amino group at the fatty acid was reported to have higher potency towards lysosomal acid ceramidase in living cells. In a search for more potent inhibitors of ceramidases, we have designed a series of compounds with structural modifications of B-13 and LCL-464. In this study, we show that the efficacy of B-13 in vitro as well as in intact cells can be enhanced by suitable modification of functional groups. Furthermore, a detailed SAR investigation on LCL-464 analogues revealed novel promising inhibitors of aCDase and nCDase. In cell culture studies using the breast cancer cell line MDA-MB-231, some of the newly developed compounds elevated endogenous ceramide levels and in parallel, also induced apoptotic cell death. In summary, this study shows that structural modification of the known ceramidase inhibitors B-13 and LCL-464 generates more potent ceramidase inhibitors that are active in intact cells and not only elevates the cellular ceramide levels, but also enhances cell death.

Novel fluorescent ceramide derivatives for probing ceramidase substrate specificity.

Ceramidases are key regulators of cell fate. The biochemistry of different ceramidases and of their substrate ceramide appears to be complex, mainly due to specific biophysical characteristics at the water-membrane interface. In the present study, we describe the design and synthesis of a set of fluorescently labeled ceramides as substrates for acid and neutral ceramidases. For the first time we have replaced the commonly used polar NBD-dye with the lipophilic Nile Red (NR) dye. Analysis of kinetic data reveal that although both the dyes do not have any noticeable preference for the substitution at acyl or sphingosine (Sph) part in ceramide towards hydrolysis by acid ceramidase, the ceramides with acyl-substituted NBD and Sph-substituted NR dyes have been found to be a better substrate for neutral ceramidase.

Novel amide- and sulfonamide-based aromatic ethanolamines: effects of various substituents on the inhibition of acid and neutral ceramidases.

In the present study we describe the design and synthesis of a series of amide- and sulfonamide-based compounds as inhibitor of recombinant acid and neutral ceramidases. Inhibition of ceramidases has been shown to induce apoptosis and to increase the efficacy of conventional chemotherapy in several cancer models. B-13, lead in vitro inhibitor of acid ceramidase has been recently shown to be virtually inactive towards lysosomal acid ceramidase in living cells at lower concentrations and for a shorter time of treatment, suggesting the development of more potent inhibitors. In this study, a detailed SAR investigation has been performed to understand the effect of different substituents on the phenyl ring of amide- and sulfonamide-based compounds that partially resemble the structure of well-known inhibitors such as B-13, D-e-MAPP as well as NOE. Our results suggest that the electronic effects of the substituents on phenyl ring in B-13 and D-e-MAPP analogues have negligible effects either in enhancing the inhibition potencies or for selectivity towards aCDase over nCDase. However, the hydrophobicity and the steric effects of longer alkyl chains (n-Pr, n-Bu or t-Bu groups) at the phenyl ring were found to be important for an enhanced selectivity towards aCDase over nCDase.

Inhibition of peroxynitrite- and peroxidase-mediated protein tyrosine nitration by imidazole-based thiourea and selenourea derivatives.

In the present study, the synthesis and characterization of a series of N-methylimidazole-based thiourea and selenourea derivatives are described. The new compounds were also studied for their ability to inhibit peroxynitrite (PN)- and peroxidase-mediated nitration of protein tyrosine residues. It has been observed that the selenourea derivatives are more efficient than the thiourea-based compounds in the inhibition of protein nitration. The higher activity of selenoureas as compared to that of the corresponding thioureas can be ascribed to the zwitterionic nature of the selenourea moiety. Single crystal X-ray diffraction studies on some of the thiourea and selenourea derivatives reveal that the C=S bonds in thioureas possess more of double bond character than the C=Se bonds in the corresponding selenoureas. Therefore, the selenium compounds can react with PN or hydrogen peroxide much faster than their sulfur analogues. The reactions of thiourea and selenourea derivatives with PN or hydrogen peroxide produce the corresponding sulfinic or seleninic acid derivatives, which upon elimination of sulfurous/selenous acids produce the corresponding N-methylimdazole derivatives.

Mechanistic investigations on the efficient catalytic decomposition of peroxynitrite by ebselen analogues.

In this study, ebselen and its analogues are shown to be catalysts for the decomposition of peroxynitrite (PN). This study suggests that the PN-scavenging ability of selenenyl amides can be enhanced by a suitable substitution at the phenyl ring in ebselen. Detailed mechanistic studies on the reactivity of ebselen and its analogues towards PN reveal that these compounds react directly with PN to generate highly unstable selenoxides that undergo a rapid hydrolysis to produce the corresponding seleninic acids. The selenoxides interact with nitrite more effectively than the corresponding seleninic acids to produce nitrate with the regeneration of the selenenyl amides. Therefore, the amount of nitrate formed in the reactions mainly depends on the stability of the selenoxides. Interestingly, substitution of an oxazoline moiety on the phenyl ring stabilizes the selenoxide, and therefore, enhances the isomerization of PN to nitrate.

Bioinorganic and medicinal chemistry: aspects of gold(I)-protein complexes.

Gold(I)-based drugs have been used successfully for the treatment of rheumatoid arthritis (RA) for several years. Although the exact mechanism of action of these gold(I) drugs for RA has not been clearly established, the interaction of these compounds with mammalian enzymes has been extensively studied. In this paper, we describe the interaction of therapeutic gold(I) compounds with mammalian proteins that contain cysteine (Cys) and selenocysteine (Sec) residues. Owing to the higher affinity of gold(I) towards sulfur and selenium, gold(I) drugs rapidly react with the activated cysteine or selenocysteine residues of the enzymes to form protein-gold(I)-thiolate or protein-gold(I)-selenolate complexes. The formation of stable gold(I)-thiolate/selenolate complexes generally lead to inhibition of the enzyme activity. The gold-thiolate/selenolate complexes undergo extensive ligand exchange reactions with other nucleophiles and such ligand exchange reactions alter the inhibitory effects of gold(i) complexes. Therefore, the effect of gold(I) compounds on the enzymatic activity of cysteine- or selenocysteine-containing proteins may play important roles in RA. The interaction of gold(I) compounds with different enzymes and the biochemical mechanism underlying the inhibition of enzymatic activities may have broad medicinal implications for the treatment of RA.