Design, Synthesis, and In Vivo Evaluation of Isosteviol Derivatives as New SIRT3 Activators with Highly Potent Cardioprotective Effects.

Cardiovascular diseases (CVDs) persist as the predominant cause of mortality, urging the exploration of innovative pharmaceuticals. Mitochondrial dysfunction stands as a pivotal contributor to CVDs development. Sirtuin 3 (SIRT3), a prominent mitochondrial deacetylase known for its crucial role in protecting mitochondria against damage and dysfunction, has emerged as a promising therapeutic target for CVDs treatment. Utilizing isosteviol, a natural ent-beyerene diterpenoid, 24 derivatives were synthesized and evaluated in vivo using a zebrafish model, establishing a deduced structure-activity relationship. Among these, derivative 5v exhibited significant efficacy in doxorubicin-induced cardiomyopathy in zebrafish and murine models. Subsequent investigations revealed that 5v selectively elevated SIRT3 expression, leading to the upregulation of SOD2 and OPA1 expression, effectively preventing mitochondrial dysfunction, mitigating oxidative stress, and preserving cardiomyocyte viability. As a novel structural class of SIRT3 activators with robust therapeutic effects, 5v emerges as a promising candidate for further drug development.

Ester Prodrugs of Malonate with Enhanced Intracellular Delivery Protect Against Cardiac Ischemia-Reperfusion Injury In Vivo.

PURPOSE: Mitochondrial reactive oxygen species (ROS) production upon reperfusion of ischemic tissue initiates the ischemia/reperfusion (I/R) injury associated with heart attack. During ischemia, succinate accumulates and its oxidation upon reperfusion by succinate dehydrogenase (SDH) drives ROS production. Inhibition of succinate accumulation and/or oxidation by dimethyl malonate (DMM), a cell permeable prodrug of the SDH inhibitor malonate, can decrease I/R injury. However, DMM is hydrolysed slowly, requiring administration to the heart prior to ischemia, precluding its administration to patients at the point of reperfusion, for example at the same time as unblocking a coronary artery following a heart attack. To accelerate malonate delivery, here we developed more rapidly hydrolysable malonate esters. METHODS: We synthesised a series of malonate esters and assessed their uptake and hydrolysis by isolated mitochondria, C2C12 cells and in mice in vivo. In addition, we assessed protection against cardiac I/R injury by the esters using an in vivo mouse model of acute myocardial infarction. RESULTS: We found that the diacetoxymethyl malonate diester (MAM) most rapidly delivered large amounts of malonate to cells in vivo. Furthermore, MAM could inhibit mitochondrial ROS production from succinate oxidation and was protective against I/R injury in vivo when added at reperfusion. CONCLUSIONS: The rapidly hydrolysed malonate prodrug MAM can protect against cardiac I/R injury in a clinically relevant mouse model.

Optimization of pyrazole-based compounds with 1,2,4-triazole-3-thiol moiety as selective COX-2 inhibitors cardioprotective drug candidates: Design, synthesis, cyclooxygenase inhibition, anti-inflammatory, ulcerogenicity, cardiovascular evaluation, and molecular modeling studies.

The cardiovascular side effects associated with COX-2 selective drugs were the worst for coxibs leading to their withdrawal from the market a few years after their discovery. Therefore, the design of new series of pyrazole (4a,b 5a,b, 7a,b, 9a,b, 10a-h, and 11a-f) substituted with a triazole moiety as selective COX-2 inhibitors with cardioprotective effect was aimed in this paper. The target compounds were prepared and evaluated in-vitro against COX-1 and COX-2 enzymes. Compound 5-(5-Methyl-1-phenyl-1H-pyrazol-4-yl)-4H-1,2,4-triazole-3-thiol (7a) showed the highest selectivity towards COX-2 enzyme (S.I. = 27.56) and was the most active anti-inflammatory agent. Interestingly, its cardiovascular profile showed the cardiac biomarkers (ALP, AST, CK-MB, and LDH), as well as inflammatory cytokines named (TNF-α and IL-6) nearly similar to the control. Besides, a histopathological study of the heart muscle and the stomach was also included. The results confirmed that compound 7a has a more favorable cardio profile than celecoxib. Moreover, docking simulation for the most selective compounds 4b, 7a, 10e, 11c, and 11e inside COX-2 active site was performed to explain their binding mode. Finally, an ADME study was applied and proved the promising activity of the new compounds as a new oral anti-inflammatory agent. In conclusion, the newly developed compound 7a represents a potential selective COX-2 NSAID candidate with minimum cardiovascular risks.

Synthesis and in vivo screening of isosteviol derivatives as new cardioprotective agents.

Isosteviol, an ent-beyerane diterpenoid, has been repeatedly reported to possess potent cardioprotective activity. With the aim of discovering new cardioprotective derivatives from isosteviol, 47 compounds, including 40 new ones, were synthesized and evaluated in vivo using the easy-handling and efficient zebrafish model. The structure-activity relationship of this type of compounds was thus discussed. Of these compounds, new derivative 15d exhibited the most pronounced efficacy in vivo. Our results indicated that 15d could effectively prevent the doxorubicin-induced morphological distortions and cardiac dysfunction in zebrafish. Its cardioprotective activity is much better than that of isosteviol, and Levosimendan in zebrafish model. The molecular mechanism underlying in H9c2 cells indicated that 15d protected cardiomyocyte death and damage through inhibiting the reactive oxygen species overproduction, restoring the mitochondrial membrane potential and maintaining morphology of mitochondrial. Thus, 15d merits further development as a potential cardioprotective clinical trial candidate. The present study is a successful example to combine synthesis, structure-activity relationship study and in vivo screening to effectively discover new cardioprotective agents from isosteviol.

Structure-Activity Relationship Study of Dexrazoxane Analogues Reveals ICRF-193 as the Most Potent Bisdioxopiperazine against Anthracycline Toxicity to Cardiomyocytes Due to Its Strong Topoisomerase IIß Interactions.

Cardioprotective activity of dexrazoxane (ICRF-187), the only clinically approved drug against anthracycline-induced cardiotoxicity, has traditionally been attributed to its iron-chelating metabolite. However, recent experimental evidence suggested that the inhibition and/or depletion of topoisomerase IIß (TOP2B) by dexrazoxane could be cardioprotective. Hence, we evaluated a series of dexrazoxane analogues and found that their cardioprotective activity strongly correlated with their interaction with TOP2B in cardiomyocytes, but was independent of their iron chelation ability. Very tight structure-activity relationships were demonstrated on stereoisomeric forms of 4,4'-(butane-2,3-diyl)bis(piperazine-2,6-dione). In contrast to its rac-form 12, meso-derivative 11 (ICRF-193) showed a favorable binding mode to topoisomerase II in silico, inhibited and depleted TOP2B in cardiomyocytes more efficiently than dexrazoxane, and showed the highest cardioprotective efficiency. Importantly, the observed ICRF-193 cardioprotection did not interfere with the antiproliferative activity of anthracycline. Hence, this study identifies ICRF-193 as the new lead compound in the development of efficient cardioprotective agents.

Combined Cardioprotective and Adipocyte Browning Effects Promoted by the Eutomer of Dual sEH/PPARγ Modulator.

The metabolic syndrome (MetS) is a constellation of cardiovascular and metabolic symptoms involving insulin resistance, steatohepatitis, obesity, hypertension, and heart disease, and patients suffering from MetS often require polypharmaceutical treatment. PPARγ agonists are highly effective oral antidiabetics with great potential in MetS, which promote adipocyte browning and insulin sensitization. However, the application of PPARγ agonists in clinics is restricted by potential cardiovascular adverse events. We have previously demonstrated that the racemic dual sEH/PPARγ modulator RB394 (3) simultaneously improves all risk factors of MetS in vivo. In this study, we identify and characterize the eutomer of 3. We provide structural rationale for molecular recognition of the eutomer. Furthermore, we could show that the dual sEH/PPARγ modulator is able to promote adipocyte browning and simultaneously exhibits cardioprotective activity which underlines its exciting potential in treatment of MetS.

Synthesis and pharmacological characterization of mitochondrial KATP channel openers with enhanced mitochondriotropic effects.

Mitochondria play a key role for deciding fate of cells and thus are considered an attractive target for pharmacological interventions focused on containment of myocardial ischemia/reperfusion (I/R) injury. Notably, the activation of mitochondrial potassium (mitoK) channels produces a mild depolarization of mitochondrial membrane, that contributes to reduce the driving force to calcium uptake and prevents the formation of mitochondrial transition membrane pore (MPTP); these events underlie anti-ischemic cardioprotection. However, an ideal mitoK channel opener should possess the fundamental requirement to be delivered at mitochondrial level; therefore, to improve the mitochondrial delivery of a previously characterized spirocyclic benzopyrane F81, new compounds have been developed. The three original triphenylphosphonium (TPP+)-derivatives of F81 (1-3), were evaluated for their cardioprotective activity on both isolated cardiac mitochondria and cardiac H9c2 cell line. Compound 1 was further investigated in an in vivo infarct model. This work confirms that the TPP+ strategy applied to mitoKATP openers could foster mitochondrial delivery and enhance the cardioprotective effects of mitochondrial activators of potassium channels.

Fabrication of Pd NPs on pectin-modified Fe3O4 NPs: A magnetically retrievable nanocatalyst for efficient C-C and C-N cross coupling reactions and an investigation of its cardiovascular protective effects.

The present report represents the synthesis of a novel Pd NPs immobilized over a natural polysaccharide (pectin) coated Fe3O4 magnetic nanocomposite material (Fe3O4@pectin/Pd) for investigating the cardiovascular protective effects. The biomolecular functionalization not only stabilizes the ferrite nanoparticles from agglomeration but also provides an environment for the biogenic reduction of Pd2+ ions. This protocol is a promising breakthrough for the synthesis of a quasi-heterogeneous catalyst, a bridge between heterogeneous and homogeneous medium. The structure, morphology and physicochemical properties of the material were characterized utilizing various analytical techniques like FT-IR, FE-SEM, TEM, VSM, EDX-elemental mapping, ICP, EDX and XPS. The catalyst showed excellent reactivity in C-C and C-N cross coupling reactions via Suzuki and Buchwald-Hartwig reactions respectively. An array of different biphenyls and aryl amines were then procured by reactions of various aryl halides with phenylboronic acid or secondary amines over the catalyst affording good to excellent yields. The catalyst was easily recoverable using an external magnet and thereafter recycled for several trials with insignificant palladium leaching or loss in catalytic performance. To investigate the cardiovascular protective activities of catalyst, the MTT assay was done on Human Aortic Endothelial Cells (HAEC), Human Coronary Artery Endothelial Cells (HCAEC), and Human Pulmonary Artery Endothelial Cells (HPAEC) cell lines. Nanocatalyst-treated cell cutlers significantly (p ≤ 0.01) decreased the caspase-3 activity, and DNA fragmentation. It raised the cell viability and mitochondrial membrane potential in the high concentration of Mitoxantrone-treated HAEC, HCAEC, and HPAEC cells. According to the above findings, nanocatalyst can be administrated as a cardiovascular protective drug for the treatment of cardiovascular diseases after approving in the clinical trial studies in humans.

Discovery of novel TNNI3K inhibitor suppresses pyroptosis and apoptosis in murine myocardial infarction injury.

Myocardial infarction (MI) injury is a highly lethal syndrome that has, until recently, suffered from a lack of clinically efficient targeted therapeutics. The cardiac troponin I interacting kinase (TNNI3K) exacerbates ischemia-reperfusion (IR) injury via oxidative stress, thereby promoting cardiomyocyte death. In this current study, we designed and synthesized 35 novel TNNI3K inhibitors with a pyrido[4,5]thieno[2,3-d] pyrimidine scaffold. In vitro results indicated that some of the inhibitors exhibited sub-micromolar TNNI3K inhibitory capacity and good kinase selectivity, as well as cytoprotective activity, in an oxygen-glucose deprivation (OGD) injury cardiomyocyte model. Furthermore, investigation of the mechanism of the representative derivative compound 6o suggested it suppresses pyroptosis and apoptosis in cardiomyocytes by interfering with p38MAPK activation, which was further confirmed in a murine myocardial infarction injury model. In vivo results indicate that compound 6o can markedly reduce myocardial infarction size and alleviate cardiac tissue damage in rats. In brief, our results provide the basis for further development of novel TNNI3K inhibitors for targeted MI therapy.

Alkylamine-Substituted Perthiocarbamates: Dual Precursors to Hydropersulfide and Carbonyl Sulfide with Cardioprotective Actions.

The recent discovery of hydropersulfides (RSSH) in mammalian systems suggests their potential roles in cell signaling. However, the exploration of RSSH biological significance is challenging due to their instability under physiological conditions. Herein, we report the preparation, RSSH-releasing properties, and cytoprotective nature of alkylamine-substituted perthiocarbamates. Triggered by a base-sensitive, self-immolative moiety, these precursors show efficient RSSH release and also demonstrate the ability to generate carbonyl sulfide (COS) in the presence of thiols. Using this dually reactive alkylamine-substituted perthiocarbamate platform, the generation of both RSSH and COS is tunable with respect to half-life, pH, and availability of thiols. Importantly, these precursors exhibit cytoprotective effects against hydrogen peroxide-mediated toxicity in H9c2 cells and cardioprotective effects against myocardial ischemic/reperfusion injury, indicating their potential application as new RSSH- and/or COS-releasing therapeutics.

Protective effects of two novel nitronyl nitroxide radicals on heart failure induced by hypobaric hypoxia.

AIMS: Hypobaric hypoxia (HH), linked to oxidative stress, impairs cardiac function. We synthesized a novel nitronyl nitroxide radical, an HPN derivative (HEPN) and investigated the protective effects of HEPN and HPN against HH-induced heart injury in mice and the underlying mechanisms of action. MAIN METHODS: Mice were administered with HPN (200 mg/kg) or HEPN (200 mg/kg) 30 min before exposed to HH. The cardiac function was measured. Serum AST, CK, LDH and cTnI were estimated. Heart tissue oxidase activity, SOD, CAT, GSH-Px, ROS and MDA were estimated. ATP content, Na+/K+-ATPase and Ca2+/Mg2+-ATPase activity was measured. The expression of HIF-1, VEGF, Nrf2, HO-1, Bax, Bcl-2, Caspase-3 was estimated. KEY FINDINGS: Results showed that pretreatment with HEPN or HPN led to a dramatic decrease in the activity of biochemical markers AST, CK, LDH and cTnI in murine serum. They increased the activity of SOD, CAT and GSH-Px and reduced the level of ROS and MDA in the hearts of mice. HEPN and HPN could increase the expression of Nrf2 and OH-1. They could maintain the ATPase activity. The Bax and Caspase-3 expression as well as the ratio of Bax/Bcl-2 were significantly downregulated and the Bcl-2 expression was upregulated by HPN or HEPN compared to the HH group. They may attenuate the HH-induced oxidant stress via free radical scavenging activity. SIGNIFICANCE: The present study showed that the nitronyl nitroxide radical HEPN and HPN may be potential therapeutic agents for treatment of HH-induced cardiac dysfunction.

Discovery of 3,3'-pyrrolidinyl-spirooxindoles as cardioprotectant prohibitin ligands.

The scaffold proteins prohibitins-1 and 2 (PHB1/2) play many important roles in coordinating many cell signaling pathways and represent emerging targets in cardiology and oncology. We previously reported that a family of natural products derivatives, flavaglines, binds to PHB1/2 to exert cardioprotectant and anti-cancer effects. However, flavaglines also target the initiation factor of translation eIF4A, which doesn't contribute to cardioprotection and may even induce some adverse effects. Herein, we report the development of a convenient and robust synthesis of the new PHB2 ligand 2'-phenylpyrrolidinyl-spirooxindole, and its analogues. We discovered that these compounds displays cardioprotective effect against doxorubicin mediated cardiotoxicity and uncovered the structural requirement for this activity. We identified in particular some analogues that are more cardioprotectant than flavaglines. Pull-down experiments demonstrated that these compounds bind not only to PHB2 but also PHB1. These novel PHB ligands may provide the basis for the development of new drugs candidates to protect the heart against the adverse effects of anticancer treatments.

Discovery of coumarin-derived imino sulfonates as a novel class of potential cardioprotective agents.

The burst of reactive oxygen species (ROS) contributes to and exacerbates cardiac injury. Exogenous supplementation of antioxidants or upregulation of endogenous antioxidant defense genes should be the potential therapies for cardiovascular disease. Sixteen coumarin-derived imino sulfonates compounds were synthesized with the ability of attenuating oxidative stress directly by reducing intracellular ROS level via promoting Nrf2 pathway. The cell-based assays showed that most of the compounds had significant protective activity against H2O2-induced oxidative injury in H9c2 cells. Compound 5h with the highest activity and low cytotoxicity was demonstrated to remarkably remove the intracellular ROS accumulation by activating expressions of Nrf2 and its downstream antioxidant proteins (ie. HO-1 and NQO1), indicating a novel promising antioxidant and Nrf2 activator. Overall, these findings demonstrated that compound 5h could serve as a potential cardioprotective agent. Moreover, our study features developing new antioxidants and Nrf2 activators by introducing a sulfonyl group and nitrogen atom to the α,ß-unsaturated carbonyl entity in coumarin, rather than adding new functional groups or active fragments to coumarin itself.

Cardioprotective effects of (E)-4-hydroxy-N'-(1-(3-oxo-3H-benzo[f]chromen-2-yl)ethylidene)benzohydrazide: a newly synthesized coumarin hydrazone against isoproterenol-induced myocardial infarction in a rat model.

The current study was carried out to evaluate the effect of pretreatment and co-treatment with a newly synthesized coumarin hydrazone, (E)-4-hydroxy-N'-(1-(3-oxo-3H-benzo[f]chromen-2-yl)ethylidene)benzohydrazide (hereinafter EK6), against isoproterenol-induced myocardial infarction in rats. Changes in biochemistry, cardiac biomarkers, electrocardiography, and histopathology after treatment with EK6 or acenocoumarol (Sintrom) were studied. Animals were randomly divided into 4 groups: vehicle control (C), isoproterenol + Sintrom (ISO + Sin), isoproterenol + EK6 (ISO + EK6), and isoproterenol (ISO). Myocardial infarction was induced by subcutaneous ISO administration at a dose of 85 mg·kg-1·day-1 with a drug-free interval of 24 h on days 6 and 7. Treatment with ISO led to significant elevation (p < 0.05) in serum levels of cardiac injury biomarkers, namely cardiac troponin-T, lactate dehydrogenase, creatine kinase-MB, alanine aminotransferase, and aspartate aminotransferase compared with levels in the vehicle control. A change in the lipid profile was also observed as a significant increase in total cholesterol and triglycerides. Furthermore, ISO caused significant alterations in the electrocardiogram pattern, including significant ST-segment elevation, significant decreased R wave amplitude, and significant increase in heart rate (16%) as well as marked changes in the histopathology of the heart tissue. Pretreatment and co-treatment with newly synthesized coumarin hydrazone restored all ISO-induced biochemical, lipid, cardiac, and histopathological changes in rats with myocardial infarction.

Toxicity, bioactivity, release of H2S in vivo and pharmaco-kinetics of H2S-donors with thiophosphamide structure.

H2S donors are substitutes of H2S with various biological activities like inhibiting the inflammatory response and protecting myocardial cells from injury. In order to confirm whether the H2S donors have drug-like properties, two series thiophosphamide H2S donors were evaluated including toxicity, bioactivity and pharmacokinetic properties in vivo and in vitro. The following results were obtained. Firstly, all the compounds released H2S under measuring condition; with the increase of pH value, the H2S release rate of all the compounds decreased and the amount reduced, but pH value had little effect on the maximum release of H2S. Secondly, in the organs and tissues of rats, the compounds released H2S in the same way as in PBS. In plasma, compound 1 reached the Cmax after administration 55 min, and no compound 1 was detected after 12 h; for compound 18, the Cmax reached only after administration 100 min, and after 6 h, compound 18 was not detected; in organs and tissues, the H2S-release rates were different from those in PBS, but the mechanism of H2S release was the same. Thirdly, in the test of toxicity, all the compounds displayed low toxicities to 5 cancer cells and W138 cell lines; compounds 1 and 18 had slight effect on the physiological tissue and function of rat liver at low concentration; the compounds had almost no effect on the hatching rate, survival rate of zebrafish embryos, and the spontaneous movement of zebrafish embryos at below 0.5 µM, but when they were over 1 µM, the compounds displayed inhibitory effect in the manner of concentration dependence. Fourthly, in the course of anti-inflammatory test, all the tested compounds significantly reduced the level of TNF-α and increased the level of IL-10; when they were 100 µM, the levels of IL-10 were three times as high as those in the control group. Among them, compounds 10 and 18 displayed stronger activities than the others. In addition, the compounds protected H9c2 cells from injure and improved myocardial injury through anti-oxidation pathway. In summary, the compounds have druglike properties due to low toxicity, better activity and good pharmacokinetic property. Therefore, they have potential to be as candidates to investigate further.

Identification of Novel Phospholipid Transfer Protein Inhibitors by High-Throughput Screening.

Atherogenesis has been recognized as a risk factor for lethal cardiovascular diseases. Plasma low-density lipoprotein levels are correlated to the occurrence of atherosclerosis, and their control is critical for both the prevention and treatment of these diseases. Phospholipid transfer protein (PLTP) is one of the key regulators of lipoprotein metabolism; PLTP-deficient mice exhibit decreased apolipoprotein B (apoB)-containing lipoprotein secretion and atherosclerosis, indicating the validity of PLTP as a promising therapeutic target. Here, we demonstrate a high-throughput screening (HTS) method to identify a novel chemotype of PLTP inhibitors. Instead of using recombinant proteins, we used human plasma as a source of enzymes in the first screening, so as to efficiently exclude promiscuous inhibitors. The selected compounds were further confirmed to target PLTP both biochemically and biophysically and were shown to inhibit apoB secretion from hepatic cells with no apparent toxicity. We believe that our approach is suitable for filtering out nonspecific inhibitors at an earlier stage of screening campaigns and that these compounds should have potential to be developed into drugs to treat dyslipidemia.

A mitochondria-targeted nitric oxide donor triggered by superoxide radical to alleviate myocardial ischemia/reperfusion injury.

A novel mitochondria-targeted superoxide-responsive nitric oxide donor was developed by incorporation of diphenylphosphinyl and triphenylphosphonium groups into diazeniumdiolate, enabling remarkable protection against ischemia/reperfusion injury in H9c2 cells and isolated rat hearts.

Cardioprotective Activity of 2,6-Diisobornyl-4-Methylphenol in Acute Myocardial Ischemia/Reperfusion in Rats.

We studied the cardioprotective effect of 2,6-diisobornyl-4-methylphenol under conditions of myocardial ischemia/reperfusion in rats. Daily administration of 2,6-diisobornyl-4-methylphenol (100 mg/kg intragastrically) over 3 days before and 5 days after modeling of myocardial ischemia/reperfusion prevented the increase in the infarction area by almost 2 times in comparison with the control by day 5 after recirculation. The type and severity of pathological changes in ECG parameters reflecting necrotic changes in the myocardium under the action of the compound significantly decreased by day 35 of the experiment. Animal survival rate during the first 24 h after ischemia/reperfusion modeling in the experimental group was by 29% higher than in the control group.

Evaluation of the Possibility of Correction of Doxorubicin-Induced Chronic Heart Failure in the Experiment with 3-Hydroxypyridine Acetylcysteinate and 3-Hydroxypyridine Succinate.

The possibility of correction of morphological changes in the myocardium and biochemical parameters of the blood with 3-hydroxypyridine acetylcysteinate in a dose of 25 mg/kg was studied in the model of doxorubicin-induced chronic heart failure in rats. It was found that 3-hydroxypyridine acetylcysteinate in a dose of 25 mg/kg produced less pronounced cardio-protective effect in experimental chronic heart failure than 3-hydroxypyridine succinate.

Spin-labeled derivatives of cardiotonic steroids as tools for characterization of the extracellular entrance to the binding site on Na+ ,K+ -ATPase.

The information obtained from crystallized complexes of the Na+ ,K+ -ATPase with cardiotonic steroids (CTS) is not sufficient to explain differences in the inhibitory properties of CTS such as stereoselectivity of CTS binding or effect of glycosylation on the preference to enzyme isoforms. The uncertainty is related to the spatial organization of the hydrophilic cavity at the entrance of the CTS-binding site. Therefore, there is a need to supplement the crystallographic description with data obtained in aqueous solution, where molecules have significant degree of flexibility. This work addresses the applicability of the electron paramagnetic resonance (EPR) method for the purpose. We have designed and synthesized spin-labeled compounds based on the cinobufagin steroid core. The length of the spacer arms between the steroid core and the nitroxide group determines the position of the reporting group (N-O) confined to the binding site. High affinity to Na+ ,K+ -ATPase is inferred from their ability to inhibit enzymatic activity. The differences between the EPR spectra in the absence and presence of high ouabain concentrations identify the signature peaks originating from the fraction of the spin labels bound within the ouabain site. The degree of perturbations of the EPR spectra depends on the length of the spacer arm. Docking of the compounds into the CTS site suggests which elements of the protein structure might be responsible for interference with the spin label (e.g., steric clashes or immobilization). Thus, the method is suitable for gathering information on the cavity leading to the CTS-binding site in Na+ ,K+ -ATPase in all conformations with high affinity to CTS.