Inhibition of Na-K-2Cl cotransport and bumetanide binding by ethacrynic acid, its analogues, and adducts

Pharmacology of Compounds Targeting Cation-Chloride Cotransporter Physiology

Transporters of the solute carrier family 12 (SLC12) carry inorganic cations such as Na+ and/or K+ alongside Cl across the plasma membrane of cells. These tightly coupled, electroneutral, transporters are expressed in almost all tissues/organs in the body where they fulfil many critical functions. The family includes two key transporters participating in salt reabsorption in the kidney: the Na-K-2Cl cotransporter-2 (NKCC2), expressed in the loop of Henle, and the Na-Cl cotransporter (NCC), expressed in the distal convoluted tubule. NCC and NKCC2 are the targets of thiazides and "loop" diuretics, respectively, drugs that are widely used in clinical medicine to treat hypertension and edema. Bumetanide, in addition to its effect as a loop diuretic, has recently received increasing attention as a possible therapeutic agent for neurodevelopmental disorders. This chapter also describes how over the past two decades, the pharmacology of Na+ independent transporters has expanded significantly to provide novel tools for research. This work has indeed led to the identification of compounds that are 100-fold to 1000-fold more potent than furosemide, the first described inhibitor of K-Cl cotransport, and identified compounds that possibly directly stimulate the function of the K-Cl cotransporter. Finally, the recent cryo-electron microscopy revolution has begun providing answers as to where and how pharmacological agents bind to and affect the function of the transporters.

Loop Diuretics Inhibit Ischemia-Induced Intracellular Ca2+ Overload in Neurons via the Inhibition of Voltage-Gated Ca2+ and Na+ Channels

One consequence of ischemic stroke is disruption of intracellular ionic homeostasis. Intracellular overload of both Na⁺ and Ca²⁺ has been linked to neuronal death in this pathophysiological state. The etiology of ionic imbalances resulting from stroke-induced ischemia and acidosis includes the dysregulation of multiple plasma membrane transport proteins, such as increased activity of sodium-potassium-chloride cotransporter-1 (NKCC-1). Experiments using NKCC1 antagonists, bumetanide (BMN) and ethacrynic acid (EA), were carried out to determine if inhibition of this cotransporter affects Na⁺ and Ca²⁺ overload observed following in vitro ischemia-acidosis. Fluorometric Ca²⁺ and Na⁺ measurements were performed using cultured cortical neurons, and measurements of whole-cell membrane currents were used to determine target(s) of BMN and EA, other than the electroneutral NKCC-1. Both BMN and EA depressed ischemia-acidosis induced [Ca²⁺]_i overload without appreciably reducing [Na⁺]_i increases. Voltage-gated Ca²⁺ channels were inhibited by both BMN and EA with half-maximal inhibitory concentration (IC₅₀) values of 4 and 36 μM, respectively. Similarly, voltage-gated Na⁺ channels were blocked by BMN and EA with IC₅₀ values of 13 and 30 μM, respectively. However, neither BMN nor EA affected currents mediated by acid-sensing ion channels or ionotropic glutamatergic receptors, both of which are known to produce [Ca²⁺]_i overload following ischemia. Data suggest that loop diuretics effectively inhibit voltage-gated Ca²⁺ and Na⁺ channels at clinically relevant concentrations, and block of these channels by these compounds likely contributes to their clinical effects. Importantly, inhibition of these channels, and not NKCC1, by loop diuretics reduces [Ca²⁺]_i overload in neurons during ischemia-acidosis, and thus BMN and EA could potentially be used therapeutically to lessen injury following ischemic stroke.

Advances in the development of novel compounds targeting cation-chloride cotransporter physiology

For about half a century, the pharmacology of electroneutral cation-chloride cotransporters has been dominated by a few drugs that are widely used in clinical medicine. Because these diuretic drugs are so good at what they do, there has been little incentive in expanding their pharmacology. The increasing realization that cation-chloride cotransporters are involved in many other key physiological processes and the knowledge that different tissues express homologous proteins with matching transport functions have rekindled interest in drug discovery. This review summarizes the methods available to assess the function of these transporters and describe the multiple efforts that have made to identify new compounds. We describe multiple screens targeting KCC2 function and one screen designed to find compounds that discriminate between NKCC1 and NKCC2. Two of the KCC2 screens identified new inhibitors that are 3-4 orders of magnitude more potent than furosemide. Additional screens identified compounds that purportedly increase cell surface expression of the cotransporter, as well as several FDA-approved drugs that increase KCC2 transcription and expression. The technical details of each screen biased them toward specific processes in the life cycle of the transporter, making these efforts independent and complementary. In addition, each drug discovery effort contributes to our understanding of the biology of the cotransporters.

Staurosporine and NEM mainly impair WNK-SPAK/OSR1 mediated phosphorylation of KCC2 and NKCC1

The pivotal role of KCC2 and NKCC1 in development and maintenance of fast inhibitory neurotransmission and their implication in severe human diseases arouse interest in posttranscriptional regulatory mechanisms such as (de)phosphorylation. Staurosporine (broad kinase inhibitor) and N-ethylmalemide (NEM) that modulate kinase and phosphatase activities enhance KCC2 and decrease NKCC1 activity. Here, we investigated the regulatory mechanism for this reciprocal regulation by mass spectrometry and immunoblot analyses using phospho-specific antibodies. Our analyses revealed that application of staurosporine or NEM dephosphorylates Thr1007 of KCC2, and Thr203, Thr207 and Thr212 of NKCC1. Dephosphorylation of Thr1007 of KCC2, and Thr207 and Thr212 of NKCC1 were previously demonstrated to activate KCC2 and to inactivate NKCC1. In addition, application of the two agents resulted in dephosphorylation of the T-loop and S-loop phosphorylation sites Thr233 and Ser373 of SPAK, a critical kinase in the WNK-SPAK/OSR1 signaling module mediating phosphorylation of KCC2 and NKCC1. Taken together, these results suggest that reciprocal regulation of KCC2 and NKCC1 via staurosporine and NEM is based on WNK-SPAK/OSR1 signaling. The key regulatory phospho-site Ser940 of KCC2 is not critically involved in the enhanced activation of KCC2 upon staurosporine and NEM treatment, as both agents have opposite effects on its phosphorylation status. Finally, NEM acts in a tissue-specific manner on Ser940, as shown by comparative analysis in HEK293 cells and immature cultured hippocampal neurons. In summary, our analyses identified phospho-sites that are responsive to staurosporine or NEM application. This provides important information towards a better understanding of the cooperative interactions of different phospho-sites.

Allenamide as a bioisostere of acrylamide in the design and synthesis of targeted covalent inhibitors

The success of acrylamide-containing drugs in treating cancers has spurred a passion to search for acrylamide bioisosteres. In our endeavour, we have identified that an allenamide group can be a reactive bioisostere of the acrylamide group. In our development of allenamide-containing compounds, we found that the most potent compound, 14, inhibited the kinase activities of both T790M/L858R double mutant and wild type EGFR in a low nM range. 14 also inhibited the growth of NCI-H1975 lung cancer cells at IC₅₀ = 33 nM, which is comparable to that of acrylamide-containing osimertinib. The western blot analysis showed that the phosphorylation of EGFR, AKT, and ERK1/2 was simultaneously inhibited in a dose-dependent manner when NCI-H1975 cells were treated with 14. By measuring the conjugate addition product formed by 14 and GSH, we obtained a reaction rate constant of 302.5 × 10^-3 min^-1, which is about 30-fold higher than that of osimertinib. Taken together, our data suggest that the allenamide-containing compounds inhibited EGFR kinases through covalent modifications. Our study indicates that the allenamide group could serve as an alternative electrophilic warhead in the design of targeted covalent inhibitors, and this bioisostere replacement may have broad applications in medicinal chemistry.

Promiscuity and selectivity in covalent enzyme inhibition: a systematic study of electrophilic fragments

Covalent ligand-target interactions offer significant pharmacological advantages. However, off-target reactivity of the reactive groups, which usually have electrophilic properties, must be minimized, and the selectivity of irreversible inhibitors is a crucial requirement. We therefore performed a systematic study to determine the selectivity of several electrophilic groups that can be used as building blocks for covalently binding ligands. Six reactive groups with modulated electrophilicity were combined with 11 nonreactive moieties, resulting in a small combinatorial library of 72 fragment-like compounds. These compounds were screened against a group of 11 enzyme targets to assess their selectivity and their potential for promiscuous binding to proteins. The assay results showed a considerably lower degree of promiscuity than initially expected, even for those members of the screening collection that contain supposedly highly reactive electrophiles.

Synthesis and radiopharmacological characterisation of a fluorine-18-labelled azadipeptide nitrile as a potential PET tracer for in vivo imaging of cysteine cathepsins

A fluorinated cathepsin inhibitor based on the azadipeptide nitrile chemotype was prepared and selected for positron emission tomography (PET) tracer development owing to its high affinity for the oncologically relevant cathepsins L, S, K and B. Labelling with fluorine-18 was accomplished in an efficient and reliable two-step, one-pot radiosynthesis by using 2-[(18) F]fluoroethylnosylate as a prosthetic agent. The pharmacokinetic properties of the resulting radiotracer compound were studied in vitro, ex vivo and in vivo in normal rats by radiometabolite analysis and small-animal positron emission tomography. These investigations revealed rapid conjugate formation of the tracer with glutathione in the blood, which is associated with slow blood clearance. The potential of the developed (18) F-labelled probe to image tumour-associated cathepsin activity was investigated by dynamic small-animal PET imaging in nude mice bearing tumours derived from the human NCI-H292 lung carcinoma cell line. Computational analysis of the obtained image data indicated the time-dependent accumulation of the radiotracer in the tumours. The expression of the target enzymes in the tumours was confirmed by immunohistochemistry with specific antibodies. This indicates that azadipeptide nitriles have the potential to target thiol-dependent cathepsins in vivo despite their disadvantageous pharmacokinetics.

Induction of mitochondrial fusion by cysteine-alkylators ethacrynic acid and N-ethylmaleimide

Mitochondrial fusion and fission are important aspects of eukaryotic cell function that permit the adoption of varied mitochondrial morphologies depending upon cellular physiology. We previously observed that ethacrynic acid (EA) induced mitochondrial fusion in cultured BSC-1 and CHO/wt cells. However, the mechanism responsible for it was not clear since EA has a number of known cellular effects including glutathione (GSH) depletion and alkylation of cysteine residues. To gain insight, we have tested the effects of a variety of compounds on EA induced cellular toxicity and mitochondrial fusion. N-acetyl cysteine (NAC), a GSH precursor, was found to abrogate both the toxic and fusion-inductive effects, whereas diethylmaleate (dEM), a GSH depletor, potentiated both these effects in a dose-dependent manner. However, treatment with dEM alone, which depleted GSH to the same degree as EA, did not induce mitochondrial fusion. These results indicate that although detoxification of EA via formation of GSH conjugates is dependant upon GSH levels, the depletion of GSH by EA is not responsible for its effect on mitochondrial fusion. Dihydro-EA (DH-EA), a saturated EA analogue, lacked EA's toxicity and effect on fusion, indicating that the alpha,beta-unsaturated ketone is central to its observed effects. N-ethylmaleimide (NEM), another well-known cysteine-alkylator, also induced mitochondrial fusion at near toxic concentrations. These data suggests that cysteine-alkylation is the causative factor for fusion and toxicity. In live BSC-1 cells, EA induced fusion of mitochondria occurred very rapidly (<20 min), which suggests that it is inducing fusion by modifying certain critical cysteine residue(s) in proteins involved in the process.

Regulation of Na-K-2Cl cotransport in cultured bovine corneal endothelial cells

We have previously demonstrated the presence of a Na(+)-K(+)-2Cl cotransporter in cultured bovine corneal endothelial cells (CBCEC) and determined that this cotransporter is located in the basolateral membrane. This transporter may contribute to volume regulation and transendothelial fluid transport. We have now investigated factors regulating the activity of the cotransporter. This activity was assessed by measuring the bumetanide-sensitive (86)Rubidium ((86)Rb) uptake in (86)Rb-containing solutions. Data were normalized to protein content determined with a Lowry protein assay. We investigated the regulation by extracellular and intracellular ion concentrations, by osmotic gradients, and by second messengers. Our results indicate that extracellular Na+ and K+ each are required for activation of the cotransporter and activate with first-order kinetics at half-maximally effective concentrations (k(1/2)) of 21.1 and 1.33 mM, respectively. Extracellular Cl- is also required for cotransport activation, but shows higher order kinetics; the k(1/2) for Cl- is 28.1 mM and the Hill coefficient 2.1. HCO(3)(-) exerts a modulating effect on cotransporter activity; at 0 HCO(3)(-) the bumetanide-sensitive K(+) uptake is reduced by 30% compared to that at 26 mm HCO(3)(-). Manipulations of the intracellular [Cl-] by preincubation in Cl- -free solution or inhibition of Cl- efflux resulted in increased uptake at low [Cl-](i) and decreased uptake at high [Cl-](i). To assess the role of protein kinases in the regulation of cotransport, we have determined the effect of protein kinase inhibitors. H-89 and KT5270, inhibitors of PKA, inhibit cotransport almost completely, while calphostin C, an inhibitor of PKC, produces a small activation of cotransport. The tyrosine kinase inhibitor genistein reduced K+ uptake while its inactive analog daidzein was without effect. The calmodulin kinase inhibitor KN-93 was without effect. We also investigated the effects of phosphatase inhibitors. Calyculin A (k(1/2)=21 nM) and okadaic acid (k(1/2)=915 nM) produced approximate doubling of K+ uptake, suggesting that phosphatase 1 is dominant. We also investigated the role of the cytoskeleton and its activation. Reduction of Ca(i)(2+) by preincubation in Ca2+ -free medium as well as by exposure to W-7, an inhibitor of the binding of Ca(2+) to calmodulin, reduced K+ uptake. Consistent with this, ML-7, a relatively specific inhibitor of the Ca2+ -calmodulin activated myosin light chain kinase, inhibited cotransport by 40%. The Ca2+ -calmodulin activated myosin light chain kinase contributes to the modulation of the cytoskeleton by regulating the actin-myosin interaction. Consistent with the above, disruption of the actin polymerization by cytochalasin D led to a decrease in K+ uptake. We conclude that extracellular Na+, K+ and Cl- are requirements for the function of the CBCEC Na(+)-K(+)-2Cl(-) cotransporter, while intracellular Cl- and extracellular HCO(3)(-) modulate its activity. Several protein kinases, including PKA, PKC, tyrosine kinase, and myosin light chain kinase, modulate the K+ uptake. Another modulating pathway for cotransport involves the state of the cytoskeleton.

Analysis of energy metabolism and mechanism of loop diuretics in the thick ascending limb of Henle's loop in dog kidneys

AIM

The thick ascending limb of Henle's loop (TALH) absorbs up to 40% of filtered NaCl in volume-expanded dogs. To examine if a fraction of this absorption is passive, NaHCO3 absorption and associated NaCl absorption in proximal tubules were inhibited by acetazolamide, a carbonic anhydrase inhibitor.

RESULTS

Ouabain, a specific inhibitor of Na,K-ATPase activity, reduced the remaining NaCl absorption and renal oxygen consumption in a ratio DeltaNa/DeltaO2 = 18, as expected for active transport. However, the responses to two loop diuretics were DeltaNa/DeltaO2 = 24 for ethacrynic acid and DeltaNa/DeltaO2 = 30 for bumetanide. Both loop diuretics induced potassium secretion. By superimposing ouabain potassium secretion was stopped and DeltaNa/DeltaO2 = 18 restored. Replacement of half of the circulating NaCl with Na2SO4 gave stop-flow pattern similar to those obtained after ethacrynic acid.

CONCLUSIONS

Low entry of some sodium ions thorugh the apical membrane is permitted despite low chloride supply or blockade by loop diuretics of chloride entry by the Na-K-2Cl transporter. Continued Na-K-ATPase activity causes secretion of potassium ions through the apical ion channel, ethacrynic acid being more kaliuretic and less natriuretic than bumetanide. Greater paracellular recycling of sodium ions after bumetanide maintains ionic balance. In contrast, under normal conditions excess entry of chloride by the Na-K-2Cl-transporter leads to paracellular back-diffusion of chloride rather than paracellular absorption of sodium ions, consistent with DeltaNa/DeltaO2 = 18 after ouabain. Thus all NaCl transport along TALH is active in vivo, whereas absorption of other cations, such as lithium, probably is passive.

NO3--induced pH changes in mammalian cells. Evidence for an NO3--H+ cotransporter

The effect of NO3- on intracellular pH (pHi) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO3- induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO3-- induced pH change was not due to modulation of Na+/H+ exchange, since it was also observed in Na+/H+ antiport-deficient mutants. Though NO3- is known to inhibit vacuolar-type (V) H+-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A1. NO3-/HCO3- exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO3-, despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B. S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448- 11454). Accumulation of intracellular NO3-, measured by the Greiss method after reduction to NO2-, indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO3- with H+ (or the equivalent counter-transport of NO3- for OH-). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO3- and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO3- and was accentuated by acidification of the extracellular space. The putative NO3--H+ cotransport was inhibited markedly by ethacrynic acid and by alpha-cyano-4-hydroxycinnamate, but only marginally by 4, 4'-diisothiocyanostilbene-2,2' disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO3--induced pH changes in mammalian cells may be related, though not identical, to the NO3--H+ cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO3- with H+ the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral.

Human monocyte interleukin-1beta posttranslational processing. Evidence of a volume-regulated response

Interleukin (IL)-1beta produced by monocytes and macrophages is not released via the normal secretory apparatus, and prior to its release, this cytokine must be proteolytically processed to generate a mature biologically active species. Biochemical mechanisms that regulate these posttranslational steps are not well understood. Lipopolysaccharide (LPS) is a poor activator of IL-1 posttranslational processing despite serving as a potent inducer of IL-1 synthesis. For example, freshly isolated human monocytes treated with LPS released <30% of their newly synthesized IL-1beta as the mature 17-kDa cytokine species, and monocytes that were aged overnight in culture prior to LPS treatment released no 17-kDa cytokine. In contrast, addition of extracellular ATP promoted IL-1beta posttranslational processing from both monocyte populations. Previous studies indicated that ATP, acting via surface P2Z-type receptors, promoted major intracellular ionic changes. To explore whether these ionic changes were required for cytokine posttranslational processing, LPS-stimulated human monocytes were maintained in ionically altered media. Hypotonic conditions promoted an efficient and selective release of mature 17-kDa IL-1beta from LPS-activated monocytes in the absence of ATP. In contrast, hypertonic conditions blocked the ATP-induced posttranslational processing reactions. Both hypotonic stress- and ATP-induced processing were blocked when NaI was substituted for NaCl within the medium; substitution with NaSCN or NaNO3 also blocked the ATP response, but these salts were less inhibitory against the hypotonic stimulus. Sodium glucuronate substitution did not inhibit cytokine processing induced by either stimulus. Removal of divalent cations from the medium did not affect the ATP response, but pretreatment of monocytes with the phosphatase inhibitor okadaic acid dose-dependently suppressed ATP-induced IL-1beta posttranslational processing. A volume-induced change to the intracellular ionic environment, therefore, may represent a key element of the mechanism by which IL-1beta posttranslational processing is initiated. The strong dependence of this cytokine release mechanism on chloride anions suggests that selective anion transporters function as important components of this response.

New specific and sensitive HPLC-assays for ethacrynic acid and its main metabolite--the cysteine conjugate--in biological material

A reversed-phase high-performance liquid chromatography (HPLC) method was developed and validated for the determination of the loop diuretic ethacrynic acid and its potentially active main metabolite, the ethacrynic acid-cysteine conjugate, in biological material. Simple and rapid sample preparation procedures were established using solid-phase extraction for the parent drug and direct injection after one washing step for the metabolite. HPLC separation was performed on a Spherisorb ODS II (3 microns) analytical column using isocratic elution with different mixtures of mobile phases (phosphoric acid-methanol-acetonitrile-tetrahydrofuran or triethylamine buffer-methanol, respectively). The analytes were detected by measuring the UV absorption of the eluate at 275 nm. Stability studies revealed that considerable amounts of ethacrynic acid may be released from the cysteine conjugate unless the urine samples are pH stabilized (pH 3-4). The assay provided high sensitivity with limits of quantification of 20 ng ml-1 for ethacrynic acid in plasma and urine, and 240 ng ml-1 for the cysteine conjugate in urine. All validation parameters were within the required limits. For the presented assays, the applicability to pharmacokinetic studies and routine analyses was proved.

Effect of daily topical ethacrynic acid on aqueous humor dynamics in monkeys

We determined the effect of 1.5% ethacrynic acid (ECA) in ointment on intraocular pressure (IOP) and outflow facility following 5 days of topical treatment in ocular normotensive cynomolgus monkeys. Twelve monkeys received a 1 cm strip of ointment containing ECA in one eye but without ECA in the other once daily for 5 days. On Day 1 and Day 5 IOP was measured immediately before and 1 and 3 h after treatment. Outflow facility (perfusion) was determined 3.5 h after treatment on Day 5. The ECA-vehicle IOP differences averaged -2.8 +/- 0.8 (S.E.M) (p < 0.01), -1.7 +/- 0.6 (p < 0.02) and -3.7 +/- 0.7 mm Hg (p < 0.001), equivalent to IOP reductions of 21 +/- 6% (p < 0.01), 13 +/- 5% (p < 0.02) and 26 +/- 5% (p < 0.002) respectively, at 0, 1 and 3 h respectively after treatment on Day 5. Facility averaged 40 +/- 15% higher (p < 0.03) in the ECA-treated compared to the vehicle-treated eyes 4 h after treatment on Day 5.

Cochlear effects of indacrinone are not altered by penicillin

Indacrinone is a loop diuretic structurally related to ethacrynic acid. Indacrinone is a racemic mixture. Previous studies have shown that the (-) enantiomer caused reduction of endocochlear potential (EP) and elevation of compound action potential (CAP) threshold (Rybak and Whitworth, 1987a). It has been demonstrated that organic acids such as penicillin, probenecid and sodium salicylate prevent the reduction of EP normally observed after furosemide administration (Rybak et al., 1992a). The present study was designed to determine whether penicillin pretreatment could prevent changes in EP and CAP threshold in (-)-indacrinone treated chinchillas. Adult chinchillas were anesthetized with ketamine and pentobarbital. A microelectrode was advanced into the scala media using the round window approach, and CAP responses to clicks were measured. One group was treated with (-)-indacrinone 100 mg/kg via the jugular vein. A second group of animals received penicillin 50 mg/kg i.v. thirty minutes before (-)-indacrinone. The mean EP change in the indacrinone-treated animals was 38.38 +/- 19.32 millivolts (mv). The reduction of EP in the group receiving penicillin was 24.43 +/- 20.74 mv (P > 0.09). The mean CAP threshold changes in animals receiving indacrinone was 20 +/- 14.14 dB whereas those pretreated with penicillin showed a threshold shift of 21.43 +/- 20.35 dB (P > 0.05). These findings are consistent with previous studies which showed that the effect of ethacrynic acid on the EP and CAP was not changed by the pretreatment with penicillin (Rybak et al., 1990).