Interactions of SARS-CoV-2 envelope protein with amilorides correlate with antiviral activity

SARS-CoV-2 is the novel coronavirus that is the causative agent of COVID-19, a sometimes-lethal respiratory infection responsible for a world-wide pandemic. The envelope (E) protein, one of four structural proteins encoded in the viral genome, is a 75-residue integral membrane protein whose transmembrane domain exhibits ion channel activity and whose cytoplasmic domain participates in protein-protein interactions. These activities contribute to several aspects of the viral replication-cycle, including virion assembly, budding, release, and pathogenesis. Here, we describe the structure and dynamics of full-length SARS-CoV-2 E protein in hexadecylphosphocholine micelles by NMR spectroscopy. We also characterized its interactions with four putative ion channel inhibitors. The chemical shift index and dipolar wave plots establish that E protein consists of a long transmembrane helix (residues 8–43) and a short cytoplasmic helix (residues 53–60) connected by a complex linker that exhibits some internal mobility. The conformations of the N-terminal transmembrane domain and the C-terminal cytoplasmic domain are unaffected by truncation from the intact protein. The chemical shift perturbations of E protein spectra induced by the addition of the inhibitors demonstrate that the N-terminal region (residues 6–18) is the principal binding site. The binding affinity of the inhibitors to E protein in micelles correlates with their antiviral potency in Vero E6 cells: HMA ≈ EIPA > DMA >> Amiloride, suggesting that bulky hydrophobic groups in the 5’ position of the amiloride pyrazine ring play essential roles in binding to E protein and in antiviral activity. An N15A mutation increased the production of virus-like particles, induced significant chemical shift changes from residues in the inhibitor binding site, and abolished HMA binding, suggesting that Asn15 plays a key role in maintaining the protein conformation near the binding site. These studies provide the foundation for complete structure determination of E protein and for structure-based drug discovery targeting this protein.

The novel coronavirus SARS-CoV-2, the causative agent of the world-wide pandemic of COVID-19, has become one of the greatest threats to human health. While rapid progress has been made in the development of vaccines, drug discovery has lagged, partly due to the lack of atomic-resolution structures of the free and drug-bound forms of the viral proteins. The SARS-CoV-2 envelope (E) protein, with its multiple activities that contribute to viral replication, is widely regarded as a potential target for COVID-19 treatment. As structural information is essential for drug discovery, we established an efficient sample preparation system for biochemical and structural studies of intact full-length SARS-CoV-2 E protein and characterized its structure and dynamics. We also characterized the interactions of amilorides with specific E protein residues and correlated this with their antiviral activity during viral replication. The binding affinity of the amilorides to E protein correlated with their antiviral potency, suggesting that E protein is indeed the likely target of their antiviral activity. We found that residue asparagine15 plays an important role in maintaining the conformation of the amiloride binding site, providing molecular guidance for the design of inhibitors targeting E protein.

Systematic evaluation of structure-property relationships and pharmacokinetics in 6-(hetero)aryl-substituted matched pair analogs of amiloride and 5-(N,N-hexamethylene)amiloride

The K+-sparing diuretic amiloride elicits anticancer activities in multiple animal models. During our recent medicinal chemistry campaign aiming to identify amiloride analogs with improved properties for potential use in cancer, we discovered novel 6-(hetero)aryl-substituted amiloride and 5-(N,N-hexamethylene)amiloride (HMA) analogs with up to 100-fold higher potencies than the parent compounds against urokinase plasminogen activator (uPA), one of amiloride's putative anticancer targets, and no diuretic or antikaliuretic effects. Here, we report the systematic evaluation of structure-property relationships (lipophilicity, aqueous solubility and in vitro metabolic stability in human and mouse liver microsomes) in twelve matched pair analogs selected from our 6-substituted amiloride and HMA libraries. Mouse plasma stability, plasma protein binding, Caco-2 cell permeability, cardiac ion channel activity and pharmacokinetics in mice (PO and IV) and rats (IV) are described alongside amiloride and HMA comparators for a subset of the four most promising matched-pair analogs. The findings combined with earlier uPA activity/selectivity and other data ultimately drove selection of two analogs (AA1-39 and AA1-41) that showed efficacy in separate mouse cancer metastasis studies.

pH-sensitive capsules as intracellular optical reporters for monitoring lysosomal pH changes upon stimulation

The concept of a long-term sensor for ion changes in the lysosome is presented. The sensor is made by layer-by-layer assembly of oppositely charged polyelectrolytes around ion-sensitive fluorophores, in this case for protons. The sensor is spontaneously incorporated by cells and resides over days in the lysosome. Intracellular changes of the concentration of protons upon cellular stimulation with pH-active agents are monitored by read-out of the sensor fluorescence at real time. With help of this sensor concept it is demonstrated that the different agents used (Monensin, Chloroquine, Bafilomycin A1, Amiloride) possessed different kinetics and mechanisms of action in affecting the intracellular pH values.

Mechanism of hCG-induced spermiation in the toad Rhinella arenarum (Amphibia, Anura)

In Rhinella arenarum spermiation occurs as a consequence of LH/FSH increase during the amplexus or by a single dose of hCG, among other gonadotropins. The present study employs an in vitro system to study the mechanism of action of hCG in the spermiation of R. arenarum. Testicular fragments were incubated for 2h at 28°C in the presence or absence of 20IU hCG with or without different PKA/PKC inhibitors and activators as well as ouabain and amiloride as Na(+)/K(+) ATPase and transcellular Na(+) transport inhibitors, respectively. Ouabain did not induce spermiation in absence of hCG and inhibited hCG-induced spermiation in a dose-dependent manner, reaching 90% inhibition with the higher concentration. In contrast, amiloride neither affected spermiation nor steroidogenesis. Activation of PKA with 8Br-cAMP induced spermiation in the absence of hCG while its inhibition with H89 blocked hCG action. On the other hand, PKC inhibition with Bi or STP did not affect hCG-induced spermiation although PKC activation significantly decreased hCG-dependent sperm release. These results suggest that PKC inhibits spermiation but also that the inhibition exerted by the kinase could be blocked by hCG. Taken together, these observations could indicate that PKA is involved in the mechanism of the gonadotropin action, mechanism also requiring the activation of a non-pumping Na(+)/K(+) ATPase pathway.

Implementation of a transit compartment model for describing drug absorption in pharmacokinetic studies

PURPOSE

To compare the performance of the standard lag time model (LAG model) with the performance of an analytical solution of the transit compartment model (TRANSIT model) in the evaluation of four pharmacokinetic studies with four different compounds.

METHODS

The population pharmacokinetic analyses were performed using NONMEM on concentration-time data of glibenclamide, furosemide, amiloride, and moxonidine. In the TRANSIT model, the optimal number of transit compartments was estimated from the data. This was based on an analytical solution for the change in drug concentration arising from a series of transit compartments with the same first-order transfer rate between each compartment. Goodness-of-fit was assessed by the decrease in objective function value (OFV) and by inspection of diagnostic graphs.

RESULTS

With the TRANSIT model, the OFV was significantly lower and the goodness-of-fit was markedly improved in the absorption phase compared with the LAG model for all drugs. The parameter estimates related to the absorption differed between the two models while the estimates of the pharmacokinetic disposition parameters were similar.

CONCLUSION

Based on these results, the TRANSIT model is an attractive alternative for modeling drug absorption delay, especially when a LAG model poorly describes the drug absorption phase or is numerically unstable.

Simultaneous determination of amiloride and hydrochlorothiazide in human plasma by liquid chromatography/tandem mass spectrometry with positive/negative ion-switching electrospray ionisation

A new method for simultaneous determination of amiloride and hydrochlorothiazide by liquid chromatography/electrospray tandem mass spectrometry (LC/MS/MS) operated in positive and negative ionization switching mode was developed and validated. Protein precipitation with acetonitrile was selected for sample preparation. The analytes were separated on a Phenomenex Curosil-PFP (250x4.6 mm, 5 microm) column by a gradient elution with a mobile phase consisting of 0.15% formic acid solution containing 0.23% ammonium acetate and methanol pumped at a flow rate of 1.0 mL.min(-1). Rizatriptan was used as the internal standard (IS) for quantification. The determination was carried out on a Waters Quattro-micro triple-quadrupole mass spectrometer operated in multiple reaction monitoring (MRM) mode using the following transitions monitored simultaneously: positive m/z 230-->171 for amiloride, m/z 270-->158 for rizatriptan, and negative m/z 296-->205 for hydrochlorothiazide. The lower limits of quantification (LLOQs) were 0.1 and 1.0 ng.mL(-1) for amiloride and hydrochlorothiazide, respectively, which were lower than other published methods by using ultraviolet (UV), fluorimetric or mass spectrometric detection. The intra- and inter-day precision and accuracy were studied at three different concentration levels and were always better than 15% (n=5). This simple and robust LC/MS/MS method was successfully applied to the pharmacokinetic study of compound amiloride and hydrochlorothiazide tablets in healthy male Chinese volunteers.

The connecting tubule is the main site of the furosemide-induced urinary acidification by the vacuolar H+-ATPase

Final urinary acidification is achieved by electrogenic vacuolar H(+)-ATPases expressed in acid-secretory intercalated cells (ICs) in the connecting tubule (CNT) and the cortical (CCD) and initial medullary collecting duct (MCD), respectively. Electrogenic Na(+) reabsorption via epithelial Na(+) channels (ENaCs) in the apical membrane of the segment-specific CNT and collecting duct cells may promote H(+)-ATPases-mediated proton secretion by creating a more lumen-negative voltage. The exact localization where this supposed functional interaction takes place is unknown. We used several mouse models performing renal clearance experiments and assessed the furosemide-induced urinary acidification. Increasing Na(+) delivery to the CNT and CCD by blocking Na(+) reabsorption in the thick ascending limb with furosemide enhanced urinary acidification and net acid excretion. This effect of furosemide was abolished with amiloride or benzamil blocking ENaC action. In mice deficient for the IC-specific B1 subunit of the vacuolar H(+)-ATPase, furosemide led to only a small urinary acidification. In contrast, in mice with a kidney-specific inactivation of the alpha subunit of ENaC in the CCD and MCD, but not in the CNT, furosemide alone and in combination with hydrochlorothiazide induced normal urinary acidification. These results suggest that the B1 vacuolar H(+)-ATPase subunit is necessary for the furosemide-induced acute urinary acidification. Loss of ENaC channels in the CCD and MCD does not affect this acidification. Thus, functional expression of ENaC channels in the CNT is sufficient for furosemide-stimulated urinary acidification and identifies the CNT as a major segment in electrogenic urinary acidification.

Nano-liposomal dry powder inhaler of Amiloride Hydrochloride

The purpose of this study was to encapsulate Amiloride Hydrochloride into nano-liposomes, incorporate it into dry powder inhaler, and to provide prolonged effective concentration in airways to enhance mucociliary clearance and prevent secondary infection in cystic fibrosis. Liposomes were prepared by thin film hydration technique and then dispersion was passed through high pressure homogenizer to achieve size of nanometer range. Nano-liposomes were separated by centrifugation and were characterized. They were dispersed in phosphate buffer saline pH 7.4 containing carriers (lactose/sucrose/mannitol), and glycine as anti-adherent. The resultant dispersion was spray dried. The spray dried powders were characterized and in vitro drug release studies were performed using phosphate buffer saline pH 7.4. In vitro and in vivo drug pulmonary deposition was carried out using Andersen Cascade Impactor and by estimating drug in bronchial alveolar lavage and lung homogenate after intratracheal instillation in rats respectively. Nano-liposomes were found to have mean volume diameter of 198 +/- 15 nm, and 57% +/- 1.9% of drug entrapment. Mannitol based formulation was found to have low density, good flowability, particle size of 6.7 +/- 0.6 microm determined by Malvern MasterSizer, maximum fine particle fraction of 67.6 +/- 0.6%, mean mass aerodynamic diameter 2.3 +/- 0.1 microm, and geometric standard deviation 2.4 +/- 0.1. Developed formulations were found to have prolonged drug release following Higuchi's Controlled Release model and in vivo studies showed maximal retention time of drug of 12 hrs within the lungs and slow clearance from the lungs. This study provides a practical approach for direct lung delivery of Amiloride Hydrochloride encapsulated in liposomes for controlled and prolonged retention at the site of action from dry powder inhaler. It can provide a promising alternative to the presently available nebulizers in terms of prolonged pharmacological effect, reducing systemic side effects such as potassium retention due to rapid clearance of the drug from lungs in patients suffering from cystic fibrosis.

Control of intracellular trafficking of ICAM-1-targeted nanocarriers by endothelial Na+/H+ exchanger proteins

Targeting nanocarriers (NC) loaded by antioxidant enzymes (e.g., catalase) to endothelial cell adhesion molecules (CAM) alleviates oxidative stress in the pulmonary vasculature. However, antioxidant protection is transient, since CAM-targeted catalase is internalized, delivered to lysosomes, and degraded. To design means to modulate the metabolism and longevity of endothelial cell (EC)-targeted drugs, we identified and manipulated cellular elements controlling the uptake and intracellular trafficking of NC targeted to ICAM-1 (anti-ICAM/NC). BAPTA, thapsigargin, amiloride, and EIPA inhibited anti-ICAM/NC uptake by EC and actin rearrangements induced by anti-ICAM/NC (required for uptake), suggesting that member(s) of Na(+)/H(+) exchanger family proteins (NHE) regulate these processes. Consistent with this hypothesis, an siRNA specific for the plasmalemma NHE1, but not the endosome-associated NHE6, inhibited actin remodeling induced by anti-ICAM/NC and internalization. Anti-ICAM/NC binding to EC stimulated formation of a transient ICAM-1/NHE1 complex. One hour after uptake, ICAM-1 dissociated from NHE1, and anti-ICAM/NC were transported to NHE6-positive vesicles en route to lysosomes. Inhibition of PKC (an activator of intracellular NHE) accelerated nanocarrier lysosomal trafficking. In contrast, monensin, which enhances the endosomal sodium influx and proton efflux maintained by NHE6, inhibited delivery of anti-ICAM/NC to lysosomes by switching their trafficking to a plasma membrane recycling pathway. This markedly prolonged the protective effect of catalase-coated anti-ICAM/NC. Therefore, 1) NHE1 and NHE6 regulate distinct phases of anti-ICAM/NC uptake and trafficking; 2) pharmacological agents affecting these regulatory elements alter the itinerary of anti-ICAM/NC intracellular trafficking; and 3) these agents modulate duration of the therapeutic effects of targeted drugs.

Catecholamine clearance from alveolar spaces of rat and human lungs

BACKGROUND

Although aerosolized beta-adrenergic agonists have been used as a therapy for the resolution of pulmonary edema, the mechanisms of catecholamine clearance from the alveolar spaces of the lung are not well known.

OBJECTIVE

To determine whether catecholamine clearance from the alveolar spaces is correlated with the fluid transport capacity of the lung.

METHODS

Albumin solution containing epinephrine (10(-7)M) or norepinephrine (10(-7)M) was instilled into the alveolar spaces of isolated rat and human lungs. Alveolar fluid clearance rate was estimated by the progressive increase in the albumin concentration over 1 h. Catecholamine clearance rate was estimated by the changes in catecholamine concentration and alveolar fluid volume over 1 h.

RESULTS

The norepinephrine clearance rate was faster than the epinephrine clearance rate in the rat and human lungs. In the rat lungs, amiloride (a sodium channel blocker) caused a greater decrease in alveolar fluid clearance and epinephrine clearance rate than propranolol (a nonselective beta-adrenergic antagonist). Although propranolol and phentolamine (an alpha-adrenergic antagonist), and 5-(N-ethyl-N-isoprophyl)amiloride (a Na+/H+ antiport blocker) changed neither the alveolar fluid clearance nor the norepinephrine clearance rate, amiloride and benzamil (a sodium channel blocker) decreased both clearance rates. As in the rat lungs, amiloride decreased alveolar fluid and norepinephrine clearance rates in the human lungs.

CONCLUSION

These results indicate that the catecholamine clearance rate from the alveolar spaces is correlated with alveolar fluid clearance in rat and human lungs.

Ionic interaction of amiloride and uridine 5'-triphosphate in nebulizer solutions

Combination therapy using nebulized amiloride hydrochloride and uridine-5'-triphosphate (UTP) trisodium salt aerosols has been investigated for the treatment of cystic fibrosis (CF). Amiloride in aqueous solution precipitates in the presence of UTP, reducing drug concentrations. Interactions between these drugs and NaCl in solution were studied using phase-solubility techniques monitored by UV spectrophotometry. Elemental analyses were employed for precipitate characterization. Amiloride solubility was reduced by more than 85% in saline. Amiloride solubility decreased with increasing UTP concentration, resulting in formation of a precipitated complex. The theoretical molar ratio of complexes range from 1-3 amiloride:1 UTP. At most concentrations only 3 amiloride:1 UTP complex was observed in precipitate. This is a reflection of low Ksp for the 3:1 complex of 2.92 x 10(-11) M4 compared with 2.09 x 10(-4) M2 for amiloride alone. Equilibration over excess bulk solid resulted in higher solubility estimates and different phase solubility diagrams than solubility studies utilizing precipitation technique. This may be explained by the absence of amiloride in the solid state and its impact on complex equilibria with UTP. The solubility suppressing effects of UTP and saline were largely additive. A number of ionic interactions increase complex solubility profile of amiloride hydrochloride in the presence of UTP and NaCl.

Functional role of the Na+/H+ exchanger in the regulation of cerebral arteriolar tone in rats

OBJECT

In vascular smooth-muscle cells, the Na+/H+ exchanger (NHE) is involved in the regulation of [Na+]i, pHi through [H+], and cell volume. Recently, investigations have determined that this exchanger contributes to ischemia and reperfusion injury in coronary circulation. Nonetheless, there is limited information on this glycoprotein in cerebral circulation, especially microcirculation. Thus, the authors in the present study examined the role of NHE in the regulation of cerebral arteriolar tone and its related mechanisms in vitro.

METHODS

The internal diameter of isolated pressurized intracerebral arterioles in rats was monitored with the aid of a microscope. To examine the basal activity of NHE two kinds of Na+/H+ exchange inhibitors (FR183998 and 5-[N,N-hexamethylene]amiloride) were administered in the arterioles. Furthermore the authors studied the effects of nitric oxide (NO) synthase inhibitor (NG methyl-L-arginine), Na+/K+ -adenosine triphosphatase (NKA) inhibitor (ouabain), and the Na+/Ca++ exchange inhibitor (SEA0400) on the vascular response induced by either of the Na+/H+ exchange inhibitors. Both of the Na+/H+ exchange inhibitors constricted the arteriole. Subsequent application of NO synthase inhibitor further decreased the diameter of the arterioles. The Na+/H+ exchange inhibitor-induced constriction was completely abolished in the presence of ouabain and SEA0400.

CONCLUSIONS

The NHE is active in the basal condition and regulates cerebral arteriolar tone through NKA and the Na+/Ca++ exchanger. Endogenous NO is not related to the activity of NHE in basal conditions.

Evaluation of Second Generation Amiloride Analogs as Therapy for Cystic Fibrosis Lung Disease

Epithelial sodium channel (ENaC) blockers have been proposed as a therapy to restore mucus clearance (MC) in cystic fibrosis (CF) airways. The therapeutic effects of the first generation ENaC blocker, amiloride, in CF patients, however, were minimal. Because the failure of amiloride reflected both its low potency and short duration of action on airway surfaces, we investigated whether the increased potency of benzamil and phenamil would produce more favorable pharmacodynamic properties. In vitro potency, maximal efficacy, rate of recovery from maximal block of ENaC, and rate of drug absorption were compared for amiloride, benzamil, and phenamil in cultured human and ovine bronchial epithelial cells. In both human and ovine bronchial epithelia, the rank order of potency was benzamil > phenamil >> amiloride, the maximal efficacy was benzamil = phenamil = amiloride, the recovery to baseline sodium transport was phenamil < benzamil << amiloride, and the rate of drug absorption was phenamil > benzamil >> amiloride. Based on greater potency, benzamil was compared with amiloride in in vivo pharmacodynamic studies in sheep, including tracheal mucus velocity (TMV) and MC. Benzamil enhanced MC and TMV, but acute potency or duration of effect did not exceed that of amiloride. In conclusion, our data support the hypothesis that ENaC blocker aerosol therapy increases MC. However, rapid absorption of benzamil from the mucosal surface offset its greater potency, making it equieffective with amiloride in vivo. More potent, less absorbable, third generation ENaC blockers will be required for an effective aerosol CF pharmacotherapy.

External Ni2 + and ENaC in A6 cells: Na+ current stimulation by competition at a binding site for amiloride and Na+

In cultured A6 monolayers from distal Xenopus kidney, external Ni2+ stimulated active Na+ uptake via the epithelial Na+ channel, ENaC. Transepithelial capacitance measurements ruled out exocytosis of ENaC-containing vesicles underlying the Ni2+ effect. Na+ current noise analysis was performed using the neutral Na(+) -channel blocker 6-chloro-3,5-diamino-pyrazine-2-carboxamide (CDPC) and amiloride. The analysis of CDPC-induced noise in terms of a three-state channel model revealed that Ni2+ elicits an increase in the number of open channels as well as in the spontaneous open probability. While Ni2+ had no influence on CDPC-blocker kinetics, the macroscopic and microscopic blocking kinetics of amiloride were affected. Ni2+ turned out to compete with amiloride for a putative binding site but not with CDPC. Moreover, external Na(+)--known to compete with amiloride and so producing the "self-inhibition" phenomenon--and Ni2+ exerted mutually exclusive analogous effects on amiloride kinetics. Na+ current kinetics revealed that Ni2+ prevents ENaC to be downregulated by self-inhibition. Co2+ behaved similarly to Ni2+, whereas Zn2+ did not. Attempts to disclose the chemical nature of the site reacting with Ni2+ suggested cysteine but not histidine as reaction partner.

Identification and characterization of the imidazoline I2b-binding sites in the hamster brown adipose tissue as a study model for imidazoline receptors

The imidazoline-type compound, MPV-1743, has been found to activate nonshivering thermogenesis (NST) in brown adipose tissue (BAT) of the genetically obese Zucker rats. The regulation of NST in BAT is linked to the catecholamine metabolism, and the imidazoline I2-binding sites have been found on the monoamine oxidase, a catecholamine metabolising enzyme. In this study, the I2-binding sites of hamster BAT have been characterised using a receptor binding assay with 3H-idazoxan as a radioligand, and the interaction of MPV-1743 with these I2-binding sites has been studied using the enantiomers of MPV 1743, that is, MPV 2088 and MPV 2089. Cirazoline was used to determine the specific binding of 3H-idazoxan to the imidazoline I2-binding sites. Rauwolscine was added in the 3H-idazoxan binding assay in order to inhibit any binding to potential alpha2-adrenergic sites. In the presence of rauwolscine mask 3H-Idazoxan labelled a population of non-adrenergic binding sites expressing the properties of the imidazoline I2b-receptor subtype similar to that found in the rat liver (cirazoline >> guanabenz = amiloride >> clonidine). The binding of 3H-idazoxan to the I2b-binding sites could be displaced by the imidazole compounds with the following affinities: detomidine (KiHigh 9.2 nM; KiLow 3200 nM), MPV-2088 (KiHigh 19 nM; IKiLow 760 nM) and MPV-2089 (KiHigh 190 nM; KiLow 1300 nM), atipamezole (3500 nM) and dexmedetomidine (Ki 8400 nM). These results have shown that the hamster BAT contains the imidazoline I2b-binding sites with heterogeneous binding properties for some test compounds. In addition, the enantiomers of MPV 1743, that is, MPV 2088 and MPV 2089, had high affinity to these BAT imidazoline I2b-binding sites. Therefore, it is suggested that the regulation of NST in the hamster BAT may be an attractive model to study the role of imidazoline I2b-binding sites.

Increasing concentration of inhaled saline with or without amiloride: effect on mucociliary clearance in normal subjects

Mucociliary clearance is determined by ciliary activity and rheology of airway surface liquid. To test the hypothesis that mucociliary clearance would increase after inhalation of an osmotically active agent that would increase the volume of airway surface liquid, we measured mucociliary clearance in 16 normal subjects after inhalation of varying tonicities of saline alone, and after pretreatment with a Na+ channel blocker (amiloride). Subjects inhaled vehicle (0.12% saline) or amiloride, followed by inhalation of 0.12, 0.9, or 7% saline. Subsequently, mucociliary clearance rates were measured by gamma scintigraphy of inhaled 99mTc Fe2O3. Mucociliary clearance of whole and peripheral lung was increased (approximately twofold) after inhalation of increasing concentrations of saline (p < 0.04). Pretreatment with amiloride increased mucociliary clearance rates (approximately twofold) after inhalation of 0.12 and 0.9% saline (p < 0.05), but not 7% saline. The rates of mucociliary clearance by pretreatment with amiloride and 7% saline alone (approximately 1.4% per minute) approached the rapid mucociliary clearance rates (approximately 2.0% per minute) reported in systemic pseudohypoaldosteronism, which has loss-of-function mutations of the epithelial Na+ channel and an increased volume of airway surface liquid. We conclude that maneuvers that increase the volume of airway surface liquid are associated with increased rates of mucociliary clearance in normal subjects.

Time-dependent effect of pneumonectomy on alveolar epithelial fluid clearance in rat lungs

OBJECTIVE

Because pneumonectomy initiates compensatory growth of the remaining lung, we determined the time-dependent effects of pneumonectomy on alveolar fluid clearance capacity.

METHODS

Alveolar fluid clearance capacity with the Evans blue-labeled albumin concentration was measured in rats 3 hours, 2 days, 7 days, 14 days, and 28 days after left pneumonectomy. The mechanisms responsible for the increase in alveolar fluid clearance were explored.

RESULTS

Alveolar fluid clearance in the remaining lung was normal through 7 days and then increased 14 and 28 days after pneumonectomy. The increase in alveolar fluid clearance at 28 days after pneumonectomy was accounted primarily by an increase in amiloride-sensitive transport. The expression of epithelial sodium channel messenger RNA was increased in the remaining lung and in type II alveolar epithelial cells isolated from rats 28 days after pneumonectomy. The number of isolated type II cells was larger in pneumonectomized rats than in control rats. Also, beta-adrenergic agonist therapy increased the rate of alveolar fluid clearance at the 3-hour and 28-day time points.

CONCLUSIONS

The capacity to remove alveolar fluid in the remaining lung is maintained at a normal level for up to 7 days after pneumonectomy in a rat, and then there is a marked increase in amiloride-sensitive alveolar fluid transport capacity that might depend, at least in part, on increased expression of epithelial sodium channels in type II cells and in part on the increased number of type II cells.

The effect of amiloride on the in vivo effective permeability of amoxicillin in human jejunum: experience from a regional perfusion technique

The purpose of this human intestinal perfusion study (in vivo) was twofold. Firstly, we aimed to determine the effective in vivo jejunal permeability (P(eff)) of amoxicillin and to classify it according to the Biopharmaceutics Classification System (BCS). Secondly, we investigated the acute effect of amiloride on the jejunal P(eff) of amoxicillin. Amoxicillin, a beta-lactam antibiotic, has been reported to be absorbed across the intestinal mucosa by both passive diffusion and active transport. A regional single-pass perfusion of the jejunum was performed using a Loc-I-Gut perfusion tube in 14 healthy volunteers. Each perfusion lasted for 200 min and was divided into two periods of 100 min each. The concentration of amoxicillin entering the jejunal segment was 300 mg/l in both periods, and amiloride, an inhibitor of the Na+/H+ exchanger, was added at 25 mg/l in period 2. The concentrations of amoxicillin and amiloride in the outlet jejunal perfusate were measured with two different HPLC-methods. Antipyrine and [14C]PEG 4000 were added as internal standards to the perfusion solution. Amiloride had no significant effect on the jejunal P(eff) of amoxicillin. The human in vivo jejunal P(eff) for amoxicillin was 0.34+/-0.11 x 10(-4) and 0.46+/-0.12 x 10(-4) cm/s in periods 1 and 2, respectively. The high jejunal P(eff) for amiloride was 1.63+/-0.51 x 10(-4) cm/s which predicts an intestinal absorption of more than 90%. Following the BCS amoxicillin was classified as a low P(eff) drug, and amiloride had no acute effect on the in vivo jejunal P(eff) of amoxicillin.

Topical amiloride accelerates healing and delays neovascularization in mechanically produced corneal ulcers in rabbits

The present investigation was undertaken to explore the ulcer healing and antiangiogenic efficacy of two dosage schedules of topically administered amiloride in mechanically produced corneal ulcers in rabbits and to compare its effect with the conventional topical antiinflammatory angiostatic agent flurbiprofen. The epithelium and superficial lamellae of the stroma of both eyes of each rabbit were cut through by a corneal trephine (8 mm diameter) up to a depth of 0.3 mm and removed after local anesthesia. The animals were randomly divided in groups of 4 rabbits each. In the eyes of 2 groups of animals, amiloride (4%) was instilled either q.i.d. or b.i.d.; in another, flurbiprofen (0.03%) was instilled twice daily whereas the saline-treated group served as control. The healing of ulcer was followed on a slit lamp regarding its size, depth, slough formation, infiltration and neovascularization on alternate days up to the 10th day with and without fluorescein staining. Healing of corneal ulcers was significantly accelerated by both dosage schedules of topical amiloride (4%) but more so following q.i.d. instillation. Topical flurbiprofen, on the other hand, delayed the healing process. Instillation of amiloride four times daily or flurbiprofen twice daily inhibited angiogenesis significantly. However, appearance of new vessels was completely prevented when amiloride (4%) was instilled twice daily. Thus topical amiloride (4%) may prove to be a cheap and better antineovascularization as well as ulcer healing agent with no apparent side effects. Inhibition of uPA by amiloride appears to be responsible for these effects.

Pharmacologic and molecular discrimination of I2-imidazoline receptor subtypes

I2-imidazoline receptors (I2-IR) are characterized by their high affinity for imidazolines and guanidines and medium affinity for imidazolidines. The differential recognition of I2-IR by amiloride led to subtype these sites as amiloride-sensitive (I2A-IR) and amiloride-insensitive (I2B-IR). I2-IR labeled with [3H]idazoxan or [3H]2-BFI in the rabbit cerebral cortex (I2A-IR) displayed higher affinities for amiloride and amiloride analogs than in the rat cerebral cortex (I2B-IR). Other drugs tested displayed biphasic curves in competition experiments, indicating the existence of high and low affinity sites for both I2-IR subtypes. The drugs (+)- and (-)-medetomidine, bromoxidine, moxonidine, and clorgyline were more potent on the high and/or low affinity sites of I2B-IR than on I2A-IR. Preincubation (30 min at 25 degrees C) with 10(-6) M isothiocyanatobenzyl imidazoline (IBI) or with 10(-6) M clorgyline reduced by 40% and 26%, respectively, the binding of [3H]2-BFI to I2B-IR, but it did not alter the binding of the radioligand to I2A-IR. These results indicated that the I2-IR subtypes differ in their pharmacologic profiles and in the nature of the imidazoline binding site involved in clorgyline and IBI alkylation. In rat cortical membranes, western blot detection of immunoreactive imidazoline receptor proteins revealed a double band of approximately 29/30 kD and three less intense bands of approximately 45, approximately 66, and approximately 85 kD. In rabbit cortical membranes the antibody detected proteins of approximately 30, approximately 57, approximately 66, and approximately 85 kD. It is suggested that I2-IR may be related to more than one receptor protein and that I2-IR subtypes differ in the nature of the proteins implicated.

Amiloride pharmacokinetics in rat

The kinetics of amiloride was investigated in plasma, urine, faeces and tissues of rats after oral (10 mg/kg) and i.v. (10 mg/kg bolus and 35 microg/h for 4-days infusion) administration. Initially the experimental data were analyzed by a multiexponential model, then a compartmental model was developed to describe the drug kinetics in plasma, urine, faeces and tissues after the i.v. bolus and the oral administration simultaneously. Aim of the model was also to predict the drug kinetics in plasma and tissues of rats after continuous i.v. infusion. The results of the prediction and the discrepancies between prediction and observed data allowed a deeper insight into the pharmacokinetics of amiloride.

Aerosolized amiloride: dose effect on nasal bioelectric properties, pharmacokinetics, and effect on sputum expectoration in patients with cystic fibrosis

Aerosolized amiloride normalizes the excessive sodium absorption cystic fibrosis (CF) respiratory epithelium. The aims of this study were to assess the dose-effect relationship and the duration for which amiloride inhibits Na+ transport, to determine acute and chronic pharmacokinetics, and to test the effect of acute aerosolized amiloride on the amount of sputum expectorated. The effect of inhaled amiloride was assessed principally by nasal potential difference (PD) measurements. Amiloride serum levels were measured in 23 patients after inhalation of different doses of aerosolized amiloride. Twenty CF patients inhaled amiloride (10(-3)M) or a placebo in a double-blinded, randomized order, and sputum production was quantitated. The results of this study showed that maximal initial PD inhibition was achieved by 6 x 10(-3)M of amiloride. The duration of inhibition of PD (effective time until return to 50% delta PD [ET50] after nasal administration) was dose dependent (10(-3)M, 39 +/- 0.8 minutes; 10(-2)M; 133 +/- 14 minutes). Amiloride serum levels were below 2.5 ng/ml in 20 of 28 patients; levels were above 5 ng/ml only within 4 hours after high dose inhalation (10(-2)M). In the double-blinded, crossover study, more sputum was expectorated after amiloride inhalation as compared with that after a placebo (P < 0.05). In conclusion, the bioelectric effects of amiloride and serum levels after inhalation are dose dependent, and amiloride is effective at inducing sputum expectoration in CF.

Airway deposition and clearance and systemic pharmacokinetics of amiloride following aerosolization with an ultrasonic nebulizer to normal airways

STUDY OBJECTIVES

Airway epithelial ion transport is an important component of the airway defense mechanism, and new therapies that target ion transport are being developed. Amiloride is an example of such a new drug, exerting a dose-dependent action to inhibit Na+ transport. Amiloride may be useful in cystic fibrosis, blocking the characteristic airway epithelial Na+ hyperabsorption that occurs in the disease. To evaluate airway and systemic delivery of amiloride via an ultrasonic nebulizer (Omron NE-UO7), we measured the airway surface concentrations of amiloride in normal volunteers via a novel approach, together with the systemic pharmacokinetics of amiloride.

DESIGN

Direct measurement of airway surface liquid, plasma, and urine amiloride concentrations following ultrasonic nebulization.

PARTICIPANTS/INTERVENTIONS

Seven normal subjects were studied in the General Clinical Research Center of the University of North Carolina. Following inhalation with amiloride (1 mg/mL, 4.5 mL) for approximately 12 min, a bronchoscopy was performed. Amiloride deposition and clearance from airway surfaces over 1 h were evaluated by transbronchoscopic sampling using preweighed filter papers. Pulmonary and systemic absorption was assessed by measuring drug concentrations in blood and urine.

RESULTS

The mean volume aerosolized was 3.5+/-0.3 mL during 12 min of aerosolization time; the mean initial concentration of amiloride on airway surfaces after nebulization was 1.6 x 10(-4) mol/L, with an elimination half life of approximately 23 min. Peak plasma concentrations of amiloride (30 min, 3.36+/-0.70 ng/mL) suggest early absorption across lung surfaces, rather than via the GI route. Mean urinary excretion of amiloride over 72 h was 0.63+/-0.07 mg, with 87% excreted in the first 24 h.

CONCLUSIONS

The ultrasonic nebulizer rapidly delivers amiloride to normal conducting airways as assessed by the transbronchoscopic sampling technique. Early blood concentrations of amiloride probably reflect initial absorption across lung surfaces and are a useful index of the efficiency of the machine.

Prediction of absolute bioavailability for drugs using oral and renal clearance following a single oral dose: a critical view

In order to determine the absolute bioavailability, both oral and intravenous administrations of a drug are often used. Recently a new method has been proposed to determine absolute bioavailability in the absence of intravenous dose. Following a single oral dose, this method requires oral and renal clearance data from normal subjects and renal failure patients. The bioavailability is calculated from a plot of oral against renal clearance following an oral dose, where the inverse of the slope is equal to absolute bioavailability. This study examines the prediction of absolute bioavailability from the proposed method for eight drugs which have a wide range of oral and renal clearance. From this study, it appears that the proposed method may not be reliable for the prediction of absolute bioavailability and further investigation is needed to test the validity of this method.

Determination and pharmacokinetics of a furosemide-amiloride drug combination

The study presents an accurate and precise HPLC assay for the determination of furosemide and amiloride in human specimens. Both drugs were extracted from human plasma with ethyl acetate; furosemide was extracted at pH 1 and amiloride at pH 12. While chromatographic separation conditions, i.e., column, mobile phase and flow-rate were the same for both investigated drugs, furosemide was detected using a UV absorbance detector, whereas amiloride, because of its very low therapeutic range, was detected with a spectrofluorimetric detector. The linearity of the furosemide and amiloride assays were confirmed over the range of 30-3000 ng/ml and 0.5-30 ng/ml, respectively. These concentrations correspond well with the therapeutic ranges of both drugs. The extraction recoveries, depending on concentration, exceed 80% for furosemide and 74% for amiloride. The reported methods were applied to pharmacokinetic investigations of the two compounds taken in form of a drug combination.

Pharmacokinetics of amiloride after inhalation and oral administration in adolescents and adults with cystic fibrosis

STUDY OBJECTIVE

To compare the pharmacokinetics and systemic exposure of nebulized and oral amiloride in adolescents and adults with mild to moderate cystic fibrosis (CF).

DESIGN

Open-label, randomized, two-way crossover, single-dose pharmacokinetic study.

SETTING

University hospital clinical research unit.

PATIENTS

Nine adolescents and 10 adults with mild to moderate CF (forced expiratory volume in 1 sec > or = 50% predicted, Brasfield score > or = 15).

INTERVENTIONS

Patients received amiloride solution orally (10 mg of amiloride 1-mg/ml solution) and by inhalation [4.5 ml amiloride of 1-mg/ml solution in 12% saline (approximately 3.8 mmol/L) by DeVilbiss 646 nebulizer] during two study phases separated by a 7- to 28-day washout period. Serial blood and urine samples were collected for 48 and 72 hours, respectively.

MEASUREMENTS AND MAIN RESULTS

After oral dosing, the mean +/- SD maximum peak concentration (Cmax) was 20.6 +/- 10.0 ng/ml at 3.2 +/- 1.2 hours in adults and 21.7 +/- 4.88 at 2.9 +/- 0.6 hours in the adolescents. Mean area under the concentration-time curve (AUC) from time zero to infinity hours was 275 +/- 115 and 254 +/- 60 ng.hr/ml in the adult and adolescent groups; half-life was 16.0 +/- 0.7 and 13.4 +/- 1.4 hours, respectively. After nebulization, 14 of 19 subjects exhibited two concentration peaks (Cmax1 and Cmax2) with mean values of 1.57 +/- 1.67 ng/ml at 0.5 +/- 0.2 hours and 1.37 +/- 1.21 ng/ml at 4.0 +/- 1.0 hours for adults, and 1.49 +/- 0.99 ng/ml at 0.5 +/- 0.1 hours and 1.52 +/- 0.81 ng/ml at 3.3 +/- 0.5 hours for adolescents. Estimated mean +/- SD dose nebulized was 1.91 +/- 0.66 and 2.28 +/- 0.30 mg in the adult and adolescent groups, respectively. Mean +/- SD AUC from time zero to the last measurable plasma amiloride concentration after inhalation was 14.4 +/- 17.6 and 15.4 +/- 10.1 ng.hr/ml in the adults and adolescents. No significant adverse events occurred during the study. Pharmacokinetic parameters were not statistically different between the adolescent and adult groups by route of administration. However significant differences in peak amiloride concentration, AUC, and urinary amiloride excretion were evident when comparing oral versus inhalation administration within each group.

CONCLUSIONS

Mean amiloride plasma concentration peaks and AUC after inhalation were significantly lower than after oral dosing. In addition, the second amiloride plasma concentration peak may be due to oral ingestion of the nebulized amiloride, whereas the earlier Cmax1 after inhalation may be due to pulmonary absorption of amiloride. These results suggest that single-dose amiloride inhalation in patients with mild to moderate CF results in minimal systemic exposure compared with oral dosing, and that drug disposition is similar in adolescents and adults with CF.

Pharmacokinetic studies of amiloride and its analogs using reversed-phase high-performance liquid chromatography

We have studied the pharmacokinetics of amiloride and its analogs. A high-performance liquid chromatographic method has been adapted for the measurement of amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA) and 5-(N, N-hexamethylene)amiloride (HMA) in mouse plasma, kidney, liver and tumor tissues. The method uses a C8 preparative solid-phase column, followed by separation using a reversed-phase C18 column (250 x 4 mm I.D., 5 microns particle size) with detection by ultraviolet absorption at 365 nm. Reversed-phase separations were performed at ambient temperature using a non-linear gradient method with two different mobile phases: mobile phase A was 100% acetonitrile while mobile phase B was 0.15 M perchloric acid at pH 2.20 (flow-rate was 1.2 ml/min). The retention times for amiloride, benzamil (used as an internal standard), EIPA and HMA are 13.4, 19.5, 21.8 and 23.5 min, respectively. The calibration curves are linear over the range of 0.1-50 microM in plasma and in tissues. The half-lives of amiloride, EIPA and HMA (and their confidence intervals) in plasma after intraperitoneal injection of drugs into mice were 68.8 +/- 0.2, 31.2 +/- 2.5 and 39.3 +/- 7.9 min, respectively. Amiloride was detected as a metabolite of EIPA but not of HMA. When EIPA was injected at a dose of 10 micrograms/g body weight, it was cleared rapidly from liver, but concentrations > 1 microM were sustained for at least 2 h in murine kidney and in a transplantable tumor.

[Assessment of the role of enterocyte ion exchange in the intestinal absorption of amoxicillin, based on a study of the interaction with amiloride]

Intestinal dipeptide carrier system has been shown in vitro to be involved in intestinal absorption of betalactam antibiotics. Given that efficiency of this transport system depends on a pH gradient (extracellular pH < intracellular pH) at the brush-border membrane of enterocytes, we assessed the effects of amiloride, a known inhibitor of the Na-H exchange, on the bioavilability of oral amoxicillin in eight normal volunteers. Following a single 10 mg oral dose of amiloride, the absolute bioavailability of oral amoxicillin turned out to decrease by 27% (p < 0.01). The extent of reduction of oral amoxicillin AUC appeared to significantly correlate (p = 0.005) with the extent of decrease in potassium renal excretion but not with variations in antibiotic renal clearance under amiloride effect. Such patterns seem to confirm in vivo the role of the Na-H exchange in betalactam absorption and to suggest the underlying regulatory function of intracellular Na concentration, the latter depending on the (NaK)-ATPase activity.

Methyl isobutyl amiloride delays normalization of brain intracellular pH after cardiac arrest in rats

OBJECTIVE

The sodium/hydrogen ion (Na+/H+) antiporter system of brain cells is responsible for reducing intracellular acid loads and regulating cellular volume. Activation of this system during reperfusion following cardiac arrest may contribute to cerebral edema and subsequent brain damage. Therefore, we wished to determine whether administration of methyl isobutyl amiloride, a known inhibitor of the Na+/H+ antiporter system, would cross the blood brain barrier and delay the return of brain intracellular pH to normal values during reperfusion after cardiac arrest in rats.

DESIGN

a) Prospective sequential evaluation of the regional brain blood flow and 3H-methyl isobutyl amiloride extraction fraction in rats; b) prospective sequential evaluation of brain intracellular pH in rats treated with methyl isobutyl amiloride compared with untreated control rats.

SETTING

A research laboratory.

SUBJECTS

Thirteen male Wistar rats: a) three rats to study regional brain blood flow and 3H-methyl isobutyl amiloride cerebral extraction; and b) ten rats to study the effect of methyl isobutyl amiloride on brain intracellular pH after cardiac arrest and reperfusion.

INTERVENTIONS

a) Rats were injected with 14C iodoantipyrine and 3H-methyl isobutyl amiloride, and their brains were subsequently analyzed to determine regional cerebral blood flow and percent of cerebral extraction of methyl isobutyl amiloride. b) Cardiac arrest was induced with potassium chloride followed by resuscitation 7 mins later in untreated control rats and rats treated with methyl isobutyl amiloride.

MEASUREMENTS AND MAIN RESULTS

a) Regional cerebral blood flow (mL/100 g/min) determined with 14C iodoantipyrine and percent of cerebral extraction of 3H-methyl isobutyl amiloride were evaluated in various regions of the brain. Mean +/- SD values were 167 +/- 15 and 7 +/- 1 for the frontal cerebral cortex; 159 +/- 10 and 7 +/- 2 for the parietal cerebral cortex, 130 +/- 17 and 8 +/- 1 for the hippocampus, 154 +/- 33 and 13 +/- 4 for the cerebellum and 166 +/- 27 and 6 +/- 1 for the striatum (mL/100 g/min). These values were determined by a dual label indicator fractionation method. b) Brain intracellular pH was measured by neutral red histophotometry after 15 mins of reperfusion following cardiac arrest. As compared with untreated control rats, methyl isobutyl amiloride-treated animals had significantly lower brain intracellular pH values after 15 mins of reperfusion. Mean +/- SD pH values were 6.78 +/- 0.18 for the rats treated with methyl isobutyl amiloride vs. normal intracellular pH of 7.11 +/- 0.07 for the untreated control rats.

CONCLUSIONS

a) Methyl isobutyl amiloride crosses the blood brain barrier of rats. b) The Na+/H+ antiporter system is operative during reperfusion after cardiac arrest in rats.

Inhibitor effects on the ionic exchanges through the human amniotic epithelial cell membranes

Direct measurements of cell podocyte/or microvillous membrane ionic exchanges were performed on the membranes of isolated human amniotic epithelial cells. The ionic exchanges were determined from the measures of cellular input conductances. The effects of various inhibitors: ouabain, amiloride, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), 4'4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), quinidine, barium, manganese were analyzed. This study shows that the ionic exchanges through the epithelial cells are regulated by the presence of Na+,K+,Cl- channels, Na+/H(+)-Cl-/HCO3- antiports, (Na-K)ATPase and Na+/Ca(2+)-Na+/Mg2+ exchangers on the 2 faces of the cells and of a Na(+)-K(+)-2Cl- cotransport on the membrane facing the amniotic cavity only.

Inhibition of the regulation of intracellular pH: potential of 5-(N,N-hexamethylene) amiloride in tumour-selective therapy

The viability of cells within the acidic microenvironment found in solid tumours is expected to depend on the regulation of intracellular pH (pHi). 5-(N,N-hexamethylene) amiloride (HMA) is a potent inhibitor of the Na+/H+ antiport, a major mechanism for the regulation of pHi. We have therefore studied the cytotoxicity of HMA in combination with nigericin, a cell-acidifying agent, for EMT-6 cells in monolayer cell culture, in spheroids and in a murine tumour model. The combination of nigericin and HMA was toxic to cells in tissue culture at extracellular pH (pHe) < or = 6.8 (as may be found in tumours) but not at pH 7.0 or above (as in most normal tissues). Compared with amiloride, the relative potency of HMA in causing in vitro cytotoxicity (approximately 100-fold) was similar to that for inhibition of the Na+/H+ antiport. The fluorescent probe Hoechst 33342 was used with flow cytometry to study the cytotoxicity of HMA and nigericin at different depths in multicellular tumour spheroids. Only small differences in the level of cell survival were observed, but higher concentrations of HMA were required as compared with those giving equal levels of survival in monolayer culture. The pharmacokinetics of HMA in mice was studied by using high-performance liquid chromatography: after intraperitoneal injection of 20 micrograms g-1, the plasma level of HMA peaked at 8 microM after about 15 min and decreased to 1 microM at 120 min; the half-life was 35 min. Nigericin and HMA, at doses of 1.25 micrograms g-1 and 10 micrograms g-1 respectively, failed to cause significant cell killing in the EMT-6 murine tumour, but the surviving fraction was reduced to approximately 0.004 when hydralazine was administered with nigericin and HMA. Local tumour irradiation (15 Gy), followed by treatment with these drugs, led to cell killing that was additive to the effects of drugs and radiation alone, so that hypoxic cells which survived radiation did not appear more sensitive to pH-dependent drug treatment. Acid-mediated therapy can lead to cell death in murine solid tumours, but further measures will be required before the strategy can be exploited clinically.

Correction of hypokalemia with antialdosterone therapy in Gitelman's syndrome

Six adult patients (4 females and 2 males, age range 26-57 years) with Gitelman's syndrome (GS) were treated with spironolactone 200-300 mg/day (n = 5) and/or amiloride 10-30 mg/day (n = 3) for 1-18 months. The patients had hypokalemia, hyperreninemia, chloride-resistant metabolic alkalosis, renal hypomagnesemia (n = 5), and hypocalciuria (n = 5). Free water clearance studies during maximal water diuresis and furosemide administration were suggestive of a solute reabsorptive defect beyond the loop of Henle. Antialdosterone therapy induced a significant increase of PK (from 2.6 +/- 0.4 to 3.4 +/- 0.4 mM; p < 0.0001) and a decrease of CK (from 21.4 +/- 13.2 to 10.6 +/- 4.8 ml/min, p < 0.02) and FEK (from 21.0 +/- 13.6 to 13.4 +/- 5.7%; p < 0.03); PMg increased from 1.38 +/- 0.38 to 1.64 +/- 0.21 mg/dl (p < 0.03) with a parallel fall of CMg (from 5.5 +/- 2.3 to 2.9 +/- 1.5 ml/min; p < 0.02) and FEMg (from 5.7 +/- 2.6 to 2.9 +/- 0.6%; p < 0.05); arterial blood pH and HCO3- did not change (P = plasma, C = clearance, FE = fractional excretion). The creatinine clearance fell (from 90.5 +/- 16.8 to 65.8 +/- 20.9 ml/min; p < 0.05), and Prenin rose (from 16.6 +/- 8.9 to 35.3 +/- 25.3 ng/ml/h; p < 0.02, as did Paldo (from 26.1 +/- 12.3 to 109 +/- 82.6 ng/dl; p < 0.01), indicating extracellular fluid volume contraction; however no significant clinical symptoms of hypovolemia ensued.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of amiloride on sodium-calcium exchange activity in rat cardiac myocytes

The effect of amiloride on Na+/Ca2+ exchange activity and adenylate energy charge in isolated rat cardiac myocytes has been studied. Amiloride inhibits Na+/Ca2+ exchange activity in rat cardiac myocytes with an IC50 of 1.76 mM when added to the external side of the cell membrane, whereas it is a weaker inhibitor when present at the cytoplasmic side. The adenylate energy charge in rat cardiac myocytes is unaffected by amiloride.

Ion transport across leech integument. I. Electrogenic Na+ transport and current fluctuation analysis of the apical Na+ channel

The dorsal skin of the leech Hirudo medicinalis was used for electrophysiological measurements performed in Ussing chambers. The leech skin is a tight epithelium (transepithelial resistance = 10.5 +/- 0.5 k omega.cm-2) with an initial short-circuit current of 29.0 +/- 2.9 microA.cm-2. Removal of Na+ from the apical bath medium reduced short-circuit current about 55%. Ouabain (50 mumol.l-1) added to the basolateral solution, depressed the short-circuit current completely. The Na+ current saturated at a concentration of 90 mmol Na+.l-1 in the apical solution (KM = 11.2 +/- 1.8 mmol.l-1). Amiloride (100 mumol.l-1) on the apical side inhibited ca. 40% of the Na+ current and indicated the presence of Na+ channels. The dependence of Na+ current on the amiloride concentration followed Michaelis-Menten kinetics (Ki = 2.9 +/- 0.4 mumol.l-1). The amiloride analogue benzamil had a higher affinity to the Na+ channel (Ki = 0.7 +/- 0.2 mumol.l-1). Thus, Na+ channels in leech integument are less sensitive to amiloride than channels known from vertebrate epithelia. With 20 mmol Na+.l-1 in the mucosal solution the tissue showed an optimum amiloride-inhibitable current, and the amiloride-sensitive current under this condition was 86.8 +/- 2.3% of total short-circuit current. Higher Na+ concentrations lead to a decrease in amiloride-blockade short-circuit current. Stimulation of the tissue with cyclic adenosine monophosphate (100 mumol.l-1) and isobutylmethylxanthine (1 mmol.l-1) nearly doubled short-circuit current and increased amiloride-sensitive Na+ currents by 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

[Comparative study of pharmacokinetics and effects on urinary secretion of electrolytes of furosemide and furosemide-amiloride in healthy subjects]

A randomized cross over study was carried out in 12 healthy volunteers to investigate simultaneously the pharmacokinetics and the effects on urinary volume and electrolyte excretion after administration of single doses of 40 mg frusemide and a combination tablet containing both 40 mg frusemide and 5 mg amiloride. From a statistical analysis of plasma levels of frusemide and amiloride measured by HPLC methodology, no significant difference between the reference drug alone, frusemide, and the combination tablets was observed in mean peak plasma levels, mean times to peak or mean areas under the plasma concentration-time curves (AUC). Frusemide and the combination tablet both produced a rapid and powerful diuresis in the 0-2 hours postdose period and did not differ significantly in urine output at any time point. However a difference in natriuretic activity was observed between frusemide and the combination with the latter producing a significantly greater sodium excretion in the 0 to 2 hours period (p less than 0.05). Potassium retaining activity throughout the 24 hours was marked after the administration of the combination, the potassium excretion being significantly less (p less than 0.05) than either control of frusemide alone. There was also a significant correlation between plasma levels of frusemide and the time course of urine and electrolyte excretion in healthy subjects.

Combination of long-acting furosemide and instant-acting amiloride: pharmacokinetics and pharmacodynamics in human subjects

The pharmacokinetics and pharmacodynamics of the combination of amiloride (2 x 2.5 mg) and long-acting furosemide (2 x 10 mg) were compared with amiloride (5 mg) and furosemide (20 mg) in 12 healthy male volunteers aged 26.2 +/- 1.6 years and weighing 68.8 +/- 6.2 kg, after random order administration. Furosemide and amiloride plasma or urine concentrations were determined by HPLC with fluorimetric detection. The rate of absorption (tmax = 3 h) and the bioavailability of the two diuretics were not significantly modified by their combination. Furosemide plasma half-life was 2.77 +/- 1.04 h after the combination treatment and 2.76 +/- 0.98 h alone, amiloride plasma half-life was respectively 15.7 +/- 4.6 h and 14.6 +/- 3.7 h. The urinary elimination of furosemide was significantly higher in the 2-4 h interval in the combination treatment, accompanying its delayed maximum effect of diuresis. A synergistic effect was observed after the combination administration of the two diuretics; between the 2nd and the 8th hour, the sodium elimination was significantly increased (P less than 0.01) and the potassium excretion was significantly decreased (P = 0.05). After a single dose, no modification of plasma or erythrocyte magnesium levels was observed. This study shows that the combination of the two drugs entails a synergy of their activities which does not involve pharmacokinetic changes.

Pulmonary deposition of nebulised amiloride in cystic fibrosis: comparison of two nebulisers

BACKGROUND Preliminary evidence suggests that regular inhalation of nebulised amiloride reduces sputum viscoelasticity, increases the clearance of sputum by mucociliary mechanisms and by coughing and reduces the rate of deterioration in lung function in patients with cystic fibrosis. These effects depend on adequate delivery of amiloride to the airways. This study was performed to quantify and compare pulmonary deposition of amiloride produced by two different nebuliser systems. METHODS The pulmonary deposition of nebulised amiloride (1 mg in 3 ml saline) was measured in eight patients with cystic fibrosis when given via a jet (System 22 with CR 60 compressor) and an ultrasonic (Fisoneb) nebuliser. Human serum albumin labelled with technectium-99m was used as an indirect marker for amiloride and its deposition in the lung was detected with a gamma camera. RESULTS Amiloride inhalation caused no side effects or changes in spirometric indices. The mean (SD) total pulmonary amiloride deposition was 57 (24) micrograms with the System 22 and 103 (53) micrograms with the Fisoneb nebuliser. Pulmonary deposition was completed more rapidly with the Fisoneb (4-5 minutes) than with the System 22 nebuliser (7-8 minutes) and the Fisoneb was preferred by the patients. CONCLUSIONS Both nebulisers appeared to deliver adequate amounts of amiloride to the lungs, but treatment with the Fisoneb nebuliser was quicker, more efficient, and more acceptable to the patients. Of the two nebulisers assessed, the Fisoneb would be preferred for clinical trials.

High-performance liquid chromatographic assay for amiloride in plasma and urine

A sensitive and simplified high-performance liquid chromatographic procedure has been developed for quantification of amiloride in rabbit plasma, as well as human plasma and urine. Following protein precipitation with perchloric acid, the supernatant was directly injected into a C18 Nucleosil column. The mobile phase consisted of methanol-water (45:55) containing 0.1 M perchloric acid, and the compound was quantitated using a fluorescence detector at excitation and emission wavelengths of 286 and 418 nm, respectively. The average recovery was 97.6%. The calibration curve was linear over the range 2.0-20.0 ng/ml. The limit of detection was 0.5 ng/ml.

Amiloride disposition in geriatric patients: importance of renal function

1. The absorption and disposition of the potassium sparing diuretic amiloride were determined in nine elderly patients aged 71 to 87 years and in eight young (25 to 38 years) subjects following oral administration of 5 mg amiloride HCl daily to steady-state. 2. The maximum and steady-state plasma amiloride concentrations were significantly (P less than 0.05 and P less than 0.001) higher in the elderly patients. The renal clearance of amiloride was lower in the elderly than in young subjects (102 +/- 36 ml min -1 vs 300 +/- 64 ml min-1, P less than 0.001) as was the urinary excretion of amiloride (36 +/- 13 vs 62 +/- 18% of the dose, P less than 0.01). 3. The steady-state plasma amiloride concentration correlated significantly (r2 = 0.61, P less than 0.001) with amiloride renal clearance and with creatinine clearance (r2 = 0.59, P less than 0.001). There was a very strong positive correlation between renal amiloride clearance and creatinine clearance (r2 = 0.76, P less than 0.001). The slope of the regression line was 2.5 indicating substantial proximal tubular secretion of amiloride. 4. Sodium and potassium excretion, along with urine volume were significantly (P less than 0.05) lower in the elderly (by 39, 45 and 34% respectively). 5. The disposition of amiloride was highly dependent on renal function, with higher plasma amiloride concentrations in the elderly reflecting diminished renal function. The dose of amiloride should be titrated to individual response, and the lower potassium excretion in the elderly patients suggests that the dose of amiloride could be reduced in this group of patients.

The pharmacokinetics of amiloride-hydrochlorothiazide combination in the young and elderly

The pharmacokinetics of amiloride and hydrochlorothiazide were studied in 12 healthy young volunteers following a single dose of a fixed combination of amiloride and hydrochlorothiazide and in 11 elderly hypertensive patients at steady-state. Following modelling of the single dose data, simulated steady-state plasma concentrations for the 2 drugs were generated to examine the effect of age and/or hypertension on pharmacokinetics. The apparent systemic plasma clearance for both amiloride and hydrochlorothiazide was significantly reduced in the elderly when compared to the young (from 753 to 325 ml.min-1, amiloride; and from 418 to 157 ml.min-1, hydrochlorothiazide). The plasma concentrations at steady state for both drugs were greatly increased in the elderly patients (Amiloride: from 7 to 25 ng.ml-1, Css,max; from 2 to 8 ng.ml-1, Css,min; and from 4 to 14 ng.ml-1, Cav; Hydrochlorothiazide: from 184 to 651 ng.ml-1, Css,max; from 31 to 121 ng.ml-1, Css,min; and from 89 to 273 ng.ml-1, Cav). The decreased clearance of the diuretics in the elderly was believed due to deterioration of renal function, and there was a significant correlation between the plasma clearance of hydrochlorothiazide and creatinine clearance in both age groups (r = 0.62, young; r = 0.72, elderly). As a result of the pharmacokinetic findings caution may be indicated in the clinical dosage of the diuretics particularly when in fixed dose combination.

Simple high-performance liquid chromatographic method for the measurement of amiloride in body fluids

A simplified rapid high-performance liquid chromatographic (HPLC) procedure has been developed for the measurement of amiloride in plasma or urine. Solid-phase extraction columns provide quick, clean and simple sample preparation, allowing ten samples to be processed in less than 5 min. The HPLC system uses a standard reversed-phase (C18) column with detection by ultraviolet absorption at 365 nm. The assay has been used to define plasma amiloride concentration-time profiles and to quantitate urine amiloride recovery in healthy men after repeated administration at two doses.

Electrophysiological study of transport systems in isolated perfused pancreatic ducts: properties of the basolateral membrane

In order to study the mechanism of pancreatic HCO3- transport, a perfused preparation of isolated intra- and interlobular ducts (i.d. 20-40 microns) of rat pancreas was developed. Responses of the epithelium to changes in the bath ionic concentration and to addition of transport inhibitors was monitored by electrophysiological techniques. In this report some properties of the basolateral membrane of pancreatic duct cells are described. The transepithelial potential difference (PDte) in ducts bathed in HCO3(-)-free and HCO3(-)-containing solution was -0.8 and -2.6 mV, respectively. The equivalent short circuit current (Isc) under similar conditions was 26 and 50 microA . cm-2. The specific transepithelial resistance (Rte) was 88 omega cm2. In control solutions the PD across the basolateral membrane (PDbl) was -63 +/- 1 mV (n = 314). Ouabain (3 mmol/l) depolarized PDbl by 4.8 +/- 1.1 mV (n = 6) within less than 10 s. When the bath K+ concentration was increased from 5 to 20 mmol/l, PDbl depolarized by 15.9 +/- 0.9 mV (n = 50). The same K+ concentration step had no effect on PDbl if the ducts were exposed to Ba2+, a K+ channel blocker. Application of Ba2+ (1 mmol/l) alone depolarized PDbl by 26.4 +/- 1.4 mV (n = 19), while another K+ channel blocker TEA+ (50 mmol/l) depolarized PDbl only by 7.7 +/- 2.0 mV (n = 9). Addition of amiloride (1 mmol/l) to the bath caused 3-4 mV depolarization of PDbl. Furosemide (0.1 mmol/l) and SITS (0.1 mmol/l) had no effect on PDbl. An increase in the bath HCO3- concentration from 0 to 25 mmol/l produced fast and sustained depolarization of PDbl by 8.5 +/- 1.0 mV (n = 149). It was investigated whether the effect of HCO3- was due to a Na+-dependent transport mechanism on the basolateral membrane, where the ion complex transferred into the cell would be positively charged, or whether it was due to decreased K+ conductance caused by lowered intracellular pH. Experiments showed that the HCO3- effect was present even when the bath Na+ concentration was reduced to a nominal value of 0 mmol/l. Similarly, the HCO3- effect remained unchanged after Ba2+ (5 mmol/l) was added to the bath. The results indicate that on the basolateral membrane of duct cells there is a ouabain sensitive (Na+ + K+)-ATPase, a Ba2+ sensitive K+ conductance and an amiloride sensitive Na+/H+ antiport. The HCO3- effect on PDbl is most likely due to rheogenic anion exit across the luminal membrane.

Effects of formulation and food on the absorption of hydrochlorothiazide and triamterene or amiloride from combination diuretic products

The absorption of three combination formulations of hydrochlorothiazide and either triamterene or amiloride was studied over a 5-year period in seven separate investigations under varying conditions of food and fasting. The most widely prescribed combination, containing 25 mg of hydrochlorothiazide and 50 mg of triamterene, demonstrated impaired absorption in the fasting state that was partially corrected by the addition of a breakfast high in fat. The increase in the fat content of the food appeared to correlate directly with the amount of both drugs absorbed from this formulation. The second formulation studied, a new combination formulation of 50 mg of hydrochlorothiazide and 75 mg of triamterene, demonstrated acceptable absorption in the fasting state that was not altered by the concurrent administration of a high-fat breakfast. The absorption of the third formulation, a combination of 50 mg hydrochlorothiazide and 5 mg amiloride, was acceptable in the fasting state and demonstrated a slight reduction in the absorption of the amiloride component when administered concurrently with a high-fat meal. The clinical and biopharmaceutic implications of these observations are discussed.

Pharmacokinetics of amiloride in renal and hepatic disease

The pharmacokinetics of the antikaliuretic amiloride has been studied in healthy controls and in patients with chronic renal failure or hepatitis. It was 40% bound to protein. In healthy volunteers 49% of an oral dose was recovered unchanged in the urine. The renal clearance of amiloride was about 3 times the creatinine clearance, which means that it was predominantly excreted via tubular secretion. Renal impairment reduced the clearance of amiloride, causing a prolongation of the t1/2 and drug accumulation in plasma. In hepatitis the t1/2 of amiloride was prolonged and the AUC increased. Urinary recovery (Ae) of amiloride was greater in hepatitis patients than in controls.