Repurposing of triamterene as a histone deacetylase inhibitor to overcome cisplatin resistance in lung cancer treatment

PURPOSE

Cisplatin is the core chemotherapeutic drug used for first-line treatment of advanced non-small cell lung cancer (NSCLC). However, drug resistance is severely hindering its clinical efficacy. This study investigated the circumvention of cisplatin resistance by repurposing non-oncology drugs with putative histone deacetylase (HDAC) inhibitory effect.

METHODS

A few clinically approved drugs were identified by a computational drug repurposing tool called "DRUGSURV" and evaluated for HDAC inhibition. Triamterene, originally indicated as a diuretic, was chosen for further investigation in pairs of parental and cisplatin-resistant NSCLC cell lines. Sulforhodamine B assay was used to evaluate cell proliferation. Western blot analysis was performed to examine histone acetylation. Flow cytometry was used to examine apoptosis and cell cycle effects. Chromatin immunoprecipitation was conducted to investigate the interaction of transcription factors to the promoter of genes regulating cisplatin uptake and cell cycle progression. The circumvention of cisplatin resistance by triamterene was further verified in a patient-derived tumor xenograft (PDX) from a cisplatin-refractory NSCLC patient.

RESULTS

Triamterene was found to inhibit HDACs. It was shown to enhance cellular cisplatin accumulation and potentiate cisplatin-induced cell cycle arrest, DNA damage, and apoptosis. Mechanistically, triamterene was found to induce histone acetylation in chromatin, thereby reducing the association of HDAC1 but promoting the interaction of Sp1 with the gene promoter of hCTR1 and p21. Triamterene was further shown to potentiate the anti-cancer effect of cisplatin in cisplatin-resistant PDX in vivo.

CONCLUSION

The findings advocate further clinical evaluation of the repurposing use of triamterene to overcome cisplatin resistance.

Drug repositioning of Clopidogrel or Triamterene to inhibit influenza virus replication in vitro

Influenza viruses cause respiratory tract infections and substantial health concerns. Infection may result in mild to severe respiratory disease associated with morbidity and some mortality. Several anti-influenza drugs are available, but these agents target viral components and are susceptible to drug resistance. There is a need for new antiviral drug strategies that include repurposing of clinically approved drugs. Drugs that target cellular machinery necessary for influenza virus replication can provide a means for inhibiting influenza virus replication. We used RNA interference screening to identify key host cell genes required for influenza replication, and then FDA-approved drugs that could be repurposed for targeting host genes. We examined the effects of Clopidogrel and Triamterene to inhibit A/WSN/33 (EC50 5.84 uM and 31.48 uM, respectively), A/CA/04/09 (EC50 6.432 uM and 3.32 uM, respectively), and B/Yamagata/16/1988 (EC50 0.28 uM and 0.11 uM, respectively) replication. Clopidogrel and Triamterene provide a druggable approach to influenza treatment across multiple strains and subtypes.

Rate-limiting biotransformation of triamterene is mediated by CYP1A2

OBJECTIVE

Triamterene (TA), a potassium-sparing diuretic, is extensively metabolized by hydroxylation in 4'-position and subsequent conjugation by cytosolic sulfotransferases. To identify the cytochrome P450 enzyme(s) catalyzing hydroxylation of triamterene (the rate-limiting step in the formation of the sulfate ester (STA)), in vitro incubation studies were performed with human liver microsomes.

METHODS

Initial rates of TA hydroxylation (0 - 300 microM) were determined during a ten-minute-incubation period with liver microsomes of two donors. The role of individual CYP enzymes was determined by pre-incubation with selective inhibitors/alternative substrates. Vice versa, the effect of TA (0 - 500 microM) on 3-demethylation of caffeine (0 - 1,000 microM) was assessed. Metabolite concentrations were estimated by reversed-phase HPLC methods.

RESULTS

TA Km values without inhibitors were 60 and 142 microM, Vmax was 177 and 220 pmol/min/mg protein, respectively. Mean inhibitor induced changes of 4'-hydroxy-TA formation were as follows: Furafylline 25 microM (CYP1A2), complete inhibition (-100%); omeprazole 250 microM (CYP1A2 inhibitor/CYP2C 19 substrate), -30%; coumarin 25 microM (CYP2A6), -11%; quinidine 25 microM (CYP2D6), -9%; ketoconazole 25 microM (CYP3A), -18%; and erythromycin 250 microM (CYP3A), -8%. In the reverse inhibition studies, TA competitively inhibited caffeine 3-demethylation with Ki values of 65 and 111 microM, respectively.

CONCLUSION

4'-hydroxylation of TA in humans appears to be mediated exclusively by CYP1A2. Inhibition or induction of CYP1A2 will change the time course of both TA and its active phase-II metabolite. The net pharmacodynamic effect of such changes is difficult to predict and needs to be evaluated in clinical studies.

Attenuation of the kaluretic properties of furosemide by triamterene (Dyrenium®) in healthy volunteers

OBJECTIVE

To examine if concomitant administration of furosemide, a loop diuretic, with the potassium- and magnesium-sparing diuretic triamterene would decrease loss of potassium and magnesium while improving diuresis.

METHODS

In this open-label, three-way crossover study, healthy subjects were randomized to receive treatment with 40 mg furosemide, with 150 mg triamterene, or treatment with 40 mg furosemide and 150 mg triamterene. Urine samples were collected 24 hours before dosing and between 0 - 1, 1 - 2, 2 - 3, 3 - 4, 4 - 6, 6 - 8, 8 - 12, and 12 - 24 hours post-dosing. Sodium and potassium levels were measured by an ion-selective electrode method. Magnesium was measured colorimetrically using a xylidyl blue reaction.

RESULTS

Co-administration of furosemide with triamterene resulted in enhanced diuresis, particularly in the first 0 - 12 hours post-dose, compared with either furosemide or triamterene alone. Compared to individual treatments, combination therapy significantly increased urinary sodium excretion (p = 0.0001) while significantly decreasing urinary potassium excretion (p = 0.0001); importantly, the magnesium-sparing characteristic of triamterene was retained with furosemide co-administration.

CONCLUSION

Triamterene, when used in combination with the loop diuretic, furosemide, preserves intracellular potassium and magnesium while enhancing the natriuretic effect of furosemide.

The G-tolerance after pharmacological hypohydration

A group of 18 healthy male subjects (25-45 y.o.) participated in the studies of +8.3 Gx tolerance (profile of ballistic reentry) and +Gz tolerance (up to physiological limit) before and after pharmacological hypohydration of organism. Moderate hypohydration was carried out by one 40 mg Furosemid (Lasix) dose, or by pharmacological complex: Furosemid (40 mg) + Hypothiazide (25 mg) + Triamteren (50 mg). Subjects removed with urine about 2 l of water, that was accompanied by increase excretion of electrolytes and caused reduction of the body mass by 2.0 +/- 0.2% and diminution of the plasma volume by 13.5% +/- 2.0%. Pharmacological hypohydration does not lead to a substantial loss in +Gx tolerance. No pathological signs, limiting +Gx tolerance up to 8.3 g were found. The longitudinal +Gz tolerance was less, than +Gx one. The limit of achieved +Gz loads after diuretics reduced by 0.6 g. The scientific and applied significance of this research is that diuretics can be safety used in pre-launch period to prevent the negative effects associated with initial phase of space flight. Besides the preliminary intake of diuretics promotes to diminish of discomfort of cosmonauts, reducing urination in a period of prelaunch waiting in spacecraft.

Vasodilatory effect of diuretics is dependent on inhibition of vascular smooth muscle carbonic anhydrase by a direct mechanism of action

Five years ago, our in vitro and in vivo studies demonstrated for the first time that diuretic agents such as furosemide, hydrochlorothiazide, amiloride, triamterene and spironolactone inhibit carbonic anhydrase (CA) I, II and renal CA IV by a direct mechanism of action. In this paper we investigate the relationship between diuretics and CA I in the vasodilatory mechanism. Both in vitro (on purified CA I, erythrocyte CA I and smooth muscle CA I) and in vivo (in human and rabbits) we studied the effect of acetazolamide, hydrochlorothiazide, indapamide, furosemide, amiloride and triamterene on purified CA I, on human erythrocyte CA I, as well as on CA I isolated from vascular smooth muscle. Our results demonstrate that in vitro all diuretics inhibit CA I by a direct mechanism of action. Inhibition reached 100% with acetazolamide, 45% with hydrochlorothiazide, 82% with indapamide, 85% with furosemide, 68% with amiloride and 58% with triamterene. In vivo, similar inhibition of erythrocyte and smooth muscle CA I was obtained, being parallel with a reduction in arterial blood pressure values. Our data show that in addition to their already known mechanisms, diuretics also inhibit CA in vascular smooth muscle. Our results suggest that this mechanism is achieved by means of pH changes induced by CA I inhibition.

Terbutaline increases open channel density of epithelial sodium channel (ENaC) in distal lung

Neonatal and adult vertebrate respiration is facilitated by alveolar fluid and sodium (Na+) absorption driven by apical sodium channels (ENaC). ENaC are characterized in Xenopus laevis lung (XLL) epithelia using voltage clamping and fluctuation analysis to non-invasively examine macroscopic transepithelial current and resistance (I(SC), R(T)), single channel current (i(Na)) and total channel density (N(T)) responses to a beta adrenergic agonist (Terbutaline). Terbutaline addition to the basolateral bath of XLL increased Na entry to > 200% of control reflecting a doubling of open channel density (N(o). These data are consistent with the notion that XLL can serve as a useful model for investigation of distal lung ENaC response to agents of physiological interest.

[The pharmacodynamic and pharmacokinetic characteristics of furosemide and furesis]

The object of the work was comparative study of the special features of the pharmacodynamic and pharmacokinetic properties of the diuretics furosemide and furesis in an ambulant regimen with the subject lying in an antiorthostatic position. Six practically healthy males were examined. They were given per os either 40 mg furosemide or one tablet of furesis (49 mg furosemide and 50 mg triamterine). Blood from the vein and urine were repeatedly tested.

Effect of quinapril and triamterene/hydrochlorothiazide on cardiac and vascular end-organ damage in isolated systolic hypertension

We compared, in a prospective double-blind randomized study, the effect of the angiotensin-converting enzyme inhibitor quinapril (QUI) with that of triamterene/hydrochlorothiazide (THCT) treatment on cardiovascular end-organ damage in subjects with untreated isolated systolic hypertension (ISH). End-organ damage measurements, performed initially and after 6 and 26 weeks of treatment, included echocardiographic determination of left ventricular mass index (LVMI) and of diastolic function and measurement of aortic distensibility and peripheral vascular resistance. Blood pressure was significantly reduced in the 44 subjects (21 QUI, 23 THCT) completing the study. Both LVMI and aortic distensibility had changed at 6 weeks, with comparable improvements in both groups. LV diastolic function showed overall no significant changes, although patterns of early filling did differ between the two drug groups. Peripheral vascular resistance appeared to increase between 6 and 26 weeks in THCT subjects only, along with a decreased aortic distensibility. Blood pressure and LV mass were rapidly and markedly reduced in both treatment groups of ISH subjects, paralleled by an improvement of aortic distensibility. In interpreting these results, the pathophysiologic alterations in ISH need to be taken into account, because these differ strongly from those in diastolic hypertension. Results of LV diastolic function and peripheral vascular resistance were less clear but appear to show less favorable changes in the THCT subjects treatment group.

Effect of cationic drugs on the renal secretion of ranitidine in the rat isolated perfused kidney

1. The rat isolated perfused kidney (IPK) was used to determine whether the renal tubular secretion of ranitidine is influenced by clinically relevant concentrations of other organic cationic drugs (amantadine, pseudoephedrine, triamterene and trimethoprim) that also undergo tubular secretion. 2. Ranitidine and [3H]-ranitidine were administered to the recirculating perfusion medium as a loading dose followed by a constant infusion to maintain clinically relevant perfusate ranitidine concentrations in the range 400-700 ng/mL. The renal clearance of ranitidine (CL[R]) was calculated, as was glomerular filtration rate (GFR), from the renal clearance of [14C]-inulin. 3. A total of 20 perfusions were performed and, in each case, ranitidine was administered for 80 min. In four control IPK, no drug other than ranitidine was administered. In the remaining IPK, amantadine, pseudoephedrine, triamterene or trimethoprim (n = 4 in each case) were administered to achieve low, medium and high concentrations during the 20-40, 40-60 and 60-80 min periods, respectively. 4. The mean (+/- SD) unbound fraction of ranitidine in the perfusion medium was 0.889 +/- 0.046 and was not altered (P>0.05) by the presence of the other drugs. 5. The CL(R)/GFR ratio for ranitidine in all kidneys was substantially greater than unity and had a mean value of 10.65 or greater in control kidneys, indicating extensive net tubular secretion. 6. The CL(R)/GFR was not affected (P>0.05) by amantadine, pseudoephedrine or triamterene at any concentration or by trimethoprim at the low concentration. However, medium (2000 ng/mL) and high (5000 ng/mL) concentrations of trimethoprim caused significant reductions in CL(R)/GFR of 20 and 28%, respectively (P<0.05). 7. The results indicate that at clinically relevant concentrations the renal tubular secretion of ranitidine is inhibited by trimethoprim, but not by amantadine, pseudoephedrine or triamterene.

Pharmacokinetics and pharmacodynamics of triamterene and hydrochlorothiazide and their combination in healthy volunteers

Although triamterene has been in clinical use for over 30 years, the linearity of triamterene kinetics was not systematically tested. Moreover, although triamterene is mostly applied concomitantly with thiazide-type diuretics the interaction of triamterene (TA) with hydrochlorothiazide (HCT) is subject to a controversial discussion. Therefore, the aim of this study was to examine the dose linearity of TA and the pharmacokinetic and pharmacodynamic interaction of triamterene and hydrochlorothiazide. In the first study 10 healthy volunteers received 0, 12.5, 25, 50, and 100 mg triamterene orally in a balanced crossover design. In the second study 0, 25, and 50 mg TA with 12.5, and 25 mg HCT, respectively, were administered to 12 healthy volunteers. Urine volume and concentration of sodium, TA, hydroxytriamterene sulfate (OH-TA ester), and HCT were measured by flame photometry and thin-layer chromatography, respectively. The observation period for each treatment was 3 days and the drug was given on the second day. Sodium excretion was increased by both drugs. Renal excretion of both TA and OH-TA ester seemed to be reduced at higher doses. However, statistical evaluation revealed no significant (p = 0.37, and p = 0.20, respectively) deviation from linearity. Renal excretion of HCT was not affected by TA and vice versa. However, renal excretion of OH-TA ester is significantly reduced when HCT is administered concomitantly. The renal excretion rate of sodium can be described by a common Emax model when the effects of the excretion rates of both TA and HCT are additive. It is concluded that the pharmacokinetics of TA is linear within the tested dose range and that pharmacodynamic additivity of HCT and TA is not due to a pharmacokinetic interaction. The results support the hypothesis of a sequential nephron blockade for both drugs acting on different tubular segments.

[Pharmacology of potassium-sparing diuretics]

Potassium sparing diuretics in combination with thiazides represent a therapeutical enrichment if applied correctly that is with respect to contraindications, interactions (especially with ACE-inhibitors and AT1-antagonists) and to pharmacokinetic properties. Triamterene has to be administered twice a day while amiloride and spironolactone are effective for 24 hours. Of course, pharmacokinetic parameters of potassium sparing diuretics have to be considered also in fixed combinations.

Potassium and magnesium retaining triamterene derivatives

This work represents a summary of studies carried out with potassium and magnesium retaining triamterene derivatives. The synthesis, analytics and physico-chemical characteristics of those substances are described. Furthermore, the relationships between the chemical structure and electrolyte excretion, pharmacokinetics, metabolism and toxicity are demonstrated. An expanded model regarding the mode of action of triamterene and its derivatives is also presented.

Effects of synthetic urokinase inhibitors on local invasion and metastasis in a murine mammary tumor model

Urokinase-type plasminogen activator (uPA) initiates an extracellular proteolytic cascade with which invasive cells eliminate barriers to movement. We have evaluated the antiinvasive and antimetastatic properties of two recently developed synthetic uPA inhibitors, B428 and B623, in a BALB/c mouse mammary carcinoma model. We used the F3II and M3 tumor cell lines, previously described by our laboratory. In vitro, noncytotoxic concentrations of B428 or B623 inhibited secreted and cell-associated uPA activity produced by tumor cells and blocked uPA-mediated whole tumor cell degradation of fibronectin, allowing deposition of extracellular fibronectin fibrils. In vivo, administration of compounds was not associated with overt toxic effects. Daily i.p. treatment with B428 (20 mg/kg/day) or B623 (7.5 mg/kg/day) for 2 weeks, beginning after tumor take, markedly blocked the invasion of the muscle and adipose layers of the subcutis and dermis in mice bearing highly invasive F3II tumors. However, these compounds neither inhibited tumor-induced angiogenesis nor reduced the incidence of spontaneous lung metastasis. Moreover, B623 enhanced the formation of experimental lung metastasis. Our results suggest that synthetic uPA inhibitors act as potent antiinvasiveness agents in vivo but may be unable to control progression of the metastatic disease in the present mammary tumor model.

Blockade of epithelial Na+ channels by triamterenes - underlying mechanisms and molecular basis

The three subunits (alpha, beta, gamma) encoding for the rat epithelial Na+ channel (rENaC) were expressed in Xenopus oocytes, and the induced Na+ conductance was tested for its sensitivity to various triamterene derivatives. Triamterene blocked rENaC in a voltage-dependent manner, and was 100-fold less potent than amiloride at pH 7.5. At -90 mV and -40 mV, the IC50 values were 5 microM and 10 microM, respectively. The blockage by triamterene, which is a weak base with a pKa of 6.2, was dependent on the extracellular pH. The IC50 was 1 microM at pH 6.5 and only 17 microM at pH 8.5, suggesting that the protonated compound is more potent than the unprotonated one. According to a simple kinetic analysis, the apparent inhibition constants at -90 mV were 0.74 microM for the charged and 100.6 microM for the uncharged triamterene. The main metabolite of triamterene, p-hydroxytriamterene sulfuric acid ester, inhibited rENaC with an approximately twofold lower affinity. Derivatives of triamterene, in which the p-position of the phenylmoiety was substituted by acidic or basic residues, inhibited rENaC with IC50 values in the range of 0.1-20 microM. Acidic and basic triamterenes produced a rENaC blockade with a similar voltage and pH dependence as the parent compound, suggesting that the pteridinemoiety of triamterene is responsible for that characteristic. Expression of the rENaC alpha-subunit-deletion mutant, Delta278-283, which lacks a putative amiloride-binding site, induced a Na+ channel with a greatly reduced affinity for both triamterene and amiloride. In summary, rENaC is a molecular target for triamterene that binds to its binding site within the electrical field, preferably as a positively charged molecule in a voltage- and pH-dependent fashion. We propose that amiloride and triamterene bind to rENaC using very similar mechanisms.

BIBW22 BS, potent multidrug resistance-reversing agent, binds directly to P-glycoprotein and accumulates in drug-resistant cells

The expression of P-glycoprotein (P-gp) in tumor cells causes a multidrug resistance (MDR) phenotype. P-gp has been shown to mediate the transport of structurally dissimilar drugs across the cell membrane in an energy-dependent manner. In this report, we show that BIBW22 BS, a phenylpteridine analog, reverses the MDR phenotype of CEM human lymphoma cells in a dose-dependent fashion. Using a photoactive analog of BIBW22 BS {[3H]azido-4-[N-(2-hydroxy-2-methylpropyl)-ethanolamino]-2, 7-bis(cis-2,6-dimethyl-morpholino)-6-phenylpteridine}, we show the photoaffinity labeling of a 170-kDa protein in drug-resistant cells immunoprecipitated with P-gp-specific monoclonal antibodies. The photolabeling of P-gp by [3H]azido-BIBW22 BS was specific and saturable. Furthermore, BIBW22 BS, vinblastine, and verapamil, but not colchicine, inhibited the photolabeling of P-gp by [3H]azido-BIBW22 BS. Drug binding studies showed that membranes from MDR cells bound more BIBW22 BS than parental drug-sensitive cells, and this binding was inhibited with vinblastine and, to a lesser extent, with uridine. However, drug transport studies demonstrated that BIBW22 BS is not a substrate for P-gp efflux pump. Interestingly, BIBW22 BS was shown to accumulate more in resistant cells. Also, BIBW22 BS accumulation in drug-sensitive and -resistant cells was not energy dependent. These results are in contrast with the observed decrease in accumulation or enhanced efflux of [3H]vinblastine seen in the same MDR cells. A comparison of [3H]azido-BIBW22 BS or [3H]azidopine photolabeled P-gp by Cleveland mapping with Staphylococcus aureus V8 protease showed differences in the photolabeled peptides. Taken together, the results of this study show that BIBW22 BS is a potent MDR-reversing agent that binds directly to P-gp but is not effluxed from drug-resistant cells.

Triamterene suppresses bombesin-enhanced peritoneal metastasis of intestinal adenocarcinomas induced by azoxymethane

The effects of combined administration of bombesin and the diuretic triamterene on the incidence of peritoneal metastasis of intestinal cancers induced by azoxymethane (AOM) and the labeling index of intestinal cancers were investigated in male inbred Wistar rats. From the start of the experiment, rats were given weekly s.c. injections of AOM (7.4 mg/kg body weight) for 10 weeks and s.c. injections of bombesin (40 micrograms/kg body weight) every other day, and from week 16, s.c. injections of triamterene (10 and 20 mg/kg body weight) every other day until the end of the experiment in week 45. Bombesin significantly increased the incidence of intestinal tumors and cancer metastasis to the peritoneum in week 45. It also significantly increased the labeling index of intestinal cancers. Although administration of both doses of triamterene with bombesin had little or no influence on the enhancement of intestinal carcinogenesis by bombesin, or the location, histologic type, depth of invasion, or labeling index of intestinal cancers, it significantly reduced the incidence of cancer metastasis. These findings indicate that triamterene suppresses cancer metastasis through a mechanism that does not affect the proliferation of intestinal cancers.

Modulation of multidrug resistance by BIBW22BS in blasts of de novo or relapsed or persistent acute myeloid leukemia ex vivo

The phenylpteridine derivative BIBW22BS (BIBW22) is a potent modulator of multidrug resistance (MDR). We investigated BIBW22 in comparison to dexniguldipine and verapamil as modifier of MDR in blasts of de novo, relapsed or persistent acute myeloid leukemia (AML) in vitro. All patients with relapsed or persistent AML had been pretreated with idarubicin and cytosine arabinoside. The degree of MDR was determined by efflux kinetics of rhodamine 123 (R123), daunorubicin, and idarubicin measured by flow cytometry (FACS). A total of 51 patients with AML, 25 de novo and 26 relapsed or persistent, were investigated. While only 6 out of 25 de novo AML blast populations showed moderate efflux of R123 and daunorubicin, 17 out of 26 blast populations of relapsed or persistent AML had an efflux between 20% and 44% within 15 min ex vivo. This efflux could be significantly inhibited by 1 microM BIBW22, 1 microM dexniguldipine, or 10 microM verapamil. For idarubicin we found an effusion of 40+/-9% within 15 min in all blast populations that could not be inhibited by the modulators. Clinically achievable drug concentrations causing only moderate side-effects are in the range of 0.5 microM dexniguldipine and 3 microM verapamil. Up to now, BIBW22 has not been investigated clinically. Thus the potential toxicity of concentrations of 0.5-1 microM BIBW22, sufficient for an optimal efflux inhibition ex vivo, is not known yet. We conclude from our ex vivo investigations in blast populations of de novo, relapsed or persistent AML that BIBW22 is a potent modulator of MDR.

Potassium and magnesium retaining potency and efficacy of triamterene and analogues in conscious saline-loaded rats

The effect of triamterene and analogues on the acute renal excretion of K+ and Mg2+ was investigated in conscious saline-loaded rats. The maximum retention (Emax) value of the potassium and magnesium retaining efficacy is almost independent of the structure of the compound and depends only on the initial basal value of ion excretion (E0). Triamterene and analogues display a comparable magnesium retaining efficacy. This can be explained by activity against the same renal Mg2+ transportation system. There was a relation between the ED50 values for the potassium and magnesium retaining properties of triamterene derivatives. Both the potassium and the magnesium retaining potency are dependent on the basicity as well as on the lipophilicity of the side chain of the triamterene derivatives. The part of the molecule responsible for the antimagnesiuretic properties of the derivatives seems to be sensitive to steric influences.

The ion conductances of colonic crypts from dexamethasone-treated rats

Whole-cell patch-clamp studies were performed in isolated colonic crypts of rats pretreated with dexamethasone (6 mg/kg subcutaneously on 3 days consecutively prior to the experiment). The cells were divided into three categories according to their position along the crypt axis: surface cells (s.c.); mid-crypt cells (m.c.) and crypt base cells (b.c.). The zero-current membrane voltage (Vm) was -56 +/- 2 mV in s.c (n = 34); -76 +/- 2 mV in m.c. (n = 47); and -87 +/- 1 mV in b.c. (n = 87). The whole-cell conductance (Gm) was similar (8-12 nS) in all three types of cells. A fractional K+ conductance accounting for 29-67% of Gm was present in all cell types. A Na+ conductance was demonstrable in s.c. by the hyperpolarizing effect on Vm of a low-Na+ (5 mmol/l) solution. In m.c. and b.c. the hyperpolarizing effect was much smaller, albeit significant. Amiloride had a concentration-dependent hyperpolarizing effect on Vm in m.c. and even more so in s.c.. It reduced Gm by approximately 12%. The dissociation constant (KD) was around 0.2 micromol/l. Triamterene had a comparable but not additive effect (KD = 30 micromol/l, n = 14). Forskolin (10 micromol/l, in order to enhance cytosolic adenosine 3', 5'-cyclic monophosphate or cAMP) depolarized Vm in all three types of cells. The strongest effect was seen in b.c.. Gm was enhanced significantly in b.c. by 83% (forskolin) to 121% [8-(4-chlorophenylthio)cAMP]. The depolarization of Vm and increase in Gm was caused to large extent by an increase in Cl-conductance as shown by the effect of a reduction in bath Cl-concentration from 145 to 32 mmol/l. This manoeuvre hyperpolarized Vm under control conditions significantly by 6-9 mV in all three types of cells, whilst it depolarized Vm in the presence of forskolin in m.c. and in b.c.. These data indicate that s.c. of dexamethasone-treated rats possess mostly a K+ conductance and an amiloride- and triamterene-inhibitable Na+ conductance. m.c. and b.c. possess little or no Na+ conductance; their Vm is largely determined by a K+ conductance. Forskolin (via cAMP) augments the Cl- conductance of m.c. and b.c. but has only a slight effect on s.c.

The interaction between short-term exercise training and a diuretic-induced hypovolemic stimulus

Nine healthy untrained males [mean (SEM) age, 20.2 (1) years; peak oxygen uptake (VO2max, 48.2 (2) ml.kg-1.min-1] took part in a study to examine whether short-term exercise training (cycle exercise 2 h.day-1 for 3 days at 60% VO2max), which normally results in an expansion of plasma volume (PV), can counteract a diuretic-induced hypovolemic stimulus (100 mg triamterene + 50 mg hydrochlorothiazide.day-1 for 5 days concurrent with exercise training) and restore PV to control levels. Resting and exercise responses (90 min, 60% VO2max) in the diuretic plus exercise training condition (D+E) were compared to a control (C) and a diuretic (D) condition in which no exercise was performed. Following the short-term training, PV was still decreased (P < 0.05) below C by -8.3 (3)% in D+E and was similar (P > 0.05) to the reduction in D [-12.4 (2)%]. The reduced PV in response to the diuretic was associated with similar (P > 0.05) elevations in resting aldosterone (ALDO) and norepinephrine (NOREPI) levels (ng.100 ml-1) in D [101 (12), 61 (4)] and D+E [85 (16), 60 (10)] above (P < 0.05) C [22 (5), 37 (4)]. During exercise, ALDO levels were increased (P < 0.05) by 66 (5) and 70 (10) ng.100 ml-1 in D and D+E, respectively, and the increase was greater (P < 0.05) than C [44 (8) ng.100 ml-1]. The rise in NOREPI during exercise was lower (P < 0.05) in D+E [164 (44) ng.100 ml-1] than in D [244 (24) ng.100 ml-1] with levels similar to C [176 (25) ng.100 ml-1]. Thus, the ALDO response to the diuretic was heightened at rest and during exercise but was not additionally affected by the short-term training session. Results suggest that 3 days of exercise training are unable to counteract the hypovolemic effects of a diuretic and restore PV to control levels despite chronic elevations in NOREPI and ALDO.

The influence of BIBW22BS, a dipyridamole derivative, on the antiproliferative effects of 5-fluorouracil, methotrexate and gemcitabine in vitro and in human tumour xenografts

Dipyridamole is known as a potent inhibitor of facilitated diffusion-mediated nucleoside transport as well as a modulator of 'classical' multidrug resistance. BIBW22BS, a derivative of dipyridamole, has been found to be 20- to 100-fold more potent in the reversal of multidrug resistance when compared to the parent compound. In parallel, we studied the efficacy of BIBW22BS in the modulation of the antiproliferative effects of 5-fluorouracil, methotrexate and gemcitabine in human cancer cell lines. BIBW22BS, at non-toxic concentrations up to 1.0 microM, increased the antiproliferative effects of 5-fluorouracil 2- to 6-fold in seven of the eight colon cancer cell lines tested in a dose-dependent manner. The addition of 1.0 microM BIBW22BS to methotrexate resulted in a slight increase in the antiproliferative effects, but inhibited the activity of gemcitabine 30- to 100-fold in various cancer cell lines. In vitro, no notable difference was found between BIBW22BS and dipyridamole in their capacity to modulate the activity of the antimetabolites studied. BIBW22BS did not affect the growth inhibition induced by 5-fluorouracil or gemcitabine in human tumour xenografts grown subcutaneously in nude mice. We confirmed the higher potency of BIBW22BS when compared to dipyridamole in the reversal of drug resistance in the Pgp-positive COLO 320 cell line.

[Effects of potassium chloride and triamterene on kaliuresis, natriuresis and diuresis in congestive heart failure]

KCl and triamterene were given to 52 patients (27 males and 25 females with CHF) aged 21-82 years. Large-dose KCl (6 g daily) apparently stimulate kaliuresis, natriuresis and diuresis. As a rule, this is associated with a rise or a fall of plasma potassium concentrations. In the latter case potassium elimination runs more actively and may involve both introduced and body's potassium. As low and moderate doses of KCl (1-4 g daily) showed neither natriuretic nor diuretic effects while enhanced kaliuresis occurred in half of the patients only, did not lower potassium concentrations in plasma they are preferable. Daily treatment with triamterene is characterized by inhibition of its K-retaining action on administration day 2-3. Within first 2 days of triamterene discontinuation all the retained K eliminates from the body. Diuretics-induced metabolic alkalosis decreased intensification of natriuresis and diuresis resultant from large-dose KCl treatment.

Biochemical modulation of 'classical' multidrug resistance by BIBW22BS, a potent derivative of dipyridamole

BACKGROUND

Modulators of the 'classical' multidrug resistance (mdr) phenotype have low efficacy in patients with solid tumors. We analyzed BIBW22BS, 4-[N-(2-hydroxy-2-met- hyl-propyl)-ethanolamino]-2,7-bis(cis-2,6-dimethyl-morpho- lino)-6-phenylpteridine, a derivative of dipyridamole, for its higher potential to modulate mdr.

MATERIALS AND METHODS

Four human malignant cell lines: BRO, A2780, GLC4, SW1573, the Pgp-positive sublines: BRO/mdr1.1, 2780AD and the non-Pgp sublines: GLC4/ADR, SW1573/2R120 were used in vitro to investigate BIBW22BS as a modulator of the antiproliferative effects of vincristine and doxorubicin and to compare the potency of BIBW22BS with that of dipyridamole, verapamil, bepridil and flunarizine. BRO/mdr1.1 s.c. well-established xenografts in nude mice were used to study the modulating properties of BIBW22BS 50 mg/kg i.v. followed after one h by vincristine 1 mg/kg i.p. or doxorubicin 8 mg/kg i.p. weekly x 2.

RESULTS

BIBW22BS was 20- to 100-fold more potent than dipyridamole in the reversal of resistance in the Pgp-positive sublines. Reversal of resistance was obtained in a dose-dependent manner and was complete at concentrations of 0.5-2.5 microM. At non-toxic, equimolar concentrations of 1.0 microM BIBW22BS showed higher modulating potency than the calcium-channel blockers. BIBW22BS did not affect resistance in the non-Pgp sublines. BRO/mdr1.1 s.c. xenografts have stable multidrug-resistance characteristics upon serial transplantation. BIBW22BS, vincristine, or doxorubicin as single agents were not effective in vivo, while the addition of BIBW22BS could significantly reduce the tumor growth expressed as the T/C% of vincristine from 109% to 48% and that of doxorubicin from 55% to 32%. However, reversal of vincristine resistance in BRO/mdr1.1 xenografts was not complete when compared to the efficacy of vincristine in BRO xenografts.

CONCLUSION

The results encourage the further preclinical development of BIBW22BS as a modulator of 'classical' multidrug resistance in cancer patients.

Improvement of the diuretic effect of triamterene via solid dispersion technique with PEG 4000

The present investigation was designed and undertaken in order to substantiate further contention concerning the universality of the utilization of PEG polymers as matrix carriers. This study could then be considered an attempt to enhance the dissolution rate of triamterene, with the subsequent enhancement in its absorption rate, via solid dispersion using PEG 4000. The approach of solid dispersions was found useful for optimizing the pharmacokinetic of triamterene in rats.

Suppression of GABAB receptor function in rat neocortical slices by amiloride

Interactions of amiloride with GABAB receptors have been examined using spontaneously discharging rat neocortical slices. These discharges were suppressed by the GABAB receptor agonist baclofen (10 microM), and were prevented by amiloride and its analogs 5-(N,N-dimethyl)-amiloride, 5-(N-methyl-N-isobutyl)-amiloride and benzamil, but not by triamterene (100-500 microM). Each of these also increased the spontaneous discharge rate and reduced the discharge amplitude. The action of amiloride and its analogs in preventing the action of baclofen, may involve allosteric modification of the receptor binding sites via guanine nucleotide-binding proteins, or an indirect effect through antagonism of co-activated adenosine A1 receptors.

Triamterene inhibits the delayed rectifier potassium current (IK) in guinea pig ventricular myocytes

In humans, proarrhythmia during therapy with action potential-prolonging drugs can be associated with hypokalemia often provoked by concomitant administration of diuretic agents. Consequently, therapy with class III antiarrhythmics and K(+)-sparing diuretics, such as triamterene, may be indicated. Triamterene, along with its K(+)-sparing properties, exhibits other pharmacological effects. In the heart, it can increase action potential duration (guinea pig atria and papillary muscles), protect against reperfusion-induced arrhythmias (rat), and increase the QT interval (humans). Therefore, studies were undertaken to assess effects of triamterene on cardiac K+ repolarizing currents. Guinea pig ventricular myocytes were superfused at 30 degrees C with Cd(2+)-containing solution to block Isi and held at -40 mV to inactivate INa. Currents were measured in the whole-cell configuration of the patch-clamp technique. The delayed rectifier outward current (IK) was elicited by short (250-millisecond) and long (5000-millisecond) depolarizing pulses, and time-independent currents were assessed by a rapid ramp test protocol. After high-voltage long pulses (+50 mV; 5000 milliseconds), tail current amplitude of the slow component of IK (IKs) was decreased 36 +/- 6% (n = 6) and 51 +/- 8% (n = 6) by triamterene 10(-5) and 10(-4) mol/L, respectively. After low-voltage short pulses (-20 mV; 250 milliseconds), tail current amplitude corresponding essentially to the rapid component of IK (IKr) was decreased only 14 +/- 11% (n = 9) and 19 +/- 10% (n = 10) by triamterene 10(-5) and 10(-4) mol/L, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Unknown effects of clinically used drugs as bias in clinical trials: antihistaminergic activity of the antihypertensive fixed-dose combination Betathiazid

In a controlled clinical trial on histamine release in anaesthesia, it was suspected that the antihypertensive fixed-dose combination drug Betathiazid masked clinical signs of histamine release. By structure analysis of its constituents (propranolol, triamterene and hydrochlorothiazide), hydrochlorothiazide was considered to be most likely an H1-antagonist. An aqueous solution of the whole drug tablet (2 x 10(-4) M propranolol, 2.9 x 10(-5) M triamterene, 1.7 x 10(-5) M hydrochlorothiazide) and of the individual substances (1 microM each) was tested in the classical H1-receptor assay using the guinea pig ileum. Betathiazid in total suppressed the contraction to histamine (78% inhibition), but not to carbachol. Propranolol and triamterene had depressive effects (14% and 38% inhibition), but hydrochlorothiazide potentiated the contractions to histamine (75% potentiation). In all cases, the type of antagonism was not competitive. Although different mechanisms may account for the modulatory effects of Betathiazid, they have to be considered in the interpretation of clinical studies, especially for relating mediator concentrations with clinical signs.

Chloroquine resistance in Plasmodium falciparum is not reversed by BIBW-22, a compound reversing the multidrug resistance phenotype in mammalian cancer cells

The pteridine derivative BIBW-22 (4-[N-(2-hydroxy-2-methyl-propyl)-ethanolamino]-2,7-bis(cis-2,6-di methyl-morpholino)-6-phenylpteridine), which had been developed for the treatment of multidrug-resistant cancer and binds to P-glycoprotein, was tested against chloroquine resistant Plasmodium falciparum strains in culture. Based on the result that BIBW-22 enhanced rather than lowered chloroquine resistance in vitro, it is concluded that chloroquine resistance in malaria parasites may not be mechanistically linked to the multidrug-resistant phenotype of chloroquine resistant P. falciparum.

BIBW 22, a dipyridamole analogue, acts as a bifunctional modulator on tumor cells by influencing both P-glycoprotein and nucleoside transport

BIBW 22, a phenylpteridine analogue of dipyridamole (DPM), enhanced vincristine cytotoxicity approximately 10 times more than DPM in a multidrug-resistant (MDR) KB V20C cell line. Using rhodamine 123 accumulation in KB V20C cells as an indicator of MDR phenotype, BIBW 22 was shown to be about 100 times more potent than DPM in inhibiting the MDR-associated efflux of rhodamine 123. Photolabeling of P-glycoprotein in KB V20C plasma membranes with 0.2 microM [3H]azidopine was strongly inhibited by 1 microM BIBW 22, indicating that this compound reverses the MDR phenotype by interfering with MDR-associated P-glycoprotein. In addition, BIBW 22 at 1 microM could also enhance the cytotoxicity of 5-fluorouracil in KB cells about 20-fold. Its potency in inhibiting nucleoside transport is 7-fold more potent than that of DPM. These results suggest that BIBW 22 is a potent bifunctional modulator which influences both P-glycoprotein and nucleoside transport in tumor cells. Potential use of this compound as a modulator of combination chemotherapy involving antimetabolites and drugs affected by MDR should be explored.

[The effect of triamterene on the ERG of Long-Evans rats]

Dopamine is an important mediator of the neuronal adaptation mechanism of the retina. Tetrahydrobiopterin (BH4) is cofactor of the rate-limiting enzymatic tyrosine-hydroxylase step in the biosynthetic pathway of dopamine. This enzymatic step causes an oxidation of BH4 to dihydrobiopterin (BH2), which is reduced again to BH4 by action of dihydropterin-reductase (DHPR) and dihydrofolate-reductase (DHFR). Triamterene, a pterin derivate, is known to induce DHFR after initial inhibition. Concomitant elevation of retinal BH4 will result in a higher dopamine concentration. ERG changes of Long-Evans rats mediated by this triamterene effect on dopamine concentration were examined after the rats received triamterene with tap water in two experimental series for 5 and 11 days. After 5 days of triamterene administration a significant decrease of scotopic a- and b-wave ERG amplitudes were registered. After 11 days of triamterene application the decrease could be shown to be even more substantial. This effect was completely reversible: 10 weeks after cessation of triamterene supply ERG amplitudes increased to previous levels. The observed transient ERG changes support the concept of biopterin as an mediator of neuronal adaptation mechanisms of the retina.

Synthesis, natriuretic, antikaliuretic and antimagnesiuretic properties of an acidic triamterene derivative

2,4,7-Triamino-6-(4-methanesulfonamidophenyl) pteridine (RPH 3048) is a new acidic triamterene derivative. Relevant physico-chemical constants were determined (solubility at pH 7.4 = 3.7 mg/l; logP at pH 7.4 = 0.2) and pharmacokinetic as well as pharmacodynamic properties were investigated, using male Wistar rats. After intravenous application of the test substances urine was collected, its volume and electrolyte composition determined, and the urine recovery of the drugs was analysed. The comparison of RPH 3048 with triamterene (CAS 396-01-0) revealed almost equipotent natriuretic and potassium-retaining effects for both drugs and an additional relative magnesium-sparing activity of RPH 3048. The urine recovery of RPH 3048 after 6 h was higher (20.6%) than that of triamterene (12.9%). No metabolite of RPH 3048 could be detected in the urine whereas a triamterene metabolite was found. Due to its good solubility in alkaline medium RPH 3048 could be dissolved (at pH 11-12) and then administered intravenously together with a loop diuretic (furosemide). Urinary electrolyte excretion following administration of two different combinations of RPH 3048 and furosemide (combination A: 12.5 mumol/kg RPH 3048 and 25 mumol/kg furosemide; combination B: 25 mumol/kg RPH 3048 and 25 mumol/kg furosemide) was compared to urinary electrolyte excretion of a control group and a group only treated with furosemide (25 mumol/kg). The additional application of RPH 3048 reduced in both groups potassium and magnesium excretion to control level but did not compromise furosemide induced natriuresis. In contrast to earlier investigations these results suggest that it is possible to develop acidic triamterene derivatives with potent antikaliuretic effects.

Effects of a new pteridine derivative on urinary sodium, potassium and magnesium excretion in conscious saline-loaded rats

1. Two recently synthesized pteridine derivatives (RPH 3036; RPH 3038) were tested in conscious saline-loaded rats and showed natriuretic and antimagnesiuretic properties but hardly reduced potassium excretion. 2. In the same model a dose-response curve was performed for RPH 3036. ED50 and Emax values were calculated for the natriuretic (ED50 = 13.4 mumol kg-1; Emax = 1.08 mmol kg-1) and antimagnesiuretic (ED50 = 11.3 mumol kg-1; Emax = -0.099 mmol kg-1) properties of RPH 3036. There were no significant changes of potassium and calcium excretion. 3. After a single dose of RPH 3036 (100 mumol kg-1) the time course of electrolyte excretion was analysed over 6 h. RPH 3036 did not show any significant effects on renal potassium and calcium excretion whereas a pronounced decrease (P less than 0.01) in renal magnesium excretion was evident during the 6 h. A moderate increase of sodium excretion was observed only after 3, 5 and 6 h. 4. A selective reduction of magnesium secretion in the late distal tubule and collecting duct was proposed as a possible mechanism of action of RPH 3036. This would explain the fast onset of action as well as the lack of antikaliuretic and anticalciuretic effects. The high selectivity of RPH 3036 makes it potentially valuable for the future investigation of renal magnesium transport.

Triamterene may preserve lymphocyte magnesium and potassium in patients with congestive heart failure

Electrolyte abnormalities are a frequent and potentially hazardous complication in the treatment of patients with congestive heart failure. Medical treatment with diuretics and/or digitalis, as well as neurohumoral activation most likely initiated by the compromised cardiac function, contributes to this alteration. The addition of potassium- and magnesium-sparing diuretics (triamterence, amiloride) to therapy with frusemide or hydrochlorothiazide is of possible value in preventing intracellular electrolyte abnormalities.

The effects of frusemide and triamterene on the hypokalaemic and electrocardiographic responses to inhaled terbutaline

Fifteen normal volunteers were given 5000 micrograms inhaled terbutaline following three separate 4 day oral treatment periods with placebo, frusemide 40 mg, and frusemide 40 mg plus triamterene 50 mg. Serum potassium (K), and electrocardiographic (ECG) responses were measured after 30 min rest and 30 min after inhalation of terbutaline. Frusemide produced significant hypokalaemia compared with placebo (means and 95% CI): 3.58 mmol l-1 (3.5-3.66) vs 3.88 mmol l-1 (3.8-3.96) (P less than 0.001), and this effect was significantly attenuated by the addition of triamterene: 3.80 mmol l-1 (3.72-3.88) (P less than 0.05). Terbutaline alone also caused significant hypokalaemia: from 3.88 mmol l-1 (3.8-3.96) to 3.35 mmol l-1 (3.24-3.46) (P less than 0.001), and a lower absolute level of K was seen when combined with frusemide: 3.13 mmol l-1 (3.02-3.24) (P less than 0.05). The addition of triamterene conferred no significant protection against the combined hypokalaemia: 3.29 mmol l-1 (3.18-3.4). Changes in T wave amplitude during the study periods showed a similar pattern of response to the hypokalaemic effects. These results show that the hypokalaemic response to terbutaline was additive to that of frusemide, and that triamterene attenuated the hypokalaemic response to frusemide, but not terbutaline.

Antiarrhythmic properties of triamterene derivatives in the coronary artery ligated rat model

Triamterene and several triamterene derivatives were tested for antiarrhythmic activity in the coronary artery ligated and reperfused (CAL-R) rat. The class-III antiarrhythmic drugs (+/-)-sotalol and amiodarone, the class-I antiarrhythmics lidocaine and quinidine as well as the potassium sparing diuretic amiloride were used as reference drugs. Triamterene at the highest dose (30 mumol/kg) revealed a 100% protection against ventricular fibrillation (VF), whereas at 10 mumol/kg no antiarrhythmic activity for triamterene could be found. For compound 4 (10 mumol/kg) a 75% protection against VF could be demonstrated, while 2, 3, and 5 revealed only a 25% protection. Compared to the reference drugs, triamterene and the derivatives 2-5 are more potent than (+/-)-sotalol, but less potent than lidocaine, quinidine and amiodarone. For amiloride as well as for the potent potassium retaining triamterene derivative 6 no antiarrhythmic activity could be shown. Therefore, we conclude different mechanisms responsible for the potassium sparing and antiarrhythmic properties of triamterene and its derivatives.

[Animal experimental and human pharmacologic studies with phase-II metabolites of triamterene]

Animal Experiments and Human Studies with the Phase-II Metabolite of Triamterene The described animal experiments as well as the pharmacological investigations in man show the diuretic efficacy of the phase-II metabolite of triamterene (CAS 396-01-0), OH-TA-ester. As demonstrated in the present investigation, a phase-II metabolite can not be considered only as an ineffective conjugation product, formed to accomplish the excretion of a foreign compound, but also it must be considered as an important factor in the therapy, if its pharmacologic efficacy has been established, moreover if as a result of disease-related alterations in the pharmacokinetics the phase-II metabolite cumulates.

Antiarrhythmic properties of benzyl-triamterene derivatives in the coronary artery ligated and reperfused rat

Triamterene (CAS 396-01-0) and a series of benzyl-triamterene derivatives were evaluated for their antiarrhythmic properties in the coronary artery ligated and reperfused (CAL-R) rat. The effects were compared with the antiarrhythmic activity of the potassium sparing diuretic amiloride and drugs out of the class-I (lidocaine) and class-III (amiodarone and sotalol). Triamterene and sotalol revealed at high doses antifibrillator activity, while the benzyl-triamterenes 2, 3, 5 and 6 could also depress ventricular extrasystoles (VES) and ventricular tachycardia (VT). At low doses the most benzyltriamterenes protected significantly against ventricular fibrillation (VF) and so they were equieffective or more effective than amiodarone or lidocaine. Amiloride showed in the CAL-R rat no antiarrhythmic activity, so that we conclude different mechanisms responsible for antikaliuretic and antiarrhythmic properties of amiloride and triamterenes. Taking into account the results of recently reported in vitro studies, where we could demonstrate antiarrhythmic activity combined with positive inotropic properties for triamterenes, the antiarrhythmic profile of these compounds may offer new possibilities for the treatment of ventricular arrhythmias.

Behavioral effects of chronic, orally administered diuretic and verapamil in baboons

Behavioral performances of six baboons were tested during chronic oral dosing with diuretic (hydrochlorothiazide/triamterene), a calcium channel blocker (verapamil), and a combination of the two drugs. Reaction times and color matching-to-sample performances as well as physiological measures were obtained in deoxycorticosterone acetate (DOCA)-salt baboons and in renovascular hypertensive baboons. Combined diuretic and verapamil impaired color matching to a small degree in comparison to baseline performance, while drug administered alone had no effect. Weekly systolic and diastolic blood pressures decreased maximally from baseline during the drug combination period, and were accompanied by maximal increases in serum sodium. The largest behavioral impairments during combination dosing were observed for colors that were most difficult to discriminate during baseline. Significant positive correlations were found between systolic blood pressure and color matching accuracy. No differences between the animal hypertension groups were found as a function of drug condition either in physiological or behavioral responses. Only the combination of diuretic and verapamil produced a deleterious effect on color discrimination, which suggests further study of commonly administered drug combination therapies in hypertension.

In vitro testing of triamterene derivatives for antiarrhythmic activity

A series of para-substituted triamterene derivatives (Table 1) were evaluated for their antiarrhythmic properties in vitro. Pharmacological evaluation of the compounds and some class-I- (quinidine, lidocaine, and propafenone) as well as class-III-antiarrhythmic drugs ((+/-)-sotalol and amiodarone) was carried out by measuring the functional refractory period (FRP), the maximal driving frequency (MDF) and the force of myocardial contractions (FC) of electrically stimulated guinea pig atria. The increase in FRP and the decrease in MDF was most pronounced with the class-I-antiarrhythmic drugs, but these compounds showed the typical negative inotropic effects, too. For the class-III-antiarrhythmics only a weak influence on FRP and MDF could be demonstrated, while FC was not altered in the presence of (+/-)-sotalol and amiodarone. Neutral substituted triamterenes like compounds 2-5 as well as most of the benzyltriamterene derivatives showed similar or stronger effects on FRP and MDF as (+/-)-sotalol and amiodarone. With the exception of 4 and 5, these effects were combined with an increase of FC. Compounds 6 and 7, well known-potent diuretics, showed no influence to FRP, MDF and FC. Therefore, we conclude different mechanisms for the antikaliuretic and cardiac activity.

Comparison of the diuretic effect and absorption of a single dose of furosemide and free and the fixed combinations of furosemide and triamterene in healthy male adults

The absorption and diuretic effect of furosemide 40 mg alone (F), and of the free (F + T) and the fixed (FT) combinations of furosemide 40 mg and triamterene 50 mg have been compared in 12 healthy young men. A slight reduction in the area under the concentration-time curve (AUC) of plasma furosemide was found for the fixed combination (AUC480) F2.58 micrograms.h.ml-1; F + T 2.46 micrograms.h.ml-1; FT 1.97 micrograms.h.ml-1. There was a significant reduction in the AUC480 of plasma triamterene (F + T 204.9 micrograms.h.l-1; FT 130.2 micrograms.h.l-1). Sodium excretion after F + T and FT was more pronounced than after F (F + T 302 mmol; FT 311 mmol; F259 mmol). When compared to F alone, there was a reduction in the 24-hour potassium excretion after F + T as well as after FT (F 121 mmol; F + T 104 mmol; FT 107 mmol). It is concluded that the absorption of triamterene was significantly reduced after ingestion of the fixed combination tablet. However, in healthy male adults this had no influence on its natriuretic and potassium-sparing effect as compared to the free combination.

Absence of magnesium sparing effect of a single dose of triamterene in combination with frusemide in healthy male adults

In a study of cross-over randomized design in 12 healthy volunteers we compared the effect on magnesium excretion after administration of single oral doses of frusemide 40 mg (F) and of frusemide 40 mg in combination with a single dose of triamterene 50 mg (F+T). F as well as F+T induced significant increases in volume, sodium and magnesium mean 24 h output as compared with 24 h renal excretion without medication. Addition of triamterene to frusemide significantly reduced the 24 h potassium excretion, but did not attenuate the excretion of magnesium. The time to reach the maximum magnesium excretion rate after F as well as after F+T administration was similar to that of the sodium excretion rate. We conclude that triamterene does not inhibit the magnesiuric effects of frusemide in healthy male adults.

Hypokalemic and ECG sequelae of combined beta-agonist/diuretic therapy. Protection by conventional doses of spironolactone but not triamterene

Salbutamol (Albuterol) and diuretics are commonly prescribed together in patients with airflow obstruction and are associated with electrocardiographic effects. We have now investigated whether the use of potassium-sparing drugs might prevent the ECG sequelae of such combined therapy. Ten healthy subjects received seven days of randomized treatments with: placebo, bendrofluazide (5 mg), bendrofluazide plus triamterene 50 mg (conventional dose), or triamterene 200 mg (high dose), and bendrofluazide plus spironolactone (100 mg). Potassium and ECG responses to inhaled salbutamol, 2 mg, were measured after each treatment period. The T-wave flattening in response to bendrofluazide and salbutamol (0.24[CI, 0.19 to 0.29]mV) was attenuated by the addition of triamterene, 200 mg (0.33[CI, 0.28 to 0.37]mV; p less than 0.05) and spironolactone 100 mg (0.42[CI, 0.37 to 0.47]mV; p less than 0.01), but not by triamterene 50 mg (0.25[CI, 0.20 to 0.30]mV). Spironolactone and high dose triamterene also diminished the frequency of U waves and ST depression. The ECG effects mirrored hypokalemic responses which were also blunted by high dose (p less than 0.01) but not low dose triamterene, as well as by spironolactone (p less than 0.001). Thus, the use of high dose triamterene and spironolactone protected against the hypokalemic and ECG sequelae of combined beta-agonist/diuretic therapy, whereas a conventional dose of triamterene had no effect. These findings may be important in the prevention of a potentially dangerous interaction in susceptible patients taking this combination of drugs.

Electrocardiographic changes induced by inhaled salbutamol after treatment with bendrofluazide: effects of replacement therapy with potassium, magnesium and triamterene

1. Twelve normal subjects were given 5 days' treatment with (a) placebo; (b) 5 mg of bendrofluazide; (c) 5 mg of bendrofluazide plus 24 mmol of potassium; (d) 5 mg of bendrofluazide plus 24 mmol of magnesium; (e) 5 mg of bendrofluazide plus 24 mmol of potassium plus 24 mmol of magnesium; (f) 5 mg of bendrofluazide plus 50 mg of triamterene. 2. After each treatment period, subjects were given two doses of inhaled salbutamol (each of 1 mg), and measurements were made 25 min after each dose. 3. Treatment with bendrofluazide plus salbutamol produced a lower absolute level of plasma potassium than salbutamol alone, although the addition of potassium, magnesium or triamterene did not attenuate this effect. Plasma magnesium was unchanged by salbutamol. 4. Salbutamol was associated with significant electrocardiographic effects, including T-wave flattening, Q-Tc interval prolongation and, in some subjects, U-waves and S-T segment depression. 5. Prior treatment with bendrofluazide potentiated the electrocardiographic effects of salbutamol. Supplementation with potassium and magnesium had no effect on electrocardiographic changes, and the addition of triamterene to bendrofluazide only attenuated electrocardiographic responses in some subjects.