Regulation of interferon-inducible proteins by doxorubicin via interferon γ-Janus tyrosine kinase-signal transducer and activator of transcription signaling in tumor cells.

Activation of the immune system is a way for host tissue to defend itself against tumor growth. Hence, treatment strategies that are based on immunomodulation are on the rise. Conventional cytostatic drugs such as the anthracycline doxorubicin can also activate immune cell functions of macrophages and natural killer cells. In addition, cytotoxicity of doxorubicin can be enhanced by combining this drug with the cytokine interferon-γ (IFNγ). Although doxorubicin is one of the most applied cytostatics, the molecular mechanisms of its immunomodulation ability have not been investigated thoroughly. In microarray analyses of HeLa cells, a set of 19 genes related to interferon signaling was significantly over-represented among genes regulated by doxorubicin exposure, including signal transducer and activator of transcription (STAT) 1 and 2, interferon regulatory factor 9, N-myc and STAT interactor, and caspase 1. Regulation of these genes by doxorubicin was verified with real-time polymerase chain reaction and immunoblotting. An enhanced secretion of IFNγ was observed when HeLa cells were exposed to doxorubicin compared with untreated cells. IFNγ-neutralizing antibodies and inhibition of Janus tyrosine kinase (JAK)-STAT signaling [aurintricarboxylic acid (ATA), (E)-2-cyano-3-(3,4-dihydrophenyl)-N-(phenylmethyl)-2-propenamide (AG490), STAT1 small interfering RNA] significantly abolished doxorubicin-stimulated expression of interferon signaling-related genes. Furthermore, inhibition of JAK-STAT signaling significantly reduced doxorubicin-induced caspase 3 activation and desensitized HeLa cells to doxorubicin cytotoxicity. In conclusion, we demonstrate that doxorubicin induces interferon-responsive genes via IFNγ-JAK-STAT1 signaling and that this pathway is relevant for doxorubicin's cytotoxicity in HeLa cells. Immunomodulation is a promising strategy in anticancer treatment, so this novel mode of action of doxorubicin may help to further improve the use of this drug among different types of anticancer treatment strategies.

The growth hormone--IGF-I axis as a mediator for the association between FTO variants and body mass index: results of the Study of Health in Pomerania.

CONTEXT: Risk alleles of the fat mass- and obesity-associated gene (FTO) are related not only to increased body mass index (BMI) values but also to mortality. It was speculated that cellular effects of the FTO gene affect most organs, especially their ability to maintain or regenerate proper function when afflicted by various diseases. FTO is highly expressed in the hypothalamus and also in the pituitary gland. The decrease in growth hormone (GH) secretion is known to cause a decrease in lean body mass in older subjects. OBJECTIVE: We hypothesized an association of rs9926289 with insulin-like growth factor (IGF)-I. DESIGN AND SETTING: Cross-sectional data from the Study of Health in Pomerania, a population-based study in the northeastern part of Germany, were used. PARTICIPANTS: For the final analyses, 3882 subjects aged 20-79 years were available. MAIN OUTCOME MEASURES: Continuous IGF-I, low IGF-I according to clinically meaningful age- and gender-specific reference values, and BMI were used as outcome measures. RESULTS: Over all age groups, a statistically significant relationship between FTO and IGF-I was found. In subjects younger than 55 years of age, homozygous carriers of the FTO risk allele exhibited lower serum IGF-I levels adjusted for 5-year age groups, gender and IGF-I binding protein 3 levels (linear regression, coefficient±s.e. for FTO AA genotype:-8.6±2.8; P=0.002). Further adjustments for obesity and diabetes did not suspend this association (coefficient:-7.8; P=0.005). As expected, the FTO AA genotype effect on BMI was reduced from 0.76 to 0.62 kg m(-2) by including IGF-I. No relationship between FTO and IGF-I levels was found in subjects aged 55 years or older (-2.7±2.4; P=0.260 for FTO AA genotype adjusted for age, gender and IGF-I binding protein 3 levels). CONCLUSION: We propose that the GH-IGF-I axis is a mediator for the relationship between FTO and BMI.

Vardenafil protects isolated rat hearts at reperfusion dependent on GC and PKG.

BACKGROUND AND PURPOSE: The type-5 PDE inhibitor vardenafil reduces myocardial infarct size in situ, following ischemia/reperfusion, when applied at reperfusion in animal models. Little is known about the underlying protective signaling. Here, we test whether vardenafil is protective in rat isolated hearts and in a cell model of calcium stress. EXPERIMENTAL APPROACH: Infarct size in rat isolated hearts was measured after a 30 min regional ischemia and 120 min reperfusion. Vardenafil (1 nM-1 microM) was infused during reperfusion. HL-1 cardiomyocytes were loaded with tetramethylrhodamine ethyl ester (TMRE), a fluorescent marker of mitochondrial membrane potential (psi m). KEY RESULTS: Vardenafil at reperfusion reduced infarct size as percentage of the ischemic zone from 45.8+/-2.0% in control hearts to 26.2+/-2.7% (P<0.001) only at 10 nM, whereas higher or lower dosages failed to protect. This protective effect was blocked by co-administration of either the GC inhibitor, 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), or the PKG inhibitor, KT-5823. HL-1 cardiomyocytes, loaded with TMRE, were treated for 80 min with the calcium ionophore, calcimycin, to induce calcium stress. This reduced the mean cell fluorescence to 63.3 +/- 3.8% of baseline values and vardenafil protected against this fall (78.6 +/- 3.6%, P<0.01). The vardenafil-induced protection of HL-1 cells was blocked by ODQ, KT-5823 or the PKG-inhibiting peptides DT-2 and DT-3, confirming a role for GC and PKG. CONCLUSIONS AND IMPLICATIONS: These results further support the hypothesis that PDE-5 inhibitors are protective in ischemic hearts, in addition to their known clinical effects in the treatment of erectile dysfunction in men.

The role of intestinal P-glycoprotein in the interaction of digoxin and rifampin.

Recent data point to the contribution of P-glycoprotein (P-gp) to digoxin elimination. On the basis of clinical observations of patients in whom digoxin levels decreased considerably when treated with rifampin, we hypothesized that concomitant rifampin therapy may affect digoxin disposition in humans by induction of P-gp. We compared single-dose (1 mg oral and 1 mg intravenous) pharmacokinetics of digoxin before and after coadministration of rifampin (600 mg/d for 10 days) in 8 healthy volunteers. Duodenal biopsies were obtained from each volunteer before and after administration of rifampin. The area under the plasma concentration time curve (AUC) of oral digoxin was significantly lower during rifampin treatment; the effect was less pronounced after intravenous administration of digoxin. Renal clearance and half-life of digoxin were not altered by rifampin. Rifampin treatment increased intestinal P-gp content 3.5 +/- 2.1-fold, which correlated with the AUC after oral digoxin but not after intravenous digoxin. P-gp is a determinant of the disposition of digoxin. Concomitant administration of rifampin reduced digoxin plasma concentrations substantially after oral administration but to a lesser extent after intravenous administration. The rifampin-digoxin interaction appears to occur largely at the level of the intestine. Therefore, induction of intestinal P-gp could explain this new type of drug-drug interaction.

Isoprenoid depletion by statins antagonizes cytokine-induced down-regulation of endothelial nitric oxide expression and increases NO synthase activity in human umbilical vein endothelial cells.

Endothelial dysfunction and atherosclerosis are associated with an inflammation-induced decrease in endothelial nitric oxide synthase (eNOS) expression. Based on the differences between hydrophobic and hydrophilic statins in their reduction of cardiac events, we analyzed the effects of rosuvastatin and cerivastatin on eNOS and inducible NO synthase (iNOS) expression and NOS activity in TNF-alpha-stimulated human umbilical vein endothelial cells (HUVEC). Both statins reversed down-regulation of eNOS mRNA and protein expression by inhibiting HMG-CoA reductase and isoprenoid synthesis. Cerivastatin tended to a more pronounced effect on eNOS expression compared to rosuvastatin. NOS activity - measured by conversion of [(3)H]-L-arginine to [(3)H]-L-citrulline - was enhanced under treatment with both drugs due to inhibition of HMG-CoA reductase. Statin-treatment reduced iNOS mRNA expression under normal conditions, but had no relevant effects on iNOS mRNA expression in cytokine-treated cells. Rosuvastatin and cerivastatin reverse the detrimental effects of TNF-alpha-induced down-regulation in eNOS protein expression and increase NO synthase activity by inhibiting HMG-CoA reductase and subsequent blocking of isoprenoid synthesis. These results provide evidence that statins have beneficial effects by increasing eNOS expression and activity during the atherosclerotic process.

Differential regulation of transport proteins in the periinfarct region following reversible middle cerebral artery occlusion in rats.

Members of various transport protein families including ATP-binding cassette transporters and solute carriers were shown to be expressed in brain capillaries, choroid plexus, astrocytes or neurons, controlling drug and metabolite distribution to and from the brain. However, data are currently very limited on how the expression of these transport systems is affected by damage to the brain such as stroke. Therefore we studied the expression of four selected transporters, P-glycoprotein (Mdr1a/b; Abcb1a/b), Mrp5 (Abcc5), Bcrp (Abcg2), and Oatp2 (Slc21a5) in a rat model for stroke. Transporter expression was analyzed by real-time polymerase chain reaction in the periinfarcted region and protein localization and cellular phenotyping were done by immunohistochemistry and confocal immunofluorescence microscopy. After stroke, P-glycoprotein staining was detected in endothelial cells of disintegrated capillaries and by day 14 in newly generated blood vessels. There was no significant difference, however, in the Mdr1a mRNA amount in the periinfarcted region compared with the contralateral site. For Bcrp, a significant mRNA up-regulation was observed from days 3-14. This up-regulation was followed by the protein as confirmed by quantitative immunohistochemistry. Oatp2, located in the vascular endothelium, was also up-regulated at day 14. For Mrp5, an up-regulation was observed in neurons in the periinfarcted region (day 14). In conclusion, after stroke the transport proteins were up-regulated with a maximum at day 14, a time point that coincides with behavioral recuperation. The study further suggests Bcrp as a pronounced marker for the regenerative process and a possible functional role of Mrp5 in surviving neurons.

Enhanced uptake of doxorubicin into bronchial carcinoma: beta-glucuronidase mediates release of doxorubicin from a glucuronide prodrug (HMR 1826) at the tumor site.

Lack of tumor selectivity is a severe limitation of cancer chemotherapy. Consequently, reducing dose-limiting organ toxicities such as the cardiac toxicity of doxorubicin (Dox) is of major clinical relevance. Approaches that would facilitate a more tumor-selective anticancer therapy by using nontoxic prodrugs that are converted to active anticancer agents at the tumor site have been the subject of intensive research. One potential method to overcome the cardiac toxicity of Dox is to apply a nontoxic, glucuronide prodrug (HMR 1826) from which Dox is released by the action of beta-glucuronidase, an enzyme present at high levels in many tumors. Using a recently developed, isolated, perfused human lung model, we compared the uptake of Dox into normal lung and lung tumors after a 2.5-h lung perfusion with doxorubicin (n = 8) and with the novel doxorubicin glucuronide prodrug (n = 8). Dox showed a poor uptake into lung tumors as compared with normal lung [mean Dox concentration at the end of perfusion, 1.78 +/- 3.11 (median, 0.66) microg/g versus 22.03 +/- 10.4 (median, 18.5) microg/g; P < 0.001]. However, after perfusion with HMR 1826, the level of Dox in tumor tissue was about 7-fold higher than after perfusion with Dox itself [14.04 +/- 12.9 (median, 12.9) microg/g versus 1.78 +/- 3.11 (median, 0.66) microg/g, P < 0.05, n = 8]. In vitro experiments showed a significantly higher beta-glucuronidase expression and activity in the tumors. The extent of in vitro cleavage of HMR 1826 by homogenized lung tissue was closely related to the content of beta-glucuronidase (r = 0.9834, P < 0.0001). When D-saccharolactone, a specific inhibitor of beta-glucuronidase, was added to the perfusate containing HMR 1826, no accumulation of Dox in lung tissue was seen. These data indicate that the high Dox levels achieved in the tumors with HMR 1826 resulted from cleavage of the prodrug by beta-glucuronidase at the tumor site. Thus, the problem of poor Dox uptake into lung tumors could be circumvented by applying the doxorubicin glucuronide prodrug. Several lines of evidence based on both ex vivo and in vitro results indicate that the approach described using a glucuronide prodrug may be useful in facilitating more selective delivery of chemotherapy to tumors in humans.

Elucidation of the mechanism enabling tumor selective prodrug monotherapy.

Elucidation of the mechanism enabling tumor selective PMT in vivo with appropriate glucuronyl-spacer-doxorubicin prodrugs, such as HMR 1826, is important for the design of clinical studies, as well as for the development of more selective drugs. Enzyme histochemistry, immunohistochemistry, and the terminal deoxytransferase technique were applied using human cryopreserved cancer tissues, normal human, monkey, and mouse tissues, and human tumor xenografts to examine mechanisms underlying the selectivity of successful PMT with HMR 1826. It could unambiguously be shown by enzyme histochemistry that necrotic areas in human cancers are the sites in which lysosomal beta-glucuronidase is liberated extracellularly in high local concentrations. The cells responsible for the liberation of the enzyme are mainly acute and chronic inflammatory cells, as shown by IHC. Furthermore, it could be demonstrated that beta-glucuronidase liberated in necrotic areas of tumors can activate HMR 1826, resulting in increased doxorubicin deposition in human tumor xenografts or in human lung cancers subjected to extracorporal perfusion, compared to chemotherapy with doxorubicin. Additionally, the doxorubicin load to normal tissues was significantly reduced compared to chemotherapy with doxorubicin. Surprisingly, the increased doxorubicin deposition in tumors also resulted in strong antitumor effects also in cancers resistant to maximum tolerated doses of systemic doxorubicin. Finally, toxicity studies in mice and monkeys revealed an excellent tolerability of HMR 1826, up to a dose of 3 g/m2 (monkeys). These data suggest that HMR 1826 is a promising candidate for clinical development.

Dose escalation of cyclophosphamide in patients with breast cancer: consequences for pharmacokinetics and metabolism.

PURPOSE: The alkylating anticancer agent cyclophosphamide (CP) is a prodrug that undergoes a complex metabolism in humans producing both active and inactive metabolites. In parallel, unchanged CP is excreted via the kidneys. The aim of this study was to investigate the influence of dose escalation on CP pharmacokinetics and relative contribution of activating and inactivating elimination pathways. PATIENTS AND METHODS: Pharmacokinetics of CP were assessed in 12 patients with high-risk primary breast cancer who received an adjuvant chemotherapy regimen that included four courses of conventional-dose CP (500 mg/m2 over 1 hour every 3 weeks) followed by one final course of high-dose CP (100 mg/kg over 1 hour). Plasma concentrations of CP were analyzed by high-performance liquid chromatography (HPLC), 24-hour urinary concentrations of CP, and its inactive metabolites (carboxyphosphamide, dechloroethylcyclophosphamide [dechlorethylCP], ketocyclophosphamide [ketoCP]) were determined by 31-phosphorus-nuclear magnetic resonance (31P-NMR)-spectroscopy. RESULTS: There was no difference in dose-corrected area under the concentration-time curve (AUC) (216 v 223 [mumol.h/[mL.g]), elimination half-life (4.8 v 4.8 hours), systemic clearance (79 v 77 mL/min) and volume of distribution (0.49 v 0.45 L/kg) of CP between conventional- and high-dose therapy, respectively. However, during high-dose chemotherapy, we observed a significant increase in the renal clearance of CP (15 v 23 mL/min; P < .01) and in the formation clearance of carboxyphosphamide (7 v 12 mL/min; P < .05) and dechloroethylCP (3.2 v 4.2 mL/min; P < .05), whereas metabolic clearance to ketoCP remained unchanged (1.3 v 1.2 mL/min). Consequently, metabolic clearance to the remaining (reactive) metabolites decreased from 52 to 38 mL/min (P < .001). The relative contribution of the different elimination pathways to overall clearance of CP demonstrated wide interindividual variability. CONCLUSION: Overall pharmacokinetics of CP are apparently not affected during eightfold dose escalation. However, there is a shift in the relative contribution of different clearances to systemic CP clearance in favor of inactivating elimination pathways, thereby indicating saturation of bioactivating enzymes during dose escalation. Besides individual enzyme capacity, hydration and concomitant medication with dexamethasone modulated CP disposition.

The drug interaction potential of ranitidine: an update.

Ranitidine is a H2-receptor antagonist widely used in the treatment of a variety of gastrointestinal disorders. Since cimetidine--the predecessor drug of ranitidine--interacts with a variety of other agents and moreover ranitidine is often administered in combination with other drugs the interaction potential of ranitidine has been subject to extensive investigations. This review updates the information available from 1988 to present. Pharmacokinetic interactions of ranitidine with other drugs may occur at the site of absorption, metabolism and renal excretion. Most of the interactions reported at each of the three levels are minor and of low clinical significance. In view of some uncontrolled anecdotal reports, one cannot completely rule out the possibility that ranitidine might have some limited interaction potential in special patient populations under certain clinical conditions. However, it must be emphasized that numerous controlled studies have proven that ranitidine can be safely coadministered with other drugs.

Magnesium deficiency is associated with periodontal disease.

In the multifactorial pathogenesis of periodontitis, there are still unknown factors influencing the outcome of the disease. An association between magnesium and periodontitis has been suggested by preliminary studies. However, relevant clinical data are lacking. We investigated the association between magnesium status and periodontal health in a population-based analysis. We conducted a cross-sectional epidemiological investigation involving 4290 subjects aged 20-80 yrs. We recorded periodontal risk factors and determined concentrations of serum magnesium and calcium, relating them to periodontal parameters. In a matched-pair study, 60 subjects using oral magnesium-containing drugs and 120 without were compared. In subjects aged 40 yrs and older, increased serum Mg/Ca was significantly associated with reduced probing depth (p<0.001), less attachment loss (p=0.006), and a higher number of remaining teeth (p=0.005). Subjects taking Mg drugs showed less attachment loss (p<0.01) and more remaining teeth than did their matched counterparts. These results suggest that nutritional magnesium supplementation may improve periodontal health.

In-vivo indices of enzyme activity: the effect of renal impairment on the assessment of CYP2D6 activity.

Urinary drug:metabolite ratios and urinary recoveries of metabolites, have been used to assess specific enzyme activity non-invasively in vivo. These indices are potentially confounded by the effect of renal function. A recent study of the effects of renal impairment has found discrepancies between different indices used to mark CYP2D6 activity based on sparteine and dextromethorphan urinary recoveries. We have re-examined these experimental data from a theoretical viewpoint. The results suggest that the dependence of fractional urinary recovery of metabolites on renal function varies with the importance of different elimination routes. Therefore, no consistent behaviour of this index is expected when markers with different pharmacokinetics are used. However, when collecting the urine until full recovery of drug and metabolite, drug:metabolite ratios show the same degree of dependence on renal function regardless of the marker. The application of the analysis to the experimental data indicates that CYP2D6 activity is compromised in parallel with deterioration of renal function.

Rapid detection of CYP2D6 null alleles by long distance- and multiplex-polymerase chain reaction.

The CYP2D6 gene on human chromosome 22 encodes a cytochrome P450 responsible for oxidative metabolism of over 30 clinically used drugs. The CYP2D6 gene is highly polymorphic with more than 20 alleles described to date. Some of these harbour loss-of-function mutations which lead to the poor metabolizer phenotype in 5-10% of Caucasians. These individuals are at increased risk of suffering from adverse side effects or to experience therapeutic failure following drug treatment. Phenotype determination requires ingestion of a probe drug and has other inherent problems. Due to the increasing number of alleles known, comprehensive CYP2D6 genotyping using the conventional assays has become cumbersome and time consuming. We have therefore developed a streamlined and more rapid CYP2D6 genotyping procedure. Use of long distance PCR allowed the amplification of a 4666 bp fragment which contains the entire CYP2D6 gene. The 4.7 kb fragment serves as a template for a multiplex allele-specific PCR assay to simultaneously identify the five PM-associated alleles, CYP2D6*3 (A), *4 (B), *6 (T), *7 (E), and *8 (G). Together with the CYP2D6 deletion allele CYP2D6*5 (D), which can be detected in a separate PCR assay, these alleles are responsible for the PM phenotype in approximately 99% of Caucasian individuals. We tested the reliability of the procedure by analysing DNA from more than 80 individuals with known CYP2D6 genotypes. Twelve different genotypes were present among these samples and all of them were correctly identified.

Consequences of rifampicin treatment on propafenone disposition in extensive and poor metabolizers of CYP2D6.

Propafenone undergoes extensive metabolism both by phase I and phase II enzymes: cytochrome P4502D6 (CYP2D6) dependent polymorphic hydroxylation to its main metabolite 5-OH-propafenone, CYP3A4/1A2 dependent N-dealkylation and further glucuronidation and sulfation. Since CYP2D6 is not inducible by rifampicin, an important drug interaction between rifampicin and propafenone is not to be expected a priori. However, non-CYP2D6-dependent pathways may be induced as a case report described dramatically lowered plasma concentrations of propafenone with loss of dysrhythmia control associated with rifampicin treatment. Therefore, this study aimed to investigate induction properties of rifampicin on propafenone disposition in extensive metabolizers and poor metabolizers of CYP2D6. Six extensive metabolizers and six poor metabolizers ingested 600 mg rifampicin once daily for nine consecutive days. The day before the first rifampicin dose and on the day of the last rifampicin dose each individual received a single intravenous (i.v.) infusion of 140 mg unlabelled propafenone and 2 h later a single dose of 300 mg deuterated propafenone orally (p.o.). During enzyme induction maximum QRS prolongation decreased significantly after propafenone p.o. (21 +/- 7% versus 13 +/- 6% in extensive metabolizers, P < 0.01; 15 +/- 6% versus 9 +/- 6% in poor metabolizers, P < 0.01) and not after propafenone i.v. In parallel, there were no substantial differences in pharmacokinetics of propafenone i.v. by rifampicin. However, bioavailability of propafenone dropped from 30 +/- 15% to 10 +/- 8% in extensive metabolizers (P < 0.01) and from 81 +/- 6% to 48 +/- 8% in poor metabolizers (P < 0.001). Following propafenone p.o. clearances through N-dealkylation (4.1 +/- 2.1 ml/min versus 23.5 +/- 12.6 ml/min in extensive metabolizers, P < 0.01; 3.4 +/- 1.3 ml/min versus 16.0 +/- 5.5 ml/min in poor metabolizers, P < 0.001) and glucuronidation (123 +/- 48 ml/min versus 457 +/- 267 ml/min in extensive metabolizers, P < 0.05; 43 +/- 9 ml/min versus 112 +/- 34 ml/min in poor metabolizers, P < 0.01), but not 5-hydroxylation increased regardless of phenotype indicating substantial enzyme induction. Clearances to propafenone sulfate and conjugates of 5-OH-propafenone were significantly enhanced by rifampicin treatment in poor metabolizers (P < 0.01). Thus, induction of both phase I pathways (CYP3A4/1A2) and phase II pathways (glucuronidation, sulfation) of propafenone by rifampicin resulted in a clinically relevant metabolic drug interaction which was more pronounced in extensive metabolizers than in poor metabolizers with regard to percentage decrease in bioavailability of propafenone.

Analysis of CYP2D6 expression in human lung: implications for the association between CYP2D6 activity and susceptibility to lung cancer.

We have studied whether CYP2D6 is expressed in human lung tissue, using a specific and sensitive reverse transcriptase-polymerase chain reaction method and immunohistochemistry. Seven out of the eight patients were extensive metabolizers as shown by genotyping for the CYP2D6 (debrisoquine-sparteine) polymorphism. To investigate whether expression of CYP2D6 in lung tumours is different from that in normal lung tissue, tumour tissue samples were also obtained from the same eight patients. Correctly spliced CYP2D6 mRNA was detected by RT-PCR analysis in human liver and duodenum but not in any of the lung samples. In accordance with these negative results, immunoreactivity for CYP2D6 protein, using specific monoclonal and polyclonal antibodies, was very low or absent. No specific cell type of lung tissue showed strong immunoreactivity for CYP2D6, although expression of CYP3A could be clearly demonstrated in the same tissue samples. Moreover, a Western blot analysis revealed no signal in lung microsomes from two additional extensive metabolizers. Taken together, these results indicate that expression of CYP2D6 in human lung is absent or very low. These findings thus argue against a significant local metabolic activation of procarcinogenic agents by CYP2D6 in the lung.

Flecainide enantiomers: disposition in human subjects and electrophysiologic actions in vitro.

The antiarrhythmic agent flecainide is administered as a racemate. The disposition of the individual enantiomers and their electrophysiologic actions are unknown. We therefore determined through plasma levels of S-(+)-flecainide and R-(-)-flecainide in 13 patients who were receiving long-term oral flecainide therapy. In addition, the effects of the enantiomers on action potential characteristics in canine cardiac Purkinje fibers were assessed. Plasma concentrations of R-(-)flecainide were significantly higher than those of the S-(+)-enantiomer (-/+ ratio, 1.10 +/- 0.13,mean +/- SD; range, 0.89 to 1.32, p less than 0.01), suggesting that the drug undergoes enantioselective disposition. In the in vitro experiments, both enantiomers reduced phase 0 action potential Vmax (an index of the fast inward sodium current) and shortened action potential duration at 50% and 90% repolarization, but no differences between the enantiomers were detected. The time constants for development of Vmax depression were significantly longer for S-(+)-flecainide (13.4 +/- 1.5 seconds) compared with R-(-)-flecainide (8.9 +/- 0.6 seconds, p less than 0.001). Thus, although S-(+)-flecainide and R-(-)-flecainide undergo modest enantioselective disposition, they exert similar electrophysiologic effects. These studies have provided no evidence to indicate that administration of a single enantiomer, rather that the racemic drug, would offer any advantage.

In vitro characterization of the human cytochrome P-450 involved in polymorphic oxidation of propafenone.

Propafenone is a new class 1 antiarrhythmic agent. The drug is extensively metabolized. 5-Hydroxylation and N-dealkylation constitute major metabolic pathways. Recently it has been demonstrated that the in vivo metabolism of propafenone is controlled by the debrisoquin/sparteine polymorphism. To elucidate which of the above metabolic reactions is catalyzed by cytochrome P-450db1, the formation of 5-hydroxypropafenone and N-desalkylpropafenone was studied in the microsomal fraction of four human kidney donor livers previously characterized with regard to their ability to hydroxylate the beta-adrenergic antagonist bufuralol. The l'hydroxylation of bufuralol is catalyzed by the P-450db1 responsible for polymorphic debrisoquin/sparteine oxidation. The formation of 5-hydroxypropafenone but not N-desalkylpropafenone was closely related to bufuralol l'hydroxylation. Incubation with LKM1 antibodies, which selectively recognize P-450db1, inhibited 5-hydroxypropafenone formation completely whereas N-dealkylation was unimpaired. Propafenone was a strong competitive inhibitor of bufuralol l'hydroxylation. Thus it can be concluded that 5-hydroxypropafenone is formed by the cytochrome P-450 isozyme involved in polymorphic bufuralol oxidation.

Variable disposition kinetics and electrocardiographic effects of flecainide during repeated dosing in humans: contribution of genetic factors, dose-dependent clearance, and interaction with amiodarone.

We studied the influence of cytochrome P450 2D6 (CYP2D6) on the steady-state disposition kinetics and the electrocardiographic effects of flecainide at two doses and during combination with amiodarone. Seven extensive and five poor metabolizers of dextromethorphan were studied during a three-period crossover study. All subjects received 50 mg flecainide every 12 hours, alone or together with 200 mg amiodarone every 12 hours, and 100 mg flecainide every 12 hours for 5 days. Mean steady-state plasma concentration of flecainide and QRS change from predrug value did not differ significantly among extensive and poor metabolizer subjects during each study period. Except for a shortened elimination half-life and nonlinear kinetics in extensive metabolizer subjects, phenotype had no significant influence on flecainide pharmacokinetics. Combination with amiodarone resulted in an increase in mean flecainide plasma concentration and effect in subjects with both phenotypes. Our findings indicate that CYP2D6 phenotype predicts flecainide nonlinear kinetics and flecainide half-life but has no influence on electrocardiographic effects during repeated administration of flecainide or on the extent of the amiodarone-flecainide interaction.

Oral bioavailability of digoxin is enhanced by talinolol: evidence for involvement of intestinal P-glycoprotein.

OBJECTIVE: Recent data indicated that disposition of oral digoxin is modulated by intestinal P-glycoprotein. The cardioselective beta-blocker talinolol has been described to be secreted by way of P-glycoprotein into the lumen of the gastrointestinal tract after oral and intravenous administration. We therefore hypothesized that coadministration of digoxin and talinolol may lead to a drug-drug interaction based on a competition for intestinal P-glycoprotein. METHODS: Pharmacokinetics of digoxin (0.5 mg orally), talinolol (30 mg intravenously and 100 mg orally), and digoxin plus talinolol orally, as well as digoxin plus talinolol intravenously, were assessed in five male and five female healthy volunteers (age range, 23 to 30 years; body weight, 60 to 95 kg) in a changeover study with at least a 7-day washout period. Digoxin and talinolol were analyzed by fluorescence polarization immunoassay and HPLC, respectively. RESULTS: Oral coadministration of 100 mg talinolol increased the area under the concentration-time curve (AUC) from 0 to 6 hours and the AUC from 0 to 72 hours of digoxin significantly by 18% and 23%, respectively (5.85+/-1.49 versus 7.22+/-1.29 ng x h/mL and 23.0+/-3.3 versus 27.1+/-3.7 ng x h/mL, for both P<.05) and the maximum serum levels by 45%. Renal clearance and half-life of digoxin remained unchanged. Coinfusion of 30 mg talinolol with oral digoxin had no significant effects on digoxin pharmacokinetics. Digoxin did not affect the disposition of talinolol after both oral and intravenous administration. CONCLUSION: We observed a significantly increased bioavailability of digoxin with oral coadministration of talinolol, which is most likely caused by competition for intestinal P-glycoprotein.