Inhibition of P-glycoprotein-mediated drug transport: A unifying mechanism to explain the interaction between digoxin and quinidine [seecomments]

The organic cation transporter, OCTN1, expressed in the human heart, potentiates antagonism of the HERG potassium channel

BACKGROUND

Variable function and expression of drug transporters have been proposed as mechanisms contributing to variable response to drug therapy. Block of the HERG channel, encoding IKr, can lead to serious arrhythmias, and a key drug-blocking site in HERG has been identified on the intracellular face of the pore. We begin to advance the hypothesis that active drug uptake enhances IKr block.

METHODS AND RESULTS

Reverse transcriptase-polymerase chain reaction identified expression in the human atrium and ventricle of 14 of 31 candidate drug uptake and efflux transporters, including OCTN1 (SLC22A4), a known uptake transporter of the HERG channel blocker quinidine. In situ hybridization and immunostaining localized OCTN1 expression to cardiomyocytes. The IC50 for quinidine block of IKr in CHO cells transfected with HERG alone was significantly higher than cells transfected with HERG + OCTN1 (0.66 +/- 0.15 microM versus 0.14 +/- 0.06 microM [52% absolute increase in drug block; 95% confidence interval, 0.4-0.64 microM]), and this effect was further potentiated by a common genetic variant of OCTN1, L503F. In the absence of OCTN1, quinidine block could be 91% +/- 5% washed out, but with the transporter, washout was incomplete (57% +/- 6%). OCTN1 coexpression also facilitated HERG block by flecainide and ibutilide, but not erythromycin.

CONCLUSIONS

Coexpression of the organic cation transporter, OCTN1, expressed in human cardiac myocytes, intensifies quinidine-induced HERG block. These findings establish a critical hypothesis that variable drug transporter activity may be a potential risk factor for torsade de pointes.

Oral cyclosporin A inhibits CD4 T cell P-glycoprotein activity in HIV-infected adults initiating treatment with nucleoside reverse transcriptase inhibitors

PURPOSE

P-glycoprotein limits the tissue penetration of many antiretroviral drugs. The aim of our study was to characterize the effects of the P-glycoprotein substrate cyclosporin A on T cell P-glycoprotein activity in human immunodeficiency virus-infected participants in the AIDS Clinical Trials Group study A5138.

METHODS

We studied P-glycoprotein activity on CD4 and CD8 T cells in 16 participants randomized to receive oral cyclosporin A (n=9) or not (n=7) during initiation antiretroviral therapy (ART) that did not include protease or non-nucleoside reverse transcriptase inhibitors.

RESULTS

CD4 T cell P-glycoprotein activity decreased by a median of 8 percentage points with cyclosporin A/ART (difference between cyclosporin A/ART vs. ART only, P= 0.001). Plasma trough cyclosporin A concentrations correlated with the change in P-glycoprotein activity in several T cell subsets.

CONCLUSIONS

Oral cyclosporin A can inhibit peripheral blood CD4 T cell P-glycoprotein activity. Targeted P-glycoprotein inhibition may enhance the delivery of ART to T cells.

[Mechanisms of pharmacokinetic drug-drug interactions]

Pharmacokinetic drug-drug interactions occur when a drug alters the disposition (absorption, distribution, elimination) of a coadministered agent. Pharmacokinetic interactions may result in the increase or the decrease of plasma drug concentrations. These modifications are variable in intensity but can lead to contraindications of the association. The mechanisms of pharmacokinetic interactions involve drug metabolizing enzymes, drug transporters and orphan nuclear receptors that regulate at the transcriptional level the expression of enzymes and transporters. The increase of drug plasma concentrations is generally related to the inhibition of enzymes and/or drug transport. The decrease of drug concentrations reflects the activation of orphan nuclear receptors by inducers that lead to the increase of the expression of enzymes and drug transporters. Inhibition of drug metabolism or transport is quite immediate (24-48h) while induction is a slower process (7-10 days). Complex situations may be observed with drugs that are both inducers and inhibitors (rifampin, ritonavir). They can cause the decrease and the increase of the exposure of the combined agent depending on the duration of the association.

Drug interactions at the blood-brain barrier: fact or fantasy?

There is considerable interest in the therapeutic and adverse outcomes of drug interactions at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). These include altered efficacy of drugs used in the treatment of CNS disorders, such as AIDS dementia and malignant tumors, and enhanced neurotoxicity of drugs that normally penetrate poorly into the brain. BBB- and BCSFB-mediated interactions are possible because these interfaces are not only passive anatomical barriers, but are also dynamic in that they express a variety of influx and efflux transporters and drug metabolizing enzymes. Based on studies in rodents, it has been widely postulated that efflux transporters play an important role at the human BBB in terms of drug delivery. Furthermore, it is assumed that chemical inhibition of transporters or their genetic ablation in rodents is predictive of the magnitude of interaction to be expected at the human BBB. However, studies in humans challenge this well-established paradigm and claim that such drug interactions will be lesser in magnitude but yet may be clinically significant. This review focuses on current known mechanisms of drug interactions at the blood-brain and blood-CSF barriers and the potential impact of such interactions in humans. We also explore whether such drug interactions can be predicted from preclinical studies. Defining the mechanisms and the impact of drug-drug interactions at the BBB is important for improving efficacy of drugs used in the treatment of CNS disorders while minimizing their toxicity as well as minimizing neurotoxicity of non-CNS drugs.

Properties of gap junction blockers and their behavioural, cognitive and electrophysiological effects: animal and human studies

Gap junctions play an important role in brain physiology. They synchronize neuronal activity and connect glial cells participating in the regulation of brain metabolism and homeostasis. Gap junction blockers (GJBs) include various chemicals that impair gap junction communication, disrupt oscillatory neuronal activity over a wide range of frequencies, and decrease epileptic discharges. The behavioural and clinical effects of GJBs suggest that gap junctions can be involved in the regulation of locomotor activity, arousal, memory, and breathing. Severe neuropsychiatric side effects suggest the involvement of gap junctions in mechanisms of consciousness. Unfortunately, the available GJBs are not selective and can bind to targets other than gap junctions. Other problems in behavioural studies include the possible adverse effects of GJBs, for example, retinal toxicity and hearing disturbances, changes in blood-brain transport, and the metabolism of other drugs. Therefore, it is necessary to design experiments properly to avoid false, misleading or uninterpretable results. We review the pharmacological properties and electrophysiological, behavioural and cognitive effects of the available gap junction blockers, such as carbenoxolone, glycyrrhetinic acid, quinine, quinidine, mefloquine, heptanol, octanol, anandamide, fenamates, 2-APB, several anaesthetics, retinoic acid, oleamide, spermine, aminosulfonates, and sodium propionate. It is concluded that despite a number of different problems, the currently used gap junction blockers could be useful tools in pharmacology and neuroscience.

Mini-series: I. Basic science. Uncertainty and inaccuracy of predicting CYP-mediated in vivo drug interactions in the ICU from in vitro models: focus on CYP3A4

Drug-drug interactions (DDIs) contribute significantly to the incidence of adverse drug reactions. Important advances in the knowledge of human drug-metabolizing enzymes have fueled the integration of in vitro drug metabolism and clinical DDIs studies for use in drug development programs and in the clinical setting. The activity of cytochrome P450 (CYP) 3A4 and P-glycoprotein are critical determinant of drug clearance, interindividual variability in drug disposition and clinical efficacy, and appears to be involved in the mechanism of numerous clinically relevant DDIs. Cell-based in vitro models are being increasingly applied in elucidating the pharmacokinetic profile of drug candidates during the preclinical steps of drug development. Human liver, intestinal samples and recombinant human CYP3A4 are now readily available as in vitro screening tools to predict the potential for in vivo DDIs. Although it is easy to determine in vitro metabolic DDIs, the interpretation and extrapolation of in vitro interaction data to in vivo situations requires a good understanding of pharmacokinetic principles. Clinicians and pharmacokineticists should recognize that in vitro models may not be clinically relevant in all situations. In the current article, research will be presented on drug metabolism and DDIs along with examples illustrating the utility of specific in vitro or in vivo approaches. In addition, the impact and clinical relevance of complexities such as dosing-route dependent effects, multi-site kinetics of drug-metabolizing enzymes and non-CYP determinants of metabolic clearance will be addressed.

An Interethnic Comparison of Polymorphisms of the Genes Encoding Drug-Metabolizing Enzymes and Drug Transporters: Experience in Singapore

Much of the interindividual variability in drug response is attributable to the presence of single nucleotide polymorphisms (SNPs) in genes encoding drug-metabolizing enzymes and drug transporters. In recent years, we have investigated the polymorphisms in a number of genes encoding phase I and II drug-metabolizing enzymes including CYPIA1, CYP3A4, CYP3A5, GSTM1, NAT2, UGT1A1, and TPMT and drug transporter (MDR1) in three distinct Asian populations in Singapore, namely the Chinese, Malays, and Indians. Significant differences in the frequencies of common alleles encoding these proteins have been observed among these three ethnic groups. For example, the frequency of the variant A2455G polymorphism of CYP1A1 was 28% in Chinese and 31% in Malays, but only 18% in Indians. CYP3A4*4 was detected in two of 110 Chinese subjects, but absent in Indians and Malays. Many Chinese and Malays (61-63%) were homozygous for the GSTM1*0 null genotype compared with 33% of Indians. The frequency of the UGTIA1*28 allele was highest in the Indian population (35%) compared to similar frequencies that were found in the Chinese (16%) and Malay (19%) populations. More importantly, our experience over the years has shown that the pharmacogenetics of these drug-metabolizing enzymes and MDR1 in the Asian populations are different from these in the Caucasian and African populations. For example, the CYP3A4*1B allele, which contains an A-290G substitution in the promoter region of CYP3A4, is absent in all three Asian populations of Singapore studied, but occurs in more than 54% of Africans and 5% of Caucasians. There were no difference in genotype and allelic variant frequencies in exon 12 of MDR1 between the Chinese, Malay, and Indian populations. When compared with other ethnic groups, the distribution of the wild-type C allele in exon 12 in the Malays (34.2%) and Indians (32.8%) was relatively high and similar to the Japanese (38.55%) and Caucasians (41%) but different from African-Americans (15%). The frequency of wild-type TT genotype in Asians (43.5% to 52.1%) and Japanese (61.5%) was much higher than those found in Caucasians (13.3%). All the proteins we studied represent the primary hepatic or extrahepatic enzymes, and their polymorphic expression may be implicated in disease risk and the disposition of drugs or endogenous substances. As such, dose requirements of certain drugs may not be optimal for Asian populations, and a second look at the factors responsible for this difference is necessary.

Meta-analysis of the effect of MDR1 C3435T polymorphism on cyclosporine pharmacokinetics

The published data revealed conflicting results of the polymorphism of MDR1 exon 26 SNP C3435T on the pharmacokinetics of cyclosporine; thus, the aim was to conduct a meta-analysis of significant magnitude to investigate the influence of SNP C3435T on the pharmacokinetics of cyclosporine. A literature search was conducted to locate the relevant papers by using the PubMed electronic source from 1997 and onwards. The pharmacokinetic parameters, including AUC(0-4), AUC(0-12), AUC(0-inf), C(max), CL/F and trough concentration (C(0)), were extracted and a meta-analysis was performed by using Stata version 9.1. A total of 14 papers concerning 1036 individuals were included in the meta-analysis. The overall results showed no major influence of SNP C3435T on the pharmacokinetic parameters, including AUC(0-4), AUC(0-inf), CL/F, C(max) and C(0), although AUC(0-12) was lower in subjects with CC genotype. A subanalysis by ethnic population showed that C(0) was lower in Caucasian individuals harbouring CC genotype. In conclusion, our meta-analysis of available studies has thus far failed to demonstrate a definitive correlation between the SNP C3435T in MDR1 gene and alterations in P-glycoprotein function that can result in altered pharmacokinetics of cyclosporine, although it was indicated in this meta-analysis that the carrier of CC genotype of the SNP C3435T of MDR1 had lower cyclosporine exposure presented as AUC(0-12) than those with at least one T allele. There seems to be ethnic differences in the relationship between the SNP C3435T of MDR1 and cyclosporine pharmacokinetics.

Animal models and intestinal drug transport

BACKGROUND

Intestinal drug metabolism and transport are now well recognized determinants of drug disposition in humans. During the last decade, various animal models lacking drug transporters have been generated in order to investigate the role of transporters for drug absorption, distribution and elimination.

OBJECTIVE

In this review the use of the animal models for the investigation of intestinal drug transport will be discussed.

METHODS

Publications describing the use of knockout animals (e.g., P-glycoprotein, Bcrp, and Oct1) regarding intestinal drug transport and animals characterized by mutations in transporters genes (e.g., Mrp2) were mainly considered for this review.

RESULTS/CONCLUSION

Knockout mouse models for ABC transporters are highly valuable tools to investigate the role of intestinal efflux transporters for the bioavailability of various compounds.

Interaction of oral antidiabetic drugs with hepatic uptake transporters: focus on organic anion transporting polypeptides and organic cation transporter 1

OBJECTIVE

The uptake of drugs into hepatocytes is a key determinant for hepatic metabolism, intrahepatic action, their subsequent systemic plasma concentrations, and extrahepatic actions. In vitro and in vivo studies indicate that many drugs used for treatment of cardiovascular diseases (e.g., oral antidiabetic drugs, statins) are taken up into hepatocytes by distinct organic anion transporters (organic anion transporting polypeptides [OATPs]; gene symbol SLCO/SLC21) or organic cation transporters (OCTs; gene symbol SLC22). Because most patients with type 2 diabetes receive more than one drug and inhibition of drug transporters has been recognized as a new mechanism underlying drug-drug interactions, we tested the hypothesis of whether oral antidiabetic drugs can inhibit the transport mediated by hepatic uptake transporters.

RESEARCH DESIGN AND METHODS

Using stably transfected cell systems recombinantly expressing the uptake transporters OATP1B1, OATP1B3, OATP2B1, or OCT1, we analyzed whether the antidiabetic drugs repaglinide, rosiglitazone, or metformin influence the transport of substrates and drugs (for OATPs, sulfobromophthalein [BSP] and pravastatin; for OCT1, 1-methyl-4-phenylpyridinium [MPP(+)] and metformin).

RESULTS

Metformin did not inhibit the uptake of OATP and OCT1 substrates. However, OATP-mediated BSP and pravastatin uptake and OCT1-mediated MPP(+) and metformin uptake were significantly inhibited by repaglinide (half-maximal inhibitory concentration [IC(50)] 1.6-5.6 micromol/l) and rosiglitazone (IC(50) 5.2-30.4 micromol/l).

CONCLUSIONS

These in vitro results demonstrate that alterations of uptake transporter function by oral antidiabetic drugs have to be considered as potential mechanisms underlying drug-drug interactions.

Role of p-glycoprotein inhibition for drug interactions: evidence from in vitro and pharmacoepidemiological studies

OBJECTIVES

We determined in vitro the potency of macrolides as P-glycoprotein inhibitors and tested in hospitalised patients whether coadministration of P-glycoprotein inhibitors leads to increased serum concentrations of the P-glycoprotein substrates digoxin and digitoxin.

METHODS

In vitro, the effect of macrolides on polarised P-glycoprotein-mediated digoxin transport was investigated in Caco-2 cells. In a pharmacoepidemiological study, we analysed the serum digoxin and digitoxin concentrations with and without coadministration of P-glycoprotein inhibitors in hospitalised patients.

RESULTS

All macrolides inhibited P-glycoprotein-mediated digoxin transport, with concentrations producing 50% inhibition (IC(50)) values of 1.8, 4.1, 15.4, 21.8 and 22.7 micromol/L for telithromycin, clarithromycin, roxithromycin, azithromycin and erythromycin, respectively. Coadministration of P-glycoprotein inhibitors was associated with increased serum concentrations of digoxin (1.3 +/- 0.6 vs 0.9 +/- 0.5 ng/mL, p < 0.01). Moreover, patients receiving macrolides had higher serum concentrations of cardiac glycosides (p < 0.05).

CONCLUSION

Macrolides are potent inhibitors of P-glycoprotein. Drug interactions between P-glycoprotein inhibitors and substrates are likely to occur during hospitalisation.

Implications of ABC transporters on the disposition of typical veterinary medicinal products

The ATP-Binding Cassette (ABC) transporters ABCB1, ABCC2 and ABCG2 are efflux transporters that facilitate the excretion of drugs, contribute to the function of biological barriers and maintain low cytoplasmic substrate concentrations in cells. ABC transporters modulate drug absorption, distribution and elimination according to the level of expression in the intestine, liver, kidney, and at biological barriers such as the blood-brain barrier. Moreover individual transporters are known to convey multi-drug resistance to tumour cells. While these diverse functions have been described in laboratory animal studies and in humans, the available information is very limited in animal species that are typical veterinary patients. This brief review summarizes the available data on organ distribution and expression levels in animals, genetic defects in dogs resulting in a non-functional P-gp expression, and describes examples of kinetic investigations directed to assess the clinical relevance of species differences in ABC-transporter expression.

Genetic mutations that prevent pain: implications for future pain medication

Part of the interindividual variability in pain therapy has been associated with genetic polymorphisms. Several genetic variants prevent or at least decrease pain in their carriers as compared with carriers of the respective wild-type or common alleles by impeding the generation, transmission and processing of nociceptive information or by increasing the local availability of active analgesics or their pharmacodynamic effects. Complete prevention of pain has so far been seen in six distinct rare hereditary syndromes, namely the ‘channelopathy-associated insensitivity to pain’, caused by 13 currently identified variants in the SCN9A gene coding for the α-subunit of the voltage-gated sodium channel, and five maladies belonging to the hereditary sensory and autonomic neuropathy (HSAN) I–V syndromes, caused by various mutations in several genes. Reduced pain in the average population has been associated with frequent variants in the µ-opioid receptor gene (OPRM1), catechol-O-methyltransferase gene (COMT), guanosine triphosphate cyclohydrolase 1/dopa-responsive dystonia gene (GCH1), transient receptor potential cation channel, subfamily V, member 1 gene (TRPV1) or the melanocortin-1 receptor gene (MC1R). Duplications/amplifications of the cytochrome P450 2D6 (CYP2D6) gene leading to increased enzyme function may cause intense opioid effects of codeine up to toxicity. The COMT V158M variant has been associated with decreased morphine requirements for analgesia. Inactivating MC1R variants have been associated with increased opioid analgesia of the µ-opioid receptor agonist morphine-6-glucuronide and, in women only, of κ-opioid agonists. Finally, variants in the P-glycoprotein gene (ABCB1) conferring decreased transporter function have been associated with increased respiratory depressive effects of fentanyl. In summary, a finite number of genetic variants that prevent pain by decreasing nociception or increasing analgesia have been identified. Given the complex biological and psychological nature of pain, we will see in the near future how much of the interindividual variance in pain and analgesia is due to identifiable genetic causes, and to what extent genetics enters clinical pain therapy.

The association of ABCB1 polymorphisms and elevated serum digitoxin concentrations in geriatric patients

OBJECTIVE

Digitoxin is a known substrate of the efflux pump P-glycoprotein (gene name: ABCB1). P-glycoprotein expression was shown to be modulated by single nucleotide polymorphisms in the ABCB1 gene, but it remains unclear whether these polymorphisms influence digitoxin blood levels. Our objective was to examine the association of ABCB1 C3435T genotype and elevated serum digitoxin concentrations (SDC) in a cohort of 77 geriatric patients consecutively admitted to a geriatric department over a 12-month period.

METHODS

The impact of ABCB1 3435 CC, CT, and TT genotypes on SDC and SDC normalized for daily digitoxin dosage and body weight was assessed by multivariate regression analysis.

RESULTS

Among participants, 18 (23%) had the CC, 36 (47%) the CT, and 23 (30%) the TT genotype. Adjusting for relevant covariates, no significant association of ABCB1 C3435T genotype and SDC or normalized SDC was detected. Mean SDC was 22.4 ng/ml (95% CI 18.9-25.9) for the TT, 21.8 ng/ml (95% CI 18.1-25.5) for the CT, and 25.7 ng/ml (95% CI 20.6-30.8) for the CC genotype. The means for normalized SDC were 5.2 kg.l(-1) (95% CI 4.3-6.1) for the TT, 6.1 kg.l(-1) (95% CI 4.7-7.5) for the CT, and 6.2 kg.l(-1) (95% CI 4.6-7.7) for the CC genotype.

CONCLUSION

In this sample of frail geriatric patients, the impact of ABCB1 C3435T genotype on serum digitoxin concentration was not of major relevance. Regular monitoring of digitoxin blood levels and surveillance of appropriate drug use remain the best ways to prevent digitoxin intoxications in the elderly.

Intestinal expression of cytochrome P450 enzymes and ABC transporters and carbamazepine and phenytoin disposition

OBJECTIVES

Interindividual variability in intestinal absorption and bioavailability might contribute to inadequate control of seizures under treatment with carbamazepine and phenytoin. We therefore correlated intestinal expression levels and genetics of CYP3A4, CYP2C9/19, MDR1 and MRP2 with dose requirement and plasma levels of carbamazepine and phenytoin.

MATERIALS AND METHODS

Epileptic patients on carbamazepine (n = 29) or phenytoin (n = 15) were stratified into a 'high'-dose (carbamazepine > or =800 mg/day, phenytoin > or =300 mg/day) and a 'low'-dose group (carbamazepine < or =600 mg/day, phenytoin < or =200 mg/day). Duodenal biopsies and DNA were obtained for Western blotting and genotyping studies.

RESULTS

Low carbamazepine plasma levels showed a trend towards higher intestinal MDR1 expression (P = 0.06). Furthermore, carbamazepine dose was positively correlated with MRP2 expression (P = 0.1). Moreover, MDR1 expression and carbamazepine and phenytoin dose requirement was influenced by the genotype in position 2677 and 3435 of the MDR1 gene.

CONCLUSION

Differences in intestinal MDR1 and MRP2 expression may influence carbamazepine and phenytoin disposition and may account for interindividual pharmacokinetic variability.

Induction of intestinal P-glycoprotein by St John's wort reduces the oral bioavailability of talinolol

St John's wort (SJW) is known to induce cytochrome P450 (CYP) 3A4 and P-glycoprotein through pregnane X-receptor activation. Our study evaluated the effects of long-term SJW administration on oral and intravenous pharmacokinetics of the nonmetabolized in vivo probe of P-glycoprotein, talinolol, in relation to intestinal P-glycoprotein expression. In a controlled, randomized study (N=9), the pharmacokinetics of oral (50 mg) and intravenous talinolol (30 mg) was determined before and after 12 days SJW (900 mg daily, Jarsin 300). Duodenal biopsies were taken and MDR1 genotypes assessed. SJW reduced the oral talinolol bioavailability by 25% (P=0.049) compared with water control. A 93% increase in oral clearance (P=0.177) and a 31% reduction in area under the serum concentration time curve (AUC; P=0.030) were observed. Renal and nonrenal clearance (CLNR), elimination half-life, peak serum drug concentration (Cmax), and time to reach Cmax were not significantly altered. After intravenous talinolol, SJW affected only CLNR (35% increase compared with water, P=0.006). SJW increased MDR1 messenger ribonucleic acid (mRNA) as well as P-glycoprotein levels in the duodenal mucosa. Subjects with the combined MDR1 genotype comprising 1236C>T, 2677G>T/A, and 3435C>T polymorphisms had lower intestinal MDR1 mRNA levels and displayed an attenuated inductive response to SJW as assessed by talinolol disposition. Long-term SJW decreased talinolol AUC with a corresponding increase in intestinal MDR1 expression, suggesting that SJW has a major inductive effect on intestinal P-glycoprotein. Interestingly, the magnitude of induction appeared to be affected by MDR1 genotype.

[Cognitive impairment induced by digoxin intake in patients older than 65 years]

Cognitive impairment or clinical signs of dementia in an old patient who receives digoxin, should suggest a digitalis intoxication. Symptoms can be present although a normal digoxin serum concentration. It is recommended to stop the treatment to obtain a regression of dementia symptoms.

RHABDOMYOLYSIS AFTER ADDITION OF DIGITOXIN TO CHRONIC SIMVASTATIN AND AMIODARONE THERAPY

Rhabdomyolysis is a well known side effect of statin therapy. Several drugs may increase its risk by drug-drug interactions. In particular, patients with heart disease receive more and more different compounds to cope with all the pathomechanisms involved and may therefore be of high risk for side effects. We report a case of rhabdomyolysis in a patient with heart failure on a multi-drug regimen caused by a drug interaction between chronic statin therapy (simvastatin), amiodarone and newly administrated digitoxin. The patient recovered fully after cessation of simvastatin therapy, the other drugs were given continuously. Potential mechanisms of this event are discussed. Most interesting in this case is that rhabdomyolysis occurred only after starting digitoxin after long-term therapy with the statin.

Increased??Absorption??of??Digoxin from??the??Human??Jejunum Due??to??Inhibition??of??Intestinal Transporter-Mediated Efflux

BACKGROUND AND OBJECTIVES

The contribution of transport in the small intestine by the apically located efflux pump P-glycoprotein to variable drug absorption in humans is still poorly understood. We therefore investigated whether inhibition of intestinal P-glycoprotein-mediated efflux by quinidine leads to increased absorption of the P-glycoprotein substrate digoxin.

METHODS

Using a multilumen perfusion catheter, we investigated the impact of P-glycoprotein inhibition on absorption of two compounds: the P-glycoprotein substrate digoxin and the marker for passive transcellular absorption antipyrine. Two 20cm adjacent jejunal segments were isolated with the multilumen perfusion catheter in seven healthy subjects. Unlabelled and deuterated digoxin and antipyrine, respectively, were simultaneously infused into either of the intestinal segments. One of the segments was additionally perfused with the P-glycoprotein inhibitor quinidine. Intestinal perfusates were collected for 3 hours, and drug concentrations were determined in the intestinal perfusates, plasma and urine.

RESULTS

Quinidine did not affect the disposition of antipyrine. In contrast, coadministration of quinidine into one jejunal segment caused a considerable increase in the amount of digoxin absorbed from this segment compared with the absorption from the other quinidine-free segment (22.3 +/- 8.9% vs 55.8 +/- 21.2% of the dose; p < 0.05). Accordingly, the area under the plasma concentration-time curve and the maximum plasma concentration of digoxin were considerably higher when luminal quinidine was coadministered (p < 0.05 and p < 0.001, respectively). Differences in digoxin absorption from the two intestinal segments were also reflected by pronounced differences in urinary digoxin elimination (5.5 +/- 3.3% vs 19.2 +/- 8.1% of the dose; p < 0.01).

CONCLUSIONS

P-glycoprotein inhibition in enterocytes increases systemic exposure of orally administered drugs that are P-glycoprotein substrates. These data highlight the importance of the small intestine as an active barrier against xenobiotics.

Characterization of beta-adrenoceptor antagonists as substrates and inhibitors of the drug transporter P-glycoprotein

Transporter proteins such as P-glycoprotein are major determinants of intracellular drug concentrations. Moreover, inhibition or induction of transporters is an important mechanism underlying drug interactions in humans. However, very little is known whether beta-adrenoceptor antagonists are substrates and/or inhibitors of P-glycoprotein. Therefore, we investigated the P-glycoprotein-mediated transport of propranolol, metoprolol, bisoprolol, carvedilol and sotalol in P-glycoprotein-expressing Caco-2 monolayers and inhibition of P-glycoprotein-mediated digoxin transport by the beta-adrenoceptor antagonists. A significant inhibition of polarized, basal to apical drug transport by the P-glycoprotein inhibitor PSC-833 was observed for bisoprolol (0.5 and 5 microm) and carvedilol (0.5 microm). Moreover, propranolol and carvedilol inhibited P-glycoprotein-mediated digoxin transport with IC(50) values of 24.8 and 0.16 microm, respectively, whereas metoprolol and sotalol had no effect. Bisoprolol significantly inhibited directional digoxin transport at 50 and 250 microm by 31% and 44%, respectively. Taken together, P-glycoprotein is likely to be one determinant of bisoprolol and carvedilol disposition in humans. In addition, the beta-adrenoceptor antagonists propranolol and carvedilol significantly inhibit P-glycoprotein function thereby possibly contributing to drug interactions in humans (e.g. digoxin-carvedilol and cyclosporine-carvedilol).

Metabolism of ATP-binding cassette drug transporter inhibitors: complicating factor for multidrug resistance

Membrane transport proteins belonging to the ATP-binding cassette (ABC) family of transport proteins play a central role in the defence of organisms against toxic compounds, including anticancer drugs. However, for compounds that are designed to display a toxic effect, this defence system diminishes their effectiveness. This is typically the case in the development of cellular resistance to anticancer drugs. Inhibitors of these transporters are thus potentially useful tools to reverse this transporter-mediated cellular resistance to anticancer drugs and, eventually, to enhance the effectiveness of the treatment of patients with drug-resistant cancer. This review highlights the various types of inhibitors of several multidrug resistance-related ABC proteins, and demonstrates that the metabolism of inhibitors, as illustrated by recent data obtained for various natural compound inhibitors, may have considerable implications for their effect on drug transport and their potential for treatment of drug resistance.

Drug-drug interactions involving membrane transporters in the human kidney

The kidneys play a critical role in the elimination of xenobiotics. Factors affecting the ability of the kidney to eliminate drugs may result in marked changes in the pharmacokinetics of a given compound. Drug-drug interactions due to competitive inhibition of renal organic anion or cation secretion systems have been noticed clinically for a long time. However, our understanding of the physical sites of interactions, that is, the specific transport proteins that the interacting drugs act on, has just begun very recently. This review summarises the latest progress in molecular identification and functional characterisation of major drug transporters in the human kidney. In particular, the review focuses on relating cloned renal drug transporters to clinically observed drug-drug interactions. The authors' opinion on the current status and future directions of research in these areas is also offered.

The role of the transporter P-glycoprotein for disposition and effects of centrally acting drugs and for the pathogenesis of CNS diseases

The MDR1 gene product P-glycoprotein is an ATP-dependent efflux pump, which transports its substrates out of cells. It is not only expressed in tumor cells, but also in cells of normal tissues. For example, it is located in the apical membrane of enterocytes, in endothelial cells forming the blood-brain and blood-testis barriers and in the apical membrane of placental syncytiotrophoblast. Since P-glycoprotein transports a wide range of drugs (e.g. antidepressants, antiepileptics, HIV protease inhibitors, cyclosporine, digoxin), its location in these tissues limits bioavailability of orally administered drugs and prevents entry of xenobiotics into the brain, testis and the fetus. Recent data highlight the role of intestinal P-glycoprotein for drug interactions (e.g. digoxin), of P-glycoprotein expressed in the blood-brain barrier for drug penetration into the CNS (e.g. loperamide, amitriptyline), the role of pharmacological inhibition of P-glycoprotein function to increases drug concentrations in sanctuary sites (e.g. for the HI virus) and for the potential role of MDR1 polymorphisms for P-glycoprotein expression, drug disposition, adverse drug reactions and disease risk. Taken together, active drug transport is now considered as an important additional mechanism limiting drug accumulation in multiple tissues including the CNS.

Pharmacogenetics of antiarrhythmic therapy

Individuals vary widely in their responses to therapy with most drugs. Indeed, responses to antiarrhythmic drugs are so highly variable that study of the underlying mechanisms has elucidated important lessons for understanding variable responses to drug therapy in general. Variability in drug response may reflect variability in the relationship between a drug dose and the concentrations of the drug and metabolite(s) at relevant target sites; this is termed pharmacokinetic variability. Another mechanism is that individuals vary in their response to identical exposures to a drug (pharmacodynamic variability). In this case, there may be variability in the target molecule(s) with which a drug interacts or, more generally, in the broad biological context in which the drug-target interaction occurs. Variants (polymorphisms and mutations) in the genes that encode proteins that are important for pharmacokinetics or for pharmacodynamics have now been described as important contributors to variable drug actions, including proarrhythmia, and these are described in this review. However, the translation of pharmacogenetics into clinical practice has been slow. To this end, the creation of large, well-characterised DNA databases and appropriate control groups, as well as large prospective trials to evaluate the impact of genetic variation on drug therapy, may hasten the impact of pharmacogenetics and pharmacogenomics in terms of delivering personalised drug therapy and to avoid therapeutic failure and serious side effects.

Establishment and characterization of the transformants stably-expressing MDR1 derived from various animal species in LLC-PK1

PURPOSE

Stable transformants expressing human multidrug resistance 1 (MDR1), monkey MDR1, canine MDR1, rat MDR1a, rat MDR1b, mouse mdr1a, and mouse mdr1b in LLC-PK1 were established to investigate species differences in P-glycoprotein (P-gp, ABCB1) mediated efflux activity.

METHODS

The seven cDNAs of MDR1 from five animals were cloned, and their transformants stably expressing the series of MDR1 in LLC-PK1 were established. Transport studies of clarithromycin, daunorubicin, digoxin, erythromycin, etoposide, paclitaxel, propranolol, quinidine, ritonavir, saquinavir, verapamil, and vinblastine were performed by using these cells, and efflux activity was compared among the species.

RESULTS

Except for propranolol, all compounds showed efflux activity in all transformants, and were judged to be substrates of P-gp. There were slight interspecies and interisoforms differences in the substrate recognition. However, the efflux ratio among the series of the MDR1 stably expressing cells showed good correlation as represented between human and monkey MDR1, and poor correlation as represented between human MDR1 and mouse mdr1a, and human and canine MDR1.

CONCLUSIONS

Results in the present study indicate that all MDR1 stably expressing cells have efflux activity for various P-gp substrates, and that interspecies differences and similarities of the P-gp substrate efflux activity may exist.

Intestinal absorptive transport of the hydrophilic cation ranitidine: a kinetic modeling approach to elucidate the role of uptake and efflux transporters and paracellular vs. transcellular transport in Caco-2 cells

PURPOSE

The mechanism of intestinal drug transport for hydrophilic cations such as ranitidine is complex, and evidence suggests a role for carrier-mediated apical (AP) uptake and saturable paracellular mechanisms in their overall absorptive transport. The purpose of this study was to develop a model capable of describing the kinetics of cellular accumulation and transport of ranitidine in Caco-2 cells, and to assess the relative contribution of the transcellular and paracellular routes toward overall ranitidine transport.

METHODS

Cellular accumulation and absorptive transport of ranitidine were determined in the absence or presence of uptake and efflux inhibitors and as a function of concentration over 60 min in Caco-2 cells. A three-compartment model was developed, and parameter estimates were utilized to assess the expected relative contribution from transcellular and paracellular transport.

RESULTS

Under all conditions, ranitidine absorptive transport consisted of significant transcellular and paracellular components. Inhibition of P-glycoprotein decreased the AP efflux rate constant (k21) and increased the relative contribution of the transcellular transport pathway. In the presence of quinidine, both the AP uptake rate constant (k12) and k21 decreased, resulting in a predominantly paracellular contribution to ranitidine transport. Increasing the ranitidine donor concentration decreased k12 and the paracellular rate constant (k13). No significant changes were observed in the relative contribution of the paracellular and transcellular routes as a function of ranitidine concentration.

CONCLUSIONS

These results suggest the importance of uptake and efflux transporters as determinants of the relative contribution of transcellular and paracellular transport for ranitidine, and provide evidence supporting a concentration-dependent paracellular transport mechanism. The modeling approach developed here may also be useful in estimating the relative contribution of paracellular and transcellular transport for a wide array of drugs expected to utilize both pathways.

Induction of rat intestinal P-glycoprotein by spironolactone and its effect on absorption of orally administered digoxin

The effect of the diuretic spironolactone (SL) on expression and function of intestinal P-glycoprotein (P-gp), as well as its impact on intestinal absorption of digoxin, was explored. Rats were treated with daily doses of 200 micromol/kg b.wt. of SL intraperitoneally for 3 consecutive days. The small intestine was divided into four equal segments of approximately 25 cm, with segment I being the most proximal. Brush-border membranes were isolated and used in analysis of P-gp expression by Western blot analysis. P-gp content increased in the SL group by 526, 292, 210, and 622% over controls for segments I, II, III, and IV, respectively. Up-regulation of apical P-gp was confirmed by immunofluorescence microscopy. P-gp transport activity was explored in intestinal sacs prepared from segment IV using two different model substrates. Serosal to mucosal transport (efflux) of rhodamine 123 was 140% higher, and mucosal to serosal transport (absorption) of digoxin was 40% lower in the SL group, both indicating increased P-gp function. In vivo experiments showed that intestinal absorption of a single dose of digoxin administered p.o. was attenuated by SL pretreatment. Thus, concentration of digoxin in portal and peripheral blood was lower in SL versus control groups, as well as its accumulation in kidney and liver. Urinary excretion of digoxin was significantly decreased in the SL group, probably reflecting decreased systemic availability of digoxin for subsequent urinary elimination. We conclude that SL induces P-gp expression with potential impact on intestinal absorption of substrates with therapeutic application.

Scientific perspectives on drug transporters and their role in drug interactions

Recently, increased interest in drug transporters and research in this area has revealed that drug transporters play an important role in modulating drug absorption, distribution, and elimination. Acting alone or in concert with drug metabolizing enzymes they can affect the pharmacokinetics and pharmacodynamics of a drug. This commentary will focus on the potential role that drug transporters may play in drug-drug interactions and what information may be needed during drug development and new drug application (NDA) submissions to address potential drug interactions mediated by transporters.

P-glycoprotein modulates aldosterone plasma disposition and tissue uptake

Aldosterone plays an important role in the pathophysiology of numerous cardiovascular disorders including heart failure and hypertension. Because aldosterone's actions are primarily mediated by its interaction with an intracellular mineralocorticoid receptor, factors affecting the cellular uptake and distribution of aldosterone may be important determinants of the hormone's activity. P-glycoprotein (P-gp) is an ATP-binding cassette efflux transporter encoded by the ABCB1 (also known as MDR1) gene in humans. P-gp is expressed on the luminal membrane of the capillary endothelial cells of tissues that are targets for aldosterone, including the brain and heart, where it attenuates cellular uptake of substrates. Recent in vitro evidence indicates P-gp transports aldosterone. Therefore, in this study we tested the hypothesis that P-gp modulates the uptake of aldosterone into the brain and heart by comparing the plasma and tissue distribution of [3H]-aldosterone in wild-type and P-gp-deficient [mdr1a/1b (-/-)] mice. Compared with wild-type mice, [3H]-aldosterone activity in the plasma, brain, and heart was significantly (P < 0.05) higher in the mdr1a/1b (-/-) animals. The area under the plasma or tissue concentration-time curves in the mdr1a/1b (-/-) mice was 2.0, 1.6, and 1.6-fold higher in the brain, heart, and plasma, respectively, than in wild-type controls. Our results demonstrate that P-gp plays an important role in aldosterone plasma disposition and modestly limits its uptake into the brain. The increased exposure of the brain and heart to aldosterone in the absence of P-gp suggests P-gp may play a key role in modulating aldosterone's effects in these organs.

Pharmacokinetic analysis of transcellular transport of quinidine across monolayers of human intestinal epithelial Caco-2 cells

To investigate the mechanism responsible for the intestinal absorption of a lipophilic organic cation, quinidine, we performed a pharmacokinetic analysis of transcellular transport across Caco-2 cell monolayers grown on a porous membrane. Basolateral-to-apical transport of the drug was almost constant in the concentration range of 100 nM-100 microM. Transcellular transport was greater in the apical-to-basolateral direction than in the opposite direction. Apical-to-basolateral transport was greater at a concentration of 100 microM than 100 nM. The calculated influx clearance value of the apical membrane was much greater than the other influx/efflux clearance values of cell membranes, and was 5.6-fold the influx clearance value of the basolateral membrane at the drug concentration of 100 microM. We also investigated the uptake of quinidine at the apical membrane of Caco-2 cells grown on plastic dishes. The uptake was markedly increased by alkalization of the apical medium at 37 degrees C, and was decreased at low temperature (4 degrees C). In addition, it was inhibited by diphenhydramine and levofloxacin, but not by carvedilol, rifamycin SV, or L-carnitine. These findings indicated that the influx at the apical membrane was the direction-determining step in the transcellular transport of quinidine across Caco-2 cell monolayers, and that some specific transport system was involved in this influx.

Itraconazole, gemfibrozil and their combination markedly raise the plasma concentrations of loperamide

OBJECTIVE

Loperamide is biotransformed in vitro by the cytochromes P450 (CYP) 2C8 and 3A4 and is a substrate of the P-glycoprotein efflux transporter. Our aim was to investigate the effects of itraconazole, an inhibitor of CYP3A4 and P-glycoprotein, and gemfibrozil, an inhibitor of CYP2C8, on the pharmacokinetics of loperamide.

METHODS

In a randomized crossover study with 4 phases, 12 healthy volunteers took 100 mg itraconazole (first dose 200 mg), 600 mg gemfibrozil, both itraconazole and gemfibrozil, or placebo, twice daily for 5 days. On day 3, they ingested a single 4-mg dose of loperamide. Loperamide and N-desmethylloperamide concentrations in plasma were measured for up to 72 h and in urine for up to 48 h. Possible central nervous system effects of loperamide were assessed by the Digit Symbol Substitution Test and by subjective drowsiness.

RESULTS

Itraconazole raised the peak plasma loperamide concentration (Cmax) 2.9-fold (range, 1.2-5.0; P < 0.001) and the total area under the plasma loperamide concentration-time curve (AUC(0-infinity)) 3.8-fold (1.4-6.6; P < 0.001) and prolonged the elimination half-life (t(1/2)) of loperamide from 11.9 to 18.7 h (P < 0.001). Gemfibrozil raised the Cmax of loperamide 1.6-fold (0.9-3.2; P < 0.05) and its AUC(0-infinity) 2.2-fold (1.0-3.7; P < 0.05) and prolonged its t(1/2) to 16.7 h (P < 0.01). The combination of itraconazole and gemfibrozil raised the Cmax of loperamide 4.2-fold (1.5-8.7; P < 0.001) and its AUC(0-infinity) 12.6-fold (4.3-21.8; P < 0.001) and prolonged the t(1/2) of loperamide to 36.9 h (P < 0.001). The amount of loperamide excreted into urine within 48 h was increased 3.0-fold, 1.4-fold and 5.3-fold by itraconazole, gemfibrozil and their combination, respectively (P < 0.05). Itraconazole, gemfibrozil and their combination reduced the plasma AUC(0-72) ratio of N-desmethylloperamide to loperamide by 65%, 46% and 88%, respectively (P < 0.001). No significant differences were seen in the Digit Symbol Substitution Test or subjective drowsiness between the phases.

CONCLUSION

Itraconazole, gemfibrozil and their combination markedly raise the plasma concentrations of loperamide. Although not seen in the psychomotor tests used, an increased risk of adverse effects should be considered during concomitant use of loperamide with itraconazole, gemfibrozil and especially their combination.

Drug transporters in pharmacokinetics

This review deals with the drug transporters allowing drugs to enter and leave cells by carrier-mediated pathways. Emphasis is put on liver transporters but systems in gut, kidney, and blood-brain barrier are mentioned as well. Drug-drug interactions on carriers may provoke significant modification in pharmacokinetics as do carrier gene polymorphisms yielding functional carrier protein mutations. An integrated phase concept should reflect the interplay between drug metabolism and drug transport.

Effect of organophosphate pesticide diazinon on expression and activity of intestinal P-glycoprotein

Organophosphate insecticide diazinon is widely used in agricultural practices, submitting farmers to repeated exposure. Because efflux pumps, as P-glycoprotein (P-gp), serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs, we analyzed the ability of diazinon to act as efflux modulator. Oral administration of diazinon (2-20 mg/kg, 5 days, or 10 mg/kg, 2-12 days) increased intestinal mdr1a mRNA of rats, in both dose- and time-dependent manner, and increased the expression of intestinal P-gp. Using the intestinal cell-line Caco-2, we found that 100 microM diazinon significantly inhibited digoxin and vinblastine secretive flux through the cell monolayers, whereas digoxin and vinblastine absorptive flux increased. The 25 microM diazinon was transported preferentially in basolateral (BL) to apical (AP) direction, suggesting a net secretion. The efflux rate significantly decreased in the presence of metabolic inhibitors sodium azide and 2-deoxy-d-glucose, P-gp inhibitors cyclosporin A and valspodar, but not in the presence of MRPs inhibitor MK571. Repeated exposure of Caco-2 cells to diazinon increased P-glycoprotein expression and activity. These results suggested the involvement of P-gp in the transfer of diazinon, leading to potential consequences for xenobiotic interactions, and showed that repeated exposure to low doses of pesticide may lead to up-regulated P-gp functions in the intestine of mammals.

IN VITRO P-GLYCOPROTEIN INHIBITION ASSAYS FOR ASSESSMENT OF CLINICAL DRUG INTERACTION POTENTIAL OF NEW DRUG CANDIDATES: A RECOMMENDATION FOR PROBE SUBSTRATES

Because modulation of P-glycoprotein (Pgp) through inhibition or induction can lead to drug-drug interactions by altering intestinal, central nervous system, renal, or biliary efflux, it is anticipated that information regarding the potential interaction of drug candidates with Pgp will be a future regulatory expectation. Therefore, to be able to utilize in vitro Pgp inhibition findings to guide clinical drug interaction studies, the utility of five probe substrates (calcein-AM, colchicine, digoxin, prazosin, and vinblastine) was evaluated by inhibiting their Pgp-mediated transport across multidrug resistance-1-transfected Madin-Darby canine kidney cell type II monolayers with 20 diverse drugs having various degrees of Pgp interaction (e.g., efflux ratio, ATPase, and calcein-AM inhibition). Overall, the rank order of inhibition was generally similar with IC(50) values typically within 3- to 5-fold of each other. However, several notable differences in the IC(50) values were observed. Digoxin and prazosin were the most sensitive probes (e.g., lowest IC(50) values), followed by colchicine, vinblastine, and calcein-AM. Inclusion of other considerations such as a large dynamic range, commercially available radiolabel, and a clinically meaningful probe makes digoxin an attractive probe substrate. Therefore, it is recommended that digoxin be considered as the standard in vitro probe to investigate the inhibition profiles of new drug candidates. Furthermore, this study shows that it may not be necessary to generate IC(50) values with multiple probe substrates for Pgp as is currently done for cytochrome P450 3A4. Finally, a strategy integrating results from in vitro assays (efflux, inhibition, and ATPase) is provided to further guide clinical interaction studies.

Meta-analysis of the influence of MDR1 C3435T polymorphism on digoxin pharmacokinetics and MDR1 gene expression

AIMS

Studies revealing conflicting results of the functional significance of MDR1 exon 26 C3435T SNP on the disposition of digoxin in different ethnic groups led us to perform a meta-analysis on published data investigating the influence of C3435T SNP on the pharmacokinetics of digoxin and the expression of MDR1.

METHODS

Meta-analysis was performed on data from published studies investigating the influence of MDR1 C3435T SNP on digoxin pharmacokinetics, as well as MDR1 expression in Caucasian and Japanese populations. The following outcomes were included: exposures to digoxin measured by area under the concentration-time curve and maximum concentration, the mean intestinal MDR1 mRNA expression and P-gp expression in the absence of digoxin administration.

RESULTS

The overall results of the meta-analysis in Caucasian and Japanese subjects suggested no major influence of the C3435T SNP on exposure levels of digoxin as determined by AUC(0-4 h) or AUC(0-24 h) although C(max) values for digoxin were lower in wild-type (CC) subjects compared with subjects harbouring TT genotypes. Subgroup analysis by ethnic populations showed the oral availability of digoxin to be lower in wild-type Caucasian populations compared with wild-type Japanese subjects. No causal relationships were detected between the C3435T SNP and MDR1 mRNA or protein expression.

CONCLUSIONS

Our meta-analysis of available studies indicates that the synonymous MDR1 C3435T SNP does not affect the pharmacokinetics of digoxin and the expression of MDR1 mRNA. Future studies should focus on the impact of MDR1 haplotypes on the pharmacokinetics of MDR1 substrates rather than the C3435T SNP alone.

Development, validation and utility of an in vitro technique for assessment of potential clinical drug-drug interactions involving P-glycoprotein

Regulatory interest is increasing for drug transporters generally and P-glycoprotein (Pgp) in particular, primarily in the area of drug-drug interactions. To aid in both identifying and discharging the potential liabilities associated with drug-transporter interactions, the pharmaceutical industry has a growing requirement for routine and robust non-clinical assays. An assay was designed, optimised and validated to determine the in vitro inhibitory potency of new chemical entities (NCEs) towards human Pgp-mediated transport. [3H]-Digoxin was established as a suitable probe substrate by investigating its characteristics in the in vitro system (MDCKII-MDR1 cells grown in 24-multiwell inserts). The inhibitory potencies (apparent IC50) of known Pgp inhibitors astemizole, GF120918, ketoconazole, itraconazole, quinidine, verapamil and quinine were determined over at least a 1000-fold concentration range. Validation was carried out using manual and automatic techniques. [3H]-Digoxin was found to be stable and have good mass balance in the system. In contrast to [A-->B] transport, [3H]-digoxin [B-->A] transport rates were readily measured with good reproducibility. There was no evidence of saturation of transport up to 10 microM digoxin and 30 nM digoxin was selected for routine assay use, reflecting clinical therapeutic concentrations. IC50 values ranged over approximately 100-fold with excellent reproducibility. Results from manual and automated versions were in close agreement. This method is suitable for routine use to assess the in vitro inhibitory potency of NCEs on Pgp-mediated digoxin transport. Comparison of IC50 values against clinical interaction profiles for the probe inhibitors indicated the in vitro assay is predictive of clinical digoxin-drug interactions mediated via Pgp.

Proarrhythmia

The concept that antiarrhythmic drugs can exacerbate the cardiac rhythm disturbance being treated, or generate entirely new clinical arrhythmia syndromes, is not new. Abnormal cardiac rhythms due to digitalis or quinidine have been recognized for decades. This phenomenon, termed "proarrhythmia," was generally viewed as a clinical curiosity, since it was thought to be rare and unpredictable. However, the past 20 years have seen the recognition that proarrhythmia is more common than previously appreciated in certain populations, and can in fact lead to substantially increased mortality during long-term antiarrhythmic therapy. These findings, in turn, have moved proarrhythmia from a clinical curiosity to the centerpiece of antiarrhythmic drug pharmacology in at least two important respects. First, clinicians now select antiarrhythmic drug therapy in a particular patient not simply to maximize efficacy, but very frequently to minimize the likelihood of proarrhythmia. Second, avoiding proarrhythmia has become a key element of contemporary new antiarrhythmic drug development. Further, recognition of the magnitude of the problem has led to important advances in understanding basic mechanisms. While the phenomenon of proarrhythmia remains unpredictable in an individual patient, it can no longer be viewed as "idiosyncratic." Rather, gradations of risk can be assigned based on the current understanding of mechanisms, and these will doubtless improve with ongoing research at the genetic, molecular, cellular, whole heart, and clinical levels.

Mechanisms, manifestations, and management of digoxin toxicity in the modern era

Because of the common use of digoxin and because of its narrow therapeutic index, digoxin toxicity has been prevalent historically and, therefore, most clinicians are well aware of the classical dose/concentration-related signs and symptoms of toxicity. Yet, in the modern era the incidence of digoxin toxicity has been declining for a variety of reasons, including a new (lower) therapeutic range, the development of more effective drug therapies for heart failure, and more accurate dosing methods. In addition, digoxin toxicity, once commonly fatal, can now be quickly and effectively treated by the emergency administration of antidigoxin Fab fragments. Indeed, it may be possible to expand the use of Fab fragments to select patients with non-life-threatening digoxin toxicity, in order to save costs and improve patient comfort. Most cases of digoxin toxicity are caused by inappropriately high dosages, which are usually prescribed in the setting of renal dysfunction, while other cases can be attributed to system errors such as multiple prescriptions, poor patient counseling, or errors in transcribing. With modern computerized prescribing systems, such as direct physician order entry and prompts that alert the clinician to the potential for error, it is possible to decrease the incidence of digoxin toxicity even further. A realistic goal is to nearly eradicate once commonplace digoxin toxicity or at least make its occurrence a rare event.