西沙必利与其他药物的相互作用

目的　了解促胃肠动力药西沙必利与其他药物合用时的相互作用。方法　通过对近期文献的阅读、分析和归纳,加以综述。结果　西沙必利由细胞色素P-450 (CYP) 3A4代谢,有较强的首过效应,所以许多CYP3A4底物或(和)抑制剂都能抑制西沙必利的代谢,使其血药浓度升高,从而可能引起心脏QT间期延长、心律失常,甚至导致扭转型室速(TdP)。药效学研究表明,西沙必利与可以引起QT间期延长的药物合用后,也可能增加心脏的毒性反应。结论　西沙必利应避免与CYP3A4抑制剂、CYP3A4底物及易引起QT间期延长的药物合用。如果必需合用,应密切观察合用后的情况,并进行心电监护或血药浓度的监测,以保证临床用药的安全有效     

抗真菌药与心血管药物的相互作用

抗真菌药在与心血管药物相互合用时，经常会引起药动学或药效学方面的相互作用。咪唑类抗真菌药中，酮康唑和伊曲康唑为细胞色素P450（CYP）3A4的抑制剂，同时也是P－糖蛋白（P－gp)的抑制剂，所以与CYP3A4底物如Ca^2 拮抗剂，许多抗心律失常药，辛伐他汀，洛伐他汀，阿托他汀等互用后有可能抑制上述这些底物的代谢，导致血药浓度升高或消除降低，严重时引起不良反应，或者抑制由P－gp转运的药物（如地高辛）的消除；氟康唑为CYP2C9抑制剂，有可能影响由CYP2C9介导的药物如氟伐他汀，氯沙坦和irbesartan等的代谢，烯丙胺类抗真菌药特比萘芬可以影响由CYP2D6的活性，所以与普罗帕酮，恩卡因，美西律，西苯唑啉（cibenzoline)等抗心律失常药及许多β受体拮抗剂合用，也有可能产生药动学方面的影响。     

3种选择性5-羟色胺再摄取抑制药代谢性相互作用的处方分析

目的 了解选择性5-羟色胺再摄取抑制药(SSRIs)氟西汀、舍曲林和帕罗西汀的合并用药情况,为临床合理用药提供参考. 方法 以代谢酶学为指导,从代谢性相互作用的角度对处方进行回顾性分析. 结果3种SSRIs的处方总量为1 985张,其中氟西汀并用CYP2C19抑制药或底物的处方9张,氟西汀、舍曲林和帕罗西汀并用CYP2D6底物或抑制药的处方分别为16,15和12张,氟西汀和舍曲林并用CYP3A4底物或抑制药的处方分别为69张和48张. 结论 避免与已有文献报道能抑制SSRIs代谢的药物合用,或选择无或很少相互作用的同类药物. 药师在SSRIs合并用药中应做好处方审查和患者用药教育,必要时对某些可疑相互作用药进行血药浓度监测.     

氟喹诺酮类药物所致尖端扭转型室性心动过速

氟喹诺酮类常用药物中左氧氟沙星、环丙沙星、加替沙星和莫西沙星等可以引起尖端扭转型室性心动过速(TdP).TdP的临床表现为眩晕、昏厥甚至心搏停止,心电图可见QT间期延长及TdP.其发生机制尚不明确,可能与抑制心肌细胞K<'+>离子通道,使K<'+>外流受阻有关.氟喹诺酮类常用药物所致TdP的危险因素有女性、高龄、器质性心脏病(特别是充血性心力衰竭、QT间期延长、心动过缓)、肝肾功能损害、低钾低镁血症,以及合用可以引起QT间期延长的药物等.一旦患者出现QT间期延长及TdP应立即停药,补充钾和镁抑制早期后除极,也可采用人工临时心脏起搏或异丙肾上腺素提高基础心率.意识丧失和心室颤动者,可进行体外电复律.     

3种5-羟色胺再摄取抑制剂代谢性相互作用的处方分析

目的：了解选择性5-羟色胺再摄取抑制剂(SSRIs)氟西汀、帕罗西汀和舍曲林的合并用药情况，为临床合理用药提供参考。方法：从代谢性相互作用的角度对处方进行回顾性分析。结果：氟西汀的用量大于帕罗西汀和舍曲林。合用药物中有11种与氟西汀的相互作用具临床意义，有1种与帕罗西汀的相互作用具临床意义。与氟西汀相互作用可能具临床意义的合用药物有6种。合用药物数两种以上、且各自与氟西汀相互作用已被文献报道或可能具临床意义的合并用药有11种。结论：氟西汀在目前临床合并用药中，相互作用的可能性大于帕罗西汀和舍曲林。与相互作用具临床意义的CYP3A4抑制剂或底物合用时．帕罗西汀可以作为替代药品。与治疗窗较窄的CYP2D6底物合用时．应注意帕罗西汀和氟西汀的剂量调整。舍曲林的合并用药较为安全．但仍有可能与CYP3A4底物发生相互作用。药师在SSRIs合并用药中应作好处方审查和病人用药教育。必要时对某些可疑相互作用进行血药浓度监测。     

西沙必利引起尖端扭转型室性心动过速

杨森制药公司已提醒美国的医生，西沙必利（cisaPride）有引起严重致命性心律失常，包括尖端扭转型室性心动过速（torsadesdePointes，简称TdP）的危险，其中有2例伍用团康峻。团康哩是细胞色素P4503A4酶系统的强抑制剂，可影响各种药物的代谢，包括特非那定（ter【ena山ne）、咪达叹会和三叹仑。根据未出版的临床资料证明，团康叹可抑制西沙必利的代谢，导致血药浓度急剧升高。确切的心脏毒性机制尚不清楚，但是升高的西沙必利血药浓度可延长QT间期，并诱发心律失常。在西沙必利上市后消1993年9月～1995年1月期间，美国FDA收到18例与…     

西沙必利的不良反应分析与合理用药

目的：对国内外的西沙必利的不良反应报告进行分析，希望能促进其合理应用。方法：检索《药物不良反应光盘），1980年至今的《中国生物医学文献数据库》，MIMS《中国药品手册互动光盘》及国内有关药学杂志，查阅西沙必利的不良反应及药物相互作用加以整理分析。结果：国内有关西沙必利的不良反应详细报道为12例，而美国报道西沙必利引起心律失常为341例，死亡80例。西沙必利引起心律失常的原因包括应用过量，使用肝脏P450CYP3A4酶抑制，延长QT间歇的药物等。结论：西沙必利的合理应用应避免禁忌证，注意药物的相互作用，防止其不良反应的发生。     

细胞色素P450与有害的药物相互作用

药物相互作用是引起药物不良反应的常见原因，不恰当的药物联用会造成严重的毒性反应，甚至导致死亡。细胞色素P450（CYP)介导的药物相互作用主要为诱导代谢和抑制代谢。前者致CYP的合成或活性增加，使通过这一途径消除的合用药物代谢加速．药物血浆浓度降低．延误治疗；后者使CYP受到抑制．使另一药物的代谢减少，轻者延长或加强其作用．增加不良反应，重者引发致残或致命的医疗事故。本文综述了近年来临床上由于诱导或抑制CYP而产生的有害药物相互作用，以期引起医药工作者的高度重视。     

细胞色素P450 3A4抑制作用的药动学-药效学效果及与临床的关系

当某一药物的药效或毒性作用因为给予另一药物而改变时,即这两种药物发生了相互作用。药动学相互作用的发生通常是药物代谢过程变化的结果。细胞色素P450(CYP)3A4可通过多种代谢过程氧化许多药物。定位于小肠和肝脏中的CYP3A4对吸收的和未吸收的药物安排都会产生影响。与CYP3A4抑制剂的某些相互作用还可能包括P-糖蛋白的抑制。临床上重要的CYP3A4抑     

细胞色素P450—3A4相关的药物相互作用

目的：综述与细胞色素P4503A4相关的药物相互作用。方法：检索Medline和中国药学文摘。结果：发现多种由CYP3A4催化代谢的药物之间可以竞争药物代谢酶,引起药物相互作用,CYP3A4的抑制剂和诱导剂均可以抑制或诱导CYP3A4催化代谢的药物代谢。导致有益或不良的药物相互作用。结论：CYP3A4催化代谢的药物联合使用,特别是CYP3A4抑制剂与底物联合使用时,可能因为抑制了药物的代谢而导致严重的药物不良反应。     

Drug interactions with cisapride: clinical implications

Cisapride, a prokinetic agent, has been used for the treatment of a number of gastrointestinal disorders, particularly gastro-oesophageal reflux disease in adults and children. Since 1993, 341 cases of ventricular arrhythmias, including 80 deaths, have been reported to the US Food and Drug Administration. Marketing of the drug has now been discontinued in the US; however, it is still available under a limited-access protocol. Knowledge of the risk factors for cisapride-associated arrhythmias will be essential for its continued use in those patients who meet the eligibility criteria. This review summarises the published literature on the pharmacokinetic and pharmacodynamic interactions of cisapride with concomitantly administered drugs, providing clinicians with practical recommendations for avoiding these potentially fatal events. Pharmacokinetic interactions with cisapride involve inhibition of cytochrome P450 (CYP) 3A4, the primary mode of elimination of cisapride, thereby increasing plasma concentrations of the drug. The macrolide antibacterials clarithromycin, erythromycin and troleandomycin are inhibitors of CYP3A4 and should not be used in conjunction with cisapride. Azithromycin is an alternative. Similarly, azole antifungal agents such as fluconazole, itraconazole and ketoconazole are CYP3A4 inhibitors and their concomitant use with cisapride should be avoided. Of the antidepressants nefazodone and fluvoxamine should be avoided with cisapride. Data with fluoxetine is controversial, we favour the avoidance of its use. Citalopram, paroxetine and sertraline are alternatives. The HIV protease inhibitors amprenavir, indinavir, nelfinavir, ritonavir and saquinavir inhibit CYP3A4. Clinical experience with cisapride is lacking but avoidance with all protease inhibitors is recommended, although saquinavir is thought to have clinically insignificant effects on CYP3A4. Delavirdine is also a CYP3A4 inhibitor and should be avoided with cisapride. We also recommend avoiding coadministration of cisapride with amiodarone, cimetidine (alternatives are famotidine, nizatidine, ranitidine or one of the proton pump inhibitors), diltiazem and verapamil (the dihydropyridine calcium antagonists are alternatives), grapefruit juice, isoniazid, metronidazole, quinine, quinupristin/dalfopristin and zileuton (montelukast is an alternative). Pharmacodynamic interactions with cisapride involve drugs that have the potential to have additive effects on the QT interval. We do not recommend use of cisapride with class Ia and III antiarrhythmic drugs or with adenosine, bepridil, cyclobenzaprine, droperidol, haloperidol, nifedipine (immediate release), phenothiazine antipsychotics, tricyclic and tetracyclic antidepressants or vasopressin. Vigilance is advised if anthracyclines, cotrimoxazole (trimethoprim-sulfamethoxazole), enflurane, halothane, isoflurane, pentamidine or probucol are used with cisapride. In addition, uncorrected electrolyte disturbances induced by diuretics may increase the risk of torsade de pointes. Patients receiving cisapride should be promptly treated for electrolyte disturbances.     

Prolongation of the QT Interval Related to Cisapride-Diltiazem Interaction

Cisapride, a cytochrome P450 3A4 (CYP3A4) substrate, is widely prescribed for the treatment of gastrointestinal motility disorders. Prolongation of QT interval, torsades de pointes, and sudden cardiac death have been reported after concomitant administration with erythromycin or azole antifungal agents, but not with other CYP3A4 inhibitors. A possible drug interaction occurred in a 45-year-old woman who was taking cisapride for gastroesophageal reflux disorder and diltiazem, an agent that has inhibitory effect on CYP3A4, for hypertension. The patient was in near syncope and had QT-interval prolongation. After discontinuing cisapride, the QT interval returned to normal and symptoms did not recur. We suggest that caution be taken when cisapride is prescribed with any potent inhibitor of CYP3A4, including diltiazem.     

Selection of drugs to treat gastro-oesophageal reflux disease: the role of drug interactions

Gastro-oesophageal reflux disease is probably the most common acid-peptic disease in Western countries, and the successful treatment of mild to moderate disease with pharmacotherapy has become commonplace. A large number of effective drugs are now available, and so the decision-making process for physicians increasingly relies on considerations other than pure efficacy. Cost, adverse effects and drug interactions have therefore become important, particularly in the most vulnerable patients - children, the elderly and patients who are ill and are taking medications that may influence the efficacy of antireflux therapy. Important drug interactions with antacids include the prevention of the absorption of antibacterials such as tetracycline, azithromycin and quinolones. H2 antagonists, proton pump inhibitors and prokinetic agents undergo metabolism by the cytochrome P450 (CYP) system present in the liver and gastrointestinal tract. Cimetidine is an inhibitor of CYP3A and it may cause significant interactions with drugs of narrow therapeutic range and low bioavailability that are metabolised by these enzymes. The gastroparietal proton pump inhibitors lansoprazole, omeprazole and pantoprazole are all primarily metabolised by a genetically polymorphic enzyme, CYP2C19, that is absent from approximately 3% of Caucasians and 20% of Asians. These drugs may also interact with CYP3A, but to a lesser extent. Interactions with prokinetic agents carry the greatest potential for harm. Metoclopramide is a dopamine antagonist that may cause extrapyramidal effects when administered alone at high concentrations, or when coadministered with antipsychotic agents such as haloperidol or phenothiazines. Cisapride is clearly able to prolong the electrocardiographic QT interval and cause lethal ventricular arrhythmias when its metabolism is slowed by interaction with inhibitors of CYP3A, such as erythromycin, ketoconazole or itraconazole.     

Identification of the cytochrome P450 enzymes involved in the metabolism of cisapride: in vitro studies of potential co-medication interactions

Cisapride is a prokinetic drug that is widely used to facilitate gastrointestinal tract motility. Structurally, cisapride is a substituted piperidinyl benzamide that interacts with 5-hydroxytryptamine-4 receptors and which is largely without central depressant or antidopaminergic side-effects. The aims of this study were to investigate the metabolism of cisapride in human liver microsomes and to determine which cytochrome P-450 (CYP) isoenzyme(s) are involved in cisapride biotransformation. Additionally, the effects of various drugs on the metabolism of cisapride were investigated. The major in vitro metabolite of cisapride was formed by oxidative N-dealkylation at the piperidine nitrogen, leading to the production of norcisapride. By using competitive inhibition data, correlation studies and heterologous expression systems, it was demonstrated that CYP3A4 was the major CYP involved. CYP2A6 also contributed to the metabolism of cisapride, albeit to a much lesser extent. The mean apparent K(m) against cisapride was 8.6+/-3.5 microM (n = 3). The peak plasma levels of cisapride under normal clinical practice are approximately 0.17 microM; therefore it is unlikely that cisapride would inhibit the metabolism of co-administered drugs. In this in vitro study the inhibitory effects of 44 drugs were tested for any effect on cisapride biotransformation. In conclusion, 34 of the drugs are unlikely to have a clinically relevant interaction; however, the antidepressant nefazodone, the macrolide antibiotic troleandomycin, the HIV-1 protease inhibitors ritonavir and indinavir and the calcium channel blocker mibefradil inhibited the metabolism of cisapride and these interactions are likely to be of clinical relevance. Furthermore, the antimycotics ketoconazole, miconazole, hydroxy-itraconazole, itraconazole and fluconazole, when administered orally or intravenously, would inhibit cisapride metabolism.     

Drug-drug interactions of Z-338, a novel gastroprokinetic agent, with terfenadine, comparison with cisapride, and involvement of UGT1A9 and 1A8 in the human metabolism of Z-338

In the present study, the inhibitory properties of N-[2-(diisopropylamino)ethyl]-2-[(2-hydroxy-4,5-dimethoxybenzoyl)amino]-1,3-thiazole-4-carboxamide monohydrochloride trihydrate (Z-338), a novel gastroprokinetic agent, were investigated and compared with those of cisapride to establish its potential for drug-drug interactions. There was no notable inhibition of terfenadine metabolism or of any of the isoforms of cytochrome P450 (CYP1A1/2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1 and 3A4) by Z-338 in in vitro studies using human liver microsomes. Z-338 was mainly metabolized to its glucuronide by UGT1A9 (UDP glucoronosyltransferase 1 family, polypeptide A9) and UGT1A8, and did not show marked inhibition of P-glycoprotein activity. On the other hand, cisapride strongly inhibited CYP3A4 and markedly inhibited CYP2C9. Furthermore, we used the whole-cell patch-clamp technique to investigate the effects of Z-338 and cisapride on potassium currents in human embryonic kidney (HEK) 293 cells transfected with the human ether-a-go-go-related gene (hERG). Z-338 had no significant effect on hERG-related current at the relatively high concentration of 10 microM. In contrast, the inhibition by Z-338 was very small compared with that of cisapride at 10 nM, which was a thousand-fold lower concentration. In the prediction method for the drug interaction between terfenadine and cisapride based on the K(i) and PK parameters, we suggest the possibility that terfenadine mainly affect the QT interval, since its plasma concentration would be markedly increased, but cisapride may not be changed. Thus, in contrast with cisapride, Z-338 did not inhibit CYP and the hERG channel, and is predominantly metabolized by glucuronide conjugation, Z-338 is considered unlikely to cause significant drug-drug interactions when coadministered with CYP substrates at clinically effective doses.     

Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs

Consistent with its highest abundance in humans, cytochrome P450 (CYP) 3A is responsible for the metabolism of about 60% of currently known drugs. However, this unusual low substrate specificity also makes CYP3A4 susceptible to reversible or irreversible inhibition by a variety of drugs. Mechanism-based inhibition of CYP3A4 is characterised by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-, time- and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYP isoenzymes to reactive metabolites capable of irreversibly binding covalently to CYP3A4. Approaches using in vitro, in silico and in vivo models can be used to study CYP3A4 inactivation by drugs. Human liver microsomes are always used to estimate inactivation kinetic parameters including the concentration required for half-maximal inactivation (K(I)) and the maximal rate of inactivation at saturation (k(inact)).Clinically important mechanism-based CYP3A4 inhibitors include antibacterials (e.g. clarithromycin, erythromycin and isoniazid), anticancer agents (e.g. tamoxifen and irinotecan), anti-HIV agents (e.g. ritonavir and delavirdine), antihypertensives (e.g. dihydralazine, verapamil and diltiazem), sex steroids and their receptor modulators (e.g. gestodene and raloxifene), and several herbal constituents (e.g. bergamottin and glabridin). Drugs inactivating CYP3A4 often possess several common moieties such as a tertiary amine function, furan ring, and acetylene function. It appears that the chemical properties of a drug critical to CYP3A4 inactivation include formation of reactive metabolites by CYP isoenzymes, preponderance of CYP inducers and P-glycoprotein (P-gp) substrate, and occurrence of clinically significant pharmacokinetic interactions with coadministered drugs.Compared with reversible inhibition of CYP3A4, mechanism-based inhibition of CYP3A4 more frequently cause pharmacokinetic-pharmacodynamic drug-drug interactions, as the inactivated CYP3A4 has to be replaced by newly synthesised CYP3A4 protein. The resultant drug interactions may lead to adverse drug effects, including some fatal events. For example, when aforementioned CYP3A4 inhibitors are coadministered with terfenadine, cisapride or astemizole (all CYP3A4 substrates), torsades de pointes (a life-threatening ventricular arrhythmia associated with QT prolongation) may occur.However, predicting drug-drug interactions involving CYP3A4 inactivation is difficult, since the clinical outcomes depend on a number of factors that are associated with drugs and patients. The apparent pharmacokinetic effect of a mechanism-based inhibitor of CYP3A4 would be a function of its K(I), k(inact) and partition ratio and the zero-order synthesis rate of new or replacement enzyme. The inactivators for CYP3A4 can be inducers and P-gp substrates/inhibitors, confounding in vitro-in vivo extrapolation. The clinical significance of CYP3A inhibition for drug safety and efficacy warrants closer understanding of the mechanisms for each inhibitor. Furthermore, such inactivation may be exploited for therapeutic gain in certain circumstances.     

Interaction of cisapride with the human cytochrome P450 system: metabolism and inhibition studies.

Using human liver microsomes (HLMs) and recombinant cytochrome P450s (CYP450s), we characterized the CYP450 isoforms involved in the primary metabolic pathways of cisapride and documented the ability of cisapride to inhibit the CYP450 system. In HLMs, cisapride was N-dealkylated to norcisapride (NORCIS) and hydroxylated to 3-fluoro-4-hydroxycisapride (3-F-4-OHCIS) and to 4-fluoro-2-hydroxycisapride (4-F-2-OHCIS). Formation of NORCIS, 3-F-4-OHCIS, and 4-F-2-OHCIS in HLMs exhibited Michaelis-Menten kinetics (K(m): 23.4 +/- 8.6, 32 +/- 11, and 31 +/- 23 microM; V(max): 155 +/- 91, 52 +/- 23, and 31 +/- 23 pmol/min/mg of protein, respectively). The average in vitro intrinsic clearance (V(max)/K(m)) revealed that the formation of NORCIS was 3.9- to 5. 9-fold higher than that of the two hydroxylated metabolites. Formation rate of NORCIS from 10 microM cisapride in 14 HLMs was highly variable (range, 4.9-133.6 pmol/min/mg of protein) and significantly correlated with the activities of CYP3A (r = 0.86, P =. 0001), CYP2C19, and 1A2. Of isoform-specific inhibitors, 1 microM ketoconazole and 50 microM troleandomycin were potent inhibitors of NORCIS formation from 10 microM cisapride (by 51 +/- 9 and 44 +/- 17%, respectively), whereas the effect of other inhibitors was minimal. Of 10 recombinant human CYP450s tested, CYP3A4 formed NORCIS from 10 microM cisapride at the highest rate (V = 0.56 +/- 0. 13 pmol/min/pmol of P450) followed by CYP2C8 (V = 0.29 +/- 0.08 pmol/min/pmol of P450) and CYP2B6 (0.15 +/- 0.04 pmol/min/pmol of P450). The formation of 3-F-4-OHCIS was mainly catalyzed by CYP2C8 (V = 0.71 +/- 0.24 pmol/min/pmol of P450) and that of 4-F-2-OHCIS by CYP3A4 (0.16 +/- 0.03 pmol/min/pmol of P450). Clearly, recombinant CYP2C8 participates in cisapride metabolism, but when the in vitro intrinsic clearances obtained were corrected for abundance of each CYP450 in the liver, CYP3A4 is the dominant isoform. Cisapride was a relatively potent inhibitor of CYP2D6, with no significant effect on other isoforms tested, but the K(i) value derived (14 +/- 16 microM) was much higher than the clinically expected concentration of cisapride (<1 microM). Our data suggest that CYP3A is the main isoform involved in the overall metabolic clearance of cisapride. Cisapride metabolism is likely to be subject to interindividual variability in CYP3A expression and to drug interactions involving this isoform.     

Evidence of impaired cisapride metabolism in neonates

AIMS: Cisapride has been shown to cause QTc prolongation in neonates in the absence of any of the known risk factors ascribed to children or adults (excessive dosage, drug-drug interactions). Our hypothesis was that the early neonatal liver may show defective elimination of cisapride resulting in its accumulation in the immature child. Owing to the difficulties associated with in vivo pharmacokinetic studies in a paediatric population, we explored the in vitro metabolism of cisapride by human cytochrome P450. METHODS: Experiments were conducted with recombinant CYPs stably expressed in mammalian cells and with liver microsomes obtained from human foetuses, neonates, infants and adults. Cisapride metabolites were measured by high performance liquid chromatography. RESULTS: The rate of biotransformation of cisapride was greater by recombinant CYP3A4 than by CYP3A7 (0.77 +/- 0.5 and 0.01 +/- 0.01 nmol metabolites formed in 24 h, respectively), whereas CYP1A1, 1A2, 2C8, 2C9 and 3A5 showed no activity. Norcisapride formation was significantly correlated with testosterone 6beta-hydroxylation, a CYP3A4 catalysed reaction (r = 0.71, P = 0.03) but not with the 16-hydroxylation of dehydroepiandrosterone, catalysed by CYP3A7 (r = 0.30, P = 0.29) by microsomes from a panel of livers from foetuses, neonates and infants. No or negligible cisapride metabolic activity was observed in microsomes from either foetuses or neonates aged less than 7 days, which contained mostly CYP3A7 and no CYP3A4. The metabolism of cisapride steadily increased after the first week of life in parallel with CYP3A4 activity to reach levels exceeding adult values. CONCLUSIONS: The low content of CYP3A4 in the human neonatal liver appears to be responsible for its inability to oxidize cisapride and could explain its accumulation in plasma leading to the cases of QTc prolongation reported in this paediatric population.     

Pharmacokinetic-pharmacodynamic consequences and clinical relevance of cytochrome P450 3A4 inhibition

Drug interactions occur when the efficacy or toxicity of a medication is changed by administration of another substance. Pharmacokinetic interactions often occur as a result of a change in drug metabolism. Cytochrome P450 (CYP) 3A4 oxidises a broad spectrum of drugs by a number of metabolic processes. The location of CYP3A4 in the small bowel and liver permits an effect on both presystemic and systemic drug disposition. Some interactions with CYP3A4 inhibitors may also involve inhibition of P-glycoprotein. Clinically important CYP3A4 inhibitors include itraconazole, ketoconazole, clarithromycin, erythromycin, nefazodone, ritonavir and grapefruit juice. Torsades de pointes, a life-threatening ventricular arrhythmia associated with QT prolongation, can occur when these inhibitors are coadministered with terfenadine, astemizole, cisapride or pimozide. Rhabdomyolysis has been associated with the coadministration of some 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors ('statins') and CYP3A4 inhibitors. Symptomatic hypotension may occur when CYP3A4 inhibitors are given with some dihydropyridine calcium antagonists, as well with the phosphodiesterase inhibitor sildenafil. Excessive sedation can result from concomitant administration of benzodiazepine (midazolam, triazolam, alprazolam or diazepam) or nonbenzodiazepine (zopiclone and buspirone) hypnosedatives with CYP3A4 inhibitors. Ataxia can occur with carbamazepine, and ergotism with ergotamine, following the addition of a CYP3A4 inhibitor. Beneficial drug interactions can occur. Administration of a CYP3A4 inhibitor with cyclosporin may allow reduction of the dosage and cost of the immunosuppressant. Certain HIV protease inhibitors, e.g. saquinavir, have low oral bioavailability that can be profoundly increased by the addition of ritonavir. The clinical importance of any drug interaction depends on factors that are drug-, patient- and administration-related. Generally, a doubling or more in plasma drug concentration has the potential for enhanced adverse or beneficial drug response. Less pronounced pharmacokinetic interactions may still be clinically important for drugs with a steep concentration-response relationship or narrow therapeutic index. In most cases, the extent of drug interaction varies markedly among individuals; this is likely to be dependent on interindividual differences in CYP3A4 tissue content, pre-existing medical conditions and, possibly, age. Interactions may occur under single dose conditions or only at steady state. The pharmacodynamic consequences may or may not closely follow pharmacokinetic changes. Drug interactions may be most apparent when patients are stabilised on the affected drug and the CYP3A4 inhibitor is then added to the regimen. Temporal relationships between the administration of the drug and CYP3A4 inhibitor may be important in determining the extent of the interaction.