Clarithromycin lowers plasma zidovudine levels in persons with human immunodeficiency virus infection

Effect of clarithromycin on the pharmacokinetics of pranlukast in healthy volunteers

Pranlukast is a cysteinyl leukotriene receptor antagonist that has been used to treat bronchial asthma and allergic rhinitis. In vitro data suggest that pranlukast is a substrate of CYP3A4. Thus, the effect of clarithromycin, a potent CYP3A4 inhibitor, on the pharmacokinetics of pranlukast was examined in an open-label, randomized, two-way crossover study in 16 healthy male volunteers. In treatment A, volunteers received a single, 225 mg dose of pranlukast. In treatment B, 200 mg of clarithromycin was administered twice daily for 7 days and a single, 225 mg dose of pranlukast was coadministered on day 7. Blood samples were collected up to 24 hours after treatment, and pranlukast concentrations in the plasma were measured. The geometric mean ratios [GMR] (90% confidence intervals [CIs]) for pranlukast AUC(0-infinity) and C(max) (with/without clarithromycin) were 1.06 (0.91, 1.24) and 1.17 (0.95, 1.45), respectively. In conclusion, clarithromycin and pranlukast could be coadministered without dose adjustment because clarithromycin minimally affected the pharmacokinetics of pranlukast.

Application and impact of population pharmacokinetics in the assessment of antiretroviral pharmacotherapy

Population pharmacokinetics has been an important technique used to explore and define relevant sources of variation in drug exposure and response in patient populations. This has been especially true in the area of antiretroviral therapy where the assurance of adequate and sustained drug exposure of multiple agents is highly correlated with therapeutic success. Population pharmacokinetic analyses across the four drug classes and 20 US FDA-approved products used to treat HIV have been published to date. The published reports were predominantly based on actual clinical trials conducted in HIV-infected patients with one or more agents administered. Modelling and simulation approaches have been used in the evaluation of antiretroviral agent outcomes incorporating problematic design and analysis factors such as sparse plasma sampling, data imbalance and censored data. Additional benefits of population modelling approaches applied to the investigation of antiretroviral agents include the ability to assess dosing compliance, understanding and quantifying drug-drug interactions in order to select dosing regimens and the screening of new drug candidates. Pharmacokinetic/pharmacodynamic models have been used to characterise the relationship between drug exposure and virological and immunological response, and to predict clinical outcome. These models offer the best opportunity for individualising and optimising patient therapy, particularly when adjusted for adherence/compliance. The impact of population pharmacokinetics in the area of antiretroviral therapy can be directly assessed by its role in the validation of surrogate markers such as viral RNA load, therapeutic drug monitoring and the management of individual patient outcomes via exposure-toxicity relationships. Each of these population pharmacokinetic outcomes has contributed to the current regulatory environment, specifically in the area of accelerated approval of new antiretroviral agents.

RXR-induced TNF-alpha suppression is reversed by morphine in activated U937 cells

Deficiency in vitamin A has been associated with adverse clinical outcomes in drug users with HIV-1 infection. Retinoids have been demonstrated to suppress proinflammatory cytokine production by immune cells in vitro. These effects are induced by ligand-mediated activation of the retinoid receptors--retinoic acid receptor (RAR) and retinoid X receptor (RXR). In these studies, the effects of all-trans-retinoid acid (ATRA, a RAR agonist), 9-cis-retinoic acid (9cis RA; RAR and RXR agonist), LG101305 (RXR agonist), LG100815 (RAR antagonist) and LG101208 (RXR antagonist) on TNF-alpha production by phytohemagglutanin-activated U937 cells and the modulation of these effects by morphine were examined. TNF-alpha production was suppressed in all cultures exposed to retinoid agonist and antagonist agents. For cells exposed to RXR agonists or RAR antagonist, incubation with morphine resulted in the reversal of TNF-alpha suppression and this effect was inhibited by naloxone. These data suggest that interactions between RXR and morphine are involved in the immune effects of retinoids on TNF-alpha production by activated U937 cells. Such information may be important for understanding interactions between drugs of abuse and immune function in individuals with chronic proinflammatory states such as HIV-1 infection.

Drug interactions between antiretroviral drugs and comedicated agents

HIV-infected individuals usually receive a wide variety of drugs in addition to their antiretroviral drug regimen. Since both non-nucleoside reverse transcriptase inhibitors and protease inhibitors are extensively metabolised by the cytochrome P450 system, there is a considerable potential for pharmacokinetic drug interactions when they are administered concomitantly with other drugs metabolised via the same pathway. In addition, protease inhibitors are substrates as well as inhibitors of the drug transporter P-glycoprotein, which also can result in pharmacokinetic drug interactions. The nucleoside reverse transcriptase inhibitors are predominantly excreted by the renal system and may also give rise to interactions. This review will discuss the pharmacokinetics of the different classes of antiretroviral drugs and the mechanisms by which drug interactions can occur. Furthermore, a literature overview of drug interactions is given, including the following items when available: coadministered agent and dosage, type of study that is performed to study the drug interaction, the subjects involved and, if specified, the type of subjects (healthy volunteers, HIV-infected individuals, sex), antiretroviral drug(s) and dosage, interaction mechanism, the effect and if possible the magnitude of interaction, comments, advice on what to do when the interaction occurs or how to avoid it, and references. This discussion of the different mechanisms of drug interactions, and the accompanying overview of data, will assist in providing optimal care to HIV-infected patients.

Prophylactic clarithromycin to treat mycobacterium avium in HIV patients receiving zidovudine may significantly increase mortality by suppressing lymphopoiesis and hematopoiesis

The increased mortality observed when human immunodeficiency virus (HIV)-infected individuals are treated with clarithromycin (CLA) as prophylaxis for disseminated infection with organisms of the Mycobacterium avium complex (MAC) suggests that CLA might possess immunosuppressive activities. To test this possibility, we assessed the immunological response of BALB/c mice following subchronic (28 days) oral administration of CLA alone or in combination with zidovudine (ZDV). Because normal hematopoiesis is needed to maintain the immune system, we also examined the effect of these drugs given individually or in combination on several hematological parameters. The major effect of administration of 500 mg/kg CLA was a marked decrease in the lymphocyte/neutrophil ratio, and the only evidence of hematotoxicity in mice treated with 240 mg/kg ZDV alone was mild macrocytic anemia. However, treatment with a combination of CLA and ZDV resulted in severe hematotoxicity, evidenced by a significant (p < 0.01) decrease in the number of circulating erythrocytes, neutrophils, and lymphocytes and a 67% drop in splenic cellularity (p < 0.01). Treatment with CLA or ZDV alone or both drugs in combination had no effect on lymphocyte function, determined by measuring the ex vivo proliferative activity of splenocytes in response to alloantigens or a B cell mitogen, lipopolysaccharide (LPS). However, because of the cellular depletion in the spleen, overall immune responses in this organ decreased significantly (p < 0.05) in mice treated with CLA plus ZDV. These data suggest that interactions between CLA and ZDV warrant further evaluation because these drugs are given in combination to persons with advanced HIV infection.

The evolution and role of macrolides in infectious diseases

Two of the most significant changes in the field of infectious disease management during the last few decades are the emergence of atypical and/or new pathogens that may have devastating consequences and the re-emergence of well-recognised organisms that have acquired antimicrobial resistance through a variety of mechanisms. Erythromycin, the prototype macrolide, was originally marketed approximately five decades ago as a useful alternative agent in the treatment of patients allergic to beta-lactam antibiotics. While clinically useful, its pharmacokinetic and adverse-event profile limited the use of erythromycin to these individuals. Enhancements of the macrolide structure circumvented many of the limitations of erythromycin and resulted in the development of azithromycin and clarithromycin. The clinical uses of clarithromycin and azithromycin are substantially wider than erythromycin due to the wide spectra of activity against the atypical and newer pathogens. In addition, these agents are well-tolerated and have a pharmacokinetic profile that allows once- or twice-daily administration. Studies also indicate that the more common of the two mechanisms of macrolide resistance in the US and Canada imparts only low-level resistance. The multitude of studies substantiating clinical as well as bacteriological success with these two agents indicates that, when used appropriately, they will stand the test of time and continue to be useful antimicrobial agents.

Comparative safety of the different macrolides

The macrolides are generally well tolerated when used for the treatment of acute infections. Even when given long term for prophylaxis, there are few discontinuations due to side-effects. There are isolated reports of QT(c) prolongation in patients treated with erythromycin and other 14-membered-ring macrolides. Since the 14-membered-ring macrolides are metabolized by P450 isoenzymes, there is the potential for interaction with other therapeutic agents also metabolized in this way. Pharmacokinetic studies have demonstrated interactions between either erythromycin or clarithromycin and cyclosporin, cisapride, pimozide, disopyramide, astemizole, carbamazepine, midazolam, digoxin, hydroxymethylglutaryl-coenzyme A reductase inhibitors (i.e. 'statins') and warfarin. In patients receiving such concurrent therapy, azithromycin may be superior to erythromycin and clarithromycin.

The macrolides

Erythromycin, which was introduced over 50 years ago, was the first macrolide to be used clinically. "New" macrolides, for the treatment of patients with various infectious diseases, were not clinically introduced until 40 years later. The pharmacokinetic and adverse events profile of erythromycin initially limited its use to an alternative agent for patients with allergy to beta-lactam agents. However, the emergence of atypical and/or new pathogens and the ongoing escalation of acquired antimicrobial resistance has impacted on the empirical and organism directed therapy of infectious diseases. Azithromycin and clarithromycin were developed by enhancing the basic macrolide structure. Some of the basic features associated with these new agents include a pharmacokinetic profiles that allow once or twice daily dosing with a much lower incidence of side effects and a substantially broader spectrum of activity which includes some Gram-negative bacilli, atypical pathogens and new, unconventional or uncommon pathogens. Clinical trial data has supported the use of "new" macrolides in a wide range of clinical indications, however, some specific indications are currently restricted to treatment with either azithromycin or clarithromycin. Macrolide resistance is a class effect and depending on the mechanism will confer either low or high level resistance. While resistance is problematic, it does not always result in clinical failure. The macrolides are a valuable class of antimicrobial agent and play an important role in the management of infectious diseases.

Pharmacokinetic and other drug interactions in patients with AIDS

A variety of medications are used in treating patients infected with the human immunodeficiency virus (HIV). These medications are used to control viremia and to prevent and treat opportunistic infections. An individual is often required to take numerous drugs at the same time and thus clinicians are confronted with potential drug interactions, some of which are significant. Three different groups of anti-HIV drugs are used to treat patients. These groups include nucleoside reverse transcription inhibitors, non-nucleoside reverse transcription inhibitors, and protease inhibitors. This article reviews the most relevant drug interactions that occur during the treatment of HIV-infected patients with traditional and also alternative drugs. The role of therapeutic drug monitoring in the routine management of HIV-infected patients is discussed.

The new antibiotics

It is easy to become overwhelmed by the amount of information available on the new antibiotics and difficult to keep abreast of the appropriate indications for each of them. For most patients with community-acquired infections, the first-line agent is usually not one of the newer agents, but a standard regimen, or at times, no antibiotic at all. The development of resistance is likely to parallel the extent to which these agents are prescribed. They should be used only when standard treatment fails, when compliance with treatment is a real and serious issue, or when the patient has a real allergic reaction to the standard regimen.

Drug-Drug interactions of clinical significance in the treatment of patients with Mycobacterium avium complex disease

Therapeutic and prophylactic regimens directed specifically against Mycobacterium avium complex (MAC) are increasingly being used in patients infected with the human immunodeficiency virus (HIV). Several of the drugs used in the management of MAC have been associated with significant drug interactions involving the cytochrome P450 (CYP) enzyme system. This enzyme system is also highly influenced by other drugs used in the management of patients with HIV, particularly the protease inhibitors, non-nucleoside reverse transcriptase inhibitors (NNRTIs) and azole antifungals. This article reviews the published concentrations or subtherapeutic concentrations of other drugs have been described. In particular, concurrent use of rifabutin with clarithromycin or fluconazole has resulted in increased concentrations of rifabutin and an accompanying increase in the incidence of rifabutin toxicities, including uveitis and leucopenia. Similar results have been seen when rifabutin is combined with protease inhibitors or delavirdine. The macrolides, clarithromycin and azithromycin, have also been associated with significant drug interactions. Clarithromycin has a higher affinity for CYP than azithromycin and, thus, is more frequently associated with clinically significant drug interactions. Clarithromycin is an inhibitor of CYP and may result in toxic concentrations of other drugs metabolised by this enzyme system. Such interactions have been described with rifabutin and the statin lipid-lowering agents. In addition, nevirapine and efavirenz have been shown to significantly reduce clarithromycin concentrations, whereas the protease inhibitors and delavirdine may increase clarithromycin concentrations. Other drugs used in the management of patients with MAC are not metabolised by CYP and thus have a lower incidence of interactions, although the absorption of ciprofloxacin may be impaired when it is given with products containing multivalent cations, such as didanosine. However, clinicians must remain vigilant for drug interactions when reviewing a patient's medication profile, keeping in mind both interactions that have been described in the literature and those that may be predicted based upon known pharmacokinetic profiles.

Drug Interactions with Antiretrovirals

The rapid development of new antiretroviral drugs, along with the evolution in clinical practice toward the recommended use of three- to four-drug combination regimens for achieving optimal suppression of viral replication, has brought the relevance of drug-drug interactions to the forefront of care for HIV-infected individuals. However, the routine clinical interpretation of drug interactions is complicated by our expanding knowledge of the physiologic mechanisms underlying pharmacokinetic interactions, particularly as they relate to drug transport and distribution (eg, P-glycoprotein) and biotransformation (hepatic cytochrome p450 monooxygenase induction and inhibition).

Macrolide drug interactions: an update

OBJECTIVE

To describe the current drug interaction profiles for the commonly used macrolides in the US and Europe, and to comment on the clinical impact of these interactions.

DATA SOURCES

A MEDLINE search (1975-1998) was performed to identify all pertinent studies, review articles, and case reports. When appropriate information was not available in the literature, data were obtained from the product manufacturers.

STUDY SELECTION

All available data were reviewed to provide an unbiased account of possible drug interactions.

DATA EXTRACTION

Data for some of the interactions were not available from the literature, but were available from abstracts or company-supplied materials. Although the data were not always explicit, the best attempt was made to deliver pertinent information that clinical practitioners would need to formulate practice opinions. When more in-depth information was supplied in the form of a review or study report, a thorough explanation of pertinent methodology was supplied.

DATA SYNTHESIS

Several clinically significant drug interactions have been identified since the approval of erythromycin. These interactions usually were related to the inhibition of the cytochrome P450 enzyme systems, which are responsible for the metabolism of many drugs. The decreased metabolism by the macrolides has in some instances resulted in potentially severe adverse events. The development and marketing of newer macrolides are hoped to improve the drug interaction profile associated with this class. However, this has produced variable success. Some of the newer macrolides demonstrated an interaction profile similar to that of erythromycin; others have improved profiles. The most success in avoiding drug interactions related to the inhibition of cytochrome P450 has been through the development of the azalide subclass, of which azithromycin is the first and only to be marketed. Azithromycin has not been demonstrated to inhibit the cytochrome P450 system in studies using a human liver microsome model, and to date has produced none of the classic drug interactions characteristic of the macrolides.

CONCLUSIONS

Most of the available data regarding macrolide drug interactions are from studies in healthy volunteers and case reports. These data suggest that clarithromycin appears to have an interaction profile similar to that of erythromycin. Given this similarity, it is important to consider the interaction profile of clarithromycin when using erythromycin. This is especially necessary as funds for further studies of a medication available in generic form (e.g., erythromycin) are limited. Azithromycin has produced few clinically significant interactions with any agent cleared through the cytochrome P450 enzyme system. Although the available data are promising, the final test should come from studies conducted in patients who are taking potentially interacting compounds on a chronic basis.

Clinical pharmacokinetics of clarithromycin

Clarithromycin is a macrolide antibacterial that differs in chemical structure from erythromycin by the methylation of the hydroxyl group at position 6 on the lactone ring. The pharmacokinetic advantages that clarithromycin has over erythromycin include increased oral bioavailability (52 to 55%), increased plasma concentrations (mean maximum concentrations ranged from 1.01 to 1.52 mg/L and 2.41 to 2.85 mg/L after multiple 250 and 500 mg doses, respectively), and a longer elimination half-life (3.3 to 4.9 hours) to allow twice daily administration. In addition, clarithromycin has extensive diffusion into saliva, sputum, lung tissue, epithelial lining fluid, alveolar macrophages, neutrophils, tonsils, nasal mucosa and middle ear fluid. Clarithromycin is primarily metabolised by cytochrome P450 (CYP) 3A isozymes and has an active metabolite, 14-hydroxyclarithromycin. The reported mean values of total body clearance and renal clearance in adults have ranged from 29.2 to 58.1 L/h and 6.7 to 12.8 L/h, respectively. In patients with severe renal impairment, increased plasma concentrations and a prolonged elimination half-life for clarithromycin and its metabolite have been reported. A dosage adjustment for clarithromycin should be considered in patients with a creatinine clearance < 1.8 L/h. The recommended goal for dosage regimens of clarithromycin is to ensure that the time that unbound drug concentrations in the blood remains above the minimum inhibitory concentration is at least 40 to 60% of the dosage interval. However, the concentrations and in vitro activity of 14-hydroxyclarithromycin must be considered for pathogens such as Haemophilus influenzae. In addition, clarithromycin achieves significantly higher drug concentrations in the epithelial lining fluid and alveolar macrophages, the potential sites of extracellular and intracellular respiratory tract pathogens, respectively. Further studies are needed to determine the importance of these concentrations of clarithromycin at the site of infection. Clarithromycin can increase the steady-state concentrations of drugs that are primarily depend upon CYP3A metabolism (e.g., astemidole, cisapride, pimozide, midazolam and triazolam). This can be clinically important for drugs that have a narrow therapeutic index, such as carbamazepine, cyclosporin, digoxin, theophylline and warfarin. Potent inhibitors of CYP3A (e.g., omeprazole and ritonavir) may also alter the metabolism of clarithromycin and its metabolites. Rifampicin (rifampin) and rifabutin are potent enzyme inducers and several small studies have suggested that these agents may significantly decrease serum clarithromycin concentrations. Overall, the pharmacokinetic and pharmacodynamic studies suggest that fewer serious drug interactions occur with clarithromycin compared with older macrolides such as erythromycin and troleandomycin.

The macrolides: erythromycin, clarithromycin, and azithromycin

In addition to erythromycin, macrolides now available in the United States include azithromycin and clarithromycin. These two new macrolides are more chemically stable and better tolerated than erythromycin, and they have a broader antimicrobial spectrum than erythromycin against Mycobacterium avium complex (MAC), Haemophilus influenzae, nontuberculous mycobacteria, and Chlamydia trachomatis. All three macrolides have excellent activity against the atypical respiratory pathogens (C. pneumoniae and Mycoplasma species) and the Legionella species. Azithromycin and clarithromycin have pharmacokinetics that allow shorter dosing schedules because of prolonged tissue levels. Both azithromycin and clarithromycin are active agents for MAC prophylaxis in patients with late-stage acquired immunodeficiency syndrome (AIDS), although azithromycin may be the preferable agent because of fewer drug-drug interactions. Clarithromycin is the most active MAC antimicrobial agent and should be part of any drug regimen for treating active MAC disease in patients with or without AIDS. Although both azithromycin and clarithromycin are well tolerated by children, azithromycin has the advantage of shorter treatment regimens and improved tolerance, potentially improving compliance in the treatment of respiratory tract and skin or soft tissue infections. Intravenously administered azithromycin has been approved for treatment of adults with mild to moderate community-acquired pneumonia or pelvic inflammatory diseases. An area of concern is the increasing macrolide resistance that is being reported with some of the common pathogens, particularly Streptococcus pneumoniae, group A streptococci, and H. influenzae. The emergence of macrolide resistance with these common pathogens may limit the clinical usefulness of this class of antimicrobial agents in the future.

Review and comparison of advanced-generation macrolides clarithromycin and dirithromycin

We reviewed English-language clinical studies, abstracts, and review articles identified from MEDLINE searches from January 1966-August 1998, and bibliographies of identified articles to compare advanced-generation macrolides dirithromycin and clarithromycin and their use for respiratory tract infections. Both agents have superior adverse effect profiles compared with erythromycin, the original macrolide. Both have broad antibacterial coverage, but clarithromycin usually has a lower MIC90 to susceptible organisms than dirithromycin; for most isolates this difference is not clinically significant. Clarithromycin has better in vitro coverage of Haemophilus influenzae, but this activity varies with formation of its bioactive metabolite, 14-hydroxyclarithromycin. Neither agent is ideal for H. influenzae eradication. The agents differ markedly in terms of pharmacokinetics, pharmacodynamics, metabolism, and cost, and thus with respect to drug interaction profiles and dosages. Dirithromycin's drug interaction profile is markedly better than clarithromycin's. Clarithromycin is dosed twice/day; dirithromycin's pharmacokinetics allow once/day dosing. Dirithromycin is less expensive with regard to both cost/day and cost/treatment regimen. Clarithromycin has been studied and approved for administration to children. In adults with respiratory tract infections who are receiving drugs that would interact with clarithromycin, and in those with renal dysfunction with or without coexisting hepatic dysfunction, dirithromycin appears to be superior in terms of safety and equivalent to clarithromycin in terms of efficacy.

A multiple drug interaction study of stavudine with agents for opportunistic infections in human immunodeficiency virus-infected patients

The effects of multiple opportunistic infection medications on stavudine pharmacokinetics were evaluated. Ten patients with CD4 counts of less than 200 cells/mm3 received stavudine (40 mg twice daily) in combination with one to three other drugs used to treat opportunistic infections. Serial blood samples for stavudine concentrations were collected after 1 week of therapy on each regimen and assayed for stavudine by using a validated high-pressure liquid chromatography method. Although the maximum concentration of drug in serum was significantly decreased when the drug was given in combination with three opportunistic infection medications, the area under the concentration-time curve did not significantly differ across various treatment regimens. Stavudine exposure was not significantly altered by multiple concomitant medications. Side effects were minor throughout the 3-month study period. The tolerability of stavudine, combined with its lack of drug interactions, makes it an attractive agent for use as part of a combination regimen.

An update on drug interactions with zidovudine

Zidovudine remains one of the most commonly prescribed agents for patients with HIV infection. A variety of drug interactions have been reported that may alter blood levels of zidovudine and concomitantly administered drugs or may involve overlapping toxicities. Several concomitant medications have been shown to alter the plasma concentrations of zidovudine, although most of these interactions are not thought to be clinically important. Use of agents that share the bone marrow-suppressive properties of zidovudine require additional monitoring for the development of hematologic toxicity. An understanding of potential mechanisms and important zidovudine-related interactions will aid in the optimal care of the HIV-infected patient.