In vitro activity and pharmacodynamics of azithromycin and clarithromycin against Streptococcus pneumoniae based on serum and intrapulmonary pharmacokinetics

A Robust and Scalable Process for the Synthesis of Substantially Pure Clarithromycin 9-(E)-Oxime with an Established Control of the (Z)-Isomer Impurity

Controlling the isomeric impurity in a key raw material is always critical to achieve the corresponding pure isomer-free targeted active pharmaceutical ingredient (API) in downstream processing. Clarithromycin 9-(E)-oxime is the key raw material for the synthesis of the 9a-lactam macrolide, which is an interesting scaffold for the synthesis of several bioactive macrolides. Here demonstrated is a scalable process for the preparation of substantially pure clarithromycin 9-(E)-oxime, with less than 1.2% of the (Z)-isomer. The process does not involve a separate time-consuming purification by a crystallization operation to purge the undesired (Z)-oxime isomer. Further, the pure clarithromycin 9-(E)-oxime obtained was subjected to the Beckmann rearrangement, thereby converting it into the pure 9a-lactam scaffold. Additionally, a few other impurities were identified and controlled at each stage. The fine-tuned process was successfully up scaled to a multikilogram scale, enabling the large-scale manufacturing of potential APIs derived from this scaffold.

In vitro activity and pharmacodynamic/pharmacokinetic parameters of clarithromycin and azithromycin: why they matter in the treatment of respiratory tract infections

Clarithromycin and azithromycin are second-generation macrolides established and widely used for treating a range of upper and lower respiratory tract infections. Extensive clinical trials data indicate that these drugs are highly effective in these applications and broadly comparable in their clinical and microbiological effectiveness. However, consideration of pharmacokinetic, metabolic, and tissue-penetration data, including the significant antibacterial activity of the metabolite 14-hydroxy-clarithromycin, plus the findings of pharmacodynamic modeling, provide evidence that the long half-life and lower potency of azithromycin predispose this agent to select for resistant isolates. Comparison of the "mutant-prevention concentrations" of clarithromycin and azithromycin, and examination of large-scale epidemiological data from Canada, also support the view that these drugs differ materially in their propensity to promote resistance among bacterial strains implicated in common respiratory infections, and that clarithromycin may offer important advantages over azithromycin that should be considered when choosing a macrolide to treat these conditions.

Azithromycin Dose To Maximize Efficacy and Suppress Acquired Drug Resistance in Pulmonary Mycobacterium avium Disease

Mycobacterium aviumcomplex is now the leading mycobacterial cause of chronic pneumonia in the United States. Macrolides and ethambutol form the backbone of the regimen used in the treatment of pulmonary disease. However, therapy outcomes remain poor, with microbial cure rates of 4% in cavitary disease. The treatment dose of azithromycin has mostly been borrowed from that used to treat other bacterial pneumonias; there are no formal dose-response studies in pulmonaryM. aviumdisease and the optimal dose is unclear. We utilized population pharmacokinetics and pharmacokinetics/pharmacodynamics-derived azithromycin exposures associated with optimal microbial kill or resistance suppression to perform 10,000 patient Monte Carlo simulations of dose effect studies for daily azithromycin doses of 0.5 to 10 g. The currently recommended dose of 500 mg per day achieved the target exposures in 0% of patients. Exposures associated with optimal kill and resistance suppression were achieved in 87 and 54% of patients, respectively, only by the very high dose of 8 g per day. The azithromycin susceptibility breakpoint above which patients failed therapy on the very high doses of 8 g per day was an MIC of 16 mg/liter, suggesting a critical concentration of 32 mg/liter, which is 8-fold lower than the currently used susceptibility breakpoint of 256 mg/liter. If the standard dose of 500 mg a day were used, then the critical concentration would fall to 2 mg/liter, 128-fold lower than 256 mg/liter. The misclassification of resistant isolates as susceptible could explain the high failure rates of current doses.

Penetration of anti-infective agents into pulmonary epithelial lining fluid: focus on antibacterial agents

The exposure-response relationship of anti-infective agents at the site of infection is currently being re-examined. Epithelial lining fluid (ELF) has been suggested as the site (compartment) of antimicrobial activity against lung infections caused by extracellular pathogens. There have been an extensive number of studies conducted during the past 20 years to determine drug penetration into ELF and to compare plasma and ELF concentrations of anti-infective agents. The majority of these studies estimated ELF drug concentrations by the method of urea dilution and involved either healthy adult subjects or patients undergoing diagnostic bronchoscopy. Antibacterial agents such as macrolides, ketolides, newer fluoroquinolones and oxazolidinones have ELF to plasma concentration ratios of >1. In comparison, β-lactams, aminoglycosides and glycopeptides have ELF to plasma concentration ratios of ≤1. Potential explanations (e.g. drug transporters, overestimation of the ELF volume, lysis of cells) for why these differences in ELF penetration occur among antibacterial classes need further investigation. The relationship between ELF concentrations and clinical outcomes has been under-studied. In vitro pharmacodynamic models, using simulated ELF and plasma concentrations, have been used to examine the eradication rates of resistant and susceptible pathogens and to explain why selected anti-infective agents (e.g. those with ELF to plasma concentration ratios of >1) are less likely to be associated with clinical treatment failures. Population pharmacokinetic modelling and Monte Carlo simulations have recently been used and permit ELF and plasma concentrations to be evaluated with regard to achievement of target attainment rates. These mathematical modelling techniques have also allowed further examination of drug doses and differences in the time courses of ELF and plasma concentrations as potential explanations for clinical and microbiological effects seen in clinical trials. Further studies are warranted in patients with lower respiratory tract infections to confirm and explore the relationships between ELF concentrations, clinical and microbiological outcomes, and pharmacodynamic parameters.

Pharmacokinetic/pharmacodynamic modeling of in vitro activity of azithromycin against four different bacterial strains

The bacterial time-kill curves of azithromycin against four bacterial strains (Streptococcus pneumoniae/penicillin-intermediate, S. pneumoniae/penicillin-sensitive, Haemophilus influenzae and Moraxella catarrhalis) were determined by in vitro infection models. Eighteen different pharmacokinetic/pharmacodynamic models were fitted to the time-kill data using non-linear regression and compared for best fit. A simple, widely used E(max) model was not sufficient to describe the pharmacodynamic effects for the four bacterial strains. Appropriate models that gave good curve fits included additional terms for saturation of the number of bacteria (N(max)), delay in the initial bacterial growth phase and/or the onset of anti-infective activity (1-exp(-zt)) as well as a Hill factor (h) that captures the steepness of the concentration-response profile. Azithromycin was highly effective against S. pneumoniae strains and M. catarrhalis while the efficacy against H. influenzae was poor. Applications of these pharmacokinetic/pharmacodynamic models will eventually provide a tool for rational antibiotic dosing regimen decisions.

Use of Monte Carlo simulation to design an optimized pharmacodynamic dosing strategy for meropenem

Prolonging the infusion of meropenem over 3 hours increases the percentage of the dosing interval that drug concentrations remain above the minimum inhibitory concentration (MIC), thereby maximizing the pharmacodynamics of this agent and adhering to drug stability constraints. Monte Carlo simulation was employed to determine pharmacodynamic target attainment rates for several prolonged infusion (PI) meropenem dosage regimens as compared with the traditional 30-minute infusion (TI) against Enterobacteriaceae, Acinetobacter species, and Pseudomonas aeruginosa populations. Percent time above the MIC (%T>MIC) exposures for 1000 mg TI q8h, 2000 mg TI q8h, 500 mg PI q8h, 1000 mg PI q12h, 1000 mg PI q8h, 2000 mg PI q12h, and 2000 mg PI q8h were simulated for 10,000 subjects. Variability in pharmacokinetic parameters and MIC distributions were derived from studies in healthy volunteers and the MYSTIC surveillance program, respectively. The probabilities of attaining bacteriostatic (30% T>MIC) and bactericidal (50% T>MIC) exposures were high for all dosage regimens against populations of Enterobacteriaceae. Against Acinetobacter species and Pseudomonas aeruginosa, the 2000-mg PI q8h dosage regimen provided the highest target attainment rates. For mild to moderate infections caused by Enterobacteriaceae, prolonged infusion regimens of 500 mg PI q8h and 1000 mg PI q12h would provide equivalent target attainment rates to the traditional 30-minute infusion while requiring less drug over 24 hours. For more serious infections presumably caused by Acinetobacter species or Pseudomonas aeruginosa, a dose of 2000 mg PI q8h is recommended because of its high bactericidal target attainment rate against these pathogens. Further study of these dosage recommendations in clinical trials is suggested.

Pharmacodynamics of Meropenem and Imipenem Against Enterobacteriaceae,Acinetobacter baumannii, andPseudomonas aeruginosa

STUDY OBJECTIVE

To compare the pharmacodynamics of meropenem and imipenem, both administered as 500 mg every 6 hours, against populations of Enterobacteriaceae, Acinetobacter baumannii, and Pseudomonas aeruginosa.

DESIGN

Ten thousand-subject Monte Carlo simulation.

INTERVENTION

Variability in total body clearance (ClT), volume of distribution as calculated by the terminal elimination rate (Vdbeta), and minimum inhibitory concentration (MIC) distributions (Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Serratia marcescens, A. baumannii, P. aeruginosa) were derived from the literature for both meropenem and imipenem. For the free drug concentrations, the percentage of the dosing interval that the drug concentrations remain above the MIC (%T>MIC) for each carbapenem-bacteria combination was calculated for 10,000 iterations, substituting a different ClT, Vdbeta, fraction of unbound drug, and MIC into the equation each time based on the probability distribution for each parameter. Probabilities of attaining targets of 30%, 50%, and 100% T>MIC were calculated.

MEASUREMENTS AND MAIN RESULTS

Meropenem free drug %T>MIC exposure was significantly greater than that of imipenem against Enterobacteriaceae and P. aeruginosa, whereas imipenem exposure was greater for A. baumannii. For both agents, free drug %T>MIC exposure was greatest against Enterobacteriaceae and less for A. baumannii and P. aeruginosa. Probabilities of target attainment for 30% and 50% T>MIC were similar between drugs for most bacteria. At 100% T>MIC, meropenem target attainments were greater than those of imipenem against Enterobacteriaceae and P. aeruginosa, and imipenem attainment was higher for A. baumannii.

CONCLUSION

The probability of attaining lower pharmacodynamic targets for most gram-negative bacteria is similar for these carbapenems; however, differences become apparent as the pharmacodynamic requirement increases. Further study of the benefits of achieving this pharmacodynamic breakpoint with a higher probability of attaining targets is necessary.

International guidelines for the treatment of community-acquired pneumonia in adults: the role of macrolides

The significance of community-acquired pneumonia (CAP) has led to the publication of guidelines from numerous international organisations. Because the macrolide class of antimicrobials is active against most of the key pathogens associated with CAP, agents from this class are commonly included in recommendations from these guidelines. However, there are differences among the various guidelines concerning the positioning of the macrolides for empirical therapy. An important factor concerning the use of macrolides for CAP is the emergence of resistance of Streptococcus pneumoniae over the past decade. The rate of S. pneumoniae resistance to macrolides ranges from 4 to 70% of strains in worldwide surveillance studies. The most common mechanisms of resistance include methylation of a ribosomal target encoded by the erm gene and efflux of the macrolides by a cell membrane protein transporter, encoded by the mef gene. S. pneumoniae strains with the mef gene are resistant at a lower level (with minimum inhibitory concentration [MIC] values generally 1-16 microg/ml) than erm resistant strains; and it is possible that such strains may be inhibited if sufficiently high levels of macrolide can be obtained at the infected site. Currently mef-associated resistance predominates in North America, whereas erm predominates in Europe. Until recently, reports of failure of treatment of CAP with macrolides has been rare, particularly for patients with low-risk for drug-resistant strains. However, since 2000, several patients treated with an oral macrolide who have subsequently required admission to the hospital for macrolide-resistant S. pneumoniae (MRSP) bacteraemia have been reported in the literature. Major issues, which are fundamental to the use of the macrolides as recommended in the various guidelines, include the importance of providing therapy for 'atypical' pathogens and the clinical significance of MRSP. Presently, the macrolides are more prominently recommended in the North American guidelines than in other parts of the world. The difference in the emphasis placed on the importance of the atypical pathogens as well as the expression of MRSP in North America compared with Europe partly explains this variance.

Azithromycin treatment failure in community-acquired pneumonia caused by Streptococcus pneumoniae resistant to macrolides by a 23S rRNA mutation

In this report, we describe an azithromycin treatment failure in community-acquired pneumonia. During the first three days of azithromycin, the patient's symptoms worsened, and she was subsequently admitted to the hospital. Blood cultures were positive for a penicillin-susceptible, macrolide-resistant S. pneumoniae. DNA sequencing revealed an A2059G mutation in domain V of the 23S rRNA. To our knowledge, this is the first clinical report of an azithromycin failure in the treatment of S. pneumoniae resistant to macrolides by this mechanism.