Cerebrospinal fluid penetration of imipenem and cilastatin (primaxin) in children with central nervous system infections

Recent advances in the ontogeny of drug disposition

Developmental changes that occur throughout childhood have long been known to impact drug disposition. However, pharmacokinetic studies in the paediatric population have historically been limited due to ethical concerns arising from incorporating children into clinical trials. As such, much of the early work in the field of developmental pharmacology was reliant on difficult-to-interpret in vitro and in vivo animal studies. Over the last 2 decades, our understanding of the mechanistic processes underlying age-related changes in drug disposition has advanced considerably. Progress has largely been driven by technological advances in mass spectrometry-based methods for quantifying proteins implicated in drug disposition, and in silico tools that leverage these data to predict age-related changes in pharmacokinetics. This review summarizes our current understanding of the impact of childhood development on drug disposition, particularly focusing on research of the past 20 years, but also highlighting select examples of earlier foundational research. Equally important to the studies reviewed herein are the areas that we cannot currently describe due to the lack of research evidence; these gaps provide a map of drug disposition pathways for which developmental trends still need to be characterized.

Nontherapeutic equivalence of a generic product of imipenem-cilastatin is caused more by chemical instability of the active pharmaceutical ingredient (imipenem) than by its substandard amount of cilastatin

Background

We demonstrated therapeutic nonequivalence of “bioequivalent” generics for meropenem, but there is no data with generics of other carbapenems.

Methods

One generic product of imipenem-cilastatin was compared with the innovator in terms of in vitro susceptibility testing, pharmaceutical equivalence, pharmacokinetic (PK) and pharmacodynamic (PD) equivalence in the neutropenic mouse thigh, lung and brain infection models. Both pharmaceutical forms were then subjected to analytical chemistry assays (LC/MS).

Results and conclusion

The generic product had 30% lower concentration of cilastatin compared with the innovator of imipenem-cilastatin. Regarding the active pharmaceutical ingredient (imipenem), we found no differences in MIC, MBC, concentration or potency or AUC, confirming equivalence in terms of in vitro activity. However, the generic failed therapeutic equivalence in all three animal models. Its E_max against S. aureus in the thigh model was consistently lower, killing from 0.1 to 7.3 million less microorganisms per gram in 24 hours than the innovator (P = 0.003). Against K. pneumoniae in the lung model, the generic exhibited a conspicuous Eagle effect fitting a Gaussian equation instead of the expected sigmoid curve of the Hill model. In the brain infection model with P. aeruginosa, the generic failed when bacterial growth was >4 log₁₀ CFU/g in 24 hours, but not if it was less than 2.5 log₁₀ CFU/g. These large differences in the PD profile cannot be explained by the lower concentration of cilastatin, and rather suggested a failure attributable to the imipenem constituent of the generic product. Analytical chemistry assays confirmed that, besides having 30% less cilastatin, the generic imipenem was more acidic, less stable, and exhibited four different degradation masses that were absent in the innovator.

Pediatric tuberculous meningitis: Model-based approach to determining optimal doses of the anti-tuberculosis drugs rifampin and levofloxacin for children

Pediatric tuberculous meningitis (TBM) is a highly morbid, often fatal disease. Standard treatment includes isoniazid, rifampin, pyrazinamide, and ethambutol. Current rifampin dosing achieves low cerebrospinal fluid (CSF) concentrations, and CSF penetration of ethambutol is poor. In adult trials, higher-dose rifampin and/or a fluoroquinolone reduced mortality and disability. To estimate optimal dosing of rifampin and levofloxacin for children, we compiled plasma and CSF pharmacokinetic (PK) and outcomes data from adult TBM trials plus plasma PK data from children. A population PK/pharmacodynamic (PD) model using adult data defined rifampin target exposures (plasma area under the curve (AUC)0-24 = 92 mg*h/L). Levofloxacin targets and rifampin pediatric drug disposition information were literature-derived. To attain target rifampin exposures, children require daily doses of at least 30 mg/kg orally or 15 mg/kg intravenously (i.v.). From our pediatric population PK model, oral levofloxacin doses needed to attain exposure targets were 19-33 mg/kg. Our results provide data-driven guidance to maximize pediatric TBM treatment while we await definitive trial results.

Even apparently insignificant chemical deviations among bioequivalent generic antibiotics can lead to therapeutic nonequivalence: the case of meropenem

Several studies with animal models have demonstrated that bioequivalence of generic products of antibiotics like vancomycin, as currently defined, do not guarantee therapeutic equivalence. However, the amounts and characteristics of impurities and degradation products in these formulations do not violate the requirements of the U.S. Pharmacopeia (USP). Here, we provide experimental data with three generic products of meropenem that help in understanding how these apparently insignificant chemical differences affect the in vivo efficacy. Meropenem generics were compared with the innovator in vitro by microbiological assay, susceptibility testing, and liquid chromatography/mass spectrometry (LC/MS) analysis and in vivo with the neutropenic guinea pig soleus infection model (Pseudomonas aeruginosa) and the neutropenic mouse thigh (P. aeruginosa), brain (P. aeruginosa), and lung (Klebisella pneumoniae) infection models, adding the dihydropeptidase I (DHP-I) inhibitor cilastatin in different proportions to the carbapenem. We found that the concentration and potency of the active pharmaceutical ingredient, in vitro susceptibility testing, and mouse pharmacokinetics were identical for all products; however, two generics differed significantly from the innovator in the guinea pig and mouse models, while the third generic was therapeutically equivalent under all conditions. Trisodium adducts in a bioequivalent generic made it more susceptible to DHP-I hydrolysis and less stable at room temperature, explaining its therapeutic nonequivalence. We conclude that the therapeutic nonequivalence of generic products of meropenem is due to greater susceptibility to DHP-I hydrolysis. These failing generics are compliant with USP requirements and would remain undetectable under current regulations.

Pharmacokinetics of antibacterial agents in the CSF of children and adolescents

The adequate management of central nervous system (CNS) infections requires that antimicrobial agents penetrate the blood-brain barrier (BBB) and achieve concentrations in the CNS adequate for eradication of the infecting pathogen. This review details the currently available literature on the pharmacokinetics (PK) of antibacterials in the CNS of children. Clinical trials affirm that the physicochemical properties of a drug remain one of the most important factors dictating penetration of antimicrobial agents into the CNS, irrespective of the population being treated (i.e. small, lipophilic drugs with low protein binding exhibit the best translocation across the BBB). These same physicochemical characteristics determine the primary disposition pathways of the drug, and by extension the magnitude and duration of circulating drug concentrations in the plasma, a second major driving force behind achievable CNS drug concentrations. Notably, these disposition pathways can be expected to change during the normal process of growth and development. Finally, CNS drug penetration is influenced by the nature and extent of the infection (i.e. the presence of meningeal inflammation). Aminoglycosides have poor CNS penetration when administered intravenously. Intrathecal gentamicin has been studied in children with more promising results, often exceeding the minimum inhibitory concentration. There are very limited data with intrathecal tobramycin in children. However, in the few patients that have been studied, the CSF concentrations were highly variable. Penicillins generally have good CNS penetration. Aqueous penicillin G reaches greater concentrations than procaine or benzathine penicillin. Concentrations remain detectable for ≥ 12 h. Of the aminopenicillins, both ampicillin and parenteral amoxicillin reach adequate CNS concentrations; however, orally administered amoxicillin resulted in much lower concentrations. Nafcillin and piperacillin are the final two penicillins with pediatric data: their penetration is erratic at best. Cephalosporins vary greatly in regard to their CSF penetration. Few first- and second-generation cephalosporins are able to reach higher CSF concentrations. Cefuroxime is the only exception and is usually avoided due to its adverse effects and slower sterilization of the CSF than third-generation agents. Ceftriaxone, cefotaxime, ceftazidime, cefixime and cefepime have been studied in children and are all able to adequately penetrate the CSF. As with penicillins, concentrations are greatest in the presence of meningeal inflammation. Meropenem and imipenem are the only carbapenems with pediatric data. Imipenem reaches higher CSF concentrations; however, meropenem is preferred due to its lower incidence of seizures. Aztreonam has also demonstrated favorable penetration but only one study has been completed in children. Both chloramphenicol and sulfamethoxazole/trimethoprim (cotrimoxazole) penetrate into the CNS well; however, significant toxicities limit their use. The small size and minimal protein binding of fosfomycin contribute to its favorable CNS PK. Although rarely used, it achieves higher concentrations in the presence of inflammation and accumulation is possible. Linezolid reaches high CSF concentrations; however, more frequent dosing might be required in infants due to their increased elimination. Metronidazole also has very limited information but it demonstrated favorable results similar to adult data; CSF concentrations even exceeded plasma concentrations at certain time points. Rifampin (rifampicin) demonstrated good CNS penetration after oral administration. Vancomycin demonstrates poor CNS penetration after intravenous administration. When combined with intraventricular therapy, CNS concentrations are much greater. Of the antituberculosis agents, isoniazid, pyrazinamide and streptomycin have been studied in children. Isoniazid and pyrazinamide have favorable CSF penetration. Streptomycin appears to produce unpredictable CSF levels. No pediatric-specific data are available for clindamycin, daptomycin, macrolides, tetracyclines, and fluoroquinolones. Daptomycin, fluoroquinolones, and tetracyclines have demonstrated favorable CNS penetration in adults; however, data are limited due to their potential pediatric-specific toxicities and newness within the marketplace. Macrolides and clindamycin have demonstrated poor CNS penetration in adults and thus have not been studied in pediatrics.

Oral infections and antibiotic therapy

Oral infections commonly originate from an odontogenic source in adults and from tonsil and lymphatic sources in children. Odontogenic infections arise from advanced dental caries or periodontal disease. Oral trauma, radiation injury, chemotherapy mucositis, salivary gland infection, lymph node abscess, and postoperative infection are potential nonodontogenic sources of infections that could potentially be life threatening. This article reviews the serious nature and potential danger that exists from oral infection and the antibiotics available to treat them are reviewed. Successful treatment requires an understanding of the microflora, the regional anatomy, the disease process, the treatment methods available, and interdisciplinary team collaboration.

[Presumptive bacterial meningitis in adults: initial antimicrobial therapy]

CSF sterilization should be obtained very rapidly to reduce both mortality and morbidity due to bacterial meningitis. Thus, antibiotic treatment should be adapted to the suspected bacterium and administered as early as possible at high dosage with - if necessary - a loading dose and continuous perfusion. The rates of abnormal susceptibility to penicillin of Streptococcus pneumoniae, Neisseria meningitis and Haemophilus influenzae are 37%, 30% and 12% respectively. Thus, ceftriaxone or cefotaxim must be used as empirical treatment. Listeria monocytogenes remains fully susceptible to aminopenicillin, so, the combination aminopenicillin and aminoglycoside is the first-line treatment. Antibiotic resistance, allergy or contra-indications, are in fact rare but in these cases, antibiotic combinations are often needed. The latter are more or less complex and clinically validated; they include molecules such as vancomycine, fosfomycin, fluoroquinolone or linezolid.

Pharmacokinetic-pharmacodynamic modeling of the electroencephalogram effect of imipenem in healthy rats

A pharmacokinetic-pharmacodynamic (PK-PD) modeling approach was developed to investigate the epileptogenic activity of imipenem in rats. Initially, animals received an intravenous infusion of imipenem at a rate of 2.65 mg min(-1) for 30 min. Blood samples were collected for drug assay, and an electroencephalogram (EEG) was recorded during infusion and postinfusion. A dramatic delay was observed between concentrations of imipenem in serum and the EEG effect; this effect was accompanied by tremors and partial seizures. Indirect-effect models failed to describe these data, which were successfully fitted using an effect compartment model. The relationship between effect and concentration at the effect site was best described by a spline function. The elimination rate constant from the effect compartment was severalfold lower than that from the central compartment. The robustness of the model was then confirmed after administering the imipenem dose over 60 and 90 min. In conclusion, the successful PK-PD modeling of the imipenem EEG effect in rats constitutes a major improvement for better prediction of the epileptogenic risk associated with this antibiotic.

Comparative cerebrospinal fluid diffusion of imipenem and meropenem in rats

The main objective of this study was to compare the cerebrospinal fluid (CSF) diffusion of imipenem and meropenem at steady state, following intravenous infusions at various rates in rats. A preliminary experiment was conducted to estimate the elimination half-lives of these two carbapenem antibiotics, and then to evaluate the infusion duration necessary to reach steady state. CSF diffusion of imipenem was essentially linear over the wide range of infusion rates (66-1,320microg min(-1)) and corresponding steady-state plasma concentrations (11.7-443.0 microg mL(-1)). Conversely the CSF diffusion of meropenem was saturable, with a predicted maximum CSF concentration equal to 1.3 microg mL(-1). Extrapolation of these data to the clinical situation may not be possible since the rats had normal blood-brain and blood-CSF barriers whereas patients with diseases such as meningitis may not. However, it is suggested that the observed differences in the diffusion characteristics of imipenem and meropenem may be partly responsible for their differences in toxicity and efficacy at the central level.

Cerebrospinal fluid pharmacokinetics of cefpodoxime proxetil in piglets

Cefpodoxime is an oral third-generation cephalosporin used for the treatment of acute upper-respiratory tract infections caused by susceptible bacteria in children. Although not indicated for the treatment of bacterial meningitis, it is used to treat other infections produced by organisms associated with meningitis and may obscure the result of cerebrospinal fluid (CSF) cultures in children who develop meningitis while receiving oral antibiotics if sufficient concentrations are achieved in the CSF. This study evaluated the disposition of cefpodoxime and penetration into CSF in piglets. Fifteen Landacre-Camborough cross piglets (10-20 days old) received cefpodoxime proxetil oral suspension (10 mg/kg). Repeated plasma and CSF samples were collected over 24 hours for quantitation of cefpodoxime by HPLC. Pharmacokinetic analysis was performed on both plasma and CSF data. The plasma concentration versus time data for cefpodoxime were best characterized using a one-compartment model with first-order absorption. The mean (+/- SD) pharmacokinetic parameters for Cmax, tmax, and AUC0-infinity were 23.3 +/- 12.9 mg/L, 3.9 +/- 1.4 h, and 237 +/- 129 mg/L.h, respectively. CSF/plasma ratios for AUC0-infinity demonstrated a mean cefpodoxime penetration of approximately 5%. CSF penetration of cefpodoxime was evident following a single oral dose of cefpodoxime proxetil suspension. Despite the small percentage of total cefpodoxime dose distributing into the CSF, the resultant concentrations approached or exceeded the MIC90 for many bacterial pathogens considered susceptible to cefpodoxime. Accordingly, clinicians should use caution in the interpretation of CSF cultures in patients who develop clinical signs and symptoms consistent with meningitis and who have been previously treated with cefpodoxime.

Antimicrobial therapy for infants and children: guidelines for the inpatient and outpatient practice of pediatriac infectious diseases

In this article, we discuss antimicrobial regimens for both outpatient and inpatient use in infants and children. A substantial number of pediatric patient visits annually result in the prescribing of antimicrobial drugs. The emergence of bacteria resistant to commonly used antimicrobial agents is a growing concern. Information on newer drugs such as meropenem, which is active against penicillin-resistant Streptococcus pneumoniae and gram-negative bacilli, and cefepime, which has activity against gram-negative bacilli including Pseudomonas aeruginosa and against gram-positive cocci is also presented. Management of patients with congenital or acquired immunodeficiencies continues to be challenging in regard to the use of antimicrobial drugs to treat various fungal and viral infections. New formulations of older drugs such as aerosolized tobramycin and amphotericin B lipid complex are available. New antiviral agents have been approved, most of which are antiretroviral agents. Childhood tuberculosis is an ongoing concern, and regimens to treat Mycobacterium tuberculosis in children are discussed.

The penetration of anti-infectives into the central nervous system

The blood-brain barrier, blood-cerebrospinal fluid (CSF) barrier, and meninges are a complex and difficult-to-study system charged with protecting the central nervous system (CNS) from toxins, including drugs. Current estimates of CNS drug exposure are limited to CSF to blood ratios, of which area-under-the curve (AUC) estimates provide the most robust measure of drug exposure. Different classes of drugs and individual drugs within classes have different CNS penetration potential that is dependent upon a variety of biologic and pharmacologic factors. Clinical data (AUC and point ratios) regarding the penetration of several anti-infective agents used for the treatment of CNS infections are provided in this article.

Pharmacokinetics and pharmacodynamics of antibiotics in meningitis

The penetration of antimicrobials into the CSF is dependent on lipid solubility, molecular size, capillary and choroid plexus efflux pumps, protein binding, and the degree of inflammation. Penicillins, certain cephalosporins, carbapenems, fluoroquinolones, vancomycin, and rifampin provide the highest ratios of CSF levels to the MBC for common infecting organisms. For beta-lactam antibiotics, it is the duration of time that CSF concentrations exceed the MBC that determines the rate of bactericidal activity. It appears that levels should exceed the MBC for more than 50% of the dosing interval. The peak/MBC and AUC/MBC ratios are important determinants of efficacy for aminoglycosides and fluoroquinolones. Once-daily dosing of aminoglycosides is as effective as multiple-daily dosing regimens in experimental meningitis, probably because of drug-induced prolonged persistent effects. Fluoroquinolones do not produce as prolonged persistent effects and are slightly less effective when administered once daily. Although steroid use can reduce the penetration and decrease the bactericidal activity of some antimicrobials, such as vancomycin, in experimental meningitis, the clinical impact of steroid use in human meningitis is still unclear.

Pharmacokinetics of anti-infective agents in paediatric patients

Various differences in drug disposition exist between children and adults. For example, the volume of distribution (Vd) for many drugs is larger in children than in adults. Other parameters, including excretion and elimination may be altered in children compared with adults. The penicillins and cephalosporins are used commonly for the treatment of infection in paediatric patients. The increased Vd in children contributes to the increased elimination half-life of these agents. Clearance of the acylureido-penicillins is increased in children with cystic fibrosis, a disease that decreases the elimination half-life for these drugs. Aminoglycosides distribute into extracellular fluid and their pharmacokinetic profile is affected by changes in Vd. The Vd for aminoglycosides is slightly higher in children than in adults. Children with cystic fibrosis, burns, or cancer have higher clearance rates and larger Vd values for aminoglycosides. Few data in the literature address the pharmacokinetics of other anti-infective agents, including vancomycin, teicoplanin, erythromycin, metronidazole, chloramphenicol, and cotrimoxazole (trimethoprim-sulfamethoxazole), in children. Similarly, there is little information regarding the pharmacokinetic profile of antivirals and antifungals in children. Dosage guidelines are available to enable the clinician to initiate anti-infective therapy in children. Subsequent dosage requirements may change based on the patient's current clinical condition. Although several studies have investigated the pharmacokinetics of anti-infectives in neonates and adults, data for children are limited. Therefore, further studies are required so that the ever growing arsenal of anti-infectives can be administered appropriately to children.

Antibiotic therapy for severe infections in infants and children

In infants and children, the absorption, distribution, metabolism, and excretion of drugs may differ considerably in comparison with these factors in adults; consequently, differences exist in therapeutic efficacy and toxicity of various antibiotic agents. Because of known toxicity, certain drugs--such as chloramphenicol in high doses, the sulfonamides, and tetracycline--should not be used in neonates. Antibiotic therapy should be modified in neonates because of biologic immaturity of organs important for the termination of drug action. Because of poor conjugation, inactivation, or excretion, the serum concentrations of many antibiotics may be higher and more prolonged in neonates than in older infants; thus, lower doses and longer intervals between administration may be necessary. In this article, we suggest dosages of antimicrobial agents for severe infections in children, older infants, and neonates. Included in the discussion are the cephalosporins, especially the third-generation cephalosporins that have assumed an important role in empiric treatment of bacterial meningitis in pediatric patients because of their ability to penetrate the central nervous system and their effectiveness against beta-lactamase-positive and negative strains of Haemophilus influenzae type b, Streptococcus pneumoniae, Neisseria meningitidis, and many gram-negative bacteria in the Enterobacteriaceae group. In patients with congenital or acquired immunodeficiencies, antifungal, antiviral, or anti-Pneumocystis agents are often added to the antimicrobial regimen for severe infections. We review the agents available for such treatment in children, the drugs used for childhood tuberculosis, and certain new antibiotics (aztreonam, ticarcillin-clavulanate, ciprofloxacin, and imipenem-cilastatin) that have proved useful in select cases but whose precise role in pediatric practice will necessitate additional clinical experience.

Antibiotic susceptibility of 629 bacterial blood and CSF isolates from Swedish infants and the therapeutic implications

Blood and CSF isolates (n = 629) from Swedish infants up to one year of age were tested in vitro against 13 antimicrobial agents in order to update the guidelines for empiric therapy of septicaemia and meningitis. Ampicillin plus gentamicin provided inadequate empiric therapy for meningitis, due to the poor CSF penetration of the aminoglycoside and the frequent occurrence of bacterial resistance to ampicillin. Ceftazidime and cefuroxime were moderately active, particularly against isolates from small infants. Cefotaxime today seemed to provide the best empiric therapy of septicaemia and meningitis in infants. Because of the occurrence of Listeria and enterococcal infections, ampicillin should initially be added and other combinations are also advisable for the occasional cases of Enterobacter, Citrobacter, Serratia, and Pseudomonas infections. For coagulase-negative staphylococci only vancomycin offered a broad activity (100% at achievable serum levels).

Clinical pharmacology of imipenem and cilastatin in premature infants during the first week of life

The first-dose and multidose pharmacokinetics of imipenem and cilastatin were evaluated in 41 premature infants during their first week of life. Premature infants (gestational age, less than or equal to 37 weeks) were assigned to receive 10-, 15-, 20-, or 25-mg/kg doses of imipenem-cilastatin (1:1) as a single- or multiple-dose regimen. A total of 39 infants received a single dose, whereas 18 infants received multiple doses. No differences were observed in pharmacokinetic parameter estimates for either agent relative to the dose administered or infant body weight; thus, the data were pooled. Elimination half-life, steady-state volume of distribution, and body clearance averaged 2.5 h, 0.5 liter/kg, and 2.5 ml/min per kg, respectively, for imipenem and 9.1 h, 0.4 liter/kg, and 0.5 ml/min per kg, respectively, for cilastatin. Similar values for these parameter estimates were observed after multidose administration, although substantial accumulation of cilastatin in serum was observed. A total of 21% of the imipenem and 43% of the cilastatin were excreted unchanged in the urine over a 12-h collection period. Corresponding renal clearances averaged 0.4 and 0.2 ml/min per kg for imipenem and cilastatin, respectively. Substantial differences were observed in the route by which imipenem was cleared from the body compared with data from adult volunteers. These data suggest that infants should receive an imipenem dose of 20 mg/kg administered every 12 h for the treatment of bacterial infections outside the central nervous system.