The economic potential of dual individualisation methodologies

Selecting antibacterials for outpatient parenteral antimicrobial therapy : pharmacokinetic-pharmacodynamic considerations

Some infectious diseases require management with parenteral therapy, although the patient may not need hospitalisation. Consequently, the administration of intravenous antimicrobials in a home or infusion clinic setting has now become commonplace. Outpatient parenteral antimicrobial therapy (OPAT) is considered safe, therapeutically effective and economical. A broad range of infections can be successfully managed with OPAT, although this form of treatment is unnecessary when oral therapy can be used. Many antimicrobials can be employed for OPAT and the choice of agent(s) and regimen should be based upon sound clinical and microbiological evidence. Assessments of cost and convenience should be made subsequent to these primary treatment outcome determinants. When designing an OPAT treatment regimen, the pharmacokinetic and pharmacodynamic characteristics of the individual agents should also be considered. Pharmacokinetics (PK) is the study of the time course of absorption, distribution, metabolism and elimination of drugs (what the body does to the drug). Clinical pharmacokinetic monitoring has been used to overcome the pharmacokinetic variability of antimicrobials and enable individualised dosing regimens that attain desirable antimicrobial serum concentrations. Pharmacodynamics (PD) is the study of the relationship between the serum concentration of a drug and the clinical response observed in a patient (what the drug does to the body). By combining pharmacokinetic properties (peak [C(max)] or trough [C(min)] serum concentrations, half-life, area under the curve) and pharmacodynamic properties (susceptibility results, minimum inhibitory concentrations [MIC] or minimum bactericidal concentrations [MBC], bactericidal or bacteriostatic killing, post-antibiotic effects), unique PK/PD parameters or indices (t > MIC, C(max)/MIC, AUC(24)/MIC) can be defined. Depending on the killing characteristics of a given class of antimicrobials (concentration-dependent or time-dependent), specific PK/PD parameters may predict in vitro bacterial eradication rates and correlate with in vivo microbiologic and clinical cures. An understanding of these principles will enable the clinician to vary dosing schemes and design individualised dosing regimens to achieve optimal PK/PD parameters and potentially improve patient outcomes. This paper will review basic principles of useful PK/PD parameters for various classes of antimicrobials as they may relate to OPAT. In summary, OPAT has become an important treatment option for the management of infectious diseases in the community setting. To optimise treatment course outcomes, pharmacokinetic and pharmacodynamic properties of the individual agents should be carefully considered when designing OPAT treatment regimens.

Antimicrobial action and pharmacokinetics/pharmacodynamics: the use of AUIC to improve efficacy and avoid resistance

In in-vitro and in animal models, antibiotics show good relationships between concentration and response, when response is quantified as the rate of bacterial eradication. The strength of these in-vitro relationships promises their utility for dosage regimen design and predictable cure of human infections. Resistance is also predictable from these parameters, fostering a rational means of using dosing adjustments to avoid or minimize the development of resistant organisms. Newly developed computerized methods for the quantitation of susceptibility allow testing of integrated kinetic-susceptibility models in patients. Our attention has focused recently on fluoroquinolones, since they are relatively non-toxic and provide the necessary range of dosage needed to elucidate correlations between concentration and response in the Intensive Care Unit patient. Studies conducted in patients with nosocomial gram-negative pneumonia reveal good correlations between bacterial eradication and integration of concentration with bacterial susceptibility. In patients, the best correlation parameters are time over MIC, and the ratio of 24-hour AUC to MIC (AUIC). Patients with serious infections like nosocomial pneumonia require bactericidal antimicrobial activity. Studies in our laboratory demonstrate that the minimum effective antimicrobial action is an area under the inhibitory titer (AUIC) of 125, where AUIC is calculated as the 24-hour serum AUC divided by the MIC of the pathogen. This target AUIC may be achieved with either a single antibiotic or it can be the sum of AUIC values of two or more antibiotics. There is considerable variability in the actual AUIC value for patients when antibiotics are given in their usually recommended dosages. Examples of this variance will be provided using aminoglycosides, fluoroquinolones, beta-lactams, macrolides and vancomycin. The achievement of minimally effective antibiotic action, consisting of an AUIC of at least 125, is associated with bacterial eradication in about 7 days for beta-lactams and quinolones. When AUIC is increased to 250, the quinolone ciprofloxacin (which displays in vivo concentration dependent bacterial killing) can eliminate the bacterial pathogen in 1-2 days. Beta lactams, even when dosed to an AUIC of 250, often require longer treatment duration to eliminate the bacterial pathogen, because the in vivo bacterial killing rate is slower with beta-lactams than with the quinolones. This remains true even at AUIC values of 250 for both compounds, which is theoretically identical dosing. Antibiotic activity indices allow clinicians to evaluate individualized patient regimens. Furthermore, antibiotic activity is a predictable clinical endpoint with predictable clinical outcome. This value is also highly predictive of the development of bacterial resistance. Antimicrobial regimens that do not achieve an AUIC of at least 125 cannot prevent the selective pressure that leads to overgrowth of resistant bacterial sub-populations. Indeed, there is considerable anxiety that conventional respiratory tract infection management strategies, which prescribe antibacterial dosages that may attain AUIC values below 125, are contributing to the pandemic rise in bacterial resistance levels.

Basis of anti-infective therapy: pharmacokinetic-pharmacodynamic criteria and methodology for dual dosage individualisation

Antimicrobial therapy should be designed on the basis of microbiological, as well as pharmacokinetic, criteria; microbiological parameters provide information about the susceptibility of the pathogen responsible for the infectious process while pharmacokinetic parameters give information about the potential ability of the drug in question to reach and remain at the sites of infection in the body. Microbiological parameters such as the minimum inhibitory concentration, minimum bactericidal concentration, bacterial titre, bactericidal rate and 'post-antibiotic effect' (PAE) must be considered. Among the pharmacokinetic parameters, the maximum serum concentration at steady state (CmaxSS), area under the concentration-time curve (AUC) and length of time that the serum concentrations exceed a particular value are the most useful in this context. Different relationships between these parameters, known as efficacy indices, have been established to predict the potential efficacy of antibacterial therapy. Antimicrobial dosage individualisation should be based on the optimisation of the efficacy index that best correlates with patient response. It seems appropriate to establish the degree of correlation among the different efficacy indices and clinical response observed in patients by means of a correlation analysis. This type of analysis can be either retrospective or prospective and may be based on linear or maximum response models. Simulation of the plasma concentration curves obtained with the particular regimen administered offers a methodology which is easy to apply and provides the pharmacokinetic information necessary to calculate the different efficacy indices. Information about the susceptibility of the pathogen to the antibacterial in question and about the response to the treatment used is also necessary for the correlation analysis. This type of analysis determines which of the indices is best correlated with efficacy and, hence, is the index to be optimised when attempting to individualise antibacterial therapy for different situations.