The antimicrobial therapy puzzle: could pharmacokinetic-pharmacodynamic relationships be helpful in addressing the issue of appropriate pneumonia treatment in critically ill patients?

Until recently, the in vitro susceptibility of microorganisms was considered the only fundamental aspect for antibiotic efficacy in treating pneumonia. However, the relevance of pharmacokinetic-pharmacodynamic relationships in optimizing drug exposure has been progressively highlighted. Antimicrobial agents were divided into concentration-dependent or time-dependent groups, with the most consistently relevant pharmacodynamic parameters for efficacy being either the ration of the plasma peak concentration to the minimum inhibitory concentration or the time the plasma concentration persists above the minimum inhibitory concentration of the etiological agent, respectively. For the adequate treatment of pneumonia, optimal pharmacodynamic exposure should be ensured also at the infection site. To investigate this, a methodologically correct approach may be to detect drug concentration levels in the epithelial lining fluid and in the alveolar macrophages for extracellular and intracellular pathogens, respectively. From this perspective, the pharmacokinetic factors--only in some instances--support the achievement of optimal exposure during the treatment of pneumonia with fixed standard dosing regimens of antimicrobials; conversely, in other instances, the pharmacokinetic factors suggest the need for an implemented dosage regimen or even the choice of a different drug.