Antifungal pharmacodynamic relationships in experimental invasive candidiasis and invasive aspergillosis: implications for antifungal therapy

Invasive candidiasis and invasive aspergillosis are both leading causes of morbidity and mortality in immunocompromised patients. Pharmacodynamics is the study of concentration-effect relationships. In recent years, there have been significant advances in the pharmacodynamics of antibacterial and antiviral agents. In contrast, less is known about the pharmacodynamics of antifungal agents. This thesis examines the pharmacodynamic relationships of antifungal agents for invasive candidiasis and invasive aspergillosis as a means for studying a range of clinically relevant questions which are difficult to resolve using other experimental platforms. The common themes are as follows: (1) the development of novel experimental pharmacodynamic models are required for an understanding of concentration-effect relationships; (2) innate immunological effectors, in combination with antifungal agents play a crucial role in determining the outcome of invasive fungal infections; (3) a delay in the administration of antifungal agents is potentially detrimental to the host; (4) critical events in the invasion of Candida spp. and Aspergillus spp. occur within the initial 24 hour period post infection; (5) mathematical models can be used to further understand and predict system behavior; and (6) population pharmacokinetics and Monte Carlo simulation provide a mechanism to further explore the clinical implications of experimental data. An introduction to the microbiology and pathogenesis of invasive fungal infections, along with the pharmacology of antifungal agents and modeling techniques used are reviewed in Chapter 1. The pharmacodynamic models of invasive candidiasis, developed in Chapter 2, which were then used to study: (1) the antifungal effect of neutrophils, and (2) the effect of a delay in the administration of antifungal agents on the exposure-response relationships. The findings were placed in a pathological context, by defining the histological appearances in the kidney. These studies demonstrated that neutrophils exerted a significant antifungal effect independent of antifungal agents. The antifungal effect of neutrophils was quantified using a mathematical model. A progressive delay in the administration of antifungal agents resulted in a diminishing antifungal effect. The data underscores the importance of adequate numbers of neutrophils and the administration of antifungal agents at the earliest possible point in time to maximize the probability of a favorable therapeutic outcome. Chapter 3 investigates the utility of combining the antifungal agents amphotericin B and 5- fluorocytosine (5FC) for the treatment of invasive candidiasis. Using a surface response methodology, the combination was shown to be additive. The clinical implications of this finding were explored using population pharmacokinetics and Monte Carlo simulation; this approach showed that the currently recommended dosage of 5FC is significantly in excess of that required to produce near maximal effect, and therefore unnecessarily exposes recipients to the risk of drug related toxicity. In Chapter 4, a unique molecular mechanism of resistance of C. albicans to 5FC was defined. In Chapter 5, a definition of a drug-exposure breakpoint for 5FC was sought (i.e. a value which separates individuals into groups with a satisfactory versus a suboptimal outcome), and the clinical implications of this finding were explored. Isolates in which the molecular mechanism of 5FC resistance had been defined in Chapter 4, were used. These experimental data were then bridged to humans to predict a susceptibility breakpoint of 5FC. Chapter 6 represents a synthesis of all of the skills acquired in previous chapters, to develop a novel in vitro model of the human alveolus. This model was used to study the pathogenesis of early invasive pulmonary aspergillosis, and the pharmacodynamic relationships of amphotericin B against A. fumigatus. A long standing problem related to the measurement of the residual Aspergillus burden following antifungal therapy was resolved with the use of the Aspergillus cell-wall antigen, galactomannan. The invasion of Aspergillus was studied using confocal microscopy and an A. fumigatus transformant expressing green fluorescent protein. The growth of Aspergillus could not be suppressed with antifungal drugs alone; rather, a combination of amphotericin B and macrophages was required to suppress growth. A large mathematical model was developed to define the concentration of amphotericin B and number of macrophages required to suppress fungal growth. The clinical implications of the experimental data were explored using population pharmacokinetics and Monte Carlo simulation. In the concluding chapter, the advantages of a pharmacodynamic approach and the future challenges are discussed