Impacts of the Immune System on Small Molecule Pharmacokinetics

Many weakly basic drug molecules accumulate in the body after repeated dosing leading to adverse events or long-term morbidity. The macrophage is a lucrative cell of the immune system implicated in many instances of bioaccumulation. Due to the endophagocytic role it occupies in maintaining homeostasis and its disseminated nature in biological systems, macrophages create a perfect environment for the accumulation of drug molecules. As a result, macrophages can induce “context-dependent” pharmacokinetics, or deviations from standard, concentration-dependent pharmacokinetics. This phenomenon can cause time, load, or dose-dependent alterations in drug distribution profiles at therapeutic doses. By examining pharmacokinetic impacts of the macrophage and macrophage storage sites, such as the spleen, we can develop more robust and reliable predictive analyses. The FDA approved drug clofazimine (CFZ), used in tuberculosis and leprosy, provides an exceptional experimental system to test the hypothesis of bioaccumulation as a mechanism underlying the context-dependent pharmacokinetics in weak bases. Under therapeutically relevant conditions, CFZ forms membrane-bound supramolecular crystal-like drug inclusions (CLDIs) within lysosomes of resident macrophages. Under daily dosing regimens, serum drug concentration remains constant while the organ-associated mass of drug continues to increase throughout the dosing duration. The pharmacokinetic analysis of CFZ within a single macrophage and macrophage containing organs can yield insights into the mechanistic underpinnings of cellular drug disposition, increasing half-life, and increasing organ associated mass of the drug under constant administration. Integrating physiochemical knowledge of weakly basic molecules with whole body distribution patterns creates a framework for predicting context-dependent pharmacokinetic changes driven by the immune system. Utilizing this approach can directly point to optimal steady-state drug loading to reduce toxicity of exogenous drug molecules. Three global approaches, outlined in this document, were taken to address the impacts of the immune system on pharmacokinetics, (1) exploring the pharmacokinetic impact of a growing macrophage population in the spleen, (2) identifying accumulation patterns underlying drug transport within a single macrophage, and (3) evaluating the pharmacokinetic influence of removing the spleen, a drug-sequestering macrophage reservoir. Through this research, CFZ concentration predictions in the spleen and serum were improved upon over static compartmental modeling approaches by increasing the half-life and expanding the volume of distribution over the course of dosing. By incorporating context-dependent adaptations seen in previous studies into CFZ pharmacokinetics models, more accurate, mechanistically driven computational predictions were observed. The mechanism behind this population driven change in pharmacokinetics was further shown to be driven by immunological factors and macrophage microenvironments. A single-cell computational macrophage model, constructed with physiologically relevant conditions, pointed to a thermodynamically favorable drug flux, leading to massive accumulation of drug into a single macrophage under therapeutic extracellular concentrations. This accumulation was shown to be disrupted in the presence of intracellular pathogens indicating infection as an important pharmacokinetic covariate in small molecule drugs which accumulate within the lysosome. And finally, by removing the spleen and thereby mechanically reducing the total volume of distribution, no tissue dependent differences in CFZ concentration or catabolism was observed. However, the immune disruption caused by drug treatment after spleen removal points to an immune sensitivity that warrants consideration during prolonged drug treatment in asplenia.PhDPharmaceuticsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/176385/1/awillmer_1.pd