Paclitaxel (Taxol®) is now widely used against breast, ovarian and non-small-cell lung cancer. Anticancer agents generally have narrow therapeutic indices, often with myelosuppression (mainly neutropenia) as dose-limiting side effect. A further complicating factor is that paclitaxel when given as Taxol® has a nonlinear pharmacokinetic (PK) behaviour in plasma. Identifying risk groups more sensitive to chemotherapy due to either a PK or pharmacodynamic (PD) interindividual variability is of importance. The aim of the thesis was to develop predictive mechanism-based PK and PD models applicable for paclitaxel. PK and PK/PD models were developed for patient data from studies with relatively frequent sampling or sparse sampling schedules. Population analyses were performed using the software NONMEM. A pharmacokinetic model describing unbound, total plasma and blood concentrations of paclitaxel from known binding mechanisms was developed and validated. The nonlinear PK in plasma could to a large extent be explained by the micelle forming vehicle Cremophor EL (CrEL) and the unbound drug showed linear PK. Besides a binding component directly proportional to concentrations of CrEL, the model included both linear and nonlinear binding components in plasma and blood. Further, relations between the PK parameters and different demographic factors, including polymorphisms in the cytochrome P450s involved in paclitaxel metabolism, were investigated. A semi-physiological PD model for chemotherapy-induced myelosuppression was developed and applied to different anticancer drugs. The model included a self-renewal for proliferating cells, transit compartments describing the delay in observed myelosuppression and a feedback parameter reflecting the effect on the bone marrow from growth factors that can result in an overshoot in white blood cells. The system-related parameters estimated showed consistency across drugs and the difference in the drug-related parameter reflected the relative bone marrow toxicity of the drugs. Relations between demographic factors and the PD parameters were identified. The developed mechanism-based models promote a better understanding of paclitaxel PK and PD and may be used as tools in dosing individualisation and in development of dosing strategies for new administration forms and new drugs in the same area.

<p>Drug transport across the intestinal epithelium is roughly predicted from permeability values obtained from Caco-2 cell monolayers. This thesis examines the important role of <i>pH</i> and extracellular additives for increasing the reliability and predictivity of the <i>in vitro</i> screening system, Caco-2.</p><p>It was shown that the passive transport of ionizable compounds may be biased by a false efflux or uptake component, when applying a physiological <i>pH</i>-gradient across the membrane. <i>pH</i> also affected the amount of compound available at the transporter-binding site. Therefore, <i>pH</i> dependence should be considered in studies of such compounds and of drug-drug interactions involving efflux transporters. It was also shown that proton-dependent apical uptake or basolateral efflux should be studied both with and without a <i>pH</i> gradient over the whole monolayers. </p><p>The two extracellular additives, bovine serum albumin (BSA) and the solubilizing agent, Cremophor^® EL, also influenced Caco-2 permeabilities. BSA applied to the receiver side increases, and to the donor side decreases drug permeation according to the drug’s protein binding capacity. Thus, the absorptive transport for both passive and active compounds is favoured, giving a physiologically sound improvement of the Caco-2 cell model. Inclusion of BSA increased both the predictivity and quality of permeability studies, particularly of highly lipophilic, BCS class II compounds. Passive and active transport processes could also be distinguished after accounting for unbound concentrations. The overall effect of Cremophor^® EL on the permeability to a drug was compound-specific and probably dependent on micellar incorporation. Cremophor^® EL can therefore not be recommended. </p><p>Neither <i>pH</i> nor BSA affect the functionality of transporters such as P-glycoprotein. However, efflux ratios of ionizable or protein bound drugs are altered in the presence of a <i>pH</i>-gradient or BSA, indicating that an experimental system without protein or <i>pH</i> gradient can over- or underestimate active and passive efflux in drug transport.</p>

Drug transport across the intestinal epithelium is roughly predicted from permeability values obtained from Caco-2 cell monolayers. This thesis examines the important role of pH and extracellular additives for increasing the reliability and predictivity of the in vitro screening system, Caco-2. It was shown that the passive transport of ionizable compounds may be biased by a false efflux or uptake component, when applying a physiological pH-gradient across the membrane. pH also affected the amount of compound available at the transporter-binding site. Therefore, pH dependence should be considered in studies of such compounds and of drug-drug interactions involving efflux transporters. It was also shown that proton-dependent apical uptake or basolateral efflux should be studied both with and without a pH gradient over the whole monolayers. The two extracellular additives, bovine serum albumin (BSA) and the solubilizing agent, Cremophor® EL, also influenced Caco-2 permeabilities. BSA applied to the receiver side increases, and to the donor side decreases drug permeation according to the drug’s protein binding capacity. Thus, the absorptive transport for both passive and active compounds is favoured, giving a physiologically sound improvement of the Caco-2 cell model. Inclusion of BSA increased both the predictivity and quality of permeability studies, particularly of highly lipophilic, BCS class II compounds. Passive and active transport processes could also be distinguished after accounting for unbound concentrations. The overall effect of Cremophor® EL on the permeability to a drug was compound-specific and probably dependent on micellar incorporation. Cremophor® EL can therefore not be recommended. Neither pH nor BSA affect the functionality of transporters such as P-glycoprotein. However, efflux ratios of ionizable or protein bound drugs are altered in the presence of a pH-gradient or BSA, indicating that an experimental system without protein or pH gradient can over- or underestimate active and passive efflux in drug transport.