<p>Established quantitative relationships between dose, plasma concentrations and response [pharmacokinetic-pharmacodynamic (PKPD) models] have a high potential in improving therapeutic indices of anticancer drug therapy and in increasing drug development efficiency. PKPD modelling is a helpful tool for characterising and understanding schedule dependence. The aim of this thesis was to develop PKPD models of anticancer drugs for tumour effects and haematological toxicity, which is the most frequent dose-limiting toxicity.</p><p>PK and haematological toxicity after several schedules were studied in rats and semi-physiological PKPD models for the whole time course of myelosuppression were developed from animal and patient data. The possibility to implant hollow fibres filled with tumour cells in immunocompetent rats was investigated for simultaneous assessment of PK, tumour response and haematological toxicity. Population data analyses were performed using the software NONMEM. </p><p>When all injections were administered within eight hours, fractionated schedules of 5-fluorouracil and epirubicin produced similar haematological toxicity in rats as a single dose, when the non-linear PK of 5-fluorouracil was accounted for. When the time interval was extended to two days for 5-fluorouracil, the fractionated regimens were more toxic. </p><p>The developed semi-physiological PKPD models included transit compartments that mimic maturation stages in bone marrow and explain the time lag. Feedback mechanisms characterised the rebound. The models successfully described myelosuppression in patients (DMDC) and rats (5-fluorouracil), after different administration schedules. Further developments made it possible to characterise the time course of myelosuppression after administration of each one of six different drugs, with parameters related to the haematopoietic system consistent across drugs. </p><p>The developed hollow fibre model in immunocompetent rats was successfully applied to monitor PK, toxicity and the time course of antitumour effects. PKPD modelling illustrated that the schedule dependence of the anticancer agent CHS 828 is partly due to dose-dependent bioavailability and partly due to a schedule-dependent PD effect.</p>

<p>Acid related diseases like GERD, duodenal-and gastric ulcers and H. Pylori-positive peptic ulcer disease are primarily managed by reducing gastric acidity. Irreversible proton pump inhibitors (PPIs) inhibit gastric acid secretion effectively throughout the day by irreversibly inhibiting the gastric proton pump, H+, K+-ATPase, in the parietal cells. Reversible gastric proton pump inhibitors are under development, but have not yet reached clinical use.</p><p>The pharmacokinetic/pharmacodynamic (PK/PD) relationships of these compounds are nonlinear, with a delay in the effect-time profile compared to the plasma concentration-time course. PK/PD-modelling was used to characterize and quantify the pharmacological effect with regard to onset, intensity and duration of effect. Models based on functional data, that discriminate between drug-and system-specific parameters, were developed. </p><p>In general, the plasma concentration-time course for each individual was approximated by linear interpolation between time-points and served as input into the pharmacodynamic models. A turnover model of irreversible inhibition of gastric acid secretion by omeprazole in the dog described the data well. The model was challenged and found to be robust under different experimental conditions. This model could predict the effect following different exposure of omeprazole and following different histamine provocation. Different fitting approaches (naïve pooling, standard two-stage and nonlinear mixed effects modelling) were compared and resulted in similar parameter estimates. For the reversible inhibitors, a kinetic binding model was finally selected. With a binding model the delay in the effect-time profile is explained by prolonged binding to the enzyme. </p><p>Use of these results in drug development can be helpful with regard to selection of drugs for further development and to predict the first clinical dose.</p>

Established quantitative relationships between dose, plasma concentrations and response [pharmacokinetic-pharmacodynamic (PKPD) models] have a high potential in improving therapeutic indices of anticancer drug therapy and in increasing drug development efficiency. PKPD modelling is a helpful tool for characterising and understanding schedule dependence. The aim of this thesis was to develop PKPD models of anticancer drugs for tumour effects and haematological toxicity, which is the most frequent dose-limiting toxicity. PK and haematological toxicity after several schedules were studied in rats and semi-physiological PKPD models for the whole time course of myelosuppression were developed from animal and patient data. The possibility to implant hollow fibres filled with tumour cells in immunocompetent rats was investigated for simultaneous assessment of PK, tumour response and haematological toxicity. Population data analyses were performed using the software NONMEM. When all injections were administered within eight hours, fractionated schedules of 5-fluorouracil and epirubicin produced similar haematological toxicity in rats as a single dose, when the non-linear PK of 5-fluorouracil was accounted for. When the time interval was extended to two days for 5-fluorouracil, the fractionated regimens were more toxic. The developed semi-physiological PKPD models included transit compartments that mimic maturation stages in bone marrow and explain the time lag. Feedback mechanisms characterised the rebound. The models successfully described myelosuppression in patients (DMDC) and rats (5-fluorouracil), after different administration schedules. Further developments made it possible to characterise the time course of myelosuppression after administration of each one of six different drugs, with parameters related to the haematopoietic system consistent across drugs. The developed hollow fibre model in immunocompetent rats was successfully applied to monitor PK, toxicity and the time course of antitumour effects. PKPD modelling illustrated that the schedule dependence of the anticancer agent CHS 828 is partly due to dose-dependent bioavailability and partly due to a schedule-dependent PD effect.

Acid related diseases like GERD, duodenal-and gastric ulcers and H. Pylori-positive peptic ulcer disease are primarily managed by reducing gastric acidity. Irreversible proton pump inhibitors (PPIs) inhibit gastric acid secretion effectively throughout the day by irreversibly inhibiting the gastric proton pump, H+, K+-ATPase, in the parietal cells. Reversible gastric proton pump inhibitors are under development, but have not yet reached clinical use. The pharmacokinetic/pharmacodynamic (PK/PD) relationships of these compounds are nonlinear, with a delay in the effect-time profile compared to the plasma concentration-time course. PK/PD-modelling was used to characterize and quantify the pharmacological effect with regard to onset, intensity and duration of effect. Models based on functional data, that discriminate between drug-and system-specific parameters, were developed. In general, the plasma concentration-time course for each individual was approximated by linear interpolation between time-points and served as input into the pharmacodynamic models. A turnover model of irreversible inhibition of gastric acid secretion by omeprazole in the dog described the data well. The model was challenged and found to be robust under different experimental conditions. This model could predict the effect following different exposure of omeprazole and following different histamine provocation. Different fitting approaches (naïve pooling, standard two-stage and nonlinear mixed effects modelling) were compared and resulted in similar parameter estimates. For the reversible inhibitors, a kinetic binding model was finally selected. With a binding model the delay in the effect-time profile is explained by prolonged binding to the enzyme. Use of these results in drug development can be helpful with regard to selection of drugs for further development and to predict the first clinical dose.

<p>Cetirizine, an H₁-antihistamine, is prescribed for allergic disorders. It exists as a racemic mixture, with levocetirizine being the active enantiomer. The central nervous system side-effects of H₁-antihistamines are caused by their penetration into the brain. In this thesis the plasma pharmacokinetics, transport across the blood-brain barrier (BBB) and H₁ receptor occupancy of cetirizine enantiomers was investigated <i>in vivo</i> in guinea pigs. The transport across the BBB was quantified using the microdialysis technique. Stereoselective brain distribution was investigated by measuring both unbound and total concentrations in plasma and brain. The time aspects of the H₁ receptor occupancy of levocetirizine was studied in the brain and the periphery.</p><p>The plasma pharmacokinetics of cetirizine was stereoselective with clearance and volume of distribution of levocetirizine being approximately half that of dextrocetirizine. This was mainly due to the differences in plasma protein binding of the enantiomers. The stereoselectivity in brain distribution indicated by the partition coefficient K_p (total AUC ratio brain to plasma) was caused by stereoselective plasma protein binding. The transport across the BBB measured in this thesis by the unbound partition coefficient K_p,uu (unbound AUC ratio brain to plasma) was the same for the two enantiomers. Binding within the brain was also not significantly different. The H₁ receptor occupancy of levocetirizine in brain lagged behind the plasma concentrations whereas it was not delayed with respect to the brain concentrations. This indicates that the delayed brain H₁ receptor occupancy of levocetirizine is caused by a slow transport across the BBB.</p><p>In summary, the results of this thesis emphasize the importance of measuring both the unbound and total concentrations in blood and brain to characterize stereoselective brain distribution. The thesis also emphasize the importance of taking local brain pharmacokinetics into consideration in understanding pharmacokinetic-pharmacodynamic relationships of drugs with central activity.</p>

<p>Cefuroxime is a renally eliminated antibiotic used against a variety of different bacterial infections. The pharmacokinetics (PK) for cefuroxime was studied in 97 hospitalized patients using population analysis. To be able to measure cefuroxime in human serum a new sensitive analytical method was developed using mass spectrometry detection. The method was validated and shown to be sensitive and selective. Cystatin C was found to be a better covariate for cefuroxime clearance compared to the traditionally used creatinine clearance (CLcr). This relation might be useful when designing dosing strategies for cefuroxime and other renally eliminated drugs. </p><p>The time-courses of the biomarkers C-reactive protein (CRP), serum amyloid A (SAA), interleukin-6 (IL-6) and body temperature were studied for the first 72 hours of cefuroxime treatment and was related to the duration of illness previous treatment with cefuroxime and to time to step-down of treatment. When duration of illness was short, CRP and SAA were showed increasing levels. None of the biomarkers could be used to differentiate between early or late step-down of therapy.</p><p>By use of known PK and pharmacodynamic (PD) principles, dosing strategies based on CLcr for cefuroxime were estimated using minimization of a risk function. The risk function was constructed with the aim to expose patients to cefuroxime concentration above minimum inhibitory concentration (MIC) for 50 % of the dosing interval and to minimize the amount of drug administered in excess to reach the aim. Based on evaluation using wild type MIC distributions for <i>Escherichia coli</i> and <i>Streptococcus pneumoniae</i> improved dosing strategies were selected.</p><p>In vitro experiments were performed exposing <i>Streptococcus pyogenes</i> to constant concentration of benzylpenicillin, cefuroxime, erythromycin, moxifloxacin or vancomycin. A semi-mechanistic PK/PD model characterizing the time-course of the antibacterial effect was developed using all data simultaneously. Internal validation showed the model being predictive and robust. </p>

<p>The pharmacokinetics and pharmacodynamics of oxycodone and morphine was investigated and related to the transport across the blood-brain barrier (BBB) in rats. The influence of a P-glycoprotein (P-gp) inhibitor on the plasma pharmacokinetics and pharmacodynamics of oxycodone was evaluated. Microdialysis experiments were conducted to evaluate the unbound pharmacokinetics, including the rate and extent of transport across the BBB, of oxycodone and morphine. Mathematical models were used to assess the pharmacokinetics and also the relationship between pharmacokinetics and pharmacodynamics of the drugs.</p><p>Oxycodone clearance, volume of distribution at steady-state, half-life, total brain tissue concentrations and tail-flick latency were all unaffected when a P-gp inhibitor was co-administered with oxycodone as compared to a control group. The lack of differences between the groups indicates that oxycodone BBB transport is not affected by P-gp inhibition. Investigating the unbound concentrations of oxycodone in brain and blood using microdialysis revealed an exciting finding. At steady-state, the unbound concentration in brain was 3 times higher than in blood (i.e. a K_p,uu of 3), indicating that active influx is involved in the BBB transport of oxycodone. In contrast, the K_p,uu of morphine was estimated to 0.56, which is an indication that active efflux mechanisms are involved in the BBB transport of morphine. This means that based on the same unbound concentration in blood, an approximately 6-fold higher unbound concentration of oxycodone compared to morphine will be reached in the brain. Using pharmacokinetic-pharmacodynamic modelling, the unbound brain concentrations of oxycodone and morphine were correlated to the tail-flick latency in vivo. The relative potency of the drugs was found to be concentration dependent with an infliction point of 55 nM.</p><p>In summary, this thesis emphasise the importance of taking the local brain pharmacokinetics into consideration when investigating the pharmacokinetics and the pharmacokinetic-pharmacodynamic relationships of centrally acting drugs.</p>

<p>Enzyme induction is a process whereby a molecule enhances the expression of enzymes. If the affected enzymes are involved in the elimination of a drug, this may result in a drug interaction. Induction is therefore of major concern during drug development and in clinical practice. </p><p>The induction process depends on the half-life of the induced enzyme, the pharmacokinetics of the inducing agent, and the relationship between the inducer’s concentration and the induction stimulus. The aim of the conducted research was to investigate these key aspects of enzyme induction and the consequences that induction has for substrate elimination.</p><p>Successful investigations of the induction process presuppose the existence of appropriate methods for the estimation of the metabolic activity. Enzyme activity measurements can be conducted in tissues with low enzyme content using the analytical method presented here. </p><p>A model was developed describing the changes in the pharmacokinetics of clomethiazole and its metabolite NLA-715, that are attributable to carbamazepine induction. The consequences of the induction was explained using a mechanistic approach, acknowledging food-induced changes in the blood flow to the liver, and interpreting in vitro generated metabolic information.</p><p>The time course of the induction process was examined in two investigations. In the first of these, the pharmacokinetics of the autoinducing drug phenobarbital and its effect on several enzymes were described in rats. This was accomplished by integrating the bidirectional interaction between drug and enzymes in a mechanistic manner. In the final investigation, the time course of the increase and cessation in enzyme activity was studied in healthy volunteers treated with carbamazepine. This investigation allowed the half-lives of CYP3A and CYP1A2 to be estimated. </p><p>The key aspects of the enzyme induction process have been examined using mechanistic induction models. These novel models improve the understanding of the induction process and its consequences for substrate elimination.</p>

Cetirizine, an H1-antihistamine, is prescribed for allergic disorders. It exists as a racemic mixture, with levocetirizine being the active enantiomer. The central nervous system side-effects of H1-antihistamines are caused by their penetration into the brain. In this thesis the plasma pharmacokinetics, transport across the blood-brain barrier (BBB) and H1 receptor occupancy of cetirizine enantiomers was investigated in vivo in guinea pigs. The transport across the BBB was quantified using the microdialysis technique. Stereoselective brain distribution was investigated by measuring both unbound and total concentrations in plasma and brain. The time aspects of the H1 receptor occupancy of levocetirizine was studied in the brain and the periphery. The plasma pharmacokinetics of cetirizine was stereoselective with clearance and volume of distribution of levocetirizine being approximately half that of dextrocetirizine. This was mainly due to the differences in plasma protein binding of the enantiomers. The stereoselectivity in brain distribution indicated by the partition coefficient Kp (total AUC ratio brain to plasma) was caused by stereoselective plasma protein binding. The transport across the BBB measured in this thesis by the unbound partition coefficient Kp,uu (unbound AUC ratio brain to plasma) was the same for the two enantiomers. Binding within the brain was also not significantly different. The H1 receptor occupancy of levocetirizine in brain lagged behind the plasma concentrations whereas it was not delayed with respect to the brain concentrations. This indicates that the delayed brain H1 receptor occupancy of levocetirizine is caused by a slow transport across the BBB. In summary, the results of this thesis emphasize the importance of measuring both the unbound and total concentrations in blood and brain to characterize stereoselective brain distribution. The thesis also emphasize the importance of taking local brain pharmacokinetics into consideration in understanding pharmacokinetic-pharmacodynamic relationships of drugs with central activity.

The pharmacokinetics and pharmacodynamics of oxycodone and morphine was investigated and related to the transport across the blood-brain barrier (BBB) in rats. The influence of a P-glycoprotein (P-gp) inhibitor on the plasma pharmacokinetics and pharmacodynamics of oxycodone was evaluated. Microdialysis experiments were conducted to evaluate the unbound pharmacokinetics, including the rate and extent of transport across the BBB, of oxycodone and morphine. Mathematical models were used to assess the pharmacokinetics and also the relationship between pharmacokinetics and pharmacodynamics of the drugs. Oxycodone clearance, volume of distribution at steady-state, half-life, total brain tissue concentrations and tail-flick latency were all unaffected when a P-gp inhibitor was co-administered with oxycodone as compared to a control group. The lack of differences between the groups indicates that oxycodone BBB transport is not affected by P-gp inhibition. Investigating the unbound concentrations of oxycodone in brain and blood using microdialysis revealed an exciting finding. At steady-state, the unbound concentration in brain was 3 times higher than in blood (i.e. a Kp,uu of 3), indicating that active influx is involved in the BBB transport of oxycodone. In contrast, the Kp,uu of morphine was estimated to 0.56, which is an indication that active efflux mechanisms are involved in the BBB transport of morphine. This means that based on the same unbound concentration in blood, an approximately 6-fold higher unbound concentration of oxycodone compared to morphine will be reached in the brain. Using pharmacokinetic-pharmacodynamic modelling, the unbound brain concentrations of oxycodone and morphine were correlated to the tail-flick latency in vivo. The relative potency of the drugs was found to be concentration dependent with an infliction point of 55 nM. In summary, this thesis emphasise the importance of taking the local brain pharmacokinetics into consideration when investigating the pharmacokinetics and the pharmacokinetic-pharmacodynamic relationships of centrally acting drugs.