Mechanistic models for myelosuppression

Development of a new pre- and post-processing tool (SADAPT-TRAN) for nonlinear mixed-effects modeling in S-ADAPT

Mechanistic modeling greatly benefits from automated pre- and post-processing of model code and modeling results. While S-ADAPT provides many state-of-the-art parametric population estimation methods, its pre- and post-processing capabilities are limited. Our objective was to develop a fully automated, open-source pre- and post-processor for nonlinear mixed-effects modeling in S-ADAPT. We developed a new translator tool (SADAPT-TRAN) based on Perl scripts. These scripts (a) automatically translate the core model components into robust Fortran code, (b) perform extensive mutual error checks across all input files and the raw dataset, (c) extend the options of the Monte Carlo Parametric Expectation Maximization (MC-PEM) algorithm, and (d) improve the numerical robustness of the model code. The post-processing scripts automatically summarize the results of one or multiple models as tables and, by generating problem specific R scripts, provide an extended series of standard and covariate-stratified diagnostic plots. The SADAPT-TRAN package substantially improved the efficiency to specify, debug, and evaluate models and enhanced the flexibility of using the MC-PEM algorithm for parallelized estimation in S-ADAPT. The parameter variability model can take any combination of normally, log-normally, or logistically distributed parameters and the SADAPT-TRAN package can automatically generate the Fortran code required to specify between occasion variability. Extended estimation features are available to avoid local minima, estimate means with negligible variances, and estimate variances for fixed means. The SADAPT-TRAN package significantly facilitated model development in S-ADAPT, reduced model specification errors, and provided useful error messages for beginner and advanced users. This benefit was greatest for complex mechanistic models.

Possible involvement of oxidative stress in 5-fluorouracil-mediated myelosuppression in mice

Certain chemotherapeutic agents subject cells to oxidative stress, thereby promoting adverse effects. However, the molecular machinery governing 5-fluorouracil (5-FU)-mediated myelotoxicity is obscure. The purpose of this study was to clarify whether 5-FU-induced myelotoxicity is a cause of oxidative stress. Treatment of mice with 5-FU (75 mg/kg, i.p.) caused a significant induction of haem oxygenase-1 and a decrease in glutathione contents in bone marrow cells, both of which are the indicators of oxidative stress. The 5-FU-mediated decrease in the myeloid colony formation was intensified in Nrf2(-/-) mice, in which antioxidant proteins were down-regulated. N-Acetylcysteine reversed the 5-FU-induced decreases in the glutathione content, number of bone marrow cells per femur and myeloid colony formation. Results from the present study reveal that 5-FU induces oxidative stress in bone marrow, which is involved, at least in part, in myelotoxicity in mice. Therefore, Nrf2-dependent genes as well as glutathione levels in bone marrow could be therapeutic targets for decreasing such side-effects in 5-FU-based chemotherapy.

A semi-physiological-based pharmacokinetic/pharmacodynamic model to describe the effects of topotecan on b-lymphocyte lineage cells

PURPOSE

To develop a semi-physiological-based model describing simultaneously the time course of immature and mature B-lymphocytes after topotecan (TPT) administration to tumor-bearing rats.

METHODS

Twenty-four tumor-bearing BDIX male rats received a single 6 mg/kg intra-peritoneal dose of TPT or saline. Mature and immature B-cell levels were measured every two days during three weeks and showed a very different temporal pattern. Both B-cell populations declined rapidly, reaching the nadir at 3-4 days after TPT administration; however, mature cells returned to baseline at day 8, while immature B-cells stayed at nadir until day 9 instead. Data were modeled using the population approach with NONMEM VI.

RESULTS

The model developed maintains the proliferation, maturation and degradation elements of previous published models for myelosuppresion. In order to describe the rapid recovery of mature cells, it includes a peripheral compartment providing a constant supply of mature cells to the bloodstream.

CONCLUSIONS

The major contribution of the model is its new structure and the dynamical consequences, demonstrating an independent behavior between mature and immature B-cells during recovery. The final model could represent a good basis for the optimization of cytotoxic drugs oriented to attain a maximum antitumor efficacy while minimizing hematological toxicity.

Model-based neutrophil-guided dose adaptation in chemotherapy: evaluation of predicted outcome with different types and amounts of information

One of the most employed approaches to reduce severe neutropenia following anticancer drug regimens is to reduce the consecutive dose in fixed steps, commonly by 25%. Another approach has been to use pharmacokinetic (PK) sampling to tailor dosing, but only rarely have model-based computer approaches utilizing collected PK and/or pharmacodynamic (PD) data been used. A semi-mechanistic model for myelosuppression that can characterize the interindividual and interoccasion variability in the time-course of neutrophils following administration of a wide range of anticancer drugs may be used in a clinical setting for model-based dose individualization. The aim of this study was to compare current stepwise procedures to model-based dose adaptation by simulations, and investigate if the overall dose intensity in the population could be increased without increasing the risk of severe toxicity. The value of various amounts of PK- and/or PD-information was compared to standard dosing strategies using a maximum a posteriori procedure in NONMEM. The results showed that when information on neutrophil counts was available, the additional improvement from PK sampling was negligible. Using neutrophil sampling at baseline and an observation near the predicted nadir increased the number of patients in the target range by 27% in comparison with a one-sided 25% dose adjustment schedule, while keeping the number of patients experiencing severe toxicity at a comparable low level after five courses of treatment. High interindividual variability did not limit the benefit of model-based dose adaptation, whereas high interoccasion variability was predicted to make any dose adaptation method less successful. This study indicates that for successful model-based dose adaptation clinically, there is no need for drug concentration sampling, and that one extra neutrophil measurement in addition to the pre-treatment value is sufficient to limit severe neutropenia while increasing dose intensity.

Pharmacodynamic effects of haematopoietic cytokines: the view of a clinical oncologist

The production of haematopoietic cells is under the tight control of a group of haematopoietic cytokines. Each cytokine has multiple actions mediated by receptors whose cytoplasmic domains contain specialized regions initiating survival, proliferation, differentiation commitment, maturation and functional activation. Granulocyte colony-stimulating factor (G-CSF), erythropoietin (EPO), and thrombopoiesis-stimulating agents are in routine clinical use to stimulate cell production and in total have been used in the management of many millions of patients. G-CSF regulates neutrophil production to maintain blood neutrophil counts in the normal range. G-CSF is used to prevent febrile neutropenia or to increase dose-density in chemotherapy regimens. Despite consistently showing a shorter duration of neutropenia, multiple prospective randomized trials have documented only modest clinical benefit. A clinical advantage of dose-dense chemotherapy has been shown only in specific chemotherapy regimens. Professional recommendations tailor the use of CSFs to patients with a high risk of adverse outcome of febrile neutropenia. EPO was used to prevent anaemia requiring red blood cell transfusion. However, recent studies strongly suggest a negative overall effect on mortality, without a plausible biological explanation. It is now proposed that its use should be restricted to patients in clinical trials. Thrombopoiesis-stimulating agents have only been recently introduced into the market for splenectomized and non-splenectomized patients with immune thrombocytopenic purpura, a rare disease. Before widely used in other conditions such as chemotherapy-induced thrombocytopenia, the lessons learned from the example of G-CSF and EPO should be taken seriously.

A semi-mechanistic model of CP-690,550-induced reduction in neutrophil counts in patients with rheumatoid arthritis

CP-690,550, a selective inhibitor of the Janus kinase family, is being developed as an oral disease-modifying antirheumatic drug for the treatment of rheumatoid arthritis (RA). A semi-mechanistic model was developed to characterize the time course of drug-induced absolute neutrophil count (ANC) reduction in a phase 2a study. Data from 264 RA patients receiving 6-week treatment (placebo, 5, 15, 30 mg bid) followed by a 6-week off-treatment period were analyzed. The model included a progenitor cell pool, a maturation chain comprising transit compartments, a circulation pool, and a feedback mechanism. The model was adequately described by system parameters (BASE(h), ktr(h), gamma, and k(circ)), disease effect parameters (DIS), and drug effect parameters (k(off) and k(D)). The disease manifested as an increase in baseline ANC and reduced maturation time due to increased demand from the inflammation site. The drug restored the perturbed system parameters to their normal values via an indirect mechanism. ANC reduction due to a direct myelosuppressive drug effect was not supported. The final model successfully described the dose- and time-dependent changes in ANC and predicted the incidence of neutropenia at different doses reasonably well.

A tool for neutrophil guided dose adaptation in chemotherapy

Chemotherapy dosing in anticancer treatment is a balancing act between achieving concentrations that are effective towards the malignancy and that result in acceptable side-effects. Neutropenia is one major side-effect of many antitumor agents, and is related to an increased risk of infection. A model capable of describing the time-course of myelosuppression from administered drug could be used in individual dose selection. In this paper we describe the transfer of a previously developed semi-mechanistic model for myelosuppression from NONMEM to a dosing tool in MS Excel, with etoposide as an example. The tool proved capable to solve a differential equation system describing the pharmacokinetics and pharmacodynamics, with estimation performance comparable to NONMEM. In the dosing tool the user provides neutrophil measures from a previous treatment course and request for the dose that results in a desired nadir in the upcoming course through a Bayesian estimation procedure.

Pharmacokinetic/pharmacodynamic modeling and simulation of neutropenia during phase I development of liposome-entrapped paclitaxel

PURPOSE

To evaluate the maximum tolerated dose (MTD), dose-limiting toxicities (DLT), and pharmacokinetics of liposome-entrapped paclitaxel easy-to-use (LEP-ETU) and to characterize the relationship between LEP-ETU concentrations and the time course of neutropenia in cancer patients.

EXPERIMENTAL DESIGN

LEP-ETU was administered to 88 patients and 63 were evaluable for pharmacokinetic/pharmacodynamic (PK/PD) analysis following 1.5- and 3-h infusions every 3 weeks (q3w; dose range, 135-375 mg/m(2)). MTD was identified using a 3 + 3, up-and-down dose-finding algorithm. PK/PD modeling was done to describe the temporal relationship between paclitaxel concentrations and neutrophil count. Simulations assessed the influence of dose and schedule on neutropenia severity to help guide dose selection.

RESULTS

The MTD of LEP-ETU was identified as 325 mg/m(2). DLTs occurring at 375 mg/m(2) consisted of febrile neutropenia and neuropathy. The C(max) and area under the plasma concentration-time curve of LEP-ETU were less than proportional with increasing dose. The PK/PD model showed that LEP-ETU inhibition of neutrophil proliferation was 9.1% per 10 mug/mL of total paclitaxel concentration. The incidence of grade 4 neutropenia increased from 33% to 42% across the dose range of 275 to 325 mg/m(2) q3w. For a dose of 110 mg/m(2) given weekly, grade 4 neutropenia was estimated to be 16% compared with 42% for the same total dose administered q3w.

CONCLUSIONS

LEP-ETU can be administered safely at higher doses than Taxol. Modeling and simulation studies predict that 325 mg/m(2) LEP-ETU q3w provides acceptable neutropenic events relative to those observed at 175 mg/m(2) Taxol q3w. A 275 mg/m(2) dose may offer an improved therapeutic index.

A pharmacodynamic model for the time course of tumor shrinkage by gemcitabine + carboplatin in non-small cell lung cancer patients

PURPOSE

This tumor response pharmacodynamic model aims to describe primary lesion shrinkage in non-small cell lung cancer over time and determine if concentration-based exposure metrics for gemcitabine or that of its metabolites, 2',2'-difluorodeoxyuridine or gemcitabine triphosphate, are better than gemcitabine dose for prediction of individual response.

EXPERIMENTAL DESIGN

Gemcitabine was given thrice weekly on days 1 and 8 in combination with carboplatin, which was given only on day 1 of every cycle. Gemcitabine amount in the body and area under the concentration-time curves of plasma gemcitabine, 2',2'-difluorodeoxyuridine, and intracellular gemcitabine triphosphate in white cells were compared to determine which best describes tumor shrinkage over time. Tumor growth kinetics were described using a Gompertz-like model.

RESULTS

The apparent half-life for the effect of gemcitabine was 7.67 weeks. The tumor turnover time constant was 21.8 week.cm. Baseline tumor size and gemcitabine amount in the body to attain 50% of tumor shrinkage were estimated to be 6.66 cm and 10,600 mg. There was no evidence of relapse during treatment.

CONCLUSIONS

Concentration-based exposure metrics for gemcitabine and its metabolites were no better than gemcitabine amount in predicting tumor shrinkage in primary lung cancer lesions. Gemcitabine dose-based models did marginally better than treatment-based models that ignored doses of drug administered to patients. Modeling tumor shrinkage in primary lesions can be used to quantify individual sensitivity and response to antitumor effects of anticancer drugs.

Using pharmacokinetic and pharmacodynamic modeling and simulation to evaluate importance of schedule in topotecan therapy for pediatric neuroblastoma

PURPOSE

The study aims to use mathematical modeling and simulation to assess the relative contribution of topotecan systemic exposure and scheduling in the activity and myelosuppression of topotecan in pediatric patients with neuroblastoma.

EXPERIMENTAL DESIGN

Pharmacokinetic and pharmacodynamic data were obtained from a phase II study for pediatric patients with high-risk neuroblastoma. The topotecan dosage was individualized to attain a topotecan lactone area under the plasma concentration-time curve between 80 and 120 ng/mL h and given over a protracted schedule (i.e., 10 days). Four mathematical models describing topotecan pharmacokinetics, tumor growth, and neutrophil and platelet dynamics were developed. The models were combined to simulate and compare different topotecan treatment strategies with respect to systemic exposure and schedule.

RESULTS

The median change in tumor volume was significantly different between schedules (5% increase for D x 5 versus 60% decrease for D x 5 x 2; P < 0.0001) when administering the same total systemic exposure. Whereas protracted schedules showed increased neutropenia (median of 7 versus 12 days below an absolute neutrophil count of 500/microL; P < 0.0001) and thrombocytopenia (median of 3 versus 10 days below a platelet count of 20,000/microL; P < 0.00001), simulations showed that delays in topotecan therapy would not be required. Simulations showed that an increase in topotecan exposure on the D x 5 schedule by 2.4-fold resulted in a modest decrease in tumor volume (i.e., median percentage change tumor volume of 24% versus 3%).

CONCLUSIONS

The present mathematical model gave an innovative approach to determining relevant topotecan schedules for possible evaluation in the clinic, which could lead to improved tumor response with minimized toxicities.

Contribution of modelling chemotherapy-induced hematological toxicity for clinical practice

Anticancer chemotherapies are responsible for numerous adverse events. Among these, hematological toxicity is one of the main causes for ending treatment. These toxicities decrease production of red blood cells (anemia), production of white blood cells (neutropenia or granulocytopenia), and production of platelets (thrombocytopenia), which may be life-threatening to the patient. Preventing such discontinuation would be valuable for treating patients more effectively. In order to achieve this goal, numerous mathematical and physiological or semiphysiological models have been developed. The complexity of models has increased over the years, from empiric E(max) models to mechanistic models including physiological mechanisms such as feedback control. This review discusses several approaches of modelling hematological toxicities illustrated with some examples: pharmacodynamic models for the hematological toxicity of 5-fluorouracil, epirubicin, melphalan, paclitaxel, topotecan, and indisulam.

Mechanism-based Pharmacokinetic/Pharmacodynamic Meta-analysis of Trabectedin (ET-743, Yondelis) Induced Neutropenia

Myelosuppression was found to be one of the main toxicities of trabectedin (ET-743, Yondelis) during phase I/II studies. Our objective was to develop a pharmacokinetic-pharmacodynamic (PK/PD) model that describes the time course of the absolute neutrophil counts (ANCs) in cancer patients receiving trabectedin. Data from 699 patients who received intravenous trabectedin as monotherapy (dose range: 0.006-1.8 mg/m2) as a 1-, 3-, or 24-h infusion every 21 days; 1- or 3-h infusion on days 1, 8, and 15 every 28 days; or a 1-h infusion daily for 5 consecutive days every 21 days were used to develop (N=405; ANCs=7,291) and validate (N=294; ANCs=5,029) the model. The PK/PD model comprised a trabectedin-sensitive progenitor cell compartment, linked to the peripheral blood compartment, through three transition compartments representing the maturation chain in the bone marrow. To capture the rebound effect due to endogenous growth factors, the model included a feedback mechanism. The model estimated three system-related parameters: ANC at baseline (Circ0), mean transit time in bone marrow (MTT), and a feedback parameter (gamma). A first-order process quantified by the rate constant k(e0) described the trabectedin concentrations at the effect compartment (C(e)), which were assumed to reduce the proliferation rate and/or to increase the killing rate of the progenitor cells according to the function alphaC(e)beta. The model was qualified and simulations were undertaken to evaluate the neutropenia schedule dependency and the effects of selected covariates. NONMEM software was used to perform the modeling and simulation analyses. For a typical man of 70 kg, the mean values (between-subject variability; %) of the Circ0, MTT, gamma, k(e0), alpha, and beta were estimated to be 4.46 x 10(9)/l (37.9%), 4.0 days (37.5%), 0.218 (41.8%), 2.09 h(-1) (77.9%), 2.00 l/microg (85.1%), and 1.26, respectively. Although in women, k(e0) was reduced by 29% and a 25% increase in body weight resulted in a 12.6% reduction in the beta parameter, the clinical relevance of these effects is limited. The model evaluation procedure indicated accurate prediction of the observed incidence of neutropenia grades 3 and 4 across the dosing regimens evaluated. Simulations indicated that trabectedin dose and interdose interval, but not infusion duration, are the main determinants of the neutropenia severity. The model-predicted time course of the ANC and its variability confirmed that neutropenia is reversible, of short duration, and non-cumulative. The extent and time course of neutropenia following six different dosing regimens of trabectedin were well predicted by the semiphysiological PK/PD model.

G-CSF Control of Neutrophils Dynamics in the Blood

White blood cell neutrophil is a key component in the fast initial immune response against bacterial and fungal infections. Granulocyte colony stimulating factor (G-CSF) which is naturally produced in the body, is known to control the neutrophils production in the bone marrow and the neutrophils delivery into the blood. In oncological practice, G-CSF injections are widely used to treat neutropenia (dangerously low levels of neutrophils in the blood) and to prevent the infectious complications that often follow chemotherapy. However, the accurate dynamics of G-CSF neutrophil interaction has not been fully determined and no general scheme exists for an optimal G-CSF application in neutropenia. Here we develop a two-dimensional ordinary differential equation model for the G-CSF-neutrophil dynamics in the blood. The model is built axiomatically by first formally defining from the biology the expected properties of the model, and then deducing the dynamic behavior of the resulting system. The resulting model is structurally stable, and its dynamical features are independent of the precise form of the various rate functions. Choosing a specific form for these functions, three complementary parameter estimation procedures for one clinical (training) data set are utilized. The fully parameterized model (6 parameters) provides adequate predictions for several additional clinical data sets on time scales of several days. We briefly discuss the utility of this relatively simple and robust model in several clinical conditions.

A Bayesian population PK-PD model of ispinesib-induced myelosuppression

The goal of the present analysis is to fit a Bayesian population pharmacokinetic pharmacodynomic (PK-PD) model to characterize the relationship between the concentration of ispinesib and changes in absolute neutrophil counts (ANC). Ispinesib, a kinesin spindle protein (KSP) inhibitor, blocks assembly of a functional mitotic spindle, leading to G2/M arrest. A first time in human, phase I open-label, non-randomized, dose-escalating study evaluated ispinesib at doses ranging from 1 to 21 mg/m(2). PK-PD data were collected from 45 patients with solid tumors. The pharmacokinetics of ispinesib were well characterized by a two-compartment model. A semimechanistic model was fit to the ANC. The PK and PD data were successfully modelled simultaneously. This is the first presentation of simultaneously fitting a PK-PD model to ANC using Bayesian methods. Bayesian methods allow for the use of prior information for some system-related parameters. The model may be used to examine different schedules, doses, and infusion times.

Successful therapy must eradicate cancer stem cells

Despite significant improvements in cancer therapy, tumor recurrence is frequent and can be due to a variety of mechanisms, including the evolution of resistance and tumor progression. Cancer stem cells have been postulated to maintain tumor growth similar to normal stem cells maintaining tissue homeostasis. Recently, the existence of these malignant stem cells has been proven for hematological as well as some solid tumors. Tumor stem cells are not targeted by standard therapy and might be responsible for treatment failure and tumor recurrence in many patients. We designed a simple mathematical model to demonstrate the importance of eliminating tumor stem cells. We explored different therapeutic scenarios to illustrate the properties required from novel therapeutic agents for successful tumor treatment. We show that successful therapy must eradicate tumor stem cells.

Relationships between sirolimus dosing, concentration and outcomes in renal transplant recipients

AIM

To explore relationships between sirolimus dosing, concentration and clinical outcomes.

METHODS

Data were collected from 25 kidney transplant recipients (14 M/11 F), median 278 days after transplantation. Outcomes of interest were white blood cell (WBC) count, platelet (PLT) count, and haematocrit (HCT). A naive pooled data analysis was performed with outcomes dichotomized (Mann-Whitney U-tests).

RESULTS

Several patients experienced at least one episode when WBC (n = 9), PLT (n = 12), or HCT (n = 21) fell below the lower limits of the normal range. WBC and HCT were significantly lower (P < 0.05) when sirolimus dose was greater than 10 mg day(-1), and sirolimus concentration greater than 12 microg l(-1). No relationship was shown for PLT and dichotomized sirolimus dose or concentration.

CONCLUSIONS

Given this relationship between sirolimus concentration and effect, linked population pharmacokinetic-pharmacodynamic modelling using data from more renal transplant recipients should now be used to quantify the time course of these relationships to optimize dosing and minimize risk of these adverse outcomes.

Clinical application of a semimechanistic-physiologic population PK/PD model for neutropenia following pemetrexed therapy

PURPOSE

The objective of these analyses was to examine the effect of variations in the explanatory factors of neutropenic response, identified by semimechanistic-physiologic population pharmacokinetic/pharmacodynamic (PK/PD) modeling, on clinically important features of the absolute neutrophil count (ANC)-time profile (e.g, the nadir of the ANC [NANC], its timing [T (Nadir)], and the timecourse of recovery [T (Rec)]).

METHODS

Correlation analyses were used to evaluate the relationship of NANC, T (Nadir), and T (Rec) as a function of overall systemic exposure (AUC) and each of the covariates contained in the population PK/PD model. Simulations using the final PK/PD model were used to generate complete ANC-time profiles. Frequency counts of NANCs from the simulated profiles were used to quantitatively explore differences in the incidence and severity of neutropenia associated with a variety of scenarios (500 mg/m2 versus 600 mg/m2, normal vitamin deficiency markers versus elevated vitamin deficiency markers, and body surface area-based versus renal function-based dosing) and to evaluate the effect of individual explanatory factors with respect to neutropenic response.

RESULTS

Information obtained from correlation analysis and simulations was helpful in quantitatively exploring the impact of dose, exposure, and/or patient characteristics on neutropenic response. The information gained from these simulations provided supportive evidence for the decision to routinely include vitamin supplementation during pemetrexed treatment as a means of managing the risk of severe neutropenia secondary to pemetrexed administration. These techniques also provided information regarding the specific T (Nadir) and T( Rec) for inclusion in product labeling and suggested that a 14-day treatment cycle might be feasible for pemetrexed.

CONCLUSION

For population PK/PD models, to provide useful information for the practicing clinician or the clinical development team, it is not sufficient to look only at influences of covariates on model parameters. Rather, the modeling results need to be carefully investigated in terms of clinically relevant measures.

Pharmacokinetic/pharmacodynamic modelling in oncological drug development

For many oncological agents, myelosuppression is the dose-limiting toxicity and the quantitative characterisation of the relationship between drug dose, plasma concentration and haematological toxicity is of importance in the drug development. Mechanism-based population pharmacokinetic-pharmacodynamic models have been developed for this purpose and the applications of these in candidate selection, first-in-man studies, prodrug and formulation development, dose finding, schedule optimisation, assessing influence of modifying agents, drug combination studies, subgroup identification and feedback individualisation are reviewed.

Pharmacokinetic–pharmacodynamic modelling of the schedule-dependent effect of the anti-cancer agent CHS 828 in a rat hollow fibre model

N-(6-Chlorophenoxyhexyl)-N'-cyano-N''-4-pyridylguanidine (CHS 828) is a novel anticancer agent that shows schedule-dependent effects in vitro and in vivo, as well as in Phase I clinical trials. A rat hollow fibre model was used to investigate whether this dependency is due to pharmacokinetic and/or pharmacodynamic factors. The effect on two cell lines, MDA-MB-231 (breast cancer) and CCRF-CEM (leukaemia) were studied after CHS 828 was administered orally as a single dose or in a 5-day schedule, at different total dose levels. The 5-day schedules were associated with greater effects on both cell lines compared with single doses. A one-compartment pharmacokinetic model, with a half-life of 2.3h and a consecutive zero- and first-order process to describe dissolution and absorption, fit the concentration data. Pharmacokinetics were dose-dependent, as the fraction absorbed decreased with increasing dose. Clearance increased with accumulative exposure. Twenty hours after administration, concentrations started to increase again, probably due to coprophagy. Pharmacokinetic-pharmacodynamic models characterized the cell growth and cell kill over time and showed that schedule-dependent antitumour effects were present also when the dose-dependent pharmacokinetics were accounted for. The prolonged schedules of CHS 828 were therefore associated with greater antitumour effects than single doses of the same total exposure.