Pharmacodynamic modeling of thrombopoietin, platelet, and megakaryocyte dynamics in patients with acute myeloid leukemia undergoing dose intensive chemotherapy

Modeling individual time courses of thrombopoiesis during multi-cyclic chemotherapy

Background

Thrombocytopenia is a major side-effect of cytotoxic cancer therapies. The aim of precision medicine is to develop therapy modifications accounting for the individual’s risk.

Methodology/Principle findings

To solve this task, we develop an individualized bio-mechanistic model of the dynamics of bone marrow thrombopoiesis, circulating platelets and therapy effects thereon. Comprehensive biological knowledge regarding cell differentiation, amplification, apoptosis rates, transition times and corresponding regulations are translated into ordinary differential equations. A model of osteoblast/osteoclast interactions was incorporated to mechanistically describe bone marrow support of quiescent cell stages. Thrombopoietin (TPO) as a major regulator is explicitly modelled including pharmacokinetics and–dynamics of TPO injections. Effects of cytotoxic drugs are modelled by transient depletions of proliferating cells.

To calibrate the model, we used population data from the literature and close-meshed individual data of N = 135 high-grade non-Hodgkin’s lymphoma patients treated with CHOP-like chemotherapies. To limit the number of free parameters, several parsimony assumptions were derived from biological data and tested via Likelihood methods. Heterogeneity of patients was explained by a few model parameters. The over-fitting issue of individual parameter estimation was successfully dealt with a virtual participation of each patient in population-based experiments.

The model qualitatively and quantitatively explains a number of biological observations such as the role of osteoblasts in explaining long-term toxic effects, megakaryocyte-mediated feedback on stem cells, bi-phasic stimulation of thrombopoiesis by TPO, dynamics of megakaryocyte ploidies and non-exponential platelet degradation. Almost all individual time series could be described with high precision. We demonstrated how the model can be used to provide predictions regarding individual therapy adaptations.

Conclusions

We propose a mechanistic thrombopoiesis model of unprecedented comprehensiveness in both, biological mechanisms considered and experimental data sets explained. Our innovative method of parameter estimation allows robust determinations of individual parameter settings facilitating the development of individual treatment adaptations during chemotherapy.

Chemotherapy is ubiquitously used to treat cancer diseases. Due to general toxicity of the drugs, chemotherapy results in a number of side effects especially with respect to blood formation. Here we study the loss of platelets during chemotherapy which is dose limiting in many situations. However, this side-effect greatly varies between patients with respect to both, severity and necessity of clinical countermeasures.We therefore developed a mathematical model to predict the time course of platelets of patients under chemotherapy and to propose possible treatment adaptations in cases of intolerable toxicity. The model is based on available biological knowledge and data of platelet formation and therapeutic effects thereon. As a major result, we could describe individual time series data of 135 patients under chemotherapy. Conversely, the model can be used to make predictions regarding alternative therapy schedules such as postponement of therapy or chemotherapy dose reductions. Our model is intended to support clinical decision making on an individual patient level.

Identification of underlying and transfusion-related platelet qualitative alterations in the hemato-oncologic patient

Hemato-oncologic patients with chemotherapy-induced thrombocytopenia are one of the populations receiving platelet transfusions. The general practice with these patients is to give prophylactic platelet transfusions when platelet counts fall below 10×10⁹PLT/L. However, in more than 40% of these patients, platelet transfusion does not prevent bleeding. The reason of the low efficacy of platelet transfusion in the context of chemotherapy patients is not entirely understood. We therefore aimed at immunophenotyping the expression of platelet surface and activation markers and thrombopoietin levels from hemato-oncologic patients before and after transfusion. A more detailed follow-up was performed in three patients that underwent autologous bone marrow transplantation. As previously reported, basal platelet activation was observed in hemato-oncologic patients. Based on flow cytometry parameters, i.e. the percentage of positivity and mean fluorescence intensity (MFI) distribution, our data provide an additional interpretation of platelet acquired qualitative changes in the hemato-oncologic patient. From our results we propose: first, the underlying activation of platelets in the hemato-oncologic patient is accompanied by loss of expression of the platelet receptors that are susceptible to protease-mediated shedding; second, soon after transfusion, the newly circulating donor platelets show additional activation, which may result in subsequent platelet receptor recycling and potential accelerated clearance of these activated platelets. In conclusion, the immunophenotype of circulating platelets changes after prophylactic platelet transfusion. Next to platelet count increment, exploration of this immunophenotype might help to explain transfusion refractory bleeding in hemato-oncologic patients. Eventually this may lead to personalization and improvement of the present platelet transfusion support regime.

A cell-level model of pharmacodynamics-mediated drug disposition

We aimed to develop a cell-level pharmacodynamics-mediated drug disposition (PDMDD) model to analyze in vivo systems where the PD response to a drug has an appreciable effect on the pharmacokinetics (PK). An existing cellular level model of PD stimulation was combined with the standard target-mediated drug disposition (TMDD) model and the resulting model structure was parametrically identifiable from typical in vivo PK and PD data. The PD model of the cell population was controlled by the production rate k in and elimination rate k out which could be stimulated or inhibited by the number of bound receptors on a single cell. Simulations were performed to assess the impact of single and repeated dosing on the total drug clearance. The clinical utility of the cell-level PDMDD model was demonstrated by fitting published data on the stimulatory effects of filgrastim on absolute neutrophil counts in healthy subjects. We postulated repeated dosing as a means of detecting and quantifying PDMDD as a single dose might not be sufficient to elicit the cellular response capable of altering the receptor pool to visibly affect drug disposition. In the absence of any PD effect, the model reduces down to the standard TMDD model. The applications of this model can be readily extended to include chemotherapy-induced cytopenias affecting clearance of endogenous hematopoietic growth factors, different monoclonal antibodies and immunogenicity effects on PK.

Alternatives, and adjuncts, to prophylactic platelet transfusion for people with haematological malignancies undergoing intensive chemotherapy or stem cell transplantation

BACKGROUND

Platelet transfusions are used in modern clinical practice to prevent and treat bleeding in people with thrombocytopenia. Although considerable advances have been made in platelet transfusion therapy since the mid-1970s, some areas continue to provoke debate especially concerning the use of prophylactic platelet transfusions for the prevention of thrombocytopenic bleeding.

OBJECTIVES

To determine whether agents that can be used as alternatives, or adjuncts, to platelet transfusions for people with haematological malignancies undergoing intensive chemotherapy or stem cell transplantation are safe and effective at preventing bleeding.

SEARCH METHODS

We searched 11 bibliographic databases and four ongoing trials databases including the Cochrane Central Register of Controlled Trials (CENTRAL, 2016, Issue 4), MEDLINE (OvidSP, 1946 to 19 May 2016), Embase (OvidSP, 1974 to 19 May 2016), PubMed (e-publications only: searched 19 May 2016), ClinicalTrials.gov, World Health Organization (WHO) ICTRP and the ISRCTN Register (searched 19 May 2016).

SELECTION CRITERIA

We included randomised controlled trials in people with haematological malignancies undergoing intensive chemotherapy or stem cell transplantation who were allocated to either an alternative to platelet transfusion (artificial platelet substitutes, platelet-poor plasma, fibrinogen concentrate, recombinant activated factor VII, desmopressin (DDAVP), or thrombopoietin (TPO) mimetics) or a comparator (placebo, standard care or platelet transfusion). We excluded studies of antifibrinolytic drugs, as they were the focus of another review.

DATA COLLECTION AND ANALYSIS

Two review authors screened all electronically derived citations and abstracts of papers identified by the review search strategy. Two review authors assessed risk of bias in the included studies and extracted data independently.

MAIN RESULTS

We identified 16 eligible trials. Four trials are ongoing and two have been completed but the results have not yet been published (trial completion dates: April 2012 to February 2017). Therefore, the review included 10 trials in eight references with 554 participants. Six trials (336 participants) only included participants with acute myeloid leukaemia undergoing intensive chemotherapy, two trials (38 participants) included participants with lymphoma undergoing intensive chemotherapy and two trials (180 participants) reported participants undergoing allogeneic stem cell transplantation. Men and women were equally well represented in the trials. The age range of participants included in the trials was from 16 years to 81 years. All trials took place in high-income countries. The manufacturers of the agent sponsored eight trials that were under investigation, and two trials did not report their source of funding.No trials assessed artificial platelet substitutes, fibrinogen concentrate, recombinant activated factor VII or desmopressin.Nine trials compared a TPO mimetic to placebo or standard care; seven of these used pegylated recombinant human megakaryocyte growth and differentiation factor (PEG-rHuMGDF) and two used recombinant human thrombopoietin (rhTPO).One trial compared platelet-poor plasma to platelet transfusion.We considered that all the trials included in this review were at high risk of bias and meta-analysis was not possible in seven trials due to problems with the way data were reported.We are very uncertain whether TPO mimetics reduce the number of participants with any bleeding episode (odds ratio (OR) 0.40, 95% confidence interval (CI) 0.10 to 1.62, one trial, 120 participants, very low quality evidence). We are very uncertain whether TPO mimetics reduce the risk of a life-threatening bleed after 30 days (OR 1.46, 95% CI 0.06 to 33.14, three trials, 209 participants, very low quality evidence); or after 90 days (OR 1.00, 95% CI 0.06 to 16.37, one trial, 120 participants, very low quality evidence). We are very uncertain whether TPO mimetics reduce platelet transfusion requirements after 30 days (mean difference -3.00 units, 95% CI -5.39 to -0.61, one trial, 120 participants, very low quality evidence). No deaths occurred in either group after 30 days (one trial, 120 participants, very low quality evidence). We are very uncertain whether TPO mimetics reduce all-cause mortality at 90 days (OR 1.00, 95% CI 0.24 to 4.20, one trial, 120 participants, very low quality evidence). No thromboembolic events occurred for participants treated with TPO mimetics or control at 30 days (two trials, 209 participants, very low quality evidence). We found no trials that looked at: number of days on which bleeding occurred, time from randomisation to first bleed or quality of life.One trial with 18 participants compared platelet-poor plasma transfusion with platelet transfusion. We are very uncertain whether platelet-poor plasma reduces the number of participants with any bleeding episode (OR 16.00, 95% CI 1.32 to 194.62, one trial, 18 participants, very low quality evidence). We are very uncertain whether platelet-poor plasma reduces the number of participants with severe or life-threatening bleeding (OR 4.00, 95% CI 0.56 to 28.40, one trial, 18 participants, very low quality evidence). We found no trials that looked at: number of days on which bleeding occurred, time from randomisation to first bleed, number of platelet transfusions, all-cause mortality, thromboembolic events or quality of life.

AUTHORS' CONCLUSIONS

There is insufficient evidence to determine if platelet-poor plasma or TPO mimetics reduce bleeding for participants with haematological malignancies undergoing intensive chemotherapy or stem cell transplantation. To detect a decrease in the proportion of participants with clinically significant bleeding from 12 in 100 to 6 in 100 would require a trial containing at least 708 participants (80% power, 5% significance). The six ongoing trials will provide additional information about the TPO mimetic comparison (424 participants) but this will still be underpowered to demonstrate this level of reduction in bleeding. None of the included or ongoing trials include children. There are no completed or ongoing trials assessing artificial platelet substitutes, fibrinogen concentrate, recombinant activated factor VII or desmopressin in people undergoing intensive chemotherapy or stem cell transplantation for haematological malignancies.

Open-label, dose-titration and continuation study to assess efficacy, safety, and pharmacokinetics of anagrelide in treatment-naïve Japanese patients with essential thrombocythemia

Although anagrelide is widely used for the treatment of essential thrombocythemia (ET) in the USA and Europe, it is not licensed in Japan. Existing literature has reported differences in polymorphism and activity of CYP1A2 in Japanese and non-Japanese ethnic groups, which may alter anagrelide metabolism. We intended to identify the optimum dosage of anagrelide in treatment-naïve Japanese patients with ET and assess its long-term safety and efficacy. Twelve patients with ET and a platelet count of ≥ 80 × 10(4)/μL were enrolled. Anagrelide was administered at an initial dose of 0.5 mg/day (weeks 1-4), then increased to 1.0 mg/day (weeks 5-8). During the following maintenance (weeks 9-52) and continuation periods (weeks 53-104), the dose was adjusted according to patient safety data and to maintain target platelet counts (<60 × 10(4)/μL). Increasing the dose led to a decrease in mean platelet count, and target platelet counts were maintained in 11 patients. Adverse events were mild or moderate, and none led to discontinuation. This cohort of Japanese patients exhibited higher pharmacokinetic exposures of anagrelide and its active metabolite than those previously documented in non-Japanese patients. These differences were modest, suggesting specific dosing regimens for Japanese patients are not required.

Pharmacokinetic and pharmacodynamic modeling of pegylated thrombopoietin mimetic peptide (PEG-TPOm) after single intravenous dose administration in healthy subjects

Pegylated thrombopoietin mimetic peptide (PEG-TPOm) is a novel, potent thrombopoietin receptor agonist with low immunotoxicity potential that protects against chemotherapy-induced thrombocytopenia in preclinical animal models. The aim of this study was to develop a population pharmacokinetic and pharmacodynamic model of PEG-TPOm following single intravenous doses in healthy subjects. Data were obtained from a double-blind, randomized, placebo-controlled study. A model based on target-mediated drug disposition and precursor pool life spans was applied. Model evaluation was performed through predictive checks and bootstrap analysis. The half-life of PEG-TPOm ranged between 18 and 36 hours, and the estimated distributional volume was 5 L. The increase in platelet counts was observed after a 4-day delay, consistent with the megakaryocyte cell life span. The platelet life span was estimated to be 5 days. After maximum platelets counts were achieved on day 9, platelets returned back to baseline on day 29. Model-based simulations were undertaken to explore pharmacodynamic effects after multiple dosing. Weekly dosing produced a sustained pharmacodynamic response, whereas an interdosing interval >or=2 weeks resulted in fluctuating pharmacodynamic profiles. Thus, the mechanistic pharmacokinetic/pharmacodynamic model was suitable for describing the complex PEG-TPOm pharmacokinetics/pharmacodynamics, including target-mediated disposition, dose-dependent platelet stimulation, and mean life spans of thrombopoietic cell populations.

Pharmacodynamic model for chemotherapy-induced anemia in rats

Anticancer agents often cause bone marrow toxicity resulting in progressive anemia which may influence the therapeutic effects of erythropoietic-stimulating agents. The objective of this study was to develop a pharmacodynamic (PD) model to describe chemotherapy-induced anemia in rats. Anemia was induced in male Wistar rats with a single intravenous (i.v.) injection of 60 mg/kg carboplatin. Hematological responses including reticulocytes, red blood cells (RBC), hemoglobin, and endogenous rat erythropoietin (EPO) were measured for up to 4 weeks. A catenary, lifespan-based, indirect response model served as a basic PD model to represent erythroid cellular populations in the bone marrow and blood involved in erythropoiesis. The model assumed that actively proliferating progenitor cells in the bone marrow are sensitive to anti-cancer agents and subject to an irreversible removal process. The removal rate of the target cells is proportional to drug activity concentrations and the cell numbers. An additional RBC loss from the circulation resulting from thrombocytopenia was described by a first-order process. The turnover process of rat EPO and EPO-mediated feedback inhibition mechanism regulated by hemoglobin changes were incorporated. Reticulocyte counts decreased rapidly and reached a nadir by day 3 after administration of carboplatin and returned to the baseline by day 13. This was followed by a gradual increase and the rebound peak occurred at about day 15. The hemoglobin nadir was approximately 9 g/dl observed at about 11-13 days compared to its normal value of 13 g/dl and hemoglobin returned to the baseline by day 30. The increase in endogenous rat EPO mirrored inversely hemoglobin changes and the maximum increase was observed soon after the hemoglobin nadir. The carboplatin-treated rats exhibited progressive anemia. The proposed model adequately described the time course of hematological changes after carboplatin in rats and can be a useful tool to explore potential strategies for the management of anemia caused by chemotherapy.

Megakaryocyte development and platelet production

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, and the proliferation, maturation and terminal differentiation of the megakaryocytic progenitors. Circulating levels of thrombopoietin (TPO), the primary growth-factor for the megakaryocyte (MK) lineage, induce concentration-dependent proliferation and maturation of MK progenitors by binding to the c-Mpl receptor and signalling induction. Decreased platelet turnover rates results in increased concentration of free TPO, enabling the compensatory response of marrow MKs to increased platelet production. C-Mpl activity is orchestrated by a complex cascade of signalling molecules that induces the action of specific transcription factors to drive MK proliferation and maturation. Mature MKs form proplatelet projections that are fragmented into circulating particles. Newly developed thrombopoietic agents operating via c-Mpl receptor may prove useful in supporting platelet production in thrombocytopenic state. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease state, and the new approaches to thrombopoietic therapy.