What is the clinical spectrum of Thrombocytopenia?
Thrombocytopenia is a hematologic pathology in which counts of platelets (thrombocytes), that intervene in blood clotting and wound healing, are below the lower limit of normal, 150x10-9/L for adults.
Thrombocytopenia clinical presentation, prevalence, degree of morbidity, and fatality rate varies according to disease subtype. This executive summary focuses on chemotherapy-induced thrombocytopenia and disseminated intravascular coagulation. For a full analysis, including other TP types - aplastic anemia, drug-induced thrombocytopenia, heparin-induced thrombocytopenia, radiation-induced thrombocytopenia, primary and secondary immune thrombocytopenia, primary and secondary inherited thrombocytopenia, thrombotic thrombocytopenic purpura and hemolytic uremic syndrome - explore your access options.
Chemotherapy-induced thrombocytopenia (CIT) is a frequent complication of cancer therapies, resulting in increased risk of bleeding, dose reduction, and treatment discontinuation negatively affecting oncologic outcomes.
Disseminated intravascular coagulation (DIC) exhibits both excessive clot formation (microvascular thrombosis) and bleeding, resulting in mortality in up to 50% of patients. Symptoms of DIC also include bruising, low blood pressure, shortness of breath, and confusion. The main driver of mortality in DIC is microvascular thrombosis that can produce multiple organ failures, predominantly acute kidney injury and acute respiratory distress syndrome.
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What do we know about the etiology of Thrombocytopenia?
The causes of thrombocytopenia are heterogenous and can be internal, external, or a combination of both.
Chemotherapy-induced thrombocytopenia (CIT) - CIT is commonly caused by cytotoxic effects of chemotherapy, damaging hematopoietic stem and progenitor cell (HSPC). Several mechanisms of CIT have been proposed, such as direct suppression of megakaryocyte progenitors in the bone marrow, alteration of the bone marrow microenvironment (stromal cells, cytokines) that supports hematopoiesis, and apoptosis-triggering DNA damage. Beyond megakaryocyte suppression, other mechanisms that may underly CIT involve immune-mediated platelet destruction (fludarabine, oxaliplatin) and splenic sequestration.
Disseminated intravascular coagulation (DIC) - DIC is secondary to an underlying condition, mainly sepsis, trauma, cancer, infections and obstetric complications. These conditions produce inflammatory triggers that cause massive release of procoagulants from endothelial cells and monocytes, leading to excess thrombin generation within the vasculature, consumption of platelet, fibrinogen, and clotting factors, and consumption coagulopathy that impairs the organism's ability to form new clots where needed. In hyperfibrinolytic DIC, that may occur in response to clotting, the excessive activity of plasmin leads to premature degradation of fibrin clots, preventing stable clot formation and facilitating uncontrolled bleeding. The innate immune response mediated by monocytes and neutrophils at the sites of tissue injury or infection plays a central role in development of DIC. The pathways activated vary according to the trigger type (cancer, trauma, infection), explaining why some DIC patients present with predominant thrombosis while others present with massive bleeding.
For the complete 35+ page in‑depth analysis, including full etiology and references, explore your access options.
How similar are human and animal skeletal systems?
Below are some of the examples of inter-species differences that negatively affect researcher's ability to mimic thrombocytopenia phenotypes, understand its mechanisms, and develop effective therapies.
Species-specific differences in immune system
Extensive differences between humans and mice were demonstrated in the structure of innate and adaptive immunity, including in balance of leukocyte subsets, defensins, Toll receptors, inducible NO synthase, and Ig subsets. For instance, mice lack the genetic equivalent of FcγRIIA activating receptor for IgG, which in humans plays a key role in recognition of IgG antibody-coated platelets by macrophages. Moreover, mouse monocytes and macrophages express a greater number of inhibitory FcyRIIB receptors than their human counterparts, limiting antibody dependent platelet clearance. As a result, mouse models of platelet-mediated immune activation may underestimate human risk and produce misleading results for monoclonal antibody therapies.
Species-specific differences in platelet production, clearance and turnover rates
The liver-produced hormone thrombopoietin (TPO) is the primary physiological regulator of platelet production. By binding to the megakaryocyte progenitor transmembrane cell receptor MPL (myeloproliferative leukemia protein) on HSC, megakaryocyte progenitors and mature megakaryocytes, TPO promotes HSC differentiation toward megakaryocyte lineage, proliferation of megakaryocyte progenitors and cytoplasmic expansion of megakaryocytes. Mice have faster platelet turnover rates compared to humans, supported by higher circulating TPO levels and faster clearance rate of TPO compared to humans. The accelerated platelet production may confer a higher resistance to certain thrombocytopenia-inducing conditions seen in humans.
Species-specific differences in platelet central signaling cascade
Exogenous factors such as vascular injury, drugs, inflammation, and infection can trigger a coordinated and tightly regulated response of platelet molecular networks known as central signaling cascade. Systems biological pathway analysis-aided multi-omics comparison of central platelet signaling cascades in mice and humans, shows major differences in expression levels and regulatory fine-tuning between the two species that may adversely affect the ability of mouse models of thrombocytopenia to recapitulate human-relevant pathophysiology and reliably predict human responses to treatments.
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Face validity - How well do animal models replicate the human disease phenotype?
Attempts to model human thrombocytopenia in animals have had a varying degree of success over the last six decades of research, depending on the disease (sub)type modeled, the species used, and the method of experimental induction.
Chemotherapy-induced thrombocytopenia (CIT) - Injected intraperitoneally, the cytotoxic chemotherapy agent fluorouracil (5-FU), that is indicated in treatment of several types of cancer, induced profound myelosuppression in mice. To enhance the severity of thrombocytopenia, a combined chemotherapy-radiation mouse model was developed, in which mice were treated with multicycle regimen of carboplatin plus irradiation, resulting in anemia and thrombocytopenia.
Disseminated intravascular coagulation (DIC) - Numerous attempts have been made to reproduce DIC-like symptoms in mice, rats, dogs, non-human primates and other animal species by employing a variety of experimental induction methods, including blood loss, hemocoagulating poisons, viral infection, tumour inoculation, thermal injury, and infusion of lipopolysaccharides (LPS), tissue factor, cytokines, and activated factor Xa-phospholipids. While coagulation activation, platelet consumption, and microvascular thrombosis were recapitulated in most animal models of acute DIC, multiple organ failures seen in human DIC were often absent or mild. With the exception of tissue factor and activated factor Xa-phospholipid infusion, most DIC in vivo models tended to overrepresent thrombotic features and underrepresent fibrinolysis.
For the complete 35+ page in‑depth analysis, including full face validity assessment and references, explore your access options.
Construct validity - How well do the mechanisms of disease induction in animals reflect the currently understood etiology of the human disease?
Thrombocytopenia has a multitude of etiologies, sometimes with no identifiable trigger. However, animal models often fail to replicate the complexity of human thrombocytopenia and its mechanisms.
Chemotherapy-induced thrombocytopenia (CIT) - Animal models of CIT replicated bone marrow suppression and impaired megakaryopoiesis observed in CIT patients. Yet, several therapies developed in animal models of CIT, such as TPO analogues and TPO receptor agonists, failed to produce desired results in cancer patients, pointing to inter-species differences in mechanisms of CIT and platelet recovery. Another concern is that CIT animal models do not capture the complexity of human CIT, that may involve immune dysregulation, genetic variants (drug metabolism) and comorbidities (anemia, infection).
Disseminated intravascular coagulation (DIC) - In patients with DIC the disease mechanisms involve a complex cascade of events, including immune activation, endothelial dysfunction, tissue factor expression, and fibrinolysis. In stark contrast, most animal models of DIC tend to be reductionist and mechanistically narrow. For instance, LPS injection and cytokine infusion to mimic sepsis-induced DIC does not replicate pathogen-host interactions that initiate a cascade of events – immune activation, endothelial injury, and dysregulation of coagulation.
For the complete 35+ page in‑depth analysis, including full construct validity assessment and references, explore your access options.
Predictive validity - How well do animal models predict safety and efficacy of therapies in human patients?
Largely owing to decades of reliance on experimental animal models of thrombocytopenia, most available treatments are non-curative and only transiently compensatory. Drug-induced thrombocytopenia and disseminated intravascular coagulation are among the most common cases found in intensive care units, and yet effective treatments for these conditions are sorely lacking.
Chemotherapy-induced thrombocytopenia (CIT) - CIT often leads to negative outcomes in cancer patients, and yet, there are no standardized guidelines for preventing or managing CIT. Administered following chemotherapy-radiation cycles to mimic human chemotherapy-radiation-induced thrombocytopenia, treatment with thrombopoietin receptor agonist (TPO-RA) peptibody analogue of TPO - romiplostim - was effective at accelerating platelet recovery in BDF1 mice. However, these TPO-RA have not received approval from the FDA or EMA for treatment of CIT. The National Comprehensive Cancer Network endorsed consideration of romiplostim as one option for patients with CIT. The International Society on Thrombosis and Haemostasis (ISTH) Subcommittee on Hemostasis and Malignancy recommends against the use of TPO-RA for the management of CIT in acute myeloid leukemia, HSCT, and lymphoma. When considering off-label use of TPO-RA for solid tumors, use of romiplostim over other TPO-RA was preferred.
Disseminated intravascular coagulation (DIC) - Despite decades of animal research, there is no medication specific for DIC, which is not surprising given the overall poor face and construct validity of animal models of DIC. For instance, in a baboon model of E. Coli-induced sepsis, site-inactivated FVIIa limited lung injury by attenuating coagulation activation, but this benefit did not translate into improved outcomes in patients with acute respiratory distress syndrome. Heparin may be considered in patients with predominant thrombosis but is not routinely used in DIC because of hemorrhage risk. Antifibrinolytics are only used in severe bleeding when fibrinolysis dominates. The small molecule TPO-RA - eltrompobag - does not address the excessive platelet consumption in DIC, and could potentially worsen microvascular thrombi.
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Ethical validity - How well do animal experiments align with human ethical principles?
Preclinical - Animal research is unethical in essence by human standards, since it involves physical constraint, psychological suffering and deprivation of freedom, social interactions, natural environment, and life purpose. In addition to this baseline, experiments inflict severe clinical harm in animals.
Clinical - Statistics consistently show that clinical success rates of drugs developed and tested in animals is very low, raising the question of whether it is ethical to put the health of patients at risk.
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Intrinsic validity - How well do animal models capture the clinical heterogeneity of the human disease?
For reasons detailed in previous sections, animal models do not recapitulate the clinical heterogeneity and inter-patient variability in thrombocytopenia phenotypes, etiologies, and responses to treatments.
For the complete 35+ page in‑depth analysis, including intrinsic validity assessment and references, explore your access options.
Extrinsic validity - How well does animal experimentation generate reliable and reproducible outcomes?
Contributing factors include flawed experimental design, variation in animal strains and experimental conditions, and lack of transparency on methodology and results of animal studies. In spite of significant investment in dissemination, various incentives and training of animal researchers, the ARRIVE - Animal Research: Reporting of In Vivo Experiments - guidelines remain poorly implemented and the majority of animal experiments is irreproducible. While in vitro methods are not immune to issues of reproducibility, the moral weight of irreproducible animal studies is not the same.
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Key takeaways
Thrombocytopenia (TP) is a hematologic pathology in which counts of platelets are below the lower limit of normal. Patients with thrombocytopenia are at an increased risk of bleeding, thrombosis, and mortality. It is a clinically heterogenous condition with a multitude of forms, etiologies, and contributing risk factors.
Over the last decades, various TP types were modelled in vivo, causing severe suffering in animals. Yet, largely owing to numerous species-specific features, including in platelet production regulation, receptors, aggregation factors, signaling cascade, genetics, and immunity, animal models of TP do not faithfully recapitulate human TP clinical features and underlying mechanisms.
Despite decades-long biomedical research in animal models of TP, effective therapies for some of the most common conditions found in ICU, such as drug-induced thrombocytopenia and disseminated intravascular coagulation are critically missing.
Human-based in vitro methods will allow to improve the predictivity of drug-induced adverse effects, deepen the understanding of TP mechanisms, develop effective therapies and tailor solutions to individual TP cases.
How is Human-Based In Vitro the Answer to Advance Biomedical Research into Thrombocytopenia
The following section explores the innovative ways in which researchers can apply advanced human-based in vitro technologies to improve the quality of life and survival of patients with thrombocytopenia. It features over 30 suggestions and examples of in vitro approaches, including recapitulating the human bone marrow niche to study human-specific features of platelet production, engineering bone marrow-on-chip to investigate the non-immune and immune-mediated mechanisms of CIT, modelling the triggers of DIC in a dual human endothelium-kidney-on-chip to capture variations in pathophysiology, and assessing the effectiveness of personalized treatments by using patient-derived cells, platelets, and plasma.
For the complete 35+ page in‑depth analysis, including examples, suggestions and references, explore your access options.
