Pharmacokinetics, pharmacodynamics, and safety of administering pegylated recombinant megakaryocyte growth and development factor to newborn rhesus monkeys

Neonatal and adult megakaryopoiesis

PURPOSE OF REVIEW

It has become increasingly clear that there are substantial biological differences between fetal/neonatal and adult megakaryopoiesis. Over the last 18 months, studies challenged the paradigm that neonatal megakaryocytes are immature and revealed a developmentally unique uncoupling of proliferation, polyploidization, and cytoplasmic maturation. Several studies also described substantial molecular differences between fetal/neonatal and adult megakaryocytes involving transcription factors, signaling pathways, cytokine receptors, and microRNAs.

RECENT FINDINGS

This review will summarize our current knowledge on the developmental differences between fetal/neonatal and adult megakaryocytes, and recent advances in the underlying molecular mechanisms, including differences in transcription factors, in the response to thrombopoietin (Tpo), and newly described developmentally regulated signaling pathways. We will also discuss the implications of these findings on the way megakaryocytes interact with the environment, the response of neonates to thrombocytopenia, and the pathogenesis of Down syndrome-transient myeloproliferative disorder (TMD) and Down syndrome-acute megakaryoblastic leukemia (DS-AMKL).

SUMMARY

A better characterization of the molecular differences between fetal/neonatal and adult megakaryocytes is critical to elucidating the pathogenesis of a group of disorders that selectively affect fetal/neonatal megakaryocyte progenitors, including the thrombocytopenia-absent radius (TAR) syndrome, Down syndrome-TMD or Down syndrome-AMKL, and the delayed platelet engraftment following cord blood transplantation.

Neonatal thrombocytopenia and megakaryocytopoiesis

Thrombocytopenia is common among sick neonates, affecting 20% to 35% of all patients admitted to the neonatal intensive care unit (NICU). While most cases of neonatal thrombocytopenia are mild or moderate and resolve within 7 to 14 days with appropriate therapy, 2.5% to 5% of NICU patients develop severe thrombocytopenia, sometimes lasting for several weeks and requiring >20 platelet transfusions. The availability of thrombopoietic agents offers the possibility of decreasing the number of platelet transfusions and potentially improving the outcomes of these infants. However, adding thrombopoietin (TPO) mimetics to the therapeutic armamentarium of neonatologists will require careful attention to the substantial developmental differences between neonates and adults in the process of megakaryocytopoiesis and in their responses to TPO. Taken together, the available data suggest that TPO mimetics will stimulate platelet production in neonates, but might do so through different mechanisms and at different doses than those established for adults. In addition, the specific groups of thrombocytopenic neonates most likely to benefit from therapy with TPO mimetics need to be defined, and the potential nonhematological effects of these agents on the developing organism need to be considered. This review summarizes our current understanding of neonatal megakaryocytopoiesis, and examines in detail the developmental factors relevant to the potential use of TPO mimetics in neonates.

Increasing platelets without transfusion: is it time to introduce novel thrombopoietic agents in neonatal care?

The Food and Drug Administration recently approved two novel thrombopoiesis-stimulating agents, Romiplostim (AMG-531, Nplate) and Eltrombopag (Promacta), for the treatment of adults with immune thrombocytopenic purpura. For physicians taking care of critically ill neonates, this offers the opportunity of decreasing platelet transfusions and potentially improving the outcomes of neonates with severe and prolonged thrombocytopenia. However, several developmental factors need to be taken into consideration. First, the population of thrombocytopenic neonates likely to benefit from these agents needs to be carefully selected. Second, the mechanisms underlying neonatal and adult thrombocytopenia differ from each other and are incompletely understood, and pre-clinical evidence suggests that the response of neonates to thrombopoietic factors might be different from that of adults. Finally, the potential non-hematopoietic effects of thrombopoietin have not been well established. Here, we will discuss these issues in detail, and will highlight the critical developmental differences between neonates and adults that need to be considered as we think about introducing these agents into neonatal care.

Thrombopoietin following transfusion of platelets in preterm neonates

Thrombocytopenia is common in the neonatal intensive care unit. Transfusion of platelets is often required. The purpose of our study was to determine changes in thrombopoietin (Tpo) following transfusion of platelets in preterm neonates. Preterm neonates undergoing platelet transfusion were randomized to receive a transfusion volume of either 10 or 15 ml/kg. Blood was obtained for Tpo measurement pre-transfusion, one and 24 hours post-transfusion. Platelet Factor 4 (PF4) was also measured to quantify platelet activation. Statistical analysis was performed using repeated measures ANOVA, and Mann-Whitney U test as appropriate. Ten infants were enrolled in each group. Gestational age, birth weight, etiology of thrombocytopenia, and timing of transfusion did not differ between the 10 and 15 ml/kg groups. There were no differences between the groups in platelet count prior to and/or following transfusion. Both transfusion volumes were equally well tolerated. Tpo and PF4 did not differ between groups at any of the study time points. When both groups were analysed together, Tpo dropped 43% (95% confidence 37-49%, p = 0.01) 1-hour post compared to pre-transfusion. In conclusion the observed decrease in Tpo following platelet transfusion suggests that Tpo kinetics in neonates is similar to adults following transfusion. PF4 was not affected by transfusion. There was not an increase in platelet count following transfusion volume of 15 ml/kg compared to 10 ml/kg.

The CBC: reference ranges for neonates

"Normal values" for blood parameters of neonates are generally unavailable, because blood is not usually drawn on healthy, normal neonates to establish normal ranges. Instead, "reference ranges" are used, consisting of the 5th to the 95th percentile values compiled from tests performed on neonatal patients with minimal pathology, under the premise that such ranges approximate normal values. In recent years, we have been seeking to establish reference ranges for various elements of the complete blood count (CBC) of neonates, using the large databases of Intermountain Healthcare, a health care system in the western United States. Establishing these reference ranges has been facilitated by using modern hematology analyzers and electronic data repositories of clinical and laboratory information. The present review brings together several of our recent reports, displaying reference ranges for elements of the CBC among neonates at various gestational and postnatal ages.

Platelet reference ranges for neonates, defined using data from over 47,000 patients in a multihospital healthcare system

OBJECTIVE

Identifying a platelet count as abnormal (thrombocytopenia or thrombocytosis) can facilitate recognizing various disease states. However, the published reference ranges for platelet counts in neonates may be imprecise, as they were generated from relatively small sample sizes and compiled before modern platelet enumeration methods.

STUDY DESIGN

We derived new neonatal reference ranges for platelet counts and mean platelet volume (MPV) measurements using electronic data accumulated during a recent 6-year period from a multihospital healthcare system.

RESULT

Platelet counts were obtained between the first and the 90th day after birth, from 47,291 neonates delivered at 22 to 42 weeks gestation. The first platelet counts obtained in the first 3 days of life, increased over the range of 22 to 42 weeks gestation. In those born < or =32 weeks gestation, the lower reference range (5th percentile) was 104,200 microl(-1), but it was 123,100 microl(-1) in late-preterm and -term neonates. Advancing postnatal age had a significant effect on platelet counts; during the first 9 weeks, the counts fit a sinusoidal pattern with two peaks; one at 2 to 3 weeks and a second at 6 to 7 weeks. The upper limit of expected counts (95th percentile) during these peaks were as high as 750,000 microl(-1).

CONCLUSION

The figures herein describe reference ranges for platelet counts and MPV determinations of neonates at various gestational ages during their first 90 days. Expected values differ substantially from the 150,000 microl(-1) to 450,000 microl(-1) range previously used to define neonatal thrombocytopenia and thrombocytosis. The new definitions will render the diagnoses of neonatal thrombocytopenia and thrombocytosis less commonly than when the old definitions were used, because the new ranges are wider than 150,000 microl(-1) to 450,000 microl(-1).

Megakaryocyte growth and development factor is a potent growth factor for primitive hematopoietic progenitors in the human fetus

Megakaryocyte growth and development factor (MGDF), or thrombopoietin, has received considerable attention as a therapeutic agent for treating thrombocytopenia or for its use in the ex vivo culture of hematopoietic stem cells. MGDF is known to support the growth of a broad spectrum of hematopoietic precursors obtained from adult or neonatal tissues, but its effects on the growth of fetal progenitors and stem cells has not been studied. Human CD38(+)CD34(2+) progenitors and CD38(-)CD34(2+) cells, a population that contains stem cells, were isolated from midgestation liver and grown under defined conditions with MGDF and various cytokines known to support the growth of primitive hematopoietic precursors. In clonal assays of colony-forming cells (CFCs), MGDF supported the growth of 15-25% of candidate stem cells when combined with granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor (GM-CSF), flk-2/flt3 ligand, or stem cell factor. MGDF was observed to strongly support the early stages of hematopoiesis and expansion of high proliferative potential CFCs. More mature progenitors were expanded nearly 78-fold in 1 wk of culture with MGDF+SCF+GM-CSF. MGDF alone was also found to support the short-term (2 d) survival of CD38(-)CD34(2+) high proliferative potential CFCs. The effects of MGDF were more modest on CD38(+)CD34(2+) progenitors with only additive increases in colony formation being observed. These findings suggest that MGDF administration in fetuses and neonates may strongly affect the growth and mobilization of primitive hematopoietic progenitors and that MGDF may find use in the ex vivo growth and expansion of fetal stem cells.

Platelet function in term and preterm neonates

Platelet dysfunction likely contributes to the pathophysiology of catastrophic hemorrhages in preterm neonates. In vitro studies have demonstrated that platelets of both term and preterm neonates are hyporesponsive to a variety of agonists. In contrast,template bleeding times of term neonates are shorter than those from adults. Very little is known about this and other tests of primary hemostasis in premature and sick neonates in the neonatal intensive care unit (NICU). This article covers the current knowledge of platelet function in preterm and term neonates and review show new agents (such as recombinant thrombopoietin and recombinant factor VIIa) may enhance neonatal platelet function.

Evaluation and treatment of severe and prolonged thrombocytopenia in neonates

Thrombocytopenia is one of the most common hematologic problems in the neonatal intensive care unit (NICU). Despite its prevalence,several basic pathophysiologic questions remain unanswered. For instance, there is a lack of evidence-based guidelines for treatment,and the kinetic mechanisms (decreased platelet production,increased platelet consumption, or sequestration) responsible for most varieties of neonatal thrombocytopenia are not well defined.Moreover, a clear correlation between degree of thrombocytopenia and the resulting bleeding risk has not been demonstrated, and no transfusion-trigger studies have been conducted in neonates. As a consequence of these deficiencies in knowledge, there is great variability in platelet transfusion practices among NICUs. This article presents an overview of the evaluation of a neonate with severe thrombocytopenia and a review of current and projected therapeutic options.

Neonatal transfusion practice

Many previously widely accepted neonatal transfusion practices are changing as neonatologists become more aware of the risks to their patients of multiple blood product transfusions. Recent literature and research on neonatal transfusion practice are here reviewed, and practical guidelines and trigger thresholds for blood products commonly used in neonatal medicine are proposed.

New TPO treatment schedules of increased safety and efficacy: pre-clinical validation of a thrombopoiesis simulation model

Thrombopoietin (TPO) immunogenicity hampers its development as a therapeutic agent for attenuating thrombocytopenia and improving platelet harvest in donors. This work was aimed at validating, in mouse and in monkey experiments, a thrombopoiesis computer-model prediction that platelet counts, similar to those obtained with accepted TPO dose scheduling, can also be achieved by new and safer schedules of significantly reduced doses. To this end we compared, in a two-arm mouse experiment, platelet increases obtained with a single intraperitoneal dosing of recombinant mouse TPO (17.5 microg/kg), with those obtained by the model-suggested protocol of a significantly reduced dose (2 microg/kg on 4 consecutive days). The two TPO regimens generated similar platelet profiles, peaking at ca. 2700 x 10(9)/l platelets. In rhesus monkeys, treated by rhesus monkey recombinant TPO (5 microg/kg on 4 consecutive days), the suggested protocol yielded effective platelet stimulation with significantly reduced immunogenicity. The model's ability to predict individual monkey responses to several new TPO administration protocols was further validated, proving sufficient robustness in providing good predictions with limited input data. The simulation tool could be used for testing the effects of different therapeutic agents on thrombopoiesis. Human trials are warranted for testing the suggested improved TPO protocol, possibly in conjunction with chemotherapy.

Neonatal thrombocytopenia: causes and management

Neonatal thrombocytopenia is a common clinical problem. Thrombocytopenia presenting in the first 72 hours of life is usually secondary to placental insufficiency and caused by reduced platelet production; fortunately most episodes are mild or moderate and resolve spontaneously. Thrombocytopenia presenting after 72 hours of age is usually secondary to sepsis or necrotising enterocolitis and is usually more severe and prolonged. Platelet transfusion remains the only treatment. There is a need for trials to define the safe lower limit for platelet count and which neonates will benefit from treatment.

Evaluation and treatment of thrombocytopenia in the neonatal intensive care unit

Phlebotomy-induced anaemia excepted, thrombocytopenia is the most common haematological abnormality in neonatal intensive care unit (NICU) patients. Roughly one-quarter of all NICU patients and half of all sick preterm neonates develop thrombocytopenia. Whereas a large number of varied precipitating conditions has been identified, early-onset thrombocytopenia (<72 h) is most commonly associated with fetomaternal conditions complicated by placental insufficiency and/or fetal hypoxia, e.g. maternal pre-eclampsia and fetal intrauterine growth restriction. The resulting neonatal thrombocytopenia is usually mild to moderate, resolves spontaneously and requires no specific therapy. Deviation from this pattern of thrombocytopenia suggests the presence of more significant precipitating conditions. The most important of these are the immune thrombocytopenias, and every NICU should develop investigation and treatment protocols to manage these cases promptly and avoid unnecessary risk of haemorrhage. In contrast, late-onset thrombocytopenia (>72 h) is almost always associated with sepsis or necrotizing enterocolitis and the associated thrombocytopenia is severe, prolonged and often requires treatment by platelet transfusion. Unfortunately, evidence-based guidelines for platelet transfusion therapy in NICU patients are currently unavailable, making it difficult to define widely accepted thresholds for transfusion and leading to a significant variation in transfusion practice between centres.

CONCLUSION

While improving this situation remains a pressing need, the growing evidence that impaired megakaryocytopoiesis and platelet production are major contributors to many neonatal thrombocytopenias suggests that recombinant haemopoietic growth factors, including thrombopoietin and interleukin-11, may be useful future therapies to ameliorate neonatal thrombocytopenia.

Mechanisms underlying thrombocytopenia in the neonatal intensive care unit

Thrombocytopenia is one of the most common hematological problems among neonates in the neonatal intensive care unit (NICU), but in the majority of cases the kinetic mechanism responsible is unclear. This review focuses on both traditional and innovative methods used to evaluate the mechanisms responsible for thrombocytopenia in neonates, and analyzes the data generated from those methods.

CONCLUSION

Results of studies using new methods for evaluating thrombocytopenia, coupled with recent descriptions of marrow megakaryocyte mass, suggest that decreased platelet production complicates most cases of thrombocytopenia among neonates in the NICU.

Thrombocytopenia in the newborn

Thrombocytopenia remains a common problem in sick newborns. A quarter of all neonates admitted to neonatal intensive care units develop thrombocytopenia, and in 20% of episodes the thrombocytopenia is severe (platelets <50 x 10(9)/L). Practical and clinically relevant classifications of neonatal thrombocytopenia have now been developed which, by highlighting the principal conditions precipitating severe thrombocytopenia (eg, sepsis, necrotizing enterocolitis, perinatal asphyxia, and the immune thrombocytopenias), aid the practicing neonatologist. Recent reviews demonstrate that many neonates with severe thrombocytopenia receive repeated platelet transfusions, although evidence of their clinical benefit is lacking, and there exists a significant variation in platelet transfusion practice between centers. These facts support the need for the development of evidence-based protocols for platelet transfusion in the newborn and stimulate continued interest in the potential of hemopoietic growth factors (, thrombopoietin and interleukin-11) to prevent or treat neonatal thrombocytopenia.

Epidemiologic and outcome studies of patients who received platelet transfusions in the neonatal intensive care unit

STUDY DESIGN

We conducted a historic cohort study of neonates who received platelet transfusions at the National Institute of Perinatology, Mexico City, from January 1997 to May 2000. We obtained descriptive and outcome data, and assessed demographic and laboratory means of predicting "good candidates" for a future recombinant thrombopoietin (rTpo) trial.

RESULTS

A minority of the transfused patients (11.4%) received only one transfusion; the majority (88.6%) received multiple transfusions. Neonates who received one or more platelet transfusions were more likely to die (24.5% mortality) than neonates who received no platelet transfusions (3.7% mortality). Regression analyses indicated that the presence of liver disease was the best predictor of a "good candidate" for rTpo administration.

CONCLUSION

The majority of neonates in our institution who receive platelet transfusions receive multiple, not single, transfusions. Receiving any platelet transfusion is a marker for high risk of death. Neonates with liver disease who receive platelet transfusions might be a reasonable group for a phase I rTpo trial.

Platelet transfusions in the neonatal intensive care unit:factors predicting which patients will require multiple transfusions

BACKGROUND

Previous studies suggest that recombinant thrombopoietin (rTPO) will increase platelet production in thrombocytopenic neonates. However, the target populations of neonates most likely to benefit should be defined. Studies suggest that rTPO will not elevate the platelet count until 5 days after the start of treatment. Therefore, the neonates who might benefit from rTPO are those who will require multiple platelet transfusions for more than 5 days. This study was designed to find means of prospectively identifying these patients.

STUDY DESIGN AND METHODS

A historic cohort study of all patients in the neonatal intensive care unit (NICU) at the University of Florida who received platelet transfusions from January 1, 1997, through December 31, 1998, was conducted.

RESULTS

Of the 1389 patients admitted to the NICU during the study period, 131 (9.4%) received platelet transfusions. Seventeen were treated with extracorporeal membrane oxygenation and were excluded from further analysis. Of the remaining 114 patients, 55 (48%) received one transfusion and 59 (52%) received more than one transfusion (21 had >4). None of the demographic factors examined predicted multiple platelet transfusions. However, two clinical conditions did; liver disease and renal insufficiency. Neonates who received one platelet transfusion had a relative risk of death 10.4 times that in neonates who received none (p = 0.0001). Neonates who received >4 platelet transfusions had a risk of death 29.9 times that in those who received no transfusions (p = 0.0001).

CONCLUSION

NICU patients with liver disease or renal insufficiency who receive one platelet transfusion are likely to receive additional transfusions. Therefore, these patients constitute a possible study population for a Phase I/II rTPO trial.

Neonatal thrombocytopenia: new insights into pathogenesis and implications for clinical management

The healthy fetus has a platelet count of greater than 150 x 10(9)/L by the second trimester of pregnancy and only 2% of term infants are thrombocytopenic at birth. Severe thrombocytopenia (platelets < 50 x 10(9)/L) occurs in fewer than three per 1000 term infants, the most important cause being alloimmune thrombocytopenia. In contrast, in infants admitted to neonatal intensive care units, thrombocytopenia develops in 25% and in up to half of sick preterm infants. Recent evidence shows that these infants mostly have evidence of underlying impaired fetal megakaryocytopoiesis and platelet production following pregnancy complications characterized by placental insufficiency or fetal hypoxia. The mechanism of this is unknown. However, many neonatal complications exacerbate this thrombocytopenic potential and 20% of thrombocytopenias in neonatal intensive care unit patients are severe. Evidence-based guidelines for platelet transfusion therapy in these patients are yet to be defined, but as platelet underproduction underlies most neonatal thrombocytopenias, recombinant hemopoietic growth factors, including thrombopoietin and interkeukin-11, may be useful future therapies.

Evaluation and treatment of thrombocytopenia in the neonatal intensive care unit

Thrombocytopenia is a very frequent problem among sick neonates, affecting up to 35% of all infants admitted to the NICU. Although multiple clinical conditions have been causally associated with neonatal thrombocytopenia, the cause of the thrombocytopenia is unclear in up to 60% of affected neonates. This article provides neonatologists with a practical approach to the thrombocytopenic neonate, with an emphasis on conditions that could be life-threatening or could have significant implications for further pregnancies. An overview of the current therapeutic modalities is also presented, including a discussion of the possible use of recombinant thrombopoietic cytokines to treat certain groups of thrombocytopenic neonates.

Dose-response relationship of megakaryocyte progenitors from the bone marrow of thrombocytopenic and non-thrombocytopenic neonates to recombinant thrombopoietin

Megakaryocyte (MK) progenitors from the marrow of adults undergo dose-dependent clonogenic proliferation in response to recombinant thrombopoietin (rTpo). It is unknown whether progenitors from the marrow of thrombocytopenic neonates display rTpo dose-dependent proliferation and whether they are more or less sensitive to rTpo than progenitors from non-thrombocytopenic neonates or adults. To assess this, we cultured marrow from four thrombocytopenic and four non-thrombocytopenic neonates, and from six healthy adults, in a serum-free system in the presence of increasing concentrations of rTpo (0-100 ng/ml). Marrow from the thrombocytopenic and non-thrombocytopenic neonates generated three times more MK colonies/105 light density cells (129 +/- 39 and 167 +/- 30 respectively) than marrow from adults (54 +/- 30, P < 0.0001) at a rTpo concentration of 50 ng/ml. Neonatal and adult samples had a rTpo dose-dependent increase in MK colonies. However, neonates reached a maximal number of colonies at a rTpo concentration of 10 ng/ml, compared with 50 ng/ml in adults, resulting in a larger area under the rTpo dose-response curve for neonatal progenitors (P = 0. 0047). Neonates also generated more large MK colonies than the adults (24% vs. 2% at 100 ng/ml).

Thrombopoietin in the fetus and neonate

C-mpl ligand or thrombopoietin (Tpo) is increasingly recognised as the major regulator of platelet homeostasis in humans. Relatively little is known about Tpo in the fetus and neonate but no evidence has yet been found to suggest any fundamental difference in Tpo structure, function and regulation in the fetus and neonate compared to older age groups. Tpo mRNA transcripts have been detected in the fetus as early as 6 weeks post conception and the liver appears to be the main site of Tpo production in both the fetus and neonate. The vast majority of healthy newborns have detectable levels of circulating Tpo and raised Tpo levels are commonly, but not consistently, found in thrombocytopenic neonates. In adults receptor binding and subsequent metabolism of Tpo is proposed as the main method of regulation of the circulating Tpo level. Preliminary studies in neonates showing increased Tpo levels most often during thrombocytopenia accompanied by reduced megakaryocytopoiesis supports this concept. In addition to this demonstrable fetal and neonatal endogenous Tpo production megakaryocyte progenitor and precursor cells from the fetus and from preterm and term newborns proliferate and differentiate extensively in-vitro in response to exogenous Tpo. Furthermore a recent study has shown a marked rise in platelet count in newborn rhesus monkeys administered one form of recombinant Tpo. Although these studies remain at an early stage together these findings strongly suggest that, as in adults, Tpo is the major regulator of platelet homeostasis in the fetus and neonate. Thrombocytopenia is common in sick neonates and progress in understanding this important clinical problem is likely to be greatly enhanced by the current and future research into Tpo production, function and regulation in the healthy and thrombocytopenic fetus and neonate.