Investigation of the Pharmacokinetics of Romiplostim in Rodents with a Focus on the Clearance Mechanism

Immune thrombocytopenia: Pathophysiology and impacts of Romiplostim treatment

Immune thrombocytopenia (ITP) is an autoimmune bleeding disease caused by immune-mediated platelet destruction and decreased platelet production. ITP is characterized by an isolated thrombocytopenia (<100 × 109/L) and increased risk of bleeding. The disease has a complex pathophysiology wherein immune tolerance breakdown leads to platelet and megakaryocyte destruction. Therapeutics such as corticosteroids, intravenous immunoglobulins (IVIg), rituximab, and thrombopoietin receptor agonists (TPO-RAs) aim to increase platelet counts to prevent hemorrhage and increase quality of life. TPO-RAs act via stimulation of TPO receptors on megakaryocytes to directly stimulate platelet production. Romiplostim is a TPO-RA that has become a mainstay in the treatment of ITP. Treatment significantly increases megakaryocyte maturation and growth leading to improved platelet production and it has recently been shown to have additional immunomodulatory effects in treated patients. This review will highlight the complex pathophysiology of ITP and discuss the usage of Romiplostim in ITP and its ability to potentially immunomodulate autoimmunity.

Repeat dose and reproductive toxicity of thrombopoietin mimic peptide in Sprague-Dawley rats

Thrombopoietin mimic peptide (TMP) is a novel thrombopoietin receptor agonist. In this report, we evaluated the potential toxicity of TMP in repeat-dose toxicity and reproductive/developmental toxicity studies (segment Ⅰ, Ⅱ, Ⅲ). TMP was administered subcutaneously to Sprague-Dawley (SD) rats at 5, 15 or 50 mcg/kg. In repeat-dose toxicity study, the rats were administrated three times a week for 26 week with a 4-week recovery. TMP could produce anti-drug antibodies and induce platelet counts increase, megakaryocyte proliferation. While platelet counts decreased gradually and returned to normal after 4 weeks in male rats. Other significant findings included myelofibrosis of bone marrow, hepatic extramedullary hematopoiesis, splenic lymphocytic depletion and bone hyperostosis. All treatment-related effects were reversed following recovery. The NOAEL of repeat-dose toxicity in female rats is 5 mcg/kg. In the reproductive/developmental toxicity (segment Ⅰ, Ⅲ), no deaths occurred, and no general toxicological effects or abnormal reproductive functions were observed. In embryo-fetal developmental toxicity study (segment Ⅱ), the number of resorbed fetuses in the 50 mcg/kg group was significantly increased. The NOAEL as related to reproductive/developmental toxicity in these rats was 15 mcg/kg.

Strategies to improve the physicochemical properties of peptide-based drugs

Peptides are a rapid-growing class of therapeutics with unique and desirable physicochemical properties. Due to disadvantages such as low membrane permeability and susceptibility to proteolytic degradation, peptide-based drugs have limited bioavailability, a short half-life, and rapid in vivo elimination. Various strategies can be applied to improve the physicochemical properties of peptide-based drugs to overcome limitations such as limited tissue residence time, metabolic instability, and low permeability. Applied strategies including backbone modifications, side chain modifications, conjugation with polymers, modification of peptide termini, fusion to albumin, conjugation with the Fc portion of antibodies, cyclization, stapled peptides, pseudopeptides, cell-penetrating peptide conjugates, conjugation with lipids, and encapsulation in nanocarriers are discussed.

Pharmacokinetics, Safety, and Pharmacodynamics of Romiplostim in Chinese Subjects With Immune Thrombocytopenia: A Phase I/II Trial

Romiplostim is approved for the treatment of immune thrombocytopenia (ITP). This study aimed to evaluate the pharmacokinetics, safety, and pharmacodynamics of romiplostim in Chinese patients with ITP. This multicenter, open-label, dose-escalation phase I/II trial enrolled ITP patients from 5 centers in China between October 2015 and August 2017. There were 2 cohorts: 1 μg/kg and 3 μg/kg weekly for 2 weeks. The end points included pharmacokinetics, platelet changes from baseline, hematological indicators, and adverse events (AEs). Sixteen participants, with 8 patients in each cohort, were enrolled. In the 1 μg/kg cohort, time to maximum concentration was 4.00 (4.00-7.83) hours, maximum serum drug concentration was 52.0 (16.0-228.0) pg/mL, and area under the serum drug concentration-time curve from time 0 to the last detectable time point was 389 (32.0-5400) pg · h/mL. In the 3 μg/kg cohort, time to maximum serum drug concentration was 11.91 (4.00-12.00) hours, maximum serum drug concentration was 105.0 (25.5-313.0) pg/mL, and half-life was 12.7 (8.2-23.6) hours. The absolute change of peak platelet count from baseline was 14 (3-40) and 72 (3-369) ×10⁹ /L in the 1 and 3 μg/kg cohorts, respectively. Seven (87.5%) and eight (100%) participants had treatment-emergent AEs in 1 μg/kg cohort and 3 μg/kg cohort, respectively. No major AEs occurred in the 2 cohorts. Romiplostim (1 and 3 μg/kg) is safe and well tolerated in Chinese patients with ITP.

Experts' review: the emerging roles of romiplostim in immune thrombocytopenia (ITP)

INTRODUCTION

The management of ITP has in recent years been transformed from reliance on immunosuppressants and splenectomy to targeted therapy with thrombopoietin receptor agonists (TPO-RA) that directly stimulate platelet production in the bone marrow. This has reduced the long-term infective complications and toxicities associated with the use of potent immunosuppressants and splenectomy. The welltolerated romiplostim, itself a novel drug construct called peptibody, has established itself, alongside other TPO-RA as the preferred 2nd line therapy in major international guidelines on treatment of ITP.

AREAS COVERED

This review summarizes the data from early licensing trials of romiplostim and discusses the real-world experience to date, the unexpected emerging data on treatment-free long-term remission achieved using TPO-RA, and the case for its early introduction in the therapeutic pathway. The emerging risk of thrombosis is also discussed.

EXPERT OPINION

The use of romiplostim and other TPO-RA will be increasingly brought forward in the management pathway of ITP with the prospect of modifying the long-term outcome of the disease by increasing sustained treatment-free remission. With the prospect of several new targeted therapies been introduced into clinical practice, TPO-RA will likely be a key component of future combination therapies for difficult cases.

Enhancing the Pharmacokinetics and Antitumor Activity of an α-Amanitin-Based Small-Molecule Drug Conjugate via Conjugation with an Fc Domain

Herein we describe the design and biological evaluation of a novel antitumor therapeutic platform that combines the most favorable properties of small-molecule drug conjugates (SMDCs) and antibody drug conjugates (ADCs). Although the small size of SMDCs, compared to ADCs, is an appealing feature for their application in the treatment of solid tumors, SMDCs usually suffer from poor pharmacokinetics, which severely limits their therapeutic efficacy. To overcome this limitation, in this proof-of-concept study we grafted an α-amanitin-based SMDC that targets prostate cancer cells onto an immunoglobulin Fc domain via a two-step "program and arm" chemoenzymatic strategy. We demonstrated the superior pharmacokinetic properties and therapeutic efficacy of the resulting Fc-SMDC over the SMDC in a prostate cancer xenograft mouse model. This approach may provide a general strategy toward effective antitumor therapeutics combining small size with pharmacokinetic properties close to those of an ADC.

Engineering the hinge region of human IgG1 Fc-fused bispecific antibodies to improve fragmentation resistance

Fc domain fusion can improve the therapeutic effects of relatively small biological molecules such as peptides, cytokines, and antibody fragments. Fc fusion proteins can also be used to enhance the cytotoxic effects of small bispecific antibodies (bsAbs). However, fragmentation of Fc fusion proteins, which mainly occurs around the hinge regions during production, storage, and circulation in the blood, is a major issue. In this study, we first investigated the mechanisms of fragmentation around the hinge region during storage using Fc-fused bsAbs with specificity for epidermal growth factor receptor and CD3 as a model. The fragmentation peaks generated by gel filtration analysis indicated that both contaminating proteases and dissolved active oxygen should be considered causes of fragmentation. We designed and constructed variants by introducing a point mutation into the upper hinge region, which reduced the cleavage caused by dissolved active oxygen, and shortened the hinge region to restrict access of proteases. These hinge modifications improved fragmentation resistance and did not affect the biological activity of the bsAbs in vitro. We confirmed the versatility of the hinge modifications using another Fc-fused bsAb. Our results show that hinge modifications to the Fc fusion protein, especially the introduction of a point mutation into the upper hinge region, can reduce fragmentation substantially, and these modifications can be used to improve the fragmentation resistance of other recombinant Fc fusion proteins.

Peptibodies: An elegant solution for a long-standing problem

Chimeric proteins composed of a biologically active peptide and a fragment crystallizable (Fc) domain of immunoglobulin G (IgG) are known as peptibodies. They present an extended half-life due to neonatal Fc receptor (FcRn) salvage pathway, a decreased renal clearance rate owing to its increased size (≈70 kDa) and, depending on the peptide used in the design of the peptibody, an active-targeting moiety. Also, the peptides therapeutic activity is boosted by the number of peptides in the fusion protein (at least two peptides) and to some peptides' alterations. Peptibodies are mainly obtained through recombinant DNA technology. However, to improve peptide properties, "unnatural" changes have been introduced to the original peptides' sequence, for instance, the incorporation of D- or non-natural amino acid residues or even cyclization thus, limiting the application of genetic engineering in the production of peptibodies, since these peptides must be obtained via chemical synthesis. This constrains prompted the development of new methods for conjugation of peptides to Fc domains. Another challenge, subject of intense research, relates to the large-scale production of such peptibodies using these new techniques, which can be minimized by their proved value. To date, two peptibodies, romiplostim and dulaglutide, have been approved and stay as the standard of care in their areas of action. Furthermore, a considerable number of peptibodies are currently in preclinical and clinical development.

A novel recombinant human thrombopoietin therapy for the management of immune thrombocytopenia in pregnancy

The aim of this study was to determine the safety and efficacy of recombinant human thrombopoietin (rhTPO) for the management of immune thrombocytopenia (ITP) during pregnancy. Pregnant patients with ITP were enrolled in the study if they had a platelet count less than 30 × 10⁹/L, were experiencing bleeding manifestations, had failed to respond to corticosteroids and/or intravenous immunoglobulin (IVIG), and had developed refractoriness to platelet transfusion. Thirty-one patients received rhTPO at an initial dose of 300 U/kg once daily for 14 days. Twenty-three patients responded (74.2%), including 10 complete responders (>100 × 10⁹/L) and 13 responders (30-100 × 10⁹/L). It appears that rhTPO ameliorated the bleeding symptoms remarkably, even in the nonresponders. rhTPO was well tolerated. Dizziness, fatigue, and pain at an injection site were reported in 1 patient each. No congenital disease or developmental delays were observed in the infants in a median follow-up of 53 (range, 39-68) weeks. In conclusion, rhTPO is a potentially safe and effective treatment choice for patients with ITP during pregnancy. Our work has paved the way for further study on the clinical application of rhTPO and other thrombopoietic agents for the management of ITP during pregnancy. This study is registered at www.clinicaltrials.gov as NCT02391272.

Steady-state volume of distribution of two-compartment models with simultaneous linear and saturated elimination

The model-independent estimation of physiological steady-state volume of distribution ([Formula: see text]), often referred to non-compartmental analysis (NCA), is historically based on the linear compartment model structure with central elimination. However the NCA-based steady-state volume of distribution ([Formula: see text]) cannot be generalized to more complex models. In the current paper, two-compartment models with simultaneous first-order and Michaelis-Menten elimination are considered. In particular, two indistinguishable models [Formula: see text] and [Formula: see text], both having central Michaelis-Menten elimination, while first-order elimination exclusively either from central or peripheral compartment, are studied. The model-based expressions of the steady-state volumes of distribution [Formula: see text] and their relationships to NCA-based [Formula: see text] are derived. The impact of non-linearity and peripheral elimination is explicitly delineated in the formulas. Being concerned with model identifiability and indistinguishability issues, an interval estimate of [Formula: see text] is suggested.

Key factors influencing ADME properties of therapeutic proteins: A need for ADME characterization in drug discovery and development

Protein therapeutics represent a diverse array of biologics including antibodies, fusion proteins, and therapeutic replacement enzymes. Since their inception, they have revolutionized the treatment of a wide range of diseases including respiratory, vascular, autoimmune, inflammatory, infectious, and neurodegenerative diseases, as well as cancer. While in vivo pharmacokinetic, pharmacodynamic, and efficacy studies are routinely carried out for protein therapeutics, studies that identify key factors governing their absorption, distribution, metabolism, and excretion (ADME) properties have not been fully investigated. Thorough characterization and in-depth study of their ADME properties are critical in order to support drug discovery and development processes for the production of safer and more effective biotherapeutics. In this review, we discuss the main factors affecting the ADME characteristics of these large macromolecular therapies. We also give an overview of the current tools, technologies, and approaches available to investigate key factors that influence the ADME of recombinant biotherapeutic drugs, and demonstrate how ADME studies will facilitate their future development.

Closed form solutions and dominant elimination pathways of simultaneous first-order and Michaelis-Menten kinetics

The current study aims to provide the closed form solutions of one-compartment open models exhibiting simultaneous linear and nonlinear Michaelis-Menten elimination kinetics for single- and multiple-dose intravenous bolus administrations. It can be shown that the elimination half-time ([Formula: see text]) has a dose-dependent property and is upper-bounded by [Formula: see text] of the first-order elimination model. We further analytically distinguish the dominant role of different elimination pathways in terms of model parameters. Moreover, for the case of multiple-dose intravenous bolus administration, the existence and local stability of the periodic solution at steady state are established. The closed form solutions of the models are obtained through a newly introduced function motivated by the Lambert W function.

Pharmacokinetics and pharmacokinetic-pharmacodynamic correlations of therapeutic peptides

Peptides, defined as polymers of less than 50 amino acids with a molecular weight of less than 10 kDa, represent a fast-growing class of new therapeutics which has unique pharmacokinetic characteristics compared to large proteins or small molecule drugs. Unmodified peptides usually undergo extensive proteolytic cleavage, resulting in short plasma half-lives. As a result of their low permeability and susceptibility to catabolic degradation, therapeutic peptides usually have very limited oral bioavailability and are administered either by the intravenous, subcutaneous, or intramuscular route, although other routes such as nasal delivery are utilized as well. Distribution processes are mainly driven by a combination of diffusion and to a lesser degree convective extravasation dependent on the size of the peptide, with volumes of distribution frequently not larger than the volume of the extracellular body fluid. Owing to the ubiquitous availability of proteases and peptidases throughout the body, proteolytic degradation is not limited to classic elimination organs. Since peptides are generally freely filtered by the kidneys, glomerular filtration and subsequent renal metabolism by proteolysis contribute to the elimination of many therapeutic peptides. Although small peptides have usually limited immunogenicity, formation of anti-drug antibodies with subsequent hypersensitivity reactions has been described for some peptide therapeutics. Numerous strategies have been applied to improve the pharmacokinetic properties of therapeutic peptides, especially to overcome their metabolic instability, low permeability, and limited tissue residence time. Applied techniques include amino acid substitutions, modification of the peptide terminus, inclusion of disulfide bonds, and conjugation with polymers or macromolecules such as antibody fragments or albumin. Application of model-based pharmacokinetic-pharmacodynamic correlations has been widely used for therapeutic peptides in support of drug development and dosage regimen design, especially because their targets are often well-described endogenous regulatory pathways and processes.

Novel ubiquitin-derived high affinity binding proteins with tumor targeting properties

Targeting effector molecules to tumor cells is a promising mode of action for cancer therapy and diagnostics. Binding proteins with high affinity and specificity for a tumor target that carry effector molecules such as toxins, cytokines, or radiolabels to their intended site of action are required for these applications. In order to yield high tumor accumulation while maintaining low levels in healthy tissues and blood, the half-life of such conjugates needs to be in an optimal range. Scaffold-based binding molecules are small proteins with high affinity and short systemic circulation. Due to their low molecular complexity, they are well suited for combination with effector molecules as well as half-life extension technologies yielding therapeutics with half-lives adapted to the specific therapy. We have identified ubiquitin as an ideal scaffold protein due to its outstanding biophysical and biochemical properties. Based on a dimeric ubiquitin library, high affinity and specific binding molecules, so-called Affilin® molecules, have been selected against the extradomain B of fibronectin, a target almost exclusively expressed in tumor tissues. Extradomain B-binding molecules feature high thermal and serum stability as well as strong in vitro target binding and in vivo tumor accumulation. Application of several half-life extension technologies results in molecules of largely unaffected affinity but significantly prolonged in vivo half-life and tumor retention. Our results demonstrate the utility of ubiquitin as a scaffold for the generation of high affinity binders in a modular fashion, which can be combined with effector molecules and half-life extension technologies.

Pharmacokinetics of Peptide-Fc fusion proteins

Peptide-Fc fusion proteins (or peptibodies) are chimeric proteins generated by fusing a biologically active peptide with the Fc-domain of immunoglobulin G. In this review, we describe recent studies that have evaluated the absorption, distribution, metabolism, and excretion characteristics of peptibodies. Key features of the pharmacokinetics of peptibodies include their extended half-life due to recycling by the neonatal Fc receptor (FcRn), a substantial contribution by renal excretion to total clearance and, for certain peptibodies, target-mediated drug disposition. The prolonged half-life of peptibodies permits less-frequent dose administration compared with small therapeutic peptides, thereby supporting patient convenience and compliance. Hence, a considerable number of peptibodies are currently in preclinical and clinical development. Investigation of the metabolism (biotransformation) of biologics is an evolving area of research: ligand-binding mass spectrometry techniques have been employed for the characterization of the peptibody romiplostim, providing a new approach to evaluation of the degradation products of biologics. Pharmacokinetic/pharmacodynamic modeling and simulation techniques have been used to predict the pharmacokinetics of peptibodies which can inform clinical decision-making, particularly selection of dosing regimens. This integrated review highlights the distinct pharmacokinetic characteristics of peptibodies and their influence on the drug development process for this emerging family of therapeutics.

Use of a thrombopoietin mimetic for chronic immune thrombocytopenic purpura in pregnancy

BACKGROUND

Romiplostim, a thrombopoietin mimetic, is a novel therapeutic option for patients with chronic immune thrombocytopenic purpura. We report on the effects of romiplostim use throughout pregnancy.

CASE

A 28-year-old primigravid woman with chronic immune thrombocytopenic purpura initiated a planned pregnancy on romiplostim. The second and third trimesters were marked by a cyclic pattern of thrombocytopenia requiring supplemental corticosteroids or intravenous immunoglobulin and resultant thrombocytosis. Increased romiplostim doses and daily corticosteroids stabilized the platelet count before induction of labor at 33 weeks of gestation. The newborn manifested intraventricular hemorrhage at birth, although no developmental delay was present on follow-up at 10 months of age.

CONCLUSION

The decreased efficacy of romiplostim monotherapy is attributed to increased target-mediated drug disposition and the physiologic changes of pregnancy. Safety concerns still exist for the developmental effects of romiplostim on the fetus.

Pharmacokinetic and pharmacodynamic modeling of romiplostim in animals

PURPOSE

Romiplostim is a novel thrombopoiesis-stimulating peptibody that targets the thrombopoietin c-Mpl receptor, resulting in increased platelet production. The pharmacodynamic-mediated disposition (PDMDD) and its stimulatory effect on platelet production in Sprague-Dawley rats, rhesus monkeys, and cynomolgus monkeys following IV bolus and SC administration at various dose levels were determined.

METHODS

The pharmacokinetic (PK) profile was described by a PDMDD model that accounts for romiplostim binding to the c-Mpl receptor. The PD model contained a series of aging compartments for precursor cells in bone marrow and platelets. The stimulatory function was described by an on-and-off function operating on the fractional receptor occupancy (RO). The threshold effect, RO(thr), and K(D) parameters were determinants of drug potency, whereas S(max) reflected drug efficacy.

RESULTS

The model implicated that receptor-mediated clearance was negligible. RO(thr) estimated occupancies were 0.288, 0.385, 0.771 for rats, rhesus, and cynomolgus monkeys, respectively. The analogous estimated values of K(D) were 4.05, 2320, and 429 ng/mL, implying that romiplostim was much more potent in rats, which was confirmed by a dose-response (ratio of peak platelet count to baseline) relationship.

CONCLUSIONS

The model adequately described romiplostim serum concentrations and platelet counts in rats, rhesus monkeys, and cynomolgus monkeys, and quantified linear clearance, PDMDD, and potency of romiplostim.

Use of romiplostim in a hemodialysis patient with primary immune thrombocytopenia

OBJECTIVE

To present the case of a patient with primary immune thrombocytopenia (ITP), renal impairment, and chronic hepatitis C virus (HCV) infection who was treated with platelet transfusions, intravenous immunoglobulin (IVIG), corticosteroids, eltrombopag, rituximab, and romiplostim in an attempt to raise platelet counts to a clinically acceptable level.

CASE SUMMARY

A 71-year-old man with end-stage renal disease (ESRD) was on maintenance hemodialysis and had long-term diabetes mellitus, chronic obstructive pulmonary disease, and other comorbidities. He was admitted with epistaxis, severe thrombocytopenia, and a platelet count of 4 × 10(9)/L. Platelet transfusions, treatment with IVIG, corticosteroids, eltrombopag, and rituximab resulted in transient and inadequate increases in platelet counts. Further bleeding manifestations, including epistaxis, melena, hematomas, and ecchymotic patches prompted treatment with blood product concentrates and a higher dose of eltrombopag, resulting in a further lack of clinical response. After 6 weeks of failed treatment attempts, initiation of weekly treatment with romiplostim 5 μg/kg resulted in rapid stabilization (within a week) of platelet counts in the range of 200 × 10(9)/L. The patient was discharged, with subsequent dose adjustment of weekly romiplostim treatment to 2.5 μg/kg, continued hemodialysis, and a return to normal daily activities.

DISCUSSION

The primary clinical concern in this elderly patient with multiple comorbidities was to lower the bleeding risk associated with consistent thrombocytopenia. Despite the lack of clinical data to support the efficacy and safety of romiplostim in patients with ITP and renal impairment, stimulation of platelet production with romiplostim was a reasonable approach in view of the bleeding risk and following nonresponse to treatment with corticosteroids, IVIG, eltrombopag, and rituximab. To our knowledge, this case represents the first successful use of romiplostim to manage primary ITP in the presence of ESRD and concurrent chronic HCV infection in a patient on hemodialysis.

CONCLUSIONS

Romiplostim appears to be a viable option for treatment of ITP in a patient with ESRD and chronic HCV infection following nonresponse to treatment with corticosteroids, IVIG, eltrombopag, and rituximab.

Romiplostim: a review of its use in immune thrombocytopenia

Romiplostim (Nplate®) is an Fc-peptide fusion protein (peptibody) that acts as a thrombopoietin receptor agonist; it has no amino acid sequence homology with endogenous thrombopoietin. This article reviews the clinical efficacy and tolerability of subcutaneous romiplostim in adults with immune thrombocytopenia (ITP), as well as summarizing its pharmacological properties. The efficacy of 12 or 24 weeks' therapy with subcutaneous romiplostim was compared with that of placebo in patients with ITP in three randomized, double-blind, multicentre, phase III trials. In the two 24-week trials, the durable platelet response rate (primary endpoint) was significantly higher with romiplostim than with placebo in both splenectomized and nonsplenectomized patients. In addition, the majority of romiplostim recipients were able to discontinue or reduce their concurrent ITP therapy, and romiplostim improved health-related quality of life (HR-QOL). In the 12-week trial in splenectomized or nonsplenectomized Japanese patients with ITP, the median number of weeks with a platelet response (primary endpoint) was significantly higher with romiplostim than with placebo. Two extension studies (with median durations of romiplostim treatment of 78 and 100 weeks) demonstrated that long-term therapy with romiplostim maintained platelet counts in the target range in patients with ITP. In a randomized, open-label, multicentre, 52-week, phase IIIb trial, romiplostim was more effective than the medical standard of care in nonsplenectomized patients with ITP who had received at least one prior ITP treatment. Significantly fewer patients receiving romiplostim versus the medical standard of care experienced treatment failure or required splenectomy (co-primary endpoints), and clinically meaningful improvements from baseline in HR-QOL scores were seen with romiplostim. Subcutaneous romiplostim was generally well tolerated in patients with ITP; in short-term trials, the majority of adverse events were of mild to moderate severity and appeared to be related to the underlying thrombocytopenia. The incidence of bleeding events of at least grade 3 severity did not significantly differ between romiplostim and placebo recipients, and was significantly lower with romiplostim than with the medical standard of care. Romiplostim did not appear to be associated with an increased risk of haematological malignancies or an increased risk of thrombotic events. Although binding antibodies against romiplostim or thrombopoietin developed in some romiplostim recipients, with neutralizing antibodies to romiplostim detected in two romiplostim recipients, antibodies cross reacting to thrombopoietin have not been detected. Romiplostim was associated with modest increases in bone marrow reticulin in some patients with ITP; reductions in reticulin were usually seen when romiplostim was discontinued. In conclusion, subcutaneous romiplostim is a valuable agent for use in patients with refractory chronic ITP.

Myelofibrosis associated with romiplostim treatment in a patient with immune thrombocytopenia

Immune thrombocytopenia is characterized by antibody-mediated platelet destruction and insufficient platelet production resulting in isolated thrombocytopenia in the absence of underlying cause. Despite many treatment options, low-to-intermediate rates of remission and high rates of resistance to treatment are seen. Approximately 20% of patients do not attain a hemostatic platelet count after splenectomy or after first- and second-line medical approaches. A new option in these cases is treatment with romiplostim. Bone marrow (BM) fibrosis has been reported in clinical trials with romiplostim. We report a case with marked reticulin fibrosis of the BM, worsening of cytopenias and dyserythropoiesis, and atypical megakaryocytes, which did not reverse following cessation of the drug. Although this could represent idiopathic myelofibrosis, unrelated to therapy, the pretreatment biopsy demonstrating no fibrosis combined with the concordant timing of the drug and fibrosis suggests the treatment with romiplostim may be causative.

Investigation of the mechanism of clearance of AMG 386, a selective angiopoietin-1/2 neutralizing peptibody, in splenectomized, nephrectomized, and FcRn knockout rodent models

Purpose

To investigate the mechanisms of clearance of AMG 386, an investigational recombinant peptide-Fc fusion protein (peptibody) that blocks tumor angiogenesis by neutralizing the interaction between angiopoietin-1 and -2 and the Tie2 receptor.

Methods

The role of the neonatal Fc receptor (FcRn) in AMG 386 clearance was assessed in wild-type and FcRn-knockout mice; the roles of the spleen and kidneys were assessed in splenectomized and 5/6th nephrectomized rats, respectively, compared with sham-operated rats. Animals were administered AMG 386 as a single intravenous dose of 3 or 10 mg/kg. Blood samples for pharmacokinetic analysis were collected periodically throughout a 504-hour postdose period.

Results

Compared with wild-type mice, AMG 386 clearance in FcRn-knockout mice was 18-fold faster at the 3-mg/kg dose (FcRn knockout, 13.2 mL/h/kg; wild-type, 0.728 mL/h/kg) and 14-fold faster at the 10-mg/kg dose (FcRn knockout, 10.7 mL/h/kg; wild-type, 0.777 mL/h/kg). Clearance in nephrectomized rats was slower than in sham-operated rats at both the 3-mg/kg dose (nephrectomized, 1.23 mL/h/kg; sham-operated, 1.75 mL/h/kg) and the 10-mg/kg dose (nephrectomized, 1.14 mL/h/kg; sham-operated, 1.65 mL/h/kg). Splenectomy had no apparent effect on the pharmacokinetics of AMG 386.

Conclusions

The FcRn is integral to maintaining circulating levels of AMG 386 in mice. Renal clearance contributed approximately 30% to total AMG 386 clearance in rats.