Thrombopoietin receptor agonist therapy in primary immune thrombocytopenia is associated with bone marrow hypercellularity and mild reticulin fibrosis but not other stromal abnormalities

Structure of the thrombopoietin-MPL receptor complex is a blueprint for biasing hematopoiesis

Thrombopoietin (THPO or TPO) is an essential cytokine for hematopoietic stem cell (HSC) maintenance and megakaryocyte differentiation. Here, we report the 3.4 Å resolution cryoelectron microscopy structure of the extracellular TPO-TPO receptor (TpoR or MPL) signaling complex, revealing the basis for homodimeric MPL activation and providing a structural rationalization for genetic loss-of-function thrombocytopenia mutations. The structure guided the engineering of TPO variants (TPOmod) with a spectrum of signaling activities, from neutral antagonists to partial- and super-agonists. Partial agonist TPOmod decoupled JAK/STAT from ERK/AKT/CREB activation, driving a bias for megakaryopoiesis and platelet production without causing significant HSC expansion in mice and showing superior maintenance of human HSCs in vitro. These data demonstrate the functional uncoupling of the two primary roles of TPO, highlighting the potential utility of TPOmod in hematology research and clinical HSC transplantation.

Congenital Neutropenia with Specific Granulocyte Deficiency Caused by Novel Double Heterozygous SMARCD2 Mutations

SMARCD2 (SWI/SNF-related, matrix-associated, actin-dependent regulator of chromatin, subfamily D, member 2) is critical for myelopoiesis. Recently, bi-allelic SMARCD2 mutations have been reported in five children, causing autosomal recessive congenital neutropenia with specific granulocytes deficiency (CN-SGD); a syndrome resulting in G-CSF resistant neutropenia, recurrent infections, and dysplastic myelopoiesis. We report a new case with CN-SGD caused by two novel heterozygous pathogenic variants in the SMARCD2 gene (c.1081del (p.Gln361Argfs*15)), and (c.217C>T (p.Arg73*)). Treatment with the weekly dosing of thrombopoietin receptor agonist, Romiplostim, along with daily G-CSF transformed her clinical course, implying potential synergism. This report advances the understanding of CN-SGD caused by SMARCD2 mutations.

Exploring the Potential of Eltrombopag: Room for More?

Since its introduction in clinical practice, eltrombopag (ELT) has demonstrated efficacy in heterogeneous clinical contexts, encompassing both benign and malignant diseases, thus leading researchers to make a more in-depth study of its mechanism of action. As a result, a growing body of evidence demonstrates that ELT displays many effects ranging from native thrombopoietin agonism to immunomodulation, anti-inflammatory, and metabolic properties. These features collectively explain ELT effectiveness in a broad spectrum of indications; moreover, they suggest that ELT could be effective in different, challenging clinical scenarios. We reviewed the extended ELT mechanism of action in various diseases, with the aim of further exploring its full potential and hypothesize new, fascinating indications.

Chronic immune thrombocytopenia in Denmark, Sweden and Norway: The Nordic Country Patient Registry for Romiplostim

BACKGROUND

Population-based cohorts of immune thrombocytopenia (ITP) are useful for understanding occurrence, clinical characteristics and long-term clinical course. This paper describes the content of the Nordic Country Patient Registry for Romiplostim (NCPRR) and provides prevalence and incidence estimates of chronic ITP (cITP).

METHODS

The NCPRR, a cohort study established in 2009, includes all adult (≥ 18 years) patients in Denmark, Sweden and Norway with cITP (defined as ITP lasting > 12 months and platelet count < 100 × 10⁹/L), combining data from national health registries and medical records. The NCPRR currently includes prevalent cITP patients diagnosed before 2009 and incident cITP patients diagnosed during 2009-2016. The registry obtains clinical information for cITP patients, including comorbidities, treatments, laboratory values, and complete follow-up for various outcomes.

FINDINGS

The NCPRR currently includes 3831 patients with cITP (1258 prevalent; 2573 incident). In 2009, the prevalence of registered cITP was 10 · 0/100,000 (95%CI:9 · 1-11 · 0) adult persons in Denmark and 10 · 7/100,000 (95% CI: 9 · 9-11 · 4) adults in Sweden. During 2009-2016, the incidence rates of cITP per 100,000 person-years were 2 · 8 (95%CI: 2 · 6-3 · 0), 1 · 8 (95%CI: 1 · 7-1 · 9) and 2 · 1 (95%CI: 1 · 9-2 · 2) in Denmark, Sweden and Norway, respectively. Fifty-eight percent of cITP patients were women. At NCPRR inclusion, 30.2% were aged ≥ 70 years, 23% had a platelet count < 50 × 10⁹/L, 17.4% were splenectomized, 41% had prior ITP therapy, and 8.6% had severe comorbidity.

INTERPRETATION

The NCPRR provides population-based data on the epidemiology and characteristics of almost 4000 cITP patients and is a valuable resource for research.

FUNDING

This study was partly funded by a research grant from Amgen to Aarhus University.

Thrombopoietin receptor-independent stimulation of hematopoietic stem cells by eltrombopag

Eltrombopag (EP), a small-molecule thrombopoietin receptor (TPO-R) agonist and potent intracellular iron chelator, has shown remarkable efficacy in stimulating sustained multilineage hematopoiesis in patients with bone marrow failure syndromes, suggesting an effect at the most immature hematopoietic stem and multipotent progenitor level. Although the functional and molecular effects of EP on megakaryopoiesis have been studied in the past, mechanistic insights into its effects on the earliest stages of hematopoiesis have been limited. We investigated the effects of EP treatment on hematopoietic stem cell (HSC) function using purified primary HSCs in separation-of-function mouse models, including a TPO-R-deficient strain, and stem cells isolated from patients undergoing TPO-R agonist treatment. Our mechanistic studies showed a stimulatory effect on stem cell self-renewal independently of TPO-R. Human and mouse HSCs responded to acute EP treatment with metabolic and gene expression alterations consistent with a reduction of intracellular labile iron pools that are essential for stem cell maintenance. Iron preloading prevented the stem cell stimulatory effects of EP. Moreover, comparative analysis of stem cells in the bone marrow of patients receiving EP showed a marked increase in the number of functional stem cells compared to patients undergoing therapy with romiplostim, another TPO-R agonist lacking an iron-chelating ability. Together, our study demonstrates that EP stimulates hematopoiesis at the stem cell level through iron chelation-mediated molecular reprogramming and indicates that labile iron pool-regulated pathways can modulate HSC function.

Immune thrombocytopenia is associated with persistently deranged fibrosis-related seromarker profiles but low bone marrow fibrosis grades: A 2-year observational study on thrombopoietin receptor agonist treatment

Bone marrow (BM) fibrosis is a potential side effect of thrombopoietin receptor agonist (TPO-RA) treatment. We aimed to investigate stromal seromarker profiles and growth factors in order to elucidate pathogenic and dynamic aspects of immune thrombocytopenia (ITP)-related BM fibrosis before and during TPO-RA treatment. Connective tissue metabolites [procollagen I and III peptides (PINP/PIIINP); hyaluronan (HYA), C-terminal-telopeptide (ICTP), and fibrosis-related growth factors (transforming growth factor-beta (TGF-beta), HGF, basic fibroblast growth factor)] were measured in blood samples acquired before initiation of TPO-RA and subsequently at 6-month intervals for up to 2 years. BM fibrosis was graded MF-0 in 8 (18%), MF-1 30 (65%), and MF-2 8 (18%) in the last available BM biopsy. In the 21 patients having more than one biopsy, the grade of fibrosis from the first to the last available biopsy decreased in 2 (10%), remained unchanged in 15 (71%), and increased in 4 (19%). Pretreatment levels of PIIINP, PINP, ICTP, and HYA were significantly increased in ITP versus controls. PINP, PIIINP, and HYA decreased on TPO-RA; ICTP remained unchanged. PINP:ICTP was lower before and during treatment compared to controls. Pretreatment, TGF-beta was lower than in controls; HGF exhibited the opposite pattern. HYA, ICTP, and TGF-beta tended to increase while PINP and platelet-derived growth factor tended to decrease with increasing fibrosis grade. In conclusion, ITP is associated with deranged patterns of extracellular matrix seromarkers and growth factors, indicating that BM stromal remodeling is enhanced. During TPO-RA treatment for up to 2 years, this profile was partially reversed while mild BM reticulin fibrosis was still present in the majority of patients. These observations likely reflect a BM injury by autoimmunity that is modified by TPO-RA.

Bone marrow morphology and disease progression in congenital thrombocytopenia: a detailed clinicopathologic and genetic study of eight cases

Patients with congenital thrombocytopenia have an increased risk of developing myeloid neoplasms. In these cases, the morphologic distinction between disease at baseline and at progression is challenging. This report analyzes clinicopathologic features of congenital thrombocytopenia with long-term follow-up at one referral center. Records from the last 20 years were searched for cases of congenital thrombocytopenia with bone marrow biopsies and peripheral blood smears. The clinical, morphologic, immunophenotypic, and molecular features were analyzed. Six adult and two pediatric patients were identified (six male, two female). Age range at first biopsy was 1-47 (median, 31) years. Underlying diseases included thrombocytopenia-absent radius syndrome, congenital thrombocytopenia with radial-ulnar synostosis, MYH9-related disorder, shortened telomere syndrome, congenital thrombocytopenia with ANKRD26 mutation, and familial platelet disorder with predisposition to acute myeloid leukemia. Four patients had myelodysplastic/myeloproliferative neoplasm-like marrow changes such as hypercellularity, increased myeloid to erythroid ratio, numerous micromegakaryocytes (highlighted by CD42b), and marrow fibrosis. Two patients had marrow hypoplasia and two had unremarkable marrow morphology. Three patients-all in the myelodysplastic/myeloproliferative neoplasm-like group-developed disease progression characterized by erythroid and myeloid dysplasia, elevated bone marrow blasts, and new cytogenetic abnormalities. Unlike non-familial myeloid neoplasms, congenital thrombocytopenia patients in the myelodysplastic/myeloproliferative neoplasm-like group had a long and indolent clinical course (average age at disease progression, 47 years). In summary, three distinct morphologic types of congenital thrombocytopenia were identified: a hyperplastic myelodysplastic/myeloproliferative neoplasm-like group, a hypoplastic bone marrow failure-like group, and a group with relatively normal marrow morphology. Emergence of cytogenetic abnormalities and dysplasia in non-megakaryocyte lineages correlated with disease progression.

Changes in bone marrow morphology in adults receiving romiplostim for the treatment of thrombocytopenia associated with primary immune thrombocytopenia

The effects of romiplostim on bone marrow morphology were evaluated in adults with immune thrombocytopenia (ITP). Patients with platelet counts <50 × 10(9)/L, ≥1 prior ITP therapies, and no collagen at baseline received weekly subcutaneous romiplostim starting at 1 μg/kg, adjusted to maintain platelet counts between 50 and 200 × 10(9)/L. Biopsies were scheduled after 1, 2, or 3 years of romiplostim (cohorts 1, 2, and 3, respectively). Irrespective of scheduled time, biopsies were performed earlier if patients discontinued or failed to achieve/maintain a response to romiplostim. Reticulin (silver stain) and collagen (trichrome stain) were graded by two hematopathologists using the modified Bauermeister scale (0-4). Of 169 patients, 131 had evaluable biopsies; 9/131 (6.9 %) had increases of ≥2 grades on the modified Bauermeister scale (cohort 1: 0/34; cohort 2: 2/39; cohort 3: 7/58), including two with collagen. Three of the nine patients had follow-up biopsies, including one patient with collagen; changes were reversible after romiplostim discontinuation. Of the nine patients, one had neutropenia detected by laboratory test and two had adverse events of anemia, both non-serious and not treatment-related. By actual exposure (as some biopsies did not occur as scheduled), the number of patients with grade increases ≥2 were year 1: 3/41, year 2: 1/38, year 3: 5/52. Twenty-four patients sustained platelet counts ≥50 × 10(9)/L for ≥6 months with no ITP medications after discontinuing romiplostim, i.e., they entered clinical remission of their ITP. In conclusion, in patients with ITP receiving romiplostim, bone marrow changes were observed in a small proportion of patients.ClinicalTrials.gov identifier: NCT#00907478.

Problems and pitfalls in grading of bone marrow fibrosis, collagen deposition and osteosclerosis - a consensus-based study

AIMS

In the era of potentially disease-modifying agents such as Janus kinase inhibitors, accurate grading and differentiation of bone marrow (BM) fibrosis has become more relevant to assess staging of disease and therapeutic effects. However, different fibrosis grading models have been used in the past without uniformity, including the proposal by the World Health Organization. Current scoring systems are based only on reticulin fibrosis. Therefore, additional assessment of collagen and the grade of osteosclerosis appear to be essential to discriminate all components of the complex BM fibrous matrix.

METHODS AND RESULTS

We evaluated problems and pitfalls regarding staining techniques and the interpretation of reticulin fibrosis on a total of 352 samples. Furthermore, we propose a minor modification of the current grading and separate scoring for collagen deposition and osteosclerosis. Reproducibility of gradings was tested among 11 haematopathologists in a blinded assessment. Overall, the inter-rater reliability of all three grading systems ranged between 0.898 and 0.926.

CONCLUSIONS

A standardized assessment of BM fibrosis with differentiation between reticulin, collagen and osteosclerosis is recommended to evaluate the various components of the fibrous matrix which may be delinked after therapy. In this regard, quality of staining and application of laboratory standards enable a highly reproducible scoring.

Evaluation of bone marrow reticulin in patients with chronic immune thrombocytopenia treated with eltrombopag: Data from the EXTEND study

Thrombopoietin receptor agonists, which raise platelet counts in patients with chronic immune thrombocytopenia, may be associated with increases in bone marrow (BM) reticulin. Patients with chronic immune thrombocytopenia participating in the Eltrombopag Extended Dosing (EXTEND) study underwent BM biopsies to identify clinically relevant BM fibrosis-related increases. Specimens were centrally reviewed by 2 hematopathologists. Two hundred thirty-two biopsy specimens were collected from 117 patients treated for ≤5.5 years. Moderate to marked reticulin fibrosis was found in 2 patients. After withdrawing from the study, the biopsy of 1 patient reverted to normal. There were no other pathologic changes identified among on-treatment specimens, and no pattern of abnormal reticulin deposition associated with eltrombopag treatment was evident.

Nuclear factor-erythroid 2, nerve growth factor receptor, and CD34-microvessel density are differentially expressed in primary myelofibrosis, polycythemia vera, and essential thrombocythemia

Because of the presence of various overlapping findings, the discrimination of polycythemia vera (PV) from prefibrotic/fibrotic primary myelofibrosis (PF/F-PMF) and essential thrombocythemia (ET) may be challenging, particularly in suboptimal bone marrow biopsy specimens. In this study, we assessed whether differences in the expression of nuclear factor-erythroid 2 (NF-E2), nerve growth factor receptor (NGFR; CD271), CD34, CD68, p53, CD3, CD20, and CD138 by immunohistochemistry could be useful in separating among them. Higher frequencies of nuclear positive erythroblasts with NF-E2 were observed in ET and PV cases (50% ± 13.3% and 41.5% ± 9.4%, respectively) when compared with both PF-PMF (21% ± 11.7%) and F-PMF (28.5% ± 10.8%). We found that with a cutoff level of at least 30% nuclear staining for NF-E2 in erythroblasts, we could reliably exclude the possibility of PMF. Conversely, NGFR+ stromal cells per high-power field (HPF) was significantly increased in F-PMF (53.5 ± 19.1/HPF) and PF-PMF (13.5 ± 3.8/HPF) compared with ET (4.4 ± 2.2/HPF) and PV (6.6 ± 3.3/HPF). Similarly, differences in CD34-microvessel density was remarkable in F-PMF and PF-PMF cases in comparison with PV and ET (49.9 ± 12.1/HPF, 29.3 ± 12.4/HPF, 13.7 ± 4.6/HPF, and 11.9 ± 5.1/HPF, respectively). Thus, the assessment of NF-E2 and NGFR expression and the evaluation of CD34-microvessel density may provide additional support in reaching a correct diagnosis in these cases of myeloproliferative neoplasms.

United Kingdom immune thrombocytopenia registry: retrospective evaluation of bone marrow fibrosis in adult patients with primary immune thrombocytopenia and correlation with clinical findings

Fibrosis has been reported in some patients with immune thrombocytopenia (ITP) treated with thrombopoietin receptor agonists (TPO-RA). However, fibrosis has also been reported in patients with various stages of ITP, who were TPO-RA treatment-naïve. In our study, we looked for fibrosis in bone marrow trephine biopsies taken at initial diagnosis from 32 adult patients with ITP. Ten of the 32 evaluated samples (31·25%) showed increased reticulin (Grade 1-2 on Bauermeister scale and Grade 0-1 on the European Consensus scale), which showed a positive correlation with ethnicity (0·3%) but did not correlate with disease severity, any clinical features or co-morbidities.

Romiplostim as a treatment for immune thrombocytopenia: a review

“Immune thrombocytopenia” (ITP) is an autoimmune disorder that leads to peripheral destruction, as well as a decreased production of platelets. ITP most commonly presents as mild mucocutaneous bleeding. Though it is rare, the leading cause of mortality in persons with ITP is intracranial hemorrhage and those that do not respond to therapy are at increased risk. Our understanding of the pathophysiology of ITP has evolved immensely, especially over the last 60 years. The discovery of the platelet-production stimulator, thrombopoietin (TPO), lent clarity to an earlier hypothesis that inhibition of platelet production at the level of the megakaryocyte, at least in part, accounts for thrombocytopenia in adults with ITP. This facilitated the development of TPO-based therapies to treat ITP. Thrombopoietin receptor agonists are one of the most recent treatments to enter the landscape. Original production of a recombinant human TPO was halted after clinical trials revealed the untoward effect of autoantibodies to the recombinant human TPO with cross-reactivity to endogenous TPO. Next-step development focused on stimulation of the TPO receptor with fewer immunogenic agents. Currently, two such thrombopoietin receptor agonists, romiplostim and eltrombopag, are licensed in the USA to treat thrombocytopenia in adults with persistent or chronic ITP. Ongoing research will assess their efficacy in other immune-mediated and nonimmune-mediated primary and secondary thrombocytopenias.

Resuscitating a dying marrow: the role of hematopoietic growth factors

The treatment landscape for myelodysplastic syndromes (MDS) has evolved over the last two decades, with a better understanding of the disease pathophysiology and the use of newer or combination therapies. For lower-risk MDS patients, hematopoietic growth factors have continued to be the mainstay of therapy. However, better patient selection criteria and decision tools to predict responses have made these therapies more beneficial to patients. As the range of newer drugs continues to expand in our treatment armamentarium for lower-risk MDS, questions still remain regarding the safety of these drugs with long-term use. This review will discuss the role of growth factors in MDS, focusing on dosing and combination strategies to improve responses, selecting the appropriate patient population, and recognizing the safety profile based on evidence from published literature.

Bone marrow fibrosis in 66 patients with immune thrombocytopenia treated with thrombopoietin-receptor agonists: a single-center, long-term follow-up

Thrombopoietin-receptor agonists increase platelet counts by stimulating the thrombopoietin receptor. Bone marrow fibrosis has been reported in patients receiving thrombopoietin-receptor agonists. This study determined the extent of myelofibrosis, its clinical relevance, and incidence of phenotypic or karyotypic abnormalities in patients with immune thrombocytopenia treated with thrombopoietin-receptor agonists. The grade of myelofibrosis was assessed before (n=15), during (n=117) and after (n=9) treatment in bone marrow biopsies from 66 patients. The proportion of bone marrow biopsies showing no fibrosis (myelofibrosis grade 0) decreased from 67% pre-treatment to 22% at last biopsy, of which 59% had grade 1 myelofibrosis and 18% had grade 2 myelofibrosis. The median duration of treatment with thrombopoietin-receptor agonists to last bone marrow biopsies was 29 months; patients who had two or more biopsies significantly more frequently had myelofibrosis grades 2/3 in the last bone marrow biopsies as compared to the first. Older age was associated with higher grades of fibrosis. No differences in blood counts or lactate dehydrogenase levels were found between patients with myelofibrosis grades 0/1 and those with grade 2. No clonal karyotypic or immunophenotypic abnormalities emerged. This study found that thrombopoietin-receptor agonists induce myelofibrosis grades 2/3 in approximately one-fifth of patients with immune thrombocytopenia, increasingly with >2 years of treatment with thrombopoietin-receptor agonists. Annual/biannual follow-up with bone marrow biopsies is, therefore, recommended in patients being treated with thrombopoietin-receptor agonists in order to enable prompt discontinuation of these drugs should grades 2/3 myelofibrosis develop. Discontinuation of thrombopoietin-receptor agonists may prevent development of clinical manifestations by stopping progression of fibrosis in grade 2/3.

Childhood immune thrombocytopenia: role of rituximab, recombinant thrombopoietin, and other new therapeutics

Childhood immune thrombocytopenia (ITP) is often considered a benign hematologic disorder. However, 30% of affected children will have a prolonged course and 5%-10% will develop chronic severe refractory disease. Until recently, the only proven therapeutic option for chronic severe ITP was splenectomy, but newer alternatives are now being studied. However, because immunosuppressive agents such as rituximab are not approved for use in ITP and the thrombopoietin receptor agonists are not yet approved in children, the decision to use alternatives to splenectomy needs to be considered carefully. This review describes the factors that should affect decisions to treat ITP at diagnosis and compares the options for the occasional child in whom ITP does not resolve within the first year.

Thrombopoietin-receptor agonists

PURPOSE OF REVIEW

Thrombopoietin-receptor agonists (TPO-RAs) have been approved for use in immune thrombocytopenia (ITP) after showing safety and efficacy. There is increasing interest to expand the role of TPO-RAs, both in ITP as well as in other thrombocytopenic disorders.

RECENT FINDINGS

In ITP, more studies are providing evidence of TPO-RA efficacy and safety, as well as their applicability to various patient groups, including children. Use of TPO-RAs in hepatitis C has shown early success in allowing treatments in patients who would otherwise be excluded due to thrombocytopenia. Use in congenital thrombocytopenias has also shown early success. The use of TPO-RAs in myelodysplastic syndrome (MDS) is questionable after reports of increasing blasts and leukemic transformation, whereas in other chemotherapy-induced thrombocytopenias (C-ITs) reports are few. Bone marrow fibrosis remains an area of active study, although the data to date suggest this is seen in a small minority of patients, and is reversible and of questionable clinical relevance. Thrombotic complications are also an area of concern and need further close follow-up.

SUMMARY

The use of TPO-RAs continues to grow as more evidence of safety and efficacy is found. More studies are needed to determine their utility in other diseases as well as to better characterize adverse events observed to date.

How I treat immune thrombocytopenia: the choice between splenectomy or a medical therapy as a second-line treatment

The paradigm for managing primary immune thrombocytopenia (ITP) in adults has changed with the advent of rituximab and thrombopoietin receptor agonists (TPO-RAs) as options for second-line therapy. Splenectomy continues to provide the highest cure rate (60%-70% at 5+ years). Nonetheless, splenectomy is invasive, irreversible, associated with postoperative complications, and its outcome is currently unpredictable, leading some physicians and patients toward postponement and use of alternative approaches. An important predicament is the lack of studies comparing second-line options to splenectomy and to each other. Furthermore, some adults will improve spontaneously within 1-2 years. Rituximab has been given to more than 1 million patients worldwide, is generally well tolerated, and its short-term toxicity is acceptable. In adults with ITP, 40% of patients are complete responders at one year and 20% remain responders at 3-5 years. Newer approaches to using rituximab are under study. TPO-RAs induce platelet counts > 50 000/μL in 60%-90% of adults with ITP, are well-tolerated, and show relatively little short-term toxicity. The fraction of TPO-RA-treated patients who will be treatment-free after 12-24 months of therapy is unknown but likely to be low. As each approach has advantages and disadvantages, treatment needs to be individualized, and patient participation in decision-making is paramount.