Eltrombopag: an update on the novel, non-peptide thrombopoietin receptor agonist for the treatment of immune thrombocytopenia

The developmental and inflammatory disease target protein ADAM17 is vulnerable to off-target interaction by the drug eltrombopag: Insights from molecular modeling

The loss of proteolytic activity of ADAM17 causes birth defects and embryonic lethality. Conversely, inhibiting ADAM17 activity represents a potential strategy for treating inflammatory and autoimmune diseases. ADAM17 has an active site cleft with a divalent Zn ion and hydrophobic S1'/S3' subsites interconnected to form an L shaped cavity. However, it is currently unknown whether the active site of ADAM17 is susceptible to off-target inhibition by the small molecule drug eltrombopag, which contains metal-binding moieties and is classified as pregnancy category C by the FDA. The in-depth molecular modeling analysis in this study revealed that the unique structural features of L-shaped S1'/S3', crucial for determining ADAM17 specificity, along with spatial constraints imposed by active site amino acid residues, create an ideal binding environment for eltrombopag. Interestingly, the structural peculiarity of L-shaped S1'/S3' cavity enabled the carboxylate group rather than the traditionally recognized metal binding domain of eltrombopag to chelate catalytic Zn of ADAM17. Further, eltrombopag's biphenyl and xylene groups embed in the S1'/S3' subsites and pyrazole and hydrazine linker occupy the interconnecting tunnel, forming a stable eltrombopag-ADAM17 complex. These novel findings from molecular modeling suggest that ADAM17 is an off-target of eltrombopag, a drug used to increase platelet production in thrombocytopenia. They stimulate further in vitro and in vivo studies to test the repurposing potential of eltrombopag as an ADAM17 inhibitor to prevent tissue destruction in autoimmune diseases in adults and whether the use of eltrombopag during pregnancy could potentially lead to developmental toxicity due to ADAM17 inhibition.

Thrombopoietin-receptor agonists for adult patients with immune thrombocytopenia: a narrative review and an approach for managing patients fasting intermittently

Introduction

Thrombopoietin-receptor agonist (TPO-RAs) currently represent the state of art for treating immune thrombocytopenia. Their different molecular structures contribute to the difference in their pharmacodynamics and pharmacokinetics. This narrative review aims to provide an overview of the current TPO-RAs approved for primary immune thrombocytopenia (romiplostim, eltrombopag, avatrombopag) and the effect of intermittent fasting in adult patients receiving TPO-RAs.

Areas covered

Literature was searched with no limits on date or language, using various combinations of keywords. Data on the pharmacokinetics, pharmacodynamics, efficacy, and safety of TPO-RAs and the effect of intermittent fasting were summarized.

Expert opinion

Switching between TPO-RAs is a useful strategy to tackle some associated limitations. Romiplostim and avatrombopag have an advantage over eltrombopag as they do not require any dietary restrictions. In cases where romiplostim and avatrombopag are unavailable, patients should be educated on the appropriate administration, possible interactions, and dietary restrictions before initiating eltrombopag.

Prolonged haematologic toxicity in CAR-T-cell therapy: A review

Chimeric antigen receptor-T-cell (CAR-T-cell) therapy is a novel immunotherapy with encouraging results for treatment of relapsed/refractory haematologic malignancies. With increasing use, our understanding of immune-mediated side effects such as cytokine release syndrome and neurotoxicity has improved; nevertheless, prolonged haematologic toxicity (PHT), with a high incidence rate, remains underrecognized. Owing to heterogeneity in populations, the CAR-T cells used and diseases treated as well as differences in the definition of PHT, its rate, risk factors and management vary across studies. In this review, we provide a narrative of PHT occurring in patients following CAR-T-cell therapy; evidence of PHT treatment strategies is also presented, with the aim of contributing to systematic understanding of PHT.

Safety and Efficacy of Eltrombopag in Children and Adults with Immune Thrombocytopenia: A Systematic Review and Meta-Analysis

Immune thrombocytopenia is an immune condition where antibodies are produced against platelets. Eltrombopag is a thrombopoietin receptor agonist that stimulates and promotes platelet production approved for treating thrombocytopenia in patients with chronic immune thrombocytopenia, where other treatments such as corticosteroids, splenectomy or immunoglobulins are inadequate. The aim of this meta-analysis was to evaluate the efficacy and safety of the eltrombopag in adults and children with immune thrombocytopenia. We included 7 studies with a total of 765 patients (606 adults and 159 children). We evaluated the number of patients that achieved a post-treatment platelet count equal or above 50x10⁹/L (primary result-target) without the need of rescue treatment for at least 4 weeks. Our data showed that patients who received eltrombopag were almost 4 times more probable in achieving the primary target when compared to patients who received placebo (RR 3.84, 95% CI 2.39 to 6.14; I² = 46%). The number of patients needed rescue treatment and the number of bleeding incidents were reduced in the group that received eltrombopag when compared to those who received placebo (RR 0.40, 95% CI 0.25 to 0.62; I² = 40%) (RR 0.74, 95% CI 0.62 to 0.89; I² = 68%). The total number of side effects did not statistically differ between the two groups (RR 0.99, 95% CI 0.90 to 1.08; I² = 14%). Our findings were similar to previously published studies and confirm that eltrombopag is safe and efficient in immune thrombocytopenia. However, more clinical trials are needed in order to enhance our findings.

A Multi-Center Study on the Efficacy of Eltrombopag in Management of Refractory Chronic Immune Thrombocytopenia: A Real-Life Experience

Objective

The aim of the present study was to evaluate the efficacy and safety of eltrombopag, an oral thrombopoietin receptor agonist, in patients with chronic immune thrombocytopenia (ITP).

Materials and Methods

A total of 285 chronic ITP patients (187 women, 65.6%; 98 men, 34.4%) followed in 55 centers were enrolled in this retrospective cohort. Response to treatment was assessed according to platelet count (/mm³) and defined as complete (platelet count of >100,000/mm³), partial (30,000-100,000/mm³ or doubling of platelet count after treatment), or unresponsive (<30,000/mm³). Clinical findings, descriptive features, response to treatment, and side effects were recorded. Correlations between descriptive, clinical, and hematological parameters were analyzed.

Results

The median age at diagnosis was 43.9±20.6 (range: 3-95) years and the duration of follow-up was 18.0±6.4 (range: 6-28.2) months. Overall response rate was 86.7% (n=247). Complete and partial responses were observed in 182 (63.8%) and 65 (22.8%) patients, respectively. Thirty-eight patients (13.4%) did not respond to eltrombopag treatment. For patients above 60 years old (n=68), overall response rate was 89.7% (n=61), and for those above 80 years old (n=12), overall response rate was 83% (n=10). Considering thrombocyte count before treatment, eltrombopag significantly increased platelet count at the 1^st, 2^nd, 3^rd, 4^th, and 8^th weeks of treatment. As the time required for partial or complete response increased, response to treatment was significantly reduced. The time to reach the maximum platelet levels after treatment was quite variable (1-202 weeks). Notably, the higher the maximum platelet count after eltrombopag treatment, the more likely that side effects would occur. The most common side effects were headache (21.6%), weakness (13.7%), hepatotoxicity (11.8%), and thrombosis (5.9%).

Conclusion

Results of the current study imply that eltrombopag is an effective therapeutic option even in elderly patients with chronic ITP. However, patients must be closely monitored for response and side effects during treatment. Since both response and side effects may be variable throughout the follow-up period, patients should be evaluated dynamically, especially in terms of thrombotic risk factors.

Mechanisms and therapeutic prospects of thrombopoietin receptor agonists

The second-generation thrombopoietin (TPO) receptor agonists eltrombopag and romiplostim are potent activators of megakaryopoiesis and represent a growing treatment option for patients with thrombocytopenic hematological disorders. Both TPO receptor agonists have been approved worldwide for the treatment of children and adults with chronic immune thrombocytopenia. In the EU and USA, eltrombopag is approved for the treatment of patients with severe aplastic anemia who have had an insufficient response to immunosuppressive therapy and in the USA for the first-line treatment of severe aplastic anemia in combination with immunosuppressive therapy. Eltrombopag has also shown efficacy in several other disease settings, for example, chemotherapy-induced thrombocytopenia, selected inherited thrombocytopenias, and myelodysplastic syndromes. While both TPO receptor agonists stimulate TPO receptor signaling and enhance megakaryopoiesis, their vastly different biochemical structures bestow upon them markedly different molecular and functional properties. Here, we review and discuss results from preclinical and clinical studies on the functional and molecular mechanisms of action of this new class of drug.

Biometric Image Analysis for Quantitation of Dividing Platelets

(1) Background: Quantification of platelet division is challenging because automated Coulter cell counters produce equivocal platelet counts. (2) Methods: We applied the flow cytometric cell tracking dye dilution assay as a popular immunological method to evaluate lymphocyte proliferation to prove and quantitate platelet division. We also devised a method relying on platelet culture in a semisolid medium which enabled dividing platelets to be identified by limiting the diffusive movement of platelets. Mixing platelets of different labeling colors in semisolid medium and counting the platelet doublets of each color combination enabled us to prove and quantitate platelet division. (3) Results: The tracking dye dilution assay revealed that 75.5 to 85.6% of platelets were dividing after 20 hours in culture. Platelets labeled with two different tracking dyes were mixed and cultured in semisolid medium for differential doublet counting. We counted platelet singlets and doublets of each color and color combination using confocal microscopy after six hours of culture and compared the relative number of two-colored doublets with binomial prediction to prove platelet division (P < 0.01). Division was suppressed by taxol, nocodazole, or cytochalasin D treatment. We derived a formula for determining the fraction of dividing platelets using the numbers of singlets and doublets of each color and color combination. The platelet division fraction ranged from 8.8 to 17.5%. (4) Conclusion: We successfully measured platelet division using a simple biometric image analysis method with possible future application to microfluidic devices.