Thrombopoietin Receptor Agonists: A Critical Review

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Thrombopoietin Receptor Agonists: A Critical Review William B. Mitchell MD, James B. Bussel MD

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Cite this article as: William B. Mitchell MD, James B. Bussel MD, Thrombopoietin Receptor Agonists: A Critical Review, Semin Hematol , http://dx.doi.org/10.1053/j.seminhematol.2014.11.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Thrombopoietin receptor agonists: a critical review

William B Mitchell, MD James B Bussel, MD Department of Pediatrics, Division of Hematology, Weill Cornell Medical College

Corresponding author: William B Mitchell Weill Cornell Medical College 525 East 68th Street P695 New York, NY 10065 [email protected] 646-570-3280

Disclosures: WB Mitchell: Nothing to disclose JB Bussel: Research funding from Amgen and Glaxow Smith Klein

Abstract: Thrombopoietic agents have created a paradigm shift in the management of chronic or poorly responsive ITP. There are ample randomized, placebocontrolled trial data as well as long term data gathered for more than 5 years: short term efficacy and safety are well documented and long-term efficacy and safety data are emerging. The purpose of this review will be to focus critically on what we know or do not know at this point about these agents. We will review evolution, efficacy and clinical use, side effects, special populations, and off target effects.

Introduction and Background:

Overall because of the high rate of efficacy and relatively low degree of toxicity, the thrombopoietin receptor agonists (TPO-RA) have provided a major advance in the management of thrombocytopenia similar to the way that erythropoietin (EPO) and granulocyte colony stimulating factor (G-CSF) have improved management of anemia and neutropenia, respectively. The main deterrents to their use are the very high cost and the presumption that once a TPO-RA is initiated, it will be needed long-term because of the absence of curative effects. The goal of this article will be to focus on the issues that complicate the use of these agents and by so doing, clarify those issues, which may point to directions of future research.

Clinical results with first generation TPO-RA generalize well to those of the second generation agents

Two first generation therapeutic TPO-RA were developed. One was a recombinant full-length TPO molecule, and the other was a recombinant peptide containing the cmpl-binding domain of TPO attached to a polyethylene glycol moiety1. Many trials were completed with these first generation agents, but they were taken off the market after the formation of antibodies that cross-reacted against native TPO in study subjects2. Second generation TPO-RA were developed to specifically not have any homology to native TPO3. Two second

generation agents are currently licensed; one is a peptide bound to an Fc molecule, and the other is a small molecule. Both of these agents have been tested in the largest ITP trials ever performed: thus far in published clinical trial results, over 300 ITP patients have received each agent4-11. Currently these agents are also being/have been evaluated for other indications.

The findings with the first generation TPO-RA generalize well to those of the second generation TPO-RA. However, there is only limited overlap in the indications for the clinical trials of the first and (current) second generation TPORA. Studies with the first generations focused on the role of a TPO-RA in chemotherapy-induced thrombocytopenia1. A number of trials in patients receiving myeloablative chemotherapy, eg for treatment of myeloid leukemia, demonstrated very limited efficacy in this setting. Consequently, trials for this indication have not been repeated with the second generation agents. Similarly, the use of TPO-RA to stimulate normal platelet donors, while highly efficacious with the first generation agents12, has not (yet) been attempted with the second generation agents. Conversely, clinical trials of the second generation agents have focused on in ITP, thrombocytopenia in hepatitis C13, and aplastic anemia14. The data from these trials has demonstrated sufficient efficacy (and safety) that these indications have been licensed for one or both agents. These indications were either not tested at all with the first generation agents, or only briefly tested and resulted at most in publication of abstracts, not manuscripts.

Overlap between first and second generation agents has been primarily seen in the area of non-myeloablative chemotherapy including myelodysplastic syndrome (MDS). This area demonstrated proof of concept in the initial trials with the first generation agents but was critiqued for improving numerical platelet counts but not reducing the need for platelet transfusions. One small randomized study suggested that the value of these agents could be in maintenance of an on-time schedule of chemotherapy, resulting in a higher cure rate. Of interest, this data was only published because of the revival of the use of TPO-RA, which led the authors of the study, based on first generation agents, to publish it after all (with much longer follow up)15.

The mechanism of regulation of TPO is similar to that of G-CSF rather than EPO Hematopoietic growth factors revolutionized treatments for malignancies, bone marrow failure syndromes, kidney failure and other causes of blood cell deficiencies. Erythropoietin was cloned and entered clinical trials in the second half of the 1980’s16. G-CSF was cloned and went into clinical trial several years later17,18. The discovery and clinical adaptation of single lineage growth factors allowed the escalation of chemotherapy beyond what would normally be tolerated without such support. At the same time, animal and human studies indicated that thrombopoietin (TPO) existed. Accordingly, there was intense interest in identifying TPO and in 1994 several groups simultaneously cloned the ligand molecule for the c-mpl receptor, TPO19-21.

The mechanism of regulation of TPO is similar to that of G-CSF. For EPO it has been demonstrated that there is a renal sensor of tissue perfusion (circulating hemoglobin and consequent oxygen delivery) that drives production of EPO when peripheral perfusion (and oxygen delivery) is too low22. In contrast, TPO and G-CSF levels seem to be based on the number of TPO and G-CSF receptors in the circulation (or in the marrow)23. This model explains the distinction in TPO levels between ITP (low platelets but increased numbers of megakaryocytes, low TPO level) and marrow failure (low platelets and low megakaryocytes, high TPO level).

The TPO receptor, cmpl, functions in a manner similar to the cytokine receptors for EPO and GCSF. Cmpl is a type 1 cytokine receptor with homology to EPO and GCSF receptors24. All three ligands activate the Jak and STAT family of proteins25-27. All three receptor cytoplasmic domains activate Shc and MAPK family proteins. GCSF expression is regulated by inflammatory signals through IL17, and EPO expression is regulated by hypoxia signals through HIF1a. In contrast, TPO expression has been thought to be stable and its expression level modulated via uptake by platelets and megakaryocytes although this is a subject of active investigations. As the platelet mass increases, more TPO is taken up, resulting in lower circulating TPO.

Efficacy of TPO agonists is excellent—and comparable to that of IVIG

First-line pharmacologic therapies for acute ITP include intravenous immunoglobulin (IVIG). In 1981 Imbach and colleagues successfully treated their pediatric ITP patients with IVIG, using the same regimen as for their hypogammaglobulinemia patients, 400 mg/kg daily for five days28. Newland and colleagues treated 25 adult ITP patients with IVIG and reported that all 25 patients had some response29. Bussel and colleagues compressed the five day IVIG regimen into a single dose of 1 g/k30. All 29 children with ITP treated on that protocol responded. These studies established a new paradigm in the clinical management of ITP. Subsequent studies have shown that most patients (70%) will respond to high dose IVIG by 72 hours, often within 24 hours. While TPO-RA take longer to manifest any effect, they also clearly have a response rate of about 80% in patients with refractory and poorly responsive ITP. It is not known how effective they would be in new onset ITP.

TPO agonists increase platelet counts in a wide variety of thrombocytopenias, not all of which are licensed as indications

Currently one or both TPO agents have been licensed for the treatment of ITP, for thrombocytopenia in the setting of hepatitis C and cirrhosis13, and recently in aplastic anemia14. Ongoing and published trials have demonstrated efficacy in patients with MYH9-RD and Wiskott-Aldrich31. Both agents, while not licensed, have shown efficacy in MDS and in non-myeloablative chemotherapy32. Use of

the first generation agents suggested they would be effective in HIV-related thrombocytopenia and highly effective in stimulating donors of platelets for transfusion.

Dosing of both agents is only partly clearly and appropriately spelled out in the package inserts. An issue with both agents is the dose to use and particularly the dose at which to start. For eltrombopag this seems relatively straightforward because the suggested starting dose (50 mg/day unless one is of East Asian descent) is in the middle of the adult therapeutic range and therefore it makes sense to start with it. There also are primarily 3 levels (75, 50, and 25 mg/day) at which to dose, although intermediate doses can be achieved by alternation between 2 doses. With romiplostim it is much more difficult to decide on a starting dose. The package insert recommends starting at 1 mcg/kg and going up by 1 mcg/kg/week. This often will be too low a dose and is not often effective. Similarly, if one starts at a higher dose (hypothetically 3 mcg/kg) and this is not enough, does one increase by 1 mcg/kg weekly or 2 mcg/kg? Part of the uncertainty for the starting dose of both agents also involves deciding how refractory a patient may be. For example, how refractory does one consider a patient who has failed splenectomy or not responded to one or more other treatments? What, if any, degree of refractoriness will be best treated by the different starting doses? These decisions will definitely play a role in determining how soon that patient will respond to treatment. For highly refractory patients we

typically begin at 3 or 5 mcg/kg/dose and increase by 2 mcg/kg/dose each week, especially if it is urgent to achieve a platelet response.

Understanding why rare patients do not respond to these agents.

A similar and important issue involves predicting for whom TPO-RA treatment will work. It may be thought that TPO-RA treatment is so effective that it will work “in everybody” but that is not the case. Duration of disease or response to previous treatments has not been shown to correlate with response to TPO-RA. Similarly, response to splenectomy or rituximab does not seem to correlate with response to TPO-RA. However, there do need to be precursors in the marrow for a response to be seen. Studies of the role of certain anti-platelet antibodies to contribute to response or lack thereof are ongoing. As more is learned about the normal physiology of platelet production, more will likely be understood about the pathophysiology of thrombocytopenic diseases in turn allowing a better understanding of the likely efficacy of TPO agents.

The rate and time course of platelet increases is not completely predictable

Prediction of response to treatment both in terms of rate and time could be possible only with the assumption that the patient would respond to treatment and he/she was given the effective dose. Then one could believe that a small increase would be seen in 4 days and a substantial increase after 7 days.

However even then in certain cases, giving the same dose again would not result in a further platelet increase and the platelet count may even fall. This speaks to a compensatory mechanism that seems to counteract the initial impact of TPORA.

In regard to the time course, very occasionally patients will seem to lose their responses over time. The reason behind this is not well understood. Similarly certain patients may improve with time and no longer require TPO treatment to maintain at least an adequate platelet count. Mechanisms that have been documented as potential reasons for improvement are an increase in regulator T cells and a shift in Fc receptor expression in favor of FcRIIB, the inhibitory FcR33.

Platelet function changes in TPO-RA produced platelets correlate with reduction of bleeding

In a study looking at platelet function in patients with ITP, platelet ADP and TRAP responses were lower than in controls34. These low levels increased to essentially normal levels of ADP and TRAP reactivity in those patients who responded to eltrombopag. Responders in this study also had significantly decreased bleeding as determined by a bleeding score. Thus, responders to TPO-RA had both increased platelet function and decreased bleeding. In a recent study comparing TEG and platelet reticulocytes to a bleeding score,

surprisingly the platelet reticulocytes were better correlated to bleeding than the findings of the TEG35.

Toxicity Approved TPO-RA agents differ in patient acceptance and ease of administration Almost all patients will have at least minor side effects. The most common is mild headache. More serious side effects are thrombosis, both venous and arterial, which were seen in the clinical trials, and increased bone marrow reticulin deposition, which is seen in up to 20% of patients. Another potentially serious side effect of eltrombopag is liver toxicity. Finally, in certain patients whose marrows may contain pre-malignant cells, use of TPO agents can induce an apparent leukemia. In a number of these cases but certainly not all, cessation of the TPO agent treatment will be sufficient to allow the leukemia to disappear.

Both agents can be difficult to dose, especially when they are being initiated and frequent (weekly) follow up with dose adjustment is the rule rather than the exception. Furthermore romiplostim requires weekly injection by a trained medical professional which may mean weekly visits to the hematologist. This can be a burden for both patient and physician.

While romiplostim is a weekly subcutaneous injection without restrictions as to diet and other medication, eltrombopag is quite different. Because its activity as a

calcium chelator is critical to its efficacy, eltrombopag must be taken on a completely empty stomach. This can be difficult to achieve on a daily basis, resulting in highly variable eltrombopag levels.

An important and unanswered question is who is vulnerable to toxicity? Patients of East Asian origin who are not dose reduced appropriately for eltrombopag can have toxicity. Patients with suspected or known hepatic disease could also have significant toxicity. Elderly patients may also have more toxicity especially thrombosis which is a particularly complicated issue because it is not clear what the mechanism is or who is really at risk for it. Furthermore, in the various clinical trials in ITP, the risk of venous and arterial thrombosis is almost equal, suggesting that multiple mechanisms are at play. The toxicity data in one study of romiplostim was most conclusive for thrombosis risk in patients over 70, but anyone with a tendency to clot would presumably be at risk36.

Fibrosis of the marrow with continued use of either agent is an important risk

A recent study of bone marrow biopsy specimens from patients before and during treatment with TPO-RA found that TPO-RA induce myelofibrosis grades 2/3 in approximately one-fifth of patients with immune thrombocytopenia37. The risk increases with increasing exposure (>2 years) to the drug. Older age was also associated with higher grades of fibrosis. Annual/biannual follow-up with

bone marrow biopsies was recommended in patients being treated with TPO-RA so that therapy could be stopped before the fibrosis progresses. Other studies have shown lower rates of fibrosis but have followed patients primarily for only 2 years. It does appear that only reticulin is laid down is laid down in typical ITP cases, not collagen. Furthermore, lysyl oxidase levels are low which correlates with the reversibility of marrow fibrosis when treatment is discontinued for 4-6 months38.

Induction of leukemia by TPO agents may be a risk in specific clinical circumstances

An important issue is whether there is a risk of inducing leukemia in treating a patient with TPO-RA. There has not been a documented case of leukemia induction in an ITP patient. The same is not true for patients with MDS or aplastic anemia, where leukemic transformation has occurred39. Since these patients have an underlying risk of leukemic transformation, it is not known whether the TPO-RA accelerates the leukemic transformation process.

The process does not seem to be a direct effect of TPO-RAs on the normal marrow per se, but rather the result of the TPO-RA stimulating abnormal cells that are already in the marrow. This distinction seems straightforward in that by the clinical diagnosis and previous bone marrow one would know which disease

the patient had, ITP or MDS. However, in practice it is often not that easy to determine. ITP that is refractory and very difficult to manage can be difficult to distinguish from MDS, and only after starting TPO-RA therapy may it become evident that it is really MDS. One could argue that even in the setting of MDS, if the phenotype looked like very refractory ITP and one could not bring up the platelet count in any other way, TPO-RA would be appropriate short-term treatment.

An area that is potentially more confusing is in the inherited thrombocytopenias. Certain of these diseases are potentially pre-leukemic, similar to MDS, but other than platelet transfusion there is no way to raise the platelet count. Therefore, TPO-RA therapy may be an option, especially for short-term for a procedure or a particular bleeding problem, but not for chronic use. TPO-RA are unlikely to create a problem when used for 2-4 weeks in the setting of inherited thrombocytopenia, and if they do, the problem would most likely regress. However, if it TPO-RA are used as a long-term treatment the potential for induction of a malignancy is there.

In summary, if it appears that if a case is clearly ITP, for example there was an adequate but too transient response to IVIG, then a marrow is very likely not needed. However, if a case is less likely to be ITP and may be inherited platelet disorder or MDS, then a bone marrow performed prior to treatment can be very useful.

Thrombocytosis is an important complication and can occur with both agents

Platelets in ITP patients are thought to be generally more thrombogenic than normal platelets. Thus a concern regarding the use of the TPO-RA is the potential for inducing thrombocytosis and increasing the risk of thrombosis. Thus the goals of therapy are not to bring the platelet count to normal, but only to hemostatic levels. Interestingly, while there were several thromboses reported in the clinical trials, these did not occur at the highest platelet counts. These findings suggest that the high platelet counts are not the only underlying etiology of the thromboses.

Thrombosis in the setting of TPO-RA may be venous as well as arterial

Patients on the eltrombopag clinical trials experienced both arterial and venous thrombosis. Of 135 patients receiving eltrombopag on the RAISE study, three developed venous thrombosis5. The EXTEND extension study followed 299 patients for up to five years and reported nine patients with venous thrombosis and seven patients with arterial thrombosis6. In the case of romiplostim, of 125 patients treated for 24 weeks, two had arterial thromboses9. Thus the risk is almost equal between arterial vs venous thrombosis. The majority of thromboses

occurred in patients with prior history of thrombosis. Of note, the thromboses did not necessarily occur at the highest platelet counts. Thus the mechanism underlying the thromboses is not simply due to thrombocytosis of overactive platelets. This problem will need further study to elucidate the mechanism.

Use of eltrombopag may require monitoring of hepatobiliary laboratory abnormalities

No increase in cataracts has been seen with eltrombopag. ITP occurs commonly in the elderly who may have taken steroids and may have smoked cigarettes, both of which predispose to cataracts. Although cataracts were seen in a species of juvenile rodents in pre-clinical studies, no cataracts were seen in 171 children treated for at least 6 months.

In contrast, monitoring of liver tests is very important. They need to be monitored carefully in the beginning, weekly to biweekly. If no abnormalities are seen, the intervals can be spread out to every 4 weeks and eventually every 8 weeks. In the Raise study, abnormalities were seen at least once in almost 10% of patients but only 3% of patients had to permanently discontinue the drug.

Special Populations: Use of TPO-RA during pregnancy

Pregnant women with ITP would in all likelihood respond to TPO agents. However it is thought that both agents would cross the placenta and affect the fetus. This reason, as well as the lack of efficacy data, has led to avoidance of this usage. In certain circumstances, however, administering these agents to a pregnant woman with very refractory ITP or with an inherited thrombocytopenia might prove useful.

Children may require higher doses of TPO-RA

Efficacy of both agents in children is at least as good as it is in adults. In studies with romiplostim, it is not clear that a higher dose in micrograms/kg is required for children than adults. In the recent studies with eltrombopag however it is clear that especially younger children require higher doses averaging 3 mg/kg/dose whereas an adult would be more in the 1 mg/kg range. However toxicity in children appears to be less than in adults. Specifically the number of children discontinuing for transaminitis was similar but no episodes of thrombosis or induction of malignancy were seen. This topic is covered in much greater detail in the chapter by Kuhne.

Other indications:

Eltrombopag has been evaluated in patients with hepatitis C. One study showed that eltrombopag allowed thrombocytopenic patients to continue taking their peginterferon-ribavirin treatment to treat their hepatitis C infection13. Romiplostim has been used with Vidaza in MPD patients and decreased their dependence on platelet transfusions.40 A recent study has demonstrated the efficacy of Eltrombopag in > 80% of 15 patients with MYH9-RD (aka May Hegglin) requiring treatment41. A study reported in abstract form has shown efficacy in patients with the XLT form of Wiskott Aldrich syndrome42. Eltrombopag was recently approved in the US for second line treatment of severe aplastic anemia. 17 of 43 patients (40%) with severe aplastic anemia responded to eltrombopag with increased counts at 3 to 4 months14. Many of the patients recovered their red blood cells and WBC, not only their platelets. Most patients who continued the drug recovered all three lineages. Five patients were able to discontinue the drug. Of note, eight patients, including 6 nonresponders and 2 responders, developed new cytogenetic abnormalities on eltrombopag, including 5 with chromosome 7 loss or partial deletion. However there have been no transformations to leukemia to date.

In summary a number of features of these agents make them difficult to use. One is a wide range of dose response although the therapeutic index is also wide. The second is the requirement for either weekly injection (permitted in the EU but not in the USA at home) or oral use with absolute dietary restrictions. The third is risk of thrombosis and induction of malignancy in certain patient

populations. A fourth is the high cost which needs to be weighed against settings where either the TPO agents are uniquely effective or more effective and less toxic than other options. Further understanding of why patients do not respond and how to combine these agents with other agents is required in the future. Expansion of indications ideally would include in vitro platelet production, stimulation of platelet donors, defining the role in HSCT (human stem cell transplantation), and ideally in MDS.

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Table 1: Platelet Production Is Suboptimal in ITP Patients

Autoantibodies inhibit Mk growth and promote apoptosis43

Autologous 111In-platelet studies show platelet production < normal in 2/3 pts44

Decreased absolute platelet retics45

TPO levels normal in 75% of ITP patients46

Damaged or Dysfunctional Mk in marrow47