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Mirabegron in MPN
Bone marrow histology
Bone marrow biopsies before and after mirabegron treatment were obtained in 20 patients of the 39 patients who consented to this subproject (51%). These included 9 PV, 4 ET, 4 PMF, 2 post-ET, and 1 post-PV MF patients. The biopsies were evaluated in a blinded fashion.
A slight decrease in reticulin fiber content from a medi- an grade of 1.0 (IQR 0-3) to 0.5 (IQR 0-2) (P=0.01) and an increase in the nestin+ MSCs cells from a median of 1.09/mm2 (IQR 0.38-3.27) to 3.95/mm2 (IQR 1.98-8.79) (P<0.0001) were observed (Figure 2). The mean change in the nestin+ cells from baseline to week 24 was 3.52/mm2 [95% confidence interval (CI): 1.65-5.39]. We found no correlation between reticulin fibrosis or nestin+ cell con- tent with time from diagnosis to study inclusion, blood counts, splenomegaly, and JAK2-V617F allele burden. The decrease in reticulin fibrosis was limited to patients with- out hydroxyurea treatment (-0.85/mm2 without hydrox- yurea vs. 0.0/m2 with hydroxyurea; P=0.042). No statisti- cally significant differences in CD34+ cell numbers were noted on paired samples before and after 24 weeks of mirabegron. Quantitative assessment of megakaryocyte numbers showed no differences between baseline to week 24 (median 25.5/mm2, IQR 16.75-34.25 vs. 22/mm2, IQR 14.38-29.63; P=0.371), but a trend towards reduction in megakaryocyte cluster formation and decrease in num- bers of large megakaryocytes with staghorn-like morphol- ogy was noted in some patients.
Discussion
Mirabegron was safe and well tolerated in patients with JAK2-mutated MPNs. However, the primary end point of reducing the JAK2-V617F allele burden was not reached (Figure 1). A slight overall hematologic improve- ment was seen in a subset of patients, but was not con- sidered clinically relevant (Table 4). In a JAK2-V617F-dri- ven mouse model of MPN, treatment with the β-3-sym- pathomimetic agonist BRL37344 lowered platelet and neutrophil counts, and decreased mutant hematopoietic progenitor numbers and spleen size.8 However, we did not observe effects on blood counts, spleen size or CD34+ cells in our phase II study. Species differences in the β3- adrenergic signaling and responsiveness of β3-adrenergic receptors towards different agonists between human and mouse could contribute to the observed discrepancies. Mirabegron is selective for the human β3-adrenergic receptor and was less effective in mice,8 whereas BRL37344 shows higher affinity for the murine β3-adren- ergic receptor.
Nevertheless, some of the effects observed in the pre- clinical JAK2-V617F mouse model treated with BRL37344, i.e. increase in nestin+ bone marrow MSCs and
decrease in myelofibrosis, were also seen in our mirabegron study: BM biopsies performed in a subset of 20 patients revealed a significant increase in the nestin+ MSCs and a decrease in reticulin fibrosis (Figure 2). Although the beneficial effect of mirabegron on reticulin fibrosis was moderate, the duration of treatment was also rather short (24 weeks), as it was mainly designed to assess the primary end point of reduction of allele burden. The question of whether a higher dose of mirabegron might have been more effective is difficult to answer. Although doses of 100 mg daily have been tested in ear- lier clinical studies, no clear dose-dependent effect has been observed, while cardiovascular symptoms and a prolongation of the QT interval were noted.9,19 The fact that nestin+ cells showed a robust increase at 24 weeks of treatment indicates that mirabegron at 50 mg daily had one of the expected biological effects that had previously been described in mouse experiments.
Surprisingly, the effect on reticulin fibrosis was limited to patients who did not receive hydroxyurea treatment (41% of patients). The mechanism of how hydroxyurea interfered with the effect of mirabegron on reticulin fibro- sis is currently unknown. Previous reports suggest that hydroxyurea alone can reduce reticulin fibrosis in some MPN patients.20,21 Selecting patients who have not previ- ously received hydroxyurea, a longer trial duration and higher dosage of mirabegron will be considered for future studies in MPN.
Despite the fact that the primary end point of reducing JAK2-V617F allele burden was not reached in this trial, the observed effects on nestin+ MSCs and reticulin fibrosis is encouraging and shows that a β-3-sympathomimetic ago- nist can modify the microenvironment where the JAK2-mutant stem cells are maintained. These results gen- erate an interest in evaluating β-3-sympathomimetic ago- nists specifically in patients with myelofibrosis not pre- treated with hydroxyurea, and possibly in combination with other substances.
Acknowledgments
The authors thank the patients for participating in the study and the local data managers for collecting patient data.
Funding
This investigator-initiated trial was supported by grants from the Rising Tide Foundation, Gateway for Cancer Research, Swiss Cancer League (KFS-3655-02-2015), and the Swiss National Science Foundation (KFS-3539-08-2014) (31003A_166613) to RCS, ERC-2014-CoG-648765 grant to SMF, and the Swiss Cancer League to JRP. The SAKK organi- zation is supported by the Swiss State Secretary for Education, Research and Innovation, Swiss Cancer Research Foundation and the Swiss Cancer League. The study drug mirabegron was provided free of charge by Astellas Pharma AG Switzerland.
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