Interest of Denosumab Treatment in Osteoporosis Associated to Systemic Mastocytosis

Systemic mastocytosis (SM) represents a heterogenous group of disease characterized by abnormal proliferation of mast cells caused by activating mutations in c-Kit receptor; a tyrosine kinase family receptor present in mast cell that control cell proliferation. Prevalence of SM is estimated to 1/20 000 person. According to the World Health Organization (WHO) criteria (Johnson et al, 2009), SM can be separated into two different groups : indolent SM and aggressive SM. Aggressive SM are defined by a poor prognostic disease either because of an important mast cell tumor mass as sarcoma mastocytosis or mast cell leukemia, or due to an association with an other myeloid hemopathy as myelodysplasic/myeloproliferative syndrome. Indolent SM are the most common cases with a very good prognostic similar to general population. Symptoms related to mast cell proliferation in indolent SM are very various (Theoharides et al., 2015) and could be divided into 2 entities : those related to mast cell proliferation in tissue as the urticaria pigmentosa and those related to the mast cell degranulation. There are many clinical relevant mediators released by mast cells after activation that could have putative effects on different systems as cardiovascular, cutaneous, neurologic, digestive, systemic, respiratory and musculoskeletal (Frenzel et al., 2013). Tryptase, histamine, prostaglandin, interleukine-6 and Tumor Necrosis Factor (TNF)-α are the most common and ubiquitous mediators released by mast cells but there are also some specific mediator targeting an organ as RANKL ; expression of RANKL by mast cells directly control regulation of osteoclast activity and is involved in osteoporosis associated to SM (Rabenhorst et al., 2013). In systemic mastocytoses, bone lesions are found in about half of patients. A third have osteoporosis defined as a lumbar spine or hip bone T score of -2,5 Standard Deviation (SD) or less. In most cases, osteoporosis was complicated at least by one vertebral fracture with pain and functional disorders (Barete et al, 2010). The median time to fragility fracture after mastocytoses diagnosis is up to 5 years, even in very young population usually considering as low risk of fracture in non mastocytoses associated osteoporosis (Van de Veer et al., 2014) By default, these patients are generally treated with bisphosphonate per os or intravenously with some efficiency on bone mineral density gain and bone turnover marker decrease (Rossini et al, 2014). However, no study with bisphosphonate in SM has showed durable prevention of new bone events and there are no randomized studies that proved their efficacy in systemic mastocytosis. Moreover, these drugs are not well tolerated in SM wether per os due to worsening of digestive symptoms by mast cells activation, or intravenously with an important number of acute phase characterized by some systemic symptoms as fever ; really experiences with intravenous biphosphonate but very increased in SM. Additional, the use of a bisphosphonate ask the question of the residual long-term effect of this drug that may be several years or even decades with a significant risk of atypical fracture secondary to a adynamic bone remodeling (Edwards et al. , 2013) and the long-term presence of crystals of bisphosphonate in the skeleton, particularly worrying in women of childbearing age. In postmenopausal osteoporosis, importance of residual long-term effect is weighted with advanced median age of patient. The fact that mostly patients with a systemic mastocytosis are young people led to restrict the use of bisphosphonate due to those effects. Among different treatment available in post menopausal osteoporosis, the use of an anti-RANKL antibody, denosumab, in SM associated osteoporosis could be interesting. First, denosumab has already shown significant efficacy in the treatment of postmenopausal osteoporosis in women (Cummings et al , 2009). Second, this treatment is proposed every 6 months by a subcutaneous injection with very few side effects and half-life drug is shorter than biphosphonate with probably low residual long-time effect. Third, as mast cells expressed mainly RANKL with a direct role on bone turnover in SM. The use of a specific drug seems to be obvious and attractive.