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Preclinical models of mastocytosis
forthcoming new standard of therapy in advanced SM is midostaurin (PKC412).48,49 This drug was approved for treatment of advanced SM by the American Food and Drug Administration and the European Medicines Agency in 2017. For ASM/MCL patients with rapid pro- gression and those resistant to 2CdA or midostaurin, poly-chemotherapy is usually recommended, followed, when possible, by allogeneic hematopoietic stem cell transplantation.50 Almost all drug-based cytoreductive therapies have been validated preclinically in vitro using MCL-like MC lines. The most widely used cells for this purpose have been (and still remain) the two HMC-1 sub- clones. However, the newly emerging MCL-like human MC lines, ROSA and MCPV-1, have also been used repeatedly in such drug-testing studies. A summary of drug-testing approaches and of results obtained with these cell lines is provided in the following paragraphs.
HMC-1 cell lines and their responses to targeted and non-targeted cytoreductive drugs
Numerous antineoplastic drugs have been tested for their effects on HMC-1 cells.51 Among conventional anti- neoplastic drugs, doxorubicin and cytosine arabinoside were the most active agents.51 Other effective agents were vinblastine, etoposide and mitomycin.51 The potent effects of these chemotherapy-type drugs, otherwise used to treat acute myeloid leukemia, formed the basis to suggest treatment of patients with rapidly progressing ASM and MCL as well as patients with SM-acute myeloid leukemia with standard induction chemothera- py, often as preparation for allogeneic stem cell transplan- tation.
The effects of interferon(s) on the growth of HMC-1 cells have also been analyzed.52 HMC-1 cell numbers decreased in the presence of interferon-γ but were unaf- fected by interferon-a,52 contrasting with the activity of interferon-a in a subset of patients with advanced SM.44 This example highlights the fact that not all drug effects observed in vitro can be translated into clinical practice and that in each case, drugs and drug combinations need to be tested in additional disease models and finally in interventional clinical trials.
Studies of the in vitro anti-proliferative activity of 2CdA on HMC-1 cells were published after this drug was used in vivo to treat patients with advanced SM. Indeed, the first reports on the in vivo effects of 2CdA in patients were published between 2001 and 2004,53-55 but it was not until 2006 that the in vitro effects of 2CdA on HMC-1 cells were described.56 While 2CdA alone produced growth- inhibitory effects on HMC-1 cells, the drug was also found to cooperate with midostaurin.56 The observation that midostaurin can induce apoptosis and growth inhibi- tion in HMC-1 cells and that efficacy was identical in HMC-1.1 and HMC-1.2 cells prompted further investiga- tions and led to the initiation of clinical trials.48,49
Human mast cell leukemia-like cell lines as models of drug resistance
Because most patients with SM harbor an activating point mutation in KIT (mostly KIT D816V) which is asso- ciated with disease pathology, considerable efforts have been made to identify drugs capable of inhibiting the kinase activity of the KIT mutant. The effects of imatinib, a drug targeting KIT WT, on cell lines harboring various KIT mutations, were investigated soon after the drug was
Table 4. Expression by the human mastocytosis-like mast cell lines of antigens aberrantly expressed or overexpressed on malignant mast cells and/or their neoplastic progenitors in patients with systemic mastocytosis.
CD Name HMC-1
ROSAKIT D816V
MCPV-1
CD13 APN +
+ +
- - - - + + - - + + + + + + + + +/- -
CD25
CD30
CD33
CD34
CD44
CD52
CD87
CD117
IL-2Ra - KI-1 - SIGLEC-3 + HPCA-1 -
PGP-1 + CAMPATH-1 +/-
UPAR +
KIT +
IL-3Ra -
+: strong expression; +/-: weak expression; -: no expression.
CD123
found to block growth of leukemic cells in Philadelphia chromosome-positive chronic myeloid leukemia. In 2000, Ma et al. reported that imatinib inhibited KIT WT at low concentrations, without significant effects on the KIT D816V mutant.57 In 2003, these findings were confirmed using HMC-1.2 cells and patient-derived KIT D816V+ MC.29,58 More recently, it was also confirmed that ROSAKIT D816V cells are insensitive to imatinib.15 Masitinib, another TKI active on KIT WT, although devoid of activity on KIT D816V in vitro,59 was administered in a randomized, dou- ble-blind, placebo-controlled, phase 3 study in a cohort of severely symptomatic ISM or SSM patients resistant to classical anti-mediator therapy.60 Interestingly, masitinib improved mediator-related symptoms in a subset of patients as compared to placebo-treated patients, regard- less of KIT mutational status.60 This clinical activity was linked to the in vitro inhibitory effects of masitinib on two molecules involved in MC activation, namely LYN and FYN.59
Given the inefficacy of imatinib on the KIT D816V mutant, several other TKI have been evaluated in vitro (and for a few of them in vivo) for their potential activity in the SM context. Dasatinib is a multikinase inhibitor highly active on BCR-ABL1, KIT and PDGFRa.61,62 The potential activity of this drug against KIT D816V was investigated in vitro in HMC-1 cells, SM patient-derived KIT D816V+ cells and ROSA cells.15,63 In each instance, dasatinib exerted in vitro cytotoxic effects at relatively low half maximal inhibitory concentrations (IC50), although the IC50 for dasatinib was higher in KIT D816V+ cells than in KIT D816V– cells.15,63 However, when evaluated in vivo in clinical trials or in individual SM patients, dasatinib unexpectedly demonstrated only marginal activity.64-66 While the in vivo effects of dasatinib have been disap- pointing, midostaurin, a potent multikinase inhibitor, has proven to be highly promising. Indeed, midostaurin decreased the proliferation of KIT D816V+ cell lines at pharmacological concentrations.15,56,63,67 In addition, the drug abrogated KIT phosphorylation in MCL-like cell lines harboring KIT D816V and induced their apopto- sis.15,56,68 Moreover, midostaurin suppressed the growth of primary human KIT D816V+ neoplastic MC.68 Finally, midostaurin was found to block IgE-dependent histamine release from MC and basophils.67,69,70 Based on these data,
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