Inclusion Criteria:

- Confirmed diagnosis of aggressive systemic mastocytosis (ASM) or mast cell leukemia
(MCL) according to WHO criteria for SM and established criteria for ASM and MCL
(Valent et al 2003), presenting with at least one measurable C-Finding.

- ECOG performance status of 0-3

- Life expectancy > 12 weeks

- ECG: QTc interval ≤ 450 ms

- Meeting the following laboratory values:

- AST and ALT must be ≤ 5 x Upper Limit of Normal (ULN) if this elevation is solely due
to ASM/MCL, otherwise AST, ALT must be ≤ 2.5 x ULN

- Serum Bilirubin must be ≤ 3 x Upper Limit of Normal (ULN) if this elevation is solely
due to ASM/MCL, otherwise serum bilirubin must be

- 1.5 x ULN

- Serum Creatinine ≤ 2.0 mg/dL

- Patients who would be suitable for imatinib therapy (e.g. having ASM with
eosinophilia and known positivity for the FIP1L1-PDGFRα fusion or known to be KIT
D816V negative) unless they have demonstrated relapse or disease progression on prior
imatinib therapy

Exclusion Criteria:

- Any other concurrent severe known disease (except carcinoma in-situ) concurrent
severe and/or uncontrolled medical condition including congestive heart failure grade
III or IV according to the NYHA classification or with ejection fraction < 50%, etc.

- Patients with any pulmonary infiltrate including those suspected to be of infectious
origin. Exception: Patients with a pleural effusion related to the disease under
study as confirmed by the investigator are permitted to enter the study

- Patients who have failed more than two prior SM therapies (not including those given
for supportive care)

- Patients who have received any investigational agent, chemotherapy, interferon-α, or
2-chlorodeoxyadenosine (2-CdA, cladribine) within 30 days prior to first dose

- Patients who would be suitable for imatinib therapy (e.g. having ASM with
eosinophilia and known positivity for the FIP1L1-PDGFRα fusion) unless they have
demonstrated relapse or disease progression on prior imatinib therapy

Other protocol-defined inclusion/exclusion criteria may apply