Treatment of Indolent Systemic Mastocytosis With PA101

Inclusion Criteria: * Diagnosis of indolent systemic mastocytosis (ISM) according to the WHO criteria and the consensus proposal (2001) * Experiencing at least one predefined qualifying symptom in at least two organ systems within 3 months of Screening despite the use of H1 and/or H2 antihistamines and other anti-mediator therapy * Experiencing symptoms with a severity score of at least 4 for at least 7 out of 14 days during the Run-in period with at least one predefined qualifying symptom each from at least two organ systems, despite the use of H1 and/or H2 antihistamines and/or other anti-mediator therapy * Willing and able to use an eDiary device daily for the duration of the study * Completed at least 5 eDiary reports during each of two consecutive weeks of the Run-in period * Willing and able to provide written informed consent prior to any study procedures Exclusion Criteria: * Advanced systemic mastocytosis (i.e., aggressive systemic mastocytosis \[ASM\], mast cell leukemia \[MCL\], or systemic mastocytosis with an associated clonal hematologic non-mast cell lineage disease \[SM-AHNMD\] ) * Current or recent history of clinically significant cardiovascular, hematological, renal, neurologic, hepatic, endocrine, psychiatric, malignant, or other illnesses that could put the patient at risk or compromise the quality of the study data as determined by the Investigator * Use of oral cromolyn sodium within 6 weeks of Screening * History of systemic corticosteroid, immunosuppressive, or anti-IgE monoclonal antibody therapy (e.g., omalizumab) within 6 months of Screening * History of anaphylaxis requiring systemic treatment (i.e., corticosteroid or epinephrine) within 12 months of Screening * Upper or lower respiratory tract infection within 4 weeks of Screening * History of malignancy within the last 5 years, except basal cell carcinoma or cervix carcinoma in situ * Major surgery within 6 months of Screening * Current or recent history (within 12 months) of excessive use or abuse of alcohol * Current or recent history (within 12 months) of abusing legal drugs or use of illegal drugs or substances * Pregnant or breastfeeding females, or if of child-bearing potential unwilling to practice acceptable means of birth control or abstinence during the study * Participation in any other investigational drug study within 4 weeks of Screening * History of hypersensitivity or intolerance to aerosol medications or cromolyn sodium