Inclusion Criteria:

- Patient must give written informed consent

- Eastern Cooperative Oncology Group (ECOG) performance status of 0-3

- Life expectancy > 12 weeks

- Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) =< 2.5 x upper
limit of normal (ULN), if caused by ASM/MCL =< 5 x ULN

- Serum direct bilirubin =< 1.5 x ULN; if considered related to ASM/MCL =< 3 x ULN

- Serum creatinine =< 2.0 mg/dL

- A diagnosis of systemic mastocytosis (SM) per 2008 World Health Organization (WHO)
Criteria

- Neoplastic mast cells must express CD30 by immunohistochemistry or flow cytometry

- Patients with ASM and MCL with or without an AHNMD are required to have at least one
of the eligible organ damage findings as defined by the international consensus
response criteria

- Both females of childbearing potential and males who have partners of childbearing
potential must agree to use an effective contraceptive method during the study and
for 30 days following the last dose of study drug

- Females of childbearing potential must have a negative serum or urine beta-human
chorionic gonadotropin (hCG) pregnancy test result within 7 days prior to the first
dose of SGN-35; females of non-childbearing potential are those who are
postmenopausal greater than 1 year or who have had a bilateral tubal ligation or
hysterectomy

Exclusion Criteria:

- Patients unwilling or unable to comply with the protocol

- Any other concurrent severe known disease (except carcinoma in-situ) or concurrent
severe and/or uncontrolled medical condition (e.g. uncontrolled diabetes, or active
uncontrolled infection) which could compromise participation in the study

- History of another primary malignancy that has not been in remission for at least 3
years (the following are exempt from the 3-year limit: non-melanoma skin cancer,
fully excised melanoma in situ [stage 0], curatively treated localized prostate
cancer, and cervical carcinoma in situ on biopsy or a squamous intraepithelial lesion
on Papanicolaou [PAP] smear)

- Patients with cardiovascular disease including congestive heart failure grade III or
IV according to the New York Heart Association (NYHA) classification, left
ventricular ejection fraction of < 50%, myocardial infarction within previous 6
months or poorly controlled hypertension

- Women who are pregnant or lactating

- Patients with >= grade 2 neuropathy

- Patients with a known hypersensitivity to any excipient contained in the drug
formulation

- Confirmed diagnosis of human immunodeficiency virus (HIV) infection or active viral
hepatitis

- Patients presenting with an AHNMD requiring immediate cytoreductive therapy or
targeted drugs (e.g. AML)

- Patients who have received any investigational agent, chemotherapy, interferon-alfa,
or 2-chlorodeoxyadenosine (2-CdA, cladribine) within 30 days prior to Day 1

- Patients who have received hematopoietic growth factor support within 14 days of Day
1 of SGN-35

- Use of prednisone (or equivalent corticosteroid dose) for SM up to 10 mg/day or its
equivalent is allowed, but it cannot have been started during screening; patients who
are on prednisone up to 10 mg/day for medical problems unrelated to SM are also
permitted on study

- Patients with the FIP1L1-PDGFRalpha fusion even with resistance to imatinib (such
patients are no longer defined as systemic mastocytosis by the WHO)

- Patients who have received any treatment with SGN-35 prior to study entry

- Patients who have had any surgical procedure, excluding central venous catheter
placement or other minor procedures (e.g. skin biopsy) within 14 days of Day 1