This phase II trial is studying how well sunitinib works in treating patients with idiopathic myelofibrosis. Sunitinib may stop the growth of abnormal cells by blocking some of the enzymes needed for cell growth and by blocking blood flow to the abnormal cells.

PRIMARY OBJECTIVE:

I. Assess the response rate and the duration of response in patients with idiopathic myelofibrosis treated with sunitinib malate.

SECONDARY OBJECTIVE:

I. Assess the safety of sunitinib malate in these patients.

OUTLINE: This is an open-label, multicenter study.

Patients receive oral sunitinib malate once daily for 6 weeks. Treatment repeats every 6 weeks in the absence of disease progression or unacceptable toxicity.

After completion of study treatment, patients are followed for 4 weeks.

Criteria:

• At least 4 weeks since prior chemotherapy or radiotherapy (6 weeks for nitrosoureas or mitomycin C)
• At least 4 weeks since prior major surgery
• At least 7 days since prior and no concurrent CYP3A4 inhibitors, including azole antifungals (ketoconazole and itraconazole), clarithromycin, erythromycin, diltiazem, verapamil, HIV protease inhibitors (indinavir, saquinavir, ritonavir, atazanavir, nelfinavir), and delavirdine
• At least 12 days since prior and no concurrent CYP3A4 inducers, including rifampin, rifabutin, carbamazepine, phenobarbital, phenytoin, Hypericum perforatum (St. John's wort), efavirenz, and tipranavir
• No prior antiangiogenic agents (e.g., bevacizumab, sorafenib, pazopanib, AZD2171, vatalanib, VEGF Trap)
• No other concurrent investigational agents
• No other concurrent anticancer agents or therapies
• No concurrent therapeutic doses of coumarin-derivative anticoagulants (e.g., warfarin) (Warfarin =< 2 mg/day for prophylaxis of thrombosis and low molecular weight heparin allowed provided INR =< 1.5)
• No concurrent combination antiretroviral therapy for HIV-positive patients
• No concurrent agents with proarrhythmic potential (e.g., terfenadine, quinidine, procainamide, disopyramide, sotalol, probucol, bepridil, haloperidol, risperidone, indapamide, and flecainide)

Inclusion Criteria:

• Diagnosis of MF (histologically confirmed) requiring therapy, including those previously treated and relapsed or refractory (no more than one prior standard acute leukemia-type chemotherapy regimen; no limit on prior MF - directed therapies) or if newly diagnosed, with intermediate or high risk according to Lille scoring system (adverse prognostic factors are: Hb < 10 g/dl, WBC < 4 or > 30 x 10^9/L; risk group: 0 = low, 1 = intermediate, 2 = high), or with symptomatic organomegaly.
• Serum bilirubin levels </= 2x upper limit of the normal (ULN). Higher levels are acceptable if these can be attributed to active hemolysis or MF
• Serum glutamic-pyruvic transaminase (SGPT) (alanine aminotransferase [ALT]) levels </= 2x ULN. Higher levels are acceptable if these can be attributed to MF.
• Serum creatinine levels </= 2x ULN.
• Eighteen years of age or older.
• ECOG performance status 0-1 (Karnofsky </= 70%).
• Patients must have QTc < 500 msec
• Women of childbearing potential and men must agree to use adequate contraception (e.g. hormonal or barrier method of birth control; abstinence) for the duration of study participation. All women of childbearing potential must have a negative pregnancy test prior to receiving sunitinib. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately.
• Ability to understand and the willingness to sign a written informed consent document
• Both men and women and members of all races and ethnic groups are eligible for this trial.

Exclusion Criteria:

• Patients who have had chemotherapy or radiotherapy within 4 weeks (6 weeks for nitrosoureas or mitomycin C) prior to beginning treatment or those who have not recovered from treatment-limiting adverse events (to grade 1 or better) due to agents administered more than 4 weeks earlier. At least 4 weeks must have elapsed since any major surgery prior to study enrollment. Continuous use of supportive care medications (i. e. growth factors, anagrelide, or hydroxyurea) is allowed.
• Patients may not be receiving any other investigational agents.
• Patients who have received prior treatment with any other specific antiangiogenic agent (e.g., bevacizumab, sorafenib, pazopanib, AZD2171, PTK787, VEGF Trap, etc.) targeting VEGF/VEGFR system. Other agents that may have some antiangiogenic effects are allowed). (i.e. thalidomide)
• Patients with abnormal QTc prolongation (defined as a QTc interval equal to or greater than 500 msec) or patients who have a history of serious ventricular arrhythmia (VT or VF>/= 3 beats in a row or other significant (as judged by treating physician) ECG abnormalities.Patients with consistently poorly controlled (during the month prior to study screening) hypertension (systolic blood pressure of 140 mmHg or higher or diastolic blood pressure of 90 mmHg or higher) despite appropriate medical management of the hypertension.
• Patients who require use of therapeutic doses of coumarin-derivative anticoagulants such as warfarin. Exceptions include: patients using warfarin of up to 2 mg daily for prophylaxis of thrombosis, and patients using low molecular weight heparin provided the patient's INR is \