RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Combining chemotherapy with G-CSF may make tumor cells more sensitive to the chemotherapy drug and may kill more tumor cells.

PURPOSE: Phase I/II trial to study the effectiveness of mitoxantrone plus G-CSF in treating patients with metastatic prostate cancer that has not responded to hormone therapy.

OBJECTIVES: I. Study the effectiveness of mitoxantrone in patients with metastatic, hormone refractory prostate cancer. II. Assess the side effects of mitoxantrone in this patient population. III. Determine whether filgrastim (granulocyte colony-stimulating factor; G-CSF) treatment can overcome chronic bone marrow suppression and facilitate a dose intensive mitoxantrone chemotherapy regimen in this patient population.

OUTLINE: This is a dose escalation study. Patients receive a 30 minute intravenous infusion of mitoxantrone once a week on day 1. On days 2-6 of each week, patients receive subcutaneous filgrastim (granulocyte colony-stimulating factor; G-CSF). Treatment continues for patients who respond to therapy and exhibit no disease progression. Dose escalation proceeds until the maximum tolerated dose (MTD) of mitoxantrone with G-CSF support is determined. The MTD is defined as the dose preceding that at which 2 or more patients experience dose limiting toxicity. After the MTD is determined, an additional 20-40 patients are enrolled at this dose level to examine their response to this therapy.

PROJECTED ACCRUAL: Approximately 12-24 patients will be accrued for the phase I portion of this protocol. An additional 20-40 patients may be accrued for the phase II portion of this study.

• Biological: filgrastim
• Drug: mitoxantrone hydrochloride

DISEASE CHARACTERISTICS: Histologically confirmed hormone refractory adenocarcinoma of the prostate Metastatic disease required

PATIENT CHARACTERISTICS: Age: Not specified Performance status: SWOG 0-2 Life expectancy: Greater than 12 weeks Hematopoietic: Absolute granulocyte count greater than 1500/mm3 Platelet count greater than 100,000/mm3 Hepatic: Bilirubin less than 2.0 mg/dL SGOT less than 2 times normal Renal: Creatinine less than 1.5 mg/dL Cardiovascular: Must have adequate cardiac function (LVEF at least 40%) Other: No other malignancy except skin lesions that have been completely excised No prior hypersensitivity to E. coli derived products

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy Endocrine therapy: See Disease Characteristics At least 1 month since LHRH agonist and/or flutamide Radiotherapy: No concurrent radiotherapy No prior extensive radiotherapy (such as whole pelvic irradiation) Surgery: Not specified