PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy Chemotherapy: At least 3 weeks since prior chemotherapy (6 weeks for mitomycin or nitrosoureas) and recovered No more than 1 prior chemotherapy regimen Prior fluorouracil and taxanes allowed Prior parenteral etoposide allowed Endocrine therapy: No concurrent anti-cancer hormonal agents Radiotherapy: At least 3 weeks since prior radiotherapy and recovered Prior radiotherapy to brain metastases allowed if stable neurological status achieved within 4 weeks of treatment No concurrent radiotherapy except for palliation of bone or brain metastases or pathological fractures of known lytic disease Surgery: Not specified Other: At least 3 weeks since prior investigational drugs and recovered No other concurrent anticancer drugs No other concurrent investigational therapy No concurrent halogenated antiviral agents such as lodenosine, fialuridine, clevudine, emtricitabine, or sorivudine No concurrent antiarrhythmic medication