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BACKGROUND: The purpose of the study was to evaluate the feasibility of increasing dose intensity by a stepwise reduction of the time intervals between chemotherapy cycles in separate patient cohorts with small-cell lung cancer. Patients received up to 6 courses of combination chemotherapy with carboplatin, etoposide, ifosfamide and vincristine followed by support with filgrastim. Dose intensity, incidence, duration and severity of neutropenic fever and infections, objective response to chemotherapy, and safety of filgrastim were determined. PATIENTS AND METHODS: 29 patients with small-cell lung cancer (limited disease: 2, extensive disease: 27) were treated with a combination of carboplatin 250 mg/m2 i.v. day 1, ifosfamide 2 g/m2 and etoposide 120 mg/m2 i.v. days 1 and 2, etoposide 120 mg/m2 orally day 3, and vincristine 1.4 mg/m2 day 14. Initially, filgrastim (5 micrograms/kg) was administered subcutaneously from day 7 to 16. With shorter treatment intervals, filgrastim was administered on days 4-16 or 4-14. RESULTS: An overall increase in dose intensity by a factor of 1.44 was achieved after reducing the treatment interval from 27 to 17 days. Further reduction to 14 days was not feasible due to persistent thrombocytopenia. Six patients (21%) developed a total of 9 febrile episodes, and 14 patients (48%) had to be withdrawn from the study before the completion of six cycles of chemotherapy. The median duration of infectious episodes was 6 days. Overall, a total of 22 of 27 evaluable patients had an objective response. Longer treatment intervals resulted in a lower probability for objective response (> or = 23 days: 10/14 patients vs. < or = 17 days: 7/7 patients). CONCLUSION: Filgrastim allows for the reduction of treatment intervals in patients with small-cell lung cancer and increased dose intensity with acceptable hematologic and nonhematologic toxicities.