Phase I trial of low-dose, prolonged continuous infusion fluorouracil plus interferon-alfa: Evidence for enhanced fluorouracil toxicity without pharmacokinetic perturbation

Purpose: To determine the maximum-tolerable dose (MTD) of fluorouracil (5- FU) administered as a low-dose, prolonged continuous intravenous infusion (PCI) plus interferon-alfa (IFN-α) that would permit treatment for at least 28 consecutive days, and to determine the effect of IFN-α on 5-FU pharmacokinetics. Patients and Methods: Twenty-six assessable patients with advanced cancer received low-dose PCI 5-FU (150, 200, 250, and 300 mg/m²/d) plus IFN-α, 5 x 10⁶ IU/m² administered subcutaneously (SC) at hour 48 of the 5-FU infusion, then thrice weekly thereafter in cohorts of at least three patients. Treatment continued until treatment-limiting toxicity (TLT) developed, such as mucositis, diarrhea, or fatigue. Escalation to the next 5- FU dose level occurred if none of three or zero to two of six patients developed TLT before day 28. Quantitation of plasma 5-FU concentration by high-performance liquid chromatography was performed in 15 patients. Data were standardized using the Cosinor method and compared before and after IFN- α administration using the paired t test. Results: The mean number of days of continuous 5-FU therapy for patients receiving 150, 200, 250, and 300 mg/m²/d of 5-FU plus IFN alfa-2a (IFN-α2a) was 75, 54, 37, and 22 days, respectively. The MTD of PCI 5-FU by our criteria that could be combined with IFN-α was 250 mg/m²/d. Comparison of the standardized pharmacokinetic data showed no significant effect of IFN-α on plasma 5-FU concentration, and no alteration of the normal circadian variation in plasma 5-FU concentration that was evident before IFN-α administration. Objective response occurred in patients with adenocarcinoma of the pancreas (n = 3), kidney (n = 2), and lung (n = 1). Conclusion: IFN-α substantially enhanced the gastrointestinal toxicity of low-dose PCI 5-FU without affecting 5-FU pharmacokinetics, contrary to previous reports using alternative 5-FU schedules in which IFN- α-related enhancement of 5-FU toxicity was attributable to reduced 5-FU clearance. Our findings suggest that under certain conditions, mechanisms other than altered 5-FU pharmacokinetics may be responsible for the ability of IFN-α to enhance the toxic effects of 5-FU.