Abstract
BACKGROUND: Ixabepilone, which stabilizes microtubules, has low susceptibility to drug resistance mediated by P-glycoprotein or βIII-tubulin. MATERIALS AND METHODS: This study was designed to determine the maximum tolerated dose (MTD) of oral ixabepilone when administered every 6 h for three doses, every 3 weeks, to patients with refractory advanced cancers. Eighteen patients were treated with escalating doses of ixabepilone: three at cohort 1 (30 mg/dose; 90 mg on Day 1), nine at cohort 2 (40 mg/dose; 120 mg on Day 1), and six at cohort 3 (50 mg/dose; 150 mg on Day 1). Serial plasma samples were collected during cycle 1 for pharmacokinetic (PK) measurements. RESULTS: Of the 18 treated patients, eight were male and ten were female. The median age was 59 years, and most had an excellent performance status (KPS 90-100; 61%). There were two dose limiting toxicities (DLT): Grade 4 febrile neutropenia at the 120 mg dose and Grade 4 neutropenic sepsis at the 150 mg dose. Because of the severity and duration of neutropenic sepsis at level 3, level 2 (120 mg) was defined as the MTD and this cohort was expanded to nine patients. High inter-individual variability in plasma drug concentrations was observed during the study, with particularly high levels in two patients with DLT. CONCLUSIONS: On the basis of this safety profile, the MTD of oral ixabepilone was defined as 120 mg given as three 40 mg doses each separated by 6 h on Day 1 of a 3-week cycle. However, the PK variability observed makes further development of this oral formulation unlikely.