Phase II clinical trial of ixabepilone in metastatic breast cancer (MBC) patients previously untreated with taxanes

Abstract

651 Background: Ixabepilone, an epothilone B analog, stabilizes microtubules by binding to tubulin. The response rate (RR) in taxane-pretreated patients at our institution was 22%. Methods: Patients (pts) were eligible if they had MBC previously untreated with taxanes and measurable disease by RECIST criteria. Ixabepilone was given at 6mg/m2/d intravenously days 1–5 every 3 weeks until unacceptable toxicity or disease progression. Primary objectives included RR and toxicity. Pts underwent pre and/or post treatment tumor biopsies for correlative studies. Acetylated α-tubulin, Tau-1, and p53 were stained with anti-acetylated α-tubulin, anti-Tau-1, and anti-p53 antibodies in samples from 13 pts. Staining was scored quantitatively using the Automated Cellular Imaging System. Results: Twenty-three pts received 197 cycles (C). Median of 7C (range 2–22) per pt were administered. Median age was 55 (range 22–79). Seven pts received 1 prior metastatic chemotherapy regimen. Ten of 23 or 43% (exact 95% confidence interval: 23.2% to 65.5%) pts had partial responses (PR), 9 (39%) stable disease (SD) (2 unconfirmed PRs), and 4 (17%) progressive disease (PD). Median time to progression was 5.3 months; median duration of response was 5.4 months from date of best response. Four pts required dose reductions for neutropenia, neuropathy or fatigue. Grade 3/4 toxicities included neutropenia (22%), fatigue (13%), anorexia (9%), infection without neutropenia (9%), motor neuropathy (4%), and muscle weakness (4%). No grade 3/4 sensory neuropathy was seen, but 35% and 13% of pts had grades 1 and 2 neuropathy respectively. Median acetylated α-tubulin at baseline was 0.2 in responders and 17.6 in non-responders (p=0.069). There were no differences in response according to Tau-1 or p53 expression at baseline. Conclusion: Ixabepilone is an effective treatment for MBC with a 43% RR in 23 pts previously untreated with taxanes. There was minimal hematologic toxicity and no grade 3 sensory neuropathy. The use of baseline level of acetylated α-tubulin to predict response may warrant further study. No significant financial relationships to disclose.