Abstract
Background. Lapatinib is an oral small molecule dual tyrosine kinase inhibitor that has been shown to improve time to progression versus capecitabine in women with HER2+ metastatic breast cancer (MBC) previously treated with trastuzumab. Objective. To describe extent, predictors, and consequences of nonadherence with lapatinib in women with MBC who were previously treated with trastuzumab. Methods. This was a retrospective observational study using data from a large health insurance claims databases spanning January 2000 to March 2010. Measures of lapatinib adherence included medication possession ratio (MPR), time to discontinuation (end of supply), time to first treatment interruption (gap during treatment of 30 days without supply), and duration of continuous therapy (time to gap of 30 days without supply or end of supply). Predictors of nonadherence to lapatinib and the association between nonadherence and outcomes, utilization, and costs were examined using multiple regression analysis. Results. A total of 666 patients met all inclusion criteria. Mean initial lapatinib dosage was 1161 mg daily; 63% received index lapatinib in combination with capecitabine. Mean MPR was 87%; 22% of patients had MPR < 80%. Median time to lapatinib discontinuation was 9.1 months (95% confidence interval = 8.0-10.2). Twenty-seven percent of patients had one or more treatment interruptions during follow-up. Median duration of continuous therapy was 5.9 months (95% confidence interval = 5.1-6.1). Concomitant therapy with a taxane was a predictor of nonadherence (odds ratio for MPR < 80% = 10.30; P < .001). There was a statistically significant association between nonadherence to lapatinib and greater number of outpatient visits (P = .028). Conclusions. In women with MBC who were previously treated with trastuzumab, mean adherence to lapatinib in typical clinical practice is relatively high overall, although there is a small group of patients with high nonadherence. Targeted efforts to improve adherence to lapatinib in this subgroup may be warranted.