Mechanism of drug resistance in relation to site of metastasis: Meta-analyses of randomized controlled trials in advanced breast cancer according to anticancer strategy

Combination of targeted agents and endocrine therapy results in concordant, superior PFS suggesting targetable endocrine resistance across metastatic sites. Discordant responses with endocrine strategy alone support use of targeted therapy, rather than change in endocrine agent at disease progression. HER2 targeted therapies may be less effective in areas of poor vascularization.