As many breast cancer patients receive adjuvant chemotherapy using anthracyclines or anthracenediones and taxanes, more therapeutic options are needed for subsequent lines of therapy. Pemetrexed (ALIMTA, multitargeted antifolate, LY231514) is a novel antifolate that inhibits several enzymes in the de novo pathways of pyrimidine and purine biosynthesis. This paper reports on a subset analysis of a phase II clinical trial of pemetrexed in heavily pretreated metastatic breast cancer (MBC) patients. Patients were required to have received prior first-line anthracycline therapy for metastatic disease. Prior adjuvant chemotherapy and prior taxanes were allowed. A substantial subset of the study population (31 of 72 patients, 43%) had also received a taxane in the metastatic setting. All patients were treated with pemetrexed, 600 mg/m2 by intravenous infusion, once every 21 days. In the study subset, 23 of 31 (74%) patients were anthracyclines failures (progression > 30 days following treatment), and eight (26%) patients were anthracyclines refractory (progression during or < or = 30 days of treatment). The median age was 55 years (range, 30-75 years) and the median World Health Organization performance status was 0. Metastases were present in the liver (61%), lung (29%), bone (6%), and soft tissue (19%). The overall response rate for this subset was 26%, with one complete response, seven partial responses, and 13 (42%) patients with stable disease. The median duration of response was 5.4 months and median survival was 12.8 months. Pemetrexed was well tolerated by patients in the study. This post hoc analysis suggests promising activity in MBC patients previously treated with both anthracyclines and taxanes. An ongoing trial is prospectively evaluating activity in this same population.