BACKGROUND: Non-small cell lung cancer (NSCLC), ovarian and breast cancers, especially in advanced stages, are difficult to treat using chemotherapy, and novel treatments are required to improve the outcome for the large numbers of patients who relapse after receiving the most effective first- and second-line treatments currently available. This paper reviews the results from three trials of ZD9331, a novel, direct-acting antifolate, in patients with relapsed or refractory solid tumours. PATIENTS AND METHODS: Patients with relapsed or refractory NSCLC, ovarian or breast cancer were included in these three open-label, multicentre trials. All three trials included an i.v. arm of ZD9331 at a dose of 130 mg/m2; the ovarian study also included a 65 mg/m2 i.v. treatment arm and the breast cancer trial included a 3 mg oral treatment arm. Patients received ZD9331 as a 30-min i.v. infusion once weekly for 2 weeks followed by 1 week without treatment (3-week cycle). Oral ZD9331 was given once daily for 28 consecutive days and repeated every 6 weeks. RESULTS: One hundred and eighty-nine patients were included in the three trials (NSCLC: n = 46; ovarian: n = 80; breast: n = 63). Neutropenia (45-59%), asthenia (25-42%) and nausea (41-59%) were amongst the most common adverse events observed in all three trials; however, in the oral treatment group of the breast cancer trial anaemia (58%) and increased alanine aminotransferase (45%) or aspartate aminotransferase (39%) were also frequent. There were no objective responses seen in the NSCLC trial; 20 of 46 patients (43.5%) experienced a best overall response of stable disease. Objective response rates (ORRs) in the ovarian trial were 2.5% (one patient) and 10% (four patients) in the 65 and 130 mg/m2 treatment arms, respectively. In the breast cancer trial ORRs were 9.7% (three patients) and 12.5% (four patients) in the oral and i.v. groups, respectively. CONCLUSIONS: ZD9331 has a manageable toxicity profile, and shows some evidence of activity in patients with relapsed or refractory NSCLC, ovarian and breast cancer.