Purpose: Diflomotecan, a homocamptothecin, targets DNA topoisomerase I. Previous clinical trials have demonstrated a variable degree of dose limiting toxicity. The purpose of this study was to further evaluate the safety and pharmacokinetic profile of a range of diflomotecan doses administered intravenously. Methods: Patients with advanced solid malignant tumours, refractory to standard therapies, with adequate haematologic, renal and hepatic function, received diflomotecan administered as a 20 min intravenous infusion every 21 days. Cohorts of six patients were recruited sequentially to one of three fixed starting dose groups-2, 4, or 7 mg, with drug administered by fixed-dose rather than dosing by body surface area. Pharmacokinetic analyses were performed on serial blood samples taken over the first 24 h after diflomotecan administration (cycles 1 and 2). Cytochrome P450 3A4 (CYP3A4) activity was determined by an erythromycin breath test (EBT) prior to diflomotecan administration in cycles 1 and 2. Results: Thirteen patients, were treated with a starting dose of either 2 mg (n = 8) or 4 mg (n = 5) of diflomotecan. Dose limiting toxicities (DLTs) were observed in 1 patient in the 2 mg starting dose level (grade 4 neutropenia which lasted for 8 days), and in 2 of 5 patients enrolled at the 4 mg starting dose level (grade 4 neutropenia for 11 days; grade 4 neutropenia leading to withdrawal from the study), and no further dose escalation was performed. Pharmacokinetic analyses revealed a less than dose-proportional increase in diflomotecan and for the two metabolites BN80942 and P-20, with a magnitude of P-20 exposure similar to the parent drug. There was a high inter-patient variability in diflomotecan exposure similar to that observed with other camptothecin derivatives. One minor response was observed in a patient with oesophageal cancer. Conclusions: Diflomotecan administered as a 20-min intravenous infusion 3-weekly is characterised by a variable pharmacokinetic profile. Alternative oral dosing schedules of diflomotecan have been shown to display a more predictable PK/PD and safety profile and should be selected for further evaluation in Phase II clinical trials.