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Background: Troxacitabine is a novel L-nucleoside analogue. Preclinical studies showed improved activity with infusions of at least 3 days compared with bolus regimens, especially at concentrations >20 ng/ml. This phase I study tested the feasibility of achieving a troxacitabine steady-state concentration of 20 ng/ml for at least 72 h in patients with solid tumors. Patients and methods: Patients with solid tumors received troxacitabine as a progressively longer infusion on days 1–4 of a 28-day cycle. The initial length of infusion and infusion rate were 48 h and 3 mg/m2/day. Results: Twenty-one patients were treated at infusion lengths that increased from 48 to 72 h and then 96 h. The infusion rate was decreased from 3 to 1.88 mg/m2/day due to toxicity. Dose-limiting toxicities consisted of grade 4 neutropenia (three) and grade 3 constipation (one). The maximum tolerated dose of continuous infusion troxacitabine in patients with solid tumors is 7.5 mg/m2 administered over 96 h. This dose level resulted in steady-state drug concentration of at least 20 ng/ml for 72 h. Conclusions: Administration of troxacitabine by continuous infusion achieved the prospectively defined target plasma concentration. Pharmacokinetics (PK) modeling coupled with real-time PK assessment was an efficient approach to conduct hypothesis-driven phase I trials. [ABSTRACT FROM AUTHOR]