Indibulin (ZIO-301/D-24851) is an orally applied small molecule with antitumor activity based upon destabilization of microtubule polymerization. The purpose of this phase I study was to determine the maximum tolerated dose (MTD) as well as the dose limiting toxicity (DLT), the pharmacokinetics, safety and tolerability of orally administered indibulin as capsule formulation in patients with advanced solid tumors. Patients received a single dose of indibulin. Seven dose-levels were evaluated: 100 mg, 150 mg, 250 mg, 350 mg and 600 mg once daily (QD), 450 mg and 600 mg twice daily (BID). After a washout period, patients received indibulin at the pre-defined daily dose for 14 days every 3 weeks (multiple dose part). A total of 28 patients entered the study. Indibulin administered as capsules was generally well tolerated. The MTD was not reached. There was a disproportionate increase of the area under the plasma concentration-time curve (AUC) with dose, with declining AUC corrected for dose starting at the 250 mg dose-level. There was no significant difference in AUC of indibulin after multiple dosing (day 1-14) compared to single administration (day-4). Inter-patient variability in AUC (102% CV) was high. A plateau in drug exposure was observed prior to reaching the MTD. Continued dose-escalation was unlikely to yield any increase in exposure of indibulin. The formulation needs optimization to increase the systemic exposure upon oral administration.