Abstract: DHP107 is a newly developed lipid-based oral formulation of paclitaxel. We evaluated the in vivo tissue pharmacokinetics (PKs) of DHP107 in mice and patients using positron emission tomography (PET). Radioisotope-labeled [3H]DHP107 and [18F]DHP107 for oral administration were formulated in the same manner as the manufacturing process of DHP107. In vivo tissue PK were assessed in healthy ICR mice and breast cancer xenografted SCID mice. Two patients with metastatic breast cancer were clinically evaluated for absorption at the target lesion after internal absorbed dose estimation. Whole-body PET/computed tomography data were acquired in healthy and xenografted mice and in patients up to 10–24 h after administration. Tissue [18F]DHP107 signals were plotted against time and PK parameters were determined. The amounts of radioactivity in various organs and excreta were determined using a beta-counter and are expressed as the percentage of injected dose (ID). Oral [18F]DHP107 was well-absorbed and reached the target lesion in mice and patients with breast cancer. Significant amounts of radioactivity were found in the stomach, intestine, and liver after oral administration of [3H]- and [18F]DHP107 in healthy mice. The [18F]DHP107 reached a peak distribution of 0.7–0.8%ID in the tumor at 5.6–7.3 h in the xenograft model. The [18F]DHP107 distribution in patients with metastatic breast cancer was the highest at 3–4 h postadministration. Systemic exposures after administration of a DHP107 therapeutic dose were comparable with those in previous studies. PET using radioisotope-labeled drug candidates is useful for drug development and can provide valuable information that can complement plasma PK data, particularly in early phase clinical trials.