Background: ModraDoc006 is a novel formulation of docetaxel and to enhance bioavailability, this tablet is co-administered with ritonavir (r), an inhibitor of cytochrome p450 3A4 and P-glycoprotein. The safety, anti-tumor activity and pharmacokinetics (PK) of ModraDoc006/r were investigated by dose-escalation in patients with mCRPC, to propose a recommended phase 2 dose (RP2D). Methods: Progressive mCRPC patients, who were treatment naïve or previously treated with either abiraterone or enzalutamide, received a maximum of 30 weekly cycles of ModraDoc006/r in a bi-daily once weekly (BIDW) schedule. Plasma docetaxel concentrations were determined at the first 2 cycles up to 48h after intake of ModraDoc006/r. Adverse events were evaluated using the Common Terminology Criteria for Adverse Events (CTCAE) version 4.03. Serum Prostate Specific Antigen (PSA) levels were assessed every 6 weeks. Results: 23 patients were included, of whom 20 were evaluable for safety and PK. In 5 patients, the initial 30-20/100-100 dose was explored, with observation of one dose limiting toxicity (DLT) (grade 3 alanine aminotransferase increase). The next dose level of 30-20/200-200 resulted in 2 DLTs in 6 patients (grade 3 diarrhea and mucositis). Subsequently, two intermediate dose levels: 30-20/200-100 and 20-20/200-100 were explored in 6 and 3 patients. At the 30-20/200-100 dose, an adequate docetaxel exposure was achieved and 1 DLT (grade 3 diarrhea) was observed, with no DLTs at the 20-20/200-100 dose. Common treatment-related AEs (occurring in > 30% of all patients) were diarrhea, nausea, vomiting, anorexia and fatigue. Five patients completed the maximum of 30 weekly treatments. In 10 patients, evaluable for anti-tumor activity after treatment with ≥9 cycles, 4 confirmed and 2 non-confirmed PSA responses ( > 50% decrease) were observed. Conclusions: The RP2D of BIDW ModraDoc006/r in mCRPC was established as 30-20/200-100 mg. These results are encouraging for further development of ModraDoc006/r as a convenient, safe and effective alternative to IV docetaxel for mCRPC patients. A phase 2b study is currently being conducted. Clinical trial information: NCT03136640