ModraDoc006, an oral docetaxel formulation in combination with ritonavir (ModraDoc006/r), in metastatic castration‐resistant prostate cancer patients: A phase Ib study

BACKGROUND: ModraDoc006 is an oral formulation of docetaxel, which is co‐administered with the cytochrome P450 3A4 and P‐glycoprotein inhibitor ritonavir (r): ModraDoc006/r. Weekly treatment with ModraDoc006/r had been evaluated in phase I trials in patients with different types of advanced solid tumors, but up to this point in time not in patients with metastatic castration‐resistant prostate cancer (mCRPC). AIM: We assessed safety and pharmacokinetics (PK) of ModraDoc006/r to establish the recommended phase 2 dose (RP2D) in patients with mCRPC. METHODS: mCRPC patients, treatment naïve or following abiraterone or enzalutamide treatment, were included. Dose‐escalation of ModraDoc006/r was based on safety and docetaxel PK. Antitumor activity was assessed by serum prostate‐specific antigen (PSA) and radiological evaluation. RESULTS: Cohort 1 (n = 5) received once weekly ModraDoc006 30 mg with ritonavir 100 mg in the morning, and ModraDoc006 20 mg with ritonavir 100 mg in the evening (30‐20/100‐100). The mean docetaxel area under the plasma concentration‐time curve (mAUC0‐inf) was 461 ng/mL × h with 1 dose limiting toxicity (DLT); grade 3 alanine transferase increase. In cohort 2 (n = 6, ModraDoc006/r 30‐20/200‐200), the mAUC0‐inf was 1687 ng/mL × h with 2 DLTs; grade 3 diarrhea and mucositis. In cohort 3A (n = 6, ModraDoc006/r 30‐20/200‐100), the mAUC0‐inf was 1517 ng/mL × h with 1 DLT; grade 3 diarrhea. In cohort 3B (n = 3, ModraDoc006/r 20‐20/200‐100), the mAUC0‐inf was 558 ng/mL × h without DLTs. The mAUC0‐inf exceeded estimated exposures of intravenous docetaxel in cohort 2 and 3A, was lower in cohort 1 and was in range in cohort 3B. PSA decreases of >50% occurred in 6/10 evaluable patients throughout the various cohorts. In five radiological evaluable patients, two confirmed partial responses were observed. CONCLUSION: The RP2D was established at weekly ModraDoc006/r 30‐20/200‐100. Observed PSA and radiological responses suggest promising clinical activity. These results have led to an ongoing randomized Phase 2b study, comparing weekly ModraDoc006/r with 3‐weekly IV docetaxel in patients with mCRPC.