Influence of probenecid on endoxifen systemic exposure in breast cancer patients on adjuvant tamoxifen treatment

Introduction: In breast cancer patients treated with the anti-estrogen tamoxifen, low concentrations of the active metabolite endoxifen are associated with more disease recurrence. We hypothesized that we could increase endoxifen concentrations by induction of its formation and inhibition of its metabolism by co-administration of probenecid. Methods: We conducted a crossover study and measured endoxifen concentrations in patients on steady-state tamoxifen monotherapy and after 14 days of combination treatment with probenecid. Eleven evaluable patients were included. Results: Treatment with tamoxifen and probenecid resulted in a 26% increase of endoxifen area under the plasma concentration–time curve from 0 to 24 h (AUC0–24h) compared to tamoxifen monotherapy (95% confidence interval [CI]: 8–46%; p < 0.01), while the maximum observed endoxifen concentration increased with 24% (95% CI: 7–44%; p < 0.01). The metabolic ratio of endoxifen to tamoxifen increased with 110% (95% CI: 82–143%; p < 0.001) after the addition of probenecid. Conclusion: Probenecid resulted in a clinically relevant increase of endoxifen concentrations in breast cancer patients treated with adjuvant tamoxifen. This combination therapy could provide a solution for patients with a CYP2D6-poor metabolizer phenotype or endoxifen concentrations below the threshold despite earlier tamoxifen dose

Therapeutic Drug Monitoring of Endoxifen for Tamoxifen Precision Dosing: Feasible in Patients with Hormone-Sensitive Breast Cancer

Background: Endoxifen is the most important active metabolite of tamoxifen. Several retrospective studies have suggested a minimal or threshold endoxifen systemic concentration of 14–16 nM is required for a lower recurrence rate. The aim of this study was to investigate the feasibility of reaching a predefined endoxifen level of ≥ 16 nM (5.97 ng/mL) over time using therapeutic drug monitoring (TDM). Methods: This prospective open-label intervention study enrolled patients who started treatment with a standard dose of tamoxifen 20 mg once daily for early breast cancer. An outpatient visit was combined with a TDM sample at 3, 4.5, and 6 months after initiation of the tamoxifen treatment. The tamoxifen dose was escalated to a maximum of 40 mg if patients had an endoxifen concentration < 16 nM. The primary endpoint of the study was the percentage of patients with an endoxifen level ≥ 16 nM at 6 months after the start of therapy compared with historical data, in other words, 80% of patients with endoxifen levels ≥ 16 nM with standard therapy. Results: In total, 145 patients were included. After 6 months, 89% of the patients had endoxifen levels ≥ 16 nM, compared with a literature-based 80% of patients with endoxifen levels ≥ 16 nM at baseline (95% confidence interval 82–94; P = 0.007). In patients with an affected CYP2D6 allele, it was not always feasible to reach the predefined endoxifen level of ≥ 16 nM. No increase in tamoxifen-related adverse events was reported after dose escalation. Conclusion: This study demonstrated that it is feasible to increase the percentage of patients with endoxifen levels ≥ 16 nM using TDM. TDM is a safe strategy that offers the possibility of nearly halving the number of patients with endoxifen levels < 16 nM

A randomized phase 2 study exploring the role of bevacizumab and a chemotherapy-free approach in HER2-positive metastatic breast cancer: The HAT study (BOOG 2008-2003), a Dutch Breast Cancer Research Group trial

BACKGROUND: To explore the role of bevacizumab and a chemotherapy-free approach, the authors evaluated the combination of bevacizumab, trastuzumab, and paclitaxel (HAT) and the regimen of trastuzumab and bevacizumab (HA) with the addition of paclitaxel after progression (HA-HAT) as first-line treatment for patients with human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer. METHODS: In a noncomparative phase 2 trial, patients were randomized between HAT and HA-HAT. The primary endpoint was the progression-free rate at 1 year (1-year PFR). In the HA-HAT group, progression-free survival (PFS) was separately established for HA (PFS1) and HAT (PFS2). RESULTS: Eighty-four patients received HAT (n = 39) or HA-HAT (n = 45). The 1-year PFR was 74.4% (95% confidence interval [CI], 61.8%-89.4%) and 62.2% (95% CI, 49.6%-89.4%) in the HAT and HA-HAT arms, respectively. The median PFS was 19.8 months (95% CI, 14.9-25.6 months) in the HAT arm and 19.6 months (95% CI, 12.0-32.0 months) in the HA-HAT arm. In the HA-HAT arm, the median PFS1 was 10.4 months (95% CI, 6.2-15.0 months), and the median PFS2 was 8.2 months (95% CI, 7.0-12.6 months). The number and severity of adverse events were comparable between the arms. CONCLUSIONS: Both HAT and HA-HAT have promising activity in patients with HER2-positive metastatic breast cancer. In particular, starting with only targeted agents and delaying chemotherapy is worth further exploration