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Simple Summary: In melanoma patients treated with dabrafenib and trametinib, dose reductions and treatment discontinuations related to adverse events (AE) occur frequently. However, the associations between patient characteristics, AE, and exposure are unclear. Our prospective study analyzed serum (hydroxy-)dabrafenib and trametinib exposure and investigated its association with toxicity and patient characteristics. Additionally, the feasibility of at-home sampling of capillary blood was assessed, and a model to convert capillary blood concentrations to serum concentrations was developed. (Hydroxy-)dabrafenib or trametinib exposure was not associated with age, sex, body mass index, or AE. Co-medication with P-glycoprotein inducers was associated with lower trough concentrations of trametinib but not (hydroxy-)dabrafenib. The applicability of the self-sampling of capillary blood was demonstrated. Our conversion model was adequate for estimating serum exposure from micro-samples. The monitoring of dabrafenib and trametinib may be useful for dose modification and can be optimized by at-home sampling and our new conversion model. Patients treated with dabrafenib and trametinib for BRAFV600-mutant melanoma often experience dose reductions and treatment discontinuations. Current knowledge about the associations between patient characteristics, adverse events (AE), and exposure is inconclusive. Our study included 27 patients (including 18 patients for micro-sampling). Dabrafenib and trametinib exposure was prospectively analyzed, and the relevant patient characteristics and AE were reported. Their association with the observed concentrations and Bayesian estimates of the pharmacokinetic (PK) parameters of (hydroxy-)dabrafenib and trametinib were investigated. Further, the feasibility of at-home sampling of capillary blood was assessed. A population pharmacokinetic (popPK) model-informed conversion model was developed to derive serum PK parameters from self-sampled capillary blood. Results showed that (hydroxy-)dabrafenib or trametinib exposure was not associated with age, sex, body mass index, or toxicity. Co-medication with P-glycoprotein inducers was associated with significantly lower trough concentrations of trametinib (p = 0.027) but not (hydroxy-)dabrafenib. Self-sampling of capillary blood was feasible for use in routine care. Our conversion model was adequate for estimating serum PK parameters from micro-samples. Findings do not support a general recommendation for monitoring dabrafenib and trametinib but suggest that monitoring can facilitate making decisions about dosage adjustments. To this end, micro-sampling and the newly developed conversion model may be useful for estimating precise PK parameters. [ABSTRACT FROM AUTHOR]

• First LC–MS/MS method to quantify hydroxy-dabrafenib in human plasma. • Practical method to simultaneously quantify dabrafenib, hydroxy-dabrafenib and trametinib in BRAF-mutated cancer patients. • Useful for plasma drug monitoring. A new liquid chromatography-tandem mass spectrometry (LC–MS/MS) method for the simultaneous quantification of dabrafenib (DAB), its main metabolite hydroxy-dabrafenib (OHD) and trametinib (TRA) in human plasma has been developed and validated. After addition of internal standard (dabrafenib-d9), extraction was achieved after protein precipitation with acetonitrile containing 1 % (v/v) formic acid. Chromatographic separation was performed on an Accucore® C18 (2.1 × 50 mm; 2.6 μm) column using a gradient elution of water acidified with 0.1 % (v/v) formic acid (A) and acetonitrile containing 0.1 % (v/v) formic acid (B) at a flow rate of 500 μL/min. The calibration ranged from 10 to 2000 ng/mL for DAB and OHD and from 5 to 50 ng/mL for TRA. This method was validated with satisfactory results including good precision (intra- and inter-assay coefficient of variation from 2.0 %–14.9 %) and good accuracy (inter- and intra-day bias between -1.2 % and 10.9 %), as well as long term stability in unprocessed plasma at −20 °C. This newly proposed method is useful for clinical research purposes as well as therapeutic drug monitoring for patients with a Rapidly Accelerated Fibrosarcoma kinase B (BRAF)-mutated cancer. [ABSTRACT FROM AUTHOR]

Patients treated with dabrafenib/trametinib (DAB/TRA) exhibit a large interindividual variability in clinical outcomes. The aims of this study were to characterize the pharmacokinetics of DAB, hydroxy-dabrafenib (OHD), and TRA in BRAF-mutated patients and to investigate the exposure–response relationship for toxicity and efficacy in metastatic melanoma (MM) patients. Univariate Fisher and Wilcoxon models including drug systemic exposure (area under the plasma concentration curve, AUC) were used to identify prognostic factors for the onset of dose-limiting toxicities (DLT), and Cox models for overall (OS) and progression-free survival (PFS). Seventy-three BRAF-mutated patients were included in pharmacokinetic (n = 424, NONMEM) and 52 in pharmacokinetic/pharmacodynamic analyses. Age and sex were identified as determinants of DAB and OHD clearances (p < 0.01). MM patients experiencing DLT were overexposed to DAB compared to patients without DLT (AUC: 9624 vs. 7485 ng∙h/mL, respectively, p < 0.01). Eastern Cooperative Oncology Group Performance Status (ECOG PS) ≥ 2 and plasma ratio AUCOHD/AUCDAB ≥ 1 were independently associated with shorter OS (HR: 6.58 (1.29–33.56); p = 0.023 and 10.61 (2.34–48.15), p = 0.022, respectively). A number of metastatic sites ≥3 and cerebral metastases were associated with shorter PFS (HR = 3.25 (1.11–9.50); p = 0.032 and HR = 1.23 (1.35–10.39), p = 0.011; respectively). TRA plasma exposure was neither associated with toxicity nor efficacy. Our results suggest that early drug monitoring could be helpful to prevent the onset of DLT in MM patients, especially in fragile patients such as the elderly. Regarding efficacy, the clinical benefit to monitor plasma ratio AUCOHD/AUCDAB deserves more investigation in a larger cohort of MM patients. [ABSTRACT FROM AUTHOR]