Dabrafenib, a selective BRAF V600 mutation kinase inhibitor, is clinically approved as monotherapy or in combination with trametinib (a MEK inhibitor) for multiple indications. These include unresectable/metastatic BRAF-mutated melanoma, metastatic non-small cell lung cancer, BRAF V600E-mutated anaplastic thyroid carcinoma, and pediatric recurrent low-grade gliomas with V600 mutations (Dummer et al., 2020; Busaidy et al., 2022; Chou et al., 2022; Atkins et al., 2023; Bouffet et al., 2023). Substantial interindividual variability in systemic exposure to dabrafenib and its metabolite hydroxy-dabrafenib has been documented in clinical cohorts (Puszkiel et al., 2019; Balakirouchenane et al., 2020). Notably, these pharmacokinetic variations show no significant correlation with demographic factors—including age, gender, or body mass index (BMI)—implicating metabolic enzyme activity and drug-drug interactions as primary determinants (Isberner et al., 2022). Cutaneous toxicity constitutes the most frequently reported adverse effect, manifesting as rash, photosensitivity reactions, and hyperkeratosis (Mackin et al., 2019; Bumbacea et al., 2022). Raman spectroscopy analyses have established a direct correlation between epidermal dabrafenib concentration and skin toxicity severity, supporting local drug accumulation as a mechanistic contributor to cutaneous adverse events (Azan et al., 2017). These findings underscore the necessity of individualized dose optimization to balance therapeutic efficacy with toxicity mitigation.

Interindividual variability in cytochrome P450 (CYP450) enzymatic activity significantly influences drug exposure levels (Harpaz et al., 2010; Thiengsusuk et al., 2020; Yu et al., 2023). Kinetic analyses using substrate depletion assays demonstrate that dabrafenib undergoes extensive metabolism primarily mediated by CYP2C8 and CYP3A4, with minor CYP2C9 involvement (Lawrence et al., 2014). Its major metabolites exhibit distinct clearance pathways: hydroxy-dabrafenib is exclusively metabolized by CYP3A4, while desmethyl-dabrafenib is predominantly cleared by CYP3A4, with additional contributions from CYP2C19 and CYP2C9. Carboxy-dabrafenib, however, remains unaffected by the evaluated CYP enzymes (Lawrence et al., 2014; Puszkiel et al., 2019). Critically, both hydroxy- and desmethyl-dabrafenib undergo further CYP3A4-mediated metabolism, suggesting CYP3A4 modulators (inducers/inhibitors) may substantially alter systemic dabrafenib exposure (Lawrence et al., 2014; Balakirouchenane et al., 2020; Nebot et al., 2021; Sorf et al., 2022). This is supported by pharmacokinetic data: ketoconazole (a potent CYP3A4 inhibitor) coadministration increased dabrafenib C_max by 33% and area under curve (AUC) by 71% (Puszkiel et al., 2019; Balakirouchenane et al., 2021).

Despite these insights, the mechanistic basis of dabrafenib’s metabolic regulation remains incompletely characterized, with limited comprehensive pharmacokinetic data available. Dabrafenib exhibits high oral bioavailability (F = 95%) after single-dose administration (Denton et al., 2013). However, repeated dosing induces time-dependent CYP3A4 auto-induction, increasing apparent clearance and reducing plasma exposure (Puszkiel et al., 2019; Balakirouchenane et al., 2021). Although dabrafenib and hydroxy-dabrafenib have been profiled in population pharmacokinetic and drug-drug interactions (DDI) studies (Ellens et al., 2017; Balakirouchenane et al., 2023), further research is needed to fully elucidate the metabolic landscape and clinically relevant interindividual variability in its disposition.

CYP3A4, the predominant CYP450 enzyme responsible for metabolizing 30%–50% of clinical drugs, exhibits distinctive tissue distribution patterns, with highest expression in the liver (constituting 30% of total hepatic CYP content) and intestine (representing 80% of intestinal CYP proteins) (Iversen et al., 2022; Wang et al., 2023). This enzyme demonstrates substantial genetic polymorphism and functional variability (Zanger and Schwab, 2013; Hakkola et al., 2020; Zhou and Lauschke, 2022). However, variant-specific effects on dabrafenib metabolism remain uncharacterized. To address this knowledge gap, we systematically characterized 48 pharmacological agents for drug-drug interaction potential and quantitatively assessed enzymatic kinetics using wild-type CYP3A4.1 alongside 21 genetic variants. This approach provides essential quantitative insights to optimize dabrafenib dosing regimens and advance personalized therapy.

In this study, we conducted an in vitro screening of 48 drugs—including antihistamines, antipyretic analgesics, hypoglycemic agents, gastrointestinal drugs, and antidepressants—to evaluate their effects on dabrafenib metabolism. All screened agents carry clinical relevance for potential co-administration with dabrafenib: antidepressants (e.g., sertraline, mirtazapine) for mood disorders; proton pump inhibitors (PPIs, e.g., omeprazole, lansoprazole) for acid reflux; NSAIDs/COX-2 inhibitors (e.g., celecoxib, diclofenac) for pain management; and antidiabetic agents (e.g., glipizide, metformin) for hyperglycemia control. High-risk scenarios include concomitant use with strong CYP2C8 inhibitors (e.g., clopidogrel-proxy quinidine) or CYP3A4 inhibitors (e.g., fluoxetine), which may elevate dabrafenib exposure and toxicity. The inclusion of metabolic probes such as dextromethorphan (CYP2D6) and chlorzoxazone (CYP2E1) further assessed enzymatic specificity, while widely used agents (e.g., acetaminophen, loratadine) reflect real-world polypharmacy patterns. Thus, this screening panel integrates mechanistic breadth with clinical utility for predicting DDIs during dabrafenib therapy. Among these, eight compounds—involving sertraline, brexpiprazole, lansoprazole, dronedarone hydrochloride, fluoxetine, vortioxetine, and loratadine—exhibited >70% inhibition of dabrafenib metabolism. The identification of these potent inhibitors provides critical data for clinical precautions regarding dabrafenib combination therapies.

Allergic reactions represent a commonly reported adverse event associated with dabrafenib (Choi et al., 2020). To mitigate these effects, loratadine, a second-generation H₁ receptor antagonist and non-sedating antihistamine with established clinical efficacy in managing dermatological allergies (Hunto et al., 2020; AlMasoud et al., 2022), was proposed for co-administration. Its favorable safety profile and demonstrated therapeutic efficacy position it as a viable option for alleviating dabrafenib-induced hypersensitivity reactions while minimizing sedative side effects. Furthermore, loratadine had been proposed as a CYP3A4 inhibitor, suggesting a potential pharmacokinetic interaction with dabrafenib (Li et al., 2010). Investigation revealed loratadine exhibited the highest inhibition extent (>90%), a finding with significant clinical implications. Consequently, given the potential for co-administration, exploration of the interaction between loratadine and dabrafenib warranted investigation.

In vitro experiments confirmed that loratadine significantly inhibited dabrafenib metabolism through a mixed-type inhibition mechanism, involving both non-competitive and uncompetitive components. This inhibitory pattern was consistent in both RLM and HLM. However, we observed notably divergent IC₅₀ values for loratadine between them. The underlying cause of this discrepancy was not explicitly investigated in the present study. Based on prior research findings, the observed IC₅₀ differences are likely attributable to interspecies variations in relative enzyme activities (Zhang et al., 2020). From a clinical relevance perspective, the IC₅₀ values obtained in HLM exceed the blood exposure levels of single-dose dabrafenib in humans, suggesting potential inhibitory effects may occur clinically (Puszkiel et al., 2019). However, the magnitude of inhibition could be modulated by tissue-specific drug distribution (e.g., intestinal/skin exposure), metabolizer phenotypes (e.g., CYP3A4 poor/rapid metabolizers), and chronic dosing cumulative effects. To further elucidate the molecular basis of this interaction, we employed molecular docking to simulate the binding of CYP3A4 with dabrafenib and loratadine. The results studied further demonstrated that dabrafenib bonded to CYP3A4 residues through hydrophobic interactions. In contrast, loratadine occupied a distinct binding site, inducing conformational changes that sterically hindered dabrafenib-CYP3A4 interactions and altered the substrate selectivity and structural specificity of dabrafenib metabolism.

In vivo experiments demonstrated that loratadine significantly altered the pharmacokinetic profile of dabrafenib in rats. Co-administration of dabrafenib with loratadine resulted in substantial increases in AUC_(0-t), AUC(0- ∞ ), C_max, and t_1/2, alongside significant decreases in CL_z/F and V_Z/F. The observed reduction in V_z/F likely stemmed from enhanced bioavailability (via gastrointestinal CYP3A4 inhibition) and diminished hepatic first-pass metabolism, consistent with prior evidence that metabolic drug interactions minimally affected distribution volume (Sodhi et al., 2021; Murata et al., 2023). These alterations elevated plasma dabrafenib concentrations, increasing the risk of toxicity due to accumulation. These findings implied that co-administration with loratadine led to elevated plasma exposure, increased bioavailability, reduced apparent clearance, slower metabolism, and potential drug accumulation, which may heighten the risk of adverse reactions due to elevated plasma concentrations. Clinical pharmacokinetic studies demonstrated that a single 150 mg oral dose of dabrafenib achieved C_max of 2,160 ng/mL Although the measured IC₅₀ values (52.40 ± 4.63 µM in HLM) substantially exceeded therapeutically relevant free plasma concentrations (≈6.3 nM), a comprehensive clinical risk assessment required consideration of: tissue-specific exposure profiles, metabolic enzyme phenotypes, accumulation potential during chronic dosing (Puszkiel et al., 2019; Balakirouchenane et al., 2021).

CYP3A4 plays an integral role in drug metabolism and exhibits considerable genetic polymorphism, resulting in diverse pharmacogenetic phenotypes that influence individual drug response (Guttman et al., 2019; Šemeláková et al., 2021; Zhang et al., 2024). Unlike other cytochrome P450 enzymes, CYP3A4 currently lacks published pharmacogenetic guidelines, primarily due to its complex enzymatic kinetics. This complexity stems partly from the enzyme’s large active site pocket, which accommodates a wide range of substrates with variable metabolic kinetics (Guengerich et al., 2021; Feng et al., 2024). This structural heterogeneity complicates the use of substrate probes and underscores the need for continued research into novel substrates to better elucidate CYP3A4’s pharmacogenetic landscape (Ducharme et al., 2021).

CYP3A4 serves as the primary enzyme responsible for the oxidative metabolism of dabrafenib. Given the significant genetic polymorphism of CYP3A4, variations in dabrafenib blood exposure levels among individuals could lead to divergent metabolic outcomes. However, the impact of CYP3A4 genetic polymorphisms on dabrafenib metabolism remained poorly characterized. This investigation utilized wild-type CYP3A4.1 as a benchmark to assess the functional impacts of 21 CYP3A4 variants on dabrafenib metabolism. Detailed enzyme kinetic analyses were performed on representative variants CYP3A4.8 and CYP3A4.28. Compared to wild-type CYP3A4.1, CYP3A4.8 exhibited significantly diminished metabolism, reflected in a substantially reduced relative clearance, while CYP3A4.28 demonstrated increased clearance. These findings suggest that patients harboring slow-metabolizing variants may require consideration of dose reduction or alternative medications during dabrafenib therapy to mitigate the risk of serious adverse reactions. Conversely, patients with fast-metabolizing variants may risk sub-therapeutic exposure. Additionally, CYP3A4.20, a known non-functional allele, was confirmed to lack catalytic activity towards dabrafenib, consistent with previous reports (Riera et al., 2018; Liu et al., 2023). Collectively, these data highlight the profound impact of CYP3A4 genetic polymorphisms on dabrafenib metabolism and underscore the necessity of genotype-guided dosing strategies to optimize therapeutic efficacy and minimize adverse effects.

This study systematically investigated the drug interaction profile of dabrafenib, revealed that loratadine exerted significant inhibitory effects on its metabolism in both in vitro and in vivo settings. Furthermore, CYP3A4 genetic polymorphisms significantly altered dabrafenib metabolic clearance, which highlighted the complex interplay between drug-drug interactions and pharmacogenetic variability in dabrafenib metabolism. Our findings highlight the necessity of genotyping CYP3A4 variants and avoiding loratadine co-administration to optimize dabrafenib dosing and minimize toxicity risks.