Metabolism and disposition of oral dabrafenib in cancer patients: proposed participation of aryl nitrogen in carbon-carbon bond cleavage via decarboxylation following enzymatic oxidation

Development and validation of an LC-MS/MS method to measure the BRAF inhibitors dabrafenib and encorafenib quantitatively and four major metabolites semi-quantitatively in human plasma

This article describes the development and validation of a liquid-chromatography coupled with tandem mass spectrometry (LC-MS/MS) assay for the simultaneous quantitation of the BRAF inhibitors dabrafenib and encorafenib, and semi-quantitation of their major metabolites (i.e., carboxy-dabrafenib, desmethyl-dabrafenib, hydroxy-dabrafenib, M42.5A) in human plasma. Analytes were extracted from human plasma by protein precipitation, followed by reversed phase high-performance liquid chromatography. Analyte detection was performed using tandem mass spectrometry with heated electrospray ionization operating in positive ion mode. The assay was validated in accordance with the current U.S. Food and Drug Administration Guidance on Bioanalytical Method Validation. Results showed that measurements were both accurate (94.6-112.0 %) and precise (within-run: 1.9-3.4 %; between-run: 1.7-12.0 %) spanning a concentration range of 5 to 2000 ng/mL for dabrafenib and 10 to 4000 ng/mL for encorafenib. Recoveries for these analytes were consistent with mean values ranging from 85.6 % to 90.9 %. The mean internal standard-normalized matrix factors for each drug ranged between 0.87 and 0.98 and were found to be precise (% RSD <6.4 %). Dabrafenib and encorafenib were stable in the final extract and in human plasma held under various storage conditions. The metabolites also passed the validation criteria for precision and selectivity. Finally, the clinical applicability of the assay was confirmed by (semi-)quantitation of all six analytes in plasma samples from cancer patients receiving standard-of-care treatment with dabrafenib and encorafenib. Reproducibility of the measured analyte concentrations in study samples was confirmed successfully by incurred sample reanalysis. In conclusion, this sensitive LC-MS/MS assay has been validated successfully and is suitable for therapeutic drug monitoring of dabrafenib and encorafenib and clinical pharmacokinetic studies with these BRAF inhibitors.

Mechanistically Coupled PK (MCPK) Model to Describe Enzyme Induction and Occupancy Dependent DDI of Dabrafenib Metabolism

Dabrafenib inhibits the cell proliferation of metastatic melanoma with the oncogenic BRAF(V600)-mutation. However, dabrafenib monotherapy is associated with pERK reactivation, drug resistance, and consequential relapse. A clinical drug-dose determination study shows increased pERK levels upon daily administration of more than 300 mg dabrafenib. To clarify whether such elevated drug concentrations could be reached by long-term drug accumulation, we mechanistically coupled the pharmacokinetics (MCPK) of dabrafenib and its metabolites. The MCPK model is qualitatively based on in vitro and quantitatively on clinical data to describe occupancy-dependent CYP3A4 enzyme induction, accumulation, and drug–drug interaction mechanisms. The prediction suggests an eight-fold increase in the steady-state concentration of potent desmethyl-dabrafenib and its inactive precursor carboxy-dabrafenib within four weeks upon 150 mg b.d. dabrafenib. While it is generally assumed that a higher dose is not critical, we found experimentally that a high physiological dabrafenib concentration fails to induce cell death in embedded 451LU melanoma spheroids.

A Review of CYP3A Drug-Drug Interaction Studies: Practical Guidelines for Patients Using Targeted Oral Anticancer Drugs

Many oral anticancer drugs are metabolized by CYP3A. Clinical drug-drug interaction (DDI) studies often only examine the effect of strong CYP3A inhibitors and inducers. The effect of moderate or weak inhibitors or inducers can be examined using physiologically based pharmacokinetic simulations, but data from these simulations are not always available early after approval of a drug. In this review we provide recommendations for clinical practice on how to deal with DDIs of oral anticancer drugs if only data from strong CYP3A inhibitors or inducers is available. These recommendations were based on reviewed data of oral anticancer drugs primarily metabolized by CYP3A and approved for the treatment of solid tumors from January 1st, 2013 to December 31st, 2015. In addition, three drugs that were registered before the new EMA guideline was issued (i.e., everolimus, imatinib, and sunitinib), were reviewed. DDIs are often complex, but if no data is available from moderate CYP3A inhibitors/inducers, a change in exposure of 50% compared with strong inhibitors/inducers can be assumed. No a priori dose adaptations are indicated for weak inhibitors/inducers, because their interacting effect is small. In case pharmacologically active metabolites are involved, the metabolic pathway, the ratio of the parent to the metabolites, and the potency of the metabolites should be taken into account.

Evaluation of the Effects of Repeat-Dose Dabrafenib on the Single-Dose Pharmacokinetics of Rosuvastatin (OATP1B1/1B3 Substrate) and Midazolam (CYP3A4 Substrate)

Dabrafenib is an oral BRAF kinase inhibitor approved for the treatment of various BRAF V600 mutation–positive solid tumors. In vitro observations suggesting cytochrome P450 (CYP) 3A induction and organic anion transporting polypeptide (OATP) inhibition prompted us to evaluate the effect of dabrafenib 150 mg twice daily on the pharmacokinetics of midazolam 3 mg (CYP3A substrate) and rosuvastatin 10 mg (OATP1B1/1B3 substrate) in a clinical phase 1, open‐label, fixed‐sequence study in patients with BRAF V600 mutation–positive tumors. Repeat dabrafenib dosing resulted in a 2.56‐fold increase in rosuvastatin maximum observed concentration (C_max), an earlier time to C_max, but only a 7% increase in area under the concentration‐time curve from time 0 (predose) extrapolated to infinite time. Midazolam C_max and AUC extrapolated to infinite time decreased by 47% and 65%, respectively, with little effect on time to C_max. No new safety findings were reported. Exposure of drugs that are CYP3A4 substrates is likely to decrease when coadministered with dabrafenib. Concentrations of medicinal products that are sensitive OATP1B1/1B3 substrates may increase during the absorption phase.

Tandem mass spectrometry of small-molecule signal transduction inhibitors: Accurate-m/z data to adapt structure proposals of product ions

Protein kinases inhibitors or, more generally, signal transduction inhibitors (STIs) can be used to treat diseases in which deregulation of the protein kinase activity plays a role, such as in cancer. A wide variety of drugs has been developed and/or is under investigation to act as protein kinase inhibitors, especially in tyrosine kinase inhibition. The bioanalysis of STIs has received considerable attention in the past 20 years. Liquid chromatography-tandem mass spectrometry (LC-MS-MS) in selected-reaction monitoring (SRM) mode is the method-of-choice in such studies. In several of these studies from us and others, structures are proposed for the product ions applied in SRM. A critical review of these proposed structures is presented using accurate-m/z data, which we have now generated with a linear-ion-trap-Orbitrap hybrid mass spectrometer. This led to adaptation and new structural proposals of 18 product ions for 13 STIs. Our investigation endorses the power of accurate-m/z analysis in structure elucidation of product ions in bioanalytical LC-MS-MS studies and for which the SRM mode in tandem-quadrupole instruments is apparently less suitable.

Biliary Excretion-Mediated Food Effects and Prediction

Many orally administered drugs with negative food effects (i.e., lower exposure under fed conditions) are often primarily or partially eliminated by biliary excretion. The aim of this study is to assess the potential correlation between a negative food effect and biliary excretion. Correlation analysis was conducted using a training dataset containing 27 drugs which met the following criteria: (1) immediate-release formulations, (2) shows a negative food effect, (3) > 10% biliary clearance, and (4) does not undergo extensive metabolism. A correlation between fed-state biliary clearance (CL_b,fed) and fasted-state biliary clearance (CL_b,fast) (y = 1.81*x, R² = 0.68) was observed. The 1.8-fold increase in biliary clearance was then used as a correction factor to improve physiologically based pharmacokinetic (PBPK) prediction of food effects for 12 test drugs. The mean deviations of predicted fed/fasting AUC ratio and C_max ratio from clinically observed values were reduced from 32.4 to 17.2% and from 63.3 to 54.3%, respectively. In contrast to the positive food effects on most biopharmaceutics classification system (BCS) class II drugs for which food-stimulated bile flow increases drug solubility and absorption, our results suggest that the elimination of biliary excreted drugs is increased by food-stimulated bile flow, resulting in negative food effects.

Five-Year Analysis of Adjuvant Dabrafenib plus Trametinib in Stage III Melanoma

BACKGROUND

In the previously reported primary analysis of this phase 3 trial, 12 months of adjuvant dabrafenib plus trametinib resulted in significantly longer relapse-free survival than placebo in patients with resected stage III melanoma with BRAF V600E or V600K mutations. To confirm the stability of the relapse-free survival benefit, longer-term data were needed.

METHODS

We randomly assigned 870 patients who had resected stage III melanoma with BRAF V600E or V600K mutations to receive 12 months of oral dabrafenib (at a dose of 150 mg twice daily) plus trametinib (2 mg once daily) or two matched placebos. The primary end point was relapse-free survival. Here, we report 5-year results for relapse-free survival and survival without distant metastasis as the site of the first relapse. Overall survival was not analyzed, since the required number of events to trigger the final overall survival analysis had not been reached.

RESULTS

The minimum duration of follow-up was 59 months (median patient follow-up, 60 months for dabrafenib plus trametinib and 58 months for placebo). At 5 years, the percentage of patients who were alive without relapse was 52% (95% confidence interval [CI], 48 to 58) with dabrafenib plus trametinib and 36% (95% CI, 32 to 41) with placebo (hazard ratio for relapse or death, 0.51; 95% CI, 0.42 to 0.61). The percentage of patients who were alive without distant metastasis was 65% (95% CI, 61 to 71) with dabrafenib plus trametinib and 54% (95% CI, 49 to 60) with placebo (hazard ratio for distant metastasis or death, 0.55; 95% CI, 0.44 to 0.70). No clinically meaningful between-group difference in the incidence or severity of serious adverse events was reported during the follow-up period.

CONCLUSIONS

In the 5-year follow-up of a phase 3 trial involving patients who had resected stage III melanoma with BRAF V600E or V600K mutations, 12 months of adjuvant therapy with dabrafenib plus trametinib resulted in a longer duration of survival without relapse or distant metastasis than placebo with no apparent long-term toxic effects. (Funded by GlaxoSmithKline and Novartis; COMBI-AD ClinicalTrials.gov number, NCT01682083; EudraCT number, 2012-001266-15.).

The Anti-Cancer Drug Dabrafenib Is a Potent Activator of the Human Pregnane X Receptor

The human pregnane X receptor (hPXR) is activated by a large set of endogenous and exogenous compounds and plays a critical role in the control of detoxifying enzymes and transporters regulating liver and gastrointestinal drug metabolism and clearance. hPXR is also involved in both the development of multidrug resistance and enhanced cancer cells aggressiveness. Moreover, its unintentional activation by pharmaceutical drugs can mediate drug–drug interactions and cause severe adverse events. In that context, the potential of the anticancer BRAF inhibitor dabrafenib suspected to activate hPXR and the human constitutive androstane receptor (hCAR) has not been thoroughly investigated yet. Using different reporter cellular assays, we demonstrate that dabrafenib can activate hPXR as efficiently as its reference agonist SR12813, whereas it does not activate mouse or zebrafish PXR nor hCAR. We also showed that dabrafenib binds to recombinant hPXR, induces the expression of hPXR responsive genes in colon LS174T-hPXR cancer cells and human hepatocytes and finally increases the proliferation in LS174T-hPXR cells. Our study reveals that by using a panel of different cellular techniques it is possible to improve the assessment of hPXR agonist activity for new developed drugs.

Clinical Pharmacokinetics and Pharmacodynamics of Dabrafenib

Dabrafenib is a potent and selective inhibitor of BRAF-mutant kinase that is approved, as monotherapy or in combination with trametinib (mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor), for unresectable or metastatic BRAF-mutated melanoma, advanced non-small cell lung cancer and anaplastic thyroid cancer harbouring the BRAF^V600E mutation. The recommended dose of dabrafenib is 150 mg twice daily (bid) under fasted conditions. After single oral administration of the recommended dose, the absolute oral bioavailability (F) of dabrafenib is 95%. Dabrafenib shows a time-dependent increase in apparent clearance (CL/F) following multiple doses, which is likely due to induction of its own metabolism through cytochrome P450 (CYP) 3A4. Therefore, steady state is reached only after 14 days of daily dose administration. Moreover, the extent of this auto-induction process is dependent on the dose, which explains why dabrafenib systemic exposure at steady state increases less than dose proportionally over the dose range of 75-300 mg bid. The main elimination route of dabrafenib is the oxidative metabolism via CYP3A4/2C8 and biliary excretion. Among the three major metabolites identified, hydroxy-dabrafenib appears to contribute to the pharmacological activity. Age, sex and body weight did not have any clinically significant influence on plasma exposure to dabrafenib. No dose adjustment is needed for patients with mild renal or hepatic impairment, whereas the impacts of severe impairment on dabrafenib pharmacokinetics remain unknown. Considering that dabrafenib is a substrate of CYP3A4/2C8 and is a CYP3A4/2B6/2C inducer, drug-drug interactions are expected with dabrafenib. The relationship between clinical outcomes and plasma exposure to dabrafenib and hydroxy-dabrafenib should be investigated more deeply.

Population Pharmacokinetics/Pharmacodynamics of Dabrafenib Plus Trametinib in Patients with BRAF-Mutated Metastatic Melanoma

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 AUC_OHD/AUC_DAB ≥ 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 AUC_OHD/AUC_DAB deserves more investigation in a larger cohort of MM patients.

A Phase I and Pharmacokinetic Study of Oral Dabrafenib in Children and Adolescent Patients with Recurrent or Refractory BRAF V600 Mutation-Positive Solid Tumors

PURPOSE

The 2-part, phase I/IIa, open-label study (NCT01677741) sought to determine the safety, tolerability, pharmacokinetics, and preliminary activity of dabrafenib in pediatric patients with advanced BRAF V600-mutated cancers.

PATIENTS AND METHODS

This phase I dose-finding part treated patients ages 1 to <18 years with BRAF V600 mutation-positive tumors with oral dabrafenib 3 to 5.25 mg/kg/day to determine the RP2D based on safety and drug exposure target.

RESULTS

Between May 2013 and November 2014, 27 patients [12 male; median age, 9 years (range, 1-17 years)] with BRAF V600-mutant solid tumors recurrent/refractory to treatment (low- or high-grade glioma, Langerhans cell histiocytosis, neuroblastoma, or thyroid cancer) were enrolled. The median treatment duration was 75.6 weeks (range, 5.6-148.7 weeks), with 63% treated for >52 weeks and 52% undergoing treatment at data cutoff date. The most common grade 3/4 adverse events suspected to be related to study drug were maculopapular rash and arthralgia (2 patients each). No dose-limiting toxicities were observed. Pharmacokinetic analyses showed a dose-dependent increase in AUC0-12 and achievement of adult exposure levels at the recommended phase II doses of 5.25 mg/kg/day (age <12 years) and 4.5 mg/kg/day (age ≥12 years) divided into 2 equal doses daily, not exceeding 300 mg daily.

CONCLUSIONS

In this first clinical trial in pediatric patients with pretreated BRAF V600-mutant tumors, dabrafenib was well tolerated while achieving target exposure levels; the average treatment duration was >1 year with many patients still on treatment. The phase II component is also closed and will be reported separately.

Small molecule inhibition of RAS/MAPK signaling ameliorates developmental pathologies of Kabuki Syndrome

Kabuki Syndrome (KS) is a rare disorder characterized by distinctive facial features, short stature, skeletal abnormalities, and neurodevelopmental deficits. Previously, we showed that loss of function of RAP1A, a RAF1 regulator, can activate the RAS/MAPK pathway and cause KS, an observation recapitulated in other genetic models of the disorder. These data suggested that suppression of this signaling cascade might be of therapeutic benefit for some features of KS. To pursue this possibility, we performed a focused small molecule screen of a series of RAS/MAPK pathway inhibitors, where we tested their ability to rescue disease-relevant phenotypes in a zebrafish model of the most common KS locus, kmt2d. Consistent with a pathway-driven screening paradigm, two of 27 compounds showed reproducible rescue of early developmental pathologies. Further analyses showed that one compound, desmethyl-Dabrafenib (dmDf), induced no overt pathologies in zebrafish embryos but could rescue MEK hyperactivation in vivo and, concomitantly, structural KS-relevant phenotypes in all KS zebrafish models (kmt2d, kmd6a and rap1). Mass spectrometry quantitation suggested that a 100 nM dose resulted in sub-nanomolar exposure of this inhibitor and was sufficient to rescue both mandibular and neurodevelopmental defects. Crucially, germline kmt2d mutants recapitulated the gastrulation movement defects, micrognathia and neurogenesis phenotypes of transient models; treatment with dmDf ameliorated all of them significantly. Taken together, our data reinforce a causal link between MEK hyperactivation and KS and suggest that chemical suppression of BRAF might be of potential clinical utility for some features of this disorder.

Evaluation of the effect of dabrafenib and metabolites on QTc interval in patients with BRAF V600-mutant tumours

AIMS

The effect of repeat oral supratherapeutic dosing of the BRAF inhibitor dabrafenib on QTc interval was assessed in patients with BRAF V600-mutant tumours.

METHODS

Part 1 of this phase 1, multicentre, 2-part study (BRF113773/NCT01738451) assessed safety/tolerability of dabrafenib 225 or 300 mg twice daily (BID) to inform part 2 dosing. Patients in part 2 received dabrafenib-matched placebo on day -1, single-dose dabrafenib 300 mg on day 1, 300 mg BID on days 2 to 7, and 300 mg on day 8 (morning), followed by 24-h Holter electrocardiographic monitoring and pharmacokinetics sample collection each dose day. Pharmacokinetics/pharmacodynamics analysis assessed combined dabrafenib and metabolite effects on QTc interval.

RESULTS

Part 1 (n = 12) determined supratherapeutic dosing, 300 mg BID, for part 2. Thirty-one patients completed part 2. Mean maximum ΔΔQTcF occurred on day 8, 10 h postdose (2.86 msec; 90% CI, -1.36 to 7.07). Categorical analysis showed no placebo and dabrafenib outliers (increase >60 msec; QTcF >500 msec). Day 1 dabrafenib 300 mg C_max and AUC_(0-∞) were ≈ 2-fold higher than with single-dose 150 mg. Day 8 AUC_(0-τ) with 300 mg BID was ≈ 2.7-fold higher than with 150 mg BID. Dabrafenib metabolites showed similar trends. Pharmacokinetics/pharmacodynamics modelling/simulation showed that median QTc increase was <5 msec (upper 90% CI, <10 msec). No unexpected toxicities occurred with supratherapeutic dosing.

CONCLUSION

Repeat oral supratherapeutic dabrafenib 300 mg BID dosing had no clinically relevant effect on QTc interval, with no new safety signals seen.

Dabrafenib plus Trametinib: a Review in Advanced Melanoma with a BRAF (V600) Mutation

The BRAF inhibitor dabrafenib (Tafinlar(®)) and the MEK inhibitor trametinib (Mekinist(®)) are indicated, as monotherapy or in combination with each other, for the treatment of patients with unresectable or metastatic melanoma with a BRAF (V600) mutation. This article reviews the therapeutic efficacy and tolerability of combination treatment with dabrafenib and trametinib in this indication and summarizes relevant pharmacological data. Dabrafenib plus trametinib significantly prolonged progression-free survival (PFS) and overall survival (OS), improved objective response rates (ORRs) and preserved health-related quality of life (HR-QOL) to a greater extent than dabrafenib (in the double-blind COMBI-d study) and vemurafenib (in the open-label COMBI-v study) in two large, randomized, phase III studies in treatment-naïve patients with unresectable or metastatic melanoma with BRAF (V600E/K) mutation. Limited treatment benefit with the combination was also seen in patients who had progressed on prior BRAF inhibitor therapy, as indicated by ORRs of ≤ 15 % and stable disease in ≤ 50 % of patients in small phase I and II studies. Combination therapy did not increase overall toxicity relative to dabrafenib or vemurafenib monotherapy, with most adverse events (AEs) mild or moderate in severity and generally manageable. Fewer skin-related AEs (e.g. cutaneous malignancies, hyperkeratinosis and hand-foot syndrome) were reported with combination therapy than with dabrafenib or vemurafenib, probably because of reduced paradoxical activation of the MAPK pathway. Thus, dabrafenib plus trametinib provides an important treatment option for patients with BRAF (V600) mutation-positive unresectable or metastatic melanoma.

Practical Recommendations for Therapeutic Drug Monitoring of Kinase Inhibitors in Oncology

Despite the fact that pharmacokinetic exposure of kinase inhibitors (KIs) is highly variable and clear relationships exist between exposure and treatment outcomes, fixed dosing is still standard practice. This review aims to summarize the available clinical pharmacokinetic and pharmacodynamic data into practical guidelines for individualized dosing of KIs through therapeutic drug monitoring (TDM). Additionally, we provide an overview of prospective TDM trials and discuss the future steps needed for further implementation of TDM of KIs.

Simple and cost-effective liquid chromatography-mass spectrometry method to measure dabrafenib quantitatively and six metabolites semi-quantitatively in human plasma

Dabrafenib is an inhibitor of BRAF V600E used for treating metastatic melanoma but a majority of patients experience adverse effects. Methods to measure the levels of dabrafenib and major metabolites during treatment are needed to allow development of individualized dosing strategies to reduce the burden of such adverse events. In this study, an LC-MS/MS method capable of measuring dabrafenib quantitatively and six metabolites semi-quantitatively is presented. The method is fully validated with regard to dabrafenib in human plasma in the range 5–5000 ng/mL. The analytes were separated on a C18 column after protein precipitation and detected in positive electrospray ionization mode using a Xevo TQ triple quadrupole mass spectrometer. As no commercial reference standards are available, the calibration curve of dabrafenib was used for semi-quantification of dabrafenib metabolites. Compared to earlier methods the presented method represents a simpler and more cost-effective approach suitable for clinical studies.

Graphical abstract

An UPLC-MS/MS method for the quantification of BRAF inhibitors (vemurafenib, dabrafenib) and MEK inhibitors (cobimetinib, trametinib, binimetinib) in human plasma. Application to treated melanoma patients

Targeted therapies for cancers are fast-growing therapies. For instance kinase inhibitors such as BRAF inhibitors (BRAFi) and MEK inhibitors (MEKi) are increasingly used to treat malignant melanoma. The metabolic profile of these drugs can result in great interindividual variability, justifying therapeutic drug monitoring (TDM). We describe a rapid and specific method for quantification of 2 BRAFi (vemurafenib, dabrafenib) and 3 MEKi (cobimetinib, trametinib and binimetinib). Chromatography was performed on a Waters Acquity-UPLC system with CORTECS C18+ column, under a gradient of 10% acetic acid in water/acetonitrile. An Acquity-TQD® with electrospray ionization was used for detection. Samples were prepared by solid phase extraction (Oasis® MCX microElution) before injection in the system. Calibration curves ranges from 0.4 to 100μg/ml for vemurafenib, from 1 to 1000ng/ml for dabrafenib, from 0.5 to 500ng/ml for cobimetinib and binimetinib, and from 0.75 to 250ng/ml for trametinib. At all concentrations the bias was within ±15% of the nominal concentrations and precision was ≤15%. All results were within the acceptance criteria of the EMA guidelines on method validation. This rapid, sensitive and specific UPLC-MS/MS method can perform simultaneous quantification of targeted therapies used in malignant melanoma and is usable for routine TDM.

Prediction of the Transporter-Mediated Drug-Drug Interaction Potential of Dabrafenib and Its Major Circulating Metabolites

The BRAF inhibitor dabrafenib was recently approved for the treatment of certain BRAF V600 mutation-positive tumors, either alone or in combination therapy with the mitogen-activated extracellular signal regulated kinase 1 (MEK1) and MEK2 inhibitor, trametinib. This article presents the dabrafenib transporter-mediated drug-drug interaction (DDI) risk assessment, which is currently an important part of drug development, regulatory submission, and drug registration. Dabrafenib and its major circulating metabolites (hydroxy-, carboxy-, and desmethyl-dabrafenib) were investigated as inhibitors of the clinically relevant transporters P-gp, BCRP, OATP1B1, OATP1B3, OCT2, OAT1, and OAT3. The DDI Guidance risk assessment decision criteria for inhibition of BCRP, OATP1B1 and OAT3 were slightly exceeded and therefore a minor DDI effect resulting from inhibition of these transporters remained possible. Biliary secretion is the major excretion pathway of dabrafenib-related material (71.1% of orally administered radiolabeled dose recovered in feces), whereas urinary excretion was observed as well (22.7% of the dose). In vitro uptake into human hepatocytes of the dabrafenib metabolites, but not of dabrafenib parent compound, was mediated, at least in part, by hepatic uptake transporters. The transporters responsible for uptake of the pharmacologically active hydroxy- and desmethyl dabrafenib could not be identified, whereas carboxy-dabrafenib was a substrate of several OATPs. Dabrafenib, hydroxy-, and desmethyl-dabrafenib were substrates of P-gp and BCRP, whereas carboxy-dabrafenib was not. Although a small increase in exposure to carboxy-dabrafenib upon inhibition of OATPs and an increase in exposure to desmethyl-dabrafenib upon inhibition of P-gp or BCRP cannot be excluded, the clinical significance of such increases is likely to be low.

Recent developments in the chromatographic bioanalysis of approved kinase inhibitor drugs in oncology

In recent years (2010-present) there has been an increase in the number of publications reporting the development, validation and use of bioanalytical methods in the rapidly expanding drug class of small molecule protein kinase inhibitors. Most reports describe the technological set-up of the methods that have allowed for drug concentration measurements from various sample types. This includes plasma, dried blood-spot, and tissue-analysis. Also method development, exploration of various techniques, as well as measurement and identification of metabolites were addressed. For the bioanalysis, a variety of sample-pretreatment methods like protein-precipitation, liquid-liquid extraction, and solid-phase extraction have been employed, all varying in complexity, cleanliness and time-consumption. Chromatographic separation, nowadays, is more focused on separating components from ion-suppressive effects, since for MS/MS detection, various components do not have to be baseline separated. For detection multiple types of detectors were used, ranging from state-of-the-art high resolution, and tandem mass spectrometry with low picogram per milliliter detection limits to the classical UV-detector with several nanograms per milliliter limits. As new bioanalytical methods have arisen that do rely on chromatographic separation, for example for high-throughput analysis, these are addressed in this review as well.

Regulatory aspects of human radiolabeled mass balance studies in oncology: concise review

Human radiolabeled mass balance studies are performed to obtain information about the absorption, distribution, metabolism, and excretion of a drug in development. The main goals are to determine the route of elimination and major metabolic pathways. This review provides an overview of the current regulatory guidelines concerning human radiolabeled mass balance studies and discusses scientific trends seen in the last decade with a focus on mass balance studies of anticancer drugs. This paper also provides an overview of mass balance studies of anticancer agents that were executed in the last 10 years.

Safe handling of oral antineoplastic medications: Focus on targeted therapeutics in the home setting

Introduction With the growing number of oral targeted therapies being approved for use in cancer therapy, the potential for long-term administration of these drugs to cancer patients is expanding. The use of these drugs in the home setting has the potential to expose family members and caregivers to them either through direct contact with the drugs or indirectly by exposure to the parent compounds and/or their active metabolites in contaminated patients' waste. Methods A systematic literature review was performed and the known adverse health effect of 32 oral targeted therapeutics is summarized. In particular, the carcinogenicity, genotoxicity, and embryo-fetal toxicity, along with the route of excretion were evaluated. Results Carcinogenicity testing has not been performed on most of the oral targeted therapeutics and the genotoxicity data are mixed. However, the majority of these drugs exhibit adverse reproductive effects, some of which are severe. Currently, available data does not permit the possibility of a health hazard from inappropriate handling of drugs and contaminated patients waste to be ignored, especially in a long-term home setting. Further research is needed to understand these issues. Conclusions With the expanding use of targeted therapies in the home setting, family members and caregivers, especially those of reproductive risk age, are, potentially at risk. Overall basic education and related precautions should be taken to protect family members and caregivers from indirect or direct exposure from these drugs. Further investigations and discussion on this subject are warranted.

Human absorption, distribution, metabolism and excretion properties of drug molecules: a plethora of approaches

Human radiolabel studies are traditionally conducted to provide a definitive understanding of the human absorption, distribution, metabolism and excretion (ADME) properties of a drug. However, advances in technology over the past decade have allowed alternative methods to be employed to obtain both clinical ADME and pharmacokinetic (PK) information. These include microdose and microtracer approaches using accelerator mass spectrometry, and the identification and quantification of metabolites in samples from classical human PK studies using technologies suitable for non-radiolabelled drug molecules, namely liquid chromatography-mass spectrometry and nuclear magnetic resonance spectroscopy. These recently developed approaches are described here together with relevant examples primarily from experiences gained in support of drug development projects at GlaxoSmithKline. The advantages of these study designs together with their limitations are described. We also discuss special considerations which should be made for a successful outcome to these new approaches and also to the more traditional human radiolabel study in order to maximize knowledge around the human ADME properties of drug molecules.

Carbon-carbon bond cleavage and formation reactions in drug metabolism and the role of metabolic enzymes

Elimination of xenobiotics from the human body is often facilitated by a transformation to highly water soluble and more ionizable molecules. In general, oxidation-reduction, hydrolysis, and conjugation reactions are common biotransformation reactions that are catalyzed by various metabolic enzymes including cytochrome P450s (CYPs), non-CYPs, and conjugative enzymes. Although carbon-carbon (C-C) bond formation and cleavage reactions are known to exist in plant secondary metabolism, these reactions are relatively rare in mammalian metabolism and are considered exceptions. However, various reactions such as demethylation, dealkylation, dearylation, reduction of alkyl chain, ring expansion, ring contraction, oxidative elimination of a nitrile through C-C bond cleavage, and dimerization, and glucuronidation through C-C bond formation have been reported for drug molecules. Carbon-carbon bond cleavage reactions for drug molecules are primarily catalyzed by CYP enzymes, dimerization is mediated by peroxidases, and C-glucuronidation is catalyzed by UGT1A9. This review provides an overview of C-C bond cleavage and formation reactions in drug metabolism and the metabolic enzymes associated with these reactions.

Imaging MS in Toxicology: An Investigation of Juvenile Rat Nephrotoxicity Associated with Dabrafenib Administration

As part of an investigative nephrotoxicity study, kidney tissues from juvenile rats orally administered dabrafenib at different age intervals between postnatal day (PND) 7 to 35 were investigated by MALDI and LDI imaging mass spectrometry (IMS) to determine the chemical composition of tubular deposits. In the youngest age group (PND 7-13), MALDI IMS demonstrated that a dabrafenib carboxylic acid metabolite was diffusely localized to the regions of tubular deposits (medulla and corticomedullary junction); however, no dabrafenib-related material was detected directly from the deposits. Rather, the LDI IMS analysis determined that the deposits were composed primarily of calcium phosphate. Based on these data, the dabrafenib associated nephrotoxicity, including the formation of tubular deposits, was determined to be age dependent. Furthermore, immature renal function was hypothesized to be responsible for the susceptibility of the youngest pups.

Dabrafenib for the treatment of melanoma

INTRODUCTION

Approximately 50% of patients with cutaneous melanoma have an activating mutation in BRAF kinase, leading to constitutive activation of the mitogen-activated protein kinase pathway and unregulated cell growth. Selective inhibitors of the mutated BRAF kinase produce response rates of approximately 50% with median progression-free survival of 6 - 7 months. BRAF-blocking therapies work rapidly, with responses seen within 2 weeks after therapy initiation, and they are associated with generally mild toxicities. The BRAF inhibitor dabrafenib recently was approved for use in patients with BRAF V600-mutated metastatic melanoma.

AREAS COVERED

This article discusses the mechanisms of action and pharmacokinetic and pharmacodynamic changes as well as clinical efficacy and safety of dabrafenib for treatment of patients with advanced melanoma including unresectable stage IIIc and stage IV patients who harbor a BRAF V600 mutation. Clinical trial data are reviewed, and efficacy of dabrafenib in patients with brain metastases and in combination with the MEK inhibitor trametinib is discussed.

EXPERT OPINION

Despite rapid and significant tumor reduction in a majority of patients with BRAF-mutant metastatic melanoma who are treated with dabrafenib, this drug's use as a single agent is limited because of its relatively short duration of response. Various combinations with drug(s) inhibiting other target kinases and/or with immunomodulating agent(s) will likely be the standard in the near future.