"This week the U.S. Food and Drug Administration approved Mekinist (trametinib) in combination with Tafinlar (dabrafenib) to treat patients with advanced melanoma that is unresectable (cannot be removed by surgery) or metastatic (late-stage)."...
Mechanism Of Action
Dabrafenib is an inhibitor of some mutated forms of BRAF kinases with in vitro IC50 values of 0.65, 0.5, and 1.84 nM for BRAF V600E, BRAF V600K, and BRAF V600D enzymes, respectively. Dabrafenib also inhibits wild-type BRAF and CRAF kinases with IC50 values of 3.2 and 5.0 nM, respectively, and other kinases such as SIK1, NEK11, and LIMK1 at higher concentrations. Some mutations in the BRAF gene, including those that result in BRAF V600E, can result in constitutively activated BRAF kinases that may stimulate tumor cell growth [see INDICATIONS AND USAGE]. Dabrafenib inhibits BRAF V600 mutation-positive melanoma cell growth in vitro and in vivo.
Dabrafenib and trametinib target two different tyrosine kinases in the RAS/RAF/MEK/ERK pathway. Use of dabrafenib and trametinib in combination resulted in greater growth inhibition of BRAF V600 mutation-positive melanoma cell lines in vitro and prolonged inhibition of tumor growth in BRAF V600 mutation positive melanoma xenografts compared with either drug alone.
After oral administration, median time to achieve peak plasma concentration (Tmax) is 2 hours. Mean absolute bioavailability of oral dabrafenib is 95%. Following a single dose, dabrafenib exposure (Cmax and AUC) increased in a dose-proportional manner across the dose range of 12 to 300 mg, but the increase was less than dose-proportional after repeat twice-daily dosing. After repeat twice-daily dosing of 150 mg, the mean accumulation ratio was 0.73 and the inter-subject variability (CV%) of AUC at steady-state was 38%.
Administration of dabrafenib with a high-fat meal decreased Cmax by 51%, decreased AUC by 31%, and delayed median Tmax by 3.6 hours as compared with the fasted state [see DOSAGE AND ADMINISTRATION].
Dabrafenib is 99.7% bound to human plasma proteins. The apparent volume of distribution (Vc/F) is 70.3 L.
The metabolism of dabrafenib is primarily mediated by CYP2C8 and CYP3A4 to form hydroxy-dabrafenib. Hydroxy-dabrafenib is further oxidized via CYP3A4 to form carboxy-dabrafenib and subsequently excreted in bile and urine. Carboxy-dabrafenib is decarboxylated to form desmethyl-dabrafenib; desmethyl-dabrafenib may be reabsorbed from the gut. Desmethyl-dabrafenib is further metabolized by CYP3A4 to oxidative metabolites. Hydroxy-dabrafenib terminal half-life (10 hours) parallels that of dabrafenib while the carboxy- and desmethyl-dabrafenib metabolites exhibited longer half-lives (21 to 22 hours). Mean metabolite-to-parent AUC ratios following repeat-dose administration are 0.9, 11, and 0.7 for hydroxy-, carboxy-, and desmethyl-dabrafenib, respectively. Based on systemic exposure, relative potency, and pharmacokinetic properties, both hydroxy- and desmethyl-dabrafenib are likely to contribute to the clinical activity of dabrafenib.
The mean terminal half-life of dabrafenib is 8 hours after oral administration. The apparent clearance of dabrafenib is 17.0 L/h after single dosing and 34.4 L/h after 2 weeks of twice-daily dosing.
Fecal excretion is the major route of elimination accounting for 71% of radioactive dose while urinary excretion accounted for 23% of total radioactivity as metabolites only.
Age, Body Weight, and Gender: Based on the population pharmacokinetics analysis, age has no effect on dabrafenib pharmacokinetics. Pharmacokinetic differences based on gender and on weight are not clinically relevant.
Pediatric: Pharmacokinetics of dabrafenib has not been studied in pediatric patients.
Renal: No formal pharmacokinetic trial in patients with renal impairment has been conducted. The pharmacokinetics of dabrafenib were evaluated using a population analysis in 233 patients with mild renal impairment (GFR 60 to 89 mL/min/1.73 m²) and 30 patients with moderate renal impairment (GFR 30 to 59 mL/min/1.73 m²) enrolled in clinical trials. Mild or moderate renal impairment has no effect on systemic exposure to dabrafenib and its metabolites. No data are available in patients with severe renal impairment.
Hepatic: No formal pharmacokinetic trial in patients with hepatic impairment has been conducted. The pharmacokinetics of dabrafenib was evaluated using a population analysis in 65 patients with mild hepatic impairment enrolled in clinical trials. Mild hepatic impairment has no effect on systemic exposure to dabrafenib and its metabolites. No data are available in patients with moderate to severe hepatic impairment.
In vitro studies show that dabrafenib is a substrate of CYP3A4 and CYP2C8 while hydroxy-dabrafenib and desmethyl-dabrafenib are CYP3A4 substrates. Coadministration of dabrafenib 75 mg twice daily and ketoconazole 400 mg once daily (a strong CYP3A4 inhibitor) for 4 days increased dabrafenib AUC by 71%, hydroxy-dabrafenib AUC by 82%, and desmethyl-dabrafenib AUC by 68%. Coadministration of dabrafenib 75 mg twice daily and gemfibrozil 600 mg twice daily (a strong CYP2C8 inhibitor) for 4 days increased dabrafenib AUC by 47%, with no change in the AUC of dabrafenib metabolites. Dabrafenib is a substrate of human P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) in vitro.
In vitro data demonstrate that dabrafenib is an inducer of CYP3A4 and CYP2B6 via activation of the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) nuclear receptors. Dabrafenib may also induce CYP2C enzymes via the same mechanism. Coadministration of dabrafenib 150 mg twice daily for 15 days and a single dose of midazolam 3 mg (a CYP3A4 substrate) decreased midazolam AUC by 74%. Coadministration of dabrafenib 150 mg twice daily for 15 days and a single dose of warfarin 15 mg decreased the AUC of S-warfarin (a CYP2C9 substrate) by 37% and the AUC of R-warfarin (a CYP3A4/CYP1A2 substrate) by 33% [see DRUG INTERACTIONS].
Dabrafenib and its metabolites, hydroxy-dabrafenib, carboxy-dabrafenib, and desmethyl-dabrafenib, are inhibitors of human organic anion transporting polypeptide OATP1B1, OATP1B3 and organic anion transporter OAT1 and OAT3 in vitro. Dabrafenib and desmethyl-dabrafenib are inhibitors of BCRP in vitro.
Coadministration of trametinib 2 mg daily with dabrafenib 150 mg twice daily resulted in a 23% increase in AUC of dabrafenib, a 33% increase in AUC of desmethyl-dabrafenib, and no change in AUC of trametinib or hydroxy-dabrafenib as compared with administration of either drug alone.
Drugs that alter the pH of the upper GI tract (e.g., proton pump inhibitors, H2-receptor antagonists, antacids) may alter the solubility of dabrafenib and reduce its bioavailability. However, no formal clinical trial has been conducted to evaluate the effect of gastric pH-altering agents on the systemic exposure of dabrafenib. When TAFINLAR is coadministered with a proton pump inhibitor, H2-receptor antagonist, or antacid, systemic exposure of dabrafenib may be decreased and the effect on efficacy of TAFINLAR is unknown.
Animal Toxicology And/Or Pharmacology
Adverse cardiovascular effects were noted in dogs at dabrafenib doses of 50 mg/kg/day (approximately five times the human exposure at the recommended dose based on AUC) or greater, when administered for up to 4 weeks. Adverse effects consisted of coronary arterial degeneration/necrosis and hemorrhage, as well as cardiac atrioventricular valve hypertrophy/hemorrhage.
BRAF V600E Mutation-Positive Unresectable Or Metastatic Melanoma
In Trial 1, the safety and efficacy of TAFINLAR as a single agent were demonstrated in an international, multicenter, randomized (3:1), open-label, active-controlled trial conducted in 250 patients with previously untreated BRAF V600E mutation-positive, unresectable or metastatic melanoma. Patients with any prior use of BRAF inhibitors or MEK inhibitors were excluded. Patients were randomized to receive TAFINLAR 150 mg orally twice daily (n = 187) or dacarbazine 1,000 mg/m² intravenously every 3 weeks (n = 63). Randomization was stratified by disease stage at baseline [unresectable stage III (regional nodal or in-transit metastases), M1a (distant skin, subcutaneous, or nodal metastases), or M1b (lung metastases) versus M1c melanoma (all other visceral metastases or elevated serum LDH)]. The main efficacy outcome measure was progression-free survival (PFS) as assessed by the investigator. In addition, an independent radiology review committee (IRRC) assessed the following efficacy outcome measures in pre-specified supportive analyses: PFS, confirmed objective response rate (ORR), and duration of response.
The median age of patients in Trial 1 was 52 years. The majority of the trial population was male (60%), white (99%), had an ECOG performance status of 0 (67%), M1c disease (66%), and normal LDH (62%). All patients had tumor tissue with mutations in BRAF V600E as determined by a clinical trial assay at a centralized testing site. Tumor samples from 243 patients (97%) were tested retrospectively, using an FDA-approved companion diagnostic test, THxID™-BRAF assay.
The median durations of follow-up prior to initiation of alternative treatment in patients randomized to receive TAFINLAR was 5.1 months and in the dacarbazine arm was 3.5 months. Twenty-eight (44%) patients crossed over from the dacarbazine arm at the time of disease progression to receive TAFINLAR.
Trial 1 demonstrated a statistically significant increase in progression-free survival in the patients treated with TAFINLAR. Table 7 and Figure 1 summarize the PFS results.
Table 7: Investigator-Assessed Progression-Free
Survival and Confirmed Objective Response Results in Trial 1
N = 187
N = 63
|Number of Events (%)||78 (42%)||41 (65%)|
|Median, months (95% CI)||5.1 (4.9, 6.9)||2.7 (1.5, 3.2)|
|HRa (95% CI)||0.33 (0.20, 0.54)|
|P-valueb||P < 0.0001|
|Confirmed Tumor Responses|
|Objective Response Rate||52%||17%|
|(95% CI)||(44, 59)||(9, 29)|
|CR, n (%)||6 (3%)||0|
|PR, n (%)||91 (48%)||11 (17%)|
|Duration of Response|
|Median, months (95% CI)||5.6 (5.4, NR)||NR (5.0, NR)|
|a Pike estimator, stratified by disease state.
b Stratified log-rank test.
CI = Confidence interval; CR = Complete response; HR = Hazard ratio; NR = Not reached; PR = Partial response.30
Figure 1: Kaplan-Meier Curves of Investigator-Assessed
In supportive analyses based on IRRC assessment and in an exploratory subgroup analysis of patients with retrospectively confirmed V600E mutation-positive melanoma with the THxID™-BRAF assay, the PFS results were consistent with those of the primary efficacy analysis.
The activity of TAFINLAR for the treatment of BRAF V600E mutation-positive melanoma, metastatic to the brain was evaluated in a single-arm, open-label, two-cohort multi-center trial (Trial 3). All patients received TAFINLAR 150 mg twice daily. Patients in Cohort A (n = 74) had received no prior local therapy for brain metastases, while patients in Cohort B (n = 65) had received at least one local therapy for brain metastases, including, but not limited to, surgical resection, whole brain radiotherapy, or stereotactic radiosurgery such as gamma knife, linear-accelerated-based radiosurgery, charged particles, or CyberKnife. In addition, patients in Cohort B were required to have evidence of disease progression in a previously treated lesion or an untreated lesion. Additional eligibility criteria were at least one measurable lesion of 0.5 cm or greater in largest diameter on contrast-enhanced MRI, stable or decreasing corticosteroid dose, and no more than two prior systemic regimens for treatment of metastatic disease. The primary outcome measure was estimation of the overall intracranial response rate (OIRR) in each cohort.
The median age of patients in Cohort A was 50 years, 72% were male, 100% were white, 59% had a pre-treatment ECOG performance status of 0, and 57% had an elevated LDH value at baseline. The median age of patients in Cohort B was 51 years, 63% were male, 98% were white, 66% had a pre-treatment ECOG performance status of 0, and 54% had an elevated LDH value at baseline. Efficacy results as determined by an independent radiology review committee, masked to investigator response assessments, are provided in Table 8.
Table 8: Efficacy Results in Patients With BRAF V600E
Melanoma Brain Metastases (Trial 3)
|Endpoint||IRRC Assessed Response|
N = 74
N = 65
|Overall Intracranial Response Rate (OIRR)|
|% (95% CI)||18 (9.7, 28.2)||18 (9.9, 30.0)|
|Duration of OIRR||(N = 13)||(N = 12)|
|Median, months (95% CI)||4.6 (2.8, NR)||4.6 (1.9, 4.6)|
|IRRC = Independent radiology review committee; CI = Confidence interval; NR = Not reached.|
BRAF V600E Or V600K Unresectable Or Metastatic Melanoma
Trial 2 was a multicenter, open-label, randomized (1:1:1) dose-ranging trial designed to evaluate the clinical activity and safety of TAFINLAR in combination with trametinib (at two different doses) and to compare the safety with TAFINLAR as a single agent in 162 patients with BRAF V600E or V600K mutation-positive, unresectable or metastatic melanoma. Patients were permitted to have had one prior chemotherapy regimen and prior aldesleukin; patients with prior exposure to BRAF or MEK inhibitors were ineligible. Patients were randomized to receive TAFINLAR 150 mg orally twice daily with trametinib 2 mg orally once daily (n = 54), TAFINLAR 150 mg orally twice daily with trametinib 1 mg orally twice daily (n = 54), or TAFINLAR 150 mg orally twice daily (n = 54). Treatment continued until disease progression or unacceptable toxicity. Patients randomized to TAFINLAR as a single agent were offered TAFINLAR 150 mg orally twice daily with trametinib 2 mg orally once daily at the time of investigator-assessed disease progression. The major efficacy outcome measure was investigator-assessed overall response rate (ORR). Additional efficacy outcome measures were investigator-assessed duration of response, independent radiology review committee (IRRC)-assessed ORR, and IRRC-assessed duration of response.
The median age of patients was 53 years, 57% were male, > 99% were white, 66% of patients had a pre-treatment ECOG performance status of 0, 67% had M1c disease, 54% had a normal LDH at baseline, and 8% had history of brain metastases. Most patients (81%) had not received prior anti-cancer therapy for unresectable or metastatic disease. Based on local laboratory or centralized testing, 85% of patients' tumors had BRAF V600E mutations and 15% had BRAF V600K mutations.
The median duration of follow-up was 14 months. Efficacy outcomes for the trial arms receiving TAFINLAR in combination with trametinib 2 mg orally once daily and TAFINLAR as a single agent, are summarized in Table 9.
Table 9: Investigator-Assessed and Independent Review
Committee-Assessed Response Rates and Response Duration in Trial 2
|Endpoints||TAFINLAR plus Trametinib
N = 54
N = 54
|Responders (ORR%)||41 (76%)||29 (54%)|
|(95% CI)||(62%, 87%)||(40%, 67%)|
|Duration of Response (months)|
|(95% CI)||(7, 15)||(5, 7)|
|Independent Radiology Review Committee Assessment|
|Responders (ORR%)||31 (57%)||25 (46%)|
|(95% CI)||(43%, 71%)||(33%, 60%)|
|Duration of Response (months)|
|(95% CI)||(7, NR)||(6, NR)|
|CI = Confidence interval; ORR = Confirmed overall response rate; NR = Not reported.|
The ORR results were similar in subgroups defined by BRAF mutation subtype, i.e., in the 85% of patients with V600E mutation-positive melanoma and in the 15% of patients with V600K mutation-positive melanoma. In exploratory subgroup analyses of the patients with retrospectively confirmed BRAF V600E or V600K mutation-positive melanoma using the THxID™-BRAF assay, the ORR results were also similar to the intent-to-treat analysis.
Last reviewed on RxList: 1/22/2014
This monograph has been modified to include the generic and brand name in many instances.
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