Dabrafenib

Absorption

After oral administration, the 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 mg to 300 mg, but the increase was less than dose-proportional after repeat twice-daily dosing. After repeated 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%.

Toxicity

Carcinogenicity studies with dabrafenib have not been conducted. Dabrafenib increased the risk of cutaneous squamous cell carcinomas in patients in clinical trials.
Dabrafenib was not mutagenic in vitro in the bacterial reverse mutation assay (Ames test) or the mouse lymphoma assay and was not clastogenic in an in vivo rat bone marrow micronucleus test.
In a combined female fertility and embryo-fetal development study in rats, a reduction in fertility was noted at doses greater than or equal to 20 mg/kg/day (equivalent to the human exposure at the recommended dose based on AUC). A reduction in the number of ovarian corpora lutea was noted in pregnant females at 300 mg/kg/day (which is approximately three times the human exposure at the recommended dose based on AUC).
Male fertility studies with dabrafenib have not been conducted; however, in repeat-dose studies, testicular degeneration/depletion was seen in rats and dogs at doses equivalent to and three times the human exposure at the recommended dose based on AUC, respectively.

Description

In May 2013, the US FDA approved dabrafenib (also referred to as GSK 2118436) for the treatment of patients with unresectable or metastatic melanoma with the BRAF^V600E mutation as detected by a FDA-approved test. Dabrafenib was identified from a screen of an oncology-directed kinase collection, followed by extensive structure–activity relationships (SAR) on an initial thiazole lead. Dabrafenib is a potent inhibitor of B-BRAF^V600E kinase (IC₅₀=0.65 nM) compared to its potency against wild-type B-raf (IC₅₀=3.2 nM). It also inhibits other kinases (e.g., CRAF) and other mutant B-raf kinases (BRAF^V600E and BRAF^V600D) with enzyme IC_50s of <5 nM and is fairly selective versus a panel of 270 kinases. Consistent with its in vitro activity, oral administration of dabrafenib inhibits the growth of B-Raf^V600E mutant melanoma (A375P) and colon cancer (Colo205) human tumor xenografts growing subcutaneously in immunocompromised mice. Key steps in the synthesis of dabrafenib are condensation of an aryl sulfonamide ester with the lithium anion of 2-chloro-4-methylpyrimidine to generate a ketone intermediate and bromination of the ketone intermediate with N-bromosuccinamide followed by cyclization with tert-butyl thioamide to afford the desired thiazole core.

Originator

GlaxoSmithKline (United States)

The Uses of Dabrafenib

Dabrafenib is an inhibitor of mutated BRAF kinase and has clinical activity with a manageable safety profile in clinical trials of phase 1 and 2 in patients with BRAF(V600)-mutated metastatic melanoma.

Background

Dabrafenib mesylate (Tafinlar) is a reversible ATP-competitive kinase inhibitor and targets the MAPK pathway. It was approved on May 29, 2013, for the treatment of melanoma with V600E or V6000K mutation. It was also used for metastatic non-small cell lung cancer with the same mutation.
In May 2018, Tafinlar (dabrafenib), in combination with Mekinist (Trametinib), was approved to treat anaplastic thyroid cancer caused by an abnormal BRAF V600E gene.

Indications

As monotherapy, dabrafenib is indicated to treat unresectable or metastatic melanoma with BRAF V600E mutation as detected by an FDA-approved test.
In combination with trametinib, dabrafenib is indicated to treat for:
Dabrafenib has limitations of use: it is neither indicated for treating patients with colorectal cancer because of known intrinsic resistance to BRAF inhibition nor wild-type BRAF solid tumours.

What are the applications of Application

Dabrafenib is a potent and selective inhibitor of Raf-B protein kinase carrying V600E mutation

Definition

ChEBI: An organofluorine compound and antineoplastic agent, used as its mesylate salt in treatment of metastatic melanoma.

brand name

Tafinlar

General Description

Class: dual threonine/tyrosine kinase; Treatment: melanoma with BRAF mutations; Oral bioavailability = 95%; Elimination half-life = 8 h; Protein binding = 99.7%

Pharmacokinetics

Dabrafenib is a kinase inhibitor that is mainly used to target BRAF V600E mutation in multiple types of cancer. Although dabrafenib and trametinib both inhibit the RAS/RAF/MEK/ERK pathway, they inhibit different effectors of the pathway, thus increasing response rate and mitigating resistance without cumulative toxicity.
The melanoma approval for use with trametinib is based on results from COMBI-AD, a Phase III study of 870 patients with Stage III BRAF V600E/K mutation-positive melanoma treated with dabrafenib + trametinib after complete surgical resection. Patients received doses of dabrafenib (150 mg BID) + trametinib (2 mg QD) combination (n = 438) or matching placebos (n = 432). After a median follow-up of 2.8 years, the primary endpoint of relapse-free survival (RFS) was met.
In the case of thyroid cancer, Dabrafenib plus Trametinib is the first regimen demonstrated to have potent clinical activity in BRAF V600E–mutated anaplastic thyroid cancer and is well tolerated. These findings represent a meaningful therapeutic advance for this orphan disease.

Pharmacokinetics

Dabrafenib exhibits an oral bioavailability of 95%, indicative of extensive absorption and low firstpass intestinal and hepatic metabolism. The excellent oral bioavailability contributes to a much lower dosage than vemurafenib (150 mg, BID vs. 960 mg, BID). It has an elimination half-life of 8 h, resulting in twice-daily dosing regimen. Dabrafenib undergoes metabolism primarily via oxidation of the t-butyl group to form hydroxydabrafenib 6, which is further oxidized to carboxydabrafenib 7. Subsequent decarboxylation furnishes the desmethyl-dabrafenib 8 via a pH-dependent decarboxylation (Fig. 4). The major route of elimination of dabrafenib is a combination of oxidative metabolism (48% of the dose) and biliary excretion.

Clinical Use

Selective inhibitor of BRAF-kinase:
Treatment of metastatic melanoma and advanced non-small cell lung cancer with a BRAF V600 mutation

Drug interactions

Potentially hazardous interactions with other drugs
Antipsychotics: avoid with clozapine, increased risk of agranulocytosis.
Oestrogens and progestogens: possibly reduced contraceptive effect.

Metabolism

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.

Metabolism

Metabolism is mainly by CYP2C8 and CYP3A4 isoenzymes to form hydroxy-dabrafenib, which is further oxidised via CYP3A4 to form carboxy-dabrafenib. Carboxy-dabrafenib can be decarboxylated via a nonenzymatic process to form desmethyl-dabrafenib. Carboxy-dabrafenib is excreted in bile and urine. Desmethyl-dabrafenib may also be formed in the gut and reabsorbed. Desmethyl-dabrafenib is metabolised by CYP3A4 to oxidative metabolites. Both hydroxyand desmethyl-dabrafenib are likely to contribute to the clinical activity of dabrafenib while the activity of carboxy-darafenib is not likely to be significant.

References

1) Huang?et al. (2013),?B-Raf and the inhibitors: from bench to bedside; J. Hematol. Oncol.,?6?1 2) Ji?et al. (2016),?Endoplasmic reticulum stress-induced autophagy determines the susceptibility of melanoma cells to dabrafenib; Drugs Des. Dev. Ther.?10?2491 3) Herr?et al.?(2015),?B-Raf inhibitors induce epithelial differentiation in BRAF-mutant colorectal cancer cells; Cancer Res.,?75?216