How BRAF and MEK Inhibitors Work — and Why They Are Combined

What a BRAF Inhibitor Blocks

In BRAF V600E or V600K cancer, the mutant kinase signals as an active monomer, independent of upstream RAS. Selective BRAF inhibitors bind the active-like conformation of this monomer and prevent it from phosphorylating MEK, collapsing ERK output in the tumour.

This is a clean on-off effect in V600-mutant cells, which is why single-agent BRAF inhibition produces rapid responses in BRAF-mutant melanoma.

The Paradox in Normal Cells

In cells with wild-type BRAF but active RAS (for example, skin cells), binding drug to one protomer of a RAF dimer allosterically transactivates the other, drug-free protomer. The net effect is increased, not decreased, ERK signalling — the paradoxical activation phenomenon.

Clinically this drives the growth of keratoacanthomas and cutaneous squamous cell carcinomas, and it is a concern in tissues where RAS is often mutated, such as the colon.

Why Add a MEK Inhibitor

MEK sits directly downstream of BRAF. A MEK inhibitor blocks the pathway below the point where paradoxical activation occurs, so combining it with a BRAF inhibitor suppresses the drug-induced skin lesions and deepens pathway inhibition in the tumour.

Combinations (dabrafenib plus trametinib, encorafenib plus binimetinib, vemurafenib plus cobimetinib) improve response rate and progression-free survival over BRAF inhibitor alone in BRAF V600-mutant melanoma and are the standard approach.

Resistance and Tumour Context

Most BRAF-mutant melanomas eventually progress, commonly through reactivation of MAPK signalling: NRAS or KRAS mutations, BRAF amplification or splice variants, MEK1/2 mutations, or upstream receptor changes. Non-MAPK routes such as PI3K-AKT activation also occur.

BRAF V600 mutations outside melanoma (colorectal, thyroid, lung, histiocytosis) respond differently; colorectal cancer, for instance, needs concurrent EGFR blockade because of rapid feedback reactivation.

Dosing, the Fever Syndrome and Monitoring

All three pairs are oral and taken continuously until progression or intolerance. The dabrafenib-trametinib combination is well known for pyrexia — recurrent fever, sometimes with rigors and low blood pressure — which is managed by interrupting both drugs until it settles and then restarting, often with corticosteroid cover, rather than by pushing through it.

Shared monitoring covers the skin (including surveillance for new keratoacanthomas and squamous cell carcinomas, which are much less frequent once a MEK inhibitor is added), left ventricular function, the eyes (retinal changes, and with vemurafenib-cobimetinib retinal vein occlusion), and liver enzymes; encorafenib-binimetinib adds creatine kinase, and vemurafenib and encorafenib require QT-interval checks.

Key Takeaways

• ·Selective BRAF inhibitors shut off ERK signalling in V600-mutant cells as active monomers.
• ·In RAS-active wild-type cells they paradoxically activate the pathway, driving skin lesions.
• ·Adding a MEK inhibitor counters the paradox, deepens inhibition and delays resistance.