MEK inhibitor

MEK inhibitors bind to the allosteric site of MEK1/2, preventing their phosphorylation and activation by RAF, thereby blocking downstream ERK signaling. This pathway is constitutively activated in many cancers due to mutations in upstream components like BRAF or RAS. Unlike ATP-competitive inhibitors, allosteric MEK inhibitors are highly selective and reduce the risk of off-target effects. However, they can paradoxically activate the pathway in certain contexts, such as in RAS-mutant cells, leading to resistance or toxicity.

MEK inhibitors are approved for BRAF-mutant melanoma (in combination with BRAF inhibitors), BRAF-mutant non-small cell lung cancer, and neurofibromatosis type 1 (NF1) in pediatric patients. They are also used in low-grade serous ovarian cancer and are being investigated in RAS-mutant cancers, where they have shown modest activity. Combination with BRAF inhibitors (e.g., dabrafenib plus trametinib) has improved progression-free survival and overall response rates compared to monotherapy, but resistance inevitably develops through reactivation of the MAPK pathway or activation of parallel pathways.

Common toxicities include rash, diarrhea, fatigue, and peripheral edema. A distinctive class effect is ocular toxicity, including serous retinal detachment and retinal vein occlusion, which requires regular ophthalmologic monitoring. Cardiomyopathy and decreased left ventricular ejection fraction are also observed, necessitating cardiac evaluation. Dermatologic side effects, such as acneiform rash and photosensitivity, are managed with topical steroids and dose adjustments. Most adverse effects are reversible upon dose reduction or discontinuation, but some, like retinal vein occlusion, can be irreversible.

Beyond oncology, MEK inhibitors are being explored in inflammatory diseases, such as rheumatoid arthritis and psoriasis, due to their role in cytokine production. In NF1, selumetinib was the first MEK inhibitor approved for a non-cancer indication, shrinking plexiform neurofibromas. Research has also revealed that MEK inhibitors can enhance immune checkpoint blockade by upregulating PD-L1 expression on tumor cells, suggesting a rationale for combination immunotherapy. Additionally, some MEK inhibitors have shown activity in KRAS-mutant pancreatic cancer models, though clinical results remain disappointing. The development of next-generation MEK inhibitors with improved selectivity and brain penetration is ongoing, with agents like pimasertib and refametinib in clinical trials.

Allosteric site

A regulatory site on an enzyme distinct from the active site, where binding of a molecule induces a conformational change that alters enzyme activity.

BRAF

A proto-oncogene encoding a serine/threonine kinase in the MAPK pathway; mutations (especially V600E) are common in melanoma and other cancers.

ERK

Extracellular signal-regulated kinase, a downstream effector of MEK that regulates cell division and survival.

Plexiform neurofibroma

A benign nerve sheath tumor that can become malignant, associated with neurofibromatosis type 1.