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درباره:
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MEK Inhibitor Mechanism of Action, Side Effects, and Uses
</h1>
<p>
Metastatic melanoma is a deadly cancer for which conventional chemotherapy
provides little benefit. However, newer therapies involving a pathway called the
mitogen-activated protein kinase (MAPK) pathway have demonstrated success in
inducing remission.
</p>
<p>
The MAPK pathway is more accurately termed the RAS-RAF-MEK-ERK pathway, and
involves the regulation of cell proliferation and survival. It is constitutionally
overactive in 30% of cancers. Two enzymes in this pathway, namely, BRAF and MEK,
are target kinases which play crucial roles in the cell cycle.
</p>
<p>
The first therapies to target this pathway were BRAF inhibitors, but intrinsic
and acquired tumor resistance quickly led to treatment failure by reactivation of
the MAPK pathway. <a href="http://www.medchemhub.com/mek-inhibitors/"
target="_self">MEK inhibitors </a>have emerged to partially overcome these
resistance mechanisms and are now used in combination with BRAF inhibitors to
extend the time to resistance.
</p>
<p>
MEK is a dual specificity threonine/tyrosine kinase, so called from the term
MAPK/ERK kinase. It is a key effector of the three-layered RAS/RAF/MEK/ERK
signaling cascade, expressed by seven genes from MAPK1 to MAPK7.
</p>
<p>
MEK inhibitors bind to and inhibit MEK, inhibiting MEK-dependent cell
signaling. This inhibition leads to cell death and the inhibition of tumor growth.
These are allosteric binding inhibitors of MEK which inhibit either MEK1 alone, or
both MEK1 and MEK2.
</p>
<h2>
What is the mechanism of action of MEK inhibitors?
</h2>
<p>
The MAPK pathway is an intracellular signaling cascade that is involved in the
proliferation and survival of tumor cells. Many mutations cause cancer development
by activating this pathway, including BRAF and NRAS mutations. MEK is a downstream
protein kinase which can be targeted to prevent reactivation of the MAPK pathway
in the presence of BRAF or RAS mutations.
</p>
<p>
Normally, ERK1/2 activation initiates a variety of cellular and nuclear
pathways, while also inhibiting Raf activity by a feedback loop to modulate the
activity of the MAPK pathway. MEK1/2 inhibition inactivates ERK1/2 and also
removes the feedback inhibition on Raf.
</p>
<p>
Drugs which selectively inhibit the <a href="http://www.medchemhub.com/mek-
inhibitors/mek-inhibitor-selumetinib.html" target="_self">MEK enzymes</a> are able
to inhibit growth and to induce the death of cells in the presence of these
mutations.
</p>
<p>
Thus, MEK1/2 is highly selective in inactivating ERK1/2 but leaves other
signaling pathways intact. In addition, the non-ATP binding site means they do not
typically need to compete with ATP, which is present in very large amounts inside
cells. A new ATP-competitive inhibitor has also been designed which is effective
in mutants that display drug resistance to the ATP-noncompetitive inhibitors.
</p>
<p>
The advantages of using combination MEK inhibitor therapy with a BRAF
inhibitor is the increased progression-free survival and lower toxicity, when
compared with the latter alone.
</p>
<p>
What are the side effects of MEK inhibitors?
</p>
<p>
Adverse reactions with MEK inhibitors occur in two stages: immediate (within
days of initiation of therapy) and chronic (following several months of exposure).
Mild toxicities need not interrupt the treatment, but moderate to severe adverse
effects may require temporary withdrawal of the drugs and re-initiation following
resolution of the reaction. Such cessation of treatment for short periods does not
seem to affect outcomes. In some studies on mice, intermittent dosing was
associated with improved survival, and perhaps less toxicity.
</p>
<p>
MEK1/MEK2 inhibitors have a tendency to cause a papulopustular rash, seen in
57% of patients. Other side effects include diarrhea in 43%, whereas
peripheral edema is observed in 26%. More serious adverse effects include
hypertension in 12%, rash in 8%, and fatigue in 4%.
</p>
<p>
Creatine phosphokinase (CPK) levels are high in some patients, though without
any evidence of underlying disease processes such as rhabdomyolysis.
</p>
<p>
Abnormal liver function tests and pneumonitis are also observed, similar to
immune checkpoint inhibitors and PD-1 inhibitors. The appearance of cough,
difficulty in breathing or abnormal chest signs must be followed up with chest
radiography or a CT scan of the chest, and if pneumonitis is present, treatment
must be stopped for a time at least.
</p>
<p>
Ocular toxicity, comprising blurring of vision and reversible
chorioretinopathy (especially central serous retinopathy, CSR) is another feature
of MEK inhibitor toxicity. Among these, retinal vein occlusion is irreversible.
For this reason, a baseline ophthalmologic examination should be recorded. If any
visual disturbance occurs, examination should be repeated and compared with
baseline findings. If retinopathy is diagnosed, the drug should be withdrawn
temporarily. If the retinal appearance and function normalizes within three weeks,
and RVO is absent, the drug may be resumed at a lower dosage.
</p>
<p>
Other adverse effects include nausea, vomiting, constipation, alopecia, and
lowered left ventricular ejection fraction. An uncommon adverse effect is the
dropped-head syndrome, where the neck extensors become progressively weak because
of focal non-inflammatory myopathy. CPK levels are high, and the condition fails
to respond to steroids but resolves when the MEK inhibitor is discontinued.
</p>
<p>
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