Medical Oncologist
When targeted therapy stops working
MET Amplification and Other — Bypass Resistance Mechanisms
When a cancer that was responding to targeted therapy starts growing again, a bypass resistance mechanism is often the reason. Identifying which mechanism is driving the resistance changes what treatment options are open to you.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed August 2026
- Different from never responding — Bypass resistance means the treatment worked at first — then the cancer adapted around the blockade.
- Rebiopsy is the key step — The cancer's molecular profile at progression is often different from the original diagnosis.
- Some mechanisms are targetable — MET amplification and certain others may be addressed with specific agents or clinical trials.
- The evidence is still developing — Clinical trial participation is often the most appropriate next step when a bypass mechanism is found.
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Bypass resistance happens when cancer cells find an alternative signalling pathway to grow despite targeted therapy. MET amplification is one of the most studied bypass routes in lung cancer. NCCN and ESMO guidance recommends rebiopsy at progression to identify the specific mechanism, because different mechanisms can lead to different next-line options.
Why does targeted therapy stop working over time?
Targeted therapy works by blocking one specific signal that cancer cells depend on to grow. When that signal is blocked, some cancer cells find a different pathway to keep growing. This alternative route is called bypass resistance.
Think of it like a road with a checkpoint. The therapy closes the main road. Some cancer cells find a side road that was always present but rarely used. Once it is the only option, those cells start using it exclusively.
Different bypass mechanisms use different side roads. MET amplification, HER2 amplification, KRAS mutations, and histologic transformation — where the cancer changes its cell type — are among the bypass routes that NCCN and ESMO recognise in current clinical guidance.
What exactly is MET amplification and why does it matter?
The MET gene gives instructions for a receptor that normally helps cells grow and repair. In bypass resistance, cancer cells increase the number of copies of the MET gene dramatically. That flood of MET activity substitutes for the pathway the targeted therapy is blocking.
MET amplification is among the recognised bypass mechanisms after EGFR-directed treatment in lung cancer, and it is being studied in other cancer types. Your molecular report will state whether MET amplification was detected and at what level.
Identifying it matters because clinical trials evaluating agents that target MET in this setting exist. A rebiopsy that confirms MET amplification may open options that a different mechanism would not.
What will your team check when resistance is suspected?
- Imaging to confirm the cancer has progressed
- Liquid biopsy — a blood test for circulating tumour DNA
- Tissue rebiopsy if liquid biopsy is inconclusive or the sample insufficient
- Molecular profiling of the new sample for known resistance mechanisms
- Review of all prior treatments and how the cancer responded to each
- Screening for open clinical trials matched to your specific mechanism
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What happens after your resistance mechanism is identified?
Some bypass mechanisms can be addressed with targeted agents — either approved in other settings or being evaluated in trials. If one of those mechanisms is found in your rebiopsy, your oncologist will discuss whether a specific agent or a combination approach applies.
For mechanisms that are not yet directly targetable, clinical trial participation is often the most appropriate next step. NCCN and ESMO guidance lists clinical trials as a preferred option in this setting, because the evidence base for many bypass mechanisms is still maturing.
Your oncologist will weigh the specific mechanism found, your performance status, prior lines of treatment, and what is currently available. No single pathway applies to all resistance patterns.
Did you know?
Resistance to targeted therapy is not a single event — it is a change in the cancer's biology. The tumour at progression can look molecularly different from the tumour at diagnosis, which is why the original biopsy may not reflect what is driving resistance now.
Rebiopsy at the point of progression is standard guidance from NCCN, ASCO, and ESMO for cancers where targeted therapy is used.
Source: NCCN Clinical Practice Guidelines in Oncology: Non-Small Cell Lung Cancer; ESMO Clinical Practice Guidelines on Metastatic NSCLC
What does each specific bypass mechanism mean for next treatment?
MET amplification
MET amplification means there are now many extra copies of the MET gene in the cancer cell. Those extra copies generate signals that substitute for the EGFR pathway the original treatment was blocking. Clinical trials are evaluating agents that target MET alongside or in place of continued EGFR-directed treatment, and eligibility for those trials depends on confirming MET amplification in a current rebiopsy sample rather than the original diagnostic one.
HER2 amplification
HER2 is a growth-signalling receptor that can become amplified when EGFR is blocked, substituting for the silenced signal and allowing the cancer to keep dividing. HER2 amplification as a bypass mechanism is distinct from a primary HER2-driven cancer, and the two situations do not automatically call for the same treatment approach. Your molecular report will state whether HER2 amplification was found, and your oncologist will use that alongside your full treatment history to determine which options are worth exploring.
KRAS mutations emerging at resistance
A KRAS mutation appearing at progression is among the more complex bypass mechanisms to address directly, because for many KRAS variants there is no approved targeted agent for that specific resistance setting. KRAS G12C specifically has agents approved in some cancer types, and if that variant is identified in your resistance biopsy, eligibility for those agents or for trials testing combinations is worth discussing with your oncologist. For other KRAS variants, clinical trial enrolment remains the clearest path while the evidence develops.
Histologic transformation
In a proportion of EGFR-mutant lung cancers, what returns at progression is not the same cancer type. The original adenocarcinoma has transformed into small-cell lung cancer, or the cancer has acquired different cellular features. This is called histologic transformation and it is only detectable on a tissue biopsy — a liquid biopsy will not show it. When transformation is confirmed, treatment typically shifts to the approach used for the new cancer type rather than continuing with the original targeted therapy.
What if no mechanism is found?
Not every resistance biopsy identifies a clear mechanism. This can happen because the circulating tumour DNA level was too low for liquid biopsy to detect a signal, because the tissue sample was too small, or because the specific change driving resistance is not yet captured in current profiling panels. When no mechanism is found, your oncologist will use the broader clinical picture — how quickly the cancer grew, prior response patterns, and your current fitness — to guide the next step. Clinical trials that do not require an identified mechanism are also available.
What if more than one mechanism is found?
Resistance biopsies sometimes identify more than one bypass mechanism at the same time. This reflects the cancer's capacity to evolve along multiple pathways simultaneously, and it generally means fewer targeted options are immediately available because no single agent addresses all of them. Multiple co-existing mechanisms make clinical trial participation particularly important. Your oncologist needs to review the full molecular report rather than just the headline finding to understand which mechanism is primary and whether any combination strategies under study cover more than one.
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Common questions
Frequently asked questions
Can bypass resistance be treated?
Yes, in a proportion of patients — but whether it is treatable and how depends entirely on which mechanism caused the resistance. Some bypass mechanisms are targetable with specific agents, either in approved settings or through clinical trials. Others are not yet directly addressable, and the next step is systemic therapy or trial enrolment. This area is moving fast; trial options that did not exist two years ago for some mechanisms now exist. Ask your oncologist specifically which mechanisms were found and what those findings open up for you.
How is bypass resistance different from primary resistance?
Primary resistance means the cancer never responded to the targeted therapy — the treatment was started and the tumour did not shrink or stabilise from the beginning. Bypass resistance, also called acquired resistance, means the cancer responded at first and then found a way around the treatment over time. The biology is different in each case. Primary resistance usually means the targeted pathway was never the main growth driver for that tumour. Acquired bypass resistance means it was, and the cancer adapted under treatment pressure.
Does bypass resistance mean my cancer has become more aggressive?
Not necessarily. Acquired resistance is a biological adaptation to treatment pressure, not a sign that the cancer has become fundamentally more dangerous. It means the treatment it was responding to is no longer working as well, and that a change in approach is needed. Many people move through a series of treatment adaptations over years. Bypass resistance is one of those transitions. It is a change in the cancer's behaviour, not a statement about your prognosis.
What is a liquid biopsy and how does it detect resistance?
A liquid biopsy is a blood test that looks for fragments of tumour DNA circulating in your bloodstream. Cancer cells shed DNA as they divide or die, and a laboratory can read those fragments to identify resistance mechanisms — including MET amplification — without a surgical procedure. It is not always sufficient on its own. If the level of circulating tumour DNA is low, the liquid biopsy may miss the mechanism entirely, and a tissue biopsy from the tumour site may be needed for a complete picture. Your team will advise which is appropriate for your situation.
Is MET amplification the same as a MET mutation?
No, and the distinction matters for treatment decisions. MET amplification means there are extra copies of the MET gene in the cancer cell, which drives increased MET signalling through sheer volume. A MET mutation — such as MET exon 14 skipping — means there is a change in the sequence of the gene itself rather than the number of copies. Both affect MET activity, but they arise through different mechanisms and are not treated identically. Your molecular report will specify which was found, and your oncologist will use that to determine which options apply.
How long does rebiopsy take before we know the next step?
Molecular profiling of a new sample typically takes one to two weeks after the laboratory receives it. If a tissue rebiopsy is needed rather than a liquid biopsy, the procedure itself adds time depending on the biopsy site and local scheduling. Most oncologists use the waiting period to begin work-up for possible next-line options and to screen for open clinical trials, so the results and the plan often come together at the same appointment. Ask your team when the sample was sent and what the expected turnaround is, so you are not waiting without a timeline.