Medical Oncologist
Resistance & Next Lines
Combination Therapy to — Overcome Resistance
When a treatment stops working, it can feel like the options are narrowing. What it actually means is that the cancer's biology has changed — and understanding exactly how it changed is what determines the next step. Combination therapy is one of the main tools for cancers that have evolved to resist a single treatment.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed August 2026
- Resistance is expected — Most cancers develop some degree of resistance over time. It is a known feature of the disease, not a sign that treatment failed.
- Biology explains it — Cancer cells accumulate mutations under treatment pressure. The ones that survive are specifically the ones the drug cannot kill.
- Combination therapy is designed for this — Targeting multiple pathways at once makes it much harder for a single mutation to escape all treatments simultaneously.
- Re-biopsy changes the plan — New tissue testing at the point of progression often reveals mutations that were not present at diagnosis, opening options that were not previously available.
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When cancer stops responding, resistant cells have taken over — they evolved to escape your current treatment. Combination therapy addresses this by targeting multiple pathways at once, making it much harder for any single mutation to escape all treatments. Your oncologist uses re-biopsy results and biomarker testing to decide which combination fits your specific resistance pattern.
Why does cancer stop responding to treatment?
Cancer cells are not static. Under the pressure of treatment, they accumulate new mutations that help some of them survive the drug that was working.
The cells most vulnerable to your treatment die. The ones with a slight survival advantage — a different protein, an alternative signalling route — survive and multiply. Over time, their descendants make up most of the tumour. This process is called clonal selection.
It does not mean the original treatment failed at its job. It means the cancer evolved around it, which is a recognised pattern in many cancer types.
This is also why increasing the dose of the same drug rarely solves the problem. The cells that remain are specifically the ones that drug cannot reach.
How your team finds the next option when treatment stops working
Confirm progression
Imaging — usually a CT or PET-CT scan — confirms that the cancer has grown or spread despite treatment. Progression must be confirmed before any plan changes.
Consider re-biopsy
A biopsy of a new or growing lesion can reveal mutations that were absent at diagnosis. Not every patient needs one, but your oncologist will tell you if it adds useful information for your specific situation.
Molecular and biomarker testing
The new sample — or sometimes a liquid biopsy using blood — is tested for markers relevant to second- or later-line treatment. The results often differ from what was tested at your original diagnosis.
Multidisciplinary review
Your case is discussed by a team that includes your oncologist, pathologist, and radiologist. The aim is to agree on what the new results show and what the evidence supports next.
A revised treatment plan
Based on testing, your oncologist recommends a new regimen — a different single agent, a combination, or a clinical trial. The recommendation is made for your tumour's current biology, not its original one.
How does combination therapy help when cancer has become resistant?
Combination therapy works by attacking the cancer through more than one mechanism at the same time. A single new mutation is unlikely to simultaneously disable all of them.
Some combinations target the same pathway at two different points — called vertical inhibition — so blocking one step does not simply push signals through another route. Others target entirely different pathways, called horizontal inhibition, reducing the chance the cancer can reroute around either blockade.
A third approach, used in some cancer types, is called synthetic lethality. Two treatments are chosen because together they remove a repair system the cancer cell cannot survive without, even though neither treatment alone would achieve this.
Which strategy applies to you depends on your cancer type and what the re-biopsy shows. Your oncologist will explain the specific reasoning behind the combination they recommend.
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Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
Surgical Oncologist
Surgical Oncologist
Dr. Vinay Mamidala
MBBS, MS(General Surgery), M.Ch(Surgical Oncology), FMAS, FARIS(Ongoing)
Surgical Oncologist
Radiation Oncologist
Radiation Oncologist
Radiation Oncologist
Hematologist
Interventional Radiologist
Dr. Mohammed Imran
Surgical Oncologist
Dr. Vajja Sandeep Kumar
MBBS, MS (General Surgery), DrNB (Surgical Oncology), FALS Oncology
Surgical Oncologist
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Terms your team may use
- Acquired resistance
- Resistance that develops during treatment, after an initial response. The cancer was sensitive when treatment started and changed its biology over time.
- Primary resistance
- When cancer does not respond to a treatment from the very beginning. The cells were already resistant before the first dose.
- Clonal selection
- The process by which treatment kills sensitive cells, leaving resistant ones behind. Those resistant cells multiply and eventually dominate the tumour.
- Bypass pathway
- An alternative signalling route a cancer cell activates when its main route is blocked by a drug. The cell reroutes signals around the blockade.
- Tumour heterogeneity
- The presence of genetically different cell populations within the same tumour. Because cells are not all identical, some may already carry resistance mutations before treatment begins.
- Synthetic lethality
- A combination strategy in which two treatments together are lethal to the cancer cell, even though either one used alone would not be sufficient.
- Liquid biopsy
- A blood test that detects tumour DNA circulating in the bloodstream. It can sometimes identify resistance mutations without requiring a surgical tissue biopsy.
What should you expect when moving to a combination regimen?
Moving to a combination regimen usually means more frequent clinic visits at first, while your team monitors tolerability and early response.
Combination regimens generally carry more side effects than single-agent treatments, because more biological processes are being disrupted at the same time. Your team will explain which side effects to watch for and when to call rather than wait.
At CION, combination chemotherapy and targeted combination regimens are given as day care where the treatment schedule allows. Response-assessment imaging is coordinated with partner imaging centres. Your oncologist will set a reassessment date — typically after two or three cycles — so any adjustment is based on evidence.
Dose modifications are a normal part of managing combination therapy. They are not a sign that treatment is failing — they are how the regimen is matched to what your body can tolerate.
Did you know?
In a proportion of patients, cells carrying resistance mutations are already present within the tumour before treatment begins. Treatment does not create resistance — it selects for cells that were already there.
This is one reason ESMO and ASCO guidance increasingly recommends re-biopsy at the point of progression, rather than assuming the cancer's molecular profile is unchanged from diagnosis.
Source: ESMO Clinical Practice Guidelines; ASCO guidance on molecular testing at progression
Explore 39 more Monitoring, Resistance & Long-Term Response topics
Resistance, Progression & Next Lines
- Being Told 'There Are No More Options': Is That Really True?
- Brain Metastases on Targeted Therapy: Does Your Drug Reach the Brain?
- Can You Continue the Same Drug After Progression?
- Combination Therapy to Overcome Resistance
- Do You Need Another Biopsy When the Cancer Progresses?
- Histologic Transformation: When Lung Cancer Changes Type
- How to Emotionally Process a Progression Scan
- Leptomeningeal Disease: Symptoms, Diagnosis and Treatment
- Life After ALK Inhibitor Resistance: Sequencing Your Options
- MET Amplification and Other Bypass Resistance Mechanisms
- Oligoprogression: When Only One or Two Spots Grow
- Primary vs Acquired Resistance: Two Very Different Problems
- Should You Change Hospitals After Progression?
- What Happens After Osimertinib Stops Working?
- What Is Sequencing and Why the Order of Drugs Matters
- Why Does Targeted Therapy Stop Working? The Biology of Resistance
Long-Term Response, Stopping & Survivorship
- Am I Still a Cancer Patient? Identity After Long-Term Response
- Bone Health, Heart Health and Late Effects to Monitor
- Can You Ever Stop Targeted Therapy If the Cancer Is Gone?
- Drug Holidays: Are Planned Breaks Safe?
- Follow-Up Schedule After Stopping Targeted Therapy
- How Long Do You Have to Stay on Targeted Therapy?
- Long-Term Effects of Taking a TKI for 5 or 10 Years
- Restarting Treatment After a Break
- Treatment-Free Remission in CML: Who Can Stop Their TKI?
- Your Survivorship Care Plan: What Should Be In It
Monitoring, Scans & Response Assessment
- Are Tumour Markers Reliable on Targeted Therapy?
- Complete Response, Partial Response, Stable Disease: What Each Means
- Do I Need Regular Brain MRIs on Targeted Therapy?
- How Doctors Measure Whether Targeted Therapy Is Working
- How Often Will I Need Scans on Targeted Therapy?
- How to Read a CT or PET Scan Report Without Panicking
- Is 'Stable Disease' Good News or Bad News?
- Scanxiety: How to Get Through the Wait for Scan Results
- Tumour Flare and Pseudoprogression: When Growth Isn't Really Growth
- Understanding PFS, OS and Median Survival Without Losing Hope
- What Happens at a Follow-Up Visit: A Walk-Through
- Which Blood Tests Are Repeated Every Month and Why
- ctDNA Monitoring: Can a Blood Test Predict Progression Early?
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Common questions
Frequently asked questions
Does needing combination therapy mean my cancer is more serious than before?
Not necessarily. The decision to use a combination is usually a statement about your cancer's current biology — specifically the resistance mechanism your oncologist is trying to overcome — rather than a direct measure of how advanced the disease is. Some patients move to combinations as a deliberate early strategy, not only after progression. Your oncologist can explain the specific reason the combination is recommended for your situation, and what it is designed to achieve.
Can my body handle two or three drugs at the same time?
That depends on which drugs, the doses, your general fitness, and your other medical history. Many people tolerate combination regimens well, particularly when dose adjustments are made early. Your team will assess your fitness before starting and monitor you closely during the first cycles. Dose reduction is a standard tool — it reduces side effects without necessarily reducing how well the treatment works, and it is used routinely as part of managing any combination, not only in difficult situations.
What is a liquid biopsy and will I need one?
A liquid biopsy is a blood test that detects fragments of tumour DNA circulating in your bloodstream. It can sometimes identify resistance mutations without a surgical tissue sample, which makes it useful when a tissue biopsy is not practical. Whether you need one depends on what information it would add and whether a tissue biopsy is an option for your situation. Your oncologist will recommend the approach most likely to give useful results for your cancer type.
What is combination therapy at this stage actually trying to achieve?
The aim varies by cancer type and how much treatment you have already had. In some situations, combination therapy is intended to achieve a deep and lasting response. In others, the goal is to control the disease, preserve quality of life, and extend the time before further progression. Your oncologist should be direct with you about what this specific regimen is designed to achieve in your case, so you can make decisions that reflect what matters most to you.
How long before we know whether the combination is working?
Most regimens are reassessed after two or three cycles — typically six to nine weeks for a three-weekly schedule. Your team will look at imaging, blood markers where relevant, and how you are feeling. If the combination is not achieving a response, the plan is adjusted. Reassessment is built into the timeline precisely so decisions are made on evidence. Ask your oncologist to tell you the reassessment date before your first cycle so you are not waiting without a clear timeline.
Should I consider a clinical trial when standard treatment has stopped working?
This is often a good time to ask about trials. Studies at the point of progression are specifically designed for patients in your situation and may test combinations or agents not yet available as standard treatment. Eligibility depends on your cancer type, your previous treatments, and your current fitness. Your oncologist can tell you whether there are relevant trials open at CION or a collaborating centre, and what enrolment would involve. Asking is always reasonable.