Tyrosine kinase inhibitors (TKIs)

What it is

Tyrosine kinase inhibitors (TKIs) are targeted cancer drugs that block abnormal kinase signals; more than 40 have been approved in humans. They are usually small-molecule medicines taken by mouth, although the exact schedule depends on the drug and cancer type. The key idea is that some cancers depend on overactive tyrosine kinases, which are enzymes that switch growth and survival pathways on. By blocking those enzymes, TKIs can slow tumour growth, trigger cancer-cell death, or reduce blood-vessel formation that feeds tumours.

TKIs are not one single medicine. They are a drug class used across blood cancers and solid tumours. Some are highly specific for one abnormal target, while others inhibit several kinases at once. A classic example is imatinib for chronic myeloid leukaemia (CML), where the BCR::ABL1 fusion protein is the main disease driver. In India, many TKI-treated cancers are managed in medical oncology or haemato-oncology clinics, and access may vary by indication, brand, and generic availability.

Common TKI target	Example cancers	Example TKIs
BCR::ABL1	CML, Ph+ ALL	imatinib, dasatinib, nilotinib, bosutinib, ponatinib
EGFR	Non-small cell lung cancer	gefitinib, erlotinib, afatinib, osimertinib
ALK	Non-small cell lung cancer	crizotinib, alectinib, brigatinib
HER2	Some breast and gastric cancers	lapatinib, tucatinib
VEGFR / multi-target	Kidney cancer, liver cancer, thyroid cancer	sunitinib, sorafenib, pazopanib, lenvatinib
KIT / PDGFRA	Gastrointestinal stromal tumour (GIST)	imatinib, sunitinib, regorafenib, avapritinib

How it works

Tyrosine kinases are enzymes that transfer phosphate groups to tyrosine residues on proteins. This phosphorylation acts like a signal switch inside cells. In normal tissue, these pathways help regulate growth, repair, metabolism, and survival. In cancer, mutations, gene fusions, amplifications, or overexpression can leave the kinase stuck in the "on" position.

TKIs work by binding to the kinase domain, often at or near the ATP-binding pocket, and preventing the enzyme from passing along growth signals. Depending on the drug, this can inhibit receptor tyrosine kinases on the cell surface, such as EGFR or VEGFR, or non-receptor kinases inside the cell, such as ABL.

This is why biomarker testing matters. A TKI is most useful when the tumour actually carries the target it is designed to block. For example, EGFR TKIs are used when lung cancer has an activating EGFR mutation, and BCR::ABL1 TKIs are used when testing confirms Philadelphia chromosome-positive disease. Without the target, the same drug may offer little or no benefit.

Evidence and uses

TKIs changed cancer treatment because they allowed therapy to be matched to tumour biology rather than only to the organ where the cancer started. In CML, BCR::ABL1-targeted TKIs turned a once life-threatening disease into a long-term controllable condition for many patients. Reviews of CML treatment describe TKI therapy as one of the clearest examples of precision oncology.

Major uses include:

1. Blood cancers: CML is the best-known example. Some acute lymphoblastic leukaemias with Philadelphia chromosome positivity also use TKIs.
2. Lung cancer: EGFR, ALK, ROS1, RET, MET, and other altered pathways may be treated with specific TKIs after molecular testing.
3. GIST: KIT or PDGFRA-driven tumours often respond to TKIs, though resistance mutations can develop.
4. Kidney, liver, thyroid, and other solid tumours: Multi-target TKIs that affect VEGFR and related pathways can slow tumour growth and angiogenesis.

Response depends on the match between drug and target, the stage of disease, prior treatment, and whether resistance mutations emerge. Some patients have deep and durable responses; others have only temporary benefit. Cancer cells can become resistant through new mutations in the kinase, activation of alternate pathways, or poor drug penetration into certain sites such as the brain.

A practical point is that different TKIs in the same disease are not interchangeable without a clinician's guidance. For example, in CML, the choice among imatinib, dasatinib, nilotinib, bosutinib, or ponatinib depends on disease phase, mutation profile, side-effect risks, and treatment goals. In lung cancer, first-line and later-line TKI choices depend on the exact mutation and whether resistance mutations appear after earlier therapy.

Safety and interactions

Although TKIs are often described as "targeted," they can still cause serious side effects. Common problems across the class include diarrhoea, nausea, rash, mouth sores, fatigue, swelling, liver test abnormalities, and low blood counts. Some TKIs have distinctive risks such as QT prolongation, high blood pressure, bleeding, clotting, heart failure, lung inflammation, or impaired wound healing.

Safety varies a lot by drug. For example:

Safety issue	Seen with some TKIs	Why it matters
Liver toxicity	many TKIs	May require blood-test monitoring or dose changes
QT prolongation	some EGFR and BCR::ABL1 TKIs	Can affect heart rhythm
Fluid retention / pleural effusion	dasatinib	May cause breathlessness
Arterial occlusive events	ponatinib, some others	Can increase risk of stroke or heart attack
Skin rash / diarrhoea	EGFR TKIs	Common and sometimes treatment-limiting
Hypertension / proteinuria	VEGFR-targeting TKIs	Needs BP and kidney monitoring

Drug interactions are important. Many TKIs are metabolised through CYP3A4 and can interact with azole antifungals, certain antibiotics, anti-seizure medicines, rifampicin, and some HIV medicines. Acid-reducing drugs such as proton pump inhibitors can reduce absorption of certain TKIs. Grapefruit and grapefruit juice may also affect levels of some TKIs. Because many cancer patients in India use supplements or traditional remedies alongside treatment, it is important to tell the oncology team about all herbs, powders, tablets, and over-the-counter products. Do not start or stop a TKI, supplement, or herbal product without checking with an oncologist or pharmacist.

When to see a clinician

Contact your cancer team promptly if you develop fever, severe diarrhoea, new rash with blistering, jaundice, chest pain, sudden shortness of breath, fainting, severe headache, leg swelling, unusual bleeding, or a rapid drop in urine output while on a TKI. These can signal infection, liver injury, heart rhythm problems, blood clots, lung toxicity, or other urgent complications.

Regular follow-up is part of safe TKI therapy. Monitoring may include complete blood counts, liver and kidney tests, ECGs, blood pressure checks, urine protein, and disease-specific tests such as BCR::ABL1 PCR in CML or repeat molecular testing at progression in solid tumours. If cost, access, or adherence is becoming difficult, discuss it early; missed doses can reduce effectiveness and may contribute to resistance.

Limitations and open questions

TKIs are highly effective in some cancers, but they are not a cure-all. Not every tumour has a targetable kinase abnormality, and even when one is present, resistance often develops over time. Some patients cannot tolerate a given TKI because of side effects or interactions with other medicines.

There are also unanswered questions about the best sequence of TKIs, how to manage minimal residual disease, and when treatment can be safely stopped in selected diseases such as CML. Evidence is strong for several approved indications, but it is weaker for off-label use, combinations, and some rare mutations. Newer TKIs continue to be developed to overcome resistance mutations and improve brain penetration, but long-term comparative data are still evolving.