Chronic myeloid leukemia (CML) is a blood cancer characterized by the uncontrolled proliferation of myeloid cells - the blood cell lineage that includes neutrophils, monocytes, and their precursors. CML accounts for about 15% of adult leukemia diagnoses, with approximately 8,450 new cases per year in the United States.
CML was the first human cancer linked to a specific chromosomal abnormality: the Philadelphia chromosome, discovered in 1960. This chromosome arises from a translocation between chromosomes 9 and 22, in which genetic material is swapped, creating a fusion gene called BCR-ABL1. This gene produces a protein with abnormally high and unregulated tyrosine kinase activity - meaning it continuously sends growth signals that prevent normal cell death and drive uncontrolled proliferation.
The discovery of BCR-ABL1 led to a revolution in cancer treatment. In 2001, the drug imatinib was approved - a small molecule that blocks BCR-ABL1's kinase activity by fitting into its ATP-binding pocket, cutting off the growth signal. Imatinib was called the 'magic bullet' by Time magazine and transformed CML from a disease with 10-20% annual mortality to one with only 1-2% annual mortality.
Today, five tyrosine kinase inhibitors (TKIs) are approved for CML: imatinib, dasatinib, nilotinib, bosutinib, and ponatinib. Despite their success, 20-30% of patients develop resistance to these drugs either before starting treatment (primary resistance) or during therapy (acquired resistance). Overcoming TKI resistance is now the central challenge in CML management.
The most common mechanism of TKI resistance is the development of point mutations in the kinase domain of BCR-ABL1. Over 100 different mutations affecting more than 50 amino acids have been identified. These mutations alter either the shape of the BCR-ABL1 protein or the specific site where TKIs bind, preventing the drug from docking and blocking the kinase.
The most clinically problematic mutation is T315I - nicknamed the 'gatekeeper' mutation because it sits at the point where all approved drugs must enter to inhibit the kinase. T315I confers resistance to all first- and second-generation TKIs (imatinib, dasatinib, nilotinib, bosutinib) and only responds to the third-generation inhibitor ponatinib. This mutation is found in 4-20% of resistant patients and is associated with disease progression to blast crisis - the most aggressive form of CML.
A particularly dangerous variant is compound mutations - two or more mutations occurring within the same BCR-ABL1 molecule (the same cancer cell clone). While each mutation alone might be sensitive to a specific TKI, the combination can create resistance to that drug. For example, T315I/E255V together are resistant to ponatinib even though each mutation alone responds to it.
Mutations in the BCR-ABL1 gene are more common with disease progression: they are found in approximately 75% of CML patients who have progressed to blast crisis. Guidelines recommend mutational testing for all patients who fail first-line imatinib or second-generation TKIs, or who progress to accelerated phase, to inform the choice of the most appropriate next therapy.
For TKIs to work, they must reach the BCR-ABL1 protein inside leukemia cells at sufficient concentrations. This depends on a balance between proteins that import drugs into cells and those that export them back out. Changes in either direction can cause resistance.
The main transporter responsible for TKI uptake is OCT1, encoded by the SLC22A1 gene. Lower OCT1 expression or activity reduces how much imatinib enters cells, making it less effective. Some studies found OCT1 activity at diagnosis predicts response, though results are variable. Patients with low OCT1 may respond better to dasatinib, which can enter cells by diffusion and does not depend on OCT1.
On the export side, P-glycoprotein (P-gp), encoded by ABCB1, and the breast cancer resistance protein (BCRP), encoded by ABCG2, pump TKIs out of cells, reducing intracellular drug concentrations. High expression of these efflux transporters is associated with poor TKI response. BCRP is particularly relevant in leukemia stem cells, where it may shield these treatment-resistant cells from TKI-induced death.
Genetic variants in transporter genes also influence drug response. For example, a specific variant (rs2231142) in the ABCG2 gene reduces BCRP protein function and is associated with better molecular responses to imatinib. Testing patients for such variants may help predict who will respond well and who needs an alternative drug from the outset.
Even when BCR-ABL1 is effectively blocked by a TKI, cancer cells can develop resistance by activating alternative signaling pathways that independently provide survival and proliferation signals. Several such pathways have been identified in CML.
The SRC family kinases - including LYN and HCK - are frequently overexpressed in TKI-resistant CML. When BCR-ABL1 is inhibited, these related kinases can take over, activating AKT (a pro-survival signal) and STAT5 (a proliferation signal). This was the rationale for developing dual SRC/ABL inhibitors like dasatinib and bosutinib, which block both BCR-ABL1 and SRC kinases simultaneously.
The JAK/STAT pathway is another escape route. Cytokines released by both leukemia cells and bone marrow niche cells activate JAK2, which then phosphorylates STAT3 and STAT5. Persistent STAT5 activation drives proliferation and upregulates P-gp, creating a double resistance mechanism. STAT3 phosphorylation contributes to the resistant phenotype through additional anti-apoptotic gene targets.
The PI3K/AKT/mTOR and RAS/MAPK pathways are also activated in resistant CML cells. AKT inactivates the apoptosis-promoting protein BAD, allowing cancer cells to survive even under drug stress. RAS mutations accumulate in blast crisis, providing BCR-ABL1-independent proliferation signals. The accumulation of reactive oxygen species (ROS) driven by BCR-ABL1 further accelerates genomic instability, generating new mutations and activating these pathways.
Leukemia stem cells (LSCs) are a small population of highly drug-resistant cells at the root of CML. Unlike the bulk of CML cells that respond to TKIs, LSCs can persist even when BCR-ABL1 is fully suppressed by medication, because they rely on alternative, BCR-ABL1-independent survival pathways.
LSCs undergo metabolic reprogramming, shifting their energy production toward mitochondrial oxidative phosphorylation (OxPHOS) rather than the aerobic glycolysis (the Warburg effect) seen in most cancer cells. This metabolic adaptation supports LSC survival and is not suppressed by TKIs. Drugs targeting mitochondrial respiration - such as tigecycline - can overcome this resistance and selectively kill LSCs.
LSCs also upregulate autophagy - a cellular self-digestion process that generates energy and building blocks when nutrients are scarce. Basal autophagy is higher in LSCs than in normal stem cells and is further increased by TKI treatment, acting as a survival mechanism against drug-induced stress. Combining TKIs with autophagy inhibitors is therefore an active area of investigation.
Fatty acid metabolism through the enzyme ALOX5 is also upregulated in CML stem cells in a BCR-ABL1-independent manner. ALOX5 modulates beta-catenin levels, a pathway linked to stem cell self-renewal. Importantly, loss of ALOX5 impairs LSC function and prevents CML development in mouse models, making it an attractive therapeutic target specifically for eliminating the resistant stem cell pool.
Epigenetic alterations - changes to gene activity that do not alter the DNA sequence itself - contribute to CML progression and TKI resistance. DNA hypermethylation of tumor suppressor genes such as p15, RASSF1A, and EBF2 is a frequent event in advanced CML, and their silencing removes important brakes on cell growth. Epigenetic changes become more common as CML progresses from chronic phase to blast crisis.
MicroRNAs (miRNAs) are small regulatory molecules that suppress gene expression. In CML, the expression of several miRNAs is altered compared to healthy individuals. Notably, miR-150 and miR-146a are reduced at diagnosis and in advanced disease, but normalize in patients who respond to TKIs - making them potential biomarkers of treatment response. Oncogenic miRNAs like miR-21 and miR-17 are upregulated in imatinib-resistant cells, helping cancer cells survive.
The bone marrow microenvironment (BMM) provides a protective shelter for CML cells. Leukemia cells remodel the niche to create a more supportive environment, changing cell adhesion molecules, altering cytokine levels, and establishing a hypoxic (low-oxygen) zone. Cytokines like IL-6, IL-7, and CXCL12 produced by bone marrow stromal cells activate survival pathways in CML cells, partially counteracting TKI-induced death.
The immunological status of patients also matters: CML cells create an immunosuppressive environment by expanding regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSCs) while exhausting anti-cancer T cells. This immune evasion may contribute to resistance by protecting residual disease even when drug concentrations are adequate.
For most cases of TKI resistance, the immediate clinical response is to perform BCR-ABL1 mutation testing and switch to a more potent TKI. Second-generation drugs (dasatinib, nilotinib, bosutinib) overcome most imatinib-resistant mutations, while ponatinib and the newer asciminib (which targets a different pocket on BCR-ABL1) address many second-generation-resistant mutations including T315I.
Combination therapy is emerging as a strategy to prevent or overcome resistance. Because TKI resistance often involves multiple mechanisms simultaneously, combining a TKI with an inhibitor targeting an alternative survival pathway (such as PI3K, hedgehog, autophagy, or BCL-2) may be more effective than sequential single-agent approaches. Preclinical data supports several such combinations.
Artificial intelligence and machine learning are beginning to contribute to resistance prediction. By analyzing clinical data, drug transport profiles, mutation patterns, and gene expression data, AI models may eventually predict which patients are at risk of resistance before it occurs, enabling pre-emptive therapy adjustment rather than reactive changes after failure.
Patient adherence to therapy remains a critical but often overlooked contributor to treatment failure. Studies show that significant proportions of patients who appear clinically resistant are simply not taking their medication consistently. Interventions to improve adherence - including patient education, simplified dosing, and regular monitoring - are essential components of CML management that complement advances in molecular medicine.