Chronic Myeloid Leukemia (CML) is a blood cancer starting in the bone marrow, caused by a genetic change that leads to an overproduction of abnormal white blood cells. Due to targeted therapies, CML is now a manageable chronic condition for most people, with a nearly normal life expectancy.
The effectiveness of these treatments is tracked using specific markers that show how well the therapy is controlling the cancer. This monitoring allows doctors to gauge the response and make timely adjustments to the treatment plan.
Defining Major Molecular Response
The cause of CML is a genetic event where a piece of chromosome 9 attaches to chromosome 22. This creates an abnormally short chromosome 22, known as the Philadelphia chromosome, and a new fusion gene called BCR-ABL1. This gene produces a protein that signals CML cells to grow uncontrollably. The goal of treatment is to shut down this protein’s activity and eliminate cells carrying the BCR-ABL1 gene.
A milestone in this process is achieving a Major Molecular Response (MMR), which is a precise measurement from a blood sample. Doctors use a sensitive laboratory test called a quantitative polymerase chain reaction (qPCR) to count BCR-ABL1 gene transcripts. These transcripts are the instruction molecules produced by the gene, and the test can detect even tiny amounts of them in the blood.
MMR is defined as a reduction in the level of BCR-ABL1 transcripts to 0.1% or less, based on a standardized value known as the International Scale (IS). Reaching this level signifies a 3-log reduction, meaning the amount of the cancer-driving gene has decreased by at least 1,000-fold from the baseline measurement. This is like reducing one million transcripts in a sample down to one thousand.
The Significance of Achieving MMR
Achieving a Major Molecular Response is a primary objective of CML therapy because it signifies that the treatment is working well to control the cancer at a molecular level. This milestone is linked with excellent long-term health outcomes. Patients who reach and maintain MMR have a high likelihood of long-term survival and are protected against the disease worsening.
A benefit of reaching MMR is the lowered risk of the CML progressing to more advanced stages, known as the accelerated or blast phase. These phases are more difficult to treat and are characterized by a rapid increase in immature blood cells. MMR serves as an indicator that the prescribed therapy, a daily pill called a tyrosine kinase inhibitor (TKI), is effectively suppressing the BCR-ABL1 gene.
The measurement provides prognostic information, confirming the treatment strategy is successful. Sustaining this response is as important as reaching it, as it indicates ongoing control of the disease.
Monitoring and Treatment Timelines
CML treatment involves a structured monitoring schedule to ensure treatments are effective. For a patient on TKI therapy, this involves having blood drawn for a qPCR test to measure BCR-ABL1 levels every three months. This frequent testing is common during the first few years of treatment to track the response.
Healthcare teams follow guidelines that set specific treatment goals. An early molecular response, defined as a BCR-ABL1 level of 10% or less, is a target within the first three to six months of starting therapy. These early benchmarks are predictors of future success.
The goal of achieving MMR is set for the 12 to 18-month mark of treatment. While some individuals may reach this milestone sooner, this timeframe is a standard checkpoint. If MMR is achieved and maintained, monitoring frequency may be reduced to every three to six months after a couple of years.
Responses Beyond MMR
After a patient achieves and sustains MMR, the next goal is often a deeper level of response. These deeper responses, defined as Deep Molecular Response (DMR), indicate that the BCR-ABL1 gene has been reduced to very low or undetectable levels. These milestones include specific points on the International Scale.
One such point is MR4, representing a 4-log reduction (BCR-ABL1 level of 0.01% on the IS). An even deeper response is MR4.5, which is a 4.5-log reduction (a level of 0.0032% on the IS). Reaching these low levels of disease opens up new possibilities for long-term management.
The significance of achieving a stable and sustained DMR is that it is a prerequisite for considering Treatment-Free Remission (TFR). TFR is the opportunity for certain patients to safely stop their daily TKI medication while remaining in remission under close medical supervision. This option is explored after a patient has maintained a deep response, like MR4.5, for a significant period, such as two years or more.
When MMR is Not Achieved
If a patient does not reach MMR within the expected timeframe, it signals the medical team to reassess the treatment plan. Several factors can contribute to a suboptimal response to the initial therapy.
One reason is the development of resistance to the TKI being used, which can happen if new mutations arise in the BCR-ABL1 gene. Another factor can be challenges with treatment adherence, as TKIs are daily oral medications. A patient may also experience side effects that make it difficult to tolerate the full dose, impacting the drug’s effectiveness.
When MMR is not achieved, the next step is an evaluation that may include testing for BCR-ABL1 mutations. Based on these findings, a clinician may recommend switching to a different TKI. Changing medications is a standard and often successful strategy to get the CML under control and on track toward achieving MMR.