Exploration of treatment‐free remission in CML, based on molecular monitoring

Abstract Background Typical chronic myelogenous leukemia (CML) is a myeloproliferative neoplasm caused by t(9; 22)(q34; q11) translocation. This chromosomal translocation forms the BCR::ABL1 fusion gene. The tyrosine kinase encoded by the BCR::ABL1 is considered to be the main pathogenic diver. BCR::ABL1 is not only a therapeutic target, but also a monitoring target. Monitoring of BCR::ABL1 reveals the progression of the disease and guides the next treatment. Now for CML, the target of treatment has been focused on treatment‐free remission (TFR). Methods We conducted a literature review of current developments of treatment‐free remission and molecular monitoring methods. Results More effective and sensitive CML monitoring methods such as digital droplet PCR (ddPCR) and next generation sequencing (NGS) have further studied the measurable residual disease (MRD) and clonal heterogeneity, which provides strong support for the exploration of TFR. We discussed some of the factors that may be related to TFR outcomes at the molecular level, along with some monitoring strategies. Conclusion Currently, predictive indicators for treatment‐free remission outcomes and recurrence are lacking in clinical practice. In future, treatment‐free remission research should focus on combining the clinical indicators with molecular monitoring and biological markers to personalize patient conditions and guide clinicians to develop individualized treatment plans, so that more patients with CML can achieve safer and stabler treatment‐free remission.

on chromosome 9 with the BCR gene fragment on chromosome 22, 1 resulting in the fusion gene, BCR::ABL1, which encodes a large number of abnormal tyrosine kinases that promote disease development.ABL overexpression is a key therapeutic target for CML.Since the emergence of tyrosine kinase inhibitors (TKIs) in 2001, prognosis of patients with CML has improved greatly. 2 This progress has improved the life expectancy of patients with CML to as much as that of the normal population. 3Several TKIs have been used clinically till date.Long-term use of imatinib as a first-line treatment for patients with primary CML-chronic phase confirmed the 10-year survival rate to be 80%-90%.Introduction of secondgeneration TKIs (such as nilotinib, dasatinib, bosutinib, and radotinib) and the third-generation TKI (such as ponatinib) further accelerated and improved the treatment response rates and response depths in patients. 4A subset of studies confirmed that patients receiving second-generation TKIs have higher and faster major molecular response (MMR) rates than those receiving imatinib. 5,6Third-generation TKIs have shown stronger therapeutic efficacy and effectiveness in some patients with drug resistance mutations. 7,8he formulation of different treatment plans must consider various factors, including patient complications, disease progression, TKI resistance, drug toxicity, and treatment costs.In the past, in pursuit of higher overall survival (OS) rates, CML treatment was often lifelong.However, lifelong use of TKI might cause adverse events, 9 which could affect the patient's physical condition and quality of life, leading to a decline in patient compliance, and affecting the final treatment response. 10Therefore, in recent years, the ultimate therapeutic goal for CML has focused on treatment-free remission (TFR), that is, stopping TKI therapy after achieving a sustained deep molecular response (DMR, Table 1) without affecting the expected therapeutic effect.The realization of TFR can reduce drug-induced adverse events, lessen the financial burden on patients, improve their quality of life, and be important in some special circumstances, such as pregnancy.Sustained DMR is a prerequisite for achieving TFR; therefore, continuous and precise molecular monitoring is essential.Monitoring methods for BCR::ABL1 have evolved from the initial reverse transcription-PCR (RT-PCR) to quantitative RT-PCR (qRT-PCR) and, more recently, to digital PCR. 11ABL kinase region mutations can be analyzed by direct sequencing (Sanger sequencing with sensitivity of 10%-20%) or next-generation sequencing (NGS) of the BCR::ABL kinase region to identify the ABL kinase region point mutations 12 and TKI resistance, and guide subsequent treatment selection.Although TFR is still in the exploratory stage, the emergence of new monitoring methods and high-quality clinical research (Tables 2 and 3) can make its widespread realization possible.

| CURRENT GUIDELINES FOR TFR
To explore TFR better, we should first understand the latest guidelines for TKI discontinuation.Both the European Leukemia Net (ELN) and the National Comprehensive Cancer Network (NCCN) include the detailed criteria for TKI discontinuation (Table 4).The 2020 ELN recommendations regarding TFR are divided into mandatory, minimal, and optimal requirements. 13First, the ELN guidelines require that patients with TKI discontinuation be in their first CML-CP, actively be engaged in structured communication before implementation, and follow the conditions for performing high-quality rapid quantitative PCR based on the International Scale (IS).The patient needs to agree with monthly monitoring for the first 6 months, followed by monitoring every 2 months from 6 to 12 months, and every 3 months thereafter.For standardized monitoring, patients need to have typical e13a2 or e14a2 BCR::ABL1 transcript, to have been on TKI for more than 5 years or on two generations of tyrosine kinase inhibitor (2GTKI) for more than 4 years, and to have achieved DMR for more than 2 years, with no prior treatment failure.TKI treatment for more than 5 years, MR4 lasting for more than 3 years, and MR4.5 lasting for more than 2 years are considered optimal conditions.The latest V3.2022NCCN guidance for CML requires that, before TKI discontinuation, patients be over 18 years old in the CML-CP, with no prior history of accelerated or blast phase CML, and have quantifiable BCR::ABL1 transcripts. 14For the monitoring method, the NCCN guidelines have more detailed criteria, requiring a qPCR assay with a sensitivity of at least MR4.5 and results to be available within 2 weeks.The NCCN guidelines require patients to have had MR4 for more than 2 years (at least four tests with at least 3 months between each test) before stopping TKI.The frequency of monitoring after discontinuation is similar to that mentioned in the 2020 ELN guidelines; however, patients who remain in the MMR would require quarterly monitoring after the  TFR should be attempted only if the condition of the patient with CML is well controlled.Currently, qRT-PCR is used internationally as the gold standard for detecting BCR::ABL transcripts, 15 and its detection results are used as important indicators for evaluating the efficacy and prognosis of patients with CML.Digital droplet PCR (ddPCR), which has emerged recently, uses a water-oil emulsion droplet system that divides the sample into thousands of individually repeated PCRs. 16This high repeatability significantly improves the sensitivity of detection and is not affected by amplification differences.
Although ddPCR technology has not been widely promoted internationally, it has been used to explore TKI discontinuation owing to its high sensitivity in detecting BCR::ABL1 transcripts. 11,17,18Several high-quality studies using qRT-PCR have demonstrated the importance of monitoring minimal residual disease (MRD) during TKI therapy.A multicenter, phase II, single-treatment arm, open-label clinical trial, conducted by Takahashi et al., enrolled 78 patients with CML-CP. 19They monitored patient MRD using qRT-PCR with an accuracy of no less than MR4.5.The results showed that the treatment-free survival curve was significantly better in patients without undetectable MRD (Undetectable MRD was defined as undetectable BCR::ABL1 transcript with MR5) than in patients with undetectable MRD (3-year treatment-free survival, 75.6% vs. 48.6%,respectively; p = 0.0126 by the log-rank test). 19A single-center, prospective, pilot cohort clinical trial conducted by Seguro et al. enrolled 31 patients with CML-CP. 20All participants were monitored in the same laboratory since diagnosis using qRT-PCR, with a sensitivity of at least MR5.The results showed that a more profound molecular response (MR4.5) was associated with a lower risk of molecular relapse. 20In the TKI discontinuation trial, qRT-PCR with a sensitivity of at least 0.01% (4 log reduction or MR4) on the International Scale (IS) was the standard definition of DMR. 21However, owing to the differences in the sensitivity of qRT-PCR in different laboratories, some clinical trials cannot guarantee that all monitored samples come from the same laboratory, which leads to uncertainty regarding whether a patient at MR4 is actually at MR4.5 or MR5.Owing to its high sensitivity and accuracy, ddPCR can overcome these limitations.Studies comparing qRT-PCR and ddPCR monitoring have shown that ddPCR highlights the high heterogeneity of MRD levels in patients belonging to the same MR class before TKI discontinuation. 11Moreover, absolute numbers of BCR-ABL1 copies/μL were detected and measured by ddPCR in MR4.5 or MR5 patients with undetectable BCR-ABL1 transcripts on qRT-PCR. 11Presently, ddPCR is effective for exploring the field of TFR.According to the conversion coefficients of ddPCR and BCR::ABL1 ratios stipulated by IS, the significant cutoff point is usually set to 0.0023%. 22The TFR study by Nicolini et al. used ddPCR as a means of molecular monitoring and showed that MRD, as determined by ddPCR, is a key factor for predicting successful treatment-free remission in patients with CML-CP. 22Similarly, the results of a nonrandomized clinical trial showed that the molecular recurrence rate of patients without UMRD by both ddPCR and qRT-PCR was 10.3% (nine of 87), which was much lower than that of patients without UMRD detected by qRT-PCR (64.3%, 36 of 56). 23Although there are some limitations to qRT-PCR, there is consistency between qRT-PCR and ddPCR regarding the effect of the depth of molecular response on TFR acquisition.More targeted and personalized TKI therapy has increased the need for a more sensitive quantitative analysis of MRD, and more high-quality clinical trials would be required to further optimize ddPCR applications, so that ddPCR can be widely used for routine MRD monitoring in patients with CML.

| BCR::ABL1 DNA
DNA-PCR for BCR::ABL1 has several advantages over RNA-PCR.First, DNA samples are more stable than RNA samples, thereby reducing degradation during sample transport.Second, DNA-PCR standardization can use upstream control genes rather than cell numbers. 24However, DNA-PCR is difficult to implement compared to RNA.Owing to the large breakpoint region between BCR and ABL in the BCR::ABL1 fusion gene, multiple primers are required to determine the specific breakpoint.A study of BCR::ABL1 genomic DNA-PCR response kinetics showed that BCR::ABL1 DNA could be detected in 48% of samples in which BCR::ABL1 mRNA could not be detected. 25ecently, there have been clinical studies incorporating BCR::ABL1 DNA-PCR. 1,26,27Ross et al. conducted a prospective trial in 40 patients with CML-CP and showed a higher incidence of TFR in patients with undetectable BCR::ABL1 DNA; however, the difference was not statistically significant. 26A recent study proposed a "traffic light" model based on DNA and RNA analysis during DMR maintenance prior to TKI discontinuation.According to the detection of BCR::ABL1 DNA and RNA, the model was divided into three levels of green, yellow, and red. 27Green indicated no DNA or RNA residue detected by ddPCR, indicating a low recurrence rate (20% relapse after 18 months).Yellow indicated that ddPCR only detected DNA and not RNA, suggesting a moderate recurrence rate (43% relapse after 18 months).Red indicated that ddPCR detected residual BCR::ABL1 DNA as well as RNA before TKI discontinuation, implying that the patients had a higher recurrence rate after discontinuation (80% relapse after 18 months). 27his traffic light stratification model provided new possibilities of prediction before TKI discontinuation.A more precise study by Pagani et al. reported similar results. 1 Sample cells were sorted into granulocytes, monocytes, B lymphocytes, T lymphocytes, and NK cells before DNA-PCR.Of the 20 patients who maintained TFR for more than 1 year, residual BCR::ABL1 DNA was detected in 18, all in the B lymphocyte population; in a small number of patients, it was also detected in monocytes, T lymphocytes, and NK cells.Notably, BCR::ABL DNA residues were not found in granulocytes of the 20 patients, although they were found in the granulocytes of all patients who relapsed within 3 months after TKI discontinuation. 1 Thus, the detection of BCR::ABL1 DNA in granulocytes seemed to predict molecular relapse during TFR.However, further studies would be required to confirm this conclusion.Moreover, whether cell sorting can be broadly performed in all laboratories would need further consideration.

| Type of BCR::ABL1 transcripts
CML originates from a translocation in chromosomes 9 and 22.In most patients, the breakpoints on chromosome 9 are located upstream of exon 2 of the ABL1 gene, whereas those on chromosome 22 are located downstream of exon 13 or 14 of the BCR gene (Figure 1).The fusion genes produced at these breakpoints are referred to as e13a2 and e14a2, which also called major BCR::ABL1 or p210.The proportion of patients with e13a2 and/or e14a2 is approximately 98%. 28ranscription type is considered a variable that may influence treatment response and TFR.e14a2 has been suggested to have a relatively better therapeutic outcome and prognosis than e13a2. 29,30owever, other studies have found no significant difference in long-term survival between patients expressing e13a2 and e14a2. 31,32 similar situation was observed with respect to whether the transcription type affected TFR; some studies found the expression of e14a2 to be associated with a higher rate of TFR, 33,34 although the results of other studies showed no significant relationship between transcription type and TFR. 20,357][38] Since the e13a2 transcript is reduced by 75 bp compared to the e14a2 transcript, qRT-PCR had a higher amplification efficiency for e13a2 than e14a2.This led us to overestimate the e13a2 BCR::ABL1 residue. 36This could partly explain why patients expressing e13a2 achieve DMR and TFR more quickly and easily, respectively.Notably, ddPCR effectively eliminates this bias.We found that an experiment using ddPCR, to detect BCR::ABL1 residues, did not reveal a significant effect of transcription type on the TFR. 35here could be a 1.8-6.5-folddifference between the different transcripts 36 ; therefore, we recommended using ddPCR for exploring the effect of transcription type on TFR.
Since standard qRT-PCR is inefficient in detecting atypical fusion genes and is not suitable for molecular monitoring, patients often require customized PCR protocols or FISH analysis. 39Most guidelines do not support attempts to discontinue treatment for patients with atypical transcripts.However, a TKI discontinuation trial by Dragani et al. in patients expressing atypical transcripts showed promising results, with five of seven patients maintaining a TFR for more than 19 months. 40This suggested that under strict monitoring, TKI discontinuation may be attempted in patients with CML who are expressing atypical transcripts.
There is currently no guideline to recommend transcript types for TFR, and there are not enough clinical trials to support the significant effects of different transcript types on TFR; therefore, we cannot recommend that transcript types be considered before considering TKI discontinuation (only e13a2 and e14a2).

| BCR::ABL1 kinase domain mutations
Mutations in the BCR::ABL1 kinase domain (KD) are considered to be the main cause of acquired TKI resistance 13,14 (Table 5 41 ).][44] Studies have found the median time for BCR::ABL1 doubling in imatinib-resistant patients to be significantly shorter than that in nonresistant patients, 42,44 which might explain why imatinib-resistant patients are more prone to molecular relapse after TKI discontinuation.The latest NCCN guidelines for TKI discontinuation no longer exclude patients with TKI resistance. 14In a recent TKI discontinuation trial on 10 patients with a history of BCR::ABL1 kinase domain mutations, five gained TFR for more than a year. 45This suggested that patients with TKI resistance may indeed achieve a TFR.Sanger sequencing is the most widely used technique to detect KD mutations, with a sensitivity of approximately 10%-20% at BCR::ABL1 ratio ≥1%. 46Therefore, if Sanger sequencing is used to detect KD mutations in patients in TKI discontinuation trials, it must be performed in the treatment phase before TKI discontinuation. 47The NGS-based BCR-ABL1 KD mutation screening conducted by Soverini et al showed evidence of mutations undetectable by Sanger sequencing in 80 out of 236 patients (34%), of whom 42 (18% of the total patient population) had low-level mutations somehow relevant for clinical decision-making, 48 which means the sensitivity and accuracy of NGS for KD mutation detection were significantly better than Sanger sequencing.However, if we choose ddPCR and next-generation sequencing with higher sensitivity (approximately 0.2% detection limit), a new problem might arise; the significance of low-level mutations detected by this high-sensitivity technology is not yet clear in case of patients without TKI resistance and it cannot, therefore, be a predictor of TFR.However, KD mutations are not present in all patients who develop TKI resistance, and some resistance mechanisms still remain unknown.Whether patients with BCR::ABL1 KD mutations can maintain long-term stable TFR remains to be confirmed in larger clinical trials.

| CML stem cells
BCR::ABL1 in patients with CML originates from a cell with intrinsic or acquired biological potential to cause leukemia, such as a CML stem cell (LSC). 49,50Although traditional TKI therapy has a strong antiproliferative effect on LSC, it has a poor ability to induce apoptosis, 51 especially in quiescent LSCs.One study reported that some signaling pathways do not depend on BCR::ABL1 to maintain the survival of LSC in the quiescent stage, 52 which makes TKI treatment ineffective.Although the ideal situation for attempting TFR is to clear the LSC from the body, there is currently no treatment specific to LSCs in the clinic.However, several clinical trials have found residual CML LSCs in most patients who maintain TFR, even when BCR::ABL is monitored using a highly sensitive assay, such as ddPCR. 53,54The data suggested that LSCs are present even in patients who achieve DMR, and that TFR does not appear to require complete eradication of LSCs in at least a subset of patients.However, it should be noted that a considerable number of patients are still unable to achieve TFR.Since very little is known about LSCs till date and there is no specific clinical therapy for LSCs, current attempts to treat TFR do not consider the impact of LSCs.There have been studies on LSC-specific therapy, 55,56 and we believe that these therapies aimed at LSCs can, in future, effectively reduce molecular recurrence after TKI discontinuation.
In recent years, the development of single-cell sequencing has led to the discovery of specific gene expression patterns in patients with poor response to TKI therapy, including some LSC phenotypes. 57For example, during treatment, Lin − CD34 + CD38 −/low CD45RA − cKIT − CD26 + LSCs showed increased expression of genes related to proliferation and decreased expression of genes related to quiescence. 58In future, we may screen patients, who are not suitable for TFR, based on the expression pedigree of LSCs. 59

DISCONTINUATION
Rigorous molecular monitoring is necessary after the formal discontinuation of TKIs.Since the molecular depth inclusion criteria for TFR trials are usually MR4 or MR4.5, we recommend that laboratories or hospitals conducting TKI withdrawal tests perform qRT-PCR or ddPCR with a sensitivity of at least MR4.5.In patients aiming at TFR, molecular relapse tends to occur within the first 6 months after TKI discontinuation. 60Therefore, adequate molecular monitoring with sufficient frequency and sensitivity should be ensured during the first year after discontinuation to prevent delays in the optimal treatment opportunity for patients with molecular relapse.However, excessive monitoring programs may reduce the patients' willingness to try TFR and impose additional financial burdens, especially for those with poor financial conditions and in areas where monitoring is expensive.Past TKI discontinuation trials tended to monitor monthly for the first 12 months 61 ; however, a model of the safe minimum frequency for molecular monitoring in patients with CML attempting TFR by Ross et al. suggested that this frequency may not be necessary. 62The model suggested that monthly testing for the first 6 months and alternate-month testing during 6-12 months would not delay the resumption of TKI therapy in relapsed patients while reducing the number of tests by nearly one-third. 62A previous study had shown that the median time for BCR::ABL1 to double in patients with molecular recurrence is approximately 1log per month. 63Therefore, if a patient begins to discontinue TKI at a stable MR4.5, and BCR::ABL1 increases at the above rate, it will take at least 2 months for patients with molecular relapse to lose their MMR.Therefore, monthly molecular monitoring during the first year after TKI discontinuation was not necessary.However, it would be worth noting that the frequency of monitoring is not static.Six months after TKI discontinuation, if the patient tends to lose MMR, the frequency of monitoring should be restored to once a month to prevent any delay in treatment.It would further be worth noting that the time required to obtain the test sample and test results is equally important.The ideal time is to obtain results within 2 weeks and act on them quickly. 14If the patient loses MMR, TKI therapy is immediately resumed along with monthly molecular monitoring.In conjunction with the current guidelines and clinical trials described above, 13,14,62 we recommend monthly molecular monitoring for the first 6 months after TKI discontinuation, bimonthly monitoring during 6-12 months, and quarterly monitoring thereafter.If a patient tends to lose MMR (e.g., the patient has lost DMR, and the most recent test result is close to MR3), monthly monitoring is recommended, regardless of the duration of discontinuation.In addition, since the international standard molecular monitoring protocol is for patients with typical CML (transcripts e13a2 and/or e14a2), the above recommendations are only applicable to this subset of patients.
Patients with rare transcripts would require a relatively individualized monitoring regimen.

| SECOND ATTEMPT TO TFR
If a patient with CML, who attempts to stop TKI therapy, has a molecular relapse, the possibility of achieving TFR in such a case may be a concern.Fortunately, the vast majority of patients in whom TFR attempts have failed can quickly regain DMR after resuming TKI therapy, creating conditions for these patients to attempt TFR for a second time.A small-scale study on 16 patients who underwent a second attempt of TFR revealed different recurrence dynamics after the second withdrawal of TKI.Five patients had a slower second recurrence than the first, whereas five others had a faster second relapse. 64Till date, the largest study on the second attempt of TFR was RE-STIM in France, which included 70 patients who had a molecular relapse after the first TKI discontinuation, regained DMR, and stopped the TKI again. 65In the end, 35% of patients in the trial maintained TFR after 36 months of follow-up, and the depth of BCR::ABL1 molecular response at 3 months after the first TFR attempt was a factor in the second TFR attempt; that is, patients who retained MR4.5 at 3 months after the first TKI discontinuation had a higher success rate (46%) in the second TFR attempt.However, the criterion for the first restoration of TKI therapy in this trial was loss of MR4.5, whereas that for the second restoration of TKI therapy was loss of MMR.Another trial involving a second TFR attempt reported that six out of 12 patients remained in MMR (median follow-up, 8.6 years), and the most recent recurrence occurred 6 years after the second TKI discontinuation. 66,67Reasons underlying the successful discontinuation, after the first TKI discontinuation failure, still remain unclear; however, one possible explanation could be the progressive exhaustion of quiescent CML stem cells. 68Currently, very few clinical trials have been conducted on the second TFR attempts.A meta-analysis published in 2020 reported that only 124 patients underwent a second attempt of TFR before publication. 69Although such limited data are insufficient to assist in formulating clinical standards for the second TFR, attempt for the latter can be suggested to be safe and feasible.Dulucq et al. estimated that the molecular relapse rates of patients after the first discontinuation at 0-6, 6-12, and 12-18 months were 35%, 8%, and 3%, respectively, based on the previous TFR clinical trials.The molecular recurrence rates after the second discontinuation were 48%, 27%, and 12%, respectively. 69Although there was no significant overall difference in molecular relapse rates between the first and second discontinuations, the time point of molecular relapse after the second discontinuation was more dispersed.Therefore, the molecular monitoring strategy for the second TFR attempt should be appropriately adjusted.We recommend molecular monitoring once a month for the first 12 months, every alternate month for 12-18 months, and every quarter during the second attempt to TFR.

| ISSUES STILL TO BE CONSIDERED
Although the abovementioned detection methods for BCR::ABL1 can guide the progression of TFR to a certain extent, it is often impossible or difficult to perform the detection techniques, such as ddPCR and DNAPCR, in conventional clinical setting.This involves not only the technical facilities of the local hospital but also the economic situation of the patients and cost-effectiveness of the treatment.Consequently, many highly sensitive detection methods exist only in clinical trials, but cannot be used for conventional treatments.Here we summarize the advantages and disadvantages of these monitoring methods again (Table 6).
1][72][73][74] In fact, this is an expected outcome; longer the treatment time and duration of DMR maintenance, better is the prognosis, and easier it is to attempt TFR.However, according to this conclusion, there will never be the best time for discontinuing TKI, since it is always better to stop TKI therapy later.An optimal time is required to stop taking the drugs.Some scholars have conducted preliminary research and recommended 6 years as the shortest duration of imatinib use, based on positive and negative predictive values. 757][78] The symptoms usually lasted over months, beginning within days or weeks of drug discontinuation.However, the mechanisms underlying TKI withdrawal syndrome remain unclear.Possible causes include c-Kit inhibition, bone remodeling, and mast cell activation. 79lthough a small number of patients require medications, such as cortisol, 80 most withdrawal symptoms are mild and self-limiting.Since patients who are aiming for TFR are often in a state of anxiety due to the fear of relapse, the withdrawal symptoms need to be explained to them in detail before stopping TKI, thereby reducing unnecessary anxiety.F I G U R E 2 Approach to treatmentfree remission.Combining clinical indicators with molecular markers and using precise and efficient molecular monitoring to achieve treatment-free remission is an ideal treatment model and it is also the focus of future research in the field of CML.

| CONCLUSIONS
Based on the current clinical guidelines and the factors influencing the TFR obtained in some clinical trials, TKI discontinuation therapy has been initiated in some patients with DMR.Herein, we discussed some of the factors that may be related to TFR outcomes at the molecular level, along with some monitoring strategies.However, till date, most of these factors have not been involved in decision-making for clinical TFR.Currently, predictive indicators for TFR outcomes and recurrence are lacking in clinical practice.In future, TFR research should focus on combining the clinical indicators with molecular monitoring and biological markers to personalize patient conditions and guide clinicians to develop individualized treatment plans, so that more patients with CML can achieve safer and stabler TFR (Figure 2).

F I G U R E 1
Typical BCR::ABL1 transcripts.(A) The approximate location of the Typical breakpoint of the BCR gene on chromosome 22.(B) The approximate location of the Typical breakpoint of the ABL gene on chromosome 9. (C) Two kinds of typical transcripts of the BCR::ABL1 fusion gene on the Philadelphia chromosome.
Milestones in CML treatment.
T A B L E 1Abbreviations: CCyR, complete cytogenetic response; DMR, deep molecular response; EMR, early molecular response; IS, international scale; McyR, major cytogenetic response; MMR, major molecular response; MR, molecular response.TA B L E 2 Clinical studies of TKI discontinuation.monthlymolecularmonitoringover the next 6 months.T A B L E 3 Information of ongoing clinical trial.Abbreviations: N/A, not available; TKI, tyrosine kinase inhibitor; TFR, treatment-free remission.T A B L E 4 Criteria for TKI discontinuation.Abbreviations: 2GTKI, 2 generation tyrosine kinase inhibitor; CP, chronic phase; MMR, major molecular response; MR, molecular response; N/A, not available.
Summary of advantages and disadvantages of monitoring methods.