Real-world Imatinib Mesylate Treatment in Patients with Chronic Myeloid Leukemia: The Importance of Molecular Monitoring and the Early Molecular Response

Introduction Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder characterized by the Philadelphia (Ph) chromosome. After the introduction of imatinib mesylate (IM) in 2000, the natural history of the disease changed. Data on the treatment of CML with IM are from randomized clinical trials. Establishing whether these results can be reproduced or if caution is needed when extrapolating data to the general population with CML is essential. Objectives To evaluate the molecular response (MR) in patients with chronic-phase CML (CML-CP) not included in clinical studies and correlate them with the responses obtained in clinical trials. Methods Between January 2007 and January 2017, 227 patients newly diagnosed with CML-CP treated with IM as first-line treatment were included. This study is an observational, retrospective, and single-center study. Results At a median follow-up time of 7.3 years, 60.3% of the 227 patients who started IM were still on IM. Early molecular response (EMR) at 3 and 6 months was achieved by 74.2% and 65%, respectively. The median time to a MMR was nine months. The MR4.0 and MR4.5 were 67.2% and 51.1%, respectively. The overall survival (OS), progression-free survival (PFS), and event-free survival (EFS) of the patients who exclusively used IM were 91%, 91%, and 85.1%, respectively. Conclusion The results presented are similar to those described in prospective and randomized trials, demonstrating that the outcomes are reproducible in the real world. EMR at 3 and 6 months reflects better long-term responses, including higher rates of deeper molecular responses. Considering treatment costs, the absence of literature evidence of an impact on overall survival demonstrated by first-line second-generation tyrosine kinase inhibitors (TKIs), and the global OS of 85.8%, imatinib mesylate (IM) is still an excellent therapeutic option.


Introduction
CML is a clonal myeloproliferative disorder of pluripotent hematopoietic stem cells, in which a reciprocal translocation occurs between chromosomes 9 and 22, t(9;22)(q34;q11.2), resulting in the Philadelphia (Ph) chromosome, which is responsible for the expression of an abnormal fusion protein with altered tyrosine kinase activity called BCR::ABL1 [1]. The development and approval of TKIs for treating CML-CP have led to their becoming the first-line therapy for CML patients. The International Randomized Study of Interferon and STI571 (IRIS) is considered a reference clinical trial for CML-CP treatment since its fundamental modifications to the treatment resulted in advances in the prognosis and altered the disease's natural course [2]. Today, four TKIs have been approved for the first-line treatment of CML-CP: imatinib, dasatinib, nilotinib, and bosutinib.
The monitoring milestones of BCR::ABL1 transcript levels at 3, 6, and 12 months determine whether the current treatment should be continued (optimal response), changed (failure/ resistance), or carefully considered for continuation or change, depending on patients' characteristics, comorbidities, and tolerance (warning). TKIs have improved patient outcomes to near-normal life and evolved into a chronic conditions with age-related comorbidities. There has been increasing focus on the quality of life, avoiding long-term organ toxicities, and identifying strategies to maximize the possibility of stopping TKI therapy (treatment-free remission -TFR) which is feasible for some patients with deeper response [3][4][5][6][7][8][9].
Data on TKI treatment for CML are from clinical trials in which frail or elderly patients with multiple comorbidities were generally excluded [10]. The main objective of this study was to compare the MRs obtained during the treatment of CML-CP with IM as the first line in patients not included in clinical studies with the responses seen in randomized clinical studies and to verify whether these results can be generalized or if caution is needed when extrapolating the data to the general population with CML. The patients were followed in a reference hematology service in a Brazilian public health hospital.

Patients and inclusion criteria
This is a single-center, retrospective study of patients diagnosed with CML-CP per World Health Organization (WHO) 2007 criteria between January 2007 and January 2017 at the Hospital das Clínicas at Faculdade de Medicina of the Universidade de Sao Paulo hematology service.
The eligibility criteria were age greater than or equal to 18 years and use of IM as a first-line treatment regardless of the initial dose, exclusive diagnosis, and follow-up at our institution. Patients who did not use the medication regularly for at least one month or were inserted in research protocols were excluded ( Figure 1).
The data was collected in a RedCap database. The study was approved by the ethics committee of the Hospital das Clínicas at Faculdade de Medicina of the Universidade de Sao Paulo.

Molecular evaluation and therapeutic response
The BCR::ABL1 transcript type was defined using the reverse transcription molecular response (RT-MR), and the molecular monitoring of BCR::ABL1 transcript levels was performed using the quantitative reverse transcription molecular response Excluded: External diagnosis: n = 6 < 18 years at diagnosis: n = 3 Other study protocol: n = 11 < 1 month IM use: n = 5 (QRT-MR) technique as described elsewhere [11]. Patients underwent clinical and molecular follow-ups every three months in the first year of treatment or until they reached a MMR. For data collection, we considered BCR::ABL1 results of samples collected at 3, 6, 12, and 18 months with a margin of one month for less or more from these milestones. Subsequently, patients were followed up every 3 to 6 months or according to clinical judgment. Molecular monitoring and therapeutic response were presented and analyzed according to the recommendation criteria of the ELN 2013 [12], but clinical decisions were by the follow-up period guidelines.

Statistical analysis
The data were last updated in June 2019. Statistical analyses were performed using the R Core Team program (2020). The population characteristics are expressed descriptively. Pearson's χ-square and Fisher's exact tests were used to compare molecular response rates. The differences in survival were estimated using the Kaplan-Meier method, and the differences between them and the cumulative rates of MRs were calculated using log-rank tests. The odds of MMR, MR4.0, and MR4.5 were calculated using the cumulative incidence process. PFS was defined as the time between the date of diagnosis and death from any cause or date of progression to the accelerated phase (AP) or blastic crisis (BC). EFS was defined as the time between the date of diagnosis and the date of death from any cause, the date of progression to AP or BC, the date of loss of MMR, the date of increase in the dose of IM, and the date of switching to a second line TKI, either due to loss of response, resistance or intolerance to IM, progression to AP or BC, and date of the last consultation. OS was defined as the time between diagnosis and death from any cause, date of the patient's last visit, or loss to follow-up. Statistical significance was defined when p <0.05.

Patient characteristics
Two hundred and twenty-seven patients were diagnosed with CML-CP. The median age was 49.6 years at diagnosis (range 18 to 89 years), with a slight male-to-female predominance of 1  Table 1.
Concerning the different BCR::ABL1 transcripts, only the e14a2 transcript impacted the molecular response at 3 and 6 months (p=0.01 and p=0.03, respectively). There was no difference in the time to reach MMR, MR4.0, and MR4.5, as well as for PFS, EFS, and OS (data not shown).

Discussion
As in the Brazilian medical literature [13], the current study corroborated the median age at diagnosis of CML being 49.6 years. It may reflect the lower age of our population pyramids compared to America or Europe, as seen in Asia and Africa [3]. The patient's age plays an important role in the treatment decision because OS, comorbidities, and the development of complications are all age-related. Patients younger than 50 Fig. 6 Overall survival of MR by 6 months (1% and 1%). Red and black curves represent respectively who did and did not reach EMR Fig. 7 Probabilities of Progression Free Survival (PFS) and Event Free Survival (EFS) of 227 patients who reached MMR by 12 months. A: PFS and MMR by 12 months; B: EFS and MMR by 12 months. Red and black curves represent respectively who reached and did not reach MMR are expected to live 30 more years, and therapy discontinuation is one of the principal goals [3,4]. Most patients have the e14a2 BCR::ABL1 transcript type, which is implicated in EMR at 3 and 6 months but without any other impact in our analyses. Publications concerning the BCR::ABL1 transcript type are heterogenous [14][15][16][17][18].
To compare the collected data of this retrospective study and analyze the effectiveness and feasibility of first-line IM in daily clinical practice, we extrapolated published evidence of prospective randomized studies in which we know that the response rates in the IM group at each time point were calculated in the intention-to-treat population [19][20][21][22][23][24] (Table 4). We verified that the proportions of patients who achieved an EMR by 3 and 6 months were higher than in those studies. They had a significantly greater probability of attaining a MMR by 12 and 18 months and later reaching MR4.0 and MR4.5 than those who did not get the same benchmarks. EMR by 3 months and MMR by 12 months did not show advantages in PFS and OS but did show a significant difference in EFS. Patients with BCR::ABL1 transcript levels < 1% at 6 months and MMR at 18 months showed an impact in the PFS and OS, as seen in the CML-IV study [19]. Like the IRIS study [22], reaching MMR at 12 months does not interfere with OS when all deaths are included, regardless of the CML-related deaths, differently from getting MMR at 18 months. An attempt to treatment discontinuation can be considered if sustained DMR of sufficiently long duration has been achieved. The younger the patient, the stronger the case for achieving TFR [3]. Patients who achieved a deep MR had a statistically significant advantage in PFS and EFS and a trend toward better OS. Most patients achieve MMR in the first two years of treatment and a deep MR in the second to the fourth year. For young patients without morbidities or women wanting to become pregnant, the goal is to reach deep MR (MR4.0 and MR4.5). Thus, achieving an EMR in 3 and 6 months is significantly important.
The second-line therapy could be started at any time, but in practice, most switches occur between the third month and the first year of treatment, and the most frequent causes are resistance and intolerance to IM [25]. Although switching to a second-line inhibitor occurred in 39.7% of patients, 60.3% (137/227) were still using IM at the end of the study. 86.7% of patients chose dasatinib as the second-generation inhibitor, previously approved than nilotinib in Brazil. The median time to change the TKI was 1.2 years. Achieving an EMR at 3 and 6 months mitigates the need to switch to second-line treatment. Another important point was that patients who continued to be treated with IM had a higher PFS than patients who changed TKIs (p<0.001), while OS was not affected ( Figure 10).
As the life expectancy of patients with CML is close to that of the general population, OS is not one of the best outcomes for comparing the results. Failure to achieve MMR has been widely accepted as a warning sign of treatment failure and grounds for therapeutic changes. However, a lack of data shows that acting upon this outcome improves clinically relevant endpoints like OS. The current practice is switching CML patients to more expensive and toxic therapy when MMR milestones are unmet [26].
In conclusion, the EMR correlates with higher rates of deeper molecular responses and possible TFR. Since adherence to life-long TKI therapy is critical for most patients with CML, TKI costs and cost-effectiveness have become crucial issues for patients and society, which are justifiably involved in drug costs [3]. OS correlated only with BCR::ABL1 transcript levels < 1% at 6 months (equivalent to CCyR) and MMR at 18 months, demonstrating that in real life, CCyR is still the primary marker of survival and the MMR does not necessarily need to be early. The results presented by this Brazilian cohort are comparable to those described in prospective  and randomized studies, and IM proved to be an excellent therapeutic choice with a known and tolerable side-effect profile and a lower financial cost, especially for our public health system, which covers 75% of the Brazilian population.
The present study has some limitations, such as its retrospective nature and the lack of risk classification, such as that performed by Sokal [27], EUTOS [28], and the ELTS studies [29] that could somehow help in understanding the reason for molecular responses in the predetermined milestones that were not reached according to the European Leukemia Net [3,12] or the NCCN [30]. The presented results must be extrapolated with caution since this is a unicentric study where the regular molecular monitoring of BCR::ABL1 transcript levels is a nonreproducible reality in most centers.

Compliance with Ethical Standards
Ethical approval The study was approved by the ethics committee of the Hospital das Clínicas at Faculdade de Medicina of the Universidade de Sao Paulo in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Conflicts of interest
Author APSF declares that she has no conflict of interest. Author FSS that she has no conflict of interest. Author ARNA has received a speaker honorarium from Novartis, Brazil. Author FMS that she has no conflict of interest. Author FVRM that he has no conflict of interest. LN that she has no conflict of interest. RRG that he has no conflict of interest. ARLR that he has no conflict of interest. MPSF that he has no conflict of interest. EMR that he has no conflict of interest. VR that he has no conflict of interest. IB that he has no conflict of interest.
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