Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm that occurs in 1 to 2 per 100,000 adults. The key to CML pathogenesis is the fusion of the Abelson murine leukemia (ABL1) gene on chromosome 9 with the breakpoint cluster region (BCR) gene on chromosome 22. This leads to the expression of an oncoprotein termed BCR-ABL [1]. In this context, the BCR‐ABL1 fusion gene is an active tyrosine kinase that advances tumor growth and replication through downstream signaling pathways such as MYC and signal transducer and activator of transcription (STAT) [2,3].

Imatinib and other tyrosine kinase inhibitors (TKIs) have markedly improved the treatment of CML [4]. Several studies have reported that TKIs improve the survival of chronic phase CML patients similar to that of the general population, thereby increasing the 10‐year survival rate from approximately 20% to 80%–90% [4,5]. Accordingly, studies have assessed whether TKIs can be safely discontinued in patients who have achieved longterm deep molecular response (DMR).

Current European LeukemiaNet (ELN) [6] guidelines stipulate that treatment can be successfully stopped if both treatment duration and DMR are adequate to make treatment-free remission feasible for a potential cure. However, among patients who become candidates for the termination of TKI therapy after achieving adequate DMR, the incidence of molecular cure remains relatively low at approximately 20%–25% for imatinib and 30%–45% for second-generation TKIs [5,7].

Adherence to TKI has also become a critical aspect of care in CML [8]. As such, there have been studies on survival outcomes according to patient adherence levels [9]. Strict adherence increases the probability of clinical success [10]. Low adherence to imatinib was reported to be associated with a lower rate of cytogenetic and molecular responses in CML and consequently a higher rate of progression and resistance [8,11].

The National Health Insurance Service (NHIS) in Korea is the compulsory health insurance scheme covering the whole Korean population [12]. The National Health Information Database (NHID), formed by NHIS, has been developed for public health and medical research purposes and provides personal information, demographics, and medical treatment data of Korean citizens [13]. NHID stores the claims data of approximately 46 million patients per year, which accounts for around 90% of the total population in Korea. It also includes claims from 80,000 healthcare service providers as of 2011.

This study aimed to examine the association of TKI adherence with overall survival (OS) outcomes and clinical parameters influencing survival outcomes in CML patients treated with TKIs. Toward this goal, we investigated practice trends and clinical outcomes in a large community-based integrated health plan in South Korea, i.e., the NHID database.

This national cohort study found that most CML patients treated with TKI were highly adherent to the treatment, and TKI adherence affected survival outcomes. To our best knowledge, this is the first study to demonstrate that TKI adherence affected OS in CML patients. Adherence to TKI was assessed variably in previous reports [16]. Retrospective studies mostly used claims data and pharmacy refill data to assess adherence, with MPR used as the primary indicator [16-18]. The average adherence reported in studies varied from 19% to 100%, with differences according to the types of measurement methods used [17,19,20]. When an MPR cut-off was used (usually in the range of 80% to 90%), 44% to 97% of patients were classified to be adherent [17,21]. The median MPR in this study was 0.99, and approximately half of the patients (49.2%) were classified to have high adherence (MPR ≥ 0.95). Despite the full medical insurance coverage including pharmacy benefits, the distribution of MPR still varied widely and was comparable with that in the United States [22]. The mean MPR in this study was 0.88, and approximately two-thirds of the patients had MPR ≥ 0.9.

With respect to the effect of TKI adherence on patient outcomes, high adherence (i.e., MPR ≥ 0.95) was associated with better EFS and OS. This result was confirmed in the multivariate Cox models, as evidenced by the lower adjusted HRs for event or mortality in patients with high TKI adherence status than those in patients with moderate or low adherence status. Poor adherence may be associated with poor response to treatment [23]. Noens et al. [11] reported that patients with suboptimal response had significantly higher mean percentages of imatinib not taken than did those with an optimal response (23% vs. 7% using pill count). In addition, a previous study [8] reported that non-adherent patients were less likely to achieve complete cytogenetic responses (26% vs. 44%, p = 0.004) at any point. The estimated 5-year EFS in patients with non-adherence was inferior to that in those with no dose interruptions (59.8% vs. 76.6%, p = 0.011) [9].

In the current study, male sex, young age at diagnosis, and high medical insurance premium were associated with survival benefits. Elderly patients (≥ 75 years) do not optimally benefit from TKI treatment, and this could be because of several contributing factors. For instance, these patients experienced more TKI toxicity owing to comorbidities and frailty. In addition, they could have lower treatment adherence and may have even discontinued treatment [24]. In our study, patients aged > 50 years had inferior OS and EFS to those aged ≤ 50 years. Patients with high insurance premium had higher adherence than did those with low insurance premium. One study on CML patients in Brazil showed that lowerincome and less economic resources are significantly associated with worse medication adherence [25]. South Korea offers universal access to health care, regardless of the ability to pay, through the NHI and Medical Aid program. The National Health Insurance is financed by compulsory contributions from the insured. The contributions are calculated by considering the insured’s income, and thus, patients paying high insurance premiums generally have a high-income [26]. We found higher rates of adherence in high-income patients than in low-income patients. Studies on the impact of economic factors on survival outcomes in CML are scarce. A previous report from Sweden demonstrated that although socioeconomic conditions influenced treatment choice, these were not associated with CML survival [27]. However, the Swedish study included not only economic but also social status (i.e., child support and study allowances).

Given that most CML patients are asymptomatic and require long-term treatment to prevent disease progression and achieve optimal response, ensuring 100% treatment adherence may be challenging. Factors such as older age, living alone, male sex, experiencing medication side effects, a longer time interval between diagnosis and treatment, a longer duration of treatment, poor side effect management, and a high number of concomitant medications are associated with low TKI adherence [8,11]. In the present study, the median MPR of the younger (< 70 years) and male patients were significantly higher than that of the other age groups and female patients. This finding is in contrast to that of previous studies [11]. However, the median MPRs (≥ 0.95) in both sexes indicated high adherence, and thus, we could not consider sex as a predictor of adherence.

Most studies on CML enrolled patients treated only with imatinib mainly because of insurance coverage [11,19,22,28], and insurance coverage is important in treatment adherence. For example, a previous study from Brazil, where universal health care coverage is challenging, showed that patients had much lower adherence because of a lack of access to drugs [28]. Given that all CML patients in Korea have pharmacy medication coverage, we could better study the association between adherence and outcomes without confounding effects of insurance problems. Interestingly, among the three TKIs, adherence was the lowest to dasatinib compared to that to imatinib or nilotinib, which involves more pills per day and more complicated dosage methods. The median MPR of patients who maintained frontline TKI was higher than that of other patients. This result is expected because maintaining frontline TKI indicates that patients have achieved optimal response or have not experienced uncontrolled adverse events during first-line TKI treatment.

This study had some limitations. First, we had to depend on claims data because of the retrospective nature of the study, and thus, we might have missed some important factors or the data might be unevenly distributed. Second, this study did not consider the CML phase during patient inclusion. Third, laboratory results were not available, and therefore, the initial CML risk scores or treatment response (including disease progression) on polymerase chain or cytogenetic testing assessments could not be assessed. Future prospective studies on TKI adherence in CML should assess these factors.

In conclusion, adherence to TKI treatment is an important factor predicting survival outcomes in CML patients. Adherence was better in patients older than 70 years, with higher income and maintenance of frontline TKI. Besides TKI adherence, male sex, young age, and good economic status were associated with better survival outcomes in Korean CML patients.