TKIs are small molecules that are used to treat a wide range of malignancies, including leukemia and solid tumors (Amir et al., 2020). TKIs have been developed as a targeted treatment for BCR-ABL1 kinase activity suppression (Cilloni and Saglio, 2012). TKI therapy for CML patients has resulted in a substantial improvement in disease outcomes and a near-normal lifespan. CML is a well-studied TK-dependent alteration model disease. As a consequence of fundamental and clinical research in clinical practice, TKIs are currently the benchmark in the treatment of CML patients and an effective example of targeted therapy (Balabanov et al., 2014). Imatinib, for example, is one of the first TKIs to be certified for the treatment of CML and is presently used as a first-line treatment, while nilotinib and dasatinib are the next discoveries (Özgür Yurttaş and Eşkazan, 2020).

The first generation TKI binds to an inactive form of BCR-ABL conformation, while second generation TKIs show binding affinity for both the active and inactive forms. The TKIs of second generation (dasatinib, nilotinib, bosutinib) are approximately 1,000 times more sensitive than imatinib in terms of efficacy against BCR-ABL resistant mutations (Kantarjian et al., 2010; Saglio et al., 2010). The binding of imatinib with BCR-ABL kinase in its ATP-binding pocket is hampered by T315I mutation, which is the result of a substitution of isoleucine for threonine at position 315 of the amino acid chain. Approximately 12 percent of adult patients with BCR-ABL leukemia have seen this mutation, and is more common in people with severe conditions. However, dasatinib and nilotinib against T315I are not effective (Soverini et al., 2006; Soverini et al., 2007).

The survival has improved significantly to the point the survival in patients with CML is approaching that of the normal population with access to TKI therapy. Patients with newly diagnosed CML-CP had an OS rate that is only marginally lower than the normal population when treated with TKIs. Patients who obtained CCyR or better, on the other hand, have an OS rate that is comparable to the general population. This highlights the need of ensuring the best possible care for CML patients of all ages to maximize the benefits of these life-changing treatments (Sasaki et al., 2015). The long-term response of patients treated with different TKI therapies and their outcomes were examined by Jain et al. using Kaplan-Meier method. Patients treated with imatinib 800, dasatinib, or nilotinib had better long-term responses and outcomes than those treated with imatinib 400. Transformation to AP and BC phases, as well as mortality from CML, are now uncommon with current TKI therapy methods when well managed. However, discontinuation of treatment was more prevalent in imatinib patients than in TKIs of the second generation (Jain et al., 2015).

In the era of TKIs, CML-CP patients who lived for a certain number of years had good clinical results. Patients may, however, quit medication for some reason throughout time, affecting OS. It’s worth noting that the good conditional long-term outcomes predicted are based on rigorous ongoing monitoring of patients for CML and other comorbidities, as well as side events related to TKI usage, as done in clinical studies. It is not reasonable to believe that this may be generalized to situations where patients are treated less strictly (Sasaki et al., 2016). A clinical study showed that patients with the best long-term ELN responses have the best long-term results, and patients who received imatinib 800, dasatinib, or nilotinib have a greater chance of attaining optimum response at various periods (Jain et al., 2016).

According to Issa et al. during TKI therapy, if non-Y CCA/Ph¯ (clonal chromosomal abnormalities in Philadelphia chromosome-negative) is detected, physicians should be alerted to the possibility of poor survival outcomes, which may need careful monitoring. For most indices of response and long-term survival, patients with CCA/Ph¯ exhibited a trend toward inferiority. Early response assessment, such as BCR-ABL, was found to have a substantial predictive influence on the multivariate analysis. Patients with CCA/Ph¯ were less likely to get this reaction (74 vs. 81%). For individuals with monosomy 7, the emergence of nondeletion Y CCA/Ph¯ following therapy with different TKI is linked to reduced FFS, EFS, TFS, and OS, as well as a low but substantial chance of developing MDS or AML (Issa et al., 2017; Sasaki et al., 2018).

COVID-19 patients have been compared to an age, sex, and condition matched control group to determine the therapeutic effects of TKIs on their outcome (Başcı et al., 2020). Neither CP CML nor BCR-ABL TKIs induces clinically significant immune suppression, and there is no evidence that CP CML patients are at greater risk of complications by SARS-CoV-2 than the normal community. The clinical diagnosis of COVID-19 is not worse in TKI-treated CML patients than it is in the non-TKI-treated individuals. TKIs should be studied in wide-scale prospective and randomized studies to see if they improve COVID-19 outcomes (Başcı et al., 2020).

The clinical parameters of various TKI drugs used to treat CML are shown in Table 1 (Gleevec, 2008; Sprycel, 2010; Iclusig, 2016; Bosulif, 2017; Tasigna, 2017).

Imatinib is generally well absorbed, with far fewer complications than traditional drug treatment. The percentage of individuals with CML in CP that had neutropenia and thrombocytopenia of grade 3/4, is approximately 13 and 7%, respectively. In children, 27 percent of patients had neutropenia, 5 percent had thrombocytopenia, and 2.5 percent had anemia, all of grade 3 or 4. By temporarily halting imatinib, the cytopenia is usually fairly controllable. Gastrointestinal disorders like diarrhea, nausea, and vomiting are examples of nonhematologic toxicity. Bone pain, edema, rashes, myalgia and elevated levels of ALT and AST, etc., are the further side effects (Millot et al., 2011).

The inhibition of the HOXA4 protein may interfere with the normal growth and proliferation of myeloid progenitors. Imatinib resistance in CML patients may be caused by an epigenetic mechanism in the BCR-ABL-independent pathway. The hypermethylation of HOXA4 gene appears to be inhibiting the clinical response to IM, and as a result, is acting as a significant inhibitor of normal leukemogenesis. Inhibition of this process might be a superior treatment option, necessitating hypomethylating drugs in CML patients with this epigenetic resistance mechanism. As a result, apart from BCR-ABL gene mutation analyses, the hypermethylation profile of the HOXA4 gene might be used as an epigenetic biomarker in CML patients for predicting reaction to imatinib treatment (Elias et al., 2013).

According to recent research, the long noncoding RNA (lncRNA) HOTAIR (HOX transcript antisense RNA) is involved in the development of both solid tumors like breast cancer and NSCLC, as well as hematopoietic malignancies like AML. The epigenetic regulatory mechanisms of HOTAIR in advanced CML, on the other hand, remain unknown. Li et al. suggested that demethylation drugs may provide a novel remedy for CML-BC, as HOTAIR is a possible therapeutic target (Li and Luo, 2018).

In the age of TKIs, the incidence of second malignancies in CML patients has increased. Sasaki et al. (2019), reported data of newly diagnosed CML patients in 2001–2014, and it was found that 5.6% of the cancer patients had more than one malignancy after the CML diagnosis. The most prevalent sites of second cancers were the male genital system, digestive system and respiratory system. CML-CP had a slightly higher relative incidence of total second malignancies than the general population, with a modest rise in the AER (Sasaki et al., 2019).

There have been reports of cardiovascular or arteriothrombotic adverse events (CV- or AT-AEs) in CML patients receiving TKI therapy. The second or third-generation TKIs, such as ponatinib, significantly increase the risk of CV-AEs (excluding hypertension) and AT-AEs in CML patients (Jain et al., 2019). Myelopoeisis epigenetic regulation is controlled by the TET2 gene, which codes for the protein TET2 methylcytosine dioxygenase. Through the oxidation of methyl-cytosine, TET proteins play a role in DNA alteration, also in normal and malignant growth. A DNA epigenetic modification termed as hydroxymethylation is catalyzed by Tet2. As a result, it had been postulated that Tet2 regulated gene expression in macrophages exposed to excessive cholesterol. As a result, somatic mutations in hematopoietic cells have a role in the onset of atherosclerosis in humans. It was suggested that clonal hematopoiesis might be a controllable risk factor, either by using cholesterol-lowering medicines or targeting certain inflammatory processes (Jaiswal et al., 2017).

Patients with severe CML are more likely to have additional cytogenetic abnormalities (ACAs) in addition to the Philadelphia chromosome. Long-term survival may be improved by combining TKI treatment with systemic chemotherapy. No matter what the blast count is, the existence of CBF translocations determines whether or not a patient has AML, because CBF rearrangements are seldom diagnosed as ACAs. In individuals with CML, CBF rearrangements as ACAs might be regarded as high-risk characteristics (Morita et al., 2021a).

In 1983, after the discovery of the first cancer-epigenetic reprogramming relations, emerging evidence indicates that genetic and epigenetic changes cause cancers and that some of them occurring before profound leukemia commences preleukemia (Feinberg et al., 2006; Feinberg and Vogelstein, 1983). The BCR-ABL1 mutation induces epigenetic reprogramming, which is important in CML, as well as converting HSCs to LSCs. Polycomb-group (PcG) proteins are one kind of epigenetic regulator that is believed to be disrupted in CML LSC (Di Carlo et al., 2019). There are two complexes in PcG proteins, namely PRC1 and PRC2 (Polycomb Repressive Complex). Given the number of studies that PRC2 plays a role in cancer (including solid tumors and multiple myeloma), it’s no surprise that a variety of therapeutics targeting this complex have appeared and are under clinical studies of Phase I and II (Fioravanti et al., 2018; Gulati et al., 2018). In comparison to TKI therapy alone, Scott et al. demonstrated significant targeting of CML stem cells using the EZH2 (a core component of PRC2) inhibitor Tazemetostat in conjunction with TKI. As a result, combined therapy may be a new clinical option for CML treatment, as shown in Figure 1 (Scott et al., 2016).

Sirtuin 1 (SIRT1) is a NAD-dependent HDAC that acts on the nonhistone target p53 in CD34⁺ CML cells and has been associated with leukemogenesis and the persistence of CML LSCs. After imatinib therapy, SIRT1 expression significantly decreased but not completely depleted in CML cell lines, paving the way for effective SIRT1 inhibition. The treatment with the SIRT1 inhibitor (e.g., Selisistat or EX-527) via activation of p53 signaling, improved the effect of TKI treatment (Chen and Bhatia, 2013; Li et al., 2012; Yuan et al., 2012). SIRT1’s function is one of the most prevalent causes of treatment resistance in several malignancies, including CML (S et al., 2021).

BCL6 is a transcription factor that can epigenetically control a variety of its targets by altering the accessibility of chromatin at promoter and enhancer sites. It is often mutated in lymphoma cells (Hatzi et al., 2013). BCL6 expression is modest in TKI-naive CML cells, but it is substantially increased in CML cell lines and primary CD34+ cells after TKI therapy, although its function in CML is unknown (Hurtz et al., 2011). In Ph+ ALL patients, the BCL6 peptide inhibitor, RI-BPI was coupled with imatinib to avoid TKI resistance and to enhance imatinib’s efficacy (Duy et al., 2011). In CD34⁺38- CML LSCs, the use of RI-BPI increased the effectiveness of imatinib treatment, and decreased the ability to form CML cell colony, thus eliminated CD34⁺38- LSCs by enhancing programmed cell death. Other BCL6 small molecule inhibitors, such as FX1, significantly reduced CML CD34⁺ cells’ colony-forming ability, in combination with TKI (Madapura et al., 2017).

It had been suggested by San José-Eneriz et al. (2009), that the concomitant use of imatinib and a de-methylating drug such as 5-aza-2′-deoxycytidine (Decitabine) might result in better outcomes in individuals with reduced expression of the pro-apoptotic BCL-2-interacting mediator (BIM). This downregulation is epigenetically mediated by BIM methylation in most CML individuals and has a negative impact on health (San José-Eneriz et al., 2009). TKI-resistant leukemic stem cells (LSCs) are still a big problem in CML, and finding ways to get rid of them is a major unsatisfied clinical need. Scott et al. (2016) showed that for the existence of LSC, EZH2 and H3K27me3 reprogramming is essential, nevertheless, it also makes LSCs vulnerable to the mutual effects of EZH2 inhibitor and TKI. That becomes a novel strategy for more efficiently targeting LSCs in TKI receiving patients (Scott et al., 2016).

DNMT inhibitors are cytosine analogs that bind to DNA and inactivate genes, inhibiting the methyltransferase enzyme reversibly. Azacytidine and decitabine are DNMT inhibitors that had been approved by the FDA for the treatment of myelodysplastic syndrome (MDS). Low doses of these inhibitors are effective, but high doses are lethal (Yang et al., 2006). Decitabine is a cytosine derivative that is used as an intravenous antineoplastic agent in the treatment of MDS. It receives FDA approval in 2006 for the treatment of MDS and was also approved by the EU for the treatment of AML. The strategy of concomitant use of TKIs with epigenetic drugs for CML therapy was initiated after the approval of the first tinib drug (imatinib) in 2001 (Figure 2).

It has been demonstrated that TKI + IC/HMA (intensive chemotherapy/hypomethylating agent) therapy provides greater response rates, reduced recurrence risk, and superior 5-years EFS/OS than TKI alone, suggesting that rather than TKI alone, the combination therapy with IC or HMA + TKI (especially received decitabine) should be taken into account for the appropriate frontline therapy for such patients. Long-term survival rates were similar between IC + TKI and HMA + TKI, with a long-term survival rate of around 30% and improved outcomes for patients who were able to undergo ASCT (allogeneic stem cell transplant). However, while being less intensive than an IC method, the combination of HMA and second/third generation TKI is extremely successful (Saxena et al., 2021). As a result of their poor prognosis, patients with CML-BP require novel treatment options (Jain et al., 2017). It was found that the use of hyper-CVAD (cyclophosphamide, vincristine sulfate, doxorubicin hydrochloride/adriamycin, and dexamethasone) with dasatinib therapy, the outcome of CML-lymphoid BP has improved, and the survival rate is equivalent to that of Ph+ ALL. Using new TKIs and targeted agents, further improvements may be made in the future (Morita et al., 2021b).

HDAC inhibitors are chemical compounds that inhibit histone deacetylases. They have a long tradition of use as mood stabilizers and anti-epileptics in psychology and neurology. They’ve recently been studied as potential cancer, viral, and infectious disease therapies (Miller et al., 2003; Blanchard and Chipoy, 2005; Mwakwari et al., 2010; Patil et al., 2010). HDAC inhibitors are derived from natural and synthetic compounds of varying target specificity and actions. HDAC inhibitors are divided into four categories: hydroxamic acids (or hydroxamates), cyclic tetrapeptides, benzamides and short-chain fatty acids. Due to the concept that BCR/ABL signaling and epigenetic changes might act as two major reasons for disease and tolerance, the combined effects of BCR/ABL inhibition utilizing imatinib with panobinostat (a multi-HDAC inhibitor) in K562 cells were studied (Housman et al., 2014). In CML-derived K562 cells, the combination of panobinostat with imatinib showed synergistic antineoplastic responses and improved therapeutic effectiveness, as predicted (Zehtabcheh et al., 2021).