Novel tyrosine-kinase inhibitors for the treatment of chronic myeloid leukemia: safety and efficacy


Introduction: Chronic myeloid leukemia (CML) is characterized by a pathognomonic chromosomal translocation, which leads to the fusion of breakpoint cluster region (BCR) and Abelson leukemia virus 1 (ABL1) genes, generating an oncoprotein with deregulated tyrosine kinase activity.

Areas covered: In the last two decades, BCR/ABL1 kinase has become the molecular target for tyrosine kinase inhibitors (TKIs), a class of drugs that impressively improved overall survival. Despite these results, a proportion of patients experiences resistance to TKIs and need to change treatment. Furthermore, TKIs are unable to eradicate leukemic stem cells, allowing the persistence of neoplastic clones. Therefore, there is still clinical need for new agents to overcome common resistance mechanisms to available drugs.

This review explores the horizon of drugs actually under investigation for CML patients resistant to conventional treatment.Expert Commentary: Radotinib is an ATP-competitive TKI that showed significant activity also in front-line setting and could find employment indications in CML. Asciminib, an allosteric ABL1 inhibitor, could demonstrate a higher capacity in overcoming common TKIs resistant mutations, including T315I, but clinical findings are needed. CML stem cell target will probably require new therapeutic strategies: combinations of several compounds acting with different mechanisms of action are actually under investigation.

Keywords: chronic myeloid leukemia, tyrosine kinase inhibitors, deep molecular response, treatment-free remission, bosutinib, radotinib, asciminib

1. Introduction

Chronic myeloid leukemia (CML) is a clonal malignancy associated with abnormal proliferation of mature granulocytes and their precursors in the bone marrow and peripheral blood. This condition is secondary to a well-defined genetic aberration, the reciprocal translocation occurring between chromosomes 9 and 22 which allows the genesis of the derivative Philadelphia chromosome (Ph), which incorporates a fusion oncogene, BCR/ABL1 [1]. This protein shows a markedly constitutive increased tyrosine kinase activity, which confers a survival and proliferative advantage to the mutated clone, key moments in leukemogenesis process [2]. Imatinib, the first ABL1 tyrosine kinase inhibitor, was discovered in 1996 and showed high in vitro selectivity and activity against BCR/ABL1 cell lines [3]. Despite the increasing number of patients achieving cytogenetic responses with this new drug, approximately 30% of imatinib-treated patients experiences resistance to therapy, half of them showing the development of point mutation on the ATP- binding domain of the oncoprotein [4,5]. In the recent years, pharmacological research for new molecules in CML field enhanced, with the incorporation of novel TKIs like nilotinib, dasatinib, ponatinib and bosutinib to the armamentarium of active drugs both in chronic phase (CP) and in advanced stages settings. This second and third generation TKIs are characterized by increased antileukemic power, which permits to achieve deeper and faster molecular responses, translating into the possible discontinuation in the long-term and specific toxicity profiles, requiring a personalized approach to optimize treatment [6].

Moreover, each molecule displays an own spectrum of activity to different ABL1 mutations, feature that allows the possibility of a genotype-driven drug selection [7]. Nevertheless, despite numerous therapeutic options for CML patients, a curative solution excluding allogeneic stem cell transplantation (alloSCT) has not been discovered yet. This failure depends on TKIs inactivity towards leukemic stem cells, a pharmacological target that actually represents the main goal to be achieved in order to fully defeat the disease [8].
Several novel molecules have been recently approved for CML patients, others are actually under investigation in different clinical trials. Aim of this review is to report safety and efficacy data of novel TKIs recently approved for the treatment of CML, discuss about their potentiality and possible therapeutic combinations in the future.

2. Radotinib

Radotinib is an oral BCR/ABL1 inhibitor that presents a structural analogy to nilotinib, actually approved in South Korea for both newly diagnosed CML patients and those resistant or intolerant to at least one TKI. Radotinib inhibits wild-type BCR/ABL1 kinase with an IC50 (half maximal inhibitory concentration) of 34 nm. Several other kinases are inactivated by the drug, but at higher concentrations: PDGFRα (IC50=75.5 nm), PDGFRβ (IC50=130 nm), c-KIT (1324 nm) and SRC (>2000 nm) [9]. Radotinib showed activity against the most common BCR/ABL1 mutations, except T315I. In vitro, radotinib showed high inhibitory capacity against CML cell lines, demonstrating higher efficacy than imatinib [10]. No dose-limiting toxicities were reported in a phase I study for a daily dose up to 1000 mg [11]. A phase II trial was conducted, in order to demonstrate safety and efficacy of radotinib in CP-CML patients resistant or intolerant to first-line TKI treatment. The study enrolled 77 patients, receiving a starting dose of 400 mg twice daily (bid). After 12 months of treatment, 65% of patients achieved a MCyR and 47% a CCyR. Most common grade 3- 4 hematologic adverse events (AEs) were thrombocytopenia (24.7% of patients) and anemia (5.2%). Grade 3-4 non-hematologic adverse events were represented by fatigue (3.9%), asthenia (3.9%), nausea (2.6%), myalgia (1.3%), rash (1.3%) and pruritus (1.3).

ALT and AST elevation, of all grades, were the most common events reported, with a rate of 85.7% and 72.7% respectively (grade 3-4: 11.7% and 10.4%). Other common grade 3-4 biochemical alterations were hyperbilirubinemia (23.4%) and hyperglycemia (19.5%). A dose reduction had to be performed in 68.8% of cases, while 42.9% permanently stopped the drug: reasons for discontinuation were laboratory tests abnormality (45.5%), disease progression (24.2%), non-hematologic AEs (9%), death (6.1%) and other reasons (15.2%) [12].

A recent update, concerning data relative to a median follow-up of 45.7 months, reported that 59.7% of patients discontinued the treatment, in the majority of cases for disease progression or no response and biochemical abnormalities (18 patients each) [13]. A phase III trial conducted in several Asiatic countries compared radotinib versus imatinib in newly Diagnosed CP-CML (RERISE), exploring for the first time the drug in front-line setting. The study included 241 patients, randomized in three arms to receive radotinib dose of 300 mg BID (79 patients), 400 mg BID (81 patients) or 400 mg QD (qd; 81 patients). By 12 months of follow-up, MMR rates were higher for patients receiving radotinib (52% in 300 mg BID arm and 46% in 400 mg BID arm) if compared to those treated with imatinib (30%), such as MR4.5 rates (15% for radotinib 300 mg BID, 14% for radotinib 400 mg BID and 9% for imatinib) and CCyR rates (91% for radotinib 300 mg BID, 82% for radotinib 400 mg BID and 77% for imatinib). Early molecular response (EMR; BCR-ABL1 transcript level ≤ 10% after 3 months of treatment) was achieved by 86% of radotinib 300 mg BID, 87% of radotinib 400 mg BID and 71% of imatinib patients, respectively. After 12 months of treatment, no progression disease was reported in any group. Overall survival (OS) and progression-free survival (PFS) rates were 100% for radotinib 300 mg BID, 80% and 81% for radotinib 400 mg BID and 99% for imatinib group, respectively. For what concerns safety profile, among hematologic grade 3-4 AEs, neutropenia was the most frequent, occurring in 19%, 24% and 30% for radotinib 300 mg BID, 400 mg BID and imatinib arm, respectively. Grade 3-4 thrombocytopenia was reported in 16%, 14% and 20% of radotinib 300 mg BID, 400 mg BID and imatinib treated patients, while grade 3-4 anemia was described in 6%, 10% and 5%, respectively.

Between non-hematologic grade 3-4 AEs, the most frequent were laboratory abnormalities and particularly hyperbilirubinemia (27%, 42% and 0%), ALT elevation (20%, 26%, 1%), hyperglycemia (11%, 11%, 4%), lipase elevation (11%, 7%, 2%) for radotinib 300 mg BID, 400 mg BID and imatinib arm, respectively. Transaminases increase and hyperbilirubinemia led to dose reduction or interruption in 55% and 78% in radotinib 300 mg BID, 68% and 82% in radotinib 400 mg BID and 19% and 73% in imatinib arm, respectively. Approximately 80% of these episodes were controlled with treatment interruption. Non-laboratoristic AEs were mostly grade 1-2 events and were skin rash (all grades: 35% and 33%), nausea (23% both), headache (19% and 31%) and pruritus (19% and 30%) for radotinib 300 mg BID and 400 mg BID arm, respectively. No vascular events were reported [14].

This trial demonstrated not only that radotinib is associated to a satisfying safety profile in CML-CP setting, but also that this drug is capable of ensuring higher and faster MMR rates if compared to imatinib, with outcomes comparable to those reached by dasatinib or nilotinib. Furthermore, radotinib annual price in South Korea is approximately of 21.500 dollars, a competitive cost if compared to 21-28.000 dollars required for the annual supply of other approved TKIs in the same country [15].However, these promising results need to be confirmed in larger international trials with longer follow-up.

3. Asciminib (ABL-001)

Asciminib is a selective allosteric inhibitor of BCR/ABL1, strongly binding (dissociation constant= 0.5-0.8 nM) to the myristoyl pocket of ABL1 kinase to block leukemic cells proliferation. Myristoylic domain physiologically acts like a negative control for kinase activity, while in CML its function is lost and, also for this reason, the enzyme results constitutively activated. Asciminib act recovering the original autoregolatory mechanism in order to induce the inactive conformation with consequent inhibition of downstream signaling. Considering that the myristoyl pocket is difficultly found in other kinases, the molecule is highly selective for BCR/ABL1 and presents an IC50 value of 1-20 nM. The rational of asciminib design was to prevent the onset of resistance: through its peculiar mechanism of action, the drug results active against all mutations involving catalytic-site, including T315I, while, at the same time, all TKIs are effective against asciminib-resistant mutation. This finding suggested that the association of asciminib with different type of TKI could improve the efficacy of both drugs [16].

A phase Ia, multicenter, international, dose escalation trial is actually open, to evaluate asciminib in CML and Philadelphia positive acute lymphoblastic leukemia (ALL) patients resistant or intolerant to two or more TKIs. A preliminary analysis reported an acceptable toxicity profile, mainly composed of grade 1-2 AEs, with uncommon grade 3-4 events characterized by increased lipases and cytopenias. Efficacy data showed that 78% of resistant patients achieved a CCyR, and 57% a MMR after 12 months of treatment. A single relapse was reported, relative to a patient that developed myristoyl-site mutations (Val468Phe and Ile502Leu). The activity of the drug, as already reported for preclinical studies, was reported against multiple TKI-resistant mutations. Due to safety and efficacy of the drug, new arms in this study have been recently opened, to explore the feasibility of a combination with imatinib, nilotinib and dasatinib. Actually, no data about these new cohorts are available [17]. A phase III randomized study has been planned to test the efficacy of asciminib versus bosutinib as third line.

4. Other TKIs

Several other TKIs have been discovered and evaluated in last years in CML setting, mostly in small phase I trial enrolling heavily pre-treated patients. Aurora kinase family has a main role in mitotic spindle formation and centrosome maturation. Danusertib, a pan-Aurora kinase inhibitor, has demonstrated activity also against ABL1 (IC50=25 nM) including T315I and other mutations. Particularly, this drug inhibits proliferation of CD34+ cells both from BP-CML patients with (IC50=25 nM) or without (IC50=9 nM) T315I mutation [18]. A phase I, dose escalation trial was conducted among 37 patients, 22 in advanced phase CML (7 in AP, 15 in BP) and 15 with Ph+ ALL. T315I mutation was detected in 54% of patients. Almost 90% of patients had developed resistance to one or more TKIs, 32% had received an alloSCT. Between evaluable patients, 20% of cases (4/20) achieved a result: a complete hematologic response (CHR) and a CCyR were reported in CML patients. Toxicity profile was characterized by grade 3-4 AEs such as febrile neutropenia (17.2%), diarrhea (14%), mucosal inflammation and stomatitis (7% each), hypotension, hyponatremia, AST increased, mental status changes (3.4%) and vomiting (3%) [19]. Despite an interesting profile of efficacy, the most limiting feature of danusertib is represented by its intravenous schedule of administration, consistent of 7 days of continuous treatment in a 14- day cycle.

Another molecule that has recently been developed is rebastinib, a TKI that presents activity against ABL1, FLT3 and TIE2. This compound acts like a Type II inhibitor of ABL1, binding both ATP pocket domain and a switch control region composed by two amino acid residues mediating the change from inactive to active conformation. The engagement of switch control domain allows rebastinib to maintain ABL1 in the inactive conformation through a mechanism that is independent of phosphorylation status of kinase domain. For these peculiar features, rebastinib inhibits in vitro both phosphorylated and unphosphorylated native (IC50= 2 and 0.82 nm, respectively) and T315I (IC50= 4 and 5 nm, respectively) ABL1 kinase. This molecule has a documented in vitro activity also against other kinases such as KDR, SRC, LYN, FGR and HCK [20]. Recently a phase I, dose- finding trial with rebastinib was conducted on 57 patients with resistant/refractory CML (52 patients) or AML (5 patients). Among CML patients, 62% had received 3 prior TKIs and 40% of patients presented a T315I mutation. Concerning safety profile, in the whole group of enrolled patients 46% of them experienced grade ≥ 3 treatment-related AEs: the most observed were muscular weakness (21%), vision blurred (5%), myalgia (5%), arthralgia, fatigue and hypertension (3%). No serious adverse events (SAEs) were reported in patients receiving daily dosage below 100 mg QD and 300 mg BID. Eighteen (35%) CML patients attained a response: CHR was achieved in 8/50 patients lacking hematologic response (16%, T315I: 4 patients), CCyR in 2/37 patients evaluable for cytogenetic response (5%, T315I: 1 patient) and MMR in 4/40 patients evaluable for molecular response (10%, T315I: 2 patients) [21]. Considering the insufficient clinical activity observed, it is reasonable to imagine that rebastinib will hardly find a place in CML setting. Efficacy and toxicity data of the most important trials discussed are reported in Table 1 and Table 2.

5. Expert Commentary

Nowadays treatment landscape in CML setting is supplied by several target drugs that can be selected according to peculiar disease and patient features, in order to optimize benefit and reduce risks of toxicities. Second generation TKIs increases the rates of deep molecular responses but there is still a significant amount of CML patients that develop resistance or intolerance to these conventional treatments. Ponatinib has shown high potency and selectivity especially against T315I mutated clones, but presents a relevant toxicity profile that circumscribes its use to selected patients, especially concerning cardiovascular risk [22]. The introduction of new effective TKIs, such as bosutinib and radotinib, could implement cure options for CML patients. Bosutinib has demonstrated high efficacy in first-line setting, assuring faster molecular responses when compared to imatinib, with a brilliant safety profile that could satisfy peculiar needing of patients presenting multiple comorbidities. Radotinib has shown activity in CML setting, but its employment has been limited only to Asiatic cohorts of patients: due to their peculiar metabolic characteristics, efficacy and safety data need to be confirmed also in other ethnic groups [23]. Furthermore, its specificity towards mutated BCR/ABL1 clones is quite similar, as expected considering structural similarity, to that of nilotinib, a feature that could markedly reduce the employment setting of this drug [9]. Each TKI has a specific safety profile and a patient-centered approach is actually the specific mainstay of therapeutic strategies.

A real innovation in CML could be the clinical introduction of BCR/ABL1 allosteric inhibitor asciminib: its unique mechanism of action has already displayed activity towards the most common mutations conferring resistance to conventional TKIs, including T315I, associated to a manageable toxicity profile. Major limitations concern the two mechanism of resistance recently identified: the development of point mutations involving myristoyl pocket and the upregulation of ABCG2, a multidrug efflux pump [24]. For these reason future findings from phase I trial combination cohorts among ATP-competitive TKIs and asciminib will be fundamental to understand if the associations could effectively overcome and/or prevent both kinasic and myristoilyc domain mutations, as already shown in vitro.

Beyond TKIs resistance/intolerance issue, one of the most discussed topics in nowadays CML treatment is TFR management. Numerous study have illustrated that, unrelated to the type of TKI used to gain a deep molecular response, nearly a half of patients that discontinues treatment will relapse, probably in relation to quiescent Ph+ hematopoietic stem cells not attacked by TKIs [25]. The importance of immune system in the prevention of relapse has not been completely understood, as both innate and adoptive immune mechanisms seem to act in leukemic cells recognition [26]. Actually, several trials are exploring the efficacy of a combined therapeutic approach with Interferon-α and TKIs: longer follow-up will clarify if this association is able to confer a superior disease control and also the correct schedule of exposure to drugs in order to obtain the maximum benefit reducing AEs.

Additional molecular pathways might be involved in resistance to TKIs or relapse after discontinuation: upregulation of JAK2/STAT5, PI3K/AKT, and MAPK signalling have been described in CML patients resistant to TKIs without evidences of ABL1 point mutations [27]. Ruxolitinib (a JAK2 inhibitor) and pioglitazone (a PPAR- agonist) are two compounds downregulating JAK2/STAT5 pathway that are actually under investigation for possible combination scheme with TKIs [28]. Both drugs raise great interest also for the reported in vitro and in vivo proapoptotic activity towards CML stem cells [29,30]. Despite this, considering the toxicity profile associated to ruxolitinib, caution is needed about the recommended dose to be administered in a combination schedule.

6. Five-year view

One of the main goals on the path to cure in CML is actually represented by the achievement of sustained deep molecular responses, an indispensable target for discontinuation. Actually, only 10-20% of patients treated with imatinib are eligible for a treatment discontinuation program [31,32]. Second generation TKIs have markedly increased the percentage of patients obtaining a MR4.5, which ranges around 40-55% in front-line setting [33]. Considering the wide landscape of available TKIs, further efforts and studies are needed to identify other molecular targets of possible interest for future development of active compounds in CML. As described before, several signaling pathways are involved in BCR/ABL1 independent resistance. Furthermore, even bone marrow microenvironment alterations have been related to the upregulation of various pathways that may lead to CML stem cell resistance to TKIs [34,35]. Epigenetic aberrations have also been reported and studied in CML: processes like DNA methylation, repair and replication, histone modification and micro RNAs deregulation are known to be crucial for neoplastic cell survival [36].

In the next years, it is possible that novel compounds targeting a molecular pathway different from BCR/ABL1 might play a role in CML treatment, especially in combination with approved TKIs in front-line setting. Analyzing the molecules already in investigational phase, the most promising one seems to be asciminib: exploiting the unique mechanism of action, it could exert a complementary and synergistic function in association with kinase domain inhibitor, especially to overcome ABL1 mutations.

Finally, it will be necessary to completely define the physiopathological role played by the immune system, with the purpose to determine which immune therapies could be useful and the ideal timing of these treatment in CML clinical course.

7. Key issues

• Despite first and second-generation tyrosine kinase inhibitors (TKIs) ability to provide a high number of cytogenetic and molecular responses in chronic myeloid leukemia (CML) patients, a portion of them results resistant or intolerant to these drugs.

• Each of the approved TKIs for CML is characterized by typical safety and mutational activity profile that allow clinicians to adopt a therapeutic tailored approach: in this setting, a novel TKI such as radotinib, might result useful especially in patients presenting multiple comorbidities.

• The first allosteric TKI asciminib, through its unique mechanism of action, seems capable to overcome and prevent common ABL1 kinase domain mutations, especially if used in combination to other TKIs.

• Several studies have reported the importance of molecular pathways different than BCR/ABL1 as crucial for TKIs resistance development: in this perspective, drugs like ruxolitinib and pioglitazone are actually under investigation.

• Bone marrow microenvironmental, epigenetic and immune system aberrations seem to play a central role in disease progression. Basing on this knowledge, it is feasible that, in the next years, new molecular targets could be identified to develop effective associations with TKIs.


This paper was not funded.

Declaration of interest

M Breccia has received honoraria from Novartis, Bristol-Myer Squibb, Pfizer, and Incyte. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. One peer review declares consultant work and research funding from Novartis, Bristol-Myer Squibb, Pfizer, and Ariad. Peer reviewers on this manuscript have no other relevant financial or other relationships to disclose.