35-Nucleotide Insertion between Exon 8 and Exon 9 Splicing Transcript, Clinical Significance in 20 Adult and Pediatric Chronic Myeloid Leukemia (CML) Ph+ Patients in Mexico City

Introduction: The development of resistance to Imatinib (IMA) can occur over months or years, this constitutes a major problem for us due to economic limitations and the availability of 2° generation tyrosine kinase inhibitors (TKIs) in our reference centers. There are various mechanisms that lead to resistance to TKIs, especially because of specific mutations in the ABL kinase domain (KD) that impede the binding of the drug. The insertion of a 35 nucleotide between exon 8 and 9 induce a splicing product and these a truncated ABL protein that lacks the last 653 residues of KD, this transcript increased kinase inhibitors resistance sample in some chronic myeloid leukemia (CML) patients and in some cases, they respond to an increased dose of IMA, although in most cases molecular response (MR) is not achieved. However, the information of clinical value in literature is scarce. Herein we inform 20 cases with such genetic abnormality.

Methods: A retrospective, multicenter study was conducted in 2 medical centers in Mexico City. A total of 150 patients diagnosed with CML between 2003 and 2024 were included. From these 150 patients, those who did not achieve a MR to IMA or who experienced molecular relapse were identified. Among these, 20 patients were identified with the BCR-ABL mutation with a 35 nucleotide insertion (BCR-ABL^Ins35bp) between ABL KD exons 8 and 9. Inclusion criteria: patients with a confirmed diagnosis of CML, availability of complete clinical and laboratory data, identification of the mutation. Exclusion criterion: incomplete data. Demographic, clinical, and laboratory data were collected from the patients' electronic medical records, including age at diagnosis, gender, date of CML diagnosis, date of identification of the mutation, treatments received, including increased doses of IMA and other TKIs such as Bosutinib, Nilotinib, Asciminib, and Dasatinib, MR prior to the increase in IMA dose or treatment change, and loss of MR. All patients started with a standard dose of IMA (400 mg), in cases of no MR, the dose was increased (600-800 mg), depending on patient tolerance. In cases of no MR to IMA, mutations were requested, and if there was no MR to the maximum dose of IMA, other TKIs were administered, depending on availability. Patients were regularly followed up with clinical visits every 3-6 months and laboratory tests to evaluate treatment response.

Results: 20 patients (13.3%) diagnosed with CML were included, 18 adults (90%) and 2 children (10%) were identified with the BCR-ABL^Ins35bp, all of whom initially received standard doses of IMA. The median age at diagnosis was 33 years (4-61 years), with an equal distribution of 10 males (50%) and 10 females (50%). Among the 20 patients with the mutation: 11 patients (55%) achieved a major molecular response (MMR) but lost this response after a median time of 10 years (3-10 years) on IMA, 9 patients (45%) did not achieve an MMR at any point. Upon increasing the dose of IMA (up to 800 mg for those who tolerated it), 8 patients (40%) showed a reduction in BCR-ABL copies, but only 3 achieved an MMR. The remaining 12 patients were switched to other TKIs, all showing a reduction in BCR-ABL copies, with 5 (41%) achieving an MMR. All patients initially started on standard doses of IMA (minimum dose 400 mg, maximum dose 800 mg). 30% of patients were subsequently treated with Bosutinib, 20% Nilotinib, 5% Dasatinib, and 25% Asciminib. Up to 4 lines of treatment were required for some patients who did not achieve an MMR. No patients progressed to the blast phase during the study period.

Conclusions: We conclude that the BCR-ABL^Ins35bp invariably leads to resistance to IMA and even second-generation TKIs. Due to economic limitations and a lack of resources, sometimes the only intervention we can perform is increasing the dose of IMA. While this doesn't achieve a MMR, it significantly reduces BCR-ABL copies, as do newer drugs like Nilotinib, Bosutinib, or Dasatinib. The recent use of Asciminib shows promise, but longer follow-up is needed; however, we have achieved good results so far. It is important to mention that, despite not always achieving an MR over the years, some patients with a CML diagnosis of up to 22 years have remained asymptomatic and with no progression to blast phase. This raises the question of how deep the MR truly needs to be to impact mortality and how treatment optimization can be achieved given economic limitations. The access to second-generation TKIs in Mexico is limited.

No relevant conflicts of interest to declare.