Background and rationale:

Bosutinib is a second generation tyrosine kinase inhibitor (TKI), which has been indicated in Italy in the second line of CML treatment, since 2014. Thanks to its safety profile, bosutinib is frequently chosen as an advanced line of treatment in fragile patients.

We collected retrospective data from patients treated with bosutinib in any line of TKI therapy, in order to study the role that this drug has in real-life, compared to clinical trials in terms of treatment line, dosage, toxicity and efficacy.

Patients and methods: We collected data from65 chronic phase CML patients treated with bosutinib between 2015-08-18 and 2023-04-27, followed at 12 Haematology centers in Sicily and Calabria. Of these patients, 42 (64.6%) were male, and had a median follow-up of 29 months (range 1-96). Mean age at diagnosis was 62, and mean age at bosutinib start was 67. Median therapies prior bosutinib treatment was 2.

Fourtyfour patients (67.7%) had cardiovascular comorbidities (CV) at diagnosis: 37 patients (55%) had at least one cardiovascular risk factor: in particular, arterial hypertension in 31 patients (70.4%), dyslipidaemia in 10 patients (22.7%), diabetes mellitus in 14 patients (31.8%), obesity (grade 3) in 2 patients (4.5%); 7 patients (10.7%) had already experienced an acute cardiovascular or cerebrovascular event. Finally, only 12 patients (18.4%) had no comorbidity at diagnosis.

Mean age at the start of bosutinib for CV-risk patients was 70 years. Among 44 patients with CV risk before starting bosutinib, 38 patients were still alive (86.4%) at last follow-up. Fifteen patients with CV risk discontinued bosutinib (34%), while none presented new CV events during treatment. In terms of efficacy, however, in patients with CV risk, the median molecular response, at the last follow-up, is MR2.

Results:

Thirty-eight patients (58.5%) discontinued bosutinib, while 27 (41.5%) were still on treatment at the time of the last follow-up. Of the 38 patients who discontinued bosutinib, 5 discontinued due to resistance (13.8%) and 19 due to intolerance (50%). Of these 19 intolerant patients, only 1 patient discontinued bosutinib due to CV event (heart failure) and only 1 patient discontinued due to gastrointestinal toxicity (grade 3 diarrhea). Most reported toxicities were as follows:

  • Skin Toxicity 3 pt 5%

  • Perpheral edema 2 pt 3%

  • Hepatototoxicity 2 pt 3%

In the efficacy evaluation, early-suspenders (bosutinib treatment duration < 6 months) were, therefore, cut out, 55 patients (84.6%) were evaluated in this sub-analysis, 34 (61.8%) male and 21 (38.2%) female. Median age in this subanalysis is 67 years. Bosutinib was second-line treatment in 35 cases (63.6%), third-line therapy in 15 cases (27.3%), fourth-line therapy in 4 cases (7.3%).

Overall, 35 patients assumed bosutinib as a second-line TKI: 22 of them were from the 1L-imatinib group (62.9%), while 13 were from the 2nd generation TKI-1L group (37.1%). Of the 22 1L-imatinib and 2L-bosutinib patients, 56.1% reached MR3 or deeper during bosutinib treatment and 38.1% reached MR4.5 or deeper. Of the 13 patients in 1L-2genTKI 2L-bosutinib group, 61.5% reached at least MR3, while 15.4% reached MR4.5 or MR5. Drop-out rate was similar between the two groups.

Conclusion

Bosutinib revealed to be safe in patients with CML in 2 nd and further lines of treatment, especially in terms of cardiotoxicity, even in subjects with a high CV risk at treatment initiation.

Furthermore, our data revealed as most patients are currently treated with a lower dose of bosutinib than standard (median dosage 302 vs 500 mg a day), obtaining good response rates in 2 nd and further lines. Subsequent eventual dose increase is evaluated on the basis of the molecular response.

No relevant conflicts of interest to declare.

2023
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