One Size Does Not Fit to All: Intolerant or Resistant CML Patients Could Benefit from Different Ponatinib Starting Dose Strategies. Multicenter Italian Experience

Background: Ponatinib is effective as salvage treatment for chronic phase (CP) Ph+ chronic myeloid leukemia (CML) patients resistant or intolerant to prior TKIs. Based on the concern for an unfavourable cardiovascular toxicity profile, a dose reduction to 15 mg daily is currently recommended in CP patients after the achievement of treatment milestones. As more flexible approaches with TKIs dosage are increasingly adopted in clinical practice, identifying specific profiles of patients eligible for different treatment strategies, in order to improve the benefit/risk ratio, would be of particular interest.

Aims: To update previous observations in an extended cohort of CML patients, resistant or intolerant to prior TKIs, describing the efficacy and toxicity of two different ponatinib schedules (de-escalated vs 15 mg starting dose). Secondly, to investigate whether specific subsets of CML patients were managed more successfully with one strategy.

Methods: A retrospective analysis of CML-CP patients treated with ponatinib 15 mg, as starting or de-escalated dose has been performed. No pre-specified criteria to switch to ponatinib, to reduce or to start therapy with a lower dose were indicated. Cytogenetic and molecular responses were evaluated according to the ELN2013 recommendations. Toxicities under ponatinib treatment leading to transient or definitive discontinuation were recorded.

Results: 68 patients were included in this analysis. Median age at ponatinib start was 57 years (range: 28-87). The median time from CML diagnosis to ponatinib treatment was 4.6 years (range: 0.4-19). Most patients (70.6%) were treated with imatinib as frontline treatment. 30/68 patients (44%) received ponatinib as 4th or subsequent line of therapy. ABL1 mutations were detected in 18 (26.5%) patients (being 44% of them positive for the T315I mutation). 50/68 (73.5%) of cases had less than MR2 at the time of ponatinib initiation, with one third exhibiting primary refractoriness to all prior treatments. Resistance to prior TKIs was the reason for switch in 85% of patients treated with higher doses of ponatinib, while 13/26 patients starting with 15 mg were purely intolerant or intolerant and resistant (p= 0.001). No other significant differences between the two groups were identified.

MR2, MR3 and MR4/MR4.5 rates were 78.6%, 59.6% and 28.6% in 42 patients treated with the "de-escalated" approach, respectively. First step of dose reduction was decided after a median time of 3.1 months, with a subsequent decrease to 15 mg after a median of 10 months. Dose modifications were guided mostly by adverse events (73%) and, less frequently, to prevent toxicity and/or following response attainment (27%). Median duration of treatment was 27 months (range: 2-81), with 5/42 patients discontinuing ponatinib (1 cerebrovascular accident, 2 severe hematologic toxicities, two cases of resistance/progression). 4 deaths were registered, three after blast crisis and one for a fatal cardiovascular event.

26 patients started ponatinib 15 mg daily, with a median duration of treatment was 15 months (range: 2-58). MR2 was obtained in 77%, MR3 in 69.3% and MR4 in 46.2% of cases, respectively. MR3 or deeper responses were maintained in 80% of patients; 7/26 lost their best acquired response, with subsequent dose escalation to higher than 15 mg in 5/26 patients. Treatment discontinuation was necessary in 3 cases (1 coronary vasospasm-induced acute coronary syndrome, 2 muscle skeletal pain).

Of note, patients with ABL mutations gained benefit only from the "de-escalated" approach, with response improvements in 10/14 of cases vs 0/4 in the "low dose" group. Similarly, a higher rate of at least a MR2 was obtained in primary refractory patients treated ab initio with higher doses of ponatinib (57.1 % vs 14.3 %). However, focusing only on pure intolerant patients, 15 mg starting dose was very effective, with all patients achieving at least a MR3 or deeper response.

Summary/Conclusion: With the limits of the retrospective nature and small sample size, these data suggest that different dose strategies could be applied to specific subsets of CP-CML patients. While mutations or refractoriness to previous treatments should address towards a "debulking approach" with 45 or 30 mg ponatinib dose, intolerant or "low level resistance" cases could benefit from lower starting dose. Further investigation in larger prospective trials is warranted.