The current real-world study confirms observations from clinical trials, on the activity and toxicity pattern of ponatinib in the treatment of relapsed/refractory CML. We were most encouraged by the relatively low frequency of ponatinib treatment-emergent AOEs, especially in the absence of previous exposure to nilotinib; none of our patients without such exposure developed post-ponatinib cardiovascular complications while the incidence was 11% in those previously exposed to nilotinib. The association between previous exposure to nilotinib and post-ponatinib AOEs in the current study was independent of age, ponatinib dose, or treatment duration; accurate information on cardiovascular risk factors was not available to assess potential confounding. By contrast, the number of TKIs received prior to ponatinib therapy did not appear to influence either response or toxicity patterns. These observations support our current practice of avoiding nilotinib as first- or second-line TKI of choice, in the treatment of CML-CP, especially considering the availability of other second generation TKIs with similar efficacy but devoid of vascular toxicity. The association of nilotinib therapy in CML with AOEs was first described in 201124 and subsequently confirmed by a larger study that reported an incidence rate of 29.4% (compared to < 5% in age-matched controls), after a median follow-up of 2 years.33 In another study of 220 patients with CML-CP receiving nilotinib therapy, vascular events were reported in 12%, with an incidence of 4.1 events per 100 patient-years and risk factors identified included older age and dyslipidemia.34 While these reports were focused on overt arterial events, the possibility of nilotinib-associated subclinical endothelial injury was raised by a more recent study that described the presence of ultrasound arterial anomalies (plaques, stenosis, occlusion) in 25 (34%) of 75 nilotinib-treated patients, with the carotid bulb being the most frequently involved territory;26 the reported anomalies were more frequent in older patients and in those with history of hypertension and cardiovascular risk factors.26 On the other hand, our current practice of using dasatinib as our second-line TKI of choice in CML-CP, with imatinib being our first-line TKI of choice, is further supported by recent demonstration of the possibility of using lower doses of dasatinib, in order to mitigate drug-associated side effects, without compromising efficacy.15, 35
Observations from the current study are also supportive of our current practice of using ponatinib as the preferred third-line TKI of choice. The prevalence of AOEs in the current real-world study was very low, in the absence of prior treatment with nilotinib. It is possible that our preference of using lower doses of the drug might have contributed to this favorable observation, although we saw no correlation between AOE and ponatinib dose, in the current study. These observations are consistent with recent clinical trial updates on long-term efficacy and toxicity of ponatinib in both chronic and advanced phase CML.27 The particular update included patients with Ph1-positive acute lymphoblastic leukemia (Ph + ALL) and CML (PACE and OPTIC trials).27 In the OPTIC trial, patients with CP-CML and resistance to ≥ 2 prior TKIs or ABL1 T315I mutation were initially dosed at ponatinib 45 or 30 mg/day and their dose reduced to 15 mg/day at time of CCyR.27 Among patients with CML-CP in the PACE (n = 257) and OPTIC (n = 93) trials starting with ponatinib 45 mg/day, the 2-year CCyR and overall survival rates were 46% and 85%, respectively, in PACE and 57% and 91%, in OPTIC, respectively, while the incidence of AOEs were 18% in PACE and 4% in OPTIC.27 The latter figure is close to what was seen in the current study whose design was more in line with that of OPTIC, as well as other retrospective studies where lower doses of ponatinib were used.30, 36 Our preference of starting with the lower dose of ponatinib was intended to minimize clinically overt and subclinical vascular injury, and does not preclude upward dose titration guided by close monitoring of response. Several other studies have also looked into the experience with ponatinib.37–39
Another important observation from the current study concerns the lack of significant correlation between post-ponatinib survival and a remission state beyond CHR, regardless of whether or not the response level was achieved before or after treatment with ponatinib. Furthermore, ponatinib-induced maintenance or achievement of a response level ≥ CHR did not appear to depend on the dose of ponatinib administered and was similar in the context of 15 or 45 mg/day start-maintenance doses. We find these observations practically relevant in terms of decision making on starting dose of ponatinib and whether or not it is necessary to increase the dose in search of DMR. A conservative approach on utilizing the lowest dose of the drug that secures durable CCyR is currently our preference. Obviously, our stance is subject to change based on additional information and we are fully cognizant of the limitations of a retrospective study of relatively small sample size with an even smaller number of informative cases.
Finally, we would like to address our position on the use of bosutinib, asciminib and AHSCT, in CML-CP. In general, our enthusiasm for bosutinib has been dampened by its frequent association with diarrhea and the results of a recent phase-3 study (ASCEMBL) that showed superiority of asciminib over bosutinib, both in terms of both efficacy and toxicity;40 incidentally, the incidence of AOEs was 1.3% with bosutinib and 5.1% with asciminib, the latter of which is not trivial. The mechanism of action for asciminib involves BCR::ABL1 inhibition through allosteric inhibition targeting the myristoyl pocket of ABL1, as opposed to direct inhibition of the ATP-binding site of ABL1, which is the case with other TKIs.28 Asciminib is currently FDA approved for use in CML-CP, in the presence of T315I (approved dose 200 mg BID, which is ridiculously expensive) or after failing ≥ 2 TKIs (approved dose 40 mg BID or 80 mg QD). Noteworthy side effects of asciminib include myelosuppression including thrombocytopenia and lipase elevation that is often not associated with clinical pancreatitis.28
Without a controlled study, it is impossible to accurately assess the value and drawbacks of asciminib vs ponatinib. Data are equally scarce on the activity of asciminib in ponatinib-exposed patients with CML-CP, and vice versa;41, 42 the same holds true for emerging new therapies that target T315I, including olverembatinib.43 In the meantime, considering the longer experience with ponatinib and possibility of using a lower dose of the drug support our preference to use ponatinib as the preferred TKI after failure of imatinib and dasatinib. In regards to AHSCT, in CML-CP, the question is not whether it works but when it ought to be offered;44 in other words, when does one determine that additional treatment attempts with yet another TKI is more likely to compromise AHSCT eligibility rather than result in net survival gain.45 Our observations from the current study, in this regard, suggest that it might be safe to defer transplant as long as durable CHR is secured; yet, patients should be referred to transplant centers for evaluation and to identify suitable HLA compatible donors, to be able to proceed with the AHSCT as soon as it becomes indicated. The more difficult question to tackle is whether or not it is always necessary to use a more potent, more expensive, and, often, more dangerous TKI, in order to improve upon a CCyR, or even a CHR, which is often achieved by a first- or second-generation TKI, and might be adequate enough to secure long-term survival.7 Finally, we are fully cognizant of the limitations of the current retrospective study including its relatively small sample size, heterogeneity in patients and treatment strategies, the lack of accurate information on cardiovascular risk factors, and variable monitoring time points.