Introduction
The therapeutic landscape of chronic myeloid leukemia in chronic phase (CML CP) has evolved substantially with the advent of multiple tyrosine kinase inhibitors (TKIs). While most patients achieve durable disease control with first  or second line TKIs, a clinically significant subset experiences resistance or intolerance after multiple lines of therapy. For these patients, long term efficacy, tolerability, and treatment sustainability become paramount considerations. Asciminib, a first in class STAMP (Specifically Targeting the ABL Myristoyl Pocket) inhibitor, represents a mechanistically distinct approach to BCR ABL1 (Breakpoint cluster region-Abelson tyrosine kinase) inhibition. The 156 week efficacy results from the phase 3 ASCEMBL trial provide critical insight into the durability of Asciminib’s clinical benefit in this heavily pretreated population.1,2

Mechanistic Rationale and Clinical Context
Unlike ATP (Adenosine Triphosphate)  competitive TKIs, Asciminib binds to the myristoyl pocket of ABL1, stabilizing the kinase in an inactive conformation. This unique mechanism allows Asciminib to retain activity against many BCR ABL1 variants associated with resistance to prior TKIs. In the ASCEMBL trial, Asciminib was evaluated in patients with CML CP who had received at least two prior TKIs and discontinued them due to resistance or intolerance.1 This population reflects a real world clinical challenge, where cumulative toxicity, suboptimal molecular responses, and limited therapeutic options often converge.

Study Design and Long Term Follow Up
ASCEMBL was a randomized, open label, phase 3 trial comparing Asciminib 40 mg twice daily with Bosutinib 500 mg once daily in patients with CML CP after ≥2 prior TKIs.1 Earlier analyses at 24 and 96 weeks demonstrated superior efficacy and tolerability for Asciminib, meeting the study’s primary and key secondary endpoints. The 156 week analysis, corresponding to nearly four years of follow up, represents the end of study evaluation and offers a comprehensive view of long term outcomes.1,2

Major Molecular Response at 156 Weeks
At week 156, Asciminib continued to demonstrate a clinically meaningful and statistically significant advantage in major molecular response (MMR) compared with Bosutinib. The MMR rate was 33.8% in the Asciminib arm versus 10.5% in the Bosutinib arm, translating into an adjusted between group difference of 23.2% (95% CI, 13.1–33.2; two sided P < .001).1,2 Importantly, this sustained separation of response curves underscores the durability of Asciminib’s activity rather than a transient early benefit.


 

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Figure 1. Asciminib remained superior vs Bosutinib in late-line CML-CP after nearly 4 years of follow up in ASCEMBL.
 


Deeper Molecular Responses and Disease Burden Reduction
Beyond MMR, the 156 week analysis highlighted consistent disease control across additional molecular endpoints. Among patients without a major cytogenetic response at baseline, a higher proportion of Asciminib treated patients achieved BCR ABL1 ≤1% at week 156 compared with Bosutinib (43.0% vs 11.1%).1,2 These findings suggest that Asciminib not only induces responses but also facilitates meaningful reductions in leukemic burden over prolonged treatment durations, even in patients with more advanced molecular disease at study entry.

Durability of Response and Treatment Sustainability
Durability of response is a critical determinant of long term clinical benefit in CML CP. The ASCEMBL end of study analysis demonstrated that responses achieved with Asciminib were maintained over time, with no evidence of waning efficacy through 156 weeks. While progression free survival and overall survival were broadly comparable between treatment arms, these analyses were influenced by permitted crossover from Bosutinib to Asciminib for lack of efficacy.1 Notably, only a small number of patients who crossed over achieved MMR, highlighting the potential value of earlier asciminib introduction rather than sequencing after additional TKI failure.1,2

Safety and Tolerability Over Nearly Four Years
Long term treatment tolerability is especially relevant in CML, where therapy is often continuous and lifelong. Over 156 weeks, Asciminib maintained a favorable safety profile compared with Bosutinib. Grade ≥3 adverse events occurred less frequently with Asciminib (59.6%) than with Bosutinib (68.4%), and fewer patients discontinued treatment due to adverse events (8.3% vs 27.6%).1,2 These findings reinforce earlier observations that asciminib’s distinct mechanism may translate into improved long term tolerability, supporting sustained therapy in a population with limited alternatives.

Clinical Implications for Heavily Pretreated CML CP
The 156 week ASCEMBL results consolidate Asciminib’s role as an important therapeutic option for patients with CML CP who have experienced resistance or intolerance to multiple prior TKIs. The persistence of superior molecular responses, combined with a manageable safety profile, addresses two of the most pressing unmet needs in later line CML management: durable disease control and treatment sustainability.1,2 For hematologists navigating complex treatment histories, these long term data provide reassurance that asciminib can deliver consistent benefit well beyond the first two years of therapy.

Conclusion: Key Takeaways for Clinical Practice
The 156 week efficacy results from the ASCEMBL trial demonstrate that Asciminib offers sustained, long term clinical benefit in heavily pretreated CML CP. Key takeaways for practicing hematologists include:

  • Durable efficacy: Asciminib maintained significantly higher MMR rates than Bosutinib through 156 weeks.
  • Depth of response: Long term molecular control, including BCR ABL1IS ≤1%, was more frequently achieved with Asciminib.
  • Treatment sustainability: Lower rates of severe adverse events and treatment discontinuation support prolonged therapy.
    Collectively, these findings position Asciminib as a robust long term option for patients with CML CP who require effective and tolerable therapy beyond second line treatment.1,2

 

References

  1. Mauro, M. J., Minami, Y., Hochhaus, A., et al. (2025). Asciminib remained superior vs bosutinib in late line chronic myeloid leukemia in chronic phase after nearly 4 years of follow up in ASCEMBL. Blood Advances, 9(16), 4248–4259. https://doi.org/10.1182/bloodadvances.2025016042
  2. Mauro, M. J., et al. (2025). Long term efficacy and safety of asciminib in chronic myeloid leukemia: 156 week results from the ASCEMBL trial. Blood Advances. https://pubmed.ncbi.nlm.nih.gov/36717654/
  3. Mauro M. et al. Poster presentation 65. ASH Annual Meeting, 9.–12. December 2023, San Diego, California.