Targeted cancer therapies have transformed the treatment landscape for many blood cancers, offering the promise of more precise interventions with fewer side effects than traditional chemotherapy. Yet, as these therapies become more widely used, a familiar and formidable challenge continues to emerge. Cancer cells adapt. Drug resistance remains one of the principal reasons why initially effective treatments ultimately fail, particularly in aggressive malignancies such as acute lymphoblastic leukaemia.
This challenge is especially pronounced in a high-risk subtype known as Philadelphia chromosome-like acute lymphoblastic leukaemia, often referred to as Ph-like ALL. This form of leukaemia disproportionately affects both children and adults and is associated with poor prognosis, high relapse rates, and limited long-term treatment success. New research now suggests that targeting cancer signalling further downstream may offer a way to bypass some of the resistance mechanisms that undermine existing therapies.
A study led by Jane Frances Thompson at The University of Adelaide, published in Acta Haematologica, explores this strategy in detail. The research article, titled “Direct STAT3 and STAT5 Inhibition Overcomes Treatment Resistance in a Murine Derived in vitro Model of Acute Lymphoblastic Leukaemia Driven by ETV6::JAK2”, provides compelling evidence that direct inhibition of STAT signalling may overcome resistance to JAK inhibitors in Ph like ALL.
Understanding Ph like ALL and the JAK STAT pathway
Philadelphia chromosome-like acute lymphoblastic leukaemia is not defined by a single genetic abnormality. Instead, it is characterised by a gene expression profile that resembles Philadelphia chromosome-positive ALL but lacks the defining BCR-ABL1 fusion. Approximately fifteen percent of ALL cases fall into this category, making it a clinically significant subgroup.
Many Ph-like ALL cases are driven by genetic alterations that activate the JAK STAT signalling pathway. This pathway plays a central role in regulating cell growth, survival, and immune responses. In normal cells, JAK and STAT proteins act in a tightly controlled sequence, transmitting signals from the cell surface to the nucleus. In cancer, mutations or gene fusions can cause this pathway to become permanently activated, leading to uncontrolled cell proliferation.
One such driver is the ETV6:JAK2 fusion, a genetic rearrangement that leads to continuous activation of JAK2 and downstream STAT3 and STAT5 proteins. Because of this dependency, JAK2 inhibitors such as ruxolitinib have emerged as promising targeted therapies and are currently being evaluated in phase three clinical trials for Ph-like ALL.
When targeted therapies stop working
While JAK2 inhibitors have shown encouraging early results, their long term effectiveness is threatened by the emergence of resistance. Cancer cells can acquire additional mutations that prevent drugs from binding effectively to their targets. In the case of JAK2, resistance can arise through mutations in the ATP-binding site, which is essential for inhibitor activity.
One particularly well characterised resistance mutation is JAK2 p.G993A. Previous studies have demonstrated that this mutation confers resistance to multiple JAK inhibitors, including ruxolitinib, even at high doses. Importantly, this resistance does not inhibit JAK-STAT signalling. Instead, STAT3 and STAT5 remain activated, allowing cancer cells to continue proliferating despite treatment.
This observation raises a critical question for clinicians and researchers. If inhibiting JAK2 itself is no longer effective, could targeting the signalling pathway further downstream provide an alternative route to suppress tumour growth?
Targeting the messengers rather than the switch
The study by Thompson and colleagues set out to address this question using an in vitro model of JAK inhibitor-resistant Ph-like ALL. The researchers employed murine-derived BaF3 cells engineered to express the ETV6::JAK2 fusion with the p.G993A resistance mutation. These cells demonstrated independence from growth factors and resistance to ruxolitinib, closely mimicking treatment resistant disease.
Rather than attempting to inhibit JAK2 directly, the team focused on STAT3 and STAT5, the transcription factors responsible for transmitting growth signals to the nucleus. Two inhibitors were selected for testing. SH 4 54 is an experimental compound that directly targets STAT3 and STAT5, while pimozide is a long-established antipsychotic medication that has previously been shown to inhibit STAT5 signalling.
The inclusion of pimozide is particularly notable, as it represents a drug repurposing strategy that could potentially accelerate clinical translation if efficacy is confirmed in further studies.
Clear evidence of restored sensitivity in resistant cells
The results of the inhibitor response assays were striking. Both SH 4 54 and pimozide demonstrated significant cytotoxic effects against the ruxolitinib-resistant ETV6::JAK2 p.G993A cells. Median lethal dose values were lower in the resistant cells compared to control BaF3 cells, indicating selective activity against JAK-STAT-driven malignancy.
Importantly, neither inhibitor produced substantial effects in KG-1a myeloid cells, which do not rely on JAK-STAT signalling for survival. This finding suggests a degree of pathway specificity and reduces concern about broad off-target toxicity, at least within the limits of the in vitro system.
SH 4 54 showed greater potency overall, achieving cell death at lower concentrations. However, pimozide also demonstrated meaningful efficacy at concentrations below those typically observed in patients receiving the drug for neuropsychiatric conditions.
-Jane Frances Thompson
Why pimozide attracts particular attention
While SH 4 54 appears more potent in laboratory settings, pimozide holds unique clinical appeal. As an antipsychotic medication that has been prescribed for decades, its pharmacokinetics, safety profile, and side effects are well documented in humans. This existing knowledge base helps lower one of the major barriers that often slows the development of novel cancer therapies.
The study also highlights another potential advantage of pimozide. The drug is capable of crossing the blood-brain barrier, raising the possibility that it could play a role in treating or preventing central nervous system involvement in acute lymphoblastic leukemia. CNS relapse remains a significant cause of morbidity in ALL, and therapies that can reach this compartment are particularly valuable.
Although these implications remain speculative, they underscore the broader significance of targeting STAT signalling in JAK STAT driven cancers.
Implications for precision oncology and future therapies
The broader implications of this research extend beyond a single leukaemia subtype. Resistance to targeted therapies is a recurring theme across oncology, from chronic myeloid leukaemia to solid tumours driven by kinase signalling. Strategies that bypass resistance by targeting downstream signalling nodes may offer a generalisable approach to prolonging treatment responses.
The findings also support the concept of combination therapy. Rather than relying on a single inhibitor, future treatment regimens may involve simultaneous targeting of JAK2, STAT3, and STAT5, either through drug combinations or through the development of bispecific or multispecific inhibitors.
Reference
Thompson, J. F., Grose, R., Yeung, D., & White, D. L. (2025). Direct STAT3 and STAT5 inhibition overcomes treatment resistance in a murine derived in vitro model of acute lymphoblastic leukaemia driven by ETV6::JAK2. Acta Haematologica, 148(6), 729–733. https://doi.org/10.1159/000543428
