Executive Summary
The treatment landscape for myeloid/lymphoid neoplasms with fibroblast growth factor receptor 1 (FGFR1) rearrangements (MLN-FGFR1) has long been characterized by poor prognosis and limited therapeutic options. Historically, these aggressive malignancies, often referred to as the 8p11 myeloproliferative syndrome, have shown resistance to conventional chemotherapy, leaving hematopoietic stem cell transplantation (HSCT) as the only potentially curative intervention. However, the FIGHT-203 trial, a phase 2 study recently published in NEJM Evidence, marks a significant shift. The study evaluated pemigatinib, a potent, selective oral inhibitor of FGFR1, 2, and 3, in patients with MLN-FGFR1. The results demonstrate remarkable efficacy, particularly in the chronic phase of the disease, where the complete response rate reached 96%. This article provides a comprehensive analysis of the study’s methodology, clinical findings, and safety profile, emphasizing its implications for clinical practice.
Introduction: The Challenge of FGFR1-Rearranged Neoplasms
Myeloid/lymphoid neoplasms with FGFR1 rearrangement are rare but clinically devastating hematologic malignancies. They are cytogenetically defined by translocations involving the FGFR1 locus on chromosome 8p11, which results in the formation of various fusion genes (e.g., ZMYM2-FGFR1, BCR-FGFR1). These fusions lead to the constitutive activation of the FGFR1 tyrosine kinase domain, driving uncontrolled cellular proliferation and survival through downstream pathways such as PI3K/AKT, MAPK/ERK, and STAT. Clinically, MLN-FGFR1 presents with a diverse range of phenotypes, including myeloproliferative neoplasms (MPN), myelodysplastic syndromes (MDS), or acute leukemias (blast phase), often accompanied by lymphadenopathy and eosinophilia. The rapid progression from chronic phase to blast phase and the high relapse rate following standard induction chemotherapy have historically necessitated urgent referral for HSCT. The unmet medical need for targeted therapies that can induce deep remissions and bridge patients to transplant—or serve as definitive therapy—is profound.
FIGHT-203: Study Design and Patient Population
The FIGHT-203 study (NCT03011372) was an open-label, single-arm, multicenter phase 2 trial designed to evaluate the safety and efficacy of pemigatinib in patients with MLN-FGFR1. The study enrolled 47 patients in the safety population, of whom 45 had centrally confirmed FGFR1 rearrangements and were included in the efficacy analysis. The cohort was diverse in terms of disease presentation: 24 patients (53%) were in the chronic phase, 18 (40%) were in the blast phase, and 3 (7%) had the rearrangement without morphologic evidence of bone marrow or extramedullary involvement (likely representing molecular relapse or minimal residual disease). Patients received an oral dose of 13.5 mg of pemigatinib once daily. Dosing followed either a continuous schedule or a 2-weeks-on/1-week-off cycle. The primary endpoint was the complete response (CR) rate, adjudicated by a central review committee using protocol-defined criteria. Secondary endpoints included the complete cytogenetic response (CCyR) rate, duration of response, and safety.
Efficacy: Robust Responses Across Disease Phases
The efficacy data from FIGHT-203 are among the most encouraging reported for this patient population. Pemigatinib demonstrated high levels of activity across all disease phases, though the depth and frequency of response were most pronounced in the chronic phase.
Chronic Phase Outcomes
In the chronic phase cohort (n=24), the central review reported an extraordinary CR rate of 96% (23 of 24 patients). Furthermore, the CCyR rate—a critical marker of molecular and cytogenetic clearance—was 88% (21 of 24 patients). These results suggest that pemigatinib is highly effective at suppressing the clone driven by the FGFR1 fusion when the disease burden is relatively stable. For many of these patients, pemigatinib provided a level of disease control that was previously unattainable with conventional hydroxyurea or interferon therapy.
Blast Phase and Extramedullary Disease
While the blast phase (n=18) is traditionally much harder to treat, pemigatinib still showed significant activity. The CR rate in this group was 44% (8 of 18 patients), and the CCyR rate was 50% (9 of 18 patients). Although lower than the chronic phase results, these figures represent a substantial improvement over historical outcomes with cytotoxic chemotherapy in the relapsed/refractory setting. Additionally, all three patients with isolated FGFR1 rearrangements without morphologic disease achieved a CCyR.
Durability of Response
The duration of response was a standout feature of the study. The median duration of CR had not been reached at the time of the primary analysis (95% CI, 27.9 months to not reached). This durability highlights the potential for pemigatinib to maintain long-term disease control, which is essential for patients waiting for a suitable donor for HSCT or for those who are not transplant candidates.
Safety and Tolerability: Managing On-Target Effects
The safety profile of pemigatinib in FIGHT-203 was consistent with its known mechanism of action as an FGFR inhibitor. The most frequent treatment-emergent adverse event (TEAE) was hyperphosphatemia, occurring in 76% of patients. This is considered an on-target effect, as FGFR inhibition interferes with phosphate homeostasis in the kidneys. Management typically involves low-phosphate diets, phosphate binders, and, when necessary, dose modifications. Stomatitis was the most common grade 3 or higher adverse event, reported in 19% of the population. Other common side effects included alopecia, diarrhea, and dry mouth. It is important to note that dose management was frequently required: 64% of patients experienced dose interruptions and 60% required dose reductions. However, only 11% (5 patients) discontinued the drug due to adverse events, suggesting that with proactive monitoring, the toxicity profile is manageable for most patients.
Expert Commentary: A New Standard for a Rare Disease?
The results of the FIGHT-203 trial represent a landmark achievement in the treatment of FGFR1-rearranged neoplasms. For years, the hematology community has struggled to manage these patients due to the lack of targeted options. Pemigatinib now provides a potent tool to achieve rapid cytogenetic and clinical remissions. One of the most critical clinical questions is how to integrate pemigatinib into the existing transplant paradigm. For chronic-phase patients, pemigatinib may serve as an effective bridge to HSCT, potentially improving post-transplant outcomes by reducing pre-transplant disease burden. In the blast phase, while pemigatinib is active, the lower response rates suggest that it might be most effective when used in combination with other agents, such as intensive chemotherapy or venetoclax-based regimens, although this requires further study. Limitations of the study include its single-arm design and the small sample size, which are inherent challenges when studying ultra-rare diseases. Nonetheless, the high response rates and the biological plausibility of targeting the driver mutation provide strong evidence for its clinical utility.
Conclusion
Pemigatinib has demonstrated profound clinical and cytogenetic activity in patients with MLN-FGFR1, particularly in those in the chronic phase of the disease. With a 96% CR rate in the chronic phase and a manageable safety profile, it offers a new therapeutic standard for a previously difficult-to-treat malignancy. While blast-phase disease remains a challenge, the responses observed in this high-risk group are significant. This trial underscores the importance of cytogenetic and molecular testing in patients presenting with atypical myeloproliferative features to identify FGFR1 rearrangements early in the disease course.
Funding and Clinical Trial Information
The FIGHT-203 trial was funded by Incyte Corporation. The study is registered at ClinicalTrials.gov under the number NCT03011372.
References
1. Verstovsek S, Kiladjian JJ, Vannucchi AM, et al. Pemigatinib for Myeloid/Lymphoid Neoplasms with FGFR1 Rearrangement. NEJM Evid. 2025 Sep;4(9):EVIDoa2500017. doi: 10.1056/EVIDoa2500017. 2. Khodadadi F, et al. FGFR1-rearranged myeloid/lymphoid neoplasms: A comprehensive review. Annals of Hematology. 2023. 3. Gotlib J, et al. The 8p11 myeloproliferative syndrome: a review. Blood. 2008.
