Partner Therapeutics Receives EMA Approval of IMREPLYS® (sargramostim, rhu GM-CSF) to Treat Exposure to Myelosuppressive Doses of Radiation (Hematopoietic Sub-syndrome of Acute Radiation Syndrome [H-ARS])

• IMREPLYS is the first treatment for H-ARS approved in Europe
• Approval based upon IMREPLYS significantly improving survival, accelerating recovery of white blood cells and platelets, and reducing rates of infection and sepsis after radiation exposure without the need for supportive care such as blood products²

LEXINGTON, Mass., Sept. 9, 2025 /PRNewswire/ -- Partner Therapeutics, Inc. (PTx) today announced that the European Commission has granted a marketing authorization in the European Union (EU) for IMREPLYS^® (sargramostim, rhu GM-CSF), the same formulation that was approved by the U.S. Food and Drug Administration (FDA) under the brand name LEUKINE^® in 2018.¹ IMREPLYS is indicated for treatment of patients of all ages acutely exposed to myelosuppressive doses of radiation with Hematopoietic Sub-syndrome of Acute Radiation Syndrome (H-ARS). The European Commission approval enables sales, including government procurement for stockpiling, of IMREPLYS in all EU member states, as well as Norway, Iceland, and Liechtenstein.

"We appreciate the European Commission's decision to approve IMREPLYS for the treatment of H-ARS in Europe," said John L. McManus, President, Health Security and Critical Care of PTx. "Given the threat of use of tactical nuclear weapons and likelihood that initial medical treatment after a nuclear incident will occur in a severe, limited-resource environment, approval of a treatment that can be administered with minimal to no supportive care represents an important milestone in preparedness and civilian protection. For PTx, this approval is an important step in our global health security strategy to make IMREPLYS available in the European market and beyond."

The marketing authorization was granted by the European Commission upon recommendation by the Committee for Medicinal Products for Human Use (CHMP) of the European Medicines Agency (EMA) in June 2025 and was based on results from three adequate and well-controlled animal studies conducted in a well-characterized model of total body irradiation (TBI)-induced H-ARS.² Sargramostim significantly improved survival, accelerated recovery rates of white blood cells and platelets, and reduced the rates of infection and sepsis. Importantly, these benefits occurred when IMREPLYS was administered with minimal supportive care, mimicking the limited resource environment following a radiological and/or nuclear mass casualty incident. No whole blood, blood products, or individualized antibiotics were provided.

ABOUT ACUTE RADIATION SYNDROME AND HEMATOPOIETIC ACUTE RADIATION SYNDROME
Acute Radiation Syndrome (ARS), also known as radiation sickness or radiation toxicity, occurs when individuals are exposed to high doses of TBI that causes multi-organ injury.³ The first signs of injury appear in organ systems with high cell turnover rates, such as the hematopoietic system and gastrointestinal tract. Without medical intervention, death from ARS can occur within days to weeks. Significant causes of death among patients with ARS including overwhelming infection and sepsis, uncontrollable bleeding and severe acute anemia, all of which contribute to multi-organ dysfunction and failure (MOD/MOF).^3,4

H-ARS occurs after whole-body or partial-body (>60%) exposure to radiation doses >0.7 Gy, causing damage to rapidly dividing tissues, including bone marrow, spleen, thymus, lymph nodes and blood cells, resulting in pancytopenia (severely low levels of white blood cells, platelets and red blood cells).³ Low white blood cell counts, including lymphocytes, eosinophils, monocytes and neutrophils, culminate in immunosuppression that leads to the development of bacterial, viral and fungal infections and ultimately sepsis.³ Low platelet counts, or thrombocytopenia, leads to hemorrhage and severe acute anemia.^3,4

ABOUT IMREPLYS / LEUKINE (SARGRAMOSTIM)^5,6
Sargramostim is a human granulocyte-macrophage colony-stimulating growth factor (GM-CSF) produced by recombinant DNA technology in a yeast (S. cerevisiae) expression system. The binding to GM-CSF receptors expressed on the surface of target cells (hematopoietic progenitors and mature immune cells), initiates an intracellular signaling cascade which induces the cellular responses (i.e., division, maturation, activation). GM-CSF is a multilineage factor and, in addition to dose-dependent effects on the myelomonocytic lineage, it can promote the proliferation and maturation of megakaryocytic and erythroid progenitors. Sargramostim (LEUKINE) was approved by the US Food and Drug Administration in 2018 to increase survival in adult and pediatric patients from birth to 17 years of age acutely exposed to myelosuppressive doses of radiation (Hematopoietic Syndrome of Acute Radiation Syndrome [H-ARS]).

The product is commercially available in the United States under the brand name LEUKINE. LEUKINE will continue to be accessible through a named patient program operated by Tanner Pharma Group outside of the United States, including in the EU for named patient use outside of the H-ARS indication.

In the European Union, Iceland, Norway, and Liechtenstein, IMREPLYS is indicated:

• For treatment of patients of all ages acutely exposed to myelosuppressive doses of radiation with Hematopoietic Sub-syndrome of Acute Radiation Syndrome (H-ARS).

In the United States, LEUKINE is indicated:

• To shorten time to neutrophil recovery and to reduce the incidence of severe and life-threatening infections and infections resulting in death following induction chemotherapy in adult patients 55 years and older with acute myeloid leukemia (AML).
• For the mobilization of hematopoietic progenitor cells into peripheral blood for collection by leukapheresis and autologous transplantation in adult patients.
• For the acceleration of myeloid reconstitution following autologous bone marrow or peripheral blood progenitor cell transplantation in adult and pediatric patients 2 years of age and older.
• For the acceleration of myeloid reconstitution following allogeneic bone marrow transplantation in adult and pediatric patients 2 years of age and older.
• For treatment of delayed neutrophil recovery or graft failure after autologous or allogeneic bone marrow transplantation in adult and pediatric patients 2 years of age and older.
• To increase survival in adult and pediatric patients from birth to 17 years of age acutely exposed to myelosuppressive doses of radiation (Hematopoietic Syndrome of Acute Radiation Syndrome [H-ARS]).

Important Safety Information for LEUKINE (Sargramostim)

Contraindications

• Do not administer LEUKINE to patients with a history of serious allergic reaction, including anaphylaxis, to human granulocyte-macrophage colony-stimulating factor, sargramostim, yeast-derived products, or any other component of LEUKINE.

Warnings and Precautions

• Serious hypersensitivity reactions, including anaphylactic reactions, have been reported with LEUKINE. If a serious allergic or anaphylactic reaction occurs, immediately discontinue LEUKINE therapy, and institute medical management. Discontinue LEUKINE permanently for patients with serious allergic reactions.
• LEUKINE can cause infusion-related reactions that may be characterized by respiratory distress, hypoxia, flushing, hypotension, syncope, and/or tachycardia. Observe closely during infusion, particularly in patients with preexisting lung disease; dose adjustment or discontinuation may be needed.
• LEUKINE should not be administered simultaneously with or within 24 hours preceding cytotoxic chemotherapy or radiotherapy or within 24 hours following chemotherapy.
• Edema, capillary leak syndrome, and pleural or pericardial effusions have been reported in patients after LEUKINE administration. LEUKINE should be used with caution in patients with preexisting fluid retention, pulmonary infiltrates, or congestive heart failure. Such patients should be monitored.
• Supraventricular arrhythmia has been reported in uncontrolled studies during LEUKINE administration, particularly in patients with a history of cardiac arrhythmia. Use LEUKINE with caution in patients with preexisting cardiac disease.
• If absolute neutrophil count (ANC) >20,000 cells/mm3 or if white blood cell (WBC) counts >50,000/mm3, LEUKINE administration should be interrupted, or the dose reduced by half. Monitor complete blood counts (CBC) with differential twice per week.
• Discontinue LEUKINE therapy if tumor progression, particularly in myeloid malignancies, is detected during LEUKINE treatment.
• Treatment with LEUKINE may induce neutralizing anti-drug antibodies. Use LEUKINE for the shortest duration needed.
• Avoid administration of solutions containing benzyl alcohol (including LEUKINE for injection reconstituted with Bacteriostatic Water for Injection, USP [0.9 % benzyl alcohol]) to neonates and low birth weight infants.

Drug Interactions

• Avoid the concomitant use of LEUKINE and products that induce myeloproliferation. Monitor for clinical and laboratory signs of excess myeloproliferative effects.

Adverse Reactions
Adverse events occurring in >10 % of patients receiving LEUKINE in controlled clinical trials and reported at a higher frequency than in placebo patients are:

• In recipients of autologous bone marrow transplantation (BMT)–asthenia, malaise, diarrhea, rash, peripheral edema, urinary tract disorder
• In recipients of allogeneic BMT–abdominal pain, chills, chest pain, diarrhea, nausea, vomiting, hematemesis, dysphagia, GI hemorrhage, pruritus, bone pain, arthralgia, eye hemorrhage, hypertension, tachycardia, bilirubinemia, hyperglycemia, increased creatinine, hypomagnesemia, edema, pharyngitis, epistaxis, dyspnea, insomnia, anxiety, high glucose, low albumin
• In patients with AML–fever, weight loss, nausea, vomiting, anorexia, skin reactions, metabolic laboratory abnormalities, edema

Please see full Prescribing Information for LEUKINE at www.leukine.com.
Please see full Summary of Product Characteristics for IMREPLYS at https://ec.europa.eu/health/documents/community-register/2025/20250821166908/anx_166908_en.pdf.

ABOUT PARTNER THERAPEUTICS
Partner Therapeutics, Inc. (PTx), an integrated biotechnology company, focuses on development and commercialization of late-stage therapeutics to improve health outcomes in treatment serious diseases and to develop host-directed therapeutic treatments from global health security threats. The company believes in delivering products and supporting medical teams with the purpose of achieving superior outcomes for patients and their families. PTx's portfolio includes sargramostim (EU: IMREPLYS; US: LEUKINE; and with Nobelpharma Co. Ltd for JAPAN: SARGMALIN®) and zenocutuzumab-zbco (US: BIZENGRI®; please click here to see Important Safety Information for BIZENGRI and accompanying full Prescribing Information, including BOXED WARNING.) Visit www.partnertx.com.

REFERENCES

1. Union Register of Medicinal Products: Imreplys. 
   https://ec.europa.eu/health/documents/community-register/html/h1924.htm
2. European Medicines Agency: Imreplys.
   https://www.ema.europa.eu/en/medicines/human/EPAR/imreplys
3. Wolbarst AB, Wiley AL Jr, Nemhauser JB, et al. Medical response to a major radiologic emergency: a primer for medical and public health practitioners. Radiology. 2010;254(3):660-677. doi:10.1148/radiol.09090330
4. Dainiak N. Medical management of acute radiation syndrome and associated infections in a high-casualty incident. J Radiat Res. 2018;59(Suppl_2):ii54-ii64. doi:10.1093/jrr/rry004
5. LEUKINE^® (sargramostim) for injection Prescribing Information. Lexington, MA: Partner Therapeutics, Inc.; August 2023. https://www.leukine.com/prescribing-information.pdf
6. IMREPLYS® (sargramostim) powder for solution for injection Summary of Product Characteristics. Dublin 2, Ireland: Partner Therapeutics, Ltd.; August 2025. https://ec.europa.eu/health/documents/community-register/2025/20250821166908/anx_166908_en.pdf

PARTNER THERAPEUTICS®, IMREPLYS®, and LEUKINE® are registered trademarks owned by Partner Therapeutics, Inc. BIZENGRI® is a registered trademark of Merus N.V. ©2025 Partner Therapeutics, All rights reserved.

SOURCE Partner Therapeutics, Inc.