Progress Patients

At X4, we are driven to successfully advance innovative therapeutics that truly make a difference in the lives of patients with few to no treatment options.

We are focused on developing treatments for chronic neutropenic disorders and WHIM syndrome.

What’s it Like Living with Chronic Neutropenia?


Our lead clinical candidate, mavorixafor, is being developed as a once-daily oral therapy and is designed to facilitate the mobilization of white blood cells from the bone marrow into the blood, increase levels of circulating neutrophils, lymphocytes, and monocytes, and improve immune system function.

We have now successfully completed a Phase 3 clinical trial of mavorixafor in people with WHIM syndrome (the 4WHIM study) and have initiated a Phase 2 clinical trial (NCT04154488) in people with chronic neutropenic disorders, including idiopathic, cyclic, and congenital neutropenia. See below for more information on these programs:

  • WHIM syndrome is both a rare combined immunodeficiency and a congenital neutropenic disorder in which the body’s immune system does not function properly and has trouble fighting infections. In many people, it is caused by variations in the CXCR4 gene that result in white blood cells getting trapped in the bone marrow, frequently causing both neutropenia (low levels of circulating neutrophils) and lymphopenia (low levels of circulating lymphocytes).
  • WHIM syndrome is named for its four common clinical findings, although not all patients experience all symptoms, and not all symptoms are required for a diagnosis:
    • Warts (related to infection with the Human Papilloma Virus (HPV)),
    • Hypogammaglobulinemia (low immunoglobulin, or IG, levels),
    • Infections (both bacterial and fungal), and
    • Myelokathexis (a hyper-dense population of immune cells in the bone marrow).
  • Approximately 30-40% of people with WHIM syndrome develop HPV-associated cancers as they age.
  • Mavorixafor is an investigational antagonist of CXCR4 being developed as a once-daily oral therapy to correct the immune dysfunction experienced by people with WHIM.
  • In November 2022 and May 2023, we reported positive results from the 4WHIM trial, which met its primary endpoint and the majority of key secondary and exploratory endpoints. Mavorixafor was generally well tolerated in the trial, with no treatment-related serious adverse events reported and no discontinuations for safety events.
  • In late October 2023, our New Drug Application (NDA) submitted to the United States Food and Drug Administration (FDA) for the approval of once-daily, oral mavorixafor to treat individuals aged 12 and older with WHIM syndrome was accepted for Priority Review, establishing a goal of six months review from the date of acceptance and assigning a Prescription Drug User Fee Act (PDUFA) target action date of April 30, 2024.
  • There are currently no approved treatments for WHIM syndrome.

Understanding WHIM Syndrome/The Patient Journey

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  • Chronic neutropenic disorders are rare blood conditions lasting more than three months, persistently or intermittently; people with chronic neutropenia are at higher risk of developing infections and certain cancers and having a reduced quality of life due to low levels of neutrophils circulating in the blood.
  • The only treatment currently approved for severe neutropenia is granulocyte colony-stimulating factor (G-CSF), an injectable therapy that has been associated with burdensome side effects.
  • Due to its demonstrated ability to mobilize neutrophils from the bone marrow into the blood, we believe mavorixafor has the potential to provide therapeutic benefit across a range of chronic neutropenic disorders.
  • We are currently conducting a Phase 2 clinical trial (NCT04154488) to evaluate the use of once-daily oral mavorixafor with or without G-CSF in people with chronic neutropenic disorders, including idiopathic (of unknown origin), cyclic, and congenital (of genetic causation) neutropenia.
  • This Phase 2 trial follows successful completion of a Phase 1b clinical trial that demonstrated the ability of a single daily dose of oral mavorixafor to raise neutrophil counts in all of the trial participants with idiopathic, cyclic, or congenital neutropenia. Mavorixafor was well tolerated in the study with no treatment-related serious adverse events reported.

Understanding Chronic Neutropenia/The Patient Journey

Chronic Neutropenia Before Diagnosis Infographic
Chronic Neutropenia After Diagnosis Infographic

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Clinical trials and the subsequent approval of a therapy provide the best way to make therapies accessible to as many patients as possible. Patients interested in an investigational product that has not yet been approved are encouraged to speak with their doctor and potentially participate in one of our current or future clinical trials. Please find information on our clinical trials by searching for X4 Pharmaceuticals at

Currently, we are only offering expanded access to patients who were enrolled in an X4-sponsored long-term extension of a trial at the time that trial was completed. The expanded access program will be administered via a single-patient Investigational New Drug (IND) application process managed by the treating physicians. A single-patient IND is a written request by a physician for individual patient use of an investigational drug that is submitted to the FDA or comparable regulatory agencies in other countries.

We encourage you to stay informed regarding our development programs; we will continue to review our expanded access policy as more information and clinical data on the safety and efficacy of mavorixafor become available.

For additional information, physicians with eligible patients can contact our Medical Affairs team at

Patients and families interested in learning more about expanded access generally can contact our Patient Affairs and Advocacy team at

patient resources

The Immune Deficiency Foundation (IDF)
Information and about genetic testing and genetic testing resources, including no-cost genetic testing for people with primary immunodeficiencies (PI)

Information and about genetic testing and genetic testing resources, including no-cost genetic testing for people with primary immunodeficiencies (PI)

A no-cost genetic testing program, sponsored by X4 Pharmaceuticals in partnership with Invitae

WHIM Syndrome Community
Private Facebook group for people living with WHIM Syndrome.

The Immune Deficiency Foundation (IDF)
Not-for-profit patient advocacy organization that provides information and resources for people with primary immunodeficiencies (PIs)

International Patient Organization for Primary Immunodeficiencies (IPOPI)
IPOPI is an international not-for-profit organization that advocates for PID patients worldwide

The Jeffrey Modell Foundation (JMF)
Not-for-profit organization devoted to early and precise diagnosis, meaningful treatments and ultimately cures for people with primary immunodeficiencies

National Neutropenia Network (NNN)
Not-for-profit organization that promotes awareness, education and research for patients and families with severe chronic neutropenia

National Organization for Rare Disorders (NORD)
A not-for-profit patient advocacy organization dedicated to improving the lives of people living with rare diseases and the organizations that serve them