Science & Pipeline-old

Advancing Candidates Unmet Needs

Leveraging our unparalleled expertise in diseases of the immune system and CXCR4 biology, we aim to bring innovative treatments to people with rare diseases of the immune system.

Our Focus on CXCR4

CXCR4, or C-X-C receptor type 4, is a cell receptor that helps regulate the movement of immune cells within the body. CXCR4 receptor stimulation by its ligand, CXCL12, has been shown to play a key role in the movement of white blood cells, including neutrophils, lymphocytes, and monocytes, throughout the body. Because antagonism of the CXCR4 receptor has been shown to increase the migration of white blood cells, we believe that therapeutic inhibition of the CXCR4/CXCL12 pathway holds the potential to benefit people with a wide variety of diseases.

Diagram illustrating the CXCR4 pathway regulation of white blood cell movement from bone marrow to the bloodstream. A bone cross-section shows CXCR4 on a cell interacting with CXCL12, with cells like lymphocytes and neutrophils nearby. An arrow indicates leukocyte movement toward the bloodstream, where various white blood cells are depicted circulating.

About Mavorixafor

Our deep understanding of the biology of the CXCR4 pathway has enabled us to develop mavorixafor, an oral, small molecule antagonist of the CXCR4 receptor. In clinical trials to date, mavorixafor has demonstrated the ability to improve the migration of neutrophils, lymphocytes, and monocytes into the bloodstream in people with moderate to severe chronic neutropenic disorders, indications with high unmet medical needs.

We have successfully developed mavorixafor, receiving U.S. approval for XOLREMDI™ (mavorixafor) in its first indication and are currently developing mavorixafor for additional patient populations as well, including for those with certain chronic neutropenic disorders.

Phase 1
Phase 2
Phase 3




WHIM Syndrome
Chronic Neutropenic Disorders
U.S. Approval [April 2024]
Poised to Begin Phase 3

*Please read the full Prescribing Information

*Please read the full Prescribing Information

Advancing Mavorixafor in Chronic Neutropenic Disorders

  • Due to its demonstrated ability to durably elevate levels of circulating white blood cells across multiple clinical trials, we believe that mavorixafor may be useful in the treatment of people with a variety of chronic neutropenic disorders.
  • 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.
  • People living with neutropenia have few treatment options; patients are commonly treated with granulocyte colony-stimulating factor (G-CSF), an injectable therapy that has been associated with burdensome side effects, including severe bone pain and myalgia.
  • Following successful completion of Phase 1b and Phase 2 clinical trials exploring the use of once-daily oral mavorixafor in the treatment of certain chronic neutropenic disorders, we are poised to begin a pivotal, global Phase 3 clinical trial.
  • The Phase 3 4WARD (NCT06056297) trial aims to evaluate the safety and efficacy of oral once-daily mavorixafor with or without G-CSF in people with congenital, autoimmune, and idiopathic chronic neutropenia and recurrent and/or serious infections. For more information, please visit

Unparalleled Expertise in CXCR4 and Immune System Biology

In addition to our headquarters in Boston, Massachusetts, we have an established research center of excellence in Vienna, Austria. Here, our scientists not only focus on new drug discovery, deepening the understanding of immune system dysfunction, and the role of the CXCR4/CXCL12 axis in maintaining healthy immunity, but also on examining the genetic causes of certain chronic neutropenic disorders and the mechanism of action of mavorixafor.

X4 Lab in Vienna