Primary Mediastinal Large B-Cell Lymphoma Refractory

Primary mediastinal large B-cell lymphoma (PMBCL) is an aggressive form of cancer that develops in the space between the lungs. While most patients respond well to initial treatment, those whose disease becomes refractory—meaning it no longer responds to standard therapy—face significant challenges that require specialized approaches.

Table of contents

• What is Primary Mediastinal Large B-Cell Lymphoma?
• When Standard Treatment Stops Working
• Treatment Options for Refractory Disease
• CAR-T Cell Therapy
• Checkpoint Inhibitor Therapy
• Emerging Treatment Approaches

What is Primary Mediastinal Large B-Cell Lymphoma?

Primary mediastinal large B-cell lymphoma is a rare subtype of aggressive B-cell lymphoma that originates in the mediastinum—the space located between the lungs in the middle of the chest. This cancer develops from thymic B lymphocytes, which are white blood cells that normally develop in a gland called the thymus^[1].

PMBCL constitutes approximately 2% to 4% of all non-Hodgkin lymphomas (NHL) and about 6% to 12% of diffuse large B-cell lymphomas (DLBCL)^[1][2]. The disease primarily affects young adults in their third to fourth decade of life, with a median age of around 35 years. Unlike many other lymphomas, PMBCL has a slight female predominance^[2][3].

• Mediastinum (space between lungs)
• Thymus gland
• Lungs (may spread locally)
• Chest wall (may spread locally)
• Pleura (may spread locally)
• Pericardium (may spread locally)

The World Health Organization recognized PMBCL as a distinct entity separate from other types of DLBCL in 2001, and it has been classified as a separate lymphoma type since 2016 due to its unique clinical, histological, and molecular characteristics^[1][6].

Clinically, PMBCL typically presents as a rapidly growing mass in the anterior mediastinum. The malignant cells express B-cell surface molecules, such as CD19, CD20, CD22, and CD79a^[1]. The growing tumor mass can cause compression of surrounding tissues, leading to symptoms such as superior vena cava syndrome (swelling of the face and abdomen due to blocked blood flow), cough, difficulty breathing (dyspnea), hoarseness, and difficulty swallowing (dysphagia)^[2].

When Standard Treatment Stops Working

The most commonly used initial therapeutic regimens for PMBCL are R-CHOP (rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone) and R-EPOCH (dose-adjusted etoposide, prednisone, vincristine, cyclophosphamide, doxorubicin, and rituximab)^[1]. The addition of rituximab to chemotherapy has significantly improved response rates and survival for patients with PMBCL^[3].

In the frontline setting, DA-EPOCH-R has been shown in prospective studies to result in outstanding outcomes, with a 5-year event-free survival rate of 93% and overall survival rate of 97%^[5]. This regimen has been recognized as the preferred frontline treatment because it can achieve excellent results without the need for consolidative radiation to the mediastinum.

However, despite these encouraging results in the initial treatment setting, some patients develop refractory disease—meaning their lymphoma does not respond to standard treatment or progresses during treatment. For these patients, the outcomes remain poor. When PMBCL becomes refractory, it often spreads beyond the mediastinum through the bloodstream (hematogenous spread) to other organs and tissues^[2].

Treatment Options for Refractory Disease

The current standard approach for patients with relapsed or refractory PMBCL has traditionally been salvage chemotherapy followed by autologous stem cell transplantation (auto-HSCT), similar to the approach used for patients with DLBCL. In this procedure, a patient’s own blood-forming stem cells are collected, stored, and then returned after high-dose chemotherapy destroys the cancer cells along with the patient’s existing bone marrow^[2].

Unfortunately, this approach does not result in high rates of cure in patients with refractory PMBCL^[5]. Because PMBCL affects a relatively young population with a skewed age distribution, it accounts for a much higher proportion of autologous hematopoietic stem cell transplantations performed for treatment of lymphoma. However, patients with relapsed disease generally have a poor prognosis^[2].

CAR-T Cell Therapy

Chimeric antigen receptor T-cell therapy, commonly known as CAR-T cell therapy, has emerged as a successful strategy for patients with refractory or relapsed PMBCL^[1][5]. This innovative treatment involves collecting a patient’s own immune cells called T cells, genetically modifying them in a laboratory to recognize and attack cancer cells, and then infusing them back into the patient’s body.

Two CAR-T cell products have been used in refractory PMBCL: axicabtagene ciloleucel and lisocabtagene maraleucel^[5]. These products are designed to target CD19, a protein found on the surface of B cells, including the cancer cells in PMBCL.

Recent data from the French national DESCAR-T registry showed that patients with relapsed or refractory PMBCL treated with anti-CD19 CAR-T cells achieved meaningful responses^[4]. This real-world evidence supports the use of CAR-T cell therapy in this difficult-to-treat patient population.

Checkpoint Inhibitor Therapy

The characteristic molecular features identified in PMBCL have opened new treatment opportunities. One of the most important discoveries is that PMBCL cells frequently have copy number alterations of a specific region of chromosome 9 (9p24.1), which leads to increased expression of key genes including programmed death-ligand 1 (PD-L1), PD-L2, and JAK2. These alterations help the cancer cells evade the immune system^[5][6].

This discovery has led to the successful use of PD-1 inhibitors—drugs that block the interaction between cancer cells and the immune system, allowing the immune system to recognize and attack the cancer. In the relapsed setting, the single-agent PD-1 inhibitor pembrolizumab has demonstrated high and durable remission rates in patients with refractory PMBCL^[5].

PMBCL cells also express CD30, a protein found on the cell surface, although usually in a patchy fashion^[5][6]. While the CD30 antibody drug-conjugate brentuximab vedotin (BV) as a single agent has been deemed inactive in PMBCL, combinations of BV and PD-1 inhibitors have shown higher response rates than PD-1 inhibitor alone^[5].

Emerging Treatment Approaches

Research and development of novel therapies for refractory PMBCL are ongoing, and some studies have achieved encouraging results^[1]. The understanding of molecular mechanisms underlying PMBCL pathogenesis has helped identify targets for directed therapy in the future^[3].

PMBCL shares many biological similarities with classical Hodgkin lymphoma, including constitutive activation of the JAK-STAT and NF-κB pathways. Both pathways are involved in cell growth and survival, and drugs targeting these pathways are being investigated^[6].

However, despite these advances, randomized controlled trials with larger sample sizes are still needed to fully establish the effectiveness of newer treatments. The relatively low number of patients with PMBCL is a main obstacle in conducting randomized prospective trials, so therapeutic decisions have often been based on retrospective studies^[3].

Positron emission tomography-computed tomography (PET-CT) plays an important role in assessing treatment response and guiding subsequent treatment strategy for patients with refractory PMBCL^[1]. This imaging technique combines metabolic information with detailed anatomical images to help doctors determine whether treatment is working.

Diffuse large B-cell lymphoma
Classical Hodgkin lymphoma
Nodular sclerosing Hodgkin lymphoma