Classical Hodgkin lymphoma is a B cell derived lymphoid malignancy characterized by the presence of Reed Sternberg cells, large, distinctive binucleate or multinucleate tumor cells, surrounded by a reactive inflammatory infiltrate comprising lymphocytes, plasma cells, eosinophils, and neutrophils that collectively constitute the bulk of the tumor mass. Despite Reed Sternberg cells representing only a minority of cells within Hodgkin lymphoma tumors, they orchestrate the immunosuppressive and pro inflammatory tumor microenvironment through the production of cytokines and chemokines that recruit and educate the surrounding reactive cell populations. This unique cellular architecture, so distinct from most other lymphomas and solid tumors, has important implications for the immunobiology of the disease and its particular sensitivity to immune checkpoint inhibition.
Classical Hodgkin lymphoma is among the most curable malignancies in oncology, with combination chemotherapy regimens such as ABVD achieving cure in the majority of patients with early and intermediate stage disease. However, approximately 15 to 20 percent of patients experience relapse following first line chemotherapy, and a further subset fails to achieve complete response to initial treatment. For relapsed and refractory disease, high dose chemotherapy with autologous stem cell transplantation has been the standard salvage approach, producing long term remissions in approximately 50 percent of eligible patients. Those who relapse following or are ineligible for autologous transplantation historically had very limited effective treatment options, with poor outcomes and no established curative approach until the emergence of novel targeted and immune based therapies.
Molecular Basis of PD 1 Checkpoint Sensitivity
Classical Hodgkin lymphoma exhibits a remarkably specific genetic basis for its immunotherapy sensitivity that distinguishes it from most other malignancies where PD L1 upregulation is driven by inflammatory cytokines in the tumor microenvironment. The 9p24.1 chromosomal region, which encodes the genes for PD L1, PD L2, and the JAK2 kinase, is amplified or copy number gained in the vast majority of classical Hodgkin lymphoma cases. This genetic amplification drives constitutive, high level PD L1 expression on Reed Sternberg cells that is independent of the inflammatory PD L1 upregulation seen in other tumors. Additionally, Epstein Barr virus infection, present in approximately 40 percent of classical Hodgkin lymphoma cases, further amplifies PD L1 expression through activation of downstream signaling pathways, creating multiple overlapping mechanisms for extreme PD L1 upregulation in this disease.
The consequence of this constitutive, genetically driven PD L1 overexpression is the creation of a profoundly immunosuppressive tumor microenvironment in which T cells infiltrating the lymphoma are held in a functionally exhausted state by continuous PD 1 ligand engagement. Tumor infiltrating T cells in Hodgkin lymphoma express high levels of multiple exhaustion markers and produce minimal cytokines or cytotoxic molecules despite being potentially reactive against tumor antigens. Pembrolizumab and other PD 1 blocking antibodies relieve this T cell exhaustion with particular effectiveness in Hodgkin lymphoma, producing the highest objective response rates observed in any hematological malignancy treated with single agent checkpoint inhibition, reflecting the extreme degree of PD 1 pathway driven immune suppression in this disease.
Clinical Evidence for Pembrolizumab in Relapsed Hodgkin Lymphoma
The KEYNOTE 087 trial was a pivotal study evaluating pembrolizumab in relapsed or refractory classical Hodgkin lymphoma across three cohorts defined by prior treatment history: patients who relapsed after autologous stem cell transplantation and brentuximab vedotin, patients who were ineligible for transplantation and had failed brentuximab vedotin, and patients who had not received brentuximab vedotin but had failed transplantation. Pembrolizumab at 200 milligrams every three weeks produced objective response rates of 69 to 78 percent across all three cohorts, with complete response rates of 22 to 28 percent. These response rates were substantially higher than those historically observed with other agents in heavily pretreated relapsed Hodgkin lymphoma, providing the efficacy evidence that supported regulatory approval of Keytruda for this indication.
The durability of responses achieved with pembrolizumab in Hodgkin lymphoma has been a particularly encouraging feature of the clinical data. Updated follow up from KEYNOTE 087 has demonstrated that a meaningful proportion of complete responders maintain their remissions for multiple years, raising the possibility that pembrolizumab may achieve functional cure in some patients with relapsed or refractory disease for whom no curative option had previously existed. The integration of pembrolizumab responses as a bridge to allogeneic stem cell transplantation in eligible patients has created a treatment pathway that leverages pembrolizumab’s high response rates to achieve transplant eligibility in patients who might not otherwise have achieved sufficient disease control for transplantation.
Pembrolizumab in Earlier Treatment Lines
The success of pembrolizumab in heavily pretreated relapsed Hodgkin lymphoma has prompted investigation of its use in earlier treatment lines where greater disease control and potentially curative intent can be pursued. The KEYNOTE 204 trial compared pembrolizumab with brentuximab vedotin as second line therapy in relapsed or refractory classical Hodgkin lymphoma, demonstrating superior progression free survival for pembrolizumab with a hazard ratio of 0.65 and a median progression free survival of 13.2 months versus 8.3 months. These results positioned pembrolizumab as a competitive alternative to brentuximab vedotin in the second line setting and raised questions about optimal sequencing of these effective agents.
Frontline incorporation of PD 1 inhibitors into first line treatment of classical Hodgkin lymphoma represents an exciting investigational direction, with multiple trials evaluating the substitution or combination of pembrolizumab with standard chemotherapy components to improve primary cure rates while potentially reducing long term chemotherapy and radiation related toxicity. The long term complications of standard Hodgkin lymphoma treatment, including secondary malignancies, cardiovascular disease, and pulmonary toxicity that can emerge decades after treatment, represent a significant burden in a patient population that is typically young and expected to have decades of life ahead following successful treatment. Replacing chemotherapy components with immunotherapy may reduce this long term toxicity burden if efficacy can be maintained.
Management Considerations and Toxicity
Pembrolizumab in classical Hodgkin lymphoma is generally well tolerated compared to intensive salvage chemotherapy regimens, with immune related adverse events representing the primary safety concern. Thyroid dysfunction, pneumonitis, and infusion reactions are among the more commonly observed immune related adverse events in this population. The immunological context of Hodgkin lymphoma, characterized by significant immune dysregulation, may influence the character and frequency of immune related adverse events, and clinicians managing Hodgkin lymphoma patients on Keytruda must maintain vigilance for these complications while managing the challenges of distinguishing immune related pneumonitis from infection or lymphoma related pulmonary involvement.
The use of pembrolizumab as a bridge to allogeneic stem cell transplantation creates specific management challenges, as allogeneic transplantation following checkpoint inhibitor therapy has been associated with higher rates of severe graft versus host disease in some series. The optimal timing of transplantation relative to last pembrolizumab dose, the selection of conditioning regimen and graft versus host disease prophylaxis in this context, and the monitoring strategies appropriate for post transplant patients previously treated with checkpoint inhibitors are active clinical research questions that inform the safe integration of pembrolizumab into the comprehensive management of relapsed Hodgkin lymphoma.
Conclusion
Classical Hodgkin lymphoma represents perhaps the most striking example of the predictive power of tumor molecular biology for immunotherapy responsiveness, with the genetically driven constitutive PD L1 overexpression of Reed Sternberg cells creating an extreme checkpoint dependent immune suppression that is highly sensitive to PD 1 blockade. Pembrolizumab has demonstrated exceptional response rates in relapsed and refractory disease, establishing Keytruda as a critical component of modern Hodgkin lymphoma management. The ongoing investigation of pembrolizumab in earlier treatment lines and in combination with targeted and cellular therapies holds the promise of further improving the already excellent cure rates achievable in this disease while reducing the long term treatment related morbidity that affects Hodgkin lymphoma survivors.
