What is the most appropriate third-line treatment for a patient with follicular lymphoma without transformation who has progressed after two prior systemic regimens?

Third-Line Treatment for Follicular Lymphoma Without Transformation

For third-line treatment of follicular lymphoma without transformation, CD19-directed CAR T-cell therapy (axicabtagene ciloleucel, tisagenlecleucel, or lisocabtagene maraleucel) or CD3×CD20 bispecific antibodies (mosunetuzumab or epcoritamab) are the preferred options, with CAR T-cell therapy offering the advantage of single-infusion fixed-duration treatment and bispecific antibodies providing readily available "off-the-shelf" therapy without need for bridging chemotherapy.

Algorithmic Approach to Third-Line Selection

The choice between available third-line options should follow this decision pathway:

Primary Considerations

Timing and Disease Characteristics:

• If progression occurred <24 months from initial diagnosis (POD24), this indicates poor prognosis and warrants aggressive novel approaches 1
• Confirm CD19 and CD20 expression via re-biopsy before selecting therapy 1
• Assess disease pace: rapidly progressing disease may favor CAR T-cell therapy over bispecific antibodies due to step-up dosing requirements for the latter 1

FDA-Approved Third-Line Options (≥2 Prior Systemic Therapies)

Tier 1 - Preferred Immunotherapies:

1. CAR T-cell therapies (axicabtagene ciloleucel, tisagenlecleucel, lisocabtagene maraleucel) 1

   • Advantages: Single infusion, fixed duration, no continuous therapy required
   • Disadvantages: Requires manufacturing time (may need bridging chemotherapy), risk of CRS and ICANS, prolonged cytopenias
   • Best for: Patients who can wait for manufacturing, desire fixed-duration therapy

2. CD3×CD20 Bispecific Antibodies:

   • Mosunetuzumab (fixed duration) 1, 2
   • Epcoritamab (continuous until progression) 1, 2
   • Overall response rates: ~80%, complete response rates: 60-70% 2
   • Advantages: Immediately available, better safety profile than CAR T (minimal ICANS), no bridging therapy needed
   • Disadvantages: Step-up dosing required (challenging in rapidly progressive disease), CRS in ~50% (rarely high-grade), continuous therapy with epcoritamab 1, 2
   • Best for: Patients needing immediate treatment, those ineligible for CAR T-cell therapy

3. Zanubrutinib + obinutuzumab (BTK inhibitor combination) 1

   • Continuous therapy until progression
   • Alternative for patients who cannot receive or have failed other immunotherapies

Tier 2 - Alternative Options:

1. Loncastuximab tesirine + rituximab (CD19-directed antibody-drug conjugate) 1
   • Phase II data: 67% complete response rate, 95% PFS at 12 months 1
   • Appropriate for: Patients ineligible for or lacking access to CAR T-cell therapy or bispecific antibodies
   • Caution: Based on single-arm trial with small sample size (n=39); panel consensus was mixed on inclusion 1
   • Rare CD19 antigen loss suggests prior use doesn't preclude subsequent CAR T-cell therapy 1

Special Populations

Rituximab-Refractory Disease:

• Obinutuzumab-based regimens preferred over rituximab-based options 3
• Consider obinutuzumab maintenance (1g every 8 weeks for 12 doses) 1

Alkylating Agent-Refractory Disease:

• Associated with worse outcomes: 5-year OS 72% vs 81% in non-refractory patients 4
• Prioritize novel immunotherapies (CAR T or bispecific antibodies) over re-challenging with chemotherapy

Elderly or High Comorbidity Burden:

• Bispecific antibodies may be preferred over CAR T-cell therapy due to better safety profile 2
• Radioimmunotherapy remains an option 3

Key Clinical Nuances

Comparative Efficacy Considerations

The 2025 NCCN guidelines acknowledge that while CAR T-cell therapy showed somewhat favorable preliminary results in indirect comparisons, "limitations and uncertainties remain because of intrinsic differences in study design" 1. Both CAR T and bispecific antibodies demonstrate high response rates (ORR ~80%, CR 60-70%) with durable responses at 2 years 2.

Safety Profile Differences

• CAR T-cell therapy: CRS, ICANS, prolonged cytopenias (often related to bridging chemotherapy) 1
• Bispecific antibodies: CRS in ~50% (rarely high-grade), minimal ICANS, infections, neutropenia 1, 2
• Both approaches carry infection risk requiring vigilant monitoring 2

Resistance Mechanisms

Critical pitfall: Loss of CD19 or CD20 expression can occur with targeted therapies 1

• Re-biopsy recommended before initiating CD19-directed CAR T-cell therapy to confirm CD19 positivity 1
• CD20 loss may impact bispecific antibody efficacy 1
• However, CD19 loss after loncastuximab tesirine appears uncommon 1

What NOT to Do

• Avoid autologous stem cell transplantation (ASCT) in third-line unless patient had brief first remission after rituximab-containing regimen 3
• Do not use lenalidomide + rituximab in third-line—this is a second-line option for non-rituximab-refractory disease 3, 1
• Do not re-challenge with same class of therapy without considering cross-resistance patterns 3

Practical Implementation

For most patients in third-line, initiate evaluation for CAR T-cell therapy while simultaneously considering bispecific antibody availability. If disease is rapidly progressive or patient cannot wait for CAR T manufacturing, proceed directly with bispecific antibody (mosunetuzumab for fixed duration or epcoritamab if continuous therapy acceptable). Reserve loncastuximab tesirine + rituximab for patients who are truly ineligible for or lack access to the preferred immunotherapies 1.

The historical median PFS of 17 months and 5-year OS of 75% with conventional third-line therapies 4 underscores the critical need to utilize these novel immunotherapies, which offer substantially improved outcomes in this heavily pretreated population.