CTLA-4 Versus PD-1 Blockade: Different Brakes, Different Effects
CTLA-4 and PD-1 are both inhibitory receptors on T cells, and antibodies against each are established cancer treatments. They work at different points in the immune response, which explains why combining them increases both efficacy and side effects.
Quick Answer
CTLA-4 and PD-1 are both inhibitory receptors on T cells, and antibodies against each are established cancer treatments. They work at different points in the immune response, which explains why combining them increases both efficacy and side effects.
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Immuno-Oncology and Tumour Metabolism
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CTLA-4 and PD-1 blockade compared. Toxicity comparisons are drawn from melanoma trials and differ by dose, disease and schedule.
| Feature | CTLA-4 blockade | PD-1 blockade |
|---|---|---|
| Site of action | Lymph node, priming phase | Tumour and periphery, effector phase |
| Effect | Broadens T-cell activation; hits regulatory T cells | Reinvigorates antigen-engaged T cells |
| Example drugs | Ipilimumab, tremelimumab | Nivolumab, pembrolizumab |
| Severe immune toxicity alone | Higher and dose-dependent | Lower |
| Relationship to PD-L1 | Weak | Imperfect but present |
Where Each Acts
CTLA-4 acts early, mainly in lymph nodes, where it competes with the activating receptor CD28 for shared ligands and raises the threshold for T-cell activation. It also supports the suppressive function of regulatory T cells. Blocking CTLA-4 broadens the pool of T cells that become activated.
PD-1 acts later, in peripheral tissue and the tumour itself. When its ligand PD-L1 is present, PD-1 shuts down T cells that are already engaging antigen. Blocking PD-1 reinvigorates T cells at the tumour site.
Efficacy and Toxicity of the Combination
In advanced melanoma, combined nivolumab and ipilimumab produced higher response rates and longer progression-free survival than either agent alone, at the cost of grade 3 to 4 immune-related adverse events in a majority of patients compared with a minority on PD-1 alone.
The combination is now used in melanoma, renal cell carcinoma, mismatch-repair-deficient colorectal cancer, mesothelioma and others, with the trade-off between added benefit and added toxicity considered for each patient.
Practical Differences
CTLA-4 blockade tends to cause more colitis and hypophysitis; PD-1 blockade more pneumonitis and thyroid dysfunction, though overlap is wide. CTLA-4 toxicity is more dose-dependent.
PD-L1 expression predicts PD-1 benefit imperfectly and predicts CTLA-4 benefit poorly, so biomarker use differs between the two classes.
How the Choice Is Made
PD-1 or PD-L1 blockade alone is the more common starting point because it is better tolerated. Adding CTLA-4 blockade is reserved for situations where the extra response rate is judged worth the toxicity — first-line melanoma with high disease burden or brain metastases, mismatch-repair-deficient colorectal cancer, or mesothelioma among them.
Lower-dose or less frequent ipilimumab schedules, and substituting a LAG-3 antibody for CTLA-4 in melanoma, are ways the field has tried to keep some combination benefit with less toxicity. The decision is individualised and takes account of comorbidity, the acceptable risk of a serious inflammatory event, and access to close monitoring.
Key Takeaways
- ·CTLA-4 blockade broadens T-cell activation in lymph nodes; PD-1 blockade reinvigorates T cells in tumours.
- ·Combining them raises response rates and severe immune toxicity together.
- ·The two classes have different toxicity profiles and different relationships to PD-L1 expression.
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Frequently asked questions
What is the main difference between CTLA-4 and PD-1 blockade?
CTLA-4 blockade acts early, in lymph nodes, broadening which T cells get activated; PD-1 blockade acts later, in the tumour, reinvigorating T cells that are already engaging antigen.
Why is the combination more toxic?
It releases restraint at two stages of the immune response at once. In melanoma, combined nivolumab and ipilimumab caused grade 3 to 4 immune-related events in a majority of patients versus a minority on PD-1 alone.
Do the two classes cause different side effects?
There is wide overlap, but CTLA-4 blockade tends to cause more colitis and hypophysitis and is more dose-dependent, while PD-1 blockade causes relatively more pneumonitis and thyroid dysfunction.
References
- 1Combined nivolumab and ipilimumab or monotherapy in untreated melanoma. N Engl J Med, 2015. PubMed
- 2Relatlimab and nivolumab versus nivolumab in untreated advanced melanoma. N Engl J Med, 2022. PubMed
- 3Management of immune-related adverse events in patients treated with immune checkpoint inhibitor therapy. J Clin Oncol, 2018. PubMed
Continue Reading
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