# CD8 PET to stop or switch immunotherapy early

Source: https://onco.cc/ideas/idea-cd8-pet-io/  
OnCo record `idea-cd8-pet-io` (Idea). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Scan for T cells inside the tumour a few weeks after starting immunotherapy. If they have not arrived, change course.

## Summary

Scan for T cells inside the tumour a few weeks after starting immunotherapy, and change course if they have not arrived. Tracers such as 89Zr-crefmirlimab, a CD8 minibody, and 18F-AraG image T-cell infiltration and activation, and an early increase has tracked response in phase 2 studies. Response to checkpoint blockade requires T-cell infiltration, CT changes lag by months and pseudoprogression confounds RECIST, so an absent CD8 PET signal at week 4 could identify non-responders early. The test is a prospective trial in NSCLC or melanoma with week-4 CD8 PET, randomising PET non-responders to continue or switch. At early-clinical maturity it addresses the bottleneck No one can predict who responds to immunotherapy.

## Fields

- Kind: Idea
- Last checked: 2026-09-04
- Hypothesis: Absence of CD8 PET increase at week 4 identifies non-responders with >85% specificity, enabling early switch to alternative therapy.
- Rationale: Response to checkpoint blockade requires T-cell infiltration; CT changes lag by months; pseudoprogression confounds RECIST.
- Proposed test: Run a prospective trial in NSCLC or melanoma with week-4 CD8 PET; randomise PET-non-responders to continue vs switch.
- Maturity: early-clinical

## Sources

- Farwell et al., CD8-targeted PET imaging of tumour-infiltrating T cells in patients with cancer: a phase 1 first-in-human study of 89Zr-Df-IAB22M2C (Journal of Nuclear Medicine 2021): https://doi.org/10.2967/jnumed.121.262485

## Connected records

- cancers: [Melanoma](https://onco.cc/cancers/melanoma/), [Non-small-cell lung cancer](https://onco.cc/cancers/nsclc/)
- technologies: [Immune checkpoint inhibitors](https://onco.cc/technologies/checkpoint-inhibitor/), [Immuno-PET](https://onco.cc/technologies/immuno-pet/)
- targets: [PD-1](https://onco.cc/targets/pd1/)
- key papers: [CD8-Targeted PET Imaging of Tumor-Infiltrating T Cells in Patients with Cancer: A Phase I First-in-Humans Study of 89 Zr-Df-IAB22M2C, a Radiolabeled Anti-CD8 Minibody](https://onco.cc/key-papers/paper-farwell-j-nucl-med/)
- bottlenecks: [No one can predict who responds to immunotherapy](https://onco.cc/bottlenecks/b-immunotherapy-response/)
- ideas: [Kill combination arms early using circulating tumour DNA, before waiting for scans](https://onco.cc/ideas/idea-tr2-ctdna-futility-gates/), [Use a blood test at six weeks to decide whether to keep going](https://onco.cc/ideas/idea-bio2-ctdna-six-week-io-switch/)

---
JSON: https://onco.cc/api/v1/entities/idea-cd8-pet-io.json