# Precision targeting of T cell cytotoxicity with PET

> **NIH NIH R01** · UNIVERSITY OF CALIFORNIA, SAN FRANCISCO · 2021 · $524,205

## Abstract

The recent clinical success of inhibitors against immune checkpoint proteins (e.g. CTLA-4, PD-L1), which are
thought to stimulate T cell responses against tumors, has revolutionized cancer therapy. Yet even among
patients with high tumor mutational burden, only approximately 20-30% of patients achieve deep response, and
discerning responders from non-responders is challenging with conventional imaging. On this basis, there is an
urgent unmet need to develop biomarkers that distinguish responsive and treatment resistant patients, as well
as identify patients at risk for undesired immune related adverse events. We hypothesized that an imaging
biomarker capable of selectively measuring the biology that T cells use to impart cytotoxicity might address
these unmet needs. Since antitumor T cell cytotoxicity is conferred primarily by the pro-apoptotic serine protease
granzyme B, we have developed a peptide-based chemosensor we term “restricted interaction peptide” that
enables spatiotemporal measurements of granzyme B proteolytic activity as the enzyme traverses the
immunological synapse between T cell and target cell. Upon proteolytic cleavage of the full length, pro-form of
the restricted interaction peptide (termed GB1) by granzyme B, a radiolabeled antimicrobial peptide is liberated
and undergoes a spontaneous conformational shift that results in stable (and non-toxic) membrane association.
We have shown that radiolabeled GB1 detects T cell activation in tumors and normal tissues elicited by systemic
immune checkpoint inhibitors. Following on these encouraging preclinical data, we have now assembled a
multidisciplinary team to conduct translational studies to evaluate the utility of granzyme B biochemistry as a
biomarker. Over three specific aims, we will (1) perform IND enabling studies for 64Cu-GB1, (2) conduct a phase
0 first in human study to determine tracer safety, pharmacokinetics, and dosimetry, and (3) execute a phase I
study to determine the accuracy for detection of urothelial and renal cancers undergoing a productive immune
response due to treatment with standard of care immune checkpoint inhibitors. If successful, this project will
establish a new paradigm for the measurement of T cell cytotoxicity in vivo that could have implications for the
clinical management of other problematic human disorders like bacterial or viral (HIV, SARS-CoV) infections.
Moreover, the imaging approach is entirely new, and favorable data emerging from this project could motivate
further studies to develop restricted interaction peptides to measure the enzymology of other disease associated
proteases in vivo with PET.

## Key facts

- **NIH application ID:** 10179211
- **Project number:** 1R01CA258297-01
- **Recipient organization:** UNIVERSITY OF CALIFORNIA, SAN FRANCISCO
- **Principal Investigator:** Rahul Aggarwal
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $524,205
- **Award type:** 1
- **Project period:** 2021-03-01 → 2026-02-28

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10179211

## Citation

> US National Institutes of Health, RePORTER application 10179211, Precision targeting of T cell cytotoxicity with PET (1R01CA258297-01). Retrieved via AI Analytics 2026-05-22 from https://api.ai-analytics.org/grant/nih/10179211. Licensed CC0.

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