# Identification of small molecule inhibitors to exonuclease 1 for breast cancer treatment

> **NIH NIH U01** · BECKMAN RESEARCH INSTITUTE/CITY OF HOPE · 2023 · $575,395

## Abstract

ABSTRACT Our overall goal, which is fully responsive to PAR-20-271, is to develop a selective and effective
inhibitor of the multi-functional DNA repair enzyme exonuclease 1 (EXO1) that can be used both as a research
tool (chemical probe) and as a pre-clinical starting point toward the development of a potential cancer therapeutic
drug. There is no EXO1-specific small molecule inhibitor listed in the Chemical Probe Portal or other literature.
We will achieve our goal through discovery research, from implementing a primary high-throughput screen (HTS)
that we have already developed, to validating hits via a well-developed “critical path” of secondary assays, to
performing early hit-to-lead optimization via purchase of commercially available analogs of validated chemical
scaffolds and limited focused medicinal chemistry. EXO1 represents a druggable target, as it contains
functionally essential exonuclease activity for double-strand break response and repair (DSBRR) for processing
of stalled replication forks, which are critical pathways by which cells counteract endogenous DNA damage and
replication stress. Compared to normal cells, cancer cells carry a significantly higher burden of double-strand
breaks and replication stress, which generates a therapeutic window for treating cancer. To exploit this, current
therapeutic approaches primarily target proteins acting in repair pathways or in checkpoint signaling pathways
controlling repair. Many cancer cells are already defective in DSBRR; thus, EXO1 inhibition will cause cancer
cell-specific cell death through a synthetic lethality mechanism. Furthermore, EXO1is will display greater
specificity than currently used PARP inhibitors, because PARPs participate in a wide array of other cellular
processes, whereas EXO1 does not. Our group was the first to clone the human EXO1 gene and to characterize
its biochemical properties. We have expressed and purified the full-length and active EXO1 enzyme at scale,
developed a robust fluorescence-based enzyme inhibition assay, and performed a pilot HTS in our own core
facility. Thus, in collaboration with the Prebys Center of Sanford Burnham Prebys Medical Discovery Institute,
we are well positioned to 1) identify inhibitors of EXO1 exonuclease by performing HTS of a well-curated
~320,000 compound library; 2) validate hits for potency and selectivity; 3) perform “structure-activity relationship
(SAR)-by-catalog” and limited focused medicinal chemistry and benchmark absorption, distribution, metabolism,
and excretion (ADME)/pharmacokinetic (PK) characterization of best probes; and 4) determine the mode of
action (MOA) and biological effects of validated EXO1i candidate probes. All of our Aims are responsive to and
within the scope of PAR-20-271. The development of novel EXO1is will not only allow us to provide a critical tool
(i. e. chemical probe) to test mechanistic insights into the replication-repair interface but will also support
development of a novel chemoth...

## Key facts

- **NIH application ID:** 10735307
- **Project number:** 1U01CA276195-01A1
- **Recipient organization:** BECKMAN RESEARCH INSTITUTE/CITY OF HOPE
- **Principal Investigator:** DAVID HORNE
- **Activity code:** U01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2023
- **Award amount:** $575,395
- **Award type:** 1
- **Project period:** 2023-08-02 → 2026-07-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10735307

## Citation

> US National Institutes of Health, RePORTER application 10735307, Identification of small molecule inhibitors to exonuclease 1 for breast cancer treatment (1U01CA276195-01A1). Retrieved via AI Analytics 2026-05-25 from https://api.ai-analytics.org/grant/nih/10735307. Licensed CC0.

---

*[NIH grants dataset](/datasets/nih-grants) · CC0 1.0*