Lab-Grown Mini Tumors Help Predict Breast Cancer's Response to Therapy

Researchers at UC San Francisco have developed a new method to predict how different types of breast cancer will respond to treatment, using patient data from an innovative clinical trial called I-SPY and rapid experimentation on lab-grown mini tumors.

The advance could hasten more personalized treatments for breast cancer, including the triple negative type, which is especially aggressive and hard to treat.

In their study, which appeared Aug. 6 in Cell Reports Medicine, the researchers found that the organoids were able to mimic a tumor’s response to treatment.

“The breast tumor organoids modeled how corresponding patient tumors responded to therapies and identified candidate combination therapies for cancers that don’t respond to standard treatment,” said the study’s senior author Jennifer M. Rosenbluth, MD, PhD, a medical oncologist and the Sulochana Pradhan, MD, Endowed Professor in Breast Cancer at UCSF. “These findings support organoid modeling as a bridge between clinical biomarkers and precision treatment strategies in breast cancer.”

The researchers created the breast cancer organoids by placing patient-derived tumor cells into a gel designed to support the original tumor's biology. Over several weeks, the cells clustered into tiny spheres that reflected the structure and biology of the original tumor. Each cluster contained up to thousands of cells — too small to see clearly without a microscope — the largest appearing as translucent clusters in the gel.

The researchers then used the organoids to test what they had learned from the I-SPY2 trial about how patients with different types of tumors responded to current therapies. These included immunotherapy, PARP-inhibitors, platinum chemotherapy drugs, and dual-HER2 targeted therapies.

Since many of the organoids were derived from triple-negative breast cancer tumors, researchers used them to validate a model that predicted whether a given tumor would respond to a treatment called veliparib-platinum chemotherapy (VP) that is often used to treat triple-negative breast cancer.

The team screened 386 small-molecule inhibitors on an organoid that was resistant to VP and found a promising compound called ABT-263, which helps to eliminate damaged cells.

The organoid drug screen also revealed other promising hits, including a class of drugs called HSP90 inhibitors; and the researchers were able to link what they observed in the lab to a subset of I-SPY patients who had responded better to these types of drugs.

“The breast cancer organoids were found to express important cancer biomarkers — many of which can be targeted with drugs,” said Tam Binh V. Bui, MD, MSc, the study’s first author, who is a PhD candidate at UCSF. “These organoids allowed us to study the effects of drugs directly in human tissue and prioritize the most promising therapies for this subtype.”

The study did not test how organoid-guided treatment decisions would perform over time. And the organoids could not reflect the complexity of a whole organ and could not mimic the environment inside the body, which includes blood vessels, immune cells and other processes that influence the signals that the tumor cells receive.

But the researchers hope that one day physicians may be able to use patient-derived organoids to develop personalized approaches to cancer care.

“By combining computational analyses of large molecular and clinical datasets with organoid model systems, this proof-of-principle study demonstrated the utility of matching I-SPY2 resistance biomarkers and signatures to residual disease tumor organoid cultures,” Rosenbluth said. “Our findings highlight the value of a reverse translational approach that integrates patient-level clinical trial data and testing in organoid models to inform drug discovery and future personalized treatment strategies for patients.”

About I-SPY Trial Consortium:

For more than a decade, the UCSF-led I-SPY trial consortium has worked to accelerate the development of new therapeutics for early-stage, high-risk breast cancer. The current I-SPY 2.2 trial tests multiple cancer therapies simultaneously among different breast cancer subtypes. The trial uses clinical biomarkers of response and resistance – measurable features of the tumor that predict its response to therapy. Using these biomarkers, researchers have been able to take a more personalized approach to therapy to optimize treatment based on individual patient responses.

Reference: Bui TBV, Wolf DM, Bruck MC, et al. Biomarker-guided responses in patient-derived organoids predict effective therapies in breast cancer. CR Med. 2026. doi: 10.1016/j.xcrm.2026.102973

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