The study advances a molecular strategy the research team has been developing for years. Instead of simply blocking or removing a cancer-promoting protein, the scientists designed a small molecule that connects BCL6 to proteins capable of activating a cell’s built-in death program.

Researchers believe this type of molecular rewiring could eventually be applied beyond lymphoma to other cancers and possibly autoimmune diseases. However, TCIP3 is still an experimental compound and is not ready for use in people.

“We’re basically trying to fight cancer at its cause, which means we’re taking the driving force of cancer and rewiring it to activate the mechanisms of cell death,” said Gerald Crabtree, MD, PhD, David Cohn Professor of Pathology and professor of developmental biology.

Crabtree shares senior authorship on the study with Nathanael Gray, Krishnan Shah Family Professor and professor of chemistry and systems biology; Stephen Hinshaw, assistant professor of molecular and cellular physiology; and Michael Green, director of Lymphoma/Myeloma Translational and Laboratory Research at MD Anderson Cancer Center. Graduate student Meredith Nix and postdoctoral researcher Sai Golisankar are the study’s lead authors.

How BCL6 helps lymphoma cells survive

Diffuse large B-cell lymphoma is the most common form of non-Hodgkin lymphoma, a type of blood cancer. The disease is often driven by a protein called BCL6.

In healthy immune cells, BCL6 binds to DNA and temporarily switches off genes that would normally stop cell growth or trigger cell death. This temporary suppression allows immune cells to multiply during an immune response.

After the immune threat has passed, other proteins modify BCL6 so it can no longer silence those genes. Excess immune cells then die through apoptosis, a controlled form of programmed cell death that removes unwanted, damaged or cancerous cells without causing inflammation or harming nearby tissue.

In lymphoma cells, BCL6 can remain permanently active. By continuously suppressing genes linked to cell death, it allows malignant cells to keep multiplying.

TCIP3 uses chemically induced proximity to reprogram BCL6

Rather than simply removing BCL6’s suppression, Crabtree, Gray and their colleagues wanted to activate the same cell death genes more strongly. They used a technique known as chemically induced proximity, which uses chemical bonds to connect molecules that would normally interact only rarely, if at all.

The researchers created TCIP3, a molecule that acts like a two-sided key.

“One side binds to BCL6,” Nix explained. “The other side binds to one of two proteins called P300 and CBP and adds a chemical tag called an acetyl mark to nearby proteins.”

When an acetyl tag is added to BCL6, the protein loses its ability to repress genes involved in cell death. P300 and CBP also add acetyl tags near histones, structures that help package DNA inside cells. These changes loosen the histones’ grip on DNA, making it more accessible to the transcription factors needed to activate genes.

This mechanism differs from existing drugs that target BCL6 by blocking or degrading it.

“We’re not only relieving the inhibition brought on by BCL6, but we’re also actively promoting the expression of these cell death genes, so we have a really powerful compound,” Nix said, comparing the approach with releasing a car’s brakes while keeping a foot on the gas pedal.

Why TCIP3 acts like molecular glue

To understand why TCIP3 was so effective, the researchers examined the compound at the atomic level. They crystallized the molecular complex and used X-rays to observe how TCIP3 binds to its target proteins.

The structural analysis revealed that the proteins formed additional chemical contacts with one another after TCIP3 bound to them. These interactions stabilized the entire complex, making the hybrid molecule more effective than the researchers had expected.

“Using structural studies and biophysical measurements, we confirmed that TCIP3 acts as a type of molecular glue, anchoring these proteins to each other,” Gourisankar said.

Using this structural information, chemists strengthened the connection between the two parts of the molecule. The redesigned compound maintained favorable interactions instead of bending and losing energy. The resulting TCIP3 killed lymphoma cells grown in the laboratory at very low concentrations.

Lymphoma tumors disappeared in mice after 11 days

The researchers next tested TCIP3 in mice implanted with human lymphoma cells. After the cells grew into tumors, the animals received TCIP3 twice a day.

“By day 11, the tumors treated with TCIP3 had completely disappeared, whereas the tumors in the control animals remained,” Nix said.

The treated mice showed no obvious signs of toxicity, and blood tests found no increase in inflammatory signals. TCIP3 also eliminated germinal centers, clusters of rapidly dividing immune cells that depend heavily on BCL6 and resemble the types of cells that can become abnormal in lymphoma.

This result suggested another possible application for the technology. Germinal center cells also play an important role in autoimmune diseases such as rheumatoid arthritis and myasthenia gravis. Researchers believe molecules similar to TCIP3 could eventually be investigated as potential treatments for these conditions.

A broader strategy for reprogramming cancer cells

TCIP3 still requires further chemical purification and testing in additional animal species before researchers can consider human clinical trials. The compound has not yet been tested as a treatment in people.

The broader strategy uses bivalent, or double-headed, molecules to redirect the activity of cancer-driving proteins rather than simply blocking them. The researchers are now searching for additional cancer-promoting proteins that could be targeted through the same type of molecular matchmaking.

“This could be a powerful approach to address other cell death suppressors and transcription factors that control genes that we want to activate in cancer,” Nix said.

Researchers from MD Anderson Cancer Center and the AI-powered drug discovery platform Deep Origin contributed to the study.

Research funding and disclosures

The research was supported by the National Institutes of Health through grants CA276167, CA163915, R01CA3044298, MH126720-01, S10OD028697-01, R01CA201380 and 1K99CA296700-01. Additional support came from the Howard Hughes Medical Institute, the Mary Kay Foundation, the Williams Foundation, the Victor Family Fund, Ed and Beatriz Schweitzer, the David L. Sze and Kathleen Donahue Interdisciplinary Fellowship, and the PhRMA Foundation Drug Discovery Predoctoral Fellowship.

Crabtree is the founder and scientific adviser of Shenandoah Therapeutics, which licensed the TCIP technology described in the study from Stanford University. Gray is a founder, scientific adviser and board member of Shenandoah Therapeutics.