Rabinowitz Discovers New Approach to Augment Leading Cancer Therapy

A revolutionary treatment called checkpoint blockade immunotherapy has proven both remarkable and bewildering in the fight against cancer: remarkable because some patients respond to it with great success, bewildering because others receive little benefit.

Part of the reason involves whether the patient’s cancer has enough mutations to activate the immune system. But many other patients fail to respond for unknown reasons.

Now, a team of researchers in the Rabinowitz Lab has put forward an explanation and a possible end-run around the factors that limit the treatment’s success through a process called metabolic supplementation.

They discovered that methanol—a substance that in large quantities can be toxic to human beings—shows great efficacy among mice in bolstering immunotherapy to treat and even cure certain cancers.

Methanol has only one carbon atom. In the body, it serves as a “one-carbon unit” donor that is metabolized into another one-carbon substance, the mammalian metabolite formic acid. Formic acid itself is a valuable building block for the body’s immune system. Strikingly, it’s often severely depleted in cancer patients.

While further research is needed to probe methanol’s potential and to find the “goldilocks” dosage that could buttress human health without causing harm, the science behind the lab work is compelling, said Professor of Chemistry and Director of the Ludwig Princeton Branch Joshua Rabinowitz.

“It’s a rare day when you find something in the lab that could possibly be directly translated to the clinic,” said Rabinowitz. “It’s also rare that you see something that doesn’t just slow cancer but actually, in mice, produces cures. And the cases where that has happened have often led to ultimate clinical success.

“A lot of caution is required here,” he added. “But I think it will be great to get an answer on whether this can help patients receiving checkpoint blockade, and whether it can take another big fraction of patients who would have succumbed to cancer and give them a path to a cure.”

Promising Results Published in Cell Chemical Biology

In their recent paper in the journal Cell Chemical Biology, researchers laid out how they supplemented mice diets with one-carbon units (1C units) in the form of methanol. They were able to administer a safe dosage because only a modest amount of methanol was needed to achieve the therapeutic benefit as mice metabolize methanol more effectively than humans. What they found was that treated mice undergoing checkpoint blockade therapy through their process were not only better, but that a “substantial number” were cured of their cancers outright.

How do researchers think this is working? T cells are part of the body’s disease-fighting squadron. They attack invaders in the form of viruses, bacteria, and our own self-made cancer cells. But in a cruel biological twist, cancer cells also have their own arsenal. They throw T cells off the trail with blocking signals. Checkpoint blockade uses drugs to block those signals—basically, blocking the blockers—which permits T cells to identify, track, and fight cancer cells.

Checkpoint blockade works especially well in cancers that express antigens, mutations that allow the T cells to easily “recognize” them. But even patients who carry what seems like an actionable number of tumor antigens can respond poorly to checkpoint blockade.

This has not been readily understood.

“We think that one of the reasons is metabolic barriers to T cell action, to immune cell action, and that’s the part we’re trying to tackle,” said Rabinowitz. “And we believe that we’ve discovered a way to overcome that barrier by supplementing with 1C units in the form of methanol.”

The possibilities behind methanol

Methanol is metabolized in the body into formic acid where it serves as an intermediate in the synthesis of nucleotides like ATP. The Rabinowitz Lab investigated methanol’s role in mice as a possible precursor to usable 1C units in hopes of discovering that it would augment immunotherapy. And in fact, it does.

The risk is in the dosage: too much methanol could lead to formate poisoning. Too little may not have the desired impact.

“The good news is that we have a pretty big goldilocks range between the amount of formate we think you need and the amount where it will be toxic,” said Rabinowitz. “What we found is that we could give methanol to mice in safe amounts, that it would be converted into formic acid, that the formic acid would be taken up by the immune cells and incorporated into their nucleotides and ultimately their RNA and DNA.

“By alleviating the insufficiency of these building blocks, it allowed the immune system to more effectively eliminate the tumor. This was definitely an exciting day in the lab … an exciting day with caution. We have to think very carefully about how to advance this safely into humans.”

Their paper reveals one possibility for reducing risk by changing the isotopic “flavor” of the hydrogens on the methanol, which controls the rate at which it produces formic acid.

Xincheng Xu, a graduate student with the lab, is lead author on the paper. Xu was largely responsible for developing the isotope-tracing techniques that allowed researchers to explore nucleotide metabolism in T cells in vivo. These techniques provided the rationale for metabolic supplementation.

Like Rabinowitz, Xu cautioned against heralding methanol as an answer quite yet.

“As a proof-of-concept in mice, which are far less prone to methanol/formate poisoning, methanol demonstrated efficacy as a formate prodrug,” said Xu. “It’s particularly exciting that metabolic supplementation holds promise for enhancing standard-of-care cancer therapy.

“I also look forward to more metabolic barriers to effective anti-cancer immune response being unveiled and overcome in the future.”

The paper, “One-carbon unit supplementation fuels purine synthesis in tumor-infiltrating T cells an augments checkpoint blockade,” was published in May 2024. The authors are: Xincheng Xu, Zihong Chen, Caroline Bartman, Xi Xing, Kellen Olszewski, and Joshua Rabinowitz. This research was supported by Ludwig Cancer Research, R01CA163591 from the National Cancer Institute, DP1DK113643 from the National Institute of Diabetes, Digestion and Kidney Disease, and SU2CAACR-DT-20-16 from Stand Up to Cancer.