In 2013, renowned Boston Children's Hospital pain researcher Clifford Woolf, MB, BCh, Ph.D., and chemist Kai Johnsson, Ph.D., his fellow co-founder at Quartet Medicine, believed they held the key to non-narcotic pain relief

Woolf had shown that tetrahydrobioptrin—a protein also known as BH4—is a primary natural modulator of neuropathic and inflammatory pain sensitivity. The quartet was founded on the premise that inhibiting BH4 production could prevent the progression of acute pain to chronic pain in millions of patients, without the threat of addiction or tolerance.

In animal models of autoimmune disease and human cell lines, the researchers were able to inhibit T cell proliferation by blockading the BH4 pathway pharmacologically. In models of cancer, they were able to enhance T cell responses by elevating BH4 levels.

"By targeting BH4, we can suppress T cell activity in inflammatory conditions and increase their activity in the case of cancer," says Woolf, director of the F.M. Kirby Neurobiology Center at Boston Children's, who co-led the study. "The ability to target the same pathway in opposite directions is significant and represents a whole new therapeutic approach."

An immunological thermostat

Specifically, the researchers found that BH4 regulates the balance of available iron for mitochondria. To transition to an activated state, T cells need higher levels of mitochondrial energy; to produce it, mitochondria need higher levels of iron.

When T cells are under pressure, the body produces more BH4, increasing the supply of available iron, allowing the cells to divide and activate. When BH4 levels are low, mitochondria can't get the iron they need and T cell activity is suppressed. In the case of cancer, the study revealed that a metabolite produced by tumors works to block BH4, inhibiting T cell activation and cancer surveillance. It also showed that this response could be countered by augmenting BH4.

"The beauty of it is that the effect is upstream of specific types of T cell function," says Woolf. "Most drugs being developed now to treat autoimmune conditions are targeting specific kinds of T cells. This covers them all."

The team found that the BH4 pathway is only active in cases of infection or when proliferation needs to occur—and is not required for the normal formation of T cells. Finally, the paper reports the development of a highly potent small molecule, QM385, that inhibits the BH4 pathway, blocking T cell proliferation and autoimmunity.

Hiding in plain sight

Shane Cronin, a post-doc researcher from Ireland, arrived in the Woolf lab in 2006. He had trained in Vienna with noted immunologist Josef Penninger, MD, Ph.D. at IMBA, and now planned to shift his focus to the neurobiology of pain.

Woolf's team had just had its first major BH4 publication, which characterized the pathway as a key modulator of pain. To identify compounds that inhibit the expression of BH4, Woolf devised a drug screen using GFP fluorescent mice and asked Cronin to oversee the project.

The screen yielded plenty of hits—and for Cronin, an odd sense of déjà vu. The results pointed to the same compounds Cronin had used in his previous immunology lab to regulate T cell function.

When an opportunity to move back to Vienna presented itself, Cronin saw his chance. Penninger agreed to accept Cronin back into his lab at IMBA and threw his full support and knowledge behind the project. Cronin now had access to the resources and experience of one of Europe's leading immunology labs.

'Binary' therapeutic potential

Together, Penninger, Woolf, Cronin and the other members of the BH4 group, spent the next eight years extending their finding into models of immune-related diseases—contact dermatitis, multiple sclerosis, colitis—and finally cancer.

"There was no magic moment—just eight years of collaborative effort, putting together a puzzle, taking it apart, starting again," says Cronin. "But I guess that's the beauty of science—starting with a 'that's odd' moment and finding something incredible."

Working with Penninger, who co-led the study with Woolf, Cronin probed the binary therapeutic potential of BH4. If T cells proliferated in immune-related diseases, he wondered, what about cancer, where the same cells are often suppressed? Penninger and Cronin were able to boost BH4 levels in several mouse models of cancer, and the effect was immediate. Tumors shrank, and the metastatic spread all but ceased.

"As a trained immunologist who was involved in defining some of the paradigmatic T cell activation pathways, I had this idea that I knew it all and what was left to discover would only be details," says Penninger, who now leads the Life Sciences Institute of the University of British Columbia, Vancouver. "It was like opening an entirely new door in T cell biology—a door we can now rationally close to treat autoimmunity or keep open for T cells to kill cancer."