Rejuvenating lung stem cells for idiopathic pulmonary fibrosis

Deep inside the lung, little honeycombs of alveoli exchange carbon dioxide for oxygen, providing the body with a literal breath of fresh air. While this gas exchange responsibility lies primarily with the type 1 alveolar epithelial cells, their companions — type 2 alveolar epithelial cells (AEC2s) — exist for when things get tough. In addition to secreting surfactant to prevent the alveoli from collapsing during breathing, AEC2s are the stem cells of the lung (1). They regenerate the lung epithelial tissue after an injury occurring from exposure to a toxic air pollutant or an infection.

Michael Bollong wears a navy blue suit jacket and glasses while standing outside. — Michael Bollong at Scripps Research uses chemical biology to find new drugs for regenerative medicine.
Credit: Scripps Research

But sometimes, often due to aging, AEC2s stop proliferating and can no longer replace the injured epithelial cells in the lower airway, leading to a progressive buildup of scar tissue in the lungs. Researchers have now found that in people with idiopathic pulmonary fibrosis (IPF) — a disorder characterized by progressive and irreversible scarring of the lungs — AEC2s lose the ability to differentiate and repopulate the lower airway (2). While there are medications that slow fibrosis progression in IPF, there is no drug that targets the underlying cause of the disease.

Michael Bollong, a chemical biologist at Scripps Research, wondered if there was a way to turn back the clock on AEC2s and if restoring their regenerative ability might be a way to strike IPF at its source.

“Aging made these cells less capable of regenerating, so could we find some mechanism that would make these cells grow like they would have when the individual was younger?” he asked. “Could it overcome that roadblock?”

Through a phenotypic screen of a massive drug-repurposing library, Bollong and his colleagues identified a class of drugs that restored AEC2s’ability to regenerate (3). Now, with an optimized compound called CMR316 entering a Phase 1 clinical trial, a long-awaited cure for IPF may be on the horizon.

How did you first get interested in studying lung stem cells?

My goal since starting my own laboratory has been to think about how we tackle regenerative medicine as a field. In the context of the lung, researchers debated about which cells were the actual stem cells of the lower airway for a number of years. They didn’t reach a consensus until 2013 when folks figured out that AEC2s were the ones repairing the alveolar epithelium (4). At the same time, researchers in the IPF field were starting to think that the cause of the disease was not something idiopathic about the fibroblasts, but actually was related to a dysfunction of the stem cell population in the lung, the AEC2s.

How did you look for a drug that could restore AEC2 regeneration?

We did a high-content imaging screen — a type of phenotypic screen — of the drug repurposing library ReFRAME (3). The ReFRAME library is a unique asset of Scripps Research and Calibr, the drug discovery arm of Scripps Research, that contains practically every drug that has been in a Phase 1 clinical trial; that’s around 13,000 molecules. A fraction of those drugs in the library can be bought from chemical suppliers, but for the others, the Calibr team did patent mining and synthesized the molecules that they thought went into the clinic. It’s an amazing resource for us to go in and say that these were molecules that have been in people. They have defined targets, known pharmacology, and safety. It’s a great starting place for a drug.

Aging made these cells less capable of regenerating, so could we find some mechanism that would make these cells grow like they would have when the individual was younger?
– Michael Bollong, Scripps Research

We took aged, donor AEC2s and screened them in a monolayer without mitogens (molecules that enhance or induce cell division), and we looked for anything that would make these cells proliferate. We found things that were generally proliferative, and we threw those out. But one of the most efficacious, non-toxic, and selective class of drugs we identified was the dipeptidyl peptidase-4 (DPP4) inhibitors. That was the lightbulb.

What is DPP4, and what does it do in AEC2s?

DPP4 is an extracellular protease. It's most famous for degrading incretin hormones such as glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), but it also degrades several dozen substrates that are signaling molecules. DPP4 is expressed in a couple of cell types, but it's expressed very strongly in AEC2s and in the epithelial cells that line the lung lumen. We asked what growth factors AEC2s make and express, and among those molecules we identified IGF-1 and IL-6, both of which have very well-established roles as mitogens and are necessary for expanding and differentiating AEC2s into type one cells. It became clear that by blocking the degradation of IGF-1 and IL-6 by DPP4, the DPP4 inhibitors restored AEC2s’s regenerative capacity.

Could you then just repurpose commercial DPP4 inhibitors to regenerate AEC2s in IPF?

Initially, we thought we might be able to directly repurpose the available oral DPP4 inhibitors, but when we did animal pharmacokinetics studies and then looked at target engagement in the lung, the doses we needed to use were too high. DPP4 is located extracellularly in the lung lumen, and because the oral DPP4 inhibitors have high membrane permeability, they didn’t stay in the lung lumen for very long.

We thought that maybe we could reformulate the drug to deliver it directly to the lung as a nebulized solution. We chemically modified the drug so that it had very low membrane permeability so that it could engage DPP4 in the lung lumen and nowhere else.

Now that your nebulized DPP4 inhibitor, CMR316, has entered a Phase 1 clinical trial, what has been the most rewarding part of this drug development process?

It's incredibly rewarding to come up with an idea and then to see it being tested in patients. One of my favorite parts of this project was that this was a completely internally conducted exercise. We initiated the project in my laboratory in collaboration with Peter Schultz's group. Once we got some initial proof of concept, we started working with Calibr, and we did everything from the medicinal chemistry to the efficacy work in-house with no external pharma sponsorship. To do that, as one of the first projects that started in my laboratory, was very gratifying.

This interview has been condensed and edited for clarity.