Around seventeen percent of adults worldwide suffer from osteoarthritis, and the number rises sharply with age. For over fifty years, no medication has been available that stops cartilage breakdown in joints. Researchers at Stanford University have now described a mechanism that drives this deterioration in the journal Science and demonstrated that a small molecule can slow—and even reverse—it.
The enzyme that makes joints age
Cartilage is not a static material. It requires signaling molecules to renew itself, and one of them is called prostaglandin E2, or PGE2. This messenger molecule stimulates cartilage cells to regenerate. Helen Blau and her team at Stanford Medicine discovered something crucial: with increasing age, the concentration of an enzyme called 15-PGDH increases significantly in knee cartilage. The abbreviation stands for 15-hydroxyprostaglandin dehydrogenase. This enzyme breaks down PGE2 before it can exert its regenerative effect.
In the study published in Science last November, the team measured twice the concentration of 15-PGDH in old mice knee cartilage compared to young animals. Even after joint injuries, enzyme levels doubled. Researchers call enzymes that increase with age and damage tissue "gerozymes." The team had already identified 15-PGDH as such a gerozyme in other tissue types in 2023. For cartilage, it was an open question.
What the experiments showed
The team tested a small synthetic inhibitor of 15-PGDH. Old mice whose cartilage had already degraded received the inhibitor twice weekly for four weeks. The cartilage rebuilt itself as healthy hyaline cartilage, the high-quality form needed for smooth joint movement. The animals moved more stably and put more weight on the treated leg, both signs of reduced pain.
In a second experimental setup, young mice were injured at the knee joint. Without treatment, these animals developed osteoarthritis within four weeks. Mice treated with the inhibitor developed arthritis far less frequently. That the substance works against injury-induced arthritis is clinically significant: many human osteoarthritis cases arise after sports injuries or accidents.
Particularly relevant for translation to humans: the team tested the inhibitor on human cartilage tissue removed during knee replacement surgery. After one week of treatment, samples showed fewer cells expressing cartilage-breakdown genes, and early signs of new cartilage cells were detectable. Helen Blau commented on the finding: while PGE2 is known for its role in inflammation, at normal biological concentrations it apparently promotes regeneration. The 15-PGDH inhibitor allows exactly that.
A comparison: the joint disease that already had its turning point
Those with rheumatoid arthritis, an immunologically triggered joint inflammation, have experienced a different medicine since 1998. That year, etanercept, the first drug from the TNF-alpha inhibitor class, was approved. Since then, biologics have fundamentally transformed treatment of this condition: remission is now achievable for many patients, something unthinkable in the pre-biologic era. Rheumatoid arthritis was once one of the leading causes of premature work disability in Europe.
Osteoarthritis, the mechanically caused form of joint degeneration, has not yet experienced this turning point. Despite more than fifty years of research, not a single approved disease-modifying osteoarthritis drug (DMOAD) exists. What patients receive are pain relievers, physical therapy, and in end-stage disease, knee replacement. Knee replacement surgeries are among the most common orthopedic procedures worldwide.
How dramatically things can change once the molecular key is found is shown by hepatitis C. Before 2014, chronic hepatitis C was barely curable; treatments lasted up to forty-eight weeks and had significant side effects. Since the introduction of direct-acting antivirals, the cure rate exceeds ninety-five percent in eight to twelve weeks. The mechanism was clear, the target molecule known. The 15-PGDH finding follows this logic: osteoarthritis is not fate, but a process with an enzyme at its origin.
Three steps to clinical use
The first and most difficult hurdle for any new drug—safety proof in humans—has already been cleared for the 15-PGDH inhibitor. An oral version of the substance was tested in a phase-1 study for age-related muscle weakness and rated as safe. This significantly accelerates the path to arthritis trials.
Still needed for approval: a phase-2 study examining dosage and efficacy in humans, which the researchers say should begin within eighteen months; a subsequent phase-3 study with hundreds of patients over several years; and approval by the FDA and EMA. Following a successful phase-1 program, this pathway typically takes seven to ten years.
One important limitation remains: mouse studies frequently fail to translate to humans. The human tissue samples in this study support the hypothesis but cannot replace clinical trials. What the study accomplishes above all is direction-finding: osteoarthritis is not inevitable wear and tear, but a process with controllable molecular biology. That is the starting point.