Royal Veterinary College-led research suggests that blocking CDK8/19 could protect cartilage, reduce inflammation and modify several processes involved in osteoarthritis — but clinical trials are still needed.
A new study led by researchers at the Royal Veterinary College (RVC) has identified a promising drug target that could eventually change how osteoarthritis is treated in both human and veterinary medicine.
Published in Bone Research on 25 August 2026, the research found that inhibiting proteins known as cyclin-dependent kinases 8 and 19 (CDK8/19) produced beneficial effects across cartilage cells, inflammatory pathways and bone-related processes associated with osteoarthritis (OA). The strongest in-vivo evidence came from mice that naturally develop the disease.
The findings are particularly significant because there is currently no approved disease-modifying osteoarthritis drug that reliably stops or reverses the underlying disease process. Existing treatments primarily aim to control pain, preserve mobility and improve quality of life.
However, the research remains preclinical. The inhibitor has not yet been shown to reverse osteoarthritis in people or living horses, and it is not an approved OA treatment.
Key findings at a glance
| Area studied | What researchers found | Why it matters |
|---|---|---|
| Cartilage cells | CDK8/19 inhibition promoted genes associated with healthy cartilage while suppressing processes linked to cartilage degeneration and abnormal mineralisation | Suggests a potential cartilage-protective effect |
| Human cell models | Beneficial changes were maintained even under inflammatory, OA-like conditions | Provides evidence that the mechanism is relevant to human biology |
| Horse cartilage cells | Primary equine chondrocytes showed favourable cartilage-related gene changes after treatment | Supports further investigation for equine osteoarthritis |
| Inflammation | The inhibitor reduced several inflammatory responses in laboratory macrophage models | OA affects the whole joint, not cartilage alone |
| Osteoarthritic mice | Treated mice showed improved gait and treadmill performance and lower blood markers associated with inflammation and oxidative stress | Provides in-vivo evidence of therapeutic potential |
| Joint structure | Treatment altered cartilage composition and reduced abnormal mineralised growth-plate bridges in the mouse model | Suggests effects beyond simple pain relief |
| Clinical use | Not yet established | Human and veterinary trials are still required |
Why osteoarthritis remains such a major treatment challenge
Osteoarthritis is the world’s most common form of arthritis. Rather than being simply a matter of cartilage “wearing out”, it is now understood as a whole-joint disease involving cartilage degeneration, changes in the bone beneath the cartilage, inflammation, altered cellular metabolism and other structural changes.
The World Health Organization estimates that approximately 528 million people were living with osteoarthritis worldwide in 2019, more than double the estimated number in 1990. The knee is the most commonly affected joint, followed by the hip and hand.
Common symptoms include:
- joint pain;
- stiffness;
- swelling;
- reduced range of motion;
- difficulty walking or exercising;
- progressive loss of function.
The disease is also a major problem in veterinary medicine.
Published equine literature commonly estimates that OA affects around 50% of horses aged 15 years or older, with prevalence increasing substantially in very old horses. Osteoarthritis is also a major contributor to equine lameness, loss of athletic performance and reduced quality of life.
That makes a therapy capable of changing the underlying biology of OA — rather than simply controlling symptoms — an important research goal for both human and animal medicine.
What are CDK8 and CDK19?
CDK8 and CDK19 are closely related protein kinases involved in regulating gene transcription, essentially influencing which cellular programmes are switched on or off.
The RVC-led team became interested in the pathway after screening compounds that target transcriptional and epigenetic mechanisms.
One of the initial promising compounds was BI-1347. Further experiments led researchers to concentrate on another potent CDK8/19 inhibitor known as MSC2530818, or MSC-818.
The researchers were particularly interested in the pathway because of its effects on chondrocytes — the cells responsible for producing and maintaining cartilage.
Chondrocyte hypertrophy may be an important part of osteoarthritis
Healthy articular chondrocytes normally maintain the cartilage matrix that allows joints to move smoothly.
In osteoarthritis, however, some chondrocytes begin behaving more like cells involved in normal bone growth. They enter a process known as chondrocyte hypertrophy, which is associated with altered cartilage metabolism, mineralisation and progressive joint damage.
The researchers found that blocking CDK8/19 shifted chondrocytes toward what they described as a more pro-anabolic, anti-catabolic and anti-hypertrophic state.
In simpler terms, treated cells showed:
more activity associated with maintaining healthy cartilage, while showing less activity associated with cartilage destruction and abnormal mineralisation.
That is one reason CDK8/19 inhibition is being considered as a potential disease-modifying osteoarthritis strategy, rather than simply another method of controlling pain.
Evidence was found in human osteoarthritis tissue
The research did not rely exclusively on mouse cells.
When investigators examined human osteoarthritic cartilage, they found CDK8, MED12 and phosphorylated STAT1 concentrated around cartilage lesions, supporting the possibility that this biological pathway is involved in human OA pathology.
Human mesenchymal cells differentiated toward cartilage also responded to CDK8/19 inhibition in laboratory experiments. Under inflammatory conditions designed to mimic aspects of OA, treatment increased the expression of several cartilage-associated genes while suppressing ADAMTS5, an enzyme involved in cartilage matrix breakdown.
This makes the findings more relevant to human disease than results obtained from a mouse model alone.
Nevertheless, experiments on human cells and human tissue samples are not the same as treating a human patient. Clinical efficacy and safety remain unknown.
Why the study is also important for horses
One particularly interesting aspect of the research is its direct examination of equine cartilage cells.
Researchers isolated primary articular chondrocytes from six skeletally mature horses aged between three and 24 years. The horses had been euthanised for reasons unrelated to the research or joint injury.
When these cells were exposed to MSC-818, researchers observed favourable changes in several genes associated with cartilage health.
Under inflammatory conditions involving interleukin-1 beta (IL-1β), the treatment increased expression of cartilage-related markers including COL2A1 and MATN3 while reducing expression of ADAMTS5.
These results provide a biological reason to explore CDK8/19 inhibition as a possible treatment for equine osteoarthritis.
But there is an important distinction:
The study tested horse cartilage cells in the laboratory. It did not treat horses with osteoarthritis.
Clinical studies in horses would therefore be required before anyone could determine whether the approach improves lameness, pain, mobility or long-term joint health in real equine patients.
What happened when researchers treated mice with osteoarthritis?
The most important in-vivo part of the study involved male STR/Ort mice, a strain that spontaneously develops osteoarthritis as it ages.
This differs from some experimental models in which OA has to be artificially produced through surgery or injury. The STR/Ort model therefore allows scientists to investigate processes associated with naturally developing disease, although mouse OA still cannot perfectly reproduce osteoarthritis in humans or horses.
The researchers began treatment when the mice were 22 weeks old, after OA pathology had begun to develop.
Six mice received MSC-818 at 5 mg/kg orally three times per week for 12 weeks, while five control animals received the vehicle treatment.
Mobility improved
At week 11, researchers assessed the mice using a treadmill-based gait analysis system.
All MSC-818-treated animals completed the treadmill task successfully, compared with 60% of the control group.
Treatment also corrected aspects of gait asymmetry associated with disease.
These functional results are important because a treatment that changes laboratory biomarkers without improving movement may have limited clinical relevance.
Blood markers of inflammation and oxidative stress fell
Researchers also measured two blood biomarkers:
- IL-1β, associated with inflammation;
- malondialdehyde (MDA), a marker of oxidative stress.
Both were reduced following CDK8/19 inhibitor treatment.
The treated mice also experienced less of the abnormal weight gain seen in the STR/Ort disease model.
Changes were also visible inside the joint
Improved mobility was accompanied by biological and structural changes.
Histological examination revealed alterations in cartilage composition, including enhanced deposition of proteoglycans, important molecules that help cartilage retain water and withstand mechanical loading.
Researchers also observed increases in chondromodulin-1 and matrillin-3, proteins associated with cartilage biology.
Using high-resolution micro-computed tomography (micro-CT), the researchers examined small bridges of mineralised tissue that develop in the growth plates of STR/Ort mice and are associated with worsening disease in this model.
Treated mice developed fewer mineralised bridges, and those that did form were less dense.
Together, the findings indicate that CDK8/19 inhibition was influencing multiple aspects of the disease rather than simply changing how the animals moved.
The treatment may affect inflammation as well as cartilage
Osteoarthritis is increasingly recognised as a disorder involving far more than cartilage.
Bone, synovial tissue, immune cells, mechanical loading and inflammatory signalling can all contribute to disease progression.
The researchers therefore investigated the effect of CDK8/19 inhibition on macrophages, immune cells involved in inflammatory responses.
When laboratory-derived macrophages were exposed to inflammatory stimuli, MSC-818 reduced the activity of several inflammatory genes, including IL1B, IL6 and TNFA under certain experimental conditions.
The study also found effects involving osteoclasts and osteoblasts — cells responsible for bone resorption and formation.
That combination is significant because it suggests that targeting CDK8/19 could potentially influence the entire osteoarthritic joint environment, not just cartilage.
Does this mean scientists have found a cure for osteoarthritis?
No.
The findings are promising, but describing CDK8/19 inhibition as an established cure for arthritis would go well beyond the evidence.
The study provides preclinical evidence of disease-modifying potential.
It does not yet demonstrate that:
- CDK8/19 inhibitors reverse osteoarthritis in humans;
- the drugs reverse osteoarthritis in horses;
- long-term treatment is safe in either species;
- improvements will persist after treatment stops;
- the same dose or delivery method would work in patients;
- the treatment is superior to existing OA management strategies.
There is another important nuance in the mouse data. Although researchers observed meaningful improvements in mobility, biomarkers and several joint parameters, the study reported no significant effect on the conventional OARSI histological score. The authors suggest this may reflect limitations of that scoring method in capturing some cellular and whole-joint treatment effects, but the result reinforces the need for larger confirmatory studies.
The mouse experiment was also small, involving 11 animals in total, and used male mice only. Those factors are important when interpreting how confidently the results can be translated to much larger and biologically different species.
Why a disease-modifying osteoarthritis drug would be a major advance
Modern OA treatment can be highly valuable for controlling symptoms, but it generally focuses on management rather than reversal of established structural disease.
Depending on the patient and affected joint, current strategies may include exercise and rehabilitation, weight management when appropriate, pain-relieving or anti-inflammatory medication, injections and, in severe human cases, joint replacement surgery.
Researchers around the world are investigating potential disease-modifying osteoarthritis drugs (DMOADs) capable of slowing structural deterioration or improving joint biology.
So far, however, no disease-modifying pharmaceutical has entered routine clinical use that reliably halts OA progression.
CDK8/19 inhibition therefore joins a highly important — and difficult — area of osteoarthritis drug development.
What needs to happen next?
The RVC researchers say the next objective is to move from preclinical investigation toward proof-of-concept studies in patient populations.
Future research will need to establish several things before CDK8/19 inhibition could become a real-world osteoarthritis therapy:
- Safety — determining whether effective doses can be administered without unacceptable adverse effects.
- Optimal dosing — establishing the amount, frequency and treatment duration required.
- Long-term efficacy — showing that improvements represent durable disease modification rather than temporary changes.
- Structural benefit — demonstrating meaningful preservation or restoration of joint tissues.
- Pain and mobility outcomes — proving that biological improvements translate into benefits that matter to patients.
- Cross-species effectiveness — independently establishing efficacy in humans, horses or other animals rather than assuming results will translate between species.
- Larger controlled studies — confirming the findings in considerably larger populations.
What the study means for horse owners today
For horse owners dealing with arthritis, the research is encouraging — but it does not change current treatment recommendations yet.
There is currently no commercially approved CDK8/19 osteoarthritis therapy that owners or veterinarians should attempt to use on the basis of this study.
Horses showing stiffness, declining performance, shortened stride, difficulty turning, joint swelling or persistent lameness should still be assessed by a veterinarian. Treatment and management depend on factors including the affected joint, severity of disease, age, workload and the individual horse’s health.
The main significance of this research is what it may make possible in the future: a therapy designed to alter the biological processes driving osteoarthritis, rather than addressing symptoms alone.
A promising step — but not yet a treatment
The RVC-led study provides unusually broad evidence for CDK8/19 as an osteoarthritis target.
Researchers observed beneficial effects in mouse cartilage models, human-derived cells, equine cartilage cells, inflammatory cell models and living mice with spontaneous osteoarthritis. They also identified CDK8-related activity around lesions in human osteoarthritic cartilage.
That cross-species and whole-joint evidence makes CDK8/19 inhibition an intriguing candidate for further development.
For now, however, the appropriate conclusion is not that osteoarthritis has been cured, but that scientists have identified a credible new pathway that may eventually lead to a disease-modifying treatment.
Whether that promise translates into a safe and effective therapy for people, horses or other animals will depend on the next stages of research.
Frequently Asked Questions
What is the new osteoarthritis treatment being studied?
Researchers are investigating drugs that inhibit CDK8 and CDK19, proteins involved in regulating cellular gene activity. A compound called MSC-818 produced promising results in laboratory experiments and an osteoarthritis mouse model.
Can CDK8/19 inhibitors cure osteoarthritis?
There is currently no evidence that they cure osteoarthritis in people or horses. The research is preclinical and demonstrates therapeutic potential rather than established clinical efficacy.
Was the treatment tested in horses?
Not in living horses. Researchers tested the inhibitor on primary cartilage cells obtained from six horses, where it produced several favourable gene-expression changes under normal and inflammatory conditions.
Was it tested on human osteoarthritis?
Researchers studied human-derived cartilage-forming cells and examined human osteoarthritic cartilage tissue. However, no human patients were treated with the drug as part of this study.
When could a new osteoarthritis drug become available?
There is currently no reliable timetable. Proof-of-concept studies, safety assessment and controlled clinical trials would be required before regulatory approval could even be considered.
Are there already drugs that stop osteoarthritis progression?
As of 2026, there is no approved disease-modifying pharmaceutical in routine clinical use that reliably stops or reverses OA progression. Current management primarily addresses pain, function and associated risk factors.
