Diacerein Fails to Outperform Placebo in Patients with Inflammatory Phenotype Knee Osteoarthritis

Highlights

The multicenter, randomized, double-blind, placebo-controlled trial conducted in Australia found that diacerein, an interleukin-1β (IL-1β) blocker, did not significantly improve knee pain over 24 weeks compared to placebo in patients with symptomatic knee osteoarthritis (OA) and MRI-confirmed effusion-synovitis. Although diacerein is theoretically suited for the inflammatory phenotype of OA, the between-group mean difference in pain reduction was only -1.3 mm on a 100-mm visual analogue scale (VAS), failing to reach the minimal clinically important improvement threshold of 15 mm. Furthermore, diacerein was associated with a significantly higher incidence of gastrointestinal adverse events, particularly diarrhea, which affected nearly 40 percent of the treatment group. These findings challenge the clinical utility of diacerein in managing inflammatory knee OA.

Background: The Challenge of Inflammatory Phenotypes in Knee Osteoarthritis

Knee osteoarthritis (OA) remains one of the leading causes of global disability, characterized by cartilage degradation, subchondral bone remodeling, and chronic low-grade inflammation. Despite its prevalence, pharmacological options for OA are notoriously limited, often restricted to symptomatic relief with nonsteroidal anti-inflammatory drugs (NSAIDs) or intra-articular corticosteroids, both of which carry long-term safety concerns. In recent years, the medical community has shifted toward a phenotype-driven approach, recognizing that OA is not a monolithic disease but a collection of distinct subtypes. One such subtype is the inflammatory phenotype, characterized by effusion and synovitis, which is often associated with more severe pain and faster structural progression.

Central to this inflammatory process is the cytokine interleukin-1β (IL-1β), a potent mediator of cartilage catabolism and synovial inflammation. Diacerein, an anthraquinone derivative, acts as an inhibitor of IL-1β and has been marketed in several countries as a slow-acting drug for osteoarthritis (SYSADOA). While some previous studies and meta-analyses suggested a modest benefit, the evidence has been inconsistent, and regulatory bodies like the European Medicines Agency (EMA) have previously restricted its use due to safety concerns regarding severe diarrhea and liver disorders. This trial aimed to provide a definitive answer by specifically targeting the population most likely to benefit from IL-1β blockade: those with MRI-confirmed effusion-synovitis.

Study Design: Targeting Effusion-Synovitis with IL-1β Blockade

The study was a multicenter, randomized, double-blind, placebo-controlled clinical trial conducted at four specialized centers in Australia. From June 2019 to September 2022, researchers screened and enrolled 262 participants who met the clinical criteria for knee OA and demonstrated substantial knee pain (at least 40 mm on a 100-mm VAS) alongside MRI-confirmed effusion-synovitis. The inclusion of MRI imaging was a critical design choice, ensuring that the study population specifically represented the inflammatory phenotype of the disease.

Participants were randomized in a 1:1 ratio to receive either 50 mg of diacerein once daily or an identical placebo for the initial two weeks. If the medication was well-tolerated, the dosage was increased to 50 mg twice daily for the remainder of the 24-week trial. This titration period was designed to mitigate the known gastrointestinal side effects of the drug. The primary endpoint was the change in knee pain over 24 weeks, measured by the VAS (0-100 mm). Secondary outcomes included changes in physical function, quality of life, and structural changes on MRI, though the focus remained on the clinical experience of pain.

Key Findings: Efficacy and Safety Outcomes

The trial results were remarkably clear regarding the lack of superiority of diacerein over placebo. Of the 262 randomized participants (mean age 54.9 years; 56.1% female), 231 completed the full 24-week follow-up, reflecting a high retention rate for a long-term OA study.

Primary Outcome: Pain Reduction

Both groups experienced a reduction in pain over the 24-week period; however, there was no statistically significant difference between them. The diacerein group showed a mean pain reduction of -19.9 mm, while the placebo group showed a reduction of -18.6 mm. The between-group mean difference was a negligible -1.3 mm (95% CI, -9.8 to 7.3). Given that the predefined minimal clinically important improvement (MCII) for the VAS is 15 mm, the 1.3 mm difference is not only statistically insignificant but also clinically irrelevant. The data suggest that the observed improvement in both groups was likely due to the placebo effect or the natural fluctuation of OA symptoms rather than the pharmacological action of the drug.

Safety and Tolerability

Safety data revealed a significant burden of adverse events in the diacerein arm. Gastrointestinal symptoms were reported by 41.7% of participants in the diacerein group compared to 25.4% in the placebo group. Diarrhea was the most frequent complaint, occurring in 38.6% of those taking diacerein versus 22.3% in the placebo group. Additionally, 9.8% of participants receiving diacerein reported changes in urine color (chromaturia), a known but benign side effect of the drug’s metabolites. While no new major safety signals were identified, the high rate of diarrhea remains a significant barrier to patient adherence and overall quality of life during treatment.

Expert Commentary: Contextualizing the Failure of Diacerein

The failure of diacerein in this well-designed trial raises important questions about the role of IL-1β in osteoarthritis. Theoretically, the inflammatory phenotype should respond to IL-1β inhibition. However, the results align with other high-profile failures of IL-1 targeting agents in OA, such as the studies on canakinumab and lutikizumab. These results suggest that while IL-1β is present in the synovial fluid of OA patients, it may not be the primary driver of the subjective pain experience, or perhaps systemic inhibition is insufficient to reach therapeutic concentrations within the avascular cartilage and synovial space.

Mechanistic Plausibility vs. Clinical Reality

One possible explanation for the trial’s outcome is that the pain in knee OA is multifactorial, involving peripheral sensitization, central sensitization, and structural damage that cannot be reversed by anti-inflammatory therapy alone. Even in patients with effusion-synovitis, the pain may be driven by mechanical factors or other cytokines (such as TNF-alpha or IL-6) that diacerein does not adequately address. Furthermore, the placebo effect in OA trials is notoriously strong, often obscuring the benefits of active treatments unless the effect size is substantial.

Limitations and Generalizability

The study was robust in its use of MRI to select participants, which is a strength compared to older trials that relied solely on radiographic or clinical criteria. However, the 24-week duration, while sufficient for assessing symptomatic relief, may not be long enough to observe potential structural disease-modifying effects (DMOAD). Nevertheless, since the primary indication for diacerein in clinical practice is pain management, the lack of efficacy in this endpoint is a significant blow to its clinical rationale.

Summary and Clinical Implications

In conclusion, this randomized clinical trial demonstrates that diacerein is no more effective than placebo in reducing knee pain for patients with symptomatic OA and MRI-confirmed inflammation. The modest theoretical benefits of IL-1β inhibition did not translate into clinical reality, and the high incidence of gastrointestinal side effects further diminishes the drug’s value proposition. For clinicians, these findings suggest that diacerein should not be recommended as a primary treatment for knee pain in this population. Future research should perhaps focus on alternative pathways or more localized delivery systems for anti-inflammatory agents in osteoarthritis.

Funding and Trial Registration

This study was supported by grants from the National Health and Medical Research Council (NHMRC) of Australia. The trial was registered with the Australian New Zealand Clinical Trials Registry (ACTRN12618001656224). The authors declare no relevant financial conflicts of interest that influenced the conduct of this research.

References

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3. Bruyère O, Honvo G, Veronese N, et al. An updated algorithm recommendation for the management of knee osteoarthritis from the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO). Semin Arthritis Rheum. 2019;49(3):337-350.