Metformin Linked to Reduced Osteoarthritis Risk

Patients taking metformin for type 2 diabetes had a lower risk of developing osteoarthritis than did patients taking a sulfonylurea, according to a cohort study published in JAMA Network Open. The findings jibe with those seen in a 2022 systematic review of preclinical and observational human studies finding potentially protective effects of metformin on osteoarthritis.

"Our study provides further, robust epidemiological evidence that metformin may be associated with protection in the development and progression of osteoarthritis in individuals with type 2 diabetes," wrote Matthew C. Baker, MD, MS, an assistant professor of medicine in immunology and rheumatology at Stanford (Calif.) University, and his colleagues.

The findings also fit with the results of a poster presented at the Osteoarthritis Research Society International 2023 World Congress, although that abstract's findings did not reach statistical significance.

In the published study, the researchers analyzed deidentified claims data from Optum's Clinformatics Data Mart Database between December 2003 and December 2019. The database includes more than 15 million people with private insurance or Medicare Advantage Part D but does not include people with Medicaid, thereby excluding people from lower socioeconomic groups.

The researchers included all patients who were at least 40 years old, had type 2 diabetes, were taking metformin, and had been enrolled in the database for at least 1 uninterrupted year. They excluded anyone with type 1 diabetes or a prior diagnosis of osteoarthritis, inflammatory arthritis, or joint replacement. The authors then compared the incidence of osteoarthritis and joint replacement in these 20,937 participants to 20,937 control participants who were taking a sulfonylurea, matched to those taking metformin on the basis of age, sex, race, a comorbidity score, and duration of treatment. More than half the overall population (58%) was male with an average age of 62.

Patients needed to be on either drug for at least 3 months, but those who were initially treated with metformin before later taking a sulfonylurea could also be included and contribute to both groups. Those who first took a sulfonylurea and later switched to metformin were included only for the sulfonylurea group and censored after their switch to ensure the sulfonylurea group had enough participants. The comparison was further adjusted for age, sex, race, ethnicity, geographic region, education, comorbidities, and outpatient visit frequency.

The results revealed that those who were taking metformin were 24% less likely to develop osteoarthritis at least 3 months after starting the medication than were those taking a sulfonylurea (P < .001). The rate of joint replacements was not significantly different between those taking metformin and those taking a sulfonylurea. These two results did not change in a sensitivity analysis that compared patients who only ever took metformin or a sulfonylurea (as opposed to those who took one drug before switching to the other).

"When stratified by prior exposure to metformin within the sulfonylurea group, the observed benefit associated with metformin ... was attenuated in the people treated with a sulfonylurea with prior exposure to metformin, compared with those treated with a sulfonylurea with no prior exposure to metformin," the authors further reported. A possible reason for this finding is that those taking a sulfonylurea after having previously taken metformin gained some protection from the earlier metformin exposure, the authors hypothesized.

This observational study could not show a causative effect from the metformin, but the researchers speculated on potential mechanisms if a causative effect were present, based on past research.

"Several preclinical studies have suggested a protective association of metformin in osteoarthritis through activating AMP-activated protein kinase signaling, decreasing the level of matrix metalloproteinase, increasing autophagy and reducing chondrocyte apoptosis, and augmenting chondroprotective and anti-inflammatory properties of mesenchymal stem cells," the authors wrote.

Among this study's limitations, however, was the lack of data on body mass index, which is associated with osteoarthritis in the literature and may differ between patients taking metformin versus a sulfonylurea. The researchers also did not have data on physical activity or a history of trauma to the joints, though there's no reason to think these rates might differ between those taking one or the other medication.

Another substantial limitation is that all patients had type 2 diabetes, making it impossible to determine whether a similar protective effect from metformin might exist in people without diabetes.

Nonsignificant lower risk for posttraumatic knee osteoarthritis

Similar to the published study, the OARSI poster compared 5-year odds of incident osteoarthritis or total knee replacement surgery between patients taking metformin and those taking sulfonylureas, but it focused on younger patients, aged 18-40 years, who underwent anterior cruciate ligament or meniscus surgery.

Using data from MarketScan commercial insurance claims databases between 2006 and 2020, the authors identified 2,376 participants who were taking metformin or a sulfonylurea when they underwent their surgery or began taking it in the 6 months after their surgery. More than half the participants were female (57%) with an average age of 35.

Within 5 years, 10.8% of those taking metformin developed osteoarthritis, compared with 17.9% of those taking a sulfonylurea. In addition, 3% of those taking metformin underwent a total knee replacement, compared with 5.3% of those taking a sulfonylurea. After adjustment for age, sex, obesity, and a history of chronic kidney disease, liver disease, and depression, however, both risk difference and odds ratios were not statistically significant.

Risk of osteoarthritis was 17% lower in patients taking metformin (95% confidence interval, –0.18 to 0.09), whose odds of osteoarthritis were approximately half the odds of those taking a sulfonylurea (OR, 0.5; 95% CI, 0.21-1.67). Risk of a total knee replacement was 10% lower in metformin users (95% CI, –0.28 to 0.08) with a similar reduction in odds, compared with those taking a sulfonylurea (OR, 0.53; 95% CI, 0.2-1.44).

In this study, the researchers did not specifically determine whether the participants were diagnosed with diabetes, but they assumed all, or at least most, were, according to S. Reza Jafarzadeh, PhD, DVM, an assistant professor of medicine at Boston University.

"The goal was not to only focus on the diabetes population, but on people who received that exposure [of metformin or sulfonylureas]," Dr. Jafarzadeh said in an interview. Dr. Jafarzadeh noted that a larger randomized controlled trial is underway to look at whether metformin reduces the risk of osteoarthritis independent of whether a patient has diabetes.

The published study was funded by grants from the National Institutes of Health, the Department of Veterans Affairs, and Stanford University, and the authors reported no disclosures. The poster at OARSI was funded by NIH and the Arthritis Foundation, and the authors reported no disclosures.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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