The study, of 89 arthritis patients started on infliximab (Remicade), found obese patients improved less than leaner ones.
Of the 15 obese patients, half responded to 16 weeks of infliximab treatment meaning they showed a significant reduction in their disease activity score in 28 joints.
In contrast, three-quarters of the 66 normal-weight and overweight patients responded, as did seven of the eight underweight patients.
Even after adjustment for baseline symptom scores, weight was still related to the likelihood of improvement.
Obese have less response
The findings, published in Arthritis & Rheumatism, raise the possibility that obese patients are less likely to see results with infliximab or other tumour necrosis factor (TNF) blockers - including etanercept (Enbrel), adalimumab (Humira) and golimumab (Simponi).
It is not clear why obese individuals might have less of a response to TNF blockers. But it's possible that adipocytokines inflammation-promoting proteins produced by fat tissue, could play a role, Dr Paul P. Tak, one of the researchers on the new study, said.
The explanation would not appear to rest in the drug itself, according to Dr Tak, of the University of Amsterdam in the Netherlands.
Intravenous doses of infliximab are adjusted according to patients' body weight which is why Dr Tak's team focused on the drug for this study. So the lesser response among obese patients is unlikely to signal a need for a higher dose, the researchers say.
According to Tak's team, the study appears to be the first to look at how body weight might affect the response to TNF blockers in patients with any auto-immune disease.
Larger studies needed
Larger studies are still needed to confirm and extend the current findings; for now, they are interesting from a scientific point of view, Dr Tak said, because they suggest that fat tissue could play a role in promoting the inflammation seen in RA.
For patients and doctors, he added, they serve as an alert that TNF blockers might be relatively less effective for obese people. In general, patients on anti-TNFs are expected to show a noticeable improvement within three to four months, after which alternative medications should be considered if there is no response.
Tak has served as a consultant to several drug companies, including Humira maker Abbott Laboratories and Wyeth, which makes Enbrel.
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