Rheumatoid arthritis (RA) is a chronic, systemic autoimmune disease defined by persistent synovial inflammation, progressive cartilage and bone erosion, and extra-articular manifestations. Current treatments, such as disease-modifying anti-rheumatic drugs (DMARDs), glucocorticoids, and non-steroidal anti-inflammatory drugs (NSAIDs), control disease activity but are limited by toxicity, reduced response over time, and are expensive, demonstrating the need for safer and more effective adjuncts. The marine environment is a rich, largely untapped source of structurally diverse bioactive molecules that could be used to develop new therapeutics. This review compiles current evidence on anti-inflammatory and immunomodulatory compounds from marine organisms relevant to RA, including macroalgae, true marine microalgae, marine microorganisms (bacteria and fungi, including deep-sea taxa), sea cucumbers, sponges, mussels, corals, and jellyfish. Recurring mechanisms of action include inhibition of the NF-κB, MAPK, and JAK/STAT signalling cascades; suppression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2); reduced production of tumour necrosis factor-α (TNF-α), interleukin (IL)-1β, and IL-6; and activation of the Nrf2 antioxidant response. Particular attention is given to functional lipids (eicosapentaenoic and docosahexaenoic acids, prostaglandin-like oxylipins) and pigments (astaxanthin, fucoxanthin, β-carotene) from marine microalgae and heterotrophic protists, which recent literature identifies as the most clinically advanced marine leads. To clarify translational status, compounds are grouped by their development stage (marketed nutraceutical, clinical trial, or preclinical) and summarised in a dedicated table. Some compounds, such as green-lipped mussel extract, microalgal omega-3 oils, and astaxanthin, have reached the stage of randomised controlled trials for arthritis. However, most other potential treatments are still in the early, preclinical phase. It is worth noting that ocean-derived compounds appear generally safe, but more thorough studies, especially in living organisms and in clinical settings, are needed to confirm their effectiveness for rheumatoid arthritis before any claims can be made about their therapeutic benefits.
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