(R)-2-[1-(3-Ethoxy-4-methoxyphenyl)-2-methylsulfonylethyl]-4-acetylaminoisoindoline-1,3-dione, known as (R)-Apremilast or R-Apremilast, is the R-enantiomer of the active pharmaceutical ingredient Apremilast. It is a small molecule, orally administered phosphodiesterase 4 (PDE4) inhibitor. The compound features a phthalimide core structure substituted with an acetylaminogroup and a chiral side chain containing a methoxyethoxy phenyl ring and a methylsulfonyl group. This compound is primarily investigated as a potential active pharmaceutical ingredient (API) for the treatment of autoimmune and inflammatory diseases. Its mechanism of action involves the selective inhibition of PDE4, leading to increased intracellular cyclic AMP (cAMP) levels, which subsequently modulates the production of pro-inflammatory cytokines (e.g., TNF-α, IL-17, IL-23) and anti-inflammatory mediators. While the S-enantiomer (S-Apremilast) is the marketed drug for conditions like psoriasis and psoriatic arthritis, the R-enantiomer is a key subject of research to explore potential differences in efficacy, pharmacokinetics, or side effect profiles, representing a significant impurity and metabolite of the racemic drug. As a defined chemical entity with targeted biological activity, (R)-Apremilast is crucial in pharmaceutical development for stereochemical purity control, metabolic studies, and as a reference standard. It is not a general-purpose synthetic building block but a specific drug substance or impurity standard used in the context of drug discovery, quality control, and regulatory submissions.