Molecular structure of 3-O-acetyl-11-keto-β-boswellic acid and its ʟ-alanine conjugate (AKBA-Ala) and their impact on lipoxygenases.

3-O-Acetyl-11-keto-β-boswellic acid (AKBA) is an anti-inflammatory constituent of frankincense. Its potency is attributed to its ability to interact with enzymes in the inflammation cascade, such as lipoxygenase and cyclooxygenase, to decrease formation of pro-inflammatory products while increasing the production of pro-resolving mediators. However, the availability of AKBA in vivo may be limited by factors such as poor solubility and protein binding in serum. A prodrug approach, wherein AKBA is conjugated to amino acids through its carboxylate moiety, could be a suitable way to improve solubility and half-life in blood while maintaining anti-inflammatory efficacy. In this study, we describe a robust protocol for the preparation of high-purity amino acid-AKBA conjugates, exemplified by synthesis of the ʟ-alaninate derivative AKBA-Ala. The alanine conjugate was characterized by NMR spectroscopy, ESI-MS, HPLC, and X-ray crystallography, representing the first reported solid-state structure of an amino acid-AKBA conjugate. In comparison to AKBA, biological investigations of AKBA-Ala demonstrated a reduced inhibition (40% weaker at 10 µM) of 5-lipoxygenase (5-LOX) product formation in human neutrophils, but slightly enhanced production (120% higher at 10 µM) of specialized pro-resolving mediators (SPM) in human M2 macrophages. Overall, the anti-inflammatory potency of AKBA-Ala remains comparable to that of AKBA.