Differential inhibition of cyclooxygenases-1 and -2 by meloxicam and its 4′-isomer

Abstract.

Objective and Design: Two structurally related compounds, meloxicam (Mel) and its structural 4′-isomer (4′-Mel), were compared to examine the role of a slightly different chemical structure on cyclooxygenase (COX) selectivity in in vitro and in vivo experimental models.¶Material or Subjects: In vitro studies were performed using human whole blood obtained from healthy volunteers, in vivo studies were performed in rats.¶Treatment: A concentration-response curve was obtained in the whole blood assay for Mel, 4′-Mel, indomethacin, piroxicam and diclofenac. These were used to calculate the respective IC₅₀ values of either prostaglandin E₂ (PGE₂) or thromboxane B₂ (TxB₂). Similarly, a dose-response curve was obtained for Mel, 4′-Mel and piroxicam when measuring in vivo prostaglandin production, anti-inflammatory activity and gastric tolerance to determine the dose resulting in a 50% reduction of the each parameter.¶Methods: COX selectivity was investigated in vitro using a human whole blood assay. PGE₂ synthesis in vivo was measured in inflammatory exudate, in the stomach and kidneys of rats. Anti-inflammatory effects were measured in an adjuvant arthritis model and gastric tolerance was tested in an ulcerogenicity model in vivo in rats.¶Results: In the human whole blood assay, the ratio of IC₅₀ values for COX-1 vs. COX-2 inhibition was 13 for Mel and 1.8 for 4′-Mel. In inflammatory exudate in rats, Mel and 4′-Mel inhibited PGE₂ synthesis to a similar extent, ID₅₀ values ∼ 0.3 mg/kg. In contrast, Mel was a weaker inhibitor of PG synthesis than 4′-Mel in the rat stomach and in the rat kidney. Paw swelling was reduced by 50% with 0.1 and 0.2 mg/kg for Mel and 4′-Mel, respectively, in the rat adjuvant arthritis model. Gastric tolerance (UD₅₀) was 2.4 mg/kg for Mel and 0.4 mg/kg for 4′-Mel.¶Conclusions: These data demonstrate that the in vitro and in vivo pharmacological profile of meloxicam is structurally dependent and that minor structural changes can lead to significant differences in the selectivity for COX-1 and COX-2 in vitro and to different profiles in vivo suggesting different therapeutic potential.