Plants of the Asteraceae family have been used in traditional medicine for centuries due to their main antimicrobial and analgesic activities. A liniment from Artemisia californica has recently been tested on patients affected by either acute pain or chronic pain conditions with great success. The aim of this study was to evaluate the anti-inflammatory activity of sesquiterpene lactones (SLs), representing the majority in the Asteraceae family. Leucodin, α-santonin and sclareolide (three SLs) were chosen to undergo chemical modifications. This pool of molecules underwent molecular modeling experiments using an in-house program, WATGEN, predicting the water network and its contribution to the overall affinity of the enzyme-ligand complex. The anti-inflammatory activity and the ability of compounds to modulate COX-2 expression have been evaluated in LPS-stimulated RAW 264.7 cells and in RIF-1 cells treated according to the Photodynamic Therapy (PDT) protocols using Photoprin (PH) as photosensitizer. Furthermore, commercially available assay kits were used to evaluate the concentration of PGE-2 and the direct inhibition of COX-2. All the tested molecules fit well in the enzyme binding pocket, but to get a substantial inhibition of the expression and activity of the enzyme as well as a reduction in the PGE2 concentration, high concentrations of the compounds are needed. The only exceptions being leucodin itself and FP6 , one of the α-santonin derivatives, presenting a CF 3 functional group. We believe that this class of compounds has some interesting potential in the treatment of pain and inflammation. Although, the activity seems to be due to a mechanism related to the expression of the COX enzymes rather than on a direct inhibition.