## 4 Discussion  
Assessment of in vitro penetration is a crucial step in selecting the appropriate drug derivative, its dose, and the vehicle of topical composition. The degree of penetration of the active substance may vary depending on the properties of the compound, the vehicle, and additional components used [2,29]. The compositions applied on the skin containing NSAIDs allow for pain mitigation in the application area, simultaneously minimizing the risk of systemic adverse effects. To minimize the incidence of adverse effects of drugs with this group, especially when managing pain associated with osteoarthritis of the peripheral joints, drugs applied on the skin are recommended. One such drug is naproxen, characterized by high effectiveness for various inflammatory conditions [12,30]. Even though naproxen is a very popular topical drug, its poor solubility in water (0.051 mg cm\({}^{-3}\)[31]) has limited its bioavailability. Previous studies have shown that the structural modification of naproxen by forming its ion pair, in which the counterion is the L-proline alkyl ester cation, such as isopropyl ester L-prolinium (S)-naproxenate, [ProOiPr][NAP] and other salts of this drug ([ProOPr][NAP], [ProOBu][NAP], [ProOEt][NAP]) enhanced the penetration of pig skin compared to the control-unmodified naproxen [12,13]. Other studies also present that the ion pairs better permeate the lipids of the _stratum corneum_ and then dissociate in the lower layers of the epidermis [12,32,33]. In our previous naproxen skin permeation studies, we used a PBS solution (pH 7.4) [12] or 70% ethanol [13] as a carrier of acid naproxen (NAP) and its amino acids ester salts. The compositions based on Celugel(r) or Pentravan(r) comprising acid or salt form of naproxen were also revealed. Furthermore, the significantly higher permeation of naproxen for composition comprising [ProOiPr][NAP], than for commercially available topical gels in the permeation studies through the artificial STRAT-M(r) membrane was disclosed [13]. Going further to obtain more realistic application conditions of topical analgesic compositions, we decided to check the penetration of naproxen through human skin and not as before through artificial STRAT-M(r) membrane [13]. Furthermore, to increase the penetration of naproxen through skin, in the presented studies we selected for testing two penetration enhancers as components of the topical compositions based on Pentravan(r) and comprising unmodified naproxen and its salt with L-proline isopropyl ester. The permeation of naproxen through human skin differs significantly in comparison with the naproxen permeation through STRAT-M(r) for the same compositions used in the permeation tests. For the Pen + NAP composition, the cumulative mass of naproxen in the receptor fluid after 24 h, was 434.2 \(\pm\) 4.5 and 339.7 \(\pm\) 16.3 ug NAP cm\({}^{-2}\), respectively, when STRAT -M(r) and human skin were used in the tests. Greater differences in the amount of permeated naproxen with the change of membrane were observed for the Pen + [ProOiPr][NAP] composition applied, where the cumulative mass was 3338.9 \(\pm\) 49.0 and 791.2 \(\pm\) 83.8.\(\upmu\)sNAP cm\({}^{-2}\), respectively. Despite this difference, the naproxen salt still provides greater and faster permeation of naproxen compared with acid naproxen. Pentravan(r) is a commonly used pharmacy oil-in-water emulsion base. Pentravan(r) is a liposomal cream base with a transdermal mechanism of active ingredient absorption. The key component of this medium is the LIPOIL complex, i.e., lecithin-based liposomes, which facilitates the penetration of drugs through the layers of the epidermis. After applying this medium to the skin, liposomes penetrate through the SC through intracellular channels into the deeper layer of epidermal tissue and the highly vascularised dermis. Lipids accumulated in the extracellular structure of the epidermis constitute the route of delivery of the active substance [4,17].