Interactions of essential fatty acids with the non-polar and polar lipids of the tear film

Purpose : Essential fatty acids (EFAs) as dietary supplements have been shown to be effective in treating dry-eye by reducing inflammation at the ocular surface. Their topical application in eye drops to deliver fatty acids directly to the ocular surface is currently under investigation. Our previous research has shown that EFAs at the ocular surface can alter the biophysical properties of tear lipids. This study aimed at investigating how EFAs interact with the non-polar and polar lipids of the tear film taking cholesterol ester and phospholipid as examples. Methods : EFAs (linoleic acid and α-linolenic acid), cholesterol oleate, and dipalmitoyl phosphatidylcholine (DPPC) were spread separately on an artificial tear solution in a Langmuir trough maintained at 35°C. The lipid film at the air-tear interface was compressed and expanded to record pressure-area isocycles. EFAs were mixed with cholesterol oleate and DPPC in equal mole fraction ratios and interactions between them were studied by recording pressure-area isocycles of the mixed films. Results : Linoleic acid and α-linolenic acid added to cholesterol oleate decreased its molecular area by about 10 Å2 and the maximum surface pressure by about 15 mN/m. The plateau at about 45 mN/m corresponded to a bilayer that was highly compressible with rearrangement of lipid molecules within the layer without further increase in pressure in the mixed film and with further increase in pressure in the cholesterol oleate film. Linoleic acid and α-linolenic acid added to DPPC decreased its molecular area by about 10 Å2 and 17 Å2, respectively, but did not much affect the maximum surface pressure. The plateau at about 45 mN/m corresponded to a condensed film that was highly compressible with molecules leaving the ordered condensed state to forming a bilayer without further increase in pressure in the mixed film and the DPPC film. Conclusions : EFAs interact with the non-polar and polar lipids of the tear film causing their condensation. They maintain characteristics of cholesterol oleate and DPPC in the mixed lipid films. At higher compressions lipid molecules rearrange themselves and form bilayers that are highly compressible at the air-tear interface. Future studies will focus on studying combinations of EFAs with more lipid classes of the tear film to gain a thorough understanding of the interaction of EFAs with tear lipids for their topical application in dry-eye