Studies on Permeation Enhancers for Ocular Peptide Delivery Systems: Pz-peptide as a Novel Enhancer of Ocular Epithelial Paracellular Permeability in the Pigmented Rabbit

The objective of this study was to determine whether Pz-peptide, an enhancer of hydrophilic solute permeability in the intestine, could elevate the paracellular permeability of the cornea and conjunctiva in the pigmented rabbit. The in vitro penetration of four hydrophilic solutes, mannitol (MW 182), fluorescein (MW 376), FD-4 (FITC-dextran, 4 KDa), and FD-10 (FITC-dextran, 10 KDa) across the pigmented rabbit cornea and conjunctiva was studied either in the presence or absence of 3 mM enhancers. Drug penetration was evaluated using the modified Ussing chamber. The conjunctiva was more permeable than the cornea to all four markers. EDTA and cytochalasin B showed higher effects on marker transport than Pz-peptide, but Pz-peptide elevated the corneal transport of mannitol, fluoresein, and FD-4 by 50%, 26%, and 50%, respectively, without affecting FD-10 transport. Possibly due to the leakier nature of the conjunctiva, 3 mM Pz-peptide elevated the transport of only FD-4 by about 45%, without affecting the transport of other markers. Furthermore, the transport of Pz-peptide itself across the cornea and conjunctiva increased with increasing concentration in the 1-5 mM range, suggesting that Pz-peptide enhanced its own permeability, possibly by elevating paracellular permeability. Effects of ion transport inhibitors on Pz-peptide transport were then investigated. PZ-peptide penetration was not changed by mucosal addition of $10\;{\mu}M$ amiloride or $10\;{\mu}M$ hexamethylene amiloride, inhibiting serosal $Na^{+}$ exit by $100\;{\mu}M$ ouabain, or replacing $Na^{+}$ with choline chloride in the mucosal side buffer. These results seggested that Pz-peptide enhanced the paracellular permeability of rabbit cornea and conjunctiva and further indicate that ion transporters were not involved in the Pz-peptide induced elevation of paracellular marker permeability.