Aliphatic β-Nitroalcohols for Therapeutic Corneoscleral Cross-Linking: Permeability Considerations

Abstract

Purpose: : Our recent tissue cross-linking studies have raised the possibility of using aliphatic β-nitro alcohols (BNAs) for pharmacologic, therapeutic corneoscleral cross-linking. The present study was performed in order to determine the transcorneal permeability of the various BNAs and to explore the use of permeability enhancing agents.

Methods: : Fresh rabbit corneas were obtained post-mortem and mounted in an 8mm i.d. Franz corneal diffusion chamber with jacketed temperature control. Trans-corneal permeability was then evaluated by assaying the level of BNA (modified Greiss colorimetric nitrite assay) on the recipient chamber over time (30, 60, 90, 120 min). The apparent permeability coefficient (P_tot) was determined using the equation: P_tot=V(dc/dt)/AC_o, where V is the recipient chamber volume, dc/dt is the flux across the cornea, A is the surface area of the exposed cornea, and Co is the donor chamber initial concentration (100mM). Two mono-nitroalcohols, [2-nitroethanol (2NE), 2-nitro-1-propanol (2NProp)], a nitro-diol [2-methyl-2-nitro-1,3-propanediol (MNPD)], and a nitro-triol [2-hydroxymethyl-2-nitro-1,3-propanediol (HNPD)] were studied. In addition, the permeability enhancing agents proparacaine and benzalkonium chloride were used in conjunction with the BNAs.

Results: : Our previous unpublished work using live rabbits has shown that a 100mM concentration of these four BNAs is well tolerated (no redness or irritation observed). Thus, the P_tot was determined for the 4 compounds using a concentration of 100mM in the donor chamber. The P_tot (+/-S.E.) for each compound was as follows: P_tot=4.33x10^-5 (+/-9.82x10^-6) cm/sec for 2NE (MW=91), P_tot=9.34x10^-6 (+/- 2.16x10^-7) cm/sec for 2NProp (MW=105), P_tot=4.37x10^-6 (+/-1.86x10^-7) cm/sec for MNPD (nitro-diol, MW=135), and P_tot=8.95x10^-7 (+/-1.93x10^-8) cm/sec for HNPD (nitro-triol, MW=151). The effects of permeability enhancing agents will also be reported.

Conclusions: : The results suggest that the corneal epithelium is permeable to BNA compounds with the greatest permeability noted for 2NE and the nitro-diol. The findings are consistent with previous literature indicating that the small size of these compounds (i.e. <10Å) favors their passage through the corneal epithelium. This information will help to guide dosing regimens for topical cross-linking studies in living rabbit eyes.

Keywords: keratoconus • extracellular matrix • drug toxicity/drug effects