Influence of Oral 7-Methylxanthine on Lens-induced and Form Deprivation Myopia in Chickens

Abstract

 

Purpose

 

7-methylxanthine (7-MX), a non-selective adenosine antagonist, is approved as an anti-myopia oral medication in Denmark. To learn more about its mechanisms, this study examined its ability to inhibit lens-induced (LIM) and form deprivation myopia (FDM) in young chickens.

 

Methods

 

Forty-five young White-Leghorn chickens were divided into four groups, groups 1 and 2 being assigned -10 D lenses (n=10 each), and groups 3 and 4, white diffusers (n=12 & 13 resp.). All optical treatments were monocular, applied to left eyes, starting eight days after hatching, and extending for 10 days. From the beginning of the treatment period, groups 2 and 4 received at 9 am and 3 pm each day, two doses of oral 7-MX (30 mg/kg, samples provided by K Trier). Sterile distilled water was used as the diluent, with xanthan-gum added to ensure uniform suspension of 7-MX. Control groups (1 and 3) received the xanthan-gum solution without 7-MX, with the dosing schedule as per 7-MX. Baseline refractive errors and axial ocular dimensions were measured before treatments, using static retinoscopy and high frequency A-scan ultrasonography respectively, and measurements were repeated every other day thereafter.

 

Results

 

7-MX had a small inhibitory effect on LIM but no effect on FDM, as reflected in the refractions recorded at the end (LIM: -6.43 ± 1.30 D, 7-MX versus control, -8.30 ± 1.53 D; FDM: -10.19 ± 7.89 D, 7-MX versus -10.77 ± 7.20 D, control)(Figure 1). For LIM groups, the interocular difference in optical length was smaller in the 7-MX group. And final intergroup difference (0.12 mm) was mainly of a product of differences in anterior chamber depth (0.11 mm). Both groups showing similar interocular differences in other key ocular parameters (vitreous chamber depth and lens thickness). No significant intergroup differences in these parameters were noted for the FDM groups.

 

Conclusions

 

For the dose tested, 7-MX showed only a weak inhibitory effect against LIM and no effect against FDM in the chicken. This result adds weight to evidence suggesting a scleral site of action for the anti-myopia effect of 7-MX, because unlike mammalian and primate eyes, avian eyes have an additional cartilage layer, which largely determines eye size.

 

 

Figure 1. Interocular changes in mean spherical equivalent refractive error (SER), normalized to baseline values, for the lens-wearing group (LIM, A) and form deprived groups (FDM, B).

 

Keywords: 605 myopia • 503 drug toxicity/drug effects