Subconjunctival mitomycin C injection alters bleb survival and morphology but not intraocular pressure in a rabbit XEN63 model

Background Bleb fibrosis limits the success of subconjunctival drainage devices, and devices implanted through an intact conjunctiva require an antifibrotic adjunct to be delivered via injection. To our knowledge, no preclinical model of the 63 µm XEN gel stent has been reported. In this study, we compared XEN63 implantation alone with the XEN63–adjunctive subconjunctival mitomycin C combination in a closed-conjunctiva rabbit model. Methods Twelve New Zealand albino rabbits (24 eyes) were randomized at the animal level (six rabbits per group), with both eyes of a given rabbit receiving the same allocation. The treated eyes received 0.1 mL of mitomycin C (0.1 mg/mL) beside the implant without washout. Masked observers assessed the intraocular pressure and bleb morphology on postoperative days 1 to 4, 7, 14, 21, and 28, and a masked pathologist graded fibrosis, inflammation, neovascularization, and lymphocyte density at day 30. The primary and secondary outcomes were absolute intraocular pressure and bleb survival, respectively, and analyses accounted for intereye correlation. Results The primary outcome did not differ between groups (mean difference, 0.09 mmHg; 95% CI, −1.55 to 1.72). Bleb failure occurred in 10 of 12 eyes in the control group and 5 of 12 eyes in the treatment group (day-28 survival, 16.7% vs. 58.3%; animal-level permutation p = 0.015; hazard ratio, 0.31); in sensitivity analyses accounting for device malposition, p -values ranged from 0.045 to 0.316. The treated blebs were larger, more elevated, and avascular. No difference in histological fibrosis grade was detected, whereas the lymphocyte density was approximately 2.5-fold higher in eyes from the treatment group than in the control group eyes. Dysfunctional stent malpositioning occurred in 37.5% of eyes, all of which experienced bleb failure. In an exploratory pooled analysis, the pressure fell by 2.73 mmHg per grade of bleb height, and the between-group difference disappeared once morphology entered the model. Conclusion Adjunctive mitomycin C injection appeared to alter the probability that a bleb persisted rather than altering the pressure achieved by a functioning bleb. Because the pressure range targeted in human eye surgery overlaps the untreated baseline of the normotensive rabbit eye, tonometry identifies overfiltration rather than function. Bleb survival and morphological quality, rather than absolute pressure, should be the primary endpoints of preclinical antifibrotic studies in this model.