High-intensity corneal collagen crosslinking with riboflavin and UVA in rat cornea

Photoactivated Chromophore for Keratitis-Corneal Cross-linking (PACK-CXL)-A Scoping Review Based on Preclinical Studies

Purpose

Photoactivated chromophore for keratitis-corneal cross-linking (PACK-CXL) stabilizes the corneal stroma and eliminates microorganisms. Numerous PACK-CXL protocols, using different energy sources and chromophores, have been applied in preclinical studies, including live animal studies, with various experimental designs and endpoints. So far, a systematic mapping of the applied protocols and consistency across studies seems lacking but is essential to guide future research.

Methods

The scoping review protocol was in line with the JBI Manual for Evidence Synthesis. Electronic databases were searched (Embase, MEDLINE, Scopus, Web of Science) to identify eligible records, followed by a two-step selection process (title and abstract screening, full text screening) for record inclusion. We extracted information on (1) different PACK-CXL protocol characteristics; (2) infectious pathogens tested; (3) study designs and experimental settings; and (4) endpoints used to determine antimicrobial and tissue stabilizing effects. The information was charted in frequency maps.

Results

The searches yielded 3654 unique records, 233 of which met the inclusion criteria. With 103 heterogeneous endpoints, the researchers investigated a wide range of PACK-CXL protocols. The tested microorganisms reflected pathogens commonly associated with infectious keratitis. Bacterial solutions and infectious keratitis rabbit models were the most widely used models to study the antimicrobial effects of PACK-CXL.

Conclusions

If preclinical PACK-CXL studies are to guide future translational research, further cross-disciplinary efforts are needed to establish, promote, and facilitate acceptance of common endpoints relevant to PACK-CXL.

Translational Relevance

Systematic mapping of PACK-CXL protocols in preclinical studies guides future translational research.

Effect of Corneal Collagen Cross-Linking on Subsequent Corneal Fungal Infection in Rats

Purpose

The purpose of this study was to determine whether corneal collagen cross-linking (CXL) alters fungal susceptibility and increases the severity of keratitis through macrophage activation in rats.

Methods

Four weeks following CXL pretreatment, the corneal epithelium of adult rats was removed and inoculated with Candida albicans (C. albicans; CXL + inoculation group). The non-CXL-pretreated corneas were also inoculated with C. albicans (inoculation group). Clinical scoring and histopathological examination were performed to determine the severity of fungal keratitis. Immunofluorescence and confocal microscopy imaging were applied to determine the effects of CXL treatment on corneal local macrophage content. Real-time polymerase chain reaction (RT-PCR) and Western blots were used to evaluate mRNA and protein expression. Flow cytometry assays were performed to detect M1- and M2-type macrophages.

Results

CXL pretreatment (CXL + inoculation) resulted in higher infection success rate and more severe fungal keratitis than inoculation alone (inoculation group). On days 1, 3, and 7 following fungal infection, the increase in macrophage infiltration and IL-1β, MMP-9, and VEGFA expression was greater in the CXL + inoculation group than in the inoculation group. Number of M1- and M2-type macrophages, M1 to M2 ratio, M1-type macrophage genes, inducible nitric oxide synthase (iNOS), and tumor necrosis factor (TNFα) expression were higher in the CXL + inoculation group compared with the inoculation group.

Conclusions

Our data demonstrate that CXL may increase the colonization of macrophages and activate more M1-type macrophages to increase fungal susceptibility and severity of keratitis.

Translational Relevance

This study may aid long-term risk assessment and treatment of the complications of CXL.

How Modifications of Corneal Cross-Linking Protocols Influence Corneal Resistance to Enzymatic Digestion and Treatment Depth

Purpose

The purpose of this study was to determine the effects of the Photoactivated Chromophore for Keratitis Corneal Cross-Linking (PACK-CXL) protocol modifications on corneal resistance to enzymatic digestion and treatment depth.

Methods

Eight hundred one ex vivo porcine eyes were randomly divided into groups of 12 to 86 corneas, treated with various epi-off PACK-CXL modifications, including acceleration (30 > 2 minutes, 5.4 J/cm2), increased fluence (5.4 > 32.4 J/cm2), deuterium oxide (D2O) supplementation, different carrier types (dextran versus hydroxypropyl methylcellulose [HPMC]), increased riboflavin concentration (0.1 > 0.4%), and riboflavin replenishment during irradiation (yes/no). Control group eyes did not receive PACK-CXL. A pepsin digestion assay was used to determine corneal resistance to enzymatic digestion. A phalloidin fluorescent imaging assay was used to determine the PACK-CXL treatment effect depth. Differences between groups were evaluated using a linear model and a derivative method, respectively.

Results

PACK-CXL significantly increased corneal resistance to enzymatic digestion compared to no treatment (P < 0.03). When compared to a 10 minute, 5.4 J/cm2 PACK-CXL protocol, fluences of 16.2 J/cm2 and higher increased corneal resistance to enzymatic digestion by 1.5- to 2-fold (P < 0.001). Other protocol modifications did not significantly change corneal resistance. A 16.2 J/cm2 fluence also increased collagen compaction in the anterior stroma, whereas omitting riboflavin replenishment during irradiation increased PACK-CXL treatment depth.

Conclusions

Increasing fluence will likely optimize PACK-CXL treatment effectiveness. Treatment acceleration reduces treatment duration without compromising effectiveness.

Translational Relevance

The generated data help to optimize clinical PACK-CXL settings and direct future research efforts.

Corneal Collagen Cross-Linking Pretreatment Mitigates Injury-Induced Inflammation, Hemangiogenesis and Lymphangiogenesis In Vivo

Purpose

The purpose of this study was to evaluate if corneal collagen cross-linking (CXL) pretreatment dampens suture-induced hemangiogenesis and lymphangiogenesis driven by inflammation.

Methods

Four weeks after CXL pretreatment, suture emplacement was performed in rats. The time dependent effects were compared of this procedure in three groups: (1) suture-induced neovascularization (SNV group); (2) CXL treatment prior to suture-induced neovascularization (CXL + SNV group); (3) Normal control (NC group). Serial morphometric measurements evaluated suture-induced hemangiogenesis and lymphangiogenesis. CD45 and CD68 immunofluorescent staining pattern changes determined immune cell activation, stromal leucocyte, and macrophage infiltration. The real-time quantitative polymerase chain reaction (RT-qPCR) determined angiogenic and lymphangiogenic gene expression level changes. Western blots evaluated protein expression levels of vascular endothelial cell CD31 and lymphatic vessel endothelial hyaluronan receptor (LYVE-1).

Results

On days 7 and 14 after suture emplacement, the rises in angiogenesis, lymphangiogenesis, CD45+ and CD68+ cell infiltration were less in the CXL pretreated (CXL + SNV) group than in the untreated (SNV) group. Angiogenic and lymphangiogenic mRNA levels and CD31 and LYVE-1 protein and proinflammatory cytokines were also suppressed, confirming that CXL pretreatment improved the wound healing response.

Conclusions

CXL pretreatment inhibits injury-induced angiogenesis and lymphangiogenesis. These reductions suggest that prior CXL therapy decrease ocular inflammation reactivated by secondary trauma.

Translational Relevance

CXL pretreatment induces increases in stromal stiffness which in turn reduces trauma or microbial driven increases in inflammation, angiogenesis, and lymphangiogenesis. These beneficial effects suggest that this novel procedure may improve therapeutic management of trauma-induced corneal disease in a clinical setting.

Experimental in-vitro investigation on Epi-Off-Crosslinking on porcine corneas

Aim

To evaluate quantitatively the effects of the Epi-Off-CXL irradiance dose on the stromal stiffening of pig corneas.

Setting

Laboratory of Biological structures (LaBS), Politecnico di Milano, Milano, Italy.

Methods

Inflation tests have been carried on 90 excised and de-epithelized pig corneas, monitoring the change of configuration of the corneal dome at specific pressures. Test have been carried out twice on each cornea, once before and once after Epi-Off-CXL performed at a constant irradiance of 9 mW/cm² and variable UV-A exposure times. Corneas were grouped according to the exposure time (2.5, 5, 10, 15 and 20 min), proportional to the irradiation dose (1.35, 2.7, 5.4, 8.1, and 10.8 J/cm²). A theoretical model based on linearized shell theory has been used to estimate the increment of the corneal stiffness.

Results

The linearized shell theory allowed to establish a quantitative relation between the increment of the stiffness parameters and the irradiation dose. Relative to the pre-treatment values, in all experiments the post-treatment corneal stiffness revealed a pronounced increase. In general, the stiffness gain increased with the exposure time. No significant differences in stiffening was observed between tests conducted at 2.5, 5, and 10 min exposure.

Conclusions

Qualitatively, the effectiveness of accelerated CXL treatments observed in pig corneas complies very well with in-vivo clinical results in humans, suggesting that experimental data in pigs can be very useful for the design of the procedure in humans. A larger irradiation dose provides a larger increment of the corneal stiffness. Due to the biological variability of the tissues, however, it is difficult to distinguish quantitatively the level of the reinforcement induced by accelerated protocols (low doses with < = 10 min exposure), less prone to induce damage in the corneal tissue. Therefore, the definition of personalized treatments must be related to the actual biomechanics of the cornea.

Enzymatic Digestion of Porcine Corneas Cross-linked by Hypo- and Hyperosmolar Formulations of Riboflavin/ultraviolet A or WST11/Near-Infrared Light

Purpose

To assess enzymatic digestion rate after Riboflavin (RF) and Water-Soluble-Taurine (WST11) based corneal cross-linking (CXL), with or without the addition of high molecular weight dextran (RF-D and WST-D).

Methods

Eighty-eight paired porcine corneas were cross-linked by either RF (n = 11) or RF-D (n = 11) and ultraviolet light (UVA), or WST11 (n = 11) or WST-D (n = 11) and near-infrared (NIR) light, or used as paired control (n = 44). Corneal buttons of treated and paired control eyes were placed in a 0.3% collagenase solution. Time to full digestion and remaining dry sample weight after six hours were compared.

Results

A strong treatment effect was seen with all four formulations, as all controls had been fully digested whilst all treated samples were still visible at the experiment's endpoint. After irradiation, central corneal thickness was significantly higher in samples treated with hypo-osmolar formulations, compared to dextran enriched formulations (P < 0.001). Dry sample weight after digestion was nonsignificantly different between corneas treated by the four different formulations (P = 0.102). Average dry sample weight was 1.68 ± 0.6 (n = 10), 2.19 ± 0.50 (n = 8), 1.48 ± 0.76 (n = 11), and 1.54 ± 0.60 (n = 9) mg, for RF, RF-D, WST11, and WST-D treated samples, respectively. Enzymatic resistance was similar for RF and WST based CXL (P = 0.61) and was not affected by the addition of dextran (P = 0.221).

Conclusions

Both RF and WST11 based CXL significantly increases resistance to enzymatic digestion, with similar effect for hypo-osmolar and hyperosmolar (dextran enriched) formulations.

Translational Relevance

Our findings indicate these formulations are interchangeable, paving the way for the development of novel PACK-CXL protocols for thin corneas and deep-seated infections.

Corneal Cross-Linking: The Science Beyond the Myths and Misconceptions

PURPOSE

There has been a recent explosion in the variety of techniques used to accomplish corneal cross-linking (CXL) for the treatment of ectatic corneal diseases. To understand the success or failure of various techniques, we review the physicochemical basis of corneal CXL and re-evaluate the current principles and long-standing conventional wisdom in the light of recent, compelling, and sometimes contradictory research.

METHODS

Two clinicians and a medicinal chemist developed a list of current key topics, controversies, and questions in the field of corneal CXL based on information from current literature, medical conferences, and discussions with international practitioners of CXL.

RESULTS

Standard corneal CXL with removal of the corneal epithelium is a safe and efficacious procedure for the treatment of corneal ectasias. However, the necessity of epithelium removal is painful for patients, involves risk and requires significant recovery time. Attempts to move to transepithelial corneal CXL have been hindered by the lack of a coherent understanding of the physicochemistry of corneal CXL. Misconceptions about the applicability of the Bunsen-Roscoe law of reciprocity and the Lambert-Beer law in CXL hamper the ability to predict the effect of ultraviolet A energy during CXL. Improved understanding of CXL may also expand the treatment group for corneal ectasia to those with thinner corneas. Finally, it is essential to understand the role of oxygen in successful CXL.

CONCLUSIONS

Improved understanding of the complex interactions of riboflavin, ultraviolet A energy and oxygen in corneal CXL may provide a successful route to transepithelial corneal CXL.

Corneal Collagen Cross-Linking With Riboflavin and UVA Regulates Hemangiogenesis and Lymphangiogenesis in Rats

Purpose

The purpose of this study was to determine whether corneal collagen crosslinking (CXL) inhibits hemangiogenesis and lymphangiogenesis during acute corneal inflammation in an in vivo rat model.

Methods

Inflammatory corneal neovascularization was induced by suture placement into a rat cornea. At day 3 after suture, a CXL protocol using riboflavin and UVA was administered after mechanical epithelial debridement. Hemangiogenesis and lymphangiogenesis were analyzed morphometrically. CD45 and CD68 immunostaining evaluated corneal leucocyte and macrophage immune cell infiltration, respectively. A TUNEL assay detected stromal cell apoptosis. Quantitative RT-PCR analysis identified angiogenic and lymphangiogenic genes as well as proinflammatory cytokine expression. Western blot analysis characterized vascular endothelial cell CD31 and lymphatic vessel endothelial hyaluronan receptor (LYVE-1) protein expression.

Results

CXL treatment significantly reduced corneal pathologic suture-induced hemangiogenesis and lymphangiogenesis 7 days after suture emplacement, but this procedure failed to affect hemangiogenesis and lymphangiogenesis 14 days after suture. Increased cell apoptosis and reduced CD45+ and CD68+ cell infiltration were evident in CXL-treated rats on days 7 and 14 after suture emplacement. CXL treatment significantly decreased angiogenic and lymphangiogenic mRNA expression levels and both CD31 and LYVE-1 protein expression levels, whereas it increased proinflammatory cytokine levels on day 7 after suture emplacement. However, on day 14 after corneal neovascularization, angiogenic and lymphangiogenic mRNA gene expression levels were upregulated along with hematic CD31 and lymphatic LYVE-1 protein expression.

Conclusions

CXL treatment only temporarily inhibits corneal inflammatory-associated hemangiogenesis and lymphangiogenesis in vivo. Such insight suggests that future studies are warranted to develop novel CXL strategies with longer-lasting effectiveness in attenuating hemantic- and lymphatic-related corneal diseases.