Cystinosis is a rare lysosomal storage disorder with a worldwide incidence of 0.5-1/100,000 live births [1]. Cystinosis is characterised as a lysosomal storage disorder, resulting in the accumulation of intra-lysosomal cystine [2]. Cystinosis is categorised by age of disease onset and severity of kidney involvement. Cystinosis is classified into infantile nephropathic cystinosis, juvenile nephropathic cystinosis and ocular (non-nephropathic) cystinosis. Infantile nephropathic cystinosis is responsible for 95% of cystinosis [3]. It is the most severe form of disease, leading to renal Fanconi syndrome and kidney failure within the first decade of life if not receiving treatment [4]. Juvenile nephropathic cystinosis is slightly milder than infantile cystinosis and typically develops between the ages of 10 and 12 years of age [3]. Ocular cystinosis is a non-nephropathic form of cystinosis and is commonly diagnosed in adulthood [5].

Cystinosis is caused by a dysfunctional lysosomal transport protein, named cystinosin [2]. Cystinosin is coded for by the CTNS gene, located on chromosome 17p13 [2]. Cystinosin is a lysosomal 7 transmembrane protein, functioning to export cystine from the lysosome. It is driven by a H⁺ electrochemical gradient [6]. The dysfunctional cystinosin protein results in the intra-lysosomal accumulation of cystine and the production of pathogenic cystine crystals, leading to downstream organ damage [7]. Organs commonly damaged in cystinosis include the kidney, cornea, thyroid, bone marrow, muscles, spleen, and peripheral nerves [3].

All 3 clinical forms of cystinosis are due to bi-allelic pathogenic variants in the CTNS gene, with a specific pathogenic variant being detected in over 95% of cases [8]. Variant analysis revealed that ocular cystinosis is allelic with the nephropathic form of disorder [9]. Over 140 pathogenic variants of the CTNS gene have been described with most pathogenic variants associated with infantile cystinosis [10]. Ocular cystinosis patients harbour a mix of classical cystinosis and novel specific ocular cystinosis pathogenic variants [9]. In the Tunisian population 7 different CTNS pathogenic variants have been documented in association with ocular cystinosis – three missense pathogenic variants, one duplication, one frameshift variant, one large deletion and one splice site pathogenic variant [11].

Pathogenic cystinosis variants have a significant geographical distribution, with more developed countries harbouring most of the data [10]. The most common pathogenic variant in North America and Northern Europe is the 57kb deletion, accounting for 50-70% of this population [12]. However, this large 57kb deletion is only responsible for 17% and 0% of cystinosis patients in Italy and Egypt respectively [13,14]. CTNS nonsense variants represent 15% of the known CTNS pathogenic variants, with the most common nonsense variant being the p.W138x variant [15]. This has been attributed to disease in 50% of the French-Canadian cystinosis population [15].

Ocular non-nephropathic cystinosis was first described in 1957 [16]. It was originally termed ‘adult’ or ‘benign’ cystinosis. Ocular cystinosis presents as cystine crystal deposits in the cornea in the absence of renal disease [9]. Cystine crystals in the cornea are pathognomonic for ocular cystinosis [17]. Complications of cystine crystal deposition in the cornea commonly include photophobia, visual impairment, corneal scarring, cataracts and keratitis [18]. The crystals do not usually influence visual acuity until the cystine crystals reach the centre of the cornea [19]. Upon entry of the cystine crystals to bowman’s membrane, foreign body sensations and inflammatory symptoms of recurrent corneal erosions may occur [19].

Cystine crystals can be found in other structures of the eye as well as the cornea, including the conjunctiva, iris, ciliary body, choroid, retina, and optic nerve [17]. Cystinosis can cause retinal changes which may progress to photoreceptor degeneration and thus impaired colour and night vision [17]. Older patients may exhibit moderate to severe constriction of visual fields and reduction of rod and cone mediated ERG responses [20]. Patches of depigmentation and symmetrical pigment epithelial mottling are the most common posterior segment manifestations of cystinosis [20], with epithelial mottling starting in the peripheries and progressing towards the posterior pole over time [21]. The frequency of complications is reduced with early diagnosis and treatment of cysteamine therapy [22]. However, if ocular cystinosis is left untreated it can result in blindness [18].

Unlike in the kidney, the formation of cystine crystals in the cornea are not well understood. Corneal crystals appear in the periphery and progress centripetally over time [19]. The majority of crystals are located in the corneal stroma, which is comprised predominantly of collagen and keratocytes [23]. Corneal cystine crystals are located extracellularly [24] and are needle like in formation, in contrast to the intracellular rectangular or hexagonal crystals found in other organs. The needle like stromal crystals are orientated parallel to the stomal lamellae [25], suggesting a role for collagen in the deposition of cystine crystals [26]. Additionally, crystal production reaches a saturation point in the cornea after which no further crystallisation occurs. Dixon et al. propose the requirement of collagen in the production of cystine crystals, and thus when no more collagen is available crystal deposition plateaus [26].

Two mechanisms have been proposed for the lack of kidney involvement in ocular cystinosis. It is speculated that ocular cystinosis patients are heterozygous for one severe and one mild CTNS pathogenic variant. This spares patients from kidney involvement. Nephropathic cystinosis patients have two severe pathogenic variants, predisposing them to kidney damage. Firstly, patients may have significant residual cystinosin activity in every other tissue of the body (e.g. the G197R variant). Alternatively, substantial cystinosin activity may be present in the kidney because of tissue specific expression of factors that promote the production of a normal CTNS protein (e.g. IVS10-3 CàG variant) [9].

Extra renal complications of nephropathic cystinosis include ocular deposition of corneal cystine crystals, and endocrine dysregulation such as diabetes, hypothyroidism, and retarded growth [3]. Corneal cystine crystals can be seen in the cornea as young as 16 months of age in nephropathic cystinosis [27]. While initially asymptomatic, symptoms of ocular involvement can begin within the first few years of life [28], with most nephropathic cystinosis patients experiencing photophobia after the first decade of life [17]. A large cross sectional study reports that 75-87% of nephropathic cystinosis patients over 20 years of age showed anterior segment involvement separate from corneal and conjunctival crystal deposits [29]. There is an age dependant tendency for crystals to deposit anteriorly in childhood, while crystals are located posteriorly in the corneal stroma in adulthood [30]. The rest of this review will focus on the diagnosis and treatment of ocular (non-nephropathic) cystinosis and ocular involvement in nephropathic cystinosis as a singular entity.

The Ophthalmology Cystinosis Forum in 2017 recommend early detection of ocular cystinosis to be comprised of photophobia assessment, visual acuity, slit lamp examination, and tonometry ophthalmic examinations. Additionally, in vivo confocal microscopy and anterior segment optical coherence tomography were highlighted as valuable options to evaluate corneal cystine crystals [31].

There has been further focus on the use of spectral domain coherence tomography based retinochoroidal cystine crystal score (RCCCS) as an objective biomarker for systemic disease control in cystinosis [32]. Keidel et al. demonstrate the RCCCS being negatively correlated with cysteamine intake, highlighting how the RCCCS determines treatment response. Notably, the RCCCS is also positively correlated with cystatin C, a stable parameter of renal function, possibly an indication of nephropathic cystinosis disease [32].

Follow up of cystine crystal deposition can prove challenging. Slit lamp examination plays a large role in the diagnosis of ocular cystinosis. However, it poses difficulties regarding accurate assessment over time, tracking disease progression and treatment efficacy. Additionally, the corneal crystal score lacks accurate estimates in low levels of deposition and in assessing alterations in crystal deposition [33]. Recent work has been focused on improving the diagnosis and monitoring of cystinosis. Corneal densitometry obtained through Penta Cam can estimate the level of corneal cystine crystal deposition, offering a potential way to assess disease progression and or treatment effectiveness/adherence [33]. Furthermore, anterior segment optical coherence tomography (AS-OCT) can serve as an objective quantification of corneal cystine crystal deposition and may be able to serve as a novel biomarker for ocular disease control and treatment monitoring [34].

Ocular cystinosis typically spares the kidney and other organs [35]. However, the coexistence of juvenile and ocular forms of cystinosis were described in one family, representing a possible continuum between mild forms of cystinosis. Thus, renal monitoring is recommended in every patient with seemingly isolated ocular cystinosis [36].

Early diagnosis and treatment are paramount to prevent disease progression [22]. The primary treatment for cystinosis is oral and topical cysteamine. Cysteamine is a cystine depleting drug, functioning to reduce intra-lysosomal accumulation of cystine. Cysteamine drives a disulphide exchange reaction which results in the formation of cysteine and cystine-cysteamine mixed disulphide which can leave the lysosome through a ‘system c’ transporter [35]. In nephropathic forms of cystinosis, cysteamine delays the progression of disease to end stage renal failure, extra renal complications, and the need for renal transplant [37]. Due to the highly vascularised nature of the macula and retina, oral cysteamine therapy has significantly reduced the rates of damage to these structures [38]. However, the avascular structure of the cornea is a barrier to oral cysteamine therapy, with cysteamine failing to reduce corneal cystine crystal accumulation. Thus, topical cysteamine therapy is necessary to reduce corneal crystal density and alleviate symptoms [39].

Topical administration of cysteamine dissolves cystine crystals, improving ocular symptoms and enhancing quality of life [27]. Ocular cystinosis management requires frequent topical cysteamine administration. This is a result of eyedrops being rapidly cleared from the ocular surface due to constant blinking and nasolacrimal drainage, reducing contact time on the eye [18]. This represents a significant obstacle to patient compliance and treatment adherence. The shelf life of cysteamine once exposed to the air from the eye drop bottle is short, causing further issues [40].

There are two main cysteamine preparations. ‘Cystaran’ (0.44% cysteamine ophthalmic solution) was approved by the FDA as an orphan drug for the treatment of corneal cysteine crystal deposits in 2012 [41]. However, administration is required hourly, presenting many compliance issues [41]. ‘Cystadrops’ (0.55% cysteamine ophthalmic solution) was approved by the EMA and FDA in in 2017 and 2020 respectively [42]. ‘Cystadrops’ facilitates 4-time daily dosing, as the addition of carmellose sodium increases the viscosity of the formulation, allowing a longer residence time on the ocular surface and reducing frequency of administration [42]. A systematic review and meta-analysis carried out by Kaur et al. backs up the efficacy and safety of topical cysteamine in ocular cystinosis [43]. Liang et al. led a 45 month follow up study for patients receiving ‘Cystadrops’, demonstrating normal visual acuity scores, a reduction in photophobia, and a decrease in corneal cystine crystal scores over time (stabilising at 27 months) [44]. In this study 47 non serious adverse effects and 4 serious adverse effects were reported [44]. Notably, the use of topical 0.55% cysteamine may be limited in severe nephropathic cystinosis with Al-Hemidan et al. demonstrating its administration failing to improve photophobia in this subgroup of patients to a clinically significant level [45]. If therapy is discontinued, corneal cystine crystals accumulation will increase [1]. Patients not treated with cysteamine may require a corneal transplant. Topical cysteamine treatment should be continued post-transplant, as there is a risk of cystinosin deficient cells invading the transplanted cornea [1].

‘Cystadrops’ are currently only indicated in cystinosis for adults and children from 2 years of age. A phase 3 clinical trial was completed to determine if ‘Cystadrops’ administration may be of value to patients less than 2 years of age (NCT04125927). Considering cystine crystals can be observed on slit lamp examination in nephropathic cystinosis patients at 16 months, treatment available at this point would be beneficial [27]. No results have yet been posted.

As mentioned above, the frequent applications of cysteamine eye drops are significantly demanding and compromising to quality of life. Many studies are focused on alternative treatment options for ocular cystinosis. The primary focus is to reduce the need for continuous reapplication of topical cysteamine. Prodrug preparations of cysteamine have been developed to increase the half-life and bioavailability of the drug. Such prodrugs include N-acyl or glutaric acid derivatives of cysteamine [46,47]. Carbohydrate-cysteamine thiazolidines have been proposed as another pro drug for the treatment of cystinosis [48]. While Ramazani et al. describe the extracellular release of cysteamine as a barrier to cellular uptake and drug efficacy, Iwata et al. demonstrate the extracellular location of corneal cystine crystals [24], potentially warranting further research for this pro drug formulation in the specific context of ocular cystinosis. Currently aminothiol cysteamine preparation remains the only available treatment for cystinosis [43].

Many different drug delivery systems have been studied to prolong the contact time of cysteamine with the eye and thus reduce the frequency of administration. These drug delivery systems include gel eye drops, hydrogels, contact lenses and nano wafers.

One such delivery system includes a sustained release microsphere/thermoresponsive gel eyedrop [49]. It has a high ocular biodistribution in the cornea and aqueous humour, with a low systemic distribution of cysteamine. This sustained release formulation maintains drug release over 12 hours. Furthermore, this product was demonstrated to be tolerable in rabbit studies, with any observed side effects being transient and diminishing within 10-30 min [50].

The porosity and high-water content of hydrogels make them suitable for encapsulation of water-soluble drugs such as cysteamine. Hydrogels can be produced from natural or synthetic materials. Natural polymer-based hydrogels present the obvious advantage of biocompatibility and minimised toxicity, however synthetic polymers allow for increased drug release times and thus reduced administration frequency [18].

Natural hydrogels have been synthesised from many materials including hydroxypropyl methylcellulose [51], sodium hyaluronate [52] and hydroxyethyl cellulose [52]. Natural hydrogels have inherent biocompatibility, biodegradability and bioactivity and are safe in most of the population. However, natural hydrogels can cause allergic reactions in rare cases [53]. Natural polysaccharide hydrogels allow for the controlled release of cysteamine over a longer time period [54], and increased stability for up to 30 days [55]. Interestingly, Luaces-Rodriguez et al. demonstrate that these natural polysaccharide hydrogels could act as corneal absorption promoters [55].

Synthetic hydrogels can be designed to achieve the desired mechanical properties, reducing the frequency of topical cysteamine administration. Buchan et al. designed a hydrogel composed of carbomere 934 for the topical administration of cysteamine on the ocular surface. It is transparent and bio adhesive, reducing blinking resistance and increasing contact time on the surface of the eye. Importantly, they demonstrated a first order kinetics release of the active cysteamine drug from the hydrogel, offering potential for hydrogels to be incorporated into clinical care. The addition of cysteamine to the hydrogel did not destroy any of its properties, thus offering a new way to administer cysteamine to the corneal surface and increase contact time of cysteamine with the cornea [56]. McKenzie et al. agree on the suitability of carbomere 934 for the ophthalmic delivery of cysteamine [52].

The increased interest in precision and personalised medicine over recent years has prompted the innovation of smart hydrogels. Smart hydrogels respond to external stimuli such as pH, temperature, light, and the concentration of biomolecules. Such stimuli can be used to promote hydrogel drug release [57]. Iohara et al. demonstrate the use of thermoresponsive hydrogels for use as an ocular drug delivery system, highlighting a path for smart hydrogels in the future of ocular cystinosis treatment [58]. Furthermore, research regarding genetic therapy is advancing in many diseases. Graceffa et al. have synthesised novel fibrin hydrogels, functioning as a gene delivery system for the treatment of cystinosis. The hydrogel releases a non-viral plasmid, encoding for CTNS, targeting the core pathophysiology of cystinosis [59].

Micro vesicles have been proposed to aid the delivery of lysosomal transport protein cystinosin to the eye. Ex vivo rabbit studies have yielded supportive results for the use of micro vesicles as a delivery system for cystinosin, demonstrating them to have therapeutic value in treating cystinotic corneal keratopathy [60]. Furthermore, micro vesicles have the advantage of twice monthly administration [60].

Nano wafers are small transparent circular discs that can be applied on the ocular surface with a fingertip [61]. Nano wafer’s release drugs on the ocular surface in a controlled manner. Cysteamine is stored in nano reservoirs, allowing a tightly controlled drug release system. Once all the drug has been released, the nano wafer dissolves and fades away [62]. Cysteamine loaded nano wafer therapy has proved efficacious in Ctns ^-/- mice, achieving a 90% reduction in cystine crystal volume in comparison to cysteamine eye drop formulations which reached a 55% reduction [62]. The authors attribute this increased cystine crystal clearance to increased drug residence time on the ocular surface [62].

Contact lenses are another potential drug delivery system for use in the treatment of ocular cystinosis. Recent studies have investigated the use of contact lenses as a drug delivery system for many different ocular diseases such as glaucoma [63] and diabetic retinopathy [64]. The advantages of contact lenses as a drug delivery system include increased bioavailability and increased contact time of the drug with the ocular surface [65]. Hydrogel based soft contact lenses are proposed to have the most therapeutic efficacy [66]. When a contact lens is placed over the cornea, a thin fluid layer is formed between the lens and the cornea [65]. The drug enters the fluid layer, increasing bioavailability of the drug to 50% when compared to the administration of topical drops (bioavailability of 1-5%) [18]. Increased bioavailability allows for reduced frequency of administration, promoting compliance. However, the main limitation of contact lenses as a drug delivery system lies in the fact that most drugs have a low affinity to the polymers that are used in contact lenses. Low drug affinity results in insufficient drug loading and too rapid drug delivery [18].

Many attempts have been made to modify contact lenses to improve drug affinity. This includes the incorporation of polymeric nanoparticles [67], diffusion barriers (vitamin E) [68] and molecular imprinting [69]. There has been a big focus on the addition of vitamin E to contact lenses to increase cysteamine retention time. Vitamin E is a biocompatible hydrophobic molecule that exhibits low solubility in water and creates a tortuous pathway prolonging the diffusion time through the contact lenses [18]. Additionally, vitamin E incorporation has the added benefit of blocking UV radiation [70]. Studies by Hsu et al. and Dixon et al. have demonstrated a positive association between the addition of vitamin E and the duration of cysteamine release [71,72]. Hsu et al. suggest that a single vitamin E modified silicone hydrogel contact lens worn for 2 hours could achieve the same therapeutic effect as hourly administration of cysteamine eye dops [71].

Furthermore, vitamin E loaded contact lenses have been developed in combination with carbon black tinted lenses, successfully reducing photophobia [73]. Further optimisation of contact lenses as a cysteamine delivery system are needed, with Vitamin E loaded contact lenses representing an optimistic option to prolong drug release.

There have been promising advancements in liposome and nanoparticle-based therapy options in other ocular diseases, namely glaucoma [74] and age-related macular degeneration [65]. Notably, a drugless ocular cystinosis treatment consisting of gold nanoparticle contact lenses has been developed. The gold nanoparticle contact lenses function by withdrawing cystine from the surrounding area [76]. In vitro studies provide hope for a drugless cystinosis treatment with one- or five-hour wear of contact lenses at a time [76].

Another approach by Thoene et al. described the infection of Spodoptera frugiperda cells (SF9) with baculovirus containing human wild type CTNS spontaneously produced a functional cystinosin protein in micro vesicles. Furthermore, the addition of green fluorescent protein tagged cystinosin containing micro vesicles to ex vivo rabbit ocular globes resulted in punctate green fluorescence in the corneal keratocytes. This supports the potential therapeutic use of cystinosin containing micro vesicles, with the added benefit of twice monthly administration [60]. Iglesias et al. demonstrate the ability of stem cell derived micro vesicles to reprogram the biology of mutant tissues. They show micro vesicle-based transfer of the cystinosin protein to human cystinotic cells in vitro, and a dose dependant reduction in cystine accumulation [77].

There is also the potential for the use of combined treatment with cysteamine and mTOR inhibitor everolimus [78,79]. Dual treatment targets cystinosis from two different pathways. Isolated cysteamine treatment ameliorates cystine accumulation, with no therapeutic effect on aberrant autophagy or dysregulated apoptosis. Conversely, mTOR inhibitor everolimus targets the pathological pattern of apoptosis and autophagy seen in cystinosis. However, it does not rescue the defect in cystine loading [18]. Thus, the combination of cysteamine and everolimus presents a solution to target cystinosis from different pathological pathways [78].

Cystinosis is a rare lysosomal storage disorder, characterised by increased intra lysosomal cystine. Ocular cystinosis is a subtype of cystinosis in which cystine crystals deposit in the cornea, causing visual impairment and can ultimately lead to blindness. Additionally, ocular cystinosis is one of the predominant complications of nephropathic cystinosis. The main stay of ocular cystinosis treatment is topical cysteamine, a cystine depleting agent. However, the efficacy of cysteamine is limited by the avascular nature of the cornea [80]. Additionally, the need for frequent reapplication poses a challenge to patient compliance. Early and effective treatment is necessary to reduce the risk of blindness and improve quality of life [22]. Many studies on future treatment options for ocular cystinosis are focusing on increasing the contact time of cysteamine with the ocular surface. This is being achieved in many ways, namely using pro drugs, contact lenses, hydrogels and nanoparticles. While there is much exciting research ongoing in this area, more studies are needed to bring these products to clinical use.