This retrospective case series, carried out in a tertiary medical center in central Taiwan, studied the therapeutic effects of DEX implant treatment on DME in eyes that had been unsuccessfully treated with intravitreal ranibizumab. In this study, DEX implants showed effectiveness for the treatment of DME. Patients were assessed at monthly intervals postoperatively, and anatomic improvements as gauged by CRT were sustained throughout the entire course of follow-up. Even though improvements in CRT did not correlate with significantly improved BCVA, neither did BCVA decrease over the course of treatment and follow-up. DEX implants were well-tolerated, with only a few cases of increased IOP that were manageable with antihypertensive eyedrops.
Currently, there is no optimal treatment regimen for DEX implant therapy for DME.14 In the MEAD study, the protocol allows as-needed (pro re nata, PRN) retreatment with DEX implant with a frequency of no more than once every 6 months.12 As the out-of-pocket expense for our patients was about 40,000 NTD (1,370 USD) for each DEX implant during the study, we treated most eyes with one dose of DEX implant, followed by PRN injections when macular edema reoccurred. During the mean follow-up of 7.4 ± 4.6 months, almost 80% of the patients received only one DEX implant, and only two eyes received three injections.
The various available treatments for DME include anti-VEGFs, laser, surgery, and corticosteroids, with each targeting different pathogenic mechanisms of the disease.4 Our study suggests two main explanations for the observed benefit of DEX implant after unsuccessful ranibizumab treatment: the pharmacologic and pharmacokinetic properties of the DEX implant and the possible tachyphylaxis or tolerance to ranibizumab. First, inflammation plays a prominent role in the pathogenesis of DME. Many features of inflammation, such as the leukocyte recruitment and adhesion to vascular endothelium (leukostasis), increased blood flow and vascular permeability, tissue (macular) edema, neovascularization, and upregulation of inflammatory mediators, have been described in both human and animal models of diabetic retinopathy.13–18 Intravitreally administered corticosteroids act to ameliorate DME in multiple ways. As established anti-inflammatory agents, they reduce the production of pro-inflammatory factors, limit vascular permeability, and inhibit the expression of VEGFs.19 The DEX implant, a sustained-release drug delivery system for the potent corticosteroid dexamethasone, was developed to reduce the need for frequent intraocular injections due to the short half-life of intravitreally injected dexamethasone (< 4 hours).20 The implant releases DEX into the vitreous for up to 6 months.21 In a previous study, Lazic et al. demonstrated the therapeutic efficacy of DEX implant for DME resistant to the anti-VEGF bevacizumab.22 The use of bevacizumab for DME is off-label, and therefore we examined patients who were initially treated with ranibizumab, which is FDA-approved for DME. Second, the patients' tachyphylaxis/tolerance to the previously administered ranibizumab might be another possible mechanism for the observed therapeutic effect after switching to intravitreal DEX implant. Even though there is a difference between tachyphylaxis and tolerance, both terms have long been presented as phenomena of reduced drug efficacy and are used synonymously in the literature.23 Tachyphylaxis/tolerance in chronic treatment with bevacizumab and ranibizumab was first described in 2007 for age-related macular degeneration.24,25 Tachyphylaxis/tolerance to ranibizumab might be a result of the neutralization of ranibizumab by the formation of circulating antibodies, the desensitization of the target tissue to the drug, or the reactivation of DME driven by another pathway.26,27 These effects might be circumvented with the use of pharmaceuticals aiming at other DME-associated pathways.
In our study, DEX implant treatment showed a generally favorable safety profile. Historically, adverse events that are most commonly associated with corticosteroid therapy include cataracts and steroid-induced glaucoma. Although our study found DEX implants did not seem to have caused cataract in any of the patients, the mean follow-up period was ~ 7 months, which may not be sufficient for the development of cataracts. Some patients in this study experienced transient increase in IOP that were successfully managed with topical medication. There was no case of serious ocular or systemic adverse events such as endophthalmitis, noninfectious endophthalmitis, vitreous hemorrhage, retinal tear, retinal detachment, or sustained IOP elevations.
In this study, we found three clinical factors correlated with the treatment responses. Our results suggested that patients with lower baseline HbA1c had better anatomic improvement after treatment. The importance of glycemic control in the management of diabetic retinopathy was emphasized by previous studies.28, 29 The other two baseline predictive factors were thicker CRT and worse BCVA. Both factors correlated with better responses to DEX treatment in their respective aspects. Campos et al. also found that lower baseline BCVA predicted a higher visual acuity gain.30 It may be suggestive of a ‘ceiling effect’ and can’t be ruled out completely in this prediction model; that is, smaller improvements are required to achieve good vision in patients with better starting vision, while those with lower BCVA at baseline have greater capacity to achieved better vision outcome. Another limit of visual acuity improvement was the uncertant optimal timing for DEX switching in patients with DME non-responder to intravitreal ranibizumab. If we could know the patient with DME who is more appropriate receiving DEX implant, and perform early switch to DEX implant, that may improve their visual acuity outcome more.Limitations of this study include the small sample size and short-term follow-up, the uncontrolled retrospective design of the study, the nonstandard treatment protocols, and a lack of consistent performance of fluorescein angiography prior to switching to DEX implant treatment. Nevertheless, this study showed that intravitreal DEX implant treatment was effective immediately after switch and safe in cases of refractory DME resistant to ranibizumab. Switching to DEX implant can be considered in eyes with DME that do not respond to anti-VEGF treatments. Furthermore, higher baseline CRT and worse BCVA were found to be the predictive factors for better therapeutic responses. However, further studies are necessary to determine the optimal timing for DEX switching in patients with DME non-responder to intravitreal ranibizumab and to shed light on the long-term outcomes of this treatment modality.