Impact of Vital Dyes on Cell Viability and Transduction Efficiency of AAV Vectors Used in Retinal Gene Therapy Surgery: An In Vitro and In Vivo Analysis

Sodium Fluorescein as an Optical Label to Evaluate Subretinal Injection

PURPOSE

There is renewed interest in subretinal drug delivery as the result of novel and emerging treatments for retinal diseases, including retinal gene therapy. However, our knowledge of the distribution of subretinally delivered drugs is incomplete; herein, we describe a qualitative and quantitative means of surveying the early intraocular distribution of subretinally delivered drugs using dilute sodium fluorescein (NaFl).

METHODS

Sodium fluorescein 10% was serially diluted and mixed with a solution containing tissue plasminogen activator (tPA) at a final concentration of 0.1 mg/mL NaFl and 0.5 mg/mL of tPA. Because this solution was to be used in the context of subretinal tPA injection in the treatment of subretinal hemorrhage, fluorophotometry with, and without, the presence of human whole blood was performed to derive a formula to calculate the concentration of NaFl based on the fluorescence of aspirated intraocular fluid. Videos of subretinal tissue plasminogen activator surgery in a case are presented as a qualitative demonstration of the technique and vitreous cavity fluid collected at case completion underwent fluorophotometry to estimate the loss of therapeutic solution.

RESULTS

Although the presence of hemoglobin in blood suppresses fluorescence of NaFl, we demonstrate that centrifuging admixtures of blood with NaFl negates the optical effects of blood and yields identical fluorescence versus concentration plots to those of NaFl solution alone. We also demonstrate that NaFl at 0.1 mg/mL can be readily used to qualitatively assess drug losses before, during, and after subretinal injection. Furthermore, we describe how it may be used to quantitatively estimate the total loss of therapeutic solution during subretinal injection using fluorophotometry on aspirated fluid from the vitreous cavity (loss estimated as 4% in the case presented).

CONCLUSION

Sodium fluorescein at a concentration of 0.1 mg/mL can be used to quantitatively and qualitatively assess the fate of subretinally injected drugs during subretinal injection surgery.

Accurate Quantification of AAV Vector Genomes by Quantitative PCR

The ability to accurately determine the dose of an adeno-associated viral (AAV) therapeutic vector is critical to the gene therapy process. Quantitative PCR (qPCR) is one of the common methods to quantify the AAV vector titre, but different variables can lead to inconsistent results. The aim of this study was to analyze the influence of the conformation of the DNA used as the standard control, and the enzymatic digestion was performed to release the viral genome from the protein capsid on the physical genome titration of a clinically relevant AAV8.RPGR vector, made to good laboratory practice standards in an academic setting. The results of this study showed that the conformation of the DNA used as standard has a significant impact on the accuracy of absolute quantification by qPCR. The use of supercoiled undigested plasmid DNA template generated a higher apparent titer, as compared to the use of linearized plasmid as the standard. In contrast to previous studies, the pre-treatment of the samples with Proteinase K, in addition to the high temperature step used after DNase I digestion, resulted in a reduction on AAV titers. Ideally, all AAV documentation should state which form of reference plasmid and which pre-treatment of the samples have been used to calculate titers, so that appropriate comparisons relating to dose toxicity and transduction efficacy can be made in the clinical scenario.

Outcome Measures Used in Ocular Gene Therapy Trials: A Scoping Review of Current Practice

Multiple gene therapy trials are occurring for a variety of ophthalmic diseases around the world. The safety of gene therapy in the eye has been established, and the next step is to reliably assess efficacy. This is primarily done through the use of imaging techniques and visual function measures. Standardized visual function assessments, however, were originally developed for a clinical setting and may not be suitable for detecting and quantifying therapeutic changes. This scoping review takes a comprehensive look at current practice in terms of the outcome measures defined at trial registration. These were compared to the outcome measures reported in the literature. All published trials reported the pre-registered primary outcome measure. A range of additional secondary outcomes were reported that were not originally planned. Gaps in gene therapy assessment exist and further discussion are required to find a way forward, particularly as more conditions progress to phase 2 and 3 trials. Several factors impacting on trial design and outcome measure choice are discussed.

The Blunt End: Surgical Challenges of Gene Therapy for Inherited Retinal Diseases

PURPOSE

To review barriers to effective transduction of cells in the subretinal plane during gene therapy surgery for inherited retinal dystrophies (IRD).

DESIGN

Perspective.

METHODS

Case-based learning in clinical trials and commercial applications of gene therapy in a tertiary care, university-affiliated hospital. MEDLINE search for publications relevant to retinal surgical technique for gene therapy, clinical trials results for gene therapy of IRD, adenoviral-associated viral vector design, and immune response to viral vectors.

RESULTS

The most important surgical issues are safe access to the subretinal space, intraoperative visualization with optical coherence tomography to protect the macula, and quantitation of viral dose. Other issues for retinal surgeons are patient selection, target zone planning, and control of inflammation. Vector-related issues that may affect the precision of treatment involve capsid interaction with the innate and adaptive immune systems and selective targeting of cell types.

CONCLUSIONS

Most current gene therapy vectors for monogenic IRD require physical proximity to target tissues under the retina in order to work. New surgical skills and new instrumentation are under development. So far, the host immune response does not seem to cause rejection of genes delivered by viral vectors but the efficiency of transduction can only be indirectly assessed by long-term visual outcomes.