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After obtaining a Master’s degree in Drug Development (Pharmaceutical Sciences), I started my PhD at the Ghent Research Group on Nanomedicines. This group, part of the General Lab of Biochemistry and Physical Pharmacy, focuses on the design and evaluation of nanomedicines for advanced delivery of therapeutics in context of retinal diseases, lung diseases, cancer vaccination and intraperitoneal cancer. The first 2 years of my PhD were dedicated to deciphering the intracellular processing of Quantum Dot nanoparticles with an emphasis on the autophagy pathway. Next, my focus switched to examination of drug delivery barriers at the back of the eye.
Many ocular disorders leading to blindness could benefit from delivery of therapeutics to the retina. A preferable administration route for delivery to the retina is intravitreal (IVT) injection, a technique that is considered safe, minimally invasive and relatively easy to perform. However, the transfer of drug or gene carriers into the retina after IVT injection remains limited, primarily because of the presence of the vitreoretinal (VR) interface. Figure 1 shows that this interface consists of three structures: peripheral vitreous, the inner limiting membrane (ILM) and Müller cell endfeet. Both the vitreous and ILM represent important drug delivery barriers: the vitreous may hamper the mobility of carriers preventing them to reach the retina [1], while the ILM forms a physical border between the vitreous and the retina and works as a sieve that impedes the transfer of carriers into the retina.[2]
While many viral and non-viral gene carriers are able of successfully penetrating through the entire VR interface in rodents, IVT injection in larger animals rarely results in effective gene expression. This is due to the fact that the anatomy of the rodent eye and the structure of its vitreous and ILM is less representative for large animal or human physiology. There is therefore a great need for models that can realistically mimic these barriers. To address this need, my PhD project focused on the development of a ‘vitreoretinal explant’, an ex vivo explant model that is bovine-derived and guarantees an intact VR interface by keeping the vitreous attached to the retina at all times (Figure 2A). After explant culture, drugs or drug carriers can be IVT injected ex vivo into the vitreous of the VR explant after which their potential to cross the VR interface as well as their transport route into the retina can be examined by microscopy (Figure 2B).
After validation of the vitreoretinal explant (viability and ILM integrity), we applied the model to look into the penetration of nanoparticles in function of their physicochemistry. Here, we discovered that the transfer through the VR barrier is size-dependent since 40 nm negatively charged polystyrene beads were more easily taken up in the retina than 100 and 200 nm sized particles (Figure 3).
In addition, we found that removing the vitreous, as commonly done for culture of conventional explants, leads to an overestimation of particle uptake since more nanoparticles could enter the retina including larger aggregates (Figure 4, - vitreous). We further confirmed that the ultimate barrier to overcome for retinal uptake is the ILM, since damaging this matrix resulted in a massive increase in particle transfer into the retina (Figure 4 – ILM).
In conclusion, we have successfully developed a highly relevant ex vivo model which by keeping the VR interface more intact than conventional explants, can be applied as a representative test set-up to assess the potential of promising drug and gene delivery carriers to cross the VR interface. By doing so, we have provided researchers with a large animal model that can provide valuable information without costly and labor-intensive in vivo studies. We hope that this advanced ex vivo model can aid in the rational design of optimal retinal drug carriers by tuning the carrier features in line with the barriers’ properties. Our lab therefore gladly welcomes visitors willing to learn this and/or other advanced drug delivery models we have developed[4].
With the end of my PhD in sight, my postdoctoral project will focus on advanced approaches to overcome the ILM and the formulation and evaluation of novel nanoparticle formulations.
Pharmacist, PhD student
Lab of General Biochemistry and Physical Pharmacy
Faculty of Pharmaceutical Sciences, Ghent University
Ottergemsesteenweg 460,
9000 Ghent, Belgium
E-mail: Karen.peynshaert[at]ugent.be
Tel: +32 (0)9 264 80 49
Website: http://www.biofys.ugent.be