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Title: The vitreous as a depot to prolong the action of ocular therapies
Author: Shiekh Hassan Awwad, S.
ISNI:       0000 0004 7231 2054
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2016
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Prolonging therapeutic levels of a drug within the vitreous to treat blinding diseases is one of the most important goals in ophthalmic drug development. Intravitreal (IVT) injections of therapeutic proteins and the use of steroid implants in the vitreous cavity are currently the best clinical methods to achieve prolonged exposure in the back of the eye. Therapeutic biologics registered for ophthalmic use by IVT injection comprise a PEGylated-aptamer (Pegaptanib), antibody fragment (ranibizumab), and an Fc fusion (alfibercept). The monoclonal antibody (mAb), bevacizumab is also widely used as an unlicensed medicine to treat age-related macular degeneration (AMD). It is anticipated that ophthalmic protein-based medicines, which tend to be potent and have a rapid onset of action, will continue to be developed as molecular mechanisms involved in blinding diseases become better understood. Mass exchange within the eye is dominated by aqueous flow, which is secreted at 2.0–2.5 μL/min into the vitreous from the ciliary body. There are two main drug elimination pathways from the vitreous: (a) the aqueous outflow into the anterior chamber and (b) permeation through the retina via retinal-choroid-sclera (RCS) pathways. Therapeutic proteins are high molecular weight (MW) and charged molecules, and they clear predominantly by the anterior route. Proteins consequently have longer half-lives (t1/2) i.e. days in the vitreous cavity than low MW drugs i.e. hours, many of which are lipophilic and permeable. There is much research focused on developing new strategies to increase the vitreous residence times of macromolecular drugs to decrease the frequency of IVT injections. A two-compartment, aqueous outflow model scaled to the human eye called the PK-Eye was created to aid the development of new ocular drug formulations and delivery systems. The PK-Eye was shown to be particularly useful in evaluating protein PK and stability properties, and for the preclinical evaluation of novel, long-acting formulations, which are particularly difficult to evaluate in animal models because of the generation of anti-drug antibodies (ADAs). Ranibizumab and bevacizumab were used to validate the PK-Eye using simulated vitreous to give clearance t1/2s that were comparable to human values. The model was also used to determine the outflow clearance t1/2 of triamcinolone acetonide (TA, Kenalog®),which is clinically administered as a suspension. The clearance t1/2 for TA was overestimated by the PK-Eye compared to the human t1/2 since the model does not measure RCS clearance. Prolonging the vitreous residence time of low MW molecules relies on at least a 2-step process: (i) dissolution or release from an implant and (ii) clearance of solubilised drug from the vitreous. The first step will often be the rate-limiting step that defines the overall clearance time of a candidate preparation. Once the active is in the vitreous solution, the model has the capacity to provide a good estimate of clearance by the anterior outflow route. To model RCS clearance, invitro in vivo correlations (IVIVCs)can be developed using the PK-Eye data for outflow clearance times obtained from suspensions implants. The PK -Eye clearance t1/2for Kenalog ® and was used with TA permeability data in the literature to better estimate the human clearance time for TA. This was an important first step for developing IVIVCs that would be needed for ocular DDS of small molecules. Other DDS using electrospun (ES) fibres and microparticles were investigated using small MW drugs to examine the utility of the PK-Eye. Permeability and clearance data from the PK-Eye for dexamethasone release from microparticles were used to estimate the possible human clearance t1/2. The utility of the PK-Eye was also examined to evaluate therapeutic proteins. If more stable protein formulations are developed, higher molar concentrated IVT doses may become more routine where a therapeutic tail can be exploited to extend duration of action. Increasing doses of bevacizumab were examined in the PK-Eye where it was possible to observe a therapeutic tail. Hydrogels and in-situ forming gels have been extensively investigated for a wide range of applications. The highly hydrated state of hydrogels could be a good environment for maintaining protein stability. PEGylated-ranibizumab (PEG10-Fabrani) when encapsulated within a crosslinked NIPAAM in-situforming gel displayed an extended clearance time in the PK-Eye compared to non-encapsulated PEG10-Fabrani. In contrast, no difference in clearance times were observed between NIPAAM hydrogel encapsulated and non-encapsulated bevacizumab. Since PEG is a random coil, PEG10-Fabraniappeared to be more entangled within the hydrogel compared to bevacizumab, which may explain the prolonged clearance times that were observed. Considering that an in situ forming gel collapses on injection, controlling the burst release of the encapsulated protein is a challenge. To maintain the protein in a highly hydrated state, affinity based drug delivery was examined in the hope that the vitreous could be made into a depot that could extend the duration of action. Both endogeneous and exogeneous affinity anchors were examined. Much effort was focused on developing a bispecific antibody mimetic with one Fab targeted to collagen II as an endogenous depot anchoring site and the other Fab targeted to VEGF to inhibit angiogenesis. Implantable exogeneous anchors were also examined. Proof of concept experiments using the PK-Eye to anchor albumin as a model anchor suggest that more work is justified to develop an affinity based drug delivery strategy in the vitreous.
Supervisor: Brocchini, S. ; Khaw, P. T. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available