The major challenge faced by today's pharmacologist and formulation scientist is ocular drug delivery. Topical eye drop is the most convenient and patient compliant route of drug administration, especially for the treatment of anterior segment diseases. Delivery of drugs to the targeted ocular tissues is restricted by various precorneal, dynamic and static ocular barriers. Also, therapeutic drug levels are not maintained for longer duration in target tissues. In the past two decades, ocular drug delivery research acceleratedly advanced towards developing a novel, safe and patient compliant formulation and drug delivery devices/techniques, which may surpass these barriers and maintain drug levels in tissues. Anterior segment drug delivery advances are witnessed by modulation of conventional topical solutions with permeation and viscosity enhancers. Also, it includes development of conventional topical formulations such as suspensions, emulsions and ointments. Various nanoformulations have also been introduced for anterior segment ocular drug delivery. On the other hand, for posterior ocular delivery, research has been immensely focused towards development of drug releasing devices and nanoformulations for treating chronic vitreoretinal diseases. These novel devices and/or formulations may help to surpass ocular barriers and associated side effects with conventional topical drops. Also, these novel devices and/or formulations are easy to formulate, no/negligibly irritating, possess high precorneal residence time, sustain the drug release, and enhance ocular bioavailability of therapeutics. An update of current research advancement in ocular drug delivery necessitates and helps drug delivery scientists to modulate their think process and develop novel and safe drug delivery strategies. Current review intends to summarize the existing conventional formulations for ocular delivery and their advancements followed by current nanotechnology based formulation developments. Also, recent developments with other ocular drug delivery strategies employing in situ gels, implants, contact lens and microneedles have been discussed.

Ocular drug delivery systems: An overview

Glial proliferation and activation are associated with disease progression in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar dementia. In this study, we describe a unique platform to address the question of cell autonomy in transactive response DNA-binding protein (TDP-43) proteinopathies. We generated functional astroglia from human induced pluripotent stem cells carrying an ALS-causing TDP-43 mutation and show that mutant astrocytes exhibit increased levels of TDP-43, subcellular mislocalization of TDP-43, and decreased cell survival. We then performed coculture experiments to evaluate the effects of M337V astrocytes on the survival of wild-type and M337V TDP-43 motor neurons, showing that mutant TDP-43 astrocytes do not adversely affect survival of cocultured neurons. These observations reveal a significant and previously unrecognized glial cell-autonomous pathological phenotype associated with a pathogenic mutation in TDP-43 and show that TDP-43 proteinopathies do not display an astrocyte non-cell-autonomous component in cell culture, as previously described for SOD1 ALS. This study highlights the utility of induced pluripotent stem cell-based in vitro disease models to investigate mechanisms of disease in ALS and other TDP-43 proteinopathies.

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Astrocyte pathology and the absence of non-cell autonomy in an induced pluripotent stem cell model of TDP-43 proteinopathy

Pluripotent stem cells (PSCs) hold great promise for drug discovery and regenerative medicine owing to their ability to differentiate into any cell type in the body. After more than three decades of research, including delays due to the potential tumorigenicity of PSCs and inefficiencies in differentiation methods, the field is at a turning point, with a number of clinical trials across the globe now testing PSC-derived products in humans. Ocular diseases dominate these first-in-man trials, and Phase l/ll results are showing promising safety data as well as possible efficacy. In addition, the advent of induced PSC (iPSC) technology is enabling the development of a wide range of cell-based disease models from genetically predisposed patients, thereby facilitating drug discovery. In this Review, we discuss the recent progress and remaining challenges for the use of PSCs in regenerative medicine and drug development.

Current status of pluripotent stem cells: moving the first therapies to the clinic

Glioblastoma remains the most common and devastating primary brain tumor despite maximal therapy with surgery, chemotherapy, and radiation. The glioma stem cell (GSC) subpopulation has been identified in glioblastoma and likely plays a key role in resistance of these tumors to conventional therapies as well as recurrent disease. GSCs are capable of self-renewal and differentiation; glioblastoma-derived GSCs are capable of de novo tumor formation when implanted in xenograft models. Further, GSCs possess unique surface markers, modulate characteristic signaling pathways to promote tumorigenesis, and play key roles in glioma vascular formation. These features, in addition to microenvironmental factors, present possible targets for specifically directing therapy against the GSC population within glioblastoma. In this review, the authors summarize the current knowledge of GSC biology and function and the role of GSCs in new vascular formation within glioblastoma and discuss potential therapeutic approaches to target GSCs.

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Glioma Stem Cells: Signaling, Microenvironment, and Therapy

Nanotherapies for the treatment of ocular diseases.

In vitro differentiation of retinal ganglion-like cells from embryonic stem cell derived neural progenitors.

Hes-1 and Hes-5 are downstream effectors of Notch signaling that are known to be involved in different aspects of neural stem cell proliferation and differentiation. Evidence has emerged that Hes-1 expression can be regulated by alternate signaling pathways independent of canonical Notch/CBF1 interaction. This context-dependent differential regulation of Hes-1 expression in neural progenitor gains a lot of importance as it would help in its exponential expansion without the requirement of interaction from neighboring cells during development. Here, we have clearly demonstrated the existence of a population of neural progenitors with Notch/CBF1-independent Hes-1 expression in vitro. Further analysis demonstrated the role of FGF2 in activating Hes-1 expression through the direct binding of ATF2, a JNK downstream target, on Hes-1 promoter. This raises the possibility for the existence of two distinct populations of neural progenitors - one maintained by Hes-1 expression exclusively through Notch-independent mechanism and the other mediating Hes-1 expression through both canonical Notch and FGF2-ATF2 pathway. This alternative pathway will insure a constant expression of Hes-1 even in the absence of canonical Notch intracellular domain-mediated signaling, thereby maintaining a pool of proliferating neural progenitors required during development.

ATF2 maintains a subset of neural progenitors through CBF1/Notch independent Hes-1 expression and synergistically activates the expression of Hes-1 in Notch-dependent neural progenitors.

The poor corneal residence time of pilocarpine, an alkaloid extracted from the leaves of the Jaborandi plant, limits its ocular application. The aim of this study was to develop, characterize, and evaluate the potential of pilocarpine entrapped by poly(DL-lactic-co-glycolic acid) (PLGA) nanoparticle carriers for ocular drug delivery. Pilocarpine-loaded nanoparticles were prepared with a double-emulsion (water in oil in water) method and characterized with transmission electron microscopy and X-ray diffraction analysis. The nanoparticles exhibited an average size of 82.7 nm with an encapsulation efficiency of 57%. Stability studies showed the absence of agglomeration and constancy in the amount of drug entrapped; this indicated the solidity of these particles for long-term use. The in vitro release studies conducted in simulated tear fluid showed the sustained release of pilocarpine. In vivo evaluation of the nanoparticles was done in a rabbit model with a miosis assay and compared to an equal dose of commercially available eye drops of pilocarpine (Pilocar drops). The in vivo miosis studies showed that the duration of miotic response increased by 40% for the nanoparticles and produced an almost 68% increase in total miotic response when compared to the eye drops. In conclusion, this study clearly demonstrated the potential of pilocarpine-loaded PLGA nanoparticles for multiplying the therapeutic effect of ophthalmic drug delivery with enhanced bioavailability and pharmacological response. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Pilocarpine‐loaded poly(DL‐lactic‐co‐glycolic acid) nanoparticles as potential candidates for controlled drug delivery with enhanced ocular pharmacological response

Fate-specific differentiation of neural progenitors attracts keen interest in modern medicine due to its application in cell replacement therapy. Though various signaling pathways are involved in maintenance and differentiation of neural progenitors, the mechanism of development of lineage-restricted progenitors from embryonic stem (ES) cells is not clearly understood. Here, we have demonstrated that neuronal vs. glial differentiation potential of ES cell-derived neural progenitors (ES-NPs) are governed by the growth factors, exposed during their proliferation/expansion phase and cannot be significantly altered during differentiation phase. Exposure of ES-NPs to fibroblast growth factor-2 (FGF2) during proliferation triggered the expression of pro-neural genes that are required for neuronal lineage commitment, and upon differentiation, predominantly generated neurons. On the other hand, epidermal growth factor (EGF)-exposed ES-NPs are not committed to neuronal fate due to decreased expression of pro-neural genes. These ES-NPs further generate more glial cells due to expression of glial-restricted factors. Exposure of ES-NPs to the same growth factors during proliferation/expansion and differentiation phase augments the robust differentiation of neurons or glial subtypes. We also demonstrate that, during differentiation, exposure to growth factors other than that in which the ES-NPs were expanded does not significantly alter the fate of ES-NPs. Thus, we conclude that FGF2 and EGF determine the neural vs. glial fate of ES-NPs during proliferation and augment it during differentiation. Further modification of these protocols would help in generating fate-specified neurons for various regenerative therapies.

Sasidharan Vidyanand

Papers

In vitro differentiation of retinal ganglion-like cells from embryonic stem cell derived neural progenitors.

ATF2 maintains a subset of neural progenitors through CBF1/Notch independent Hes-1 expression and synergistically activates the expression of Hes-1 in Notch-dependent neural progenitors.

Pilocarpine‐loaded poly(DL‐lactic‐co‐glycolic acid) nanoparticles as potential candidates for controlled drug delivery with enhanced ocular pharmacological response

Neuronal vs. glial fate of embryonic stem cell-derived neural progenitors (ES-NPs) is determined by FGF2/EGF during proliferation.