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      The cGMP Pathway and Inherited Photoreceptor Degeneration: Targets, Compounds, and Biomarkers

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          Abstract

          Photoreceptor physiology and pathophysiology is intricately linked to guanosine-3’,5’-cyclic monophosphate (cGMP)-signaling. Here, we discuss the importance of cGMP-signaling for the pathogenesis of hereditary retinal degeneration. Excessive accumulation of cGMP in photoreceptors is a common denominator in cell death caused by a variety of different gene mutations. The cGMP-dependent cell death pathway may be targeted for the treatment of inherited photoreceptor degeneration, using specifically designed and formulated inhibitory cGMP analogues. Moreover, cGMP-signaling and its down-stream targets may be exploited for the development of novel biomarkers that could facilitate monitoring of disease progression and reveal the response to treatment in future clinical trials. We then briefly present the importance of appropriate formulations for delivery to the retina, both for drug and biomarker applications. Finally, the review touches on important aspects of future clinical translation, highlighting the need for interdisciplinary cooperation of researchers from a diverse range of fields.

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          Non-syndromic retinitis pigmentosa

          Retinitis pigmentosa (RP) encompasses a group of inherited retinal dystrophies characterized by the primary degeneration of rod and cone photoreceptors. RP is a leading cause of visual disability, with a worldwide prevalence of 1:4000. Although the majority of RP cases are non-syndromic, 20-30% of patients with RP also have an associated non-ocular condition. RP typically manifests with night blindness in adolescence, followed by concentric visual field loss, reflecting the principal dysfunction of rod photoreceptors; central vision loss occurs later in life due to cone dysfunction. Photoreceptor function measured with an electroretinogram is markedly reduced or even absent. Optical coherence tomography (OCT) and fundus autofluorescence (FAF) imaging show a progressive loss of outer retinal layers and altered lipofuscin distribution in a characteristic pattern. Over the past three decades, a vast number of disease-causing variants in more than 80 genes have been associated with non-syndromic RP. The wide heterogeneity of RP makes it challenging to describe the clinical findings and pathogenesis. In this review, we provide a comprehensive overview of the clinical characteristics of RP specific to genetically defined patient subsets. We supply a unique atlas with color fundus photographs of most RP subtypes, and we discuss the relevant considerations with respect to differential diagnoses. In addition, we discuss the genes involved in the pathogenesis of RP, as well as the retinal processes that are affected by pathogenic mutations in these genes. Finally, we review management strategies for patients with RP, including counseling, visual rehabilitation, and current and emerging therapeutic options.
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            Pharmacokinetic aspects of retinal drug delivery

            Drug delivery to the posterior eye segment is an important challenge in ophthalmology, because many diseases affect the retina and choroid leading to impaired vision or blindness. Currently, intravitreal injections are the method of choice to administer drugs to the retina, but this approach is applicable only in selected cases (e.g. anti-VEGF antibodies and soluble receptors). There are two basic approaches that can be adopted to improve retinal drug delivery: prolonged and/or retina targeted delivery of intravitreal drugs and use of other routes of drug administration, such as periocular, suprachoroidal, sub-retinal, systemic, or topical. Properties of the administration route, drug and delivery system determine the efficacy and safety of these approaches. Pharmacokinetic and pharmacodynamic factors determine the required dosing rates and doses that are needed for drug action. In addition, tolerability factors limit the use of many materials in ocular drug delivery. This review article provides a critical discussion of retinal drug delivery, particularly from the pharmacokinetic point of view. This article does not include an extensive review of drug delivery technologies, because they have already been reviewed several times recently. Instead, we aim to provide a systematic and quantitative view on the pharmacokinetic factors in drug delivery to the posterior eye segment. This review is based on the literature and unpublished data from the authors' laboratory.
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              Recessive mutations in the gene encoding the beta-subunit of rod phosphodiesterase in patients with retinitis pigmentosa.

              We have found four mutations in the human gene encoding the beta-subunit of rod cGMP phosphodiesterase (PDE beta) that cosegregate with autosomal recessive retinitis pigmentosa, a degenerative disease of photoreceptors. In one family two affected siblings both carry allelic nonsense mutations at codons 298 and 531. Affected individuals have abnormal rod and cone electroretinograms. PDE beta is the second member of the phototransduction cascade besides rhodopsin that is absent or altered as a cause of retinitis pigmentosa, suggesting that other members of this pathway may be defective in other forms of this disease.
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                Author and article information

                Journal
                Genes (Basel)
                Genes (Basel)
                genes
                Genes
                MDPI
                2073-4425
                14 June 2019
                June 2019
                : 10
                : 6
                : 453
                Affiliations
                [1 ]Institute for Ophthalmic Research, University of Tübingen, Elfriede-Aulhorn-Strasse 5-7, 72076 Tübingen, Germany; arianna.tolone@ 123456uni-tuebingen.de (A.T.); soumaya.belhadj@ 123456uni-tuebingen.de (S.B.)
                [2 ]Biolog Life Science Institute, 28199 Bremen, Germany; ar@ 123456biolog.de (A.R.); fs@ 123456biolog.de (F.S.)
                Author notes
                [* ]Correspondence: francois.paquet-durand@ 123456klinikum.uni-tuebingen.de ; Tel.: +49-7071-29-87430
                [†]

                These authors contributed equally to this work.

                Author information
                https://orcid.org/0000-0002-8185-3392
                Article
                genes-10-00453
                10.3390/genes10060453
                6627777
                31207907
                4686c695-d80f-4e03-9d44-902652e946d8
                © 2019 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 15 April 2019
                : 11 June 2019
                Categories
                Review

                retina,cyclic gmp,apoptosis,necrosis,drug delivery systems,translational medicine

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