Vitamin A (Retinol) (VitA)

Vitamin A, specifically in its retinol and retinal forms, is the essential precursor for the visual cycle, a metabolic pathway occurring between the photoreceptor cells and the retinal pigment epithelium (RPE) [NIH, 2022]. In this cycle, all-trans-retinol is converted into 11-cis-retinal, which binds to opsin proteins to form rhodopsin, the primary light-sensitive pigment in the eye [Kiser, 2014]. Dysfunctions in the transport or enzymatic processing of vitamin A can lead to the accumulation of toxic bisretinoids, such as A2E, which are major components of lipofuscin and contribute to the progression of Stargardt disease and age-related macular degeneration [Charbel Issa, 2013]. Pharmacological interventions target this pathway by either reducing the systemic delivery of retinol to the eye, inhibiting key enzymes like RPE65 to slow the cycle, or utilizing deuterated vitamin A analogs that resist the chemical dimerization leading to toxin formation [Kubota, 2020; Nature, 2021]. These strategies aim to preserve retinal health and slow vision loss in patients with inherited or age-related retinal degenerations. However, therapeutic modulation of this pathway often presents challenges such as delayed dark adaptation or night blindness due to the reduced availability of the visual chromophore [Kubota, 2020].

Other names

RetinolAll-trans-retinolVitamin A1AxerophtholVisual cycle retinoidsVitamin A within the visual cycle and retinal pigment epithelium metabolic pathway

Modulation of the visual cycle through reduction of serum retinol delivery, inhibition of isomerase enzymes, or replacement with stabilized analogs to prevent toxic byproduct accumulation [Kiser, 2014; Kubota, 2020].