Combating Retinopathy of Prematurity, the Most Common Cause of Childhood Blindness
Low-dose, systemic, intermittent drug delivery tested in animal model as way to alleviate hyperoxia-induced retinal damage in ROP.
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Recent data from two randomized prospective trials1,2 have defined the central paradox of oxygen therapy in severely premature infants — i.e., that oxygen is necessary to prevent mortality in these children but is simultaneously toxic to premature tissues such as the retina and the lung.
The discovery of hypoxia-inducible factors (HIFs) and their oxygen-dependent regulation through HIF prolyl hydroxylases offers a possible translational pathway for the growth and protection of blood vessels relevant to a broad range of diseases. These include anemia, stroke, myocardial infarction, skeletal muscle ischemia — and especially retinopathy of prematurity (ROP).
Our research team within Cleveland Clinic’s Cole Eye Institute is pursuing investigations that build on this work by raising the prospect of using low-dose, systemic, intermittent drug delivery to alleviate the hyperoxia-induced retinal damage of ROP. This article briefly reviews the basis for this work and the road ahead.
ROP is the most common cause of childhood blindness worldwide, and its incidence grows as severely premature infants are resuscitated at increasingly lower birth weights and younger gestational ages. Each year about 500,000 children are born prematurely in the U.S. and 13 million worldwide. In children born at less than 28 weeks’ gestation, the incidence of ROP can be as high as 75 percent.
ROP is a vasoproliferative disease that affects neonates with very low birth weight and early gestational age. The fetus develops in relative hypoxia in utero, a physiologic state that is disrupted by premature birth and worsened in susceptible tissues by supplemental oxygen.
The presence of excess oxygen, which corresponds to the hyperoxic phase I of ROP, causes the prolyl hydroxylase domain protein (PHD) to target key proline residues within the oxygen-dependent degradation domain (ODD) of the HIF-1α subunit for degradation by the ubiquitin-proteasome pathway. Two proline residues are targets of HIF PHD within the ODD and can interact independently with the von Hippel-Lindau tumor suppressor protein (pVHL). PHDs are a family of conserved enzymes with at least three mammalian homologues (PHD1-3) that regulate HIF activity through post-translational modification and therefore quickly respond to hyperoxia by downregulating HIF. Absence of HIF-1α results in halted downstream angiogenic pathways, including the reduction of vascular endothelial growth factor (VEGF) secretion associated with oxygen-induced vascular obliteration.
Our lab has definitively proven that systemic injection of the HIF activator and nonselective inhibitor of HIF PHD, dimethyloxalylglycine (DMOG), in two separate rodent models of ROP resulted in a dramatic inhibition of oxygen-induced retinopathy that was recapitulated by systemic PHD2 ablation.3-5
Surprisingly, hepatic HIF-1α protein levels after DMOG injection in mice were dramatically upregulated compared with levels in the brain, retina and kidney. Organ lysate obtained from DMOG-treated mice expressing a transgene of luciferase fused to the ODD confirmed that the highest luciferase activity is in the liver.4
This unusual finding of liver-specific HIF-1α activation and subsequent protection of retinovasculature initiated the hypothesis that the liver could be stimulated to protect retinal capillary beds via hepatic PHD inhibition. The simplicity of targeting a central visceral organ might justify angioprotection in diseases that require only a brief window for therapy, such as ROP.
This concept of remote protection — stimulating the liver to protect distal capillary beds — was then definitively proven using selective ablation of hepatic HIF-1α.6 This remarkable finding suggested that using a systemic agent to protect against retinal disease might simultaneously prevent hyperoxic damage to the lung, thereby improving gas exchange and decreasing supplemental oxygen requirements (Figure).
A recent National Eye Institute-supported investigation by our lab using systems pharmacology has demonstrated that different small molecules can target the liver, the eye or both. The latter pathway is especially valuable because it raises the possibility of using low-dose, intermittent drug administration to vulnerable neonates to minimize the chance of drug toxicity. We envision the use of a soluble small molecule, given intravenously once or twice a week in the first few weeks of life until corrected gestational age of 30 weeks, to gently induce the normal coordinated growth of retinal blood vessels and possibly other organ systems negatively impacted by hyperoxia.
Work discussed here is funded by Research to Prevent Blindness, the E. Matilda Ziegler Foundation for the Blind (J.E.S.), the Hartwell Foundation (J.E.S.) and the National Eye Institute (R01EY024972) (J.E.S.).
Dr. Sears is a staff physician in Cleveland Clinic’s Cole Eye Institute and a member of the Department of Cellular and Molecular Medicine in the Lerner Research Institute.
Dr. Hoppe is a researcher in the Cole Eye Institute.