Retinopathy of prematurity: the need for prevention

Introduction

Among the ∼4 million babies who are yearly born in the USA,¹ >450,000 are born prematurely, defined as birth before 37 weeks of gestation.^2,3 Preterm birth has been associated with many complications, occurring during delivery, within the first few weeks after delivery and even many years later.⁴ Preterm birth is the most frequent reason for neonatal death^5,6 and among the most common causes for death in children younger than 5 years.⁷ A significant improvement of the standards in neonatal intensive care units (NICUs) and perinatal care over the last decades has increased the survival rate of even the youngest preterm babies. Delivery after gestational week 26 has survival rates of >80% today.^8–10

Despite improving the health outcomes and survival rates in even the youngest infants, preterm birth still carries a high risk of negatively affecting most organ systems, such as the heart,^11,12 lungs,^13,14 brain,^15–17 and the eye.^18,19 The improved survival of very small and particularly vulnerable preterm infants has led to the resurgence in preterm complications including one of the major causes for blindness in children, retinopathy of prematurity (ROP).²⁰

Laser photocoagulation and the intravitreal injection of vascular endothelial growth factor (VEGF) antibodies have proven to be useful treatment options for severe late ROP. However, laser photocoagulation destroys major parts of the retina, and the injection of VEGF antibodies, though simple to apply, may cause a systemic suppression of vascular growth affecting the other organs, a process which has not been studied in sufficient depth. However, the simple mode of application of VEGF antibodies, and the so far promising clinical data should, not obscure the fact that the prevention of ROP should be the first priority.

There is a great interest in preventing ROP by more closely mimicking the intrauterine environment after preterm birth. Such preventative measures are also likely to reduce other complications besides ROP. These measures include preventing the toxic postbirth influences (eg, oxygen excess) as well as providing missing intrauterine factors (eg, insulin growth factor 1 [IGF-1]). This review is, therefore, meant to summarize the current knowledge on the prevention of ROP with a particular emphasize on the use of IGF-1 supplementation.

History of ROP

In 1984, the first part of the International Classification of Retinopathy of Prematurity was published by an international expert group, which was expanded in 1987.^21,22 This classification permits the examiner to specify the location of the late-stage neovascular disease and the extent of the developing vasculature involved. In order to assess the location, the retina is divided into three zones, with the optic disk being the center for each zone, and the extent is specified as clock hours (Figure 1). In addition to these two parameters, four stages are used to describe the severity of vascular abnormality ranging from stage 1, which refers to a demarcation line, separating the avascular anterior retina from the vascularized posterior part of the retina, to stage 4, which describes retinal detachment. The term plus disease was introduced to account for dilated veins and/or tortuous arterioles and was added to any stage that had these vessel characteristics associated with the poorer outcomes. The expansion of this initial classification in 1987 added a fifth stage that applied when a total retinal detachment was present. Stage 4 was, therefore, defined as a retinal detachment that would not affect the whole retina. In order to be able to distinguish between a retinal detachment with and without foveal involvement, stages 4a (extrafoveal) and 4b (including fovea) were introduced.

The latest revision of this classification was published in 2005 and it introduced the terms aggressive posterior ROP and preplus disease and clarified the definition of zone I.²³

ROP was initially understood and described in terms of oxygen toxicity, without understanding, at that time, the contribution of other risk factors. In the 1940s, Terry described a disease in preterm infants that was characterized by an opaque tissue behind the lens that he referred to as retrolental fibroplasia.^24,25

The term ROP was later coined secondary to the observation that premature birth is a key aspect in the development of this disease. At that time, the survival of preterm infants with respiratory problems was improved through the use of (unmonitored) oxygen supplementation.²⁶ In the 1940s, ROP was the single most common reason for childhood blindness in the developed world.²⁷ The animal studies by Ashton et al and a clinical study in 1952 and 1953 by Patz et al identified unmonitored use of high oxygen supplementation as a main driver of ROP.^28–31 These first observations were affirmed by other groups^30,32 and supported in animal models.^28,33 A restriction of the oxygen supply in neonates resulted in a decrease in the incidence of ROP but an increase in perinatal death.³⁴ The use of oxygen supplementation in the following years was more liberal in the nurseries, resulting in another epidemic of ROP that was, however, more treatable owing to improvements in neonatal intensive care^35,36 and the introduction of retinal ablative therapies for ROP.^37,38 The difficulty in balancing the oxygen delivery to minimize ROP (relatively low oxygen tension) versus minimizing the risk of developing brain hypoxia or death (relatively high oxygen tension) has not been fully resolved. With a better, if not perfect, control of supplemental oxygen, the other risk factors of ROP are more prominent. Over the last years, gestational age has remained as a prominent risk factor. With the lower gestational age, the degree of retinal vascular maturity will decrease, making the retina more susceptible to damage due to a lack of (postnatal) growth factors (GFs) and eventually decelerated vascularization.

Pathogenesis of ROP

The retinal vasculature in human is fully developed by the end of a full-term pregnancy.³⁹ After preterm birth, compared to the third trimester in utero, there are several insults that prevent the immature partially vascularized retina from vascularizing normally. The partial pressure of oxygen (PaO₂) in utero is ∼50 mmHg by the end of pregnancy when compared to ∼160 mmHg PaO₂ found in ambient room air.⁴⁰ Some infants are given as high as 100% oxygen as supplementation. When a preterm infant is born, he or she is immediately exposed to this dramatic increase in oxygen pressure. The high oxygen levels of even ambient room air and certainly with oxygen supplementation pose a risk for developing ROP. The reason behind this phenomenon is found in the development of the retinal vasculature: by the beginning of the fourth month of gestational age, vessels start to sprout radially from the optic nerve toward the ora serrata.⁴¹ This process is usually completed shortly before full-term birth.⁴² However, in preterm infants, this process is incomplete at birth. Studies suggest that a PaO₂ of ≥80 mmHg besides preventing the normal vascularization will also damage the existing newly developed retinal capillaries. Increased oxygen pressure as found in ambient room air, which is around double that found to be harmful to the retinal vasculature, causes attenuation in vessel growth and a constriction of already developed vessels.^43,44 As a result, avascular areas form in the retina.⁴⁵ This phenomenon is further amplified when preterm babies are ventilated with high oxygen concentrations. The size of the initial avascular zone is dependent on the gestational age at birth of the preterm infant. If Phase I of ROP with an impaired vessel growth could be prevented with the control of oxygen but also with a restoration of factors missing in the preterm infant, such as IGF-1, then the second neovascular phase that depends on the extent of avascular retina would not occur. Restoring the normal vascularization that would occur in utero would prevent end-stage ROP.

Following this first phase of impaired vascular growth and vasoattenuation, the increasing metabolism of the growing infant’s retina leads to a rising demand for oxygen and nutrients and a subsequent upregulation of angiogenic GFs, such as VEGF and erythropoietin.^46,47 This second phase of the disease, driven prominently by VEGF, is clinically characterized by the outgrowth of new but pathological vessels into the vitreous. These neovascular tufts are highly perfused, yet they do not contribute to the physiological needs of the infant’s retina and may be leaky due to an impaired blood–retina barrier. As the retina develops further, these pathological vessels may regress. However, fibrous scar tissue often remains, which tends to cause traction on the retina, and can, in the worst case, lead to retinal detachment and possibly blindness (Figure 2).⁴⁸

The transition of Phase I to Phase II usually occurs around the 32nd postmenstrual week, but the onset as well as the severity of the neovascular phase can be variable.^49,50 Interestingly, a multicenter trial found that the onset of retinal vascular changes in preterm infants seems to correlate more with gestational age at birth (time elapsed between the first day of last menstrual period and the day of delivery) rather than the chronological age (time elapsed from birth).^51,52 For infants born before gestational week 27, the onset of ROP may correlate a little more with the chronological age than the gestational age. However, even in these severely preterm babies, stage 3 ROP was never seen before a postmenstrual age (PMA; gestational age + chronological age) of 31 weeks.^49,53

Even mild ROP in more mature infants can disturb proper central retinal development.⁵⁴ ROP-related visual impairment both in the central part of the retina^55,56 and in the periphery^55,57 can often be detected many years after the initial onset of the disease. Thus, the area of undeveloped retina at birth and the lack of postnatal growth are strong determinants of proliferative ROP.

Risk factors for ROP

Treatment of ROP

Current treatment is not preventative but deals with the second proliferative phase and not the first phase of ROP, which sets the stage for neovascularization. The first treatment option in Phase II ROP was cryocoagulation of the avascular retina. The results and recommendations following this treatment approach were published in 1990 as the CRYO-ROP study.^38,87 However, today, laser photocoagulation has replaced cryotherapy in most countries.⁸⁸ Yet, the therapeutic principle stays the same: by destroying the avascular retina, the driver of an increased production of vascular GFs is reduced. When applied properly, this procedure is fairly safe and prevents most infants from developing retinal detachment (ROP stages 4 and 5). This intervention at early Phase II is important, as repairing late stage (4 and 5) ROP is unfortunately often associated with poor clinical outcomes.⁸⁹

Anti-VEGF therapy has been a clinical treatment option in most proliferative eye disease in adults and is today the most important therapy for neovascular age-related macular degeneration. Not surprisingly, anti-VEGF therapy has also found its way into (off-label) Phase II ROP treatment with neovascularization. However, treating preterm infants is challenging, and the right dose or right drug has yet to be determined. The CARE-ROP trial (clinical trial identifier: NCT02134457) investigates, among other questions, if two different lower doses (compared to the BEAT-ROP study) of an even smaller molecule, the anti-VEGF fragment ranibizumab, are similarly effective in the treatment of neo-vascular Phase II ROP. Phase I trial of de-escalating doses of bevacizumab for Phase II ROP is also underway (clinical trial identifier: NCT02390531). The dose issue is critical as anti-VEGF treatment leaks into the systemic circulation to suppress systemic VEGF levels for many weeks with the potential of suppressing normal brain and other organs’ growth in fragile preterm neonates.

IGF-1 and its deficiency in the preterm infant

IGF-1 has a dominant role in somatic growth,⁹⁰ especially in the third trimester of pregnancy, therefore, in the growth of preterm infants. IGF-1 levels drop to very low levels after preterm birth and remain low for many weeks postnatally. To understand the effects of lack of IGF-1 after preterm birth, genetic deletion of IGF-1 in mice is illustrative. Mice that do not produce IGF-1 are 40% smaller in body weight at birth compared to the wild-type controls.⁹¹ Additionally, body weight and bone length in human fetuses with a gestational age of 15–37 weeks correlate positively with serum IGF-1 concentrations.^92,93 While some data suggest a role for IGF-1 in the development of many tissues even from the start of the first trimester,^94,95 the increase in circulating fetal IGF-1 during the third trimester suggests that IGF-1 is particularly important at the later stages of pregnancy.

IGF-1 in the fetus during pregnancy is supplied by different sources, including fetal tissue. It is also received through swallowed amniotic fluid that contains IGF-1.^96,97 Nutritional intake by the mother also affects IGF-1 production, especially in response to increasing the levels of (placental) growth hormone, which in turn may regulate fetal growth.⁹⁸ The IGF-1 supply through the mother is particularly important in preterm infants, as the fetus at this early stage does not have an adult pattern of growth hormone (GH) regulation and, thus, is lacking this source of IGF-1 production at birth. Hence, it does not come as a surprise that preterm born infants have a very low serum level of IGF-1 compared to full-term babies. Without external supplementation of IGF-1, these preterm infants do not reach in-utero IGF-1 levels of infants of correspondent gestational age.^93,99–101 In these preterm infants, the IGF-1 levels correlate with postnatal weight. The loss of this important GF after preterm birth and the association of low IGF-1 levels with the later development of ROP suggest that the replacement of IGF-1 may prevent ROP by stimulating normal postnatal growth in the retina.

Being a single chain polypeptide, IGF-1 is similar in structure to (pro)insulin. However, both IGF-1 and insulin have their own distinctive receptors that share a homology of ∼60%.¹⁰² The IGF-1 receptor has a 1,000× higher affinity for IGF-1 than for insulin, while the insulin receptor has a 100× higher affinity for insulin than for IGF-1.¹⁰³ Aside from IGF-1 and growth hormone, the IGF system also includes IGF-2, two receptors, types 1 and 2, IGF-binding proteins (IGFBPs), and IGFBP proteases.¹⁰³ There are six IGFBPs, and ∼98% of all IGF-1 is bound to IGFBP-3,¹⁰⁴ which is also the most abundant form of all IGFBPs.^105,106 IGFBP-3 is important in regulating the action of IGF-1 as it can prolong the half-life of IGF-1¹⁰⁷ due to a stronger affinity of IGFBP-3 to IGF-1 than IGF-1 has to its receptor. The effect of IGF-1 will be augmented when released in proximity to the IGF-1 receptor and attenuated when IGF-1 stays bound to IGFBP-3. IGF-1’s mechanism of action is believed to be at least partly through MAPK and AKT signaling pathways, which stimulate cell growth and proliferation. IGF-1 receptor signals through multiple pathways. One key pathway is regulated by phosphatidylinositol-3 kinase and its downstream partner, the mammalian target of rapamycin, responsible for upregulating the AKT and thereby driving the growth. In addition, IGF-1 is a potent inhibitor of programmed cell death.^108,109

IGF-1 also influences glucose metabolism. Insulin is the major regulator of metabolic processes under normal physiologic conditions and exerts its effect, especially in fat, muscle, and liver cells. However, even though IGFs are better known to induce growth and anabolism, they can also influence glucose homeostasis.¹¹⁰ Laboratory studies confirm that IGF-1 promotes glucose uptake in peripheral tissues.^111,112 The metabolic consequences of IGF-1 deficiency have been studied in a liver-specific IGF-1-deficient mouse model, which show a 75% reduction in circulating IGF-1 and insulin insensitivity in muscle. Treatment with IGF-1 reduces insulin concentrations and improves insulin sensitivity, providing evidence for IGF-1 as an important component of overall insulin action in peripheral tissues.¹¹³ These data suggest that IGF-1 has an important role in metabolism; therefore, it is not surprising that a clear association between preterm babies with ROP and low early postnatal IGF-1 levels is found.¹¹⁴ While full-term babies physiologically show a rapid increase in IGF-1 levels postnatally, preterm infants born before gestational week 33 have a very slow increase in IGF-1.¹¹⁵ After preterm birth, energy requirements will strongly increase, while the maternal supply of nutrition and GFs, such as IGF-1, abruptly stop and often are not replaced, resulting in disturbed glucose metabolism. A recent study shows that plasma insulin levels are inversely correlated with gestational age,¹¹⁶ and increased glucose levels during the first week of life are strongly associated with ROP. Others have shown that premature babies versus full-term infants have some clinical signs of metabolic syndrome, including decreased insulin sensitivity^117–119 and altered adiposity^120,121 that share similarities with type 2 diabetes mellitus. ROP on the other hand shares some similarities with proliferative diabetic retinopathy, such as ischemic and vasoattenuated retinal areas, that may possibly lead to neovascularization. Interestingly, even though early data on insulin treatment within the first week of life show that it seems to increase IGF-I concentrations and improve longitudinal growth, respectively¹²² as mentioned earlier, it may also pose an increased risk to develop ROP.⁸³

A direct association between low IGF-1 and poor retinal vascular growth in IGF-1-deficient mice suggests that the size of the avascular zone (Phase I) may also be dependent on the level of IGF-1, as low IGF-1 levels may add to a decrease in vascular growth in ROP.¹²³

The retina of an infant is incompletely vascularized after preterm birth and has low IGF-I concentrations and low VEGF concentrations compared to those in utero. Thus, vascularization is delayed, and the retina becomes hypoxic. Neovascularization and blindness can occur.^{48,114,123,124} The concentrations of serum IGF-I and the duration of low IGF-I strongly correlate with the severity of ROP.^{48,123,125,126} Clinical studies mentioned earlier, correlate low postnatal IGF-I with the later development of ROP. Hence, IGF-1 supplementation in infants who are at risk for ROP may have the potential to prevent or at least attenuate ROP severity.

IGF-1 in Phases I and II of ROP

The severity of the second or neovascular phase of ROP is predominantly dictated by the inadequate vascularization in Phase I and eventually by the extent of the avascular zone of the retina. IGF-1 knockout mice have a slower growth of normal retinal blood vessels than their wild-type controls.¹²³ This paucity in IGF-1 is also found in human subjects, where low IGF-1 serum levels directly correlate with the severity of ROP, and interestingly may also account for abnormal brain development and possibly aberrant neural retinal function.¹²⁷ Other data derived from studies carried out in humans, link genetic defects of the GH/IGF-1 axis to very low levels of IGF-1, accounting for a decreased retinal vascular density in these patients.¹²⁸ Additionally, IGF-1 controls the maximal VEGF-induced activation of MAPK and AKT in endo thelial cells and, therefore, modulates vessel proliferation and survival,¹²³ essential events in preventing Phase I of ROP.

The second phase of ROP is characterized by vessel proliferation driven by vascular GFs that are produced as a result of retinal hypoxia due to insufficient vascularization. Many evidence exists correlating low IGF-1 with the occurrence of Phase I ROP. Interestingly, the first data that indirectly linked IGF-1 with ROP obtained from work showing that transgenic mice expressing a GH receptor antagonist display significantly less retinal neovascularization at the second phase of ROP-like disease.¹²⁹ These findings were supported by later studies showing that an IGF-1 receptor antagonist reduces retinal neovascularization in vivo. This study also proved that IGF-1 is required for maximum VEGF-induced activation of p44/42 MAPK – an essential pathway for endothelial cell proliferation – and, thus, the neovascularization observed in Phase II of ROP. VEGF alone would be insufficient to induce maximal angiogenesis associated with ROP, and IGF-1 acts as a permissive factor in ROP and possibly in other proliferative retinopathies.¹³⁰

IGF-1 supplementation: a preventative treatment for ROP in preterm infants

Current options in ROP treatment aim at limiting the disastrous effects arising from severe development of Phase II ROP. Laser ablative therapy destroys avascular retina in order to reduce the production of hypoxia driven GFs. Later, these (although peripheral) parts of the retina will never be able to contribute to normal visual perception.

Low systemic IGF-1 levels are associated with the later development of ROP.¹¹⁴ Hence, the importance of sufficient systemic concentrations of IGF-1 reaching the retina through circulation raises the possibility of pharmacological restoration of IGF-1 to in-utero levels as a strategy for countering the vascular degeneration associated with the first phase of ROP. An interventional preclinical study of oxygen-induced retinopathy found that mice that received recombinant human insulin-like growth factor (rhIGF)-1 treatment developed less retinopathy, which supports the possible role of supplemental IGF-1 in preventing ROP.¹³¹

These observations led to the initiation of clinical trials to evaluate the effect of a complex of IGF-1 with its most important binding protein, IGFBP-3, on ROP development. While the half-life of IGF-1 in adults was calculated to be ∼17 hours,¹³² the half-life of IGF-1 in preterm infants is significantly shorter, suggesting that a continuous intravenous infusion would be beneficial. For a better understanding of the pharmacokinetics of IGF-1 and IGFBP-3 in preterm infants, a first clinical study was performed using fresh-frozen adult plasma (FFP).¹³³ FFP from healthy adults has a considerably higher concentration of IGF-1 and IGFBP-3 than those reported for very preterm infants.¹³⁴ In this study, a total of 20 extremely preterm infants were included with a mean gestational age of 20 weeks. FFP was then administered between days 1 and 7 (median day 2) at a mean volume of 11 mL/kg over a time period of ∼120 minutes (range: 90–240 minutes). This one time continuous infusion led to an increase of 133% in IGF-1 levels and 61% in IGFBP-3 levels. The first pharmacokinetic and dosing study of an intravenous IGF-1 and IGFBP-3 complex to preterm infants was carried out in Sweden. This study included preterm infants with a gestational age of 26–29 weeks and a mean birth weight of 1,022 g. This study concluded that an intravenous infusion of an equimolar preparation of rhIGF-1/rhIGFBP-3 to these very preterm infants normalized systemic IGF-1 levels to that found in utero at the corresponding gestational age. The administration of rhIGF-1/rhIGFBP-3 in this study was safe without any incidents of hypoglycemia and was well tolera ted by all infants.¹³⁵

A Phase II trial in preterm infants with a median age of 27 weeks confirmed these results and also showed that a continuous infusion of an IGF-1/IGFBP-3 complex can bring IGF-1 levels back to the lower end of normal values. However, per the protocol, continuous infusion was discontinued after 7 days, and it was observed that thereafter serum IGF-1 concentrations fell below intrauterine levels for the corresponding gestational age. Therefore, a continuous infusion for >7 days was proposed.¹³⁶ The clinical trial (clinical trial identifier: NCT01096784) is recruiting patients, and further results are expected.

Outlook and conclusion

Destroying the avascular retina with laser photocoagulation and injecting a drug in the vitreous that may leak into systemic circulation are problematic in the context of very premature infants, yet both approaches are the only therapeutic means currently available to treat severe ROP.

The results of preclinical and early clinical studies support conducting the trials to examine ROP prevention by supplementing IGF-1 and IGFBP-3 to normal in-utero levels. Clinical studies are currently ongoing to evaluate the effect of this replacement treatment on preventing ROP. If prevention is effective, anti-VEGF injections and particularly laser photocoagulation could be used only in cases where preventative interventions were missed or an insufficient response to initial treatment was observed.