Preclinical Studies and Fetal Growth Restriction Therapies

Sildenafil and Fetal Growth Restriction (FGR)

  • Preclinical studies can inform the development of therapeutics for the fetal period, particularly in the context of FGR.
  • Sildenafil (Viagra) was investigated as a potential therapeutic to improve neurodevelopmental and cardiovascular outcomes for FGR babies.
  • Sildenafil is a phosphodiesterase 5 inhibitor that potentiates nitric oxide, promoting vasodilation and increased blood flow.

Initial Hypothesis

The idea was to administer sildenafil to pregnant mothers with growth-restricted fetuses to vasodilate the placenta and increase blood flow to the fetus.

Sheep Study (2009)
  • A study was conducted on pregnant sheep in 2009 to examine the impact of sildenafil on placental blood flow.
  • Twin fetuses were used: one growth-restricted (induced via single umbilical artery ligation) and one control.
  • Uterine blood flow and fetal physiology (heart rate, blood pressure, oxygen levels) were monitored during sildenafil administration.
Findings of Sheep Study
  • FGR fetuses had lower oxygen levels, as expected due to placental insufficiency.
  • Fetal blood pressure was slightly raised in FGR fetuses.
  • No difference in uterine blood flow between groups initially.
  • Sildenafil administration led to a significant reduction in uterine blood flow in both control and FGR fetuses.
  • Blood pressure decreased in FGR fetuses, and oxygen levels did not improve (unsurprising given reduced uterine blood flow).
  • Conclusion: Sildenafil worsened hypoxia in FGR fetuses and had detrimental cardiovascular effects (PaO2PaO_2).
  • The study suggested sildenafil may not be appropriate for pregnancies complicated by intrauterine growth restriction (IUGR) and should be used with caution.

Human Clinical Trial (STRIDER)

  • Despite the sheep study, human clinical trials (STRIDER - sildenafil therapy in dismal prognosis, early-onset fetal growth restriction) were initiated due to positive rodent studies.
  • Rodent models showed sildenafil increased fetal size and prolonged pregnancy length.
  • The STRIDER trial was conducted across multiple countries (Australia, New Zealand, UK, Netherlands, Canada, Ireland) with varying inclusion criteria.
STRIDER Trial Results and Halting
  • The Australia/New Zealand and UK trials found that sildenafil did not prolong pregnancy or improve fetal growth, but they also found no evidence of harm.
  • The Dutch trial was halted in July 2018 due to 11 unexpected neonatal deaths.
  • Lung complications, specifically pulmonary hypertension, were more common in the sildenafil-treated group.
  • 17 out of 90 sildenafil-treated babies suffered serious lung conditions, compared to 3 in the placebo group.
  • 11 babies with lung complications died before discharge.
Timeline Summary
  • 2009: Sheep study showed negative impacts of sildenafil on fetal physiology.
  • 2009-2014: Positive rodent studies on sildenafil and FGR.
  • STRIDER trial commenced, based on rodent studies.
  • STRIDER trial halted in 2018 due to fetal deaths.
Retrospective Analysis
  • A comprehensive literature search for animal studies may have prevented the STRIDER trial.
  • The STRIDER trial cited a sheep study with nutritional restriction in early pregnancy that promoted placental growth but did not report fetal physiological measures.
  • The STRIDA consortium strongly advised against prescribing sildenafil for FGR pregnancies.

Additional Research on Sildenafil

  • Research showed that sildenafil did not improve oxygen levels in growth-restricted fetuses.
  • Measurements of carotid and femoral blood flow revealed that sildenafil induced global vasodilation in the fetus, rather than the expected brain-sparing mechanism.
  • Increase in femoral blood flow, indicating vasodilation in the periphery.
Working Hypothesis
  • Sildenafil vasodilates blood vessels throughout the mother's body, drawing blood away from the placenta.
  • This worsens placental blood flow and placental insufficiency, leading to negative impacts on the fetus.
  • Vasodilation of maternal blood vessels steals cardiac output away from the placenta.

Collaboration Between Preclinical Scientists and Clinicians

  • Improved collaboration is crucial to ensure preclinical work informs clinical trials.
  • Rodent models have limitations in understanding fetal physiology in response to in-utero insults.
  • Sheep models can provide explicit understanding of changes occurring within the fetus.

Summary of Lecture

  • The fetal environment during pregnancy impacts fetal development.
  • Fetuses respond to compromise with defensive adaptive physiology.
  • Some responses can be detected during pregnancy and may benefit from antenatal treatment.
  • Multiple organ systems can be affected, and treatments should consider multi-organ effects.
  • Animal studies assist in understanding the physiology of fetal compromise and long-term outcomes.

Potential Treatments for Fetal Growth Restriction: Melatonin

  • Currently, there is no approved treatment for fetal growth restriction.
  • One potential therapeutic is melatonin, which has anti-inflammatory and antioxidant benefits.

Melatonin's Mechanism of Action

  • Melatonin could increase oxidant defenses and reduce oxidative stress in FGR fetuses.
  • Melatonin impacts on a hypoxic cell by:
    • Reducing reactive oxygen species
    • Preserving mitochondria
    • Exhibiting anti-apoptotic effects
    • Exhibiting anti-inflammatory effects

Single Umbilical Artery Ligation (SUAL) Model and Melatonin

  • The SUAL model induces placental insufficiency, leading to a growth-restricted fetus.
  • Melatonin was administered to the mother throughout gestation after SUAL.
Outcomes of Melatonin Treatment in Lambs
  • FGR was effectively induced, resulting in growth-restricted lambs.
  • FGR lambs showed significant changes in the brain, including reduced myelination and oligodendrocytes.
  • Oxidative stress was increased in FGR lambs.
  • Melatonin improved myelination and reduced oxidative stress.
Behavioral Observations
  • Melatonin restored the lamb's ability to stand and get milk from its mother at a quicker rate.
  • Untreated FGR lambs had trouble standing and showed signs indicative of cerebral palsy.
  • Melatonin-treated lambs were stronger and more robust.
Cardiovascular Impact
  • FGR resulted in a decreased ability of blood vessels to dilate, a sign of endothelial dysfunction.
  • Melatonin improved blood vessel function at four weeks of age, indicating improved placental blood flow.
Melatonin's Effects
  • It improves placental blood flow.
  • It reduces oxidative stress (MDA, a marker of oxidative stress, is significantly reduced).
  • It does not prolong pregnancy but reduces the incidence of brain hemorrhage and improves cardiovascular outcomes.

Fetal Growth Restriction and Cerebral Palsy

Brain Development

  • Brain development occurs steadily throughout gestation, from neural tube formation to synaptogenesis and myelination.
  • There is critical brain development in the last weeks of gestation.
  • Gray matter development occurs earlier in gestation, while white matter development and myelination occur from approximately 20 weeks to term.
  • IUGR often occurs around 20 weeks of gestation, impacting the developing fetus throughout the last trimester.

Consequences of FGR on Brain Structure and Development

  • Both gray matter and white matter can be impacted by FGR.
  • Reduced head circumference, total gray matter volume, hippocampal and cerebellar volume, number of cells, and delayed myelination.
  • Functionally, this can result in reduced motor skills, clumsiness, and cerebral palsy.

Cerebral Palsy

  • Cerebral palsy is a physical disability affecting movement and posture.
  • It is a heterogeneous condition with multiple causal pathways.
  • It is the most common physical disability in childhood, often resulting from a lesion in the brain during development.
  • The lesion is non-progressive and predominantly affects the white matter of the brain (periventricular leukomalacia).
Causal Pathways for Cerebral Palsy
  • Growth restriction and hypoxic-ischemic encephalopathy are key causal pathways.
  • Critical events such as oxidative stress and inflammation can lead to cell death and further injury to the developing brain.
  • Infants with FGR have an eightfold increased risk of cerebral palsy.
  • Mechanisms of brain injury include inflammation, swelling, cell death, scar formation, changes in blood vessels, and the release of damaging factors.

Fetal Growth Restriction (FGR) Defined

  • Fetal growth restriction is a disease where the developing fetus does not grow to its genetic potential.
  • Important to distinguish between FGR babies and small for gestational age (SGA) babies.
  • FGR recognizes a pathological pregnancy, while SGA does not necessarily imply pathology.
  • SGA and FGR are not synonymous.
  • SGA is defined as a birth weight below the 10th centile for gestational age.
  • FGR is ideally detected by a diminishing growth velocity (trajectory of growth).

Outcomes Associated with FGR

  • Pulmonary: bronchopulmonary dysplasia (neonate), COPD (adulthood)
  • Cardiovascular: poorer transition (neonate), coronary heart disease, hypertension, diabetes (adulthood)
  • Neurological: intraventricular hemorrhage (IVH), cerebral palsy

Mechanisms of Injury in Fetal Growth Restriction

  • Fetuses have robust mechanisms to detect fetal hypoxia.
  • The carotid rim chemoreflex elicits cardiovascular adaptations to prioritize blood flow to essential organs (brain, heart, adrenal glands).
  • Peripheral blood vessels vasoconstrict, and blood vessels in vital organs vasodilate.
  • FGR occurs secondary to placental insufficiency, leading to chronic fetal hypoxia and brain sparing.
Cardiovascular Adaptations
  • Brain sparing occurs with maladaptation of the body as a result of maintained adaptations.
  • Vasodilation to essential organs and vasoconstriction to nonessential organs.
  • Doppler in humans can detect changes in blood flow patterns, indicating the severity and timing of FGR. These Doppler waveforms show loss of normal blood flow patterns.
Pathogenesis of FGR
  • A mixture of hypoxia, oxidative stress, and inflammation plays a key role in the pathogenesis of FGR.
Cellular and Biochemical Events
  • Placental insufficiency leads to reduction in oxygen and glucose delivery, and reduction in the substances the fetus is responsible for getting rid of.
  • Hypoxia leads to a cascade of cellular and biochemical events, mitochondrial dysfunction, cellular injury, and cell death.
  • Mitochondrial dysfunction results in oxidant release.
The Role of Oxidative Stress
  • The developing fetus has a poor antioxidant defense, resulting in oxidative stress.
  • Oxidative stress leads to DNA damage, lipid peroxidation, reduced nitric oxide bioavailability, and endothelial dysfunction.
Inflammatory Response
  • Inflammation leads to an increase in NF kappa b, an increase in the pro-inflammatory cascade, and tissue-specific injury.
Developmental Programming of Disease
  • David Barker (1989) found that being born small increased the risk of cardiovascular disease in adulthood.
  • Cardiovascular disease is evident throughout the lifespan of those born small.
  • Children: systolic dysfunction, decreased stroke volume and cardiac output, increased blood pressure.
  • Infants: increased cardiac globularity, altered cardiac function, increased hospital stays.

Antenatal Interventions to Improve Long-Term Outcomes Following Suboptimal Pregnancy

Learning Objectives

  • Evaluate the connection between placental insufficiency and fetal growth restriction (FGR).
  • Explain why fetuses exposed to fetal growth restriction or preterm birth are at a higher risk of cerebral palsy.
  • Discuss the potential use of melatonin to treat or reduce the impact of fetal growth restriction.
  • Discuss the role of preclinical studies in the development of treatments for pregnancy-related diseases such as fetal growth restriction.

Impact of Pregnancy Complications on Developing Fetus

  • The heart, lungs, and brain are some of the organs that can be significantly affected.
  • Pregnancy complications that can impact the developing fetus include: chorionitis, umbilical cord compression, placental insufficiency, intrauterine growth restriction, perinatal asphyxia, maternal infection, and preeclampsia.

Antenatal Therapies

  • Options include surgery and, if life-threatening, termination
  • Fetal therapies is an incredibly complex area of science and clinical care.

Requirements for Successful Therapy

  • Diagnosis of a problem
  • Understanding of the pathophysiology that is ongoing within that condition.
  • Timing of the intervention and which organs are going to be affected by that intervention at any given moment of gestation.

Types of Therapies

  • Non-invasive:
    • Preventative therapies (e.g., folic acid)
    • Therapeutic
  • Invasive:
    • Termination of pregnancy
    • Fetal endoscopic surgery
    • Open fetal surgery
The Victorian Fetal Therapy Service
  • A collaboration of expert obstetricians trained in fetal surgery.
  • Treatments include twin-to-twin transfusion syndrome, spina bifida, congenital diaphragmatic hernia, congenital lung malformations, and congenital heart defects.

Considerations for Anetenatal Therapy

  • Ethical considerations
  • Cost-benefits analyses
  • Potential harm of any drug given, and how this might impact the development of the fetus and in and and the global impact of this therapy on multiple organs
  • Assessment of long term outcome