New research published in The Journal of Physiology shows that giving sildenafil (Viagra), which cause blood vessels to dilate, impairs the foetus’s ability to redirect blood flow to essential organs in response to low levels of oxygen, impeding foetal growth.
This study was conducted in an animal model of growth restriction where placental blood flow is restricted.
It is well accepted that in a foetus, in response to low levels of oxygen throughout the body (called hypoxia) blood flow is redirected to essential organs (such as the brain and heart) and away from non-essential organs, to help sustain life.
While the study was being conducted, a large clinical study was investigating maternal sildenafil in pregnancies affected by foetal growth restriction as a potential treatment to improve foetal growth and correct impaired placental function by increasing blood flow.
This clinical study was halted early, following evidence that maternal Viagra treatment resulted in increased morbidity in babies. However, the reasons behind the increased morbidity were not understood.
The new study provides crucial insight into a potential mechanism underlying increased morbidity. It shows that prenatal sildenafil lowers levels of oxygen in the foetal blood. But despite this, the foetus does not respond by redirecting blood to essential organs. This finding may underlie the adverse outcomes observed clinically.
Viagra is a well-known drug that dilates blood vessels and its ability to increase blood flow to certain areas has been well described in other medical conditions. Therefore, the investigation into its potential for improving foetal growth by increasing placental blood flow was warranted.
However, scientists have shown that Viagra can cross the placenta and have effects on the foetus. The results of the study highlight the importance of thorough pre-clinical investigation in large animal models prior to the initiation of large clinical trials.
The study replicated the impaired placental function that was observed clinically in foetal sheep as a pre-clinical model.
Lead author Ishmael Inocencio said: “A key finding of our study was the unexpected exacerbation of growth restriction following Viagra treatment. This has important implications for the danger of administering this drug to pregnant women.”