Fetal Exposure to Cannabinoids: Exposure, Methylation and Neurodevelopmental Effects

Cannabis is a very popular drug for both recreational and medicinal use. An estimated 20% of adults in the United States report using cannabis in the past month, and this number continues to increase each year. As of 2018, medicinal use of cannabis is legal in 33 states and the District of Columbia. Recreational use is legal in 10 states, and it is decriminalized in 15 states. Hemp-derived cannabidiol (CBD) is legal in all states. Due to the rapidly changing legal status across the country, the demand for tetrahydrocannabinoid (THC) and CBD products is also rapidly increasing. Studies have shown a significant increase in marijuana use among pregnant and parenting women following state-wide legalization, and this could have significant implications for the health and development of children born to these women.

Despite its widespread use, and increased interests in THC for medical treatment, significant gaps exist in the investigator's scientific knowledge of the pharmacology and pharmacokinetics of THC and its metabolites. When inhaled, THC and CBD reach maximum concentrations within seconds. Bioavailability varies widely and is a function of inhalation time, number, interval, and duration of puffs, as well as within product variability. Importantly, smokers have greater bioavailability of THC than non-smokers. When THC and CBD are orally consumed, the bioavailability is relatively low (< 20%). However, the lipophilicity of both THC and CBD make these compounds of great interest with respect to tissue distribution. CBD is more lipophilic than THC and thus more likely to be absorbed with topic application. THC is known to cross the placenta and can reach the fetus]. The metabolism of both THC and CBD is complex. Following absorption, THC and CBD are rapidly distributed to tissues with high blood flow. Metabolism occurs in the liver via CYP2C9, CYP2C19, and CYP3A4, as well as the brain, small intestine, heart and lung. Primary metabolites undergo glucuronidation or conjugation reactions. The elimination half-lives are prolonged due to the deposition of drug and metabolites in fat.

Some previous epidemiological studies have shown that children born to mothers who report using THC during their pregnancy have a higher prevalence of neurodevelopmental and behavioral abnormalities. While these studies are sparse, there have been some studies that follow longitudinal cohorts worldwide. These epidemiological, survey-based studies have reported an increase in executive functioning deficits in infants who were exposed to THC in utero. These studies have also shown an increase in attention problems and hyperactivity in these children during early childhood and later adolescence. However, these studies have reported inconsistent outcomes. Some have reported negative effects on cognitive or executive functioning, and others have reported no significant changes. One possibility for these discrepancies is that survey-based epidemiological studies do not accurately capture the exact levels of fetal exposure due to the unreliable nature of self-reporting, making it difficult to accurately correlate prenatal exposure levels to neurodevelopment. This makes the need for a well-designed, quantitative exposure study that is followed by longitudinal neurodevelopmental outcome measurement imperative to determining true correlations between prenatal THC use and developmental outcomes in children.

It has also previously been established that cannabis use during pregnancy can be detrimental to the developing fetus. Prenatal use of THC has been shown to cause low birth weight in the absence of any change in neonatal length or head circumference, as well as increased tremors and disrupted sleep patterns. It has also been correlated with an increased need for placement in the neonatal intensive care unit. In addition to human studies, animal studies have shown changes in the endocannabinoid system (ECS) in the fetal brain during development. The ECS is present in the developing brain from the time of 5 weeks gestation, and is required for proliferation, differentiation, and migration of neurons. Any changes to this system during development could have important implications for further neural development. Several studies have observed changes in postsynaptic target selectivity, differentiation of developing axons, and disruption of position in the ECS in animal fetuses that were exposed to THC in utero. These central nervous system alterations could potentially point to long-term neurodevelopmental changes in children born to mothers who used cannabis during pregnancy. However, while animal studies are able to report exact dosing levels, animal models of prenatal cannabinoid use often do not accurately reflect human use.

While some detrimental effects to the fetus following prenatal cannabis use have been observed in both animal models and human studies, there remains the mechanistic question of what is causing these changes. Some studies have proposed epigenetic reprogramming that may occur in the mothers and the babies following prenatal cannabis use. Prior studies have focused on the effects of prenatal cannabis use increasing the likelihood of the children's substance abuse later in life, and have found positive correlations to DNA methylation and histone modification in these cases. Additionally, it has been shown that paternal cannabis use alters DNA methylation in sperm of rats and humans, which could also have important implications for development. However, to-date, there have not been any studies correlating epigenetic changes and neurodevelopmental changes in infants prenatally exposed to cannabis.

Additionally, many of the prior human studies were done at a time when THC content in marijuana was not as high as is in today's market, which could have major implications for actual exposure levels in the fetus. The initial studies that evaluated the effects of prenatal cannabis use on fetal outcomes were done in the early 1990s, at a time when average THC potency in seized marijuana in the United States was approximately 4%. The most recently reported potency of seized illicit cannabis plant material is approximately 12%. Additionally, the ratio of psychoactive THC content to non-psychoactive cannabidiol content has increased from approximately 14 to 80 between 1995 and 2014. These factors suggest a need for more updated studies that examine the effects of current high potency levels of fetal exposure to cannabis, as an increase in potency may have greater effects on the developing fetus.

While there have been some prior studies measuring the effects of fetal exposure to THC specifically, there have been no studies to-date that have evaluated the effects of CBD exposure on the developing fetus. CBD (also known as hemp) supposedly lacks psychoactive THC components, making it legal, widely available, and generally perceived to be safe.CBD is touted as a treatment for a number of conditions, including depression, anxiety, pain, and cancer. These factors, in addition to strong marketing campaigns, have most likely caused an increase in prenatal CBD use in recent years. A recent study conducted in mice found that CBD activates a large range of CYPP450 enzymes. While these data are preliminary, CYPP450 activation raises the concern of drug-drug interactions and suggests that biologic activity of CBD is not fully understood. However, there are no published studies evaluating the effects of prenatal CBD use on fetal outcomes or neurodevelopment. Additionally, many CBD products have been shown to contain varying concentrations of CBD, or are contaminated with synthetic cannabinoids (e.g. K2, "spice") or THC. The investigators will establish the exact composition of the products the pregnant women are taking in order to properly evaluate potential exposure levels in the developing fetus. The investigators will then correlate that exposure with neurodevelopmental outcomes.