The National Toxicology Program (NTP) published three new reports Sept. 25 on an herbicide, a class of pesticides, and a hormone-mimicking drug. The publications are among the first scoping reviews — a new type of NTP report.
“A scoping review identifies what studies are available on a topic in order to inform decisions on future research or analysis,” said Kembra Howdeshell, Ph.D., a scientist at NIEHS. Scoping reviews, conducted with the same rigor and transparency as systematic reviews, present a high-level overview. Systematic reviews synthesize all the data available.
The scoping reviews reveal which health effects have been studied, and in what model system, such as human or animal. “The reviews are intended to be a springboard for further studies,” said Howdeshell. For example, if a scoping review finds either no studies or studies with conflicting results on a health outcome following exposure, the information can be used to design future studies.
As a communication tool, these scoping reviews include interactive, sortable evidence maps (see sidebar) of relevant studies. Researchers can consider the health effects data using these visualizations to explore data rich and data poor areas of research.
Herbicides and health effects
Every evaluation begins with the development of a specific question to structure the scoping review. For example, is there evidence that exposure to the herbicide paraquat contributes(https://ntp.niehs.nih.gov/whatwestudy/assessments/noncancer/completed/parkinsons/index.html) to Parkinson’s disease?
NTP identified observational studies of agricultural workers and data from animal studies suggesting that long-term exposure to paraquat might lead to central nervous system toxicity. The new report(https://ntp.niehs.nih.gov/ntp/results/pubs/rr/reports/rr16_508.pdf) found the available studies could support a systematic review to reach conclusions. It includes evidence maps(https://ntp.niehs.nih.gov/whatwestudy/assessments/noncancer/completed/parkinsons/data/index.html) and identifies research gaps for future studies to address.
Scoping reviews are particularly useful when a topic is complex, such as summarizing health effects data on the seven main neonicotinoid pesticides. Neonicotinoid(https://ntp.niehs.nih.gov/whatwestudy/assessments/noncancer/completed/neonicotinoid/index.html) pesticides are a class of agricultural chemicals used to control insects on crops and domestic animals. Just as these pesticides are neurotoxic to insects, they might also harm humans.
The scoping review(https://ntp.niehs.nih.gov/publications/reports/rr/rr15/index.html) revealed that most of the research (127 of 191 relevant studies) focused on just one of the seven, called imidacloprid. The publicly available evidence has limited ability to support conclusions, primarily due to differences between health outcomes assessed across studies.
Hormone-mimicking drugs and development
Progesterone is a hormone produced by the body that is important for maintaining pregnancy, among other reproductive functions. Drugs that mimic progesterone, called progestogens(https://ntp.niehs.nih.gov/whatwestudy/assessments/noncancer/completed/progestogens/index.html), are prescribed for contraception, infertility, preventing miscarriage, and other reasons.
Hormonal imbalance during pregnancy may lead to adverse effects on the development of the fetal reproductive system. Thus, NTP asked what impact might a drug that mimics progesterone have on offspring, when exposure occurs during pregnancy?
The scoping report(https://ntp.niehs.nih.gov/publications/reports/rr/rr17/index.html) identified 212 relevant studies. The studies evaluated different health outcomes and had inconsistent results, so NTP did not recommend conducting a systematic review on the current evidence. More research is needed to clarify potential effects of prenatal exposure to these drugs.
The future of scoping reviews
Scoping reviews support evidence-based decisions and can be used for surveillance of research on emerging topics. However, tasks in the scoping and evidence mapping process are time and resource intensive.
“Further developments in text mining and machine learning capabilities will allow DNTP to gain efficiencies in our scoping reviews,“ said Vickie Walker, a scientist at NIEHS. “We will be able to tackle bigger datasets and not have to limit our questions to narrow topics based on workflow.”
Scientists at NIEHS are identifying, developing, and implementing new technologies to make the scoping review process more efficient without compromising quality.
NTP(https://ntp.niehs.nih.gov/ntp/results/pubs/rr/reports/rr16_508.pdf). 2020. NTP Research Report on the Scoping Review of Paraquat Dichloride Exposure and Parkinson’s Disease. NTP Research Report 16. Research Triangle Park, NC: National Toxicology Program.
NTP(https://ntp.niehs.nih.gov/ntp/results/pubs/rr/reports/rr15_508.pdf). 2020. NTP Research Report on the Scoping Review of Potential Human Health Effects Associated with Exposures to Neonicotinoid Pesticides. NTP Research Report 15. Research Triangle Park, NC: National Toxicology Program.
NTP(https://ntp.niehs.nih.gov/ntp/results/pubs/rr/reports/rr17_508.pdf). 2020. NTP Research Report on the Scoping Review of Prenatal Exposure to Progestogens and Adverse Health Outcomes. NTP Research Report 17. Research Triangle Park, NC: National Toxicology Program.
(Sheena Scruggs, Ph.D., is digital outreach coordinator for the NIEHS Office of Communications and Public Liaison.)