Preclinical research is an important step of the drug and treatment development process that ensures the safety of a potential treatment modality for human testing. But until recently, most preclinical research studies in animals were only conducted on one sex: typically, male. This tendency to exclude female subjects in preclinical studies had implications for clinical trials and for understanding underlying mechanisms of biological function and disease.

“For decades, clinical trials failed women,” said Catherine Christian-Hinman, an Associate Professor of Molecular & Integrative Physiology who studies the neural mechanisms linking epilepsy and comorbid reproductive endocrine disorders. “Until U.S. law changed in 1993, many clinical trials of common drugs didn’t include women, and there are ramifications when those drugs are released to the general population.” This bias extended into preclinical animal research, with the vast majority of studies across several disciplines only describing effects in male animals.

This discrepancy led to a 2015 policy by the National Institutes of Health that requires sex as a biological variable (SABV) to be considered in research design, analysis, and reporting. However, a recent review published in Epilepsy Currents by Christian-Hinman on SABV in relation to preclinical epilepsy research sheds light on the perfunctory enforcement of this policy and disparities that persist.

Christian-Hinman spoke to the School of MCB Communications Office about the history surrounding these inequalities and advocating for improvement.

Why has neuroscience research historically prioritized the study of males?

There has been a general assumption that things seen in males will translate to females adequately enough to circumvent the effort and expense of studying both male and female animals. There’s also the thought that an ovarian cycle and changes in ovarian hormones can induce more variability in female animals, which might make the research more complicated or nuanced.

How has the implementation of the NIH’s SABV policy fallen short of its intended goals?

At this point, anyone applying for an NIH grant knows they are expected to address effects on both sexes, but there are still publications — even in top journals — that study one sex only, and it’s typically male. There hasn’t been a lot of enforcement on the back end. I think the best way to address the problem is for more journals to actually enforce the NIH policy requiring research on both male and female subjects, unless there is a clear scientific justification to only study one sex.

What are some outstanding questions about the potential impact of an ovarian cycle on epilepsy?

The most prominent relationship we see is in catamenial epilepsy, which is characterized by seizure exacerbations at certain points of the menstrual cycle — usually around menstruation or ovulation. This relationship led to the hypothesis that since ovarian hormone levels are changing, the ovarian hormones must be driving the changes in seizure frequency. But treatments designed around those changes haven’t actually worked that well. So, there’s still a lot to be learned: Why does one woman with epilepsy have more seizures around menstruation, while another has more seizures around ovulation, and another’s seizure pattern is completely unpredictable?

One of your suggestions was for researchers to conduct female-only studies that confirm the results of similar studies in males, and for those confirmatory studies to be treated as novel research. Why is that important?

If female subjects haven’t been considered in a given study, we don’t actually know if they will react identically to the male subjects used. In my mind, that makes it totally different research. Removing the assumption that everything will be identical leaves the possibility that they could be different. We can’t assume whether or not there will be a difference between the sexes. The problem is that there’s a bias in publications where “replication studies” are not considered novel, so the original study using only males may be published in a more prominent journal than the female-only follow-up. The latter is just viewed as replication, even though there may be differences. And if the follow-up study is not as high-profile, the public and research community may not be aware of it.

How has your lab’s research benefited from studying both sexes?

We are able to answer so many more questions, and our research would be so much poorer if we only focused on one sex. At the end of the day, it is truly to the benefit of science to study both.