Where are the female mice in drug testing?

A study has found that female mice are massively underrepresented in drug testing trials, despite the fact that gender-specific reactions occur very frequently.

When discussing the gender imbalance in scientific testing, the focus is often on the underrepresentation of women in drug trials. However, a new study by neuroscientist Rachel Hill suggests that the problem actually arises far earlier in the testing process in the laboratory, where the vast majority of trials using mice don’t include any females at all. We talk to her to find out why this is the case and what this means for the women who end up taking the drugs.

ResearchGate: What were the key findings from your research into the gender breakup of mice used in scientific studies?

Rachel Hill: Overall my review found that a surprisingly large percentage of animal studies (around 75-80 percent) in schizophrenia research used male mice only. In addition, of the studies that did use both sexes, a large portion were underpowered. This means that the numbers from each sex involved in the study were so low that no statistically strong sex differences could be detected. Meanwhile, in 80 percent of the trials that did use a statistically significant number of male and female mice sex-specific results were actually found – so male and female mice reacted differently to the treatment.

RG: Do you feel the sample size and scope of studies you examined were sufficient to make a general statement about the gender imbalance in mice used for scientific research?

Hill: Yes, I do. I reviewed a total of 710 original articles and found that only 111 used both males and females. These articles were on a range of animal models of schizophrenia, including genetic models, drug-induced models, and developmental models.

RG: What impact could this imbalance have on humans? 

Hill: We could be missing vital information on how specific genes may interact with sex steroid hormones to influence brain function and behavior. We could also be very misinformed about how certain psychotrophic drugs act in the female brain as a large percentage of studies using drug induced models only tested males – in fact, only 9 out of 191 we reviewed included females.

RG: In what ways has it been shown that female mice have different reactions to medications and diseases than male mice?

Hill: It has been shown that female mice and rats metabolize certain drugs at a much slower rate. In addition, their response to stress and inflammation affect different areas of the brain. For examples, my review found that the male prefrontal cortex is more vulnerable to stress and inflammation during early brain development, while the female hippocampus tends to be more vulnerable – this may explain why females tend to be more susceptible to anxiety or mood disorders that are thought to arise from hippocampal dysfunction, while males tend to be more susceptible to schizophrenia, which is associated with prefrontal cortical dysfunction.

Furthermore, some of our earlier work has shown that certain genes that are known to be dysfunctional in schizophrenia are regulated by the sex steroid hormone estrogen. Here, we found that female mice deficient in this gene showed a very different response than males, including learning and memory impairments.

RG: What reasons are being given by researchers for not using female mice in their experiments?

Hill: Firstly, it’s a matter of cost – you are essentially doubling the amount you’re spending by adding females. Secondly, female mice have an estrous cycle similar to our menstrual cycle, however it is two to three days, rather than 28 days. The large changes in estrogen and progesterone associated with their cycle can affect their behavior. Therefore, many scientists claim this is too difficult to control for, so it’s easier to simply not include females. I very much disagree with this as it is easy to test at which stage of the cycle the mice are in – and this just adds more information on how steroid hormones may be influencing our experimental tests.

RG: Are any changes being implemented to avoid this gender imbalance in the future?

Hill: The National Health and Medical Research Council now ask all grant reviewers to question whether proposed animal experiments include both sexes, and, if they do not, to question why. This is a big step forward.

RG: What are you working on now?

Hill: Our research showed that the sex steroid hormones estrogen improves cognitive – or learning and memory –  function by acting on a specific group of neurons within the hippocampus of the brain. However, estrogen is not a viable treatment option due to its unwanted systemic endocrine effects. We’re working on a way around this by testing a new drug called raloxifene, which acts like estrogen in the brain, but blocks the unwanted effects of estrogen in other tissue such as breast. Our research shows that raloxifene regulates the same group of cells identified in the hippocampus in which estrogen has beneficial effects. This is highly significant as cognitive impairments are prevalent in schizophrenia and there is no current treatment that addresses this symptom. Raloxifene could be a new adjunctive treatment to be used alongside antipsychotics to help with these impairments.

Image courtesy of Nuwandalice.