New CO2 challenge setup ready for proof-of-mechanism studies with anxiolytic drugs

‘CHDR has been using the CO2 challenge in several proof-of-mechanism studies since 2018. However, its hardware and software have become outdated, and the supplier has terminated its technical support. As the Psychiatry Group wanted to continue providing the CO2 challenge in early psychiatric drug development, we assembled our own system using commercially available hardware components,’ says Ingrid van den Heuvel, Clinical Research Engineer at BEA. ‘The challenge test has essentially remained the same, and the in-house developed software allows for standardised challenge administration and data capture. The capacity has also been expanded from two to three challenge setups.’

‘As we already started building the new CO2 setups in 2024, they were ready in time for a study with a novel anxiolytic compound we are currently conducting,’ says Olivier Ducazu, Clinical Scientist at the Psychiatry Group. ‘It is the largest CO2 study CHDR has ever conducted. The multiple dose crossover study involving 60 participants will compare the potential anxiolytic effects of three dose levels of the compound with placebo and a benzodiazepine.’ This study, which started in 2025, will continue until the summer of 2026.

The CO2 challenge is a safe experimental model that can be used to investigate the potential anxiolytic effects of a drug in healthy study participants under controlled conditions. The panic model is based on the fact that adding 35% CO2 to inhaled air induces both objective and subjective symptoms of panic. The symptoms, which are transient and self-limiting, include objective changes such as an increase in heart rate, blood pressure and stress hormone production and subjective feelings of dread and fear of losing control. When a participant is administered a drug with established anxiolytic effects, such as alprazolam, such symptoms are significantly reduced or even absent. About half of all healthy individuals are sensitive enough to CO2 to participate in this challenge. ‘We only include participants who are sensitive to the effects of CO2 in our studies with this procedure. In our screening procedure, we exclude participants with relevant psychiatric disorders in their history, and we establish participants’ sensitivity to CO2. We aim to guarantee the safety of participants and the reliability of the CO2 challenge,’ Van den Heuvel explains.

‘We quantify the three most relevant aspects of panic and anxiety in response to CO2: subjective experience, sympathetic nervous system activity and neuroendocrine activation,’ says Ducazu. ‘Directly before and after CO2 inhalation, participants are asked to rate their subjective feelings of fear and discomfort on a visual analogue scale (VAS), and to rate the presence of a variety of symptoms associated with panic or anxiety. Furthermore, immediately after inhaling the gas mixture, we ask the participant to continuously indicate the degree of fear using a visual analogue slider. Blood pressure and heart rate are measured continuously prior to, during and following CO2 administration, capturing the sympathetic response associated with anxiety or panic. Finally, blood samples are taken prior to, during and following CO2 administration to determine prolactin, ACTH and cortisol as measures for CO2-induced neuroendocrine activation.’

The new CO2 setup developed by Van den Heuvel and her colleagues contains two important innovations compared with the previous setup. First, the visual analogue slider was added to allow for real-time assessment of subjective fear and anxiety as participants experience them, as opposed to the retrospective VAS assessment. Second, the continuous blood pressure measurement has been refined further by calibrating the finger cuff at the start of the measurement via a blood pressure cuff on the upper arm. ‘This calibration ensures that the finger cuff yields reliable blood pressure data,’ says Van den Heuvel.

Ducazu concludes: ‘I hope that with this CO2 challenge study, along with another study that a colleague of mine is conducting with our NeuroCart®, we will be able to contribute to the clinical development of this promising new compound by characterising both its pharmacodynamic profile and its potential anxiolytic effects.’