A study to evaluate the effects of CSX-1004 on breathing difficulties caused by fentanyl in healthy adults and people with opioid use disorder

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What is this study about?

This study is being conducted to evaluate the effect of CSX-1004 on respiratory depression, which is a condition where breathing becomes too slow or shallow, caused by fentanyl. The research focuses on two groups: healthy adults and individuals living with opioid use disorder, a chronic condition characterized by the problematic use of opioids. During the study, participants may receive either the test substance CSX-1004 or a placebo containing sodium chloride via IV infusion, which is a method of delivering fluids directly into a vein.

The investigation follows a specific design where participants receive different treatments in a set order to compare the results. Researchers will monitor various physical signs, such as the respiratory rate, which is the number of breaths taken per minute, and oxygen saturation, which measures the amount of oxygen in the blood. Other factors being observed include apnea, which is a temporary pause in breathing, and changes in pupil diameter. Additionally, participants will report their levels of alertness and how they feel regarding the effects of the substances used.

Who Can Join the Study?

You must give your written informed consent, which means signing a document that explains the study details before any tests begin.
You must be an adult, meaning you are between 18 and 55 years old.
Your body mass index (BMI), which is a measure of your weight compared to your height, must be between 18.0 and 32.0 kg/m².
You must weigh at least 50.0 kg.
You must be medically stable, meaning your health condition is steady and not changing rapidly, as confirmed by your medical history, a physical exam, an ECG (a test that records the electrical activity of your heart), and your vital signs (such as blood pressure and heart rate).
You must have a negative urine drug screen for fentanyl, which means a urine test shows no recent use of this specific strong medicine.
You must be willing and able to follow all study rules, including staying at the clinic (the medical facility) for a period and having blood sampling (having blood taken for testing).
For the specific part of the study for certain patients, you must have a diagnosis of mild or moderate opioid use disorder, which is a medical condition involving the use of drugs like opioids.
For that same part, you must report taking a daily amount of oral opioids, measured as MME (morphine milligram equivalents, a standard way to measure the strength of different opioids), between 30 to 120 mg for at least 3 months.

Who Cannot Join the Study?

You cannot join the study if you have a history of or currently have any major health problems involving your psychiatric (mental health), endocrine (hormones), hematologic (blood), hepatic (liver), immunologic (immune system), metabolic (how your body uses energy), urologic (urinary system), pulmonary (lungs), neurologic (brain and nerves), dermatologic (skin), or renal (kidneys) systems.
In the first part of the study, you are excluded if you have a history of or currently meet the medical definition for a substance use disorder, which is a condition where a person has trouble controlling the use of a substance, such as drugs or alcohol, even though it causes harm. This includes all substances except for caffeine.
In the second part of the study, you are excluded if you meet the medical definition for a substance use disorder involving any substance except for opioids, caffeine, or nicotine.
In the second part of the study, you are excluded if you have a history of or currently have severe opioid use disorder, which is a serious medical condition related to the long-term and harmful use of drugs like fentanyl or morphine.
You cannot participate if you are currently taking concomitant (other drugs taken at the same time) medications that are not allowed in this study and that cannot be safely stopped before you receive the first dose of the study drug.

Where you can join this trial?

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Other Sites

Site Name	City	Country	Status
Centre for Human Drug Research	Leiden	The Netherlands

Trial status

Country	Status	Recruitment Start
The Netherlands	Recruiting	01.05.2026

Trial locations

Investigated drugs:

Sodium Chloride

CSX-1004 is an experimental medication administered through an intravenous infusion. It is being studied to see if it can help prevent or reverse the slowing of breathing caused by the use of fentanyl.

Investigated diseases:

Drug use disorder

Opioid use disorder – This condition is characterized by a problematic pattern of using opioid medications. It often involves a strong desire or compulsion to use these substances despite the negative effects they may cause. Over time, the brain’s chemistry changes, leading to a decreased ability to control the urge to use. This process can result in a physical or psychological dependence on the drugs. The condition typically progresses as the body becomes more accustomed to the substances, requiring higher amounts to achieve the same effects.

Trial ID:

2025-524133-33-00

Protocol code:

CSX-1004.201

Trial Phase:

Therapeutic exploratory (Phase II)

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