RecruitingPhase IInterventional

A phase 1 trial in healthy adults and adults with increased inherited risk of developing blood clots

This study is about a new investigational medicine called CITY-FXI. It's the first time it will be given to people, and we want to find out if it's safe, how well the body handles it, and how it works. CITY-FXI aims to prevent harmful blood clots, like those that can cause deep vein thrombosis or pulmonary embolism. While current clot-preventing medicines work well, they can increase the risk of serious bleeding. CITY-FXI is designed to reduce a specific protein involved in clotting, potentially lowering clot risk with less bleeding. We're inviting healthy adults and adults who have certain gene changes (Factor V Leiden or prothrombin G20210A mutation) that put them at more risk of blood clots. Your participation could help us develop safer ways to prevent these serious conditions in the future.

At a glance

Status

Recruiting

Phase

Phase I

Sponsor

City Therapeutics Inc.

Enrolment target

128

Start

22 Jan 2026

Estimated completion

01 Aug 2026

Factor V Leiden or prothrombin G20210A mutation in healthy adults and adults

What is this study about?

This study is looking into a new medicine called CITY-FXI. It's being tested for the very first time in people. The main goal is to see if it's safe and how your body reacts to it. We also want to understand if it could help prevent dangerous blood clots. Blood clots are sticky clumps that can form in your blood vessels and block blood flow, which can sometimes be very serious, like in your legs (deep vein thrombosis) or lungs (pulmonary embolism).

Right now, there are medicines to prevent blood clots, and they work well. However, they can sometimes cause serious bleeding. There's a real need for new medicines that can prevent clots without this higher risk of bleeding. CITY-FXI works by targeting a specific protein in your blood called Factor XI. By lowering this protein, we hope to reduce the chances of harmful clots forming, possibly with less risk of bleeding than existing treatments. We don't yet know for sure if CITY-FXI will have this effect in people.

The study is divided into two main groups. One group will include healthy adults, and the other will include adults who have a higher chance of developing blood clots because of specific gene changes (called Factor V Leiden or prothrombin G20210A mutation). By including both groups, we can learn more about how CITY-FXI works in different people. The information we gather will be hugely important for possibly developing safer ways to prevent dangerous blood clots in the future.

Key takeaways

This is the first time the new medicine, CITY-FXI, will be given to people.
The main aim is to check the safety and how the medicine acts in the body.
It hopes to prevent harmful blood clots with potentially less bleeding risk.
You might receive the new medicine or a dummy medicine (placebo).
Participation could last around 13 months and includes regular health checks.
Results from this study could help future patients at risk of blood clots.

Who may be eligible?

We are looking for certain people to take part in this study. For the first part of the study, we need healthy men and women who are between 18 and 45 years old.

For the second part of the study, we are looking for men and women between 18 and 60 years old who have been diagnosed with either Factor V Leiden or prothrombin G20210A mutation. These are specific gene changes that can increase your risk of blood clots. If you're a woman who could get pregnant, you'll need to agree to use effective contraception.

There are also some general requirements. Everyone needs to weigh at least 50 kg and have a Body Mass Index (BMI) between 18 and 25. You also need to be able and willing to follow all the study instructions. We can't include people with serious health conditions like heart, liver, or cancer issues, or those with a history of bleeding problems. Also, if you've recently taken part in another research study involving a new medicine, you might not be able to join this one.

Could this study suit you?

Answer these quick questions to see if you may be eligible. This is a guide only — the research team makes the final call.

Are you between 18 and 45 years old (for healthy volunteers) or 18 and 60 years old (if you have Factor V Leiden or prothrombin G20210A mutation)?
Do you weigh at least 50 kg and have a BMI between 18 and 25?
If you have Factor V Leiden or prothrombin G20210A mutation, has it been confirmed by a genetic test?
Are you able and willing to follow all study instructions and attend regular appointments?
Do you NOT have a history of serious health conditions like heart, liver, or cancer problems, or bleeding disorders?
If you are a woman who could get pregnant, are you willing to use effective contraception during the study?

Answer every question to see your result.

What does participation involve?

If you join the study, your involvement could last for about 13 months, starting from your first screening visit until your very last visit. Everyone in the study will be randomly assigned to either receive the new medicine, CITY-FXI, or a dummy medicine called a placebo. A placebo looks exactly like CITY-FXI but does not contain any active ingredient.

The study involves regular visits to the clinic where you will have health checks. These checks might include blood tests, urine tests, measuring your vital signs (like blood pressure and heart rate), and sometimes an electrocardiogram (ECG) to check your heart. You will also receive injections of either CITY-FXI or the placebo. The exact number of visits and tests will depend on how your body responds and what the study doctors need to monitor.

Potential risks and benefits

For healthy volunteers in Part A, there is no direct personal medical benefit from taking part. However, you will receive regular health check-ups for free. For those in Part B (with Factor V Leiden or prothrombin G20210A mutation), there's a chance you might have a temporary reduction in your risk of blood clots, but this is not guaranteed. All participants will receive free regular health checks, and the information we learn will hopefully help many patients at risk of blood clots in the future. Like all medicines and medical procedures, there are possible risks. These could include bleeding, allergic reactions, or some soreness or pain where blood samples are taken or where you receive an injection. Since CITY-FXI has never been given to humans before, we don't fully know all its effects, especially concerning pregnancy or breastfeeding. Our understanding of the risks is based on earlier studies, mainly in animals. If you feel unwell or get new symptoms during the study, a study doctor will examine you and arrange any necessary tests and treatments. The study doctor can discuss all potential risks and benefits with you in detail, and you are always free to withdraw from the study at any time.

Locations (1)

Some site locations are approximate. We're improving this — please verify with the trial team before travelling.

Richmond Pharmacology Limited
City only
London, England

Common questions

What is CITY-FXI?

CITY-FXI is a new medicine being tested for the first time in people. It aims to prevent harmful blood clots by lowering a specific protein in your blood, potentially with less risk of bleeding than current treatments.

What are Factor V Leiden and prothrombin G20210A mutation?

These are gene changes that mean you have a higher natural risk of developing blood clots. People with these changes are invited to take part in the second part of this study.

Will I definitely get the new medicine?

No, you will be randomly assigned to either receive CITY-FXI or a placebo. A placebo looks like the real medicine but contains no active ingredients.

How long will I be involved in the study?

Your involvement, from the first check-up to the last, could last for around 13 months, depending on how your body responds.

Who is paying for this study?

The study is funded by City Therapeutics, a company based in the United States that develops new medicines.

How to find out more

. City Therapeutics

clinicaltrials@citytx.com +1-857-219-5440 Official trial record

Always speak to your GP or specialist before deciding to take part in a study.