Understanding Capillary Leak Syndrome and Side Effects

While the targeted therapy tagraxofusp (Elzonris) has revolutionized BPDCN treatment, it comes with a unique and serious side effect called Capillary Leak Syndrome (CLS). Understanding CLS is the most important thing you can do to stay safe during your first few cycles of treatment ^[1][2].

What is Capillary Leak Syndrome?

Think of your smallest blood vessels (capillaries) as pipes. Normally, these pipes are solid enough to keep your blood and essential proteins inside. In Capillary Leak Syndrome, these “pipes” temporarily become “leaky” ^[3]. Fluid and proteins (specifically albumin) leak out of your bloodstream and into the surrounding spaces in your body, like your skin, lungs, or abdomen ^[4][5].

This “leak” causes two main problems ^[3][6]:

1. Swelling and Weight Gain: Fluid builds up where it doesn’t belong (edema).
2. Low Blood Pressure: Because the fluid is leaving your blood vessels, there is less “volume” to keep your blood pressure at a healthy level.

Early Warning Signs to Watch For

The risk for CLS is highest during your very first cycle of treatment ^[1][7]. Because of this risk, you will likely be admitted to the hospital for your first 5-day cycle to ensure you have 24/7 monitoring ^[1].

Whether in the hospital or at home, you are the first line of defense in catching CLS early. You must notify your care team immediately if you notice ^[6][8]:

• Sudden Weight Gain: A gain of 2 to 3 pounds (about 1 to 1.5 kg) in a single day is a major red flag ^[9].
• New Swelling: Feeling “tightness” in your ankles, hands, or even around your face ^[6].
• Dizziness or Lightheadedness: This can be a sign that your blood pressure is dropping ^[6].
• Shortness of Breath: This may happen if fluid begins to leak into the space around your lungs ^[6].

How Your Team Monitors and Treats You

Because CLS can be life-threatening if ignored, your medical team will follow a strict monitoring protocol ^[10][11]:

• Albumin Checks: You will have frequent blood tests to check your albumin levels. If your albumin drops too low, it is a sign that a “leak” may be starting ^[12][13].
• Daily Weight: You (or your nurses) will weigh you at the same time every morning. This is the most sensitive way to catch fluid buildup early ^[12][10].
• Vitals: Your blood pressure and heart rate will be checked multiple times a day during treatment ^[12].

If CLS is suspected, your doctors will likely pause your treatment. During the acute phase of the “leak,” your blood vessels are depleted of fluid. Your team will treat this by giving intravenous steroids or albumin infusions to pull fluid back into the vessels, and they will carefully avoid giving diuretics (“water pills”), which could dangerously lower your blood pressure further ^[10][13].

Other Potential Side Effects

While CLS is the primary concern, tagraxofusp and other intensive treatments for BPDCN can cause other side effects ^[12][13]:

• Elevated Liver Enzymes: Your blood tests may show that your liver is temporarily stressed (high ALT or AST), though this often resolves on its own ^[14][13].
• Low Platelets (Thrombocytopenia): You may be at an increased risk for bruising or bleeding during treatment ^[12][13].
• Fatigue: Feeling very tired is common as your body works to fight the cancer and handle the therapy ^[14].

By being “obsessive” about your daily weight and reporting even small changes, you help your team manage these side effects effectively, keeping your treatment on track ^[11][2].