What Is the Pathophysiology of CLS During BPDCN Treatment?

During treatment for Blastic plasmacytoid dendritic cell neoplasm (BPDCN) with the targeted therapy tagraxofusp (Elzonris), patients face a serious and potentially life-threatening complication called Capillary Leak Syndrome (CLS) ^[1]. In CLS, the walls of the microscopic blood vessels (capillaries) become abnormally permeable, allowing blood plasma and essential proteins to seep out into surrounding tissues ^[2][3]. This shift of fluid causes a combination of visible swelling and rapid weight gain, while simultaneously triggering a dangerous drop in blood pressure ^[3][4]. Because CLS carries a black box warning and can be fatal, it is critical to understand its warning signs, how it alters your body’s fluid balance, and how your care team will monitor you ^[5].

⚠️ URGENT ACTION WARNING

Capillary Leak Syndrome is a medical emergency. Do not wait for your next appointment or try to diagnose yourself. Contact your oncology team immediately or seek emergency care if you experience any of the following:

• Rapid weight gain (ask your team for your specific daily threshold)
• New or worsening swelling (edema), especially in the legs, ankles, or abdomen
• Lightheadedness, dizziness, or fainting (signs of low blood pressure)
• Shortness of breath or chest discomfort
• Confusion or severe weakness
• Markedly reduced urine output

Do not attempt to self-treat. Because fluid is simultaneously leaking out of your vessels and accumulating in your tissues, taking over-the-counter diuretics (water pills), drinking extra fluids, or changing blood pressure medications on your own can be extremely dangerous.

Why Does Tagraxofusp Cause CLS?

Tagraxofusp is a targeted drug known as a fusion protein. It binds to the CD123 receptor found on BPDCN cells and delivers a truncated diphtheria toxin to destroy them ^[6]. While clinical trials show that approximately 18% to 21% of patients develop significant CLS—with many more experiencing milder fluid shifts—the exact molecular reason why tagraxofusp causes blood vessels to leak is still being studied ^[7]. Researchers propose it may be caused by inflammatory signaling that compromises the vessel walls or by direct injury to the endothelial cells lining the vessels ^[2][7]. The risk of developing CLS is highest during your first cycle of treatment, though it can still occur in later cycles ^[7].

The “Leaky Pipe” Mechanism (A Simplified Model)

To understand what is physically happening in your body, it helps to picture your blood vessels as a closed system of plumbing pipes. Normally, the walls of these pipes are tightly sealed by a lining of cells called the endothelium, keeping blood and fluid securely inside ^[2].

During CLS, this endothelial barrier fails and becomes abnormally permeable ^[2]. Using the pipe analogy, the solid pipe essentially turns into a “soaker hose.” When this happens, two critical things leak out into the surrounding tissues:

1. Water (from your blood plasma)
2. Albumin (a large protein that circulates in your blood and acts like a sponge, helping to hold water inside the vessels)

As albumin leaks out, your blood loses its ability to retain the remaining water, which can compound the fluid leak ^[3][8].

The Paradox: Swelling and Low Blood Pressure

As water and albumin seep out of the blood vessels, they pool in the empty spaces between your cells (the interstitium). This buildup causes edema (swelling) and is responsible for the rapid, noticeable weight gain on the scale ^[9].

However, this creates a dangerous “blood pressure paradox.” Even though your body looks swollen and overloaded with fluid, your blood vessels are actually running dry ^[3].

Because so much fluid has escaped the “pipes,” your effective circulating blood volume drops significantly. This lack of fluid volume causes a sharp drop in blood pressure (hypotension) ^[3][4]. If too much fluid leaves the bloodstream, it can lead to shock and prevent vital organs from getting enough oxygen ^[10][5].

Additionally, because the liquid plasma leaves the vessels but the red blood cells remain behind, your blood becomes abnormally concentrated. This is known as hemoconcentration. It does not mean your body suddenly made more red blood cells; it simply means there is less liquid diluting them ^[4]. Your doctor may notice this as an unusually high hemoglobin or hematocrit level on your lab tests, which they will interpret alongside your weight, albumin, and symptoms ^[3][4].

Monitoring and Medical Management

Because of the severe risks, your care team will monitor you rigorously. They will check your weight, blood pressure, and serum albumin levels before doses and as clinically indicated throughout your treatment ^[7][11].

If early signs of CLS or low albumin are detected, management is highly individualized. The standard protocol often involves pausing or permanently stopping the tagraxofusp ^[12]. Depending on your symptoms, your team may administer intravenous (IV) albumin to help pull fluid back into the vessels, alongside other carefully balanced measures like IV fluids, corticosteroids, or blood pressure-supporting medications (vasopressors) ^[7]. Early recognition and directed intervention may improve the chances of recovery, but treatment is not always successful, and severe cases may require intensive hospital care ^[5][7].