Until recently, treatment for familial chylomicronemia syndrome (FCS) has been limited to diet changes and standard triglyceride medicines. But even with careful management, many people with FCS still deal with belly pain, fatigue, and repeat bouts of pancreatitis.
That's because older tools don't address the biology behind FCS.
In November 2025, the FDA approved plozasiran (Redemplo) for adults with FCS. It's meant to be used with a strict low-fat diet, often under about 20 grams per day. What's new is that it's the first medicine designed to target the process that keeps triglycerides high in FCS.
You may be wondering how it works, if it's safe, and if it's right for you. Here's a look at how plozasiran went from early research to FDA approval.
Step 1: The Science Behind Plozasiran (Redemplo)
For years, scientists have studied why fat builds up to such dangerous levels in people with FCS. That research led to plozasiran. But to understand how this new medicine works, it helps to start with some triglyceride basics.
Triglycerides are a type of fat that circulate in your blood, especially after you eat. In people without FCS, the body breaks them down and clears them out of the bloodstream, using them for energy or storing them for later. A protein called ApoC-III helps control how quickly that process happens.
In FCS, triglycerides stay trapped in fat-carrying particles called chylomicrons, which build up in the blood instead of clearing out.
Older triglyceride medicines work in broad, indirect ways. They often change how the liver handles fats. For people with FCS, that often isn't enough. Those drugs don't directly address the main signal behind the extreme triglyceride buildup in FCS, which is the loss of lipoprotein lipase (LPL) activity.
To treat people with FCS, researchers focused on a new question: What if we could turn down ApoC-III itself? That shift in thinking led to the use of siRNA technology, which stands for small-interfering RNA.
How does siRNA technology work?
Inside liver cells, genes send out messages that tell the cell which proteins to make. Medicines that use siRNA technology work by interrupting specific messages. As a result, the cell makes less of a certain protein.
In the case of plozasiran, the siRNA is designed to turn down the signal that tells liver cells to make ApoC-III. As a result, ApoC-III levels go down.
With lower ApoC-III levels, the body can clear triglycerides and chylomicrons more effectively. And fat particles move out of the bloodstream instead of piling up.
Another clue that targeted ApoC-III might help people with FCS came from research on folks who naturally have lower ApoC-III levels because of their genes. Those people tend to have lower triglycerides and related problems. This told researchers that lowering ApoC-III on purpose would likely be effective and reasonably safe.
With that scientific foundation in place, researchers were ready to test the idea that led to plozasiran.
Step 2: Early Testing
Before anyone gave plozasiran to people, scientists tested the drug in lab and animal studies.
In early tests, scientists worked with liver cells to see if the drug could target the ApoC-III signal. That showed that the siRNA technology worked at a biological level.
Next came animal studies. These tests showed how the drug behaved in a living body, where the liver, blood, and fat handling all work together. Safety was also a big focus at this stage. Researchers watched closely for liver stress and other warning signs.
By the end of this early phase, scientists saw that ApoC-III could bring triglycerides down. And it appeared to do so safely. That gave researchers the confidence to move into human studies.
Step 3: Testing in People
Once plozasiran showed promise in early lab and animal studies, researchers moved into clinical trials. These are studies that test new treatments in people. Each phase is designed to answer a different question before moving forward.
Phase 1: Is it safe? Does it work?
The earliest human studies focused on safety and basic effects. These trials included people with very high triglycerides, including a small number with confirmed FCS.
After just one shot, triglyceride levels dropped fast, in some cases by more than 90%. ApoC-III levels also fell sharply. Those changes showed up within weeks.
Researchers also watched closely for side effects. Most were mild.
Phase 2: What's the right dose?
Next came a larger phase 2 trial called SHASTA-2. This study included more than 200 people with triglyceride levels above 500 mg/dL.
People in this trial received plozasiran or a placebo shot at the start and again three months later. Over time, ApoC-III levels dropped by 77%, and triglycerides fell by about half. Most people taking plozasiran were able to bring their triglycerides below 500 mg/dL – a key threshold linked to pancreatic risk.
Those improvements lasted. Even a year into the study, triglyceride levels stayed much lower than where they started.
Side effects were similar between the treatment and placebo groups. Some people had headaches, an upset stomach, or diarrhea. Some had higher blood sugar, mainly those who already had diabetes.
Phase 3: Larger trials with more people
Phase 3 trials are the final stage of testing before a drug company can seek FDA approval. These studies involve more people, run longer, and are designed to show how well a treatment works and how safe it is for the people it's meant to help.
Who was in the study?
Researchers enrolled adults with genetically confirmed or clinically diagnosed FCS who continued to have very high triglycerides despite following a very low-fat diet and using standard triglyceride-lowering medicines.
To take part, people in this trial had to:
- Have ongoing very high fasting triglyceride levels
- Be on a strict low-fat diet that wasn't helping enough on its own
- Have a history of persistent chylomicronemia
Researchers were able to study 75 people with persistent chylomicronemia, which experts consider a strong sample size for a condition that affects only a few people per million worldwide.
How did the study work?
The phase 3 trial was randomized, double-blind, and placebo-controlled. That means that the people in it were randomly assigned to receive plozasiran or a placebo (an injection with no active drug). Neither the people being studied nor researchers knew who was getting which treatment.
Plozasiran was given as a shot just under the skin once every three months. Researchers followed people in the study for 12 months. The main goal was to measure how much triglyceride levels changed over time.
What did the results show?
Compared with a placebo, people on plozasiran saw:
- An average triglyceride drop of about 80% within the first month
- Sustained triglyceride lowering over the full year
- Fewer episodes of acute pancreatitis
By the end of the study, triglyceride levels in many people were reduced by 90% or more. These drops were seen in people who'd had trouble for years controlling their triglycerides with diet alone.
The findings were published in The New England Journal of Medicine, one of the most respected medical journals in the world. Doctors who treat people with FCS say this adds confidence to the quality of the data.
What about side effects?
Most people tolerated plozasiran well. Side effects were generally mild and manageable. The most common were:
- Mild pain or redness where the needle goes into the skin
- Headache
- Nausea
- Short-term increase in blood sugar
Doctors suggest monitoring blood sugar while taking plozasiran, especially if you have diabetes or prediabetes.
Step 4: Checking by Experts
Once the clinical trials were done, the data didn't go straight to approval. Before that can happen, the data goes to the Center for Drug Evaluation and Research (CDER).
FDA reviewers at the CDER looked closely at how well the drug lowered triglycerides, how long those effects lasted, and whether fewer pancreatitis attacks followed. They also examined safety data, including side effects and lab changes like blood sugar shifts.
Because FCS is so rare, context mattered. Reviewers considered how severe the condition was, how limited current treatment options were, and how hard it had been to control triglycerides with diet and older drugs alone.
They also looked at how the studies were run. Were the people in the studies chosen carefully? Were results consistent across trials? Did the benefits show up in ways that matter to daily life and long-term health?
The goal is to make sure the drug does what it claims to and is safe for the people who need it most.
After expert review, the FDA agreed the evidence was strong enough to move forward, clearing the path for plozasiran to become an approved treatment for adults with FCS.
Step 5: Watching Over Time
After a drug reaches pharmacy shelves, doctors and researchers continue to track how it performs in real life.
This kind of monitoring helps catch side effects that may be rare or take longer to show up. It also shows how well the drug works outside of a clinical trial, where people may have other health issues or take other medicines.
Doctors report safety concerns to the FDA, which can update prescribing info if needed. This ongoing review matters because plozasiran is meant for long-term use, which means people with FCS will take it for years.
Should You Try Plozasiran (Redemplo)?
Plozasiran is worth talking to your doctor about if you're an adult with FCS and you have very high triglycerides despite a strict low-fat diet and other treatments.
It's not meant for everyone with high triglycerides. Most people can manage milder cases with diet changes and standard medicines.
Plozasiran is designed for people at the highest risk, especially those with repeated pancreatitis or triglyceride levels that stay dangerously high, which are levels above 500 mg/dL.
If you're considering it, talk with a lipid specialist, endocrinologist, or gastroenterologist who knows FCS well. They can help confirm your diagnosis, review your risks, and decide whether a targeted treatment fits your situation.
Even if plozasiran isn't right for you, its approval signals much-needed progress. For a condition that long had few choices, having a treatment built around the biology of FCS marks a meaningful step forward.