Type I hyperlipoproteinemia (T1HLP, also known as familial chylomicronemia syndrome or FCS) is a rare diseasewhere the blood triglycerides (fats) are very high. It is caused by lack of certain enzymes and proteins in the bodythat are important in disposing circulating fats from blood. Treatment of T1HLP patients who have very high levels of blood fats (≥ 1,000 mg/dL) is challenging as conventional triglyceride-lowering medications, such as fibrates and fishoil, are ineffective.

The purpose of this trial is to study the long-term efficacy and safety of orlistat for reducing blood triglyceride levels in patients with T1HLP.

The hypotheses to be tested and the specific aims are:

Hypothesis 1: As compared to placebo, Orlistat will be effective and safe in lowering fasting serum TG concentrations in patients with T1HLP.

Specific Aim 1: To investigate the efficacy and safety of Orlistat for reducing fasting serum TG levels in 28 patients with T1HLP in a double-blind, randomized, placebo-controlled, parallel design study for a period of 24 weeks.

Hypothesis 2: The efficacy and safety of Orlistat in patients with T1HLP will be maintained over a period of up to 48 weeks.

Specific Aim 2: To investigate the efficacy and safety of Orlistat in patients with T1HLP in an open-label extension study for a period of up to 48 weeks-.

After a screening evaluation, the subjects will be advised to consume an extremely low fat diet (≤15% of total energy from fat) for the entire duration of the study. After the baseline period of 8 weeks, they will be randomly assigned to placebo or Orlistat for the duration of 24 weeks (Phase 1). After Phase 1, all patients will enter an open-label extension (Phase 2) and receive Orlistat for a period of 24 weeks for a total study duration of 56 weeks. During the last week of Baseline Period, Phase 1, and Phase 2, blood lipids and chemistry panel will be analyzed for three consecutive days, and fat-soluble vitamin levels will be measured once. All the visits are going to be outpatient.

Inclusion criteria:

• Age ≥ 8 years
• Type I hyperlipoproteinemia confirmed by bi-allelic disease-causing variants in any one of the T1HLP genes (LPL, APOC2, APOA5, LMF1, GPIHBP1, or GCKR).
• Subjects with digenic inheritance with heterozygous disease-causing variants in two different T1HLP genes.
• Subjects who have a fasting TG greater than or equal to 750 mg/dL at the end of run-in period of 8 weeks will be eligible for randomization.
• Subjects who do not have confirmed genetic mutation for Type 1 hyperlipoproteinemia but have a fasting TG greater than or equal to 750 mg/dL at the end of run-in period of 8 weeks will be eligible for randomization.
• Well controlled diabetes mellitus with hemoglobin A1c < 8%
• Off orlistat for a period of 2 months
• Patients on Olezarsen and Plozasiran (APOC3 antisense oligonucleotide) can enroll if on the drug for more than 3 months
• Following methods of contraception for males and females of childbearing age will be employed Males: Being in this research may damage your sperm, which could cause harm to a child that you may father while on this study. If you take part in this study and are sexually active, you must agree to use a medically-acceptable form of birth control. Medically-acceptable forms of birth control include: (1) surgical sterilization (vasectomy), or (2) a condom used with a spermicide (a substance that kills sperm).

Females: If you are part of this study while pregnant or breast-feeding an infant, it is possible that you may expose the unborn child or infant to risks. For that reason, pregnant and breast-feeding females cannot participate in the study. If you can become pregnant, a blood or urine pregnancy test will be done, and it must be negative before you participate in this study. If you take part in this study and you are sexually active, you and any person that you have sex with must use medically-acceptable birth control (contraceptives) during the study. Medically-acceptable birth control (contraceptives) includes: (1) surgical sterilization (such as hysterectomy or "tubes tied"), (2) approved hormonal contraceptives (such as birth control pills, patch or ring; Depo-Provera, Implanon), (3) barrier methods (such as condom or diaphragm) used with a spermicide (a substance that kills sperm), or (4) an intrauterine device (IUD). If you do become pregnant during this study, you must tell the researchers immediately.

Exclusion Criteria:

• Secondary hypertriglyceridemia due to diabetes, renal disease, alcoholism and drug therapy such as estrogens and estrogen analogues, steroids, HIV-1 protease inhibitors, retinoic acid derivatives, interferons, or lasparaginase.
• On lomitapide or participating in clinical trial of volanesorsen.
• On cyclosporine
• Having serum TSH outside of the normal range if on levothyroxine supplementation.
• Use of levothyroxine to suppress TSH in individuals with thyroid cancer.
• Pregnant or lactating women
• Significant liver disease (elevated transaminases > 2 times upper limit of normal)
• Alcohol abuse (> 7 drinks or 84 g per week for women and > 14 drinks or 168 g per week for men)
• Severe anemia (hematocrit < 24%)
• Illicit drug use (cocaine, marijuana, LSD, etc.)
• Major surgery in the past three months
• Congestive heart failure
• Serum creatinine greater than 2.5 mg/dL
• Cancer within the past five years
• Gastrointestinal surgery in the past
• Current therapy with anti-coagulants, digoxin and anti-arrhythmics
• Chronic malabsorption syndromes
• Cholestasis
• Acute illnesses such as acute pancreatitis in the last 8 weeks
• Previous history of renal calcium oxalate stones