A bleeding episode is defined as any occurrence of hemorrhage that requires administration of CFCs or BPAs

A bleeding episode is defined as any occurrence of hemorrhage that requires administration of CFCs or BPAs, e.g., hemarthrosis, muscle, or mucosal bleeding.

The number of participants experiencing any TEAEs, serious TEAEs, discontinuation due to TEAEs and death will be reported

Bleeding episodes (BE): any occurrence of hemorrhage might require administration of factor/BPA. BE start time: time at which 1st BE symptoms develop; bleeding/any symptoms at same location occurred within 72 hours of last injection used to treat BE at that location was considered part of original BE and counted as 1 BE towards ABR. Bleeding began after 72 hours of last injection at that location was considered as a new event. ABR = total number of qualifying BE/total number of days in the respective period\*365.25. Estimated data were derived by using repeated measures negative binomial (NB) regression model.

A bleeding episode (BE): any occurrence of hemorrhage might require administration of factor/BPA. BE start time: time at which 1st BE symptoms develop; bleeding/any symptoms at same location that occurred within 72 hours of last injection used to treat BE at that location was considered a part of original BE and counted as 1 BE towards ABR. Any bleeding that began after 72 hours of last injection at that location was considered as a new event. ABR = total number of qualifying BE/number of days in the respective period \*365.25.

ABR for a participant during efficacy period (EP) was defined as annualized number of bleeding episodes during EP annualized to a 1-year interval of time. Treated Bleeding episode: any occurrence of hemorrhage that required administration of by-passing agents (BPA) or factor. It started from first sign of a bleed and ended no more than 72 hours after last treatment for bleed, within which any symptoms of bleeding at same location or injections less than or equal to (\<=) 72 hours apart were considered the same bleeding episode. EP was defined as time duration starting from Day 29 when antithrombin (AT) lowering capacity of fitusiran had achieved therapeutic target range to the earliest of Day 246 or last day of bleeding follow up (maximum duration of EP: from Day 29 to Day 246). This outcome measure (OM) presents estimated results (i.e., results received by applying negative binomial \[NB\] regression model on data collected during EP).

ABR for a participant during EP was defined as annualized number of bleeding episodes during EP annualized to a 1-year interval of time. ABR= number of bleeding episodes during EP divided by total number of days during EP\*365.25. A bleeding episode was defined as any occurrence of hemorrhage that required administration of BPA or factor. It started from the first sign of a bleed and ended no more than 72 hours after the last treatment for the bleed, within which any symptoms of bleeding at the same location or injections \<=72 hours apart were considered the same bleeding episode. EP was defined as time duration starting from Day 29 when the AT lowering capacity of fitusiran had achieved therapeutic target range to the earliest of Day 246 or the last day of bleeding follow up (maximum duration of EP: from Day 29 to Day 246). This OM presents observed results (i.e., descriptive statistics values based on the data collected during EP).

Characterize the AT activity at the optimal therapeutic dose

An adverse event (AE) is any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and which does not necessarily have to have a causal relationship with the treatment. An SAE is any untoward medical occurrence that results: death or life-threatening or inpatient hospitalization or prolongation of existing hospitalization or persistent or significant disability or congenital anomaly or medically important event. All AEs collected in LTE14762 were considered TEAE because all participants received dose in the parent study. AEs of special interest (AESI) are alanine aminotransferase (ALT) or aspartate aminotransferase (AST) elevations \>3× upper limit of normal (ULN) or suspected or confirmed thromboembolic events or severe or serious injection site reactions or systemic injection associated reactions or cholecystitis or cholelithiasis.

Blood samples were collected to determine the hematology laboratory significant abnormalities. Here, DFB = decrease from baseline, NB = non-black, and B = black.

Blood samples were collected to determine the clinical chemistry laboratory abnormalities. Here, mmol/L = millimoles per liter, LLN = lower limit of normal, mg/L = milligram per liter, umol/L = micromoles per liter, mL/min = milliliter per minute, m\^2 = meter square, CB = conjugated bilirubin, and DB = direct bilirubin.

Urine samples collected to determine the significant abnormalities in urine.

Participants vital signs were examined to determine the abnormalities. Vital signs included weight, supine systolic blood pressure (SSBP) and supine diastolic blood pressure (SDBP). Here, mmHg = millimeter of mercury, and IFB = increase from baseline.

Standard 12-lead ECGs were recorded after at least 15 minutes in the supine position using an electrocardiographic device. The following were assessed: heart rate, rhythm, interval between the peaks of successive QRS complexes (RR), interval from the beginning of the P wave until the beginning of the QRS complex (PR), interval from start of the Q wave to the end of the S wave (QRS), interval between the start of the Q wave and the end of the T wave (QT), QT interval corrected for heart rate (QTc) automatic correction evaluation, QRS axis, left ventricular hypertrophy criteria, right ventricular hypertrophy criteria, repolarization charges, and overall cardiac impression for each participant. Here, msec = milliseconds.

Physical examination included, at a minimum, an assessment of the participant's general appearance; skin; head, eyes, ears, nose, and throat; examinations of lymph nodes, abdomen, extremities/joints, neurological and mental status; heart and respiratory auscultation; peripheral arterial pulse; and pupil, knee, achilles, and plantar reflexes.