This is a randomized, double-blind, double-dummy, placebo- and positive-controlled parallel study to evaluate the effect of ALXN2050 on the QT interval in healthy adult participants.

Participants randomized to the treatment arm will receive ALXN2050 in a multiple-ascending doses fashion over 3 periods (treatment sequence ABC). Participants randomized to the control arms will be further randomized to 1 of 2 treatment sequences (treatment sequence DEF or GHI) to receive placebo or active control over 3 periods.

Inclusion Criteria:

1. Body mass index in the range of 18.0 to 32.0 kilograms (kg)/meter squared, inclusive, with a minimum body weight of 50.0 kg at screening.
2. No clinically significant history or presence of ECG abnormalities at screening.
3. Female participants must be of non-childbearing potential and need not employ a method of contraception.
4. Non-sterile male participants must agree to abstinence or use a highly effective method of contraception.

Exclusion Criteria:

1. Clinically significant laboratory abnormalities.
2. History of any medical or psychiatric condition or disease that might limit the participant's ability to complete or participate in this clinical study, confound the results of the study, or pose an additional risk to the participant by their participation in the study.
3. History or presence of drug or alcohol abuse within previous 2 years, current tobacco and/or nicotine user, or positive alcohol and/or drug screen at screening or Day -1 of Period 1.
4. History or presence of clinically significant seizures, head injury, or head trauma.
5. History of procedures that could alter absorption or excretion of orally administered drugs.
6. History of meningococcal infection, or a first-degree relative with a history of meningococcal infection.
7. History of significant multiple and/or severe allergies or has had an anaphylactic reaction or significant intolerability to prescription or non-prescription drugs.
8. Body temperature ≥ 38.0°Celsius at screening or prior to first dosing in Period 1 or history of febrile illness, or other evidence of infection, within 14 days prior to (first) dosing.
9. Donation of whole blood from 3 months prior to first dosing, or of plasma from 30 days before first dosing, or receipt of blood products within 6 months prior to first dosing.
10. Participation in any other investigational study drug trial in which receipt of an investigational study drug occurred within 5 half-lives (if known) or 30 days before first dosing, whichever is longer.