ALXN2420 Versus Placebo in Combination With Somatostatin Analogs in Participants With Acromegaly

Inclusion Criteria: * Documented diagnosis of acromegaly, that is, historically documented evidence of a GH-secreting pituitary adenoma based on MRI or pathology report * Must be receiving maximum, or maximally tolerated dose per treating physician judgment, of long-acting SSAs (octreotide or lanreotide LAR) and meet both of the following: 1. Received for ≥ 6 months prior to screening 2. Receiving a once-monthly regimen (approximately every 4 weeks). Note: participants on stable regimens of other durations (for example, every 3 or 6 weeks) are not eligible * Must be a partial responder to SSAs defined as \> 20% relative IGF 1 reduction during the course of SSA therapy * Serum IGF-1 levels \> 1.3 to 5\*ULN inclusive, as assessed at a central laboratory and adjusted for age and sex, based on average of 2 consecutive values obtained during the Screening Period and obtained ≥ 7 days apart Exclusion Criteria: * Had surgery for pituitary adenoma within the last 6 months before Day 1 or planning to receive surgery for pituitary adenoma during the study * Pituitary adenoma that, per Investigator's judgment, is worsening as assessed by pituitary/sellar MRI or computed tomography scan obtained ≤ 6 months prior to screening * Pituitary adenoma causing compression of the chiasm * Clinical evidence of symptomatic hyperprolactinemia that would necessitate treatment with dopamine agonists * Known hypothyroidism or hypocortisolism not adequately treated with a stable dose of thyroid or glucocorticoid hormone replacement therapy for ≥ 3 months prior to Screening * Active, clinically significant cardiac disease as judged by the Investigator * History of unstable angina, stroke, or acute myocardial infarction ≤ 3 months prior to screening * Known uncontrolled type 2 diabetes (HbA1c \> 10%) * Active malignant disease ≤ 2 years prior to screening with exception of basal and squamous cell carcinoma of the skin * Received any type of fractionated radiotherapy or a second surgical adenectomy for pituitary adenoma within the last 3 years (5 years for conventional radiation) before starting treatment and/or are planning to receive radiotherapy or a second surgical adenectomy during the study * Received pegvisomant ≤ 8 weeks prior to screening * Received dopamine agonists ≤ 4 weeks prior to screening * Received pasireotide LAR ≤ 4 months prior to screening * Clinically significant renal or hepatic disease at the time of screening, as judged by the Investigator * eGFR (CKD-EPI formula) \< 30 mL/minute/1.73 m\^2 documented based on recent value (\< 3 months prior to randomization) * Clinically significant abnormal values for hematology, biochemistry, coagulation, or urinalysis, as judged by the Investigator, including, but not limited to, total bilirubin \> 1.5\*ULN (except if in free bilirubin linked to a known Gilbert Syndrome) or AST, ALT, or alkaline phosphatase \> 2\*ULN