A Phase 1 Study of EXT608 in Healthy Adults

This is a randomized, double-blind, first-in-human, placebo-controlled single ascending dose study to study the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of EXT608 in healthy adult participants.

Participants will be enrolled into 1 of up to 6 planned single dose cohorts in ascending fashion.

Each cohort will consist of 4 participants randomized to receive either EXT608 or placebo, whereby 3 will receive a single injection of EXT608 and 1 will receive matching placebo.

Up to 6 dosing cohorts are planned with no single dose escalation to exceed a 3-fold dose increment. A Data Monitoring Board will review the seven-day safety data in each cohort before authorizing the initiation of the next cohort.

A screening visit will take place within 28 days of dosing. Eligible participants for each cohort will be admitted to the Clinical Research Unit (CRU) 1 day prior to dosing and remain in the CRU through at least 72 hours after dosing for safety and PK assessments before discharge. The total confinement period will be 4 nights, unless extended for management of Adverse Events (AEs) at the discretion of the Investigator

Inclusion criteria

• Male or female between 18 and 55 years of age.
• Body mass index between 18.5 and 32 kilogram per square meter (kg/m2), inclusive, with a body weight greater than or equal to (>=) 45 kg.
• In general good medical health with no clinically significant or relevant abnormalities,
• A clinical safety laboratory parameter of hemoglobin greater than (>) 11.7 gram per deciliter (g/dl) (females) or 13.1 g/dl (males) and less than (<) 16 g/dl (females) or 17.4 g/dl (males) or, if out of this range, deemed not clinically significant by the Investigator.
• Total serum calcium (Se-Ca) within laboratory normal limits.
• Serum parathyroid hormone (PTH) concentration within normal laboratory limits.

Exclusion criteria

• Evidence of clinically significant (CS) neurologic, cardiovascular, pulmonary, hepatic, hematopoietic disease, renal, metabolic, gastrointestinal, urologic, immunologic, endocrine disease, serious allergy, allergic skin rash, or psychiatric disorder
• History of drug abuse
• Currently using any medication including over-the-counter (OTC), herbal or homeopathic preparations
• Pregnant, nursing, or planning a pregnancy during the course of or within 3 months of completing this study
• History of cancer or other malignancy, with the exception of basal cell carcinoma that has been in remission for at least 5 years
• Positive test result for hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibody or a human immunodeficiency virus (HIV)
• Alanine transaminase (ALT) and/or aspartate transaminase (AST) >1.5 the upper limit of normal (ULN)
• Increased baseline risk for osteosarcoma