A Phase 1 Study of EXT608 in Healthy Adults

This is a Phase 1, randomized, double-blind, placebo-controlled, single ascending dose safety and tolerability study of EXT608 in healthy subjects. There will be up to 6 sequential dose escalation cohorts of 4 participants. In each cohort 3 participants will receive EXT608 and 1 participant will receive placebo.

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

Eligibility Criteria:
Inclusion Criteria:

1. Male or female between 18 and 55 years of age.
2. 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.
3. In general good medical health with no clinically significant or relevant abnormalities,
4. 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.
5. Total serum calcium (Se-Ca) within laboratory normal limits.
6. Serum parathyroid hormone (PTH) concentration within normal laboratory limits.

Exclusion Criteria:

1. 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
2. History of drug abuse
3. Currently using any medication including over-the-counter (OTC), herbal or homeopathic preparations
4. Pregnant, nursing, or planning a pregnancy during the course of or within 3 months of completing this study
5. History of cancer or other malignancy, with the exception of basal cell carcinoma that has been in remission for at least 5 years
6. Positive test result for hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibody or a human immunodeficiency virus (HIV)
7. Alanine transaminase (ALT) and/or aspartate transaminase (AST) \>1.5 the upper limit of normal (ULN)
8. Increased baseline risk for osteosarcoma