The objective of this study is to assess the safety, tolerability, pharmacokinetics and immunogenicity of either single ascending intravenous (IV) doses of ABBV-243 or single ascending subcutaneous (SC) doses of ABBV-243 in healthy adult participants (Part 1), and a single intravenous (IV) dose in healthy Asian adult volunteers (Part 2

There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.

Inclusion Criteria:

• Individuals between 18 and 60 years of age inclusive at the time of Screening.

• BMI is ≥ 18.0 to ≤ 32.0 kg/m2 after rounding to the tenth's decimal at Screening. BMI is calculated as weight in kg divided by the square of height measured in meters.

• Females, Non-Childbearing Potential are eligible as defined by meeting the following criteria:

  • Permanent sterility due to a hysterectomy, bilateral salpingectomy, or bilateral oophorectomy.
  • Non-surgical permanent infertility due to Mullerian agenesis, androgen insensitivity, or gonadal dysgenesis; investigator discretion should be applied to determining study entry.
  • Postmenopausal female who is age ≤ 55 years with no menses for 12 or more months without an alternative medical cause AND an FSH level ≥ 30 IU/L.
  • Postmenopausal female who is age > 55 years with no menses for 12 or more months without an alternative medical cause.

• Females, Childbearing Potential are defined as all other females who do not meet the above criteria and must adhere to the following:

  • Must not be pregnant or breastfeeding.
  • Must agree to avoid pregnancy while taking study treatment(s) and for at least 200 days after the last dose of study treatment.
  • Must agree to use a contraceptive method listed below (as per local regulations) that is highly effective (with a failure rate of < 1% per year, when used consistently and correctly). Participants must provide documentation to the site.
  • Bilateral tubal occlusion/ligation.
  • Intrauterine device (IUD) to be inserted at least 30 days prior to Screening.
  • Intrauterine hormone-releasing system (IUS) to be inserted at least 30 days prior to Screening.

• Part 2 only:

Healthy Japanese and Han Chinese male or female; between 18 and 60 years of age, inclusive at the time of Screening.

- Han Chinese participants must be first- or second-generation Han Chinese of full Chinese parentage. First-generation participants are defined as those born in China to two parents and four grandparents also born in China of full Chinese descent. Second-generation participants are defined as those born outside of China to two parents and four grandparents born in China of full Chinese descent.

OR

- Japanese participants must be first- or second-generation Japanese of full Japanese parentage. First-generation participants will have been born in Japan to two parents and four grandparents also born in Japan of full Japanese descent. Second-generation subjects born outside of Japan must have two parents and four grandparents born in Japan of full Japanese descent.

Exclusion Criteria:

• History: of epilepsy, any clinically significant cardiovascular, respiratory (except mild asthma as a child), renal, endocrine, hepatic, gastrointestinal, hematologic or psychiatric disease or disorder, or any uncontrolled medical illness.
• History of any clinically significant sensitivity or allergy to any medication or food.
• Evidence of dysplasia or history of malignancy (including lymphoma and leukemia) other than successfully treated non-metastatic cutaneous squamous cell, basal cell carcinoma, or localized carcinoma in situ of the cervix.
• History or evidence of active Tuberculosis (TB) disease or latent TB infection.