Inclusion Criteria:
* Must be 18 years old (or the legal age of consent in the jurisdiction in which the study is taking place) to 75 years of age, inclusive, at the time of signing the ICF.
* Must have a physician diagnosis of asthma for at least 1 year prior to the Screening visit.
* Must be on a stable regimen of medium- to high- dose inhaled corticosteroid (ICS), in combination with a long-acting β2-agonist (LABA). The regimen may include additional controller medications used for at least 12 weeks, at a stable dose and regimen, with no change in the dose or frequency of administration for at least 4 weeks prior to the Screening visit and between the Screening and Baseline visits.
* Must have a pre-bronchodilator FEV1 40 to 80% of predicted normal at the Screening and Baseline visits, prior to randomization.
* Must have an ACQ-5 score ≥ 1.5 at the Screening and Baseline visits, prior to randomization.
* Must have a FeNO level of ≥ 25 ppb at the Screening and Baseline visits.
* Must have a demonstrated evidence of reversible airway obstruction by post-bronchodilator (albuterol/salbutamol) reversibility of FEV1 ≥12% and ≥200 mL at Screening.
* Must have an absolute blood eosinophil count must be ≥ 0.30 × 10\^9/L at Screening.
* Must have a documented history of at least 1 asthma exacerbation requiring either treatment with systemic corticosteroids (intramuscular, intravenous, or oral) and/or hospitalization or an emergency/urgent medical care visit for acute asthma worsening within the past 52 weeks prior to Screening.
* Must agree to contraceptive requirements in compliance with the clinical study and local requirements.
* Must be willing and able to comply with scheduled visits, treatment plan, laboratory tests, other study-related procedures, and questionnaires, including completing the electronic diary (e-diary), for the duration of the study as required by the study protocol.
Exclusion Criteria:
* Must not have any clinically significant pulmonary disease other than asthma.
* Must not have had an asthma exacerbation, requiring either treatment with systemic corticosteroids (intramuscular, intravenous, or oral) and/or hospitalization or an emergency/urgent medical care visit for acute asthma worsening, at any time from 4 weeks prior to the Screening visit up to and including the Baseline visit.
* Must not have any other clinically significant disease, condition, or medical history that, in the opinion of the Investigator, would interfere with participant safety, study evaluations, and/or study procedures.
* Must not be pregnant or breastfeeding; must not be a woman planning to become pregnant or breastfeed during the study.
* Must not have results from clinical laboratory safety tests that are outside the local reference range at Screening.
* Must not have been dosed with any investigational drug or device in a clinical study within 8 weeks or 5 half-lives (whichever is longer) of KT-621 administration.
* Must not have any known factor, condition, or disease that might interfere with treatment compliance, study conduct or interpretation of the results.
* Must not be a current smoker of nicotine/tobacco as well as non-nicotine products, have a smoking history of ≥ 10 pack years, or use vaping products, including electronic cigarettes. Former smokers with a smoking history of \< 10 pack years and users of vaping or e-cigarette products must have stopped for at least 26 weeks prior to the Screening visit.
* Must not have a known sensitivity to any of the components of KT-621.
* Must not be a member of the investigational team or his/her immediate family.
