Inclusion Criteria:
1. Healthy, male and female participants 18-55 years of age at Screening
2. Body weight ≥45 kg and ≤120 kg and body mass index (BMI) 18 to ≤32 kg/m2 at Screening and Day -1
3. Serum IgE ≥30 IU/mL and ≤300 IU/mL at Screening
4. Nonsmoker and must not have used any tobacco products within 2 months prior to Screening. Participants who smoke ≤2 cigarettes or equivalent (e.g., cigars, vaping, nicotine patches) per week can be included in the trial at the discretion of the PI/delegate
5. Positive skin prick test at Screening (Part B MAD Cohorts only) to common aeroallergens
6. Willing and able to read, understand, and sign an informed consent form (ICF) that includes compliance with requirements and restrictions listed in the ICF and in this protocol
Exclusion Criteria:
1. Received Exl-111 in a prior cohort in this trial
2. Prior receipt of any anti-IgE therapy
3. Use of any medication, including any prescription, over-the-counter, or other supplements, within 5 days prior to dosing, extending to 30 days after last dose of trial intervention, with the exception of those approved by the PI and Sponsor or Sponsor representative.
4. Receipt of any small molecule investigational agent or trial intervention in a clinical trial within 30 days or 5 half-lives (whichever is longer) prior to Day 1
5. Receipt of any protein or antibody-based therapeutic agents (e.g., growth hormones or mAbs) within 3 months or 5 half-lives (whichever is longer) prior to Screening
6. Receipt of either of the following:
1. Any live/unattenuated vaccinations within 12 weeks prior to Screening
2. Non-living/attenuated vaccinations (including inactivated vaccines, messenger RNA vaccines) within 4 weeks prior to Screening
7. History of active systemic infection within 30 days prior to Day 1
8. Known history of positive test for hepatitis B surface antigen (HBsAg), hepatitis C antibody (HCV Ab), human immunodeficiency virus (HIV)-1 or HIV-2
9. Positive test at Screening for HBsAg, hepatitis B core antibody (HBcAb), HCV Ab, HIV antibody, QuantiFERON®-TB Gold In-Tube test
10. Active treatment for any allergic condition, except for over-the-counter (OTC) antihistamines for seasonal allergy subject to exclusion #11
11. In Part B only, use of any drug that may interfere with the skin prick test results, including antihistamine use within 5 days before each skin prick test
12. History of relevant drug hypersensitivity, or any confirmed significant allergic reactions (urticaria or anaphylaxis), as judged by the PI, against any drug, or multiple drug or food allergies.
13. History of cardiac arrhythmia or family history of sudden cardiac death or a history of additional risk factors for Torsades de Pointes (e.g., heart failure, hypokalemia, hypomagnesia, or family history of Long QT Syndrome)
14. Clinically relevant history of respiratory, gastrointestinal, renal, hepatic, hematological, lymphatic, endocrinological, neurological, cardiovascular, psychiatric, musculoskeletal, genitourinary, immunological, dermatological, ophthalmological, or connective tissue diseases or disorders that, in the opinion of the PI, might pose additional risk to the participant or confound the results of the trial
15. History of malignancy in the previous 5 years
16. History of alcohol or substance use disorder within the previous 5 years
17. History of vasovagal reactions
18. History of hereditary alpha-tryptasemia
19. Positive test result for alcohol or drugs of abuse at Screening or Day -1
20. Clinical laboratory safety test results outside the local reference range at Screening or Day -1 which are deemed to be clinically significant by PI/delegate
21. AST/ALT \>1.5 × ULN or total bilirubin \> ULN at Screening or Day -1 (excluding participants with documented history of Gilbert's syndrome)
22. Screening or Day -1 estimated glomerular filtration rate \<60 mL/min/1.73m2, according to the Chronic Kidney Disease Epidemiology Collaboration equation. Borderline results (±10 mL/min/1.73m2) may be acceptable based on PI's discretion.
23. Screening or Day -1 supine blood pressure (BP) ≥160 mmHg (systolic) or ≥95 mmHg (diastolic), following at least 5 minutes of supine rest. If BP is ≥160 mmHg (systolic) or ≥95 mmHg (diastolic), the BP should be repeated 2 more times (at least 5 minutes between measures) and the average of the 3 BP values should be used to determine the participant's eligibility
24. Screening supine 12-lead ECG demonstrates a QTcF interval \>450 msec for male participants or \>470 msec for female participants, or a QRSD interval \>120 msec or any other relevant ECG finding at Screening. If the QTcF exceeds 450 msec, or QRSD exceeds 120 msec, the ECG should be repeated 2 more times (at least 5 minutes between measures) and the average of the 3 QTcF or QRSD values should be used to determine the participant's eligibility
25. Pregnant, lactating, or breastfeeding, or plan to become pregnant while participating in the trial or within 30 days of the last dose of trial intervention
26. Unwilling or unable to comply with the contraception requirements and lifestyle restrictions
27. Employee of the Sponsor or the site or a relative of an employee at the site
