This is a double-blind, randomised, placebo-controlled trial designed to evaluate safety, tolerability, and pharmacokinetics (PK) after topical administration of single ascending doses of GZ21T in healthy volunteers.

Participants will receive a single topical application of GZ21T or placebo:

Part A:

• Cohort 1: 25.5 mg/cm2 GZ21T or placebo will be applied to 900 cm2 of the skin, corresponding to approximately 5% body surface area (BSA) and 23 g cream.
• Cohort 2: 25.5 mg/cm2 GZ21T or placebo will be applied to 1350 cm2 of the skin, corresponding to approximately 7.5% BSA and 35 g cream.
• Cohort 3: 25.5 mg/cm2 GZ21T or placebo will be applied to 1800 cm2 of the skin, corresponding to approximately 10% BSA and 46 g cream.
• Cohort 4: 25.5 mg/cm2 GZ21T or placebo will be applied to the face, corresponding to approximately 3-3.5% BSA (540 - 630% cm2) and 16 g cream.

Participants will come for 3 visits to the research clinic for screening, treatment, and follow-up.

Sentinel dosing will be applied. All participants will be carefully monitored by clinical staff during and after IMP application and will remain at the research clinic for at least 24 hours after treatment (Day 2) for safety assessments, including safety laboratory testing, 12-lead ECG, vital signs, local tolerability, physical examination and AEs, and PK assessments.

Part B:

• Cohort 1: 13 mg/cm2 GZ21T will be applied to 900 cm2 of the skin, corresponding to approximately 5% body surface area (BSA)
• Optional cohorts 2 and 3: A single dose of GZ21T decided based on the results from preceding cohorts will be applied to 900 cm2 of the skin corresponding to approximately 5% BSA. Participants will come for 3 visits to the research clinic and 1 telephone visit for screening, treatment, and follow-up.

All participants will be carefully monitored by clinical staff during and after IMP application and will remain at the research clinic for 2 hours after treatment for local tolerability and AE evaluation. On Day 2 (Visit 3), approximately 24 hours post-dose, participants will visit the research clinic for follow-up of local tolerability and AEs. A remote telephone call will be performed on Day 7 (Visit 4) to follow-up on local tolerability and AEs.

Inclusion Criteria:

1. Willing and able to give written informed consent for participation in the trial.
2. Healthy male or female participant aged 18 to 70 years, inclusive.
3. Body Mass Index (BMI) ≥ 18.5 and ≤ 30.0 kg/m2 at the time of the screening visit.
4. WOCBP must practice abstinence from heterosexual intercourse (only allowed when this is the preferred and usual lifestyle of the participant) or must agree to use a highly effective method of contraception with a failure rate of <1 % to prevent pregnancy from at least 2 weeks prior to the administration of IMP to 4 weeks after the last administration of IMP. In addition, any male partner of a female participant must, unless he is sterile (e.g., has undergone vasectomy), agree to use a condom from the first administration of IMP until 4 weeks after the last administration of IMP.

WOCBP must refrain from donating eggs from the first IMP administration until 3 months after the last IMP administration.

Exclusion Criteria:

1. History of any clinically significant disease or disorder which, in the opinion of the Investigator, may either put the participant at risk because of participation in the trial or influence the results or the participant's ability to participate in the trial.
2. Any clinically significant illness, medical/surgical procedure, or trauma within 4 weeks of the administration of IMP.
3. Any clinically significant abnormalities regarding physical examination, vital signs, 12- lead ECG, and laboratory values at the time of the screening visit, as judged by the Investigator.
4. Malignancy within the past 5 years, including removal of basal cell carcinoma.
5. Any planned major surgery within the duration of the trial.
6. Any skin condition including tattoos that may limit the evaluation of e.g., local tolerability as judged by the Investigator.
7. History of chronic urticaria, known history of urticaria triggered by specific factors or currently experiencing an episode of urticaria within the past 3 months.
8. History of psoriasis, atopic eczema and similar conditions, as judged by the Investigator.
9. Prescence of body hair or tattoos on the intended application areas, which in the opinion of the Investigator could interfere with local tolerability assessments.
10. Females who are pregnant, currently breastfeeding, or intend to become pregnant during the course of the trial.
11. Any positive result at the screening visit for serum hepatitis B surface antigen, hepatitis B and C antibodies and/or human immunodeficiency virus (HIV).
12. After 10 minutes of supine rest at the screening visit, any vital signs values outside the following ranges: - Systolic blood pressure: <90 or ≥140 mmHg, or - Diastolic blood pressure <50 or ≥90 mmHg, or - Pulse <40 or >90 bpm
13. Prolonged QTcF (>450 ms), cardiac arrhythmias or any clinically significant abnormalities in the resting ECG at the screening visit, as judged by the Investigator.
14. History of severe allergy/hypersensitivity or ongoing allergy/hypersensitivity, as judged by the Investigator, or history of hypersensitivity to drugs or food with a similar chemical structure or class to GZ21T.
15. Regular use of any prescribed or non-prescribed medications, including antacids, analgesics, herbal remedies, within 2 weeks prior to the (first) administration of IMP, except occasional intake of paracetamol (maximum 2000 mg/day and not exceeding 3000 mg/week), as well as nasal decongestants without cortisone, antihistamine, or anticholinergics for a maximum of 10 days, at the discretion of the Investigator.
16. Unwillingness to abstain from the use of topical treatment (including but not limited to corticosteroids, calcineurin inhibitors, vitamin D analogues, and retinoids) at the application site within 1 week prior to Day 1 and from the use of moisturising ointment cream, emollients, oils (including shower oil) or sunscreen within 24 hours prior to Day 1 until 1 week after IMP administration.
17. Planned treatment or treatment with another investigational drug within 3 months prior to Day 1. Participants who consented and screened but were not dosed in previous clinical trials are not to be excluded.
18. Current smokers or users of nicotine products. Irregular use of nicotine (e.g., smoking, snuffing, chewing tobacco) less than 3 times per week is allowed before screening visit.
19. Positive screening result for drugs of abuse or alcohol at the screening visit or on admission to the trial site prior to the administration of the IMP. (Positive results that are expected given the participant's medical history and prescribed medications can be disregarded as judged by the Investigator.)
20. History of alcohol abuse or excessive intake of alcohol, history or presence of drug abuse including anabolic steroids, as judged by the Investigator.
21. Plasma donation within approximately 1 month of screening or blood donation (or corresponding blood loss) during the last 3 months prior to screening, at the discretion of the Investigator.
22. The Investigator considers the participant unlikely to comply with trial procedures, restrictions and requirements.

AEs (including serious AEs [SAEs]) were collected from the start of IMP administration until the end-of-trial visit of each part, from day 1 to day 7.

The grading of the severity/intensity (grade 1 to grade 5) of AEs followed the common terminology criteria for AEs (CTCAE) v5.0. AEs were assessed as not related, unlikely related, or possibly, probably or definitely related to the IMP.

Part A: Double-blind Part B: Open-label