The main purpose of this study is to evaluate the safety and tolerability of GLPG3970 in healthy volunteers after single oral administrations of GLPG3970 (SAD), compared to placebo (part 1 and 1bis) and after multiple (for 14 days) oral administrations of GLPG3970 (MAD), compared to placebo (part 2). The effect of food (FE) (high-fat, high calorie) on the pharmacokinetics of GLPG3970 and the relative bioavailability (rBA) of an oral solution versus a solid formulation will be assessed (part 3 and 3bis). Part 4 of the study is to evaluate the safety and tolerability of GLPG3970 in subjects with moderate to severe psoriasis when administered daily for 6 weeks.

Inclusion Criteria for Part 1, 1bis, 2, 3 and 3bis:

• Male between 18-55 years of age (extremes included), on the date of signing the informed consent form (ICF).
• A body mass index (BMI) between 18-30 kg/m2, inclusive.
• Judged to be in good health by the investigator based upon the results of a medical history, physical examination, vital signs, 12-lead electrocardiogram (ECG), and fasting clinical laboratory safety tests available at screening and prior to randomization. Hemoglobin must not be below the lower limit of normal range. Bilirubin, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) must be no greater than 1.5x upper limit of normal range (ULN). Other clinical laboratory safety test results must be within the reference ranges, or test results that are outside the reference ranges need to be considered not clinically significant in the opinion of the investigator.

This list only contains the key inclusion criteria for the healthy volunteers part of the study.

Inclusion criteria for Part 4:

• Male or female between 18-65 years of age (extremes included), on the date of signing the ICF.
• Diagnosed with plaque psoriasis ≥6 months.
• Screening Psoriasis Area and Severity Index (PASI) ≥12 (moderate to severe) and affected body surface area (BSA) ≥10%.
• A body mass index (BMI) between 18-35 kg/m2, inclusive.

This list only contains the key inclusion criteria for Part 4 of the study.

Exclusion Criteria for Part 1, 1bis, 2, 3 and 3bis:

• Known hypersensitivity to the Investigational Medicinal Product (IMP) ingredients or history of a significant allergic reaction to IMP ingredients as determined by the investigator.
• Positive serology for hepatitis B virus surface antigen (HBsAg) or hepatitis C virus or history of hepatitis from any cause with the exception of hepatitis A that was resolved at least 3 months prior to first dosing of the IMP.
• History of or a current immunosuppressive condition (e.g. human immunodeficiency virus [HIV] infection).

This list only contains the key exclusion criteria for the healthy volunteers part of the study.

Exclusion criteria for Part 4:

• Subject has evidence of skin conditions other than psoriasis (e.g., eczema) at the time of screening or baseline visit that would interfere with the evaluation of psoriasis.
• Subject is unable to discontinue systemic therapies and/or topical therapies for the treatment of psoriasis and cannot discontinue phototherapy (UVB or PUVA) from signing of the ICF up to the end of the study.

This list only contains the key inclusion criteria for Part 4 of the study.