The aim of the exploratory study is to compare the dose-related efficacy of LAS41004 in a Psoriasis Plaque Test (PPT).

Key Inclusion Criteria:

• Male or female patients between 18 and 75 years of age with a diagnosis of stable plaque-type psoriasis (psoriasis vulgaris) for at least 12 months
• Psoriatic plaques that are suitable to be defined as target area lesions by the following criteria:

oPsoriatic plaques must be located on the trunk and/or extremities. Plaques that are located on the head (incl. scalp), palms, sole of the feet, intertriginous or genitoanal areas are not suitable as target areas

• Comparable psoriatic plaques with at least "2" in each score (range 0-4) for the three distinct signs: scaling, erythema and induration
• No more than 3 points difference in total score (= sum of scores for scaling, erythema and induration; Range 0-12) of the chosen comparable psoriatic plaques
• Enough surface area to the psoriatic plaque to define eight clearly distinguishable (minimum distance between test areas: 1cm) test areas of at least 1 cm² size - Effective contraceptive methods for the duration of the study and for the 4 weeks after study drug discontinuation, for a patient of childbearing potential.

Key Exclusion Criteria:

• Too small a body surface area covered with psoriasis plaques that meet the specified inclusion criteria
• Any condition that may interfere with the study assessments or ultrasound measurements of the skin and/or may have an influence on the skin immune response (incl. open wounds)
• Current diagnosis of guttate, erythrodermic or pustular psoriasis
• Patients who have received any topical anti-psoriatic drug in the test area within the last 2 weeks before treatment with the IMP.
• Patients who have received topical products in the test area containing DEET (N,N-diethyl-m-polyamide), a common component of insect repellent products within 1 week before treatment with the IMP.
• Patients who have received systemic biologics within the last 6 months before treatment with the IMP or any systemic antipsoriatic treatment (corticosteroids, ciclosporin, methotrexate, fumaric acid esters, acitretin etc)within the last 3 months before treatment with the IMP.
• Significant skin diseases other than plaque-type psoriasis (psoriasis vulgaris), including but not limited to: skin disease due to tuberculosis or syphilis, rosacea, perioral dermatitis, acne vulgaris, skin atrophy, striae distensae, increased fragility of skin blood vessels, ichthyosis, ulcers, wounds and reaction after vaccination
• Significant skin infections (including but not limited to: herpes, varicella, mycotic skin infections, bacterial skin infections or parasitic skin infections)
• Excessive sunlight exposure within 28 days prior to study entry and during the conduct of the study
• Usage of any topical formulation (incl. cosmetics, emollients, etc) on the designated target areas during the course of the study
• Vitamin A intake > 15,000 IU/day