Psoriasis is a chronic inflammatory condition driven by a complex interplay between heritable and microenvironmental factors. Genome-wide heritability for psoriasis (PsO) is estimated at up to 50%. Although bearing genetic susceptibility at birth, most patients will remain disease free for years until an exposomal trigger activates the immunological pathway that leads to the first disease flare. In rare patients, the first flare is an isolated event, but in most patients it will evolve into a chronically relapsing-remitting disease characterized by lasting lesions and recurrent flares. Primary objective is to compare efficacy of bimekizumab versus topical corticosteroids on psoriasis clinical disease activity, assessed by PGA, at week 16 and week 24.

Patients will be randomized 1:1 to receive either 320mg bimekizumab at weeks 0 (W0), W4, W8 and W12, or clobetasol ointment for 2 to 4 weeks until complete clearance of the lesion. After a maximum of 4 weeks topical clobetasol will be applied twice weekly on the site of lesion until week 16 then stopped. Patients will not receive active treatment between week 16-24. Between week 24-96 patients that flare can receive continuous topical clobetasol if needed. These patients will be considered non-responders at subsequent timepoints for the main analysis. If the patients worsen their psoriasis under the treatments given during the study and progress to moderate or severe psoriasis (PASI 10 and above) they will end their participation to the study to receive a treatment adapted to the new severity of their psoriasis. The patients will be referred to their dermatologist to receive the actual standard of care adapted to their new condition.

During the study, the following assessments will be performed and samples will be collected

Inclusion Criteria:

1. Men and women

2. ≥ 18 and <45 years

3. Plaque psoriasis without psoriatic arthritis

4. Patients with mild psoriasis PASI >2 and <6

5. Patient with at least one lesion on the elbows, or the knees, or the lower back to be recorded as target lesion (additional lesions in other areas on top are allowed)

6. Disease duration less than 6 months (short duration psoriasis) or >2 years (long duration psoriasis)

7. The psoriasis lesions should not having being treated by any topical treatment for at least 2 weeks

8. For women of childbearing potential, an effective contraception (estroprogestative pill, contraceptive implant, IUD, condoms or tubal ligation) should be used for more than one month before the inclusion in the study. A urine pregnancy test (βHCG in urines) will be performed.

   Thus, a woman who is permanently sterile and therefore unable to procreate should not be subjected to pregnancy tests.

   Women who are sexually abstinent are not requested to use a contraception. However, they must agree to take one if they want to become sexually active.

9. Affiliation to a social security system

10. Signed informed consent

11. Patient willing and able to attend all study visits

Exclusion Criteria:

1. Pregnant or breast-feeding women. Or women who plan to get pregnant during the study duration.
2. Concomitant use of topical or systemic immunosuppressive medication or steroids in the past 12 weeks
3. Personal history of skin cancer
4. Personal history of cancer of less than 5 years
5. Patients with active infection
6. Abnormal blood counts (neutrophils <1500/mm3 and platelets <150 000/mm3) and/or positive HIV, HVB and HVC testing at screening.
7. Patients with personal history of keloid scars
8. Patients with personal history of hypersentitivity to xylocaine and/or adrenalin
9. Vulnerable people: minors, adult under guardianship or deprived of freedom
10. Participants in other clinical therapeutic studies involving a drug that could interfere with the present evaluation