Study to Evaluate the Safety, Tolerability, and Pharmacokinetics of Single and Repeat Topical Administration of BOS-475 in Healthy Subjects and Patients With Psoriasis

Inclusion Criteria

• •Part A
• •Healthy male or female participants 18 to 65 years of age inclusive, at the time of signing the informed consent.
• •* Male participants must agree to use contraception as detailed in the protocol during the treatment period and for at least 90 days (a spermatogenesis cycle) after the last dose of study drug and refrain from donating sperm during this period.
• •o Male participants who have had a vasectomy with documentation of azoospermia are not required to use contraception.
• •* Female participants must be of non-child bearing potential, defined as 1) at least 12 months of spontaneous amenorrhea with follicle stimulating hormone (FSH) \> 40 milliInternational Units per milliliter (mIU/ml), or 2) having a documented tubal ligation at least 6 weeks prior to dosing; or 3) having had a surgical bilateral oophorectomy (with or without hysterectomy).
• •Participants who are healthy as determined by the Investigator or medically qualified designee based on a medical evaluation including medical history, physical examination, laboratory tests, and cardiac monitoring.
• •Participants with body mass index (BMI) within the range 18 to 30 kilograms per meters squared (kg/m\^2) (inclusive).
• •Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the Informed Consent Form (ICF) and in this protocol
• •Willing to refrain from using any topical treatments, other than those mandated by the protocol or for protocol procedures
• •Part B
• •Male or female participants with mild to moderate psoriasis 18 to 65 years of age inclusive, at the time of signing the informed consent
• •* Male participant must agree to use contraception as detailed in the protocol during the treatment period and for at least 90 days (a spermatogenesis cycle) after the last dose of study drug and refrain from donating sperm during this period.
• •o Male participants who have had a vasectomy with documentation of azoospermia are not required to use contraception.
• •* Female of non-child bearing potential is defined as 1) at least 12 months of spontaneous amenorrhea with FSH \> 40 mIU/mL, or 2) having a documented tubal ligation at least 6 weeks prior to dosing; or 3) having had a surgical bilateral oophorectomy (with or without hysterectomy).
• •Participants who have a clinical diagnosis of stable plaque psoriasis for ≥ 6 months, as confirmed by the Investigator
• •A Psoriasis Physician Global Assessment (PGA) score of ≥ 2 at screening and Day 1
• •At least 1 psoriasis plaque located on the trunk or extremities (excluding knees and elbows) that is at least 5 centimeters squared (cm\^2) in size at Screening and Day 1 with a Target Plaque Severity Score (TPSS) ≥ 5 and induration subscore ≥ 2
• •Body Surface Area (BSA) involvement of psoriasis lesions between 2% and 15%, excluding face, scalp, palms, soles, nails, and intertriginous areas at screening
• •Participants with BMI within the range 18 to 35 kg/m\^2 (inclusive)
• •Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the ICF and in this protocol
• •Willing to refrain from using any topical treatments, other than those mandated by the protocol or for protocol procedures