Apremilast in Patients With Moderate to Severe Palmoplantar Pustulosis (PPP) (APLANTUS)

Inclusion Criteria: * Male and female patients aged 18 years or more at screening visit. * Patients with chronic PPP (disease history of at least 6 months of diagnosis), who were eligible for treatment with systemic therapy defined as having PPP inadequately controlled by topical treatment and/or phototherapy and/or previous systemic therapy * Patients with chronic moderate to severe PPP defined as patients with a PPPASI ≥12 with or without concomitant plaque-type psoriasis * Negative result of a urine pregnancy test taken at screening and at baseline for all women, except those who were surgically sterile or at least 1 year postmenopausal (i.e. at least 12 consecutive months with amenorrhea without other known or suspected medical cause) * Willingness and capability of using a highly effective contraceptive measures from Screening visit until the end of at least one menstrual cycle (but not less than 28 days) following discontinuation of apremilast as defined below: * Female patient of childbearing potential (fertile, following menarche and until becoming post- menopausal unless permanently sterile) using a highly effective method of contraception OR female patients of non-childbearing potential (surgically sterilized \[e.g. hysterectomy, bilateral salpingectomy and bilateral oophorectomy\] or postmenopausal) * Male patient, and their female partner of childbearing potential, using a highly effective method of contraception * Adequate contraceptive method defined as: * A method with less than 1% failure rate (e.g. permanent sterilization, hormone implants, hormone injections, some intrauterine devices, or vasectomized partner) OR * The use of two methods of contraception (e.g. one barrier method \[condom, diaphragm or cervical/vault caps\] with spermicide and one hormonal contraceptive \[e.g. combined oral contraceptives, patch, vaginal ring, injectables and implants\]) * Patient was capable of understanding and giving written, voluntary informed consent before study screening. * Willingness and capability of complying with all study procedure requirements, as per the Investigator's judgment (e.g. patient able to swallow the apremilast tablets, blood sampling). Exclusion Criteria: * General: * Pregnant or breast-feeding women * Current or history of psychiatric disease that would interfere with the ability to comply with the study protocol or give informed consent * Patients known to have had a substance abuse (drug or alcohol) problem within the previous 12 month * Individuals who were involved in the organization of the study * Patients who were in any way dependent on the investigator * Patients who were participating in a clinical study * Relatives, partner or staff of any clinical site personnel * Disease-related: * Evidence of skin conditions (e.g. eczema) other than PPP/psoriasis that would interfere with evaluations of the effect of study medication on PPP or psoriasis. * Laboratory values from routine blood test taken within the 8 weeks prior to screening with any of the following: * Serum creatinine \>1.4 x upper limit of normal (ULN) for age and gender * Estimated Glomerular Filtration Rate (eGFR) \<30 mL/min/1.73m2 according to the Chronic Kidney Disease-Epidemiology Collaboration (CKD-EPI) equation * Pustular psoriasis lesions on the part of body other than hands or feet * Significant concurrent medical conditions at the time of screening, including: * Risk factors for renal toxicity (renal inflammation) * Severe hepatic dysfunction * Unstable angina pectoris * Uncompensated congestive heart failure * Severe pulmonary disease requiring hospitalization or supplemental oxygen therapy * Immunodeficiency disorders: primary or secondary * Known positive human immunodeficiency virus (HIV) test result, hepatitis B surface (HBS) antigen or hepatitis C virus (HCV) test result * Uncontrolled insulin-dependent diabetes mellitus * Cancer or history of cancer (except for resected cutaneous basal cell or squamous cell carcinoma) in the last 5 years * Open cutaneous ulcers * Any condition that, in the judgment of the investigator, might cause this study to be detrimental to the patient. * Medication-related: * Ultraviolet B (UVB) therapy, topical steroids, topical calcineurin inhibitors, topical Vitamin A or D analog preparations, or anthralin within 14 days of baseline. Exceptions: low potency topical corticosteroids (class I and II, according to European classification for potency of topical corticosteroids) were allowed as therapy for the face, groin, axillae in accordance with the manufacturer's suggested usage dose * Psoralen plus ultraviolet A radiation (PUVA), ciclosporin, acitretin, alitretinoin, alefacept (Amevive®), anakinra (Kineret®), systemic corticosteroids, methotrexate, fumaric acids or any other systemic anti- psoriasis therapy within 28 days of baseline * Prior (within the last 2 years) or concomitant use of antipsoriatic biologic therapy with TNF-alpha blocker and/or ustekinumab and/or ixekizumab and/or secukinumab and/or brodalumab and/or guselkumab * Concomitant use of strong cytochrome P450 3A4 (CYP3A4) enzyme inductors (e.g. rifampicin, phenobarbital, carbamazepin, phenytoin and St. John's wort) * Use of an investigational drug within 4 weeks prior to baseline or 5 pharmacokinetic/pharmacodynamics half-lives (whichever is longer) * Prior treatment with apremilast/Otezla® * Receipt of any live (attenuated) vaccine within 28 days prior to baseline * Concomitant use of any other PDE4 inhibitor * Patients with hereditary problems of galactose intolerance, lapp lactase deficiency or glucose-galactose malabsorption * For patients with skin biopsy samples taken: patients with clinically relevant coagulation disorders or medication or known hypersensitivity against local anesthetics.