Introduction. Primary Sjögren's syndrome (pSS) is an autoimmune disease characterized by chronic inflammatory infiltration of the salivary and lacrimal glands causing dry eye and mouth. Multiple systemic manifestations can also occur. Hydroxychloroquine (HCQ), an antimalarial immunomodulator, has been used for the treatment of arthralgias, myalgia and constitutional symptoms resulting from pSS. However, there are no studies that evaluated whether blood levels of HCQ could influence in the therapeutic response, as has been reported in systemic lupus erythematosus (SLE).

Objectives. Analyze in patients with pSS receiving HCQ as part of their treatment: blood levels of HCQ; adherence using a questionnaire versus blood levels and the possible correlation of blood levels with the disease activity score in a cross-sectional evaluation followed by a longitudinal six-month assessment.

Patients and methods. Observational cross-sectional evaluation followed by a six-month longitudinal assessment, including patients with pSS receiving HCQ for at least 3 months at study admission. Patients will be evaluated clinically and the HCQ blood level measured at the beginning of the study (T0), at 3 months (T3), and at 6 months (T6). As there are no previous studies on blood levels of HCQ in patients with pSS, the sample of 75 patients was calculated based on the percentage of these patients currently using HCQ in our service (nearly 50%) and in the number of patients in current follow-up (about 150). We will evaluate 75 adult patients with pSS according to the classification criteria of the American-European Consensus Group 2002 and/or the American College of rheumatology (ACR) and the European League Against Rheumatism (EULAR) 2016, of both sexes and followed up regularly at the Sjögren Syndrome outpatient Clinic of the Rheumatology Service of "Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo (HCFMUSP)". The following will be used in the clinical evaluation: the xerostomia Inventory; the xerophthalmia questionnaire - Ocular Surface Disease Index (OSDI); the EULAR Sjögren Syndrome Reported Index (ESSPRI); the EULAR Sjögren Syndrome Disease Activity Index (ESSDAI); the Sjögren's Syndrome Disease Damage Index (SSDDI); the non-stimulated and stimulated salivary flow. Blood levels of HCQ will be measured by high performance liquid chromatography and tandem mass spectrometry.

Inclusion Criteria:

• pSS according to the classification criterias
• Who have been receiving HCQ for at least 3 months before study entry
• Patients who agree to participate in the study, according to the signing of the Free and Informed Consent Term (ICF).

Exclusion Criteria:

• Present other associated systemic autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosus, systemic sclerosis, demato/polymyositis, mixed connective tissue disease, spondyloarthritis, primary biliary cholangitis and autoimmune hepatitis;
• Present a previous history of radiotherapy of the head and neck, positive serologies for HIV, hepatitis B and C, sarcoidosis, graft versus host disease and IgG4-related disease;
• Use of drugs that can interact with HCQ or interfere in HCQ blood levels (tamoxifen, antacids and digoxin), alcoholism, current infections, liver and heart failure, dialysis, severe chronic renal failure (serum creatinine ≥3 mg/dL) and pregnancy.

pSS patients according to the classification criterias regularly followed at outpatient clinic, Rheumathology division of Hospital das Clinicas da Universidade de Sao Paulo