ETA and AT1 Antagonism in ANCA-vasculitis (SPARVASC)

Inclusion Criteria: * 18 years or older * A diagnosis of ANCA-associated vasculitis that has been in remission for ≥6 months. The diagnosis of AAV will have been made in accordance with the 2012 Revised International Chapel Hill Consensus Conference Nomenclature of Vasculitides criteria. Remission will be defined as a Birmingham Vasculitis Activity Score (BVAS) of 0 for at least 2 months prior to the screening visit whilst taking prednisolone at daily dose ≤7.5mg, in conjunction with the treating clinician's assessment of clinically silent disease. * eGFR ≥25ml/min/1.73m2 at screening. * Women of childbearing potential, beginning at menarche, must agree to the use of one highly reliable method of contraception (ie, a failure rate of \<1% per year) for at least 30 days prior to the first dose of the study medication (ie, for hormonal contraception) or according to manufacturer's recommendation (ie, for an intrauterine device) until 30 days after the last dose of the study medication, and must have a negative pregnancy test at screening. Women of childbearing potential are defined as those who are fertile, following menarche and until becoming postmenopausal, unless permanently sterile; permanent sterilization methods include hysterectomy, bilateral salpingectomy, and bilateral oophorectomy. A postmenopausal state is defined as amenorrhea for more than 24 consecutive months without an alternative medical cause; women on hormone replacement therapy must have a documented plasma follicle-stimulating hormone level \>40 mIU/mL. Exclusion Criteria: * Age \<18 years * Active vasculitis * Liver disease * Untreated hypertension (defined as systolic blood pressure \>160 bpm and diastolic blood pressure \>100 bpm) * eGFR \<25ml/min/1.73m2 * Any organ transplant recipients * Haemodialysis/peritoneal dialysis patients * A requirement for any medications contraindicated whilst taking sparsentan * Congestive heart failure * Patients not medically fit to attend for study visits * Patients without mental capacity or willingness to provide informed consent * History of multiple and/or severe (clinical judgement as determined by the investigator) allergic reactions to drugs, including the study drug or food. * Patients who are pregnant or breast feeding, or those who plan to become pregnant during the study * Participation in another clinical trial for 28 days before or 90 days after the study period.