A Randomized Multicenter Study for Isolated Skin Vasculitis

Inclusion Criteria: 1. Patients with primary skin vasculitis, not associated with any significant extra-cutaneous involvement that would require specific immunosuppressive therapy. Eligible patients will have a diagnosis of either: * Isolated cutaneous small vessel (SV) or medium-sized vessel (MV) vasculitis or cutaneous polyarteritis nodosa (PAN) * IgA vasculitis (IgA, formerly Henoch-Schönlein purpura), without active and/or progressing renal involvement (stable glomerular filtration rate (GFR) \>60 ml/min; absence of, or mild-and-stable microscopic hematuria without red blood cell casts; absence of, or mild-and-stable proteinuria (\<1g/24 hours); not requiring systemic immunosuppressive therapy). These conditions, when skin-limited, are all currently treated in similar manners in practice. Mild arthralgias, myalgias, peripheral limb edema, fatigue, weight loss ≤6 lbs or 3 kg within past 3 months, low-grade fever, and mild anemia (Hb ≥ 10 g/dL) will be allowed. 2. The diagnosis of vasculitis must have been confirmed by skin biopsy prior to enrollment (earlier, at diagnosis, and/or just prior to enrollment) that has included an immunofluorescence study (in the case of small vessel vasculitis). 3. Patients must have active cutaneous vasculitis lasting for at least 1 month continuously and/or have had 2 or more flares over the six months preceding enrollment (post-inflammatory lesions such as hyperpigmentation or healing ulceration(s) are not to be considered active vasculitis). 4. Patients must have active / ongoing cutaneous vasculitis lesions at the time of enrollment (post-inflammatory lesions such as hyperpigmentation or healing ulceration(s) are not to be considered active vasculitis). 5. Patients may have a contra-indication to one of the study drug or have been treated prior to enrollment with one of the study medications but failed to respond to it (according to the study definitions of failure and if they have been on the drug at the target dose or higher for 3 months or longer) or had to stop it because of an adverse event. Such patients can be enrolled directly in the second stage of the study and be randomized to receive one of the two other study drugs. The number of such patients enrolled directly in stage 2 will be capped at 10 (10% of the total recruitment target). 6. Patients may have received systemic glucocorticoids for their cutaneous vasculitis before enrollment. For the patients on prednisone at the time of enrollment, prednisone should be stopped within a maximum of 6 weeks after enrollment and initiation of the study drug, following a pre-defined tapering schedule. Patients on long-term, low and stable dose of glucocorticoids (≤5 mg/day prednisone-equivalent) for other conditions (e.g., asthma or adrenal insufficiency) can be enrolled if the likelihood of requiring a dose increase for this other condition is low during the 6 month study period (these patients will remain on that low and stable dose during the study period, with the option to receive one short course of prednisone at higher doses for skin vasculitis flare during the first 3 months of the study period, like any other patients enrolled). 7. Participant age 18 years or greater. Exclusion Criteria: 1. Presence of significant extra-cutaneous manifestations suggestive of a systemic vasculitis or more diffuse condition. The presence of mild arthralgias, myalgias, peripheral limb edema, fatigue, weight loss ≤6 lbs or 3 kg within past 3 months, low-grade fever, and mild anemia \[Hb ≥ 10 g/dL\] are not exclusion criteria. Mild and stable microscopic hematuria without RBC casts and/or mild and stable proteinuria (\<1g/24 hours) are not exclusion criteria. These latter patients must not require systemic immunosuppressive therapy because of possible renal involvement and their GFR must be \>60 ml/min. 2. Known systemic and/or non-skin-isolated vasculitis, such as granulomatosis with polyangiitis, eosinophilic granulomatosis with polyangiitis, cryoglobulinemic vasculitis, systemic polyarteritis nodosa, central nervous system vasculitis and patients with detectable antineutrophil cytoplasmic antibody (ANCA) by immunofluorescence or ELISA. 3. Hypocomplementemic urticarial vasculitis, cryoglobulinemic vasculitis, and other known secondary skin vasculitides such as those secondary to systemic lupus erythematosus, Sjögren syndrome, another auto-immune condition, a cancer, a hematological disorder, an ongoing active infection, or an ongoing medication. Investigators should consider such underlying diagnoses and perform and interpret appropriate laboratory work-up where indicated based on clinical presentation. 4. History of significant intolerance, allergy or serious adverse events to any of the study medications: such patients can be enrolled directly in the second stage of the study and be randomized to receive one of the two other study drugs. The number of patients enrolled directly in stage 2 of the study will be capped at 10 (10%). 5. Patients who have contra-indications to two or three of the study drugs (azathioprine, colchicine, or dapsone), or have been treated prior to enrollment with two or three of the study drugs but failed to respond to them, or had to stop two or three of them because of adverse events. 6. Deficit in glucose-6-phosphate dehydrogenase (G6PD) or history of hemolytic anemia (all patients must be tested for G6PD at the screening visit to assess for their eligibility): such patients can be enrolled directly in the second stage of the study and be randomized to receive one of the two other study drugs (azathioprine or colchicine). The number of patients enrolled directly in stage 2 of the study will be capped at 10 (10%). 7. Low or absent thiopurine methyltransferase (TPMT) activity (if known, not a requirement for study entry): Patients known to have low or absent TPMT can be enrolled directly in the second stage of the study and be randomized to receive one of the two other s