Rituximab and Belimumab Combination Therapy in PR3 Vasculitis

AAV is an organ and life threatening multisystem autoimmune disease, where ANCA are strongly implicated in disease pathogenesis, causing neutrophil activation and endothelial damage. B cell depletion with rituximab, and treatment with glucocorticoids, is associated with reduction in ANCA levels and clinical remission in AAV. However, patients with proteinase 3 (PR3) ANCA subtype and/or predominantly granulomatous disease have a lower remission rate (42% vs 9% failure rate compared to other subtypes) after rituximab and glucocorticoids, with a high subsequent relapse risk of 50% by 13 months. There is a need for newer therapies to reduce the time to remission, to spare glucocorticoid use, and to promote long-lasting remission without risk of relapse. Scientific evidence suggests that dual B-cell targeted immunotherapy with both B cell depletion (i.e. rituximab as anti-CD20) and B lymphocyte stimulator (BLyS) blockade (i.e. belimumab) may be more efficacious than targeting either mechanism alone. Therefore, this mechanistic trial will assess whether dual B-cell immunotherapy by co-administration of rituximab and belimumab will result in improvements in mechanistic endpoints, functional outcomes and clinical status compared to rituximab with placebo.

The efficacy of B cell therapy depends on depletion of pathogenic B cells and the regulated reconstitution of the B cell compartment. Response to rituximab is associated with peripheral blood B cell depletion, but this is incomplete on high resolution FACS and at the disease tissue level in ANCA vasculitis patients. Early relapse is associated with a failure to become ANCA negative by 6 months, a failure of tissue B cell depletion (including PR3 specific B cells), a high proportion of memory B cells before rituximab treatment and early peripheral B cell reconstitution with a predominant memory phenotype.

Combining B cell depleting therapy (rituximab) with BLyS antagonism (belimumab) may enhance B cell targeting in AAV through several mechanisms: belimumab reduces both CD20+ and CD20- plasmablast populations in SLE patients hence combination therapy may impact a broader B cell population than targeting CD20 alone. High BLyS levels in tissue niches may also retain B cells and protect against depletion by rituximab. As observed in the BLISS studies, belimumab is associated with an early rise in peripheral blood memory B cells, possibly due to mobilisation from lymphoid tissue. Studies on tissue B cell depletion and BLyS in pre-clinical models support the concept of combining anti-CD20 and BLyS targeting and assessing tissue depletion in lymph node biopsies as well as in blood. High BLyS levels during B cell reconstitution post rituximab can promote return of autoreactive B cell resulting in more severe flares. Regulation of BLyS levels post depletion is thought to set a higher stringency for B cell reconstitution, selecting out autoreactive B cells and would directly target any BLyS driven rebound effect.

Rituximab will be dosed at Days 8 and 22, after initiation of belimumab and discontinuation of baseline immunosuppressants. Dosing at Day 8 and Day 22 is justified by: a) separation of start times for belimumab and rituximab, thereby allowing for observation of safety events which may be attributable to starting treatment with the individual agents, and b) evidence that belimumab may mobilise B cells into the periphery making them available targets for anti-CD20 treatment, therefore, starting belimumab prior to rituximab may allow more efficient peripheral B cell depletion by rituximab. Continuing belimumab therapy for 52 weeks ensures that anti-BLyS activity continues during the critical timeframe of B cell reconstitution post rituximab (median time 8.5 to 12.6 months) reducing the potential for high levels of BLyS during this time. Assessments during follow-up after completion of beliumumab / belimumab-placebo therapy allow assessment of whether immunological and clinical remission is maintained once B cell reconstitution has taken place.

A barrier to research of B cell targeted therapy has been the difficulty in obtaining sequential cells from sites where the immune dysregulation occurs or sites of inflammation. Therefore, the inclusion of both lymph node biopsies and nasal tissue biopsies in this trial will potentially permit direct characterisation of pathogenic cells at key sites, their microenvironment and, critically, the interaction of B cells with helper T cells, the primary drivers of the abnormal immune response