Pilot STUDY to Monitor Ex-vivo the Biological Effects of the C3 Inhibitor, PEGCETACOPLAN, on Activation of Complement in Fluid Phase and on Podocytes in Patients With C3G and Primary IC-MPGN

The term membranoproliferative glomerulonephritis (MPGN) defines a glomerular lesion characterized by hypercellularity, mesangial matrix expansion and thickening of the glomerular basement membrane (GBM). This pattern may be seen in the setting of various secondary conditions as well as in primary forms that are distinguished into C3 glomerulopathy (C3G) and immunoglobulin-associated MPGN (IC-MPGN), on the basis of immunofluorescence showing predominant or exclusive C3 deposits in C3G and combined C3 and Ig deposits in IC-MPGN (Smith et al, 2019) (Lu et al, 2012, Servais et al, 2012, Riedl et al, 2017, Bomback et al, 2018).

Primary C3G and IC-MPGN are rare, with an estimated incidence of 1-3 cases/million/year.

The immunofluorescence (IF)-based classification assumes that C3G arises from abnormalities in the control of the complement alternative pathway while IC-MPGN derives from glomerular deposition of immune complexes that activate predominantly the complement classical pathway (Pickering et al, 2013). This assumption may imply that C3G and IC-MPGN should be treated differently. However, there are several intersections between primary C3G and IC-MPGN. Cohort studies have not consistently identified renal outcome differences between C3G and IC-MPGN and both are characterized by a high rate of recurrence in the grafts. C3G and IC-MPGN share the same prevalence of low serum C3 levels and of complement abnormalities. These include the autoantibodies C3NeFs or C5NeFs, which stabilize the C3 and C5 convertases, anti-factor B and anti-factor H antibodies, and genetic abnormalities affecting complement regulators or the two components of the alternative pathway C3 convertase, C3 and factor B (Benigni et al, 2026; Noris and Remuzzi, 2024; Donadelli et al, 2018).

Functional studies revealed that C3G- and primary IC-MPGN- associated genetic and acquired abnormalities cause excessive activation of the alternative pathway C3 convertase (C3bBb) that cleaves C3 into the active C3b and C3a fragments, which supports a role of the complement alternative pathway in the pathogenesis of C3G and IC-MPGN as well (Noris et al, 2019; Marinozzi et al, 2017; Tortajada et al, 2013).

Finally, there are patients who present with a bioptic pattern of IC-MPGN and a subsequent biopsy shows isolated C3 staining compatible with a C3G. Taken together, these data indicate that in order to have a clear understanding of these diseases, it is necessary to go beyond the IF-based classification. In a recent study, we analyzed detailed clinical, histological and complement data from 295 patients with C3G and primary IC-MPGN and applied hierarchical clustering to group them into five distinct clusters based on shared disease etiology. Cluster 1 and 2 patients showed systemic complement activation until C5, but Cluster 2 distinguished from cluster 1 for classical pathway activation markers. Cluster 3 showed C3-restricted systemic complement activation. Cluster 4 and 5 patients shared a normal complement profile and intense glomerular C3 staining, consistent with solid-phase complement activation (Benigni et al, 2026).

Recently, pegcetacoplan, a complement factors C3 and C3b inhibitor, has been approved by the U.S. Food and Drug Administration (FDA) and EMA as treatment for patients with C3G and primary IC-MPGN in native kidneys and post-transplant C3G recurrence.

Pegcetacoplan binds C3 and its activation fragment C3b, thereby inhibiting C3 cleavage and generation of downstream complement effectors through the classical, lectin, and alternative pathways. Furthermore, pegcetacoplan directly inhibits the C3- and C5-convertases of classical/lectin, and alternative pathways by blocking C3b in these complexes. Consequently, pegcetacoplan is predicted to halt C3 and C5 activation in all 5 clusters of patients with C3G and primary IC-MPGN, which could prevent glomerular complement deposition and kidney failure.

Consistently, in phase 2 trials, pegcetacoplan showed efficacy and safety in patients with C3G or primary IC-MPGN. The primary analysis of VALIANT, a phase 3, double-blind, placebo-controlled trial involving adolescents and adults with C3G or primary IC-MPGN in native kidneys or with disease recurrence after transplantation, was published (Fakhouri et al, 2025). Results at week 26 showed a relative reduction of 68% of urinary protein-to-creatinine ratio (PCR) as compared with placebo. In addition, 49% of patients in the pegcetacoplan group met the composite renal end-point of stabilization of eGFR and ≥50% reduction in urinary PCR vs only 3% in the placebo arm.

Preliminary results of the open-label period of the VALE extension trial indicated that longer pegcetacoplan treatment will result in higher % of patients reaching the composite end-point. Finally, real-world observational cohort data confirmed the efficacy of pegcetacoplan in pediatric and adult C3G/IC-MPGN patients, although a few patients did not respond to treatment (Bassanese et al, 2026). In this regard, variability in the biological response to pegcetacoplan among patients needs to be fully characterized and collection of ex vivo data may contribute to a better understanding of the biological effects of pegcetacoplan in patients with C3G and IC-MPGN Our group has developed two ex vivo assays that evaluate complementary aspects of complement dysregulation in C3G and IC-MPGN: fluid-phase C3 convertase activity and complement deposition on cultured podocytes. In preliminary studies, both assays showed marked complement hyperactivation induced by patients' sera and robust inhibition following exposure to pegcetacoplan.

These findings provide the rationale for using these assays as pharmacodynamic tools to monitor the biological activity of pegcetacoplan during treatment. Longitudinal assessment of complement inhibition may improve understanding of treatment response variability and help identify potential biomarkers of adequate target engagement in patients with C3G and IC-MPGN.