The place of cyclical therapy for the treatment of membranous nephropathy in the era of rituximab

ABSTRACT

Primary membranous nephropathy (MN) is the most frequent cause of nephrotic syndrome in adults, due to a variety of autoantibodies, most frequently against phospholipase A2 receptor (PLA2R). In severe cases or when spontaneous remission is not achieved, immunosuppression is required. Cyclical therapy, based on glucocorticoids and cyclophosphamide on alternate months for 6 months, has proven effective to induce remission and reduce the risk of end-stage renal disease. Since the early 2000s, rituximab (RTX) has emerged as a key player in the management of MN, showing overall comparable effectiveness and likely better safety compared with the cyclical regimen, despite the lack of adequately powered trials comparing the two approaches head to head. For these reasons, RTX is now considered the agent of choice for most patients with MN. However, there are still uncertainties. Around 20–40% of patients are resistant to RTX, especially in the setting of high anti-PLA2R levels, and this drug remains relatively unexplored in patients with the most severe disease. In these scenarios, although the expanding therapeutic armamentarium is probably going to provide further options, the cyclical regimen still plays a key role as a safety net. The aim of this article is to illustrate the role of cyclical therapy in the RTX era.

Primary membranous nephropathy (MN) is an autoimmune disease caused by the deposition of immune complexes in the subepithelial space of the glomerulus [1]. It is the most frequent cause of nephrotic syndrome in adults and the disease course may be variable, with around one-third of patients entering spontaneous remission [2]. If remission is not achieved, deterioration of renal function can occur, up to end-stage renal disease (ESRD). Furthermore, persistent nephrotic syndrome exposes patients to significant complications, such as fluid overload, thrombosis and infections. Once secondary forms are ruled out, immunosuppression may be required in patients not achieving spontaneous remission or presenting with severe manifestations of nephrotic syndrome or kidney function deterioration.

The first immunosuppressive approaches employed in the 1970s and 1980s were based on alkylating agents or glucocorticoids. The studies comparing these treatments with placebo provided conflicting evidence, with remission rates ranging from ≈20 to 65% [3–6]; of note, even when effective, sustained remission was not achieved. Importantly, when the alkylating agent employed was chlorambucil, an increased risk of cancer was observed [7]. A revolution in disease management was achieved when glucocorticoids (intravenous pulses, followed by oral administration) and alkylating agents (oral cyclophosphamide or chlorambucil) were combined in the so-called cyclical regimen. This approach was based on the administration of these drugs on alternate months for an overall treatment course of 6 months. The cyclical regimen proved to be more effective than supportive treatment alone in inducing remission in three randomised controlled trials, with response rates ranging from 62 to 72% [8–11]. The benefit of this approach was also confirmed in the long-term, leading to a reduced 10-year risk of ESRD or death (8% in the cyclical regimen group versus 40% in the untreated group [10]), as well as ESRD alone (11% in the cyclical regimen versus 35% with supportive treatment alone [11]). Of note, when a cyclophosphamide- and chlorambucil-based cyclical regimen were compared in a randomised controlled trial enrolling 83 patients, clinical outcomes were similar across the two groups, both in terms of remission rates (55% and 56%, respectively) and relapse rates (30% and 25%, respectively). However, the chlorambucil-based approach was less tolerated, with six patients withdrawing treatment due to side effects compared with two in the cyclophosphamide arm [12]. Despite the effectiveness of cyclical therapy, concerns regarding the toxicity of such an approach drove researchers to explore other options, ranging from calcineurin inhibitors (CNIs) to mycophenolate mofetil and adrenocorticotropic hormone [13]. Of note, among these therapies, CNIs have proven effective in different contexts, however, their use has been limited by safety concerns, especially in patients with a decreased glomerular filtration rate (GFR), and by a high relapse risk at the time of their withdrawal [13].

A cyclophosphamide-based cyclical regimen became the recommended initial therapy in primary MN for several years, and this was also the recommendation of the 2012 Kidney Disease: Improving Global Outcomes (KDIGO) glomerulonephritis guidelines [14]. Importantly, the rationale for such an approach became stronger when the autoimmune nature of primary MN was confirmed with the 2009 report by Beck et al. [15] showing that autoantibodies directed towards the podocyte antigen M-type phospholipase A2 receptor (PLA2R) were detectable in 70% of patients and played a key pathogenetic role. In the following years, the list of putative autoantigens in MN rapidly expanded, including thrombospondin type-1 domain-containing 7A (THSD7A) [16] and neural epidermal growth factor-like 1 (NELL-1) [17]. This resulted in a smaller and smaller proportion of patients with no detectable antibodies, currently being ≈5–10% [18]. Whether antigen specificity can affect the response to immunosuppression remains unclear, with the heterogeneity and rarity of individual autoantigens hindering adequately powered studies. Overall, the recognition that primary MN is an autoantibody-mediated disease significantly supports a role for therapeutic approaches targeting B-cells [19].

The first report of rituximab (RTX) use in MN was published in 2002 [20], and several retrospective studies in the early 2010s supported its effectiveness in this context [21–24]. In 2017–2019, two randomised controlled trials (RCTs), GEMRITUX (NCT01508468) [25] and MENTOR (NCT01180036) [26], confirmed the efficacy of RTX in different settings. The GEMRITUX study showed that RTX was superior to placebo, both in terms of immunological remission at 6 months and clinical remission after a median follow-up of 17 months (65% versus 34%) [25]. The MENTOR trial compared RTX with a 12-month course of cyclosporine, showing equivalence of the two approaches at 12 months but superiority of RTX at 24 months (60% versus 20% remission rate) [26].

Interestingly, a direct comparison between the cyclical regimen and RTX had to wait until 2020. One study, STARMEN (NCT01955187), compared the cyclical regimen to a tacrolimus-based regimen for 6 months, tapered by month 9, plus RTX 1 g at day 180 [27]. Response to therapy was assessed at 24 months from baseline. A higher proportion of combined complete and partial remissions was observed with the cyclical regimen compared with the tacrolimus–RTX regimen (36/43 versus 25/43; P = .002). Of note, adverse events were more frequent in patients who received the cyclical regimen, although the rate of serious events was similar. The remission rate in the tacrolimus–RTX arm of this study (58%) was similar to those reported in other prospective studies using RTX, while the remission rate in the cyclical therapy arm (84%) was unusually high [28]. This may reflect several potential biases in the STARMEN cohort: prognostically relevant features were not equally balanced across treatment groups at baseline, possibly favouring the cyclical therapy group (more males, higher PLA2R antibody titres and more severe nephrotic syndrome in the RTX–tacrolimus arm). This, together with the lower RTX dose used compared with other trials, urges caution in the interpretation of results.

The second RCT comparing RTX and a cyclical regimen was RI-CYCLO (NCT03018535), a pilot study aimed at providing preliminary results to design a bigger trial [29]. This study included 74 patients randomized to either RTX (1 g 2 weeks apart) or cyclical therapy. No significant differences were detected in terms of combined complete and partial remission at 12 months (62% and 73%, respectively), time to remission and relapse rates at the intention-to-treat analysis. However, at the per-protocol analysis, the rate of complete remission at month 12 was lower in the RTX arm than in the cyclical regimen arm [4/32 versus 13/38 patients; odds ratio 0.28 (95% confidence interval 0.08–0.95)]. No differences in terms of safety emerged. Of note, in four cases RTX had to be discontinued for infusion reactions.

Overall, the prospective trials performed, although limited by small sample sizes, seem to support a largely similar effectiveness of cyclical therapy and RTX, with possibly some hints of a marginally higher efficacy of the cyclical regimen and a better safety profile for RTX. The possible advantages of RTX in terms of safety are supported by a large retrospective study with a 5-year follow-up: compared with 103 patients receiving steroids and cyclophosphamide, the 100 patients treated with RTX experienced fewer events (63 versus 173; P < .001), both serious (11 versus 46; P < .001) and non-serious (52 versus 127; P < .001) [30]. However, this study has to be cautiously interpreted for several reasons: its retrospective nature; the possibility of ‘centre effects’, with subsequent biases, due to the fact that the RTX and steroids–cyclophosphamide groups were enrolled in two different centres (in Italy and The Netherlands, respectively); and the fact that the steroids–cyclophosphamide group received corticosteroids for 5 months and cyclophosphamide for up to 1 year, resulting in significantly higher steroid exposure and cyclophosphamide cumulative doses (up to 36 g) than the regimen commonly employed. Despite these uncertainties, it has to be stressed that the safety profile of RTX is now well established in different contexts, even after long-term follow-ups [31]. All these findings clearly support the role of RTX as a first-line therapeutic option for the majority of patients with idiopathic MN, in line with the most recent KDIGO guidance [32]. Now the question is: is there still room for cyclical therapy?

To answer that, we need to consider several factors. To begin with, it is important to underline that a non-negligible proportion of patients with MN do not respond to RTX: this is the case for ≈20–40% of patients according to the data generated so far. Several reasons can underlie resistance to RTX. First, a significant loss of RTX in the urine can occur in the setting of nephrotic syndrome [33], especially in patients with higher proteinuria and lower serum albumin; this subgroup shows lower residual serum RTX levels 3 months after administration and lower response rates [34]. In this context, a way to overcome RTX resistance may be an increase in dosing. Some retrospective data suggest that higher doses of RTX can induce faster and improved remission rates [35]. RTX dosing may be relevant, especially in the setting of high anti-PLA2R antibody titres. An observational study showed that in patients with elevated levels of anti-PLA2R [defined as >152 relative units (RU)/ml], a cyclophosphamide-based regimen (1.5 mg/kg/day, duration 8–24 weeks) was more effective than RTX (cumulative dose 1.5–2 g) in inducing immunological remission [36]. Importantly, rituximab redosing was shown to be a successful strategy in this group of patients [37].

A second mechanism of RTX resistance may be the presence of neutralising antibodies directed towards RTX itself. In a cohort of 44 patients, 10 (23%) showed anti-RTX antibodies at 6 months after treatment [38]. These antibodies were associated with faster B-cell reconstitution at 6 months, higher proteinuria at 12 months (despite similar remission rates) and a higher relapse rate. Patients with anti-RTX antibodies may benefit from different anti-CD20 monoclonal antibodies, such as the humanised one, obinutuzumab [39].

A third mechanism of RTX resistance may reflect the biology of the B-cell clones that sustain the production of anti-PLA2R antibodies. In most cases of MN, autoantibodies are likely produced by short-lived plasma cells, derived from memory B-cells that express CD20 on their surface and are therefore targeted by RTX [40]. It has been hypothesized that in some cases of RTX resistance, long-lived plasma cells instead may be driving autoantibody production. This has been demonstrated to be the case in the setting of idiopathic thrombocytopenic purpura refractory to anti-CD20 [41]. Long-lived plasma cells are CD20 negative and therefore intrinsically resistant to RTX and other anti-CD20 agents [42, 43]. Relative resistance to RTX due to insufficient B-cell depletion in immunological niches, such as the bone marrow, lymph nodes and tissues, may play a role as well [44]. In these settings, therapeutic approaches with synergistic and broader mechanisms of action, such as cyclophosphamide and glucocorticoids, may play a role to overcome RTX resistance [40]. Although the effect of cyclophosphamide in targeting plasma cells is unclear and largely unexplored, this drug seems to be potentially effective on plasma cells, but with long-lived plasma cells being more refractory [45, 46].

The impact of glucocorticoids on the B-cell compartment also remains unknown to a great extent. High-dose glucocorticoids (especially dexamethasone) affect plasma cells and have been widely employed in multiple myeloma [47]; they also exert broad effects on B-cell activation and, to a lesser extent, proliferation [48]. Notably, the impact of high-dose glucocorticoids on the B-cell compartment is definitely not negligible, as further underlined by the observation that the risk of hypogammaglobulinemia in RTX-treated patients with anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis increases in parallel with the glucocorticoid dose employed at induction [49].

Taken together, these observations suggest that RTX, cyclophosphamide and glucocorticoids may act synergistically on the B-cell compartment, leading to speculation that a multitarget approach may be of potential interest. Prospective studies of such an approach in MN are not available at the moment. A retrospective study reported 60 patients with primary MN treated with a combination of low-dose oral cyclophosphamide (2.5 mg/kg for 1 week then 1.5 mg/kg for 7 weeks), a prednisone taper (60 mg/day withdrawn in 28 weeks) and a 2-year course of high-intensity RTX (1 g every 4 months) [50]. Remarkably, all patients achieved immunological remission by 6 months and clinical remission by 2 years, with complete remission in 83%. No significant safety concerns emerged.

How all these scanty preclinical data and observational studies focused on clarifying the mechanisms of RTX resistance in MN should be translated to guide clinical practice is still unclear. For sure, while the identification of the mechanisms of RTX resistance may inform alternative emerging therapeutic strategies, there may be a rationale for employing cyclical therapy, since the mechanisms of RTX resistance do not apply to this therapeutic regimen.

Beside RTX resistance, another important point is the management of patients with severe MN, namely the ones with severe manifestations of nephrotic syndrome and/or deteriorating kidney function. In this setting, data on the use of RTX are very scarce, while one RCT explored the role of the cyclical regimen in patients with deteriorated kidney function. This RCT included 108 patients with reduced kidney function [at least a 20% decline in estimated GFR (eGFR) in the 2 years before study entry, with creatinine <300 μmol/L (3.4 mg/dl)] [51]. The risk of further decline of kidney function was significantly lower in the 33 patients treated with cyclical therapy than in the ones treated with cyclosporine or supportive care alone. Importantly, adverse events (mainly haematological) were more frequent in the cyclical therapy arm (61% of the patients) compared with the other two (49% with cyclosporine and 42% with supportive care).

The preference, based on available evidence, for the cyclical regimen in the most severe cases of MN is reflected also in the recently published KDIGO 2021 guidelines for the management of glomerular diseases [32]. According to these guidelines, personalization of treatment according to the individual risk profile is key: RTX and CNIs may be the first-line approaches for patients at moderate risk of progressive loss of renal function (defined as normal and stable eGFR after diagnosis), while RTX, a cyclical regimen or CNIs with RTX are recommended for patients at high risk (decreased eGFR, proteinuria >8 g/day, normal eGFR associated with serum albumin <25 g/L or PLA2R antibodies >50 RU/ml) and, importantly, only the cyclical regimen is advised for patients at very high risk (life-threatening nephrotic syndrome or rapid deterioration of kidney function).

With potential toxicity being one of the major concerns regarding cyclical therapy, it has also been suggested that intravenous dosing of cyclophosphamide, instead of oral, may provide benefits in this respect, while retaining therapeutic efficacy, similar to other glomerular diseases like ANCA-associated vasculitis [52] or systemic lupus erythematosus [53]. Some small, retrospective cases series have reported encouraging outcomes with such modifications of the cyclical regimen, alone [54] or in combination with reduced dosing of corticosteroids, avoiding intravenous pulses [55–57]. However, more robust data are needed before these approaches can be widely recommended.

In conclusion, the therapeutic armamentarium for idiopathic MN is rapidly expanding, however, there is still light to be shed. While RTX is now rightly considered the therapeutic of choice for the great majority of patients, cyclical therapy still plays a central role in the management of this disease, especially in high-risk patients with deteriorating kidney function or severe manifestations of nephrotic syndrome, and in cases resistant to RTX. Early reports on multidrug approaches are very promising, offering a proof of concept that the combination of drugs with synergistic mechanisms of action can induce very high rates of immunological remission. Despite the reassuring available data, the safety of these combined regimens remains an important concern, especially in light of the relapsing course that MN often presents.

Future studies are needed to discover clinically relevant molecular endotypes and identify biomarkers predictive of treatment response, embracing the challenges of developing a precision medicine approach in MN.

FUNDING

None declared.

AUTHORS’ CONTRIBUTIONS

FA, FM and FS drafted and reviewed the article.

CONFLICT OF INTEREST STATEMENT

F.A. has received consulting fees from AstraZeneca, CSL Vifor, Novartis, Trevere Therapeutics and GlaxoSmithKline; honoraria for lectures from AstraZeneca, CSL Vifor, Novartis, Trevere Therapeutics and Otsuka and travel support from CSL Vifor. F.M. has received honoraria for lectures from AstraZeneca and travel support from CSL Vifor.