Idiopathic membranous nephropathy (IMN) is the most common cause of primary nephrotic syndrome among adults, accounting for approximately 20%–30% of cases (Ronco et al., 2021). It primarily affects individuals aged 30 to 50, with a slightly higher incidence in males. Up to 80% of MN patients have autoantibodies to M-type phospholipase A2 receptor (PLA2R) in their bloodstream. Additional autoantibodies, such as thrombospondin type-1 domain-containing 7A (THSD7A) (Couser, 2017), are increasingly being recognized. The progression of IMN varies among individuals. Some patients may experience a self-limiting disease course, with spontaneous remission occurring in approximately one-third of cases. However, others may develop persistent nephrotic syndrome. In cases of persistent nephrotic syndrome (NS), up to 40% of patients may progress to kidney failure over the course of 10 years (Caravaca-Fontan et al., 2022). For patients with this condition, immunosuppressive therapy is recommended to improve long-term kidney outcomes. Treatment options include corticosteroids, alkylating agents like cyclophosphamide, calcineurin inhibitors such as cyclosporine and tacrolimus, and rituximab, an anti-CD20 monoclonal antibody.

The use of anti-CD20 antibodies has shown promise in treating idiopathic membranous nephropathy (IMN). Rituximab (RTX), a chimeric antibody that targets CD20, a surface antigen found on B-cells that regulates their growth and differentiation, has demonstrated potential in this regard. With increased and prolonged usage, more information has been gathered regarding its potential adverse effects. Adverse reaction reports indicate that most cases of adverse reactions occur within 20–40 min following intravenous infusion. However, some adverse reactions may occur between 90 min and 1 month after drug administration, even after the injection is completed. It is important to note that these side effects can occur at any time after the injection. Possible side effects of RTX include infusion-related reactions, infections, progressive multifocal leukoencephalopathy (PML), hypersensitivity reactions, and heart problems (Paul and Cartron, 2019; Basu et al., 2022).

We present a case of hypokalaemia in a patient with refractory membranous nephropathy following the fifth RTX infusion. The patient was subsequently treated with Obinutuzumab, a humanized type II anti-CD20 monoclonal antibody that targets a different epitope on CD20 than rituximab.

We present a case of PLA2R-Ab-associated membranous nephropathy (MN) in a patient who was unresponsive to multiple treatments, including prednisolone, tacrolimus, cyclophosphamide, and mycophenolate mofetil. The patient achieved partial remission of his nephropathy after receiving successive applications of CD20 monoclonal antibodies, specifically rituximab and obituzumab. During the fifth infusion of rituximab, the patient experienced infusion-related hypokalemia, but did not experience any infusion reactions or hypokalemia after switching to obinutuzumab.

In this patient, the PLA2R antibody titers increased from 145 to 396 after treatment with 2.0 g of rituximab. Although a decreasing trend in PLA2R and urinary proteins was observed with the subsequent addition of 0.5 g of RTX, the patient developed hypokalemia while receiving the last 0.5 g of RTX. Therefore, we had to interrupt the rituximab treatment, which also affected the judgment of whether RTX was ultimately effective in this patient. Regarding the consideration of the cause of hypokalemia, the patient was not taking any diuretics at that time. Prior to each infusion of rituximab, we followed the manufacturer’s instructions and administered 300 mg of oral acetaminophen, 25 mg of intramuscular ipecac, and 40 mg of intravenous methylprednisolone to prevent infusion reactions. Glucocorticoids are the drugs most likely to cause hypokalemia prior to rituximab infusion (Kokot and Hyla-Klekot, 2008). Glucocorticoids can cause hypokalemia through multiple mechanisms. Firstly, they increase the activity of the Na + -K+ enzyme in skeletal muscle cells, resulting in an influx of potassium ions into the cells. Additionally, they have a mild mineralocorticoid effect, leading to sodium retention and potassium excretion (Ben et al., 2009). Prolonged use of glucocorticoids can result in excessive potassium loss. However, hypokalemia resulting from the potassium-excreting effects of glucocorticoids is a chronic and cumulative process. The incidence of hypokalemia is proportional to the range, duration, and dose of glucocorticoid action. In general, routine doses do not result in a sudden drop in serum potassium. Prior to the infusion of RTX, a routine dose of methylprednisolone 40 mg was administered intravenously to patients with nephrotic syndrome. Although glucocorticoid-induced hypokalemia cannot be completely ruled out, it is not considered a significant contributing factor.

However, treatment with rituximab may cause electrolyte imbalances, such as hypokalemia. The literature to date only reports three cases of hypokalemia resulting from rituximab administration. A single young patient with recurring minimal change disease (MCD) experienced hypokalaemia after her sixth rituximab dose (Guzzi et al., 2020). An elderly patient with MCD, who received rituximab in lieu of steroid therapy, developed hypokalaemia after only her third dose. Additionally, a young woman with idiopathic nephrotic syndrome (NS) was introduced to rituximab as a means of decreasing reliance on prednisone (Song et al., 2023). The diagnosis of acute hypokalaemia was made during her sixth intravenous rituximab infusion. The three patients presented with symptoms of dizziness, palpitations, fatigue and muscle spasms in the lower limbs. The medical professionals quickly identified the symptoms, and hypokalemia was diagnosed through blood tests. Intravenous and oral potassium supplementation were administered, and hypokalemia was successfully resolved without any negative after-effects. Our patient initially exhibited fatigue and weakness, which worsened after receiving the infusion. An urgent blood test confirmed the diagnosis of hypokalaemia. After receiving potassium supplements, the patient’s blood potassium levels returned to normal and their symptoms abated.

Although three occurrences of rituximab-induced hypokalemia have been reported to date, the precise mechanism behind hypokalemia triggered by rituximab remains unknown. Among rituximab-related adverse events, infusion-related adverse events were the most common, with the most frequently observed clinical manifestations including fever, chills, asthenia, headache, anaphylaxis, anaphylactoid events, bronchospasm, dyspnea, rash, urticaria, angioedema, or pruritus. These clinical manifestations are usually associated with the first infusion and the onset is usually around 30–120 min (Wong and Long, 2017). These adverse reactions are considered to be hypersensitivity reactions, and treatment with desensitizing medications has been shown to be effective (Sloane et al., 2016; Isabwe et al., 2018). However, based on our and two other reports in the literature, none of these hypokalemia associated with rituximab infusion occurred at the time of the first infusion, and therefore, speculation about the mechanism of immune-mediated hypersensitivity reactions behind this adverse reaction seems unlikely. Indeed, the mechanism by which rituximab infusion induces hypokalemia is currently unknown. Additional experimental data should be provided to elucidate the relationship between rituximab and hypokalemia. A study of patients with papular urticaria treated with rituximab found that the amplification of KCNN4 potassium channels on the surface of B cells occurred. However, it is unlikely that hypokalemia is due to their activation since B-cell KCNN4 channels inadvertently increase calcium ion influx by releasing potassium (Caillot et al., 2018). Studies have confirmed that rituximab effectively reduces intracellular Ca2+ levels and blocks intermediate conductance Ca2+-activated K(IK) channels (Wang et al., 2007). In addition to complement-dependent cytotoxicity, rituximab also stimulates FcγRIIB receptors and inhibits Kv1.3 channels, which leads to apoptosis in malignant B lymphocytes (Wang et al., 2012). Analyzing the effects of antineoplastic drugs on electrolyte imbalance could inform future exploration of the relationship between rituximab and hypokalemia. For instance, platinum compounds have been linked to imbalances in sodium, potassium, and magnesium, while alkylating agents and periwinkle alkaloids may cause hyponatremia due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH) (Verzicco et al., 2020). Targeted therapies, including monoclonal antibodies, tyrosine kinase inhibitors, immunomodulators, and mammalian target of rapamycin, have the potential to cause hyponatremia associated with SIADH and, to a lesser extent, urinary sodium loss (Jhaveri et al., 2017). Anti-epidermal growth factor receptor (EGFR) antibodies may cause significant magnesium and potassium losses in clinical settings. SAKURADA recommends monitoring serum electrolytes for up to 8 weeks after anti-EGFR antibody dosing (2016).

None of the three patients mentioned in the literature reported whether rituximab was continued following hypokalemia development. It is unclear whether continued use of rituximab will result in the reoccurrence of hypokalemia. To avoid a recurrence of this dangerous condition, we decided not to continue rituximab after consulting with the pharmacist. The decision to switch to obinutuzumab was based on the fact that, although both rituximab and obinutuzumab bind to the CD20 antigen on B cells, their mechanisms of action and side effects differ slightly due to structural differences and interactions with the immune system. Obinutuzumab, a humanized type II anti-CD20 monoclonal antibody (Basu et al., 2022), has been found to be effective in treating refractory PLA2R-related MN that does not respond to prednisolone, cyclosporine, cyclophosphamide, or rituximab (Sethi et al., 2020; Naik et al., 2023; Conversano et al., 2024). Compared to rituximab, obinutuzumab is a more potent inducer of antibody-dependent cell-mediated cytotoxicity (ADCC) and direct cell death (DCD). Obinutuzumab is believed to lead to a deeper and more enduring depletion of the body’s B-cell population and has shown effectiveness in cases where rituximab has failed (Klomjit et al., 2020; Hudson et al., 2022). However, there is ongoing debate regarding the adverse effects or toxicity of obinutuzumab compared to rituximab. Some studies suggest a high rate of toxicity associated with the use of obinutuzumab. However, Naomi A (Amudala et al., 2022) documented three cases of ANCA-associated vasculitis patients who were treated with obinutuzumab after being allergic to rituximab. Despite prior anaphylactic reactions to rituximab, all patients tolerated obinutuzumab well and maintained remission. In our case, we switched to obinutuzumab after correcting the hypokalemia, which resulted in partial remission and the absence of hypokalemia recurrence. However, it is crucial to avoid making broad conclusions based on a limited case series. Further research is needed to determine the safety profile of obinutuzumab.

The clinical features of hypokalemia can range from mild to severe, depending on the degree of potassium decline. Common symptoms associated with this condition include muscle weakness, fatigue, palpitations, muscle cramps, and cardiac arrhythmias. Severe hypokalemia can result in life-threatening complications such as paralysis and cardiac arrest. It is important to consider hypokalemia as a potential complication during RTX therapy. Although RTX has shown effectiveness in treating various medical conditions (Deng and Xu, 2023), it is important to consider the risk of hypokalemia. Vigilant monitoring of potassium levels, prompt intervention when necessary, and appropriate potassium replacement are vital to ensure the safe and successful management of patients receiving RTX therapy. In cases where hypokalemia is corrected, continued treatment with obinutuzumab can also be attempted.

In conclusion, it is essential for clinicians to be aware of the potential risk of hypokalemia during RTX therapy and its management. This knowledge will enable them to implement preventive measures and initiate appropriate treatment.