To the Editor,

With regards our scientific letter, “Cyclophosphamide-induced lupus flare in diffuse proliferative lupus nephropathy”1 we would like to thank Park at al for their contribution, and their suggestions of the possible immunological mechanisms involved in a lupus flare. Although they question whether cyclophosphamide really induces the flare or whether other mechanisms are involved, we would like to clarify that the title of the English version “Cyclophosphamide-induced lupus flare” could lead readers to believe that we consider that treatment with cyclophosphamide induced the flare, however, we wanted to communicate exactly the opposite. Indeed, as was reflected in the “Discussion” section, cyclophosphamide treatment is the best immunosuppressive agent, with the best results in inducing remission in severe forms of lupus nephropathy (LN).2 As Park at al have pointed out in their letter, the lupus flare may or may not occur after cyclophosphamide treatment. In our case, it was during the cyclophosphamide induction period (less than 15 days). The flare was confirmed following the first phosphamide dose. Therefore, after having dismissed several causes (renal vein thrombosis, infection, etc.), we brought another cyclophosphamide dose forward (the second dose was 1.5g), which caused clinical and biochemical improvements, and the patient went into remission. To date, the patient has not had any further flares. We would like to emphasise the importance of the cyclophosphamide administered: we used the Euro-Lupus Nephritis Trial regime, 500mg every 15 days.3 In this patient the flare would have been earlier, since the relapse occurred less than 15 days with 1g of the first dose of cyclophosphamide. For that reason, before considering cyclophosphamide treatment to be ineffective, it is important to bear in mind the dose that has been administered. Finally, we must highlight that we did not consider for one moment that cyclophosphamide induced the lupus flare.