BACKGROUND: Lenalidomid ranks among immunomodulatory drugs. There are a few of the more common side effects, like a higher risk of venous trombembolism or diarrhea. Other side effects are rare. The hyperbilirubinemia described in this article can be assigned to them. In our case, the increase of bilirubin was associated with unrecognized Gilbert syndrome. CASE DESCRIPTION: We report a patient with multiple myeloma and necrobio-tic xanthogranuloma (NXG) of the skin and liver. After the treatment with bortezomib, lenalidomid and dexamethasone, complete remission was attained after 4 cycles with decrease of monoclonal immunoglobulin to an unmeasurable concentration. At the same time, the dis-appearance of cutaneous and hepatic lesions of NXG on FDG-PET/CT was evident. The administration of bortezomib was stopped after 8 cycles and only continued with lenalidomide as a maintenance therapy. However, after four cycles of this therapy, bilirubin increased above the upper limit and the increase continued till the 11th month of lenadomide administration, when bilirubin reached the highest concentration of 75 μmol/l (more than the three-fold of the upper limit, grade III toxicity). The patient had asymptomatic hyperbilirubinemia with no underlying liver disease or renal impairment while being on lenalidomide therapy. Genetic studies proved mutation; insertion in the promotor gene UGT1A1 typical for Gilbert syndrome. Hyperbilirubinemia may be attributed to the unmasking of previously undia-gnosed Gilbert syndrome. Therefore, the therapy with lenalidomide was interrupted after 11 months. The bilirubin level decreased after the discontinuation of the drug. CONCLUSION: NXG disappeared after fulfilling complete remission of multiple myeloma with disappearance of monoclonal immunoglobulin. This observation supports the hypothesis that monoclonal immunoglobulin has a crucial role in the ethiopathogenesis of NXG and suggests the treatment of monoclonal gammopathy if present in a patient with NXG, hoping that this will result in xantogranuloma disappearance.

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