Iatrogenic Kaposi sarcoma (KS) is a relatively frequent occurrence following solid-organ transplants (0–5%), in part due to prolonged immunosuppression. However, its occurrence after haematopoietic stem cell transplantation (HSCT) is rare, with only 15 cases described in the literature. We report a case of KS presenting in the skin of a patient who had received a matched unrelated donor allogeneic HSCT for IgG kappa myeloma. A 55-year-old HIV-negative white man with myeloma underwent an allogeneic HSCT from a matched, unrelated donor following conditioning chemotherapy with fludarabine–melphalan and antithymocyte globulin. Post-transplant, he was maintained on ciclosporin as graft-versus-host disease (GVHD) prophylaxis. On day 14 post-transplant, he developed a widespread eczematous eruption that rapidly evolved into an erythroderma; a skin biopsy was consistent with acute GVHD. He was managed with superpotent topical steroids and a 5-day course of oral prednisolone. He began extracorporeal photopheresis (ECP) on day 33 post-transplant, having failed to respond adequately to systemic steroids. He also completed four cycles of rituximab. Ciclosporin was stopped by day 130 post-transplant. He had completed 14 cycles of ECP by day 139 post-transplant, with resolution of cutaneous GVHD; however, he had developed a rapidly growing solitary dusky-red nodule on his right neck that was excised due to diagnostic uncertainty. Histology revealed a dermal haemorrhagic exophytic vascular tumour composed of lobules and sheets of epithelioid cells with an eosinophilic cytoplasm and pleomorphic nuclei. Immunoperoxidase staining was negative for S100, MNF-116 and AE1/AE3. The same cells were strongly and diffusely positive for the vascular markers CD31, CD34 and ERG. The lesional cells were positive for human herpesvirus (HHV)-8 confirming a diagnosis of Kaposi sarcoma. He subsequently developed a second lesion on his left forearm, despite being off all immunosuppression. A repeat bone biopsy confirmed that he remains in morphological remission from his myeloma. Positron emission tomography–computed tomography has not identified extracutaneous disease. KS following HSCT is rare, but it can be fatal. Skin lesions are the dominant clinical presentation. HHV-8 has an important role in the pathogenesis, and transplant-related KS may occur secondary to reactivation of the virus as a result of significant immunosuppression following HSCT. However, it could also occur as primary infection or be transmitted with donor cells. The outcome of KS after HSCT is variable. A watch and wait strategy can be applied if immune recovery is expected. In our patient, it developed 5 months after HSCT; shortly after immunosuppression had been tapered, he remains well and will be closely monitored for further suspicious lesions.

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Adalimumab is widely used in the treatment of immune-mediated inflammatory diseases. It is the highest-spend drug in the National Health Service, costing more than £400 million annually. In 2018, adalimumab biosimilars became available, leading to substantial cost savings. However, patient tolerability of biosimilars remains underexplored. We evaluated the tolerability of adalimumab biosimilar therapy (Idacio®) in patients who were switched from Humira® in a large dermatology and rheumatology tertiary centre. We identified all patients issued with a prescription of Humira by dermatology [psoriasis and hidradenitis suppurativa (HS)] or rheumatology (rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis and Behçet disease) and analysed the number who switched to Idacio between January 2021 and March 2022. Patient electronic records were reviewed up to October 2022 to identify patients who switched back to the originator product and the documented reason for the switch. The primary outcome was the numbers of patients who were switched back from the biosimilar to the originator product. Of 769 patients issued with a prescription of Humira, 694 were switched to Idacio during the study period. Of these, 134 (19.3%) switched back to the originator product, 102 of whom (76.1%) remained on this during follow-up. The highest proportion of individuals switching back to the originator product was observed in the HS population (26.7%). The most common reasons for switching back were patient-reported loss of disease control (n = 63; 47.0%), injection pain (n = 51; 38.1%) or both (n = 10; 7.5%). The median time to switch back was 197 days (interquartile range 138–280). Of 694 patients prescribed Idacio, 32 (4.6%) were switched to an alternative biologic agent during the study period. Our experience with previous biosimilar switches within rheumatology and dermatology (etanercept, rituximab and infliximab) has resulted in few patients requiring a switch back to the originator. Our data indicate that around one in five patients in our study population switched back to the originator product. The higher proportion of patients citing injection pain may relate to the citrate excipient content or larger injection volume of the biosimilar Idacio. Loss of efficacy with time is well established with adalimumab, in part due to antidrug antibodies, and cannot be specifically attributed to the biosimilar. Switching may alter a patient’s confidence in the drug, which is particularly important when it may be a first-line therapy or the only approved biologic for a condition. Data on switch-back rates for other adalimumab biosimilars is required to understand how generalizable our findings are and the potential economic implications.