Treatment of graft versus host disease with photopheresis interferes in voriconazole therapeutic drug monitoring: A case study

Extracorporeal photopheresis is an established procedure for refractory graft-versus-host disease, a major complication associated with notable morbidity and mortality in patients with allogeneic hematopoietic stem cell transplant. Despite being implemented over a decade ago, there is scant information about potential interactions or analytical interferences with concomitant drugs in this polymedicated population. Here we report the case of a pediatric patient diagnosed with cutaneous steroid-refractory acute graft-versus-host disease after unrelated allogeneic hematopoietic stem cell transplant that was treated with photopheresis. Analytical quantification of voriconazole by HPLC-PDA the day following photopheresis treatment did not permit therapeutic drug monitoring (TDM) due to the presence of interference at the voriconazole retention time. Following investigations, it was demonstrated that the interference is likely attributable to a psoralen-based compound. The interference was not present when samples were obtained prior to photopheresis, enabling TDM. This case underscores the relevance of communication among the members of the treating team to perform reliable TDM, especially in routine clinical practice of pediatric patients with complex diseases undergoing innovative treatments. This finding is relevant to voriconazole quantification by HPLC-PDA, frequently used in laboratories based in middle-income countries.