Abstracts

Rationale: Discoloration/pigmentation of the skin, lips, nails, mucosa, and/or ocular tissues, including retina has been reported in subjects taking ezogabine/retigabine (RTG) and has generally been associated with long duration treatment. Several nonclinical investigations have been conducted towards understanding the nature and potential mechanism of the discoloration/pigmentation reported in subjects taking RTG. Methods: These nonclinical investigations have included the evaluation of potential involvement of a blue dye excipient, active pharmaceutical ingredient (API) impurity, metal chelation, or a direct effect on melanogenesis. In addition, in vitro melanin binding assays, a whole body autoradiography assay in pigmented rats, and several in vivo studies in albino and pigmented rats have been conducted. Biopsy samples from areas of discoloration from subjects taking RTG in clinical trials were evaluated histopathologically using routine (Hematoxylin and Eosin), special (Fontana-Masson with and without bleaching, Perls' Iron, and Periodic Acid Schiff), and immunohistochemical (IMH; Melan-A) staining. Chemical analysis for RTG-related material was conducted on a limited number of clinical biopsy samples. Results: There was no evidence to support the involvement of an excipient, API impurity, a direct effect on melanogenesis, or metal chelation in the discoloration/pigmentation. Available data support an association between accumulation of free and/or melanin-bound pigmented RTG-related material and the discoloration in subjects taking RTG. This hypothesis is supported by the finding of pigmented RTG-RTG and N-acetyl metabolite of RTG (NAMR)-NAMR dimers in the eye and skin of pigmented rats as well as qualitative and quantitative measurement of these dimers in biopsy samples from discolored areas of skin and urinary bladder from human subjects. In addition, imaging analysis of a urinary bladder sample with discoloration demonstrated the RTG-RTG dimer was highly localized to focal areas of brown granular pigmentation observed histologically. Histopathologic evaluation of clinical biopsy samples from areas of discoloration demonstrated a staining pattern indicative of the presence of a reducing agent. Because RTG can act as a reducing agent this is consistent with the potential presence of RTG-related material. Despite the presence of quantifiable amounts of pigmented RTG-RTG and NAMR-NAMR dimers in various rat tissues there has been no evidence of abnormal discoloration in ongoing nonclinical in vivo studies. Conclusions: These data support an association between pigmented dimerization products of RTG and the discoloration/pigmentation reported in subjects taking RTG. GSK continues to investigate the relationship between these pigmented dimers and the discoloration, and we are not ruling out the possibility of involvement of other pigmented RTG-related materials and/or other mechanisms of discoloration/pigmentation. This work was funded by GlaxoSmithKline (GSK).