Background: 5% Imiquimod cream is a topical immune response modifier that has been used off-label to treat malignant melanocytic proliferations such as lentigo maligna. To our knowledge, imiquimod has not been previously used to treat atypical nevi (AN).
Observations: Three patients each with 1 selected clinically AN were treated with imiquimod 5 nights per week for 12 weeks. The lesions were subsequently excised and sent for routine histologic and immunohistochemical analysis. None of the lesions cleared. Two were consistent with atypical compound nevus on excisional biopsy and demonstrated inflammation, while the third showed congenital features and demonstrated minimal inflammation. The AN were initially interpreted as displaying more severe histologic atypia on excisional biopsy than was present at baseline. Immunohistochemical studies revealed that the AN but not the congenital-like nevus exhibited increased staining for CD4(+) and CD8(+) cells and for a surrogate marker of interferon alpha expression.
Conclusions: Twelve weeks of imiquimod treatment failed to cause lesional resolution. A differential inflammatory response was observed between the AN and the congenital-like nevus. The character of the inflammatory infiltrate was similar to that observed with halo nevi. Uncertainties remain concerning imiquimod use for chemoprevention of AN, and the posttreatment histologic features may be misinterpreted as severe melanocytic atypia or melanoma.