Imiquimod has the potential to induce apoptosis in skin cancer cells, possibly by circumventing mechanisms developed by malignant tumors to resist apoptotic signals.

Background: Imiquimod 5% cream (Aldara^®), a novel topical immune response modifier, has been approved for the topical treatment of anogenital HPV-induced warts. In addition, several studies have demonstrated antitumoral activity in solar keratoses, superficial basal cell carcinomas and Bowen’s disease. Aim: Given the convincing therapeutic results of imiquimod when used for treating selected types of epithelial skin cancer, we became interested to study imiquimod as an adjuvant for treating cutaneous metastases of malignant melanoma. Methods: Three patients with multiple, i.e. more than 15, cutaneous in-transit metastases of malignant melanoma in unilateral localization on the leg were treated topically with imiquimod 5% cream. Results: Twice daily application under occlusive conditions for a period of 21–28 weeks resulted in >90% regression of cutaneous metastases in 2 patients. The third patient showed marked response only when topical imiquimod was intermittently supplemented by intralesional interleukin (IL)-2 for 2 weeks. Unwanted side effects were mild in all patients. Conclusion: Overall, imiquimod as a single agent or in combination with intralesional IL-2 may be a promising immunomodulatory compound for the adjuvant topical treatment of patients with multiple cutaneous metastases of malignant melanoma.

Bypassing molecular mechanisms of apoptosis deficiency may be of great utility for the successful treatment of malignant tumors. We have discovered that imiquimod, a small-molecule immunomodulator, exerts rather tumor-selective direct pro-apoptotic activity in vivo and in vitro towards cutaneous metastases of malignant melanoma, an aggressive skin tumor. This pro-apoptotic activity was not detectable with resiquimod, a closely related structural analogue whose pro-inflammatory activity is even greater than that of imiquimod. Unresponsiveness of some melanoma metastases to imiquimod in vivo corresponded to resistance towards imiquimod-induced apoptosis in vivo and in vitro. At the molecular level, the pro-apoptotic activity of imiquimod was independent of membrane-bound death receptors, but depended on Bcl-2 expression as demonstrated by overexpression of Bcl-2 in melanoma cells. Imiquimod is the first topical compound with the potential to bypass molecular mechanisms of apoptosis deficiency, a concept that may be relevant for other tumors as well.

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