Initial assessment of tumor regrowth after vismodegib in advanced Basal cell carcinoma.

Secondary (acquired) resistance of a tumor to a chemotherapeutic agent is characterized by regrowth of a tumor after initial shrinkage. This is distinct from primary resistance, in which a tumor never responds to treatment, a separate topic from our current study. The Smoothened (SMO) inhibitor, vismodegib (GDC-0449), has recently been shown to be useful in phase 1 and 2 clinical trials1–3 for locally advanced basal cell carcinomas (laBCCs) (defined as inoperable owing to multiple postsurgical recurrences or incurable with surgery without significant deformity or loss of function) or meta-static BCCs (mBCCs). Collectively, laBCCs and mBCCs are termed advanced BCCs, and vismodegib treatment was approved in 2012 by the US Food and Drug Administration for this indication. Our case series describes a previously unreported phenomenon of BCC tumor re-growth within or immediately adjacent to (within 1 cm) the prior tumor bed of a vismodegib-responsive tumor while the patient is still undergoing continuous vismodegib treatment. We call this phenomenon secondary (acquired) resistance.4