Chemosensitivity testing of primary cultures of Merkel cell cancer.

Twenty-seven tumor specimens from patients with Merkel cell carcinoma (MCC) were tested for chemosensitivity against a battery of nine cytotoxic drugs in a short-term antimetabolic assay measuring inhibition of thymidine incorporation. Dose-response curves were constructed by plotting drug concentration in micrograms/ml versus % control [3H]thymidine incorporation. Specimens were considered 'sensitive' to a drug if, at the approximate peak plasma concentration (PPC), the inhibition of [3H]thymidine was greater than 50% when compared with untreated control primary cultures. The assay revealed a 'sensitive' tumor in 19 of 20 specimens and 16 of 17 patients had a tumor that was 'sensitive' to at least one drug tested in the assay system. The highest sensitivity in order of frequency was found with doxorubicin, epirubicin, cyclophosphamide, etoposide and cisplatin. At least 40% of the tumors were 'sensitive' to these five drugs. Cyclophosphamide was chosen as the most active drug (at PPC) in 10 of 19 assays (53%), etoposide in seven of 17 (41%), doxorubicin in four of 19 (21%), chlorambucil in one of 12 (8%) and cisplatin in one of 18 (5%) of assays. Though our results are preliminary, we have identified for the first time a range of cytotoxic drugs which appear effective against MCC in vitro. Our main task now is to determine whether our in vitro predictive assay will correlate with clinical benefit to the patient.