Suicide gene therapy on spontaneous canine melanoma: correlations between in vivo tumors and their derived multicell spheroids in vitro.

To validate the use of multicellular spheroids to predict the efficacy of herpes simplex thymidine kinase/ganciclovir (HSVtk/GCV) suicide gene therapy in the respective in vivo tumors, we established and characterized 15 melanoma-derived cell lines from surgically excised melanoma tumors. Three HSVtk-lipofected cell lines were not sensitive to GCV in any culture configuration, other five displayed similar sensitivity as monolayers or spheroids, and only one resulted more sensitive when grown as spheroids. Other six cell lines manifested a relative multicellular resistance (MCR) phenotype growing as spheroids, compared with the same cells growing as monolayers. The reverse correlation between the MCR and the monolayers survival to HSVtk/GCV suggests that one of the main causes of MCR would be the rapid cell repopulation after suicide gene treatment. The high correlation of MCR with the spheroids radial growth and with the mitotic index of the respective originary tumors supported this re-growth involvement. A remarkable finding was the high correlation in HSVtk/GCV sensitivity between in vivo tumor and the corresponding derived cell lines growing as spheroids (R(2) = 0.85). This strongly encourages the implementation of spheroids as highly realistic experimental model for optimizing and predicting the in vivo response of the respective tumors to therapeutic strategies.