Interactions of doxorubicin and cis-platin in squamous carcinoma cells in culture

Doxorubicin (DXR) has a positive inoculum effect and penetrates poorly into the core of multicellular tumour spheroids (MTS). Cis-platin (DDP) displays neither of these characteristics. We evaluated whether combining these 2 agents would influence the cell kill effect at a tumour mass level. MTS were produced from a PC-10 squamous lung carcinoma cell line. MTS were exposed to either drug first for h with different intervals between exposure. Cells were then trypsinized to a single cell suspension and subjected to clonogenic assay. Combination effects were analyzed by median effect plot analysis. The more MTS ml-' medium, the lower the cell kill effect of DXR. Simultaneous exposure to the 2 drugs was synergistic. DXR exposure first followed by DDP was less efficacious than, or the same as, the simultaneous exposure. In contrast, DDP followed by DXR was more efficacious with the best cell kill at a 1 h interval between each drug. This phenomenon was observed even at non-toxic doses of DDP. The fluorescent microscopic study of DXR indicated that prior exposure of MTS to DDP resulted in increased DXR penetration into the MTS core leading to heightened synergism with this sequence. These data suggest that the proper combination of DXR plus DDP should be in sequence with DDP first. Clinical, toxicological and pharmacological trials of DDP administration first, followed by DXR, are warranted.