Growth response of B16 melanoma to in vivo treatment with 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) at the initial stage after tumor transplantation.

The changes of implanted B16 melanoma fragments in situ following early treatment with 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) were studied by quantitative histopathologic methods from Day 1 to Day 7 and at Day 14 after transplantation. During the first 3-day period there were no apparent differences between the two groups in all the parameters studied. The most striking differences were observed on Day 5 after implantation, when the drug-treated tumor showed the lowest number of morphologically intact tumor cells and the lowest level of proliferative capacity, with a high proportion of melanotic cells. The late infiltration of host macrophages was more abundant in drug-treated tumors than in controls due to an enhanced production and liberation of melanin granules. The results suggest that a 7-day growth delay of drug-treated tumors is characterized not only by a reduced number (one order of magnitude) of intact tumor cells but also by a severely suppressed proliferative capacity.