Differential effects of nitrogen mustard in vivo on the cancer and host cells in MCa‐11 tumours

We analysed the effects of nitrogen mustard (HN₂) on the growth, cell cycle distributions, and ratios of tumour cells to host cells for MCa‐11 tumours grown in vivo. Treatment of tumour‐bearing BALB/c mice with 3 mg/kg of HN₂ produced a significant slowing of MCa‐11 tumour growth. Seventy‐two hours after treatment in vivo with either 3 or 4 mg/kg of HN₂, the host cells in the treated tumours showed a significantly decreased G₀/G₁ peak and an increased G₂/M peak (P < 0.01), whereas the cancer cells in the treated tumours showed significant increases in the G₀/G₁ peak coupled with relatively decreased proportions of S and G₂/M tumour cells (P < 0.001). The ratio of the total number of cancer cells to the total number of host cells in the tumours was significantly increased 72 h after HN₂ administration (P<0.01). Thirty‐two days after treatment with HN₂, the cell cycle distributions of the host and tumour cells in the treatment and control tumours had returned to being identical, but the ratio of the total number of cancer cells to the total number of host cells remained increased in the treated tumours (P<0.01). These results show that the administration in vivo of HN₂ can lead to entirely different cell cycle effects for the host and cancer cells in the same tumour, and that the partial growth arrest of MCa‐11 tumours from HN₂ treatment may be due in part to the preferential destruction of host cells rather than solely to a direct cytotoxic effect on the cancer cells.