The effects of immune depression on chemical carcinogenesis in the rat: III. cyclophosphamide

The effect of the immunosuppressive drug cyclophosphamide upon the incidence and growth characteristics of rat mammary tumour induced by 7,12-dimethyl benz- (α)-anthracene has been investigated. Where cyclophosphamide was administered six days before or concurrently with the carcinogen, there was a decreased incidence and an increased latency of induced tumours. However, when the cyclophosphamide was commenced six days after the first dose of carcinogen there was an increased incidence of tumours and a decreased latency. While the exact explanation of this phenomenon is uncertain, it may be that cyclophosphamide exerts a maximal cytotoxic effect at the time of tumour induction or very early in its growth; when given after the tumour is established, the immunosuppressive effect on the host may outweigh any direct cytotoxic effect upon the tumour in this system.