Estramustine potentiates the radiation effect in human prostate tumor transplant in nude mice

In this study, we have investigated the combined effect of estramustine treatment and external beam radiation on human prostatic cancer tumor cells (DU 145) transplanted in nude mice. The treatment was given according to two different schedules. In the first treatment regimen, estramustine was administered intraperitoneally (i.p.) intermittently for 20 days. The radiation therapy, which was started on day 9, was given with 6 Gy fractions during an 11‐day‐long period to a total dose of 36 Gy. The combination treatment (estramustine + radiation) resulted in a significant tumor growth retardation as compared to the control group. This pronounced effect was seen neither with radiation alone nor with estramustine alone. In order to further extend the radiation treatment time, a second therapy regimen was employed. In this part of the study, estramustine was administrated i.p. intermittently for 26 days. The radiation therapy, which was started on day 6, was given with 4 Gy fractions during a 21‐day‐long period to a total dose of 40 Gy. Under these conditions, a significant tumor growth retardation was disclosed, when comparing the combination treatment (estramustine + radiation) with radiation alone. The tumors were analyzed for content of necrosis and proliferative activity. The largest proportion of necrosis was seen in the combination (estramustine + radiation) treatment group. Also, the tumors from this group expressed a decreased proliferative activity. The data indicate that estramustine acts as a radiosensitizing agent in human prostatic cancer cells in vivo. The radiosensitizing properties of the drug encourage further studies with respect to clinical application. © 1994 Wiley‐Liss, Inc.