Genistein Potentiates the Radiation Effect on Prostate Carcinoma Cells 1 Gilda G. Hillman, 2 Jeffrey D. Forman, Omer Kucuk, Mark Yudelev, Richard L. Maughan, Johanna Rubio, Andrey Layer, Samuel Tekyi-Mensah, Judith Abrams, and Fazlul H. Sarkar Departments of Radiation Oncology [G. G. H., J. D. F., M. Y., R. L. M., J. R., A. L., S. T-M.], Internal Medicine [O. K.], Integrated Biostatistics Unit [S. T-M., J. A.], and Pathology [F. H. S.], Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, and Harper Hospital, Detroit, Michigan 48201 ABSTRACT We have shown previously that genistein, the major isoflavone in soybean, inhibited the growth of human pros- tate cancer cells in vitro by affecting the cell cycle and inducing apoptosis. To augment the effect of radiation for prostate carcinoma, we have now tested the combination of genistein with photon and neutron radiation on prostate carcinoma cells in vitro. The effects of photon or neutron radiation alone or genistein alone or both combined were evaluated on DNA synthesis, cell growth, and cell ability to form colonies. We found that neutrons were more effective than photons for the killing of prostate carcinoma cells in vitro, resulting in a relative biological effectiveness of 2.6 when compared with photons. Genistein at 15 M caused a significant inhibition in DNA synthesis, cell growth, and colony formation in the range of 40 – 60% and potentiated the effect of low doses of 200 –300 cGy photon or 100 –150 cGy neutron radiation. The effect of the combined treatment was more pronounced than with genistein or radiation alone. Our data indicate that genistein combined with radi- ation inhibits DNA synthesis, resulting in inhibition of cell division and growth. Genistein can augment the effect of neutrons at doses 2-fold lower than photon doses required to observe the same efficacy. These studies suggest a poten- tial of combining genistein with radiation for the treatment of localized prostate carcinoma. INTRODUCTION Carcinoma of the prostate is the most common malignant tumor in men, with 180,400 newly diagnosed cases annually, resulting in 31,000 deaths each year (1). Localized prostate carcinoma is sensitive to conventional radiotherapy using mega- voltage photons (X-rays); however, residual disease often causes clinical relapse (2). To increase the efficacy of radiation therapy, the use of neutrons was explored. Neutrons are heavy particles produced when a charged particle, such as deuteron, is accelerated to high energy and then made to impinge on a target such as beryllium (3). The interaction of neutrons with nuclei of atoms of soft tissues sets in motion heavy secondary particles, producing dense ionizations more likely to cause critical DNA damage (double-strand breaks), which is less repairable than that induced by photons. Neutrons are both more effective in killing hypoxic tumor cells and less dependent on the cell cycle than photons (3). These differences between neutrons and pho- tons result in a greater RBE 3 for neutrons relative to photons and form the radiobiological basis for selecting neutrons for therapy of slow-growing tumors, such as adenocarcinoma of the pros- tate. In the treatment of locally advanced adenocarcinoma of the prostate, neutrons have been shown to be superior to photons in two randomized multi-institutional Phase III clinical trials (4, 5). At Wayne State University, we have improved the conditions for three-dimensional conformal neutron irradiation using a superconducting cyclotron fully rotational around the patient to produce an isocentric beam operated with a tungsten multirod collimator used to produce irregularly shaped fields (6, 7). Field apertures were designed to conform to the size and shape of the three-dimensional reconstructed tumor volume. After a series of Phase II/III trials for localized and locally advanced prostate carcinoma using these techniques, we have established a regi- men of neutrons combined with photons that results in a signif- icant decrease in tumor recurrence and lower toxicity than that observed in the previous trials (8, 9). At 3 years, no evidence of recurrence of disease was found in 91% of patients with stage T 1 disease, in 86% of stage T 2 disease, and 61% of stage T 3 /T 4 disease (9). Although the local control of stage T 3 /T 4 (61%) was improved over photon radiation alone (35– 40%), these findings showed that this treatment alone was insufficient to prevent progression of disease in a large proportion of patients. To improve the local control of advanced disease, radiation should be combined with additional antitumor agents. In this study, our goal was to test whether genistein could augment the efficacy of radiation for the treatment of prostate cancer. We have shown previously that genistein inhibits the growth of human prostate cancer cells in vitro by inducing apoptosis (10). Genistein is an isoflavone, a major metabolite of soy produced by the intestinal Received 7/31/00; revised 11/28/00; accepted 11/29/00. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 These studies were generously supported by the Patricia and E. Jan Hartmann Cancer Fund. 2 To whom requests for reprints should be addressed, at Department of Radiation Oncology, Karmanos Cancer Institute, Hudson-Weber Bldg., Rm. 515, HWRC Cancer Center, 4100 John R., Detroit, MI 48201. Phone: (313) 966-7652; Fax: (313) 966-0605; E-mail: hillmang@karmanos.org. 3 The abbreviations used are: RBE, relative biological effectiveness; CM, culture medium; NF-B, nuclear factor-B. 382 Vol. 7, 382–390, February 2001 Clinical Cancer Research Research. on December 4, 2021. © 2001 American Association for Cancer clincancerres.aacrjournals.org Downloaded from