[CANCER RESEARCH 48, 6313-6316, November 15, 1988] Boron Neutron Capture Therapy of a Murine Melanoma1 Jeffrey A. Coderre,2 John A. Kalef-Ezra, Ralph G. Fairchild, Peggy L. Micca, Lawrence E. Reinstein, and John D. Glass Medical Department, Brookhaven National Laboratory, Upton, New York 11973 fJ. A. C, J. A. K-E., K. G. F., P. L. M., J. D. G.J; Department of Physiology and Biophysics, Ml. Sinai School of Medicine, New York, New York 10029 fJ. D. G.J; Department of Radiation Oncology, State University of New York, Stony Brook, New York 11794 fL. E. R.J; and University ofloannina. Medical School, Medical Physics Laboratory, loannina, Greece [J. A. K-E.] ABSTRACT Boron neutron capture therapy has been carried out on BALB/c mice carrying the Harding-Passey melanoma s.c. on the thigh. /7-Boronophen- ylalanine (BPA), a boronated analogue of natural melanin precursors, was used to target boron selectively to melanoma. BPA was administered to the mice either via i.p. injection or p.o. by intubation. '"It concentrations in tumor ranged from 15 to 40 ppm depending on the route and timing of administration. Irradiations with a predominantly thermal neutron beam were performed at the Brookhaven Medical Research Reactor. In the absence of BPA, only transient tumor growth delays were observed at low neutron fluences. At 5 x Id'" n/m2, 4 of 22 tumors irradiated in the absence of BPA underwent long-term tumor growth control; after p.o. administration of BPA (40 ppm '"H in the tumor), the fraction of tumors controlled increased to 11 of 19. The average dose to the tumor in the latter group was 17.8 Gy, of which 14.8 Gy were due to the '"15neutron capture reaction. The biological effectiveness of the absorbed dose from the neutron capture reaction, at the 50% tumor control level, was found to be twice that of 100 kVp X-rays. INTRODUCTION The major dose-limiting factor in cancer radiation therapy is the tolerance level of normal tissues within or close to the radiation field. In BNCT,3 thermal neutrons interact with boron via the '°B(n,a)7Li reaction (1) to produce short-range (<10 firn), densely ionizing heavy charged particles which have a large relative biological effectiveness (2-7). In principal, selec tive localization of 10Bwithin the tumor should allow most of the dose to be restricted to the tumor (7, 8). However, initial clinical trials of BNCT for the treatment of glioblastoma mul tiforme carried out between 1953 and 1961 at Brookhaven National Laboratory and the Massachusetts General Hospital were disappointing (9-11). Poor results were attributed to two major factors: (a) the use of boron-containing compounds which showed no selective accumulation in tumor; and (¿>) the rapid attenuation in tissue of the incident thermal neutron beam. The high thermal neutron fluence used resulted in exces sive surface tissue exposure; viable tumor was found at depth following the neutron irradiations. The substantial levels of boron in blood during irradiation contributed to the damage to normal brain vasculature (tumorrblood ratio, <1). The development of boron-containing compounds with the ability to concentrate in tumor tissue has renewed interest in BNCT. Clinical trials of BNCT with the sulfhydryl boronohy- dride Na2Bi2HuSH have been under way since 1968 in Japan under the direction of Professor H. Hatanaka. Median post operative survival is claimed to be longer than postoperative survival of comparable patients treated with conventional ra- Received 12/14/87; revised 7/12/88; accepted 8/15/88. 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. 1This work was supported in part by Grants CA42446 (J. A. C.) and DK10080 (J. D. G.) form the NIH and by Contract DE-AC02-76CH00016 with the United States Department of Energy. Accordingly the United States Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution or allow others to do so for United States Government purposes. 2To whom requests for reprints should be addressed. 3 The abbreviations used are: BNCT, boron neutron capture therapy; BPA, p- boronophenylalanine; BMRR, Brookhaven Medical Research Reactor. diotherapy. Some long-term survivals (>8 years) have been reported after operation and BNCT of malignant brain tumors (12). Further improvements are expected in BNCT from the use of epithermal neutron beams to improve tissue penetration (13, 14). The potential of BNCT should be realized when epithermal neutron beams can be combined with boron-containing com pounds that concentrate in tumor with low boron levels in blood and surrounding normal tissues. A number of laboratories in the United States, Europe, Japan, and Australia are engaged in the development of such compounds. We have recently shown that a boron-containing amino acid analogue, BPA (15), is transiently concentrated in a murine melanoma model with optimal conditions for tumor treatment occurring about 6h after injection (16). Within the tumor, BPA accumulation was greatest in areas identified by trinateti thymidine autoradiog- raphy as rapidly dividing tumor tissue and peaked 6 h after a single i.p. injection. Tumor to nontumor boron concentration ratios were 3:1 to 15:1. The amount of 10B in tumor ranged from 15 to 30 ppm which is estimated to be adequate for BNCT in vivo ( 13). We have found that p.o. delivery of BPA results in signifi cantly higher amounts of 10B in tumor than we were able to obtain via i.p. injections. We now report the results of BNCT experiments carried out at the BMRR using a murine mela noma and the biochemically targeted boron carrier BPA. MATERIALS AND METHODS Murine Melanoma. Adult female BALB/c mice ( 15-20 g, 8-16 weeks old) in which the Harding-Passey melanoma had been implanted s.c. on the thigh were used. This tumor has been maintained in our laboratory by serial transplantation for over 10 years and is reproducible with respect to melanin content and uptake of melanin-affinic agents. Melanin content (0.68% melanin by weight) is analogous to that found in human melanotic melanoma (0.1-0.8%; average value, 0.35%) (17). The Harding-Passey melanoma grows at the original s.c. implantation site and does not metastasize. The s.c. location of this heavily pigmented tumor facilitates volume measurements. Tumors weighing between 20 and 80 mg were chosen for therapy experiments. The initial tumor dimensions ranged from 2.5 to 6 mm with an average thickness of about 2 mm. This corresponds to 14-18 days of growth. Tumors that have grown to this size do not undergo spontaneous remission. Mice with untreated tumors or mice with tumors which eventually resumed growth following the irradiation procedures were sacrificed when the tumors exceeded 2-3 cm3. Mice whose tumors did not regrow after therapy were monitored until they neared the end of their normal life- span (an additional 18 months). To test our presumption that thermal neutron irradiation in the presence of BPA increases the radiosensitivity of the Harding-Passey melanoma, tumor-bearing mice were divided into four groups: (a) untreated controls; (b) irradiated with conventional X-rays; (c) irradi ated at the BMRR in the absence of BPA; and (¡I)irradiated at the BMRR following administration of BPA. A total of 187 tumor-bearing mice were used; 40 mice served as untreated controls to document the normal growth rate of this tumor, 56 to determine the sensitivity of the Harding-Passey melanoma to conventional X-rays, 39 to determine the response of the melanoma to neutron irradiation at the BMRR, and 52 to determine the response to irradiation in the presence of BPA. 6313 on March 21, 2016. © 1988 American Association for Cancer Research. cancerres.aacrjournals.org Downloaded from