S Bacg et a1.2 demonstrated the radioprotective

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1 RADIOPROTECTION OF TUMOR AND NORMAL TISSUES BY THIOPHOSPHATE COMPOUNDS THEODORE L. PHILLIPS, MD,* LAWRENCE KANE, BS,+ JOELLA F. UTLEY, MD* The efficacy of the radioprotective thiophosphate. compounds WR638 (sodium hydrogen 5[2 aminoethyl] phosphorothioate) and WR2721 (S2-[3-aminopropylamino] ethyl phosphorothioate) was determined for a wide range of normal tissue and tumor endpoints. Dose-modifying factors (D.M.F.) were obtained for mouse skin, small intestine, bone marrow, esophagus, lung, and kidney. D.M.F. values were also obtained for the P-388 leukemia, the EMT-6 mouse mammary carcinoma assayed in vitro, and the EMT-6 mammary carcinoma assaysed for cure. The D.M.F. values obtained for normal tissue ranged from 1.2 for lung to 3 for bone marrow CFU's. The D.M.F. values for tumor ranged from 1.3 for cure of the EMT-6 carcinoma to 2.2 for mean survival time in the P-388 leukemia. There appears to be a decreasing dose-modifying factor as a function of increasing dose in both tumor and normal tissue systems. The degree of protection afforded tumors appears to relate to their vascularization and their anoxic cell component. Although the D.M.F. obtained for cure of the solid EMT-6 carcinoma would appear to be lower than for normal tissues with single doses, the data presently available do not allow one to predict that such a beneficial effect would occur with the multiple small doses used clinically. INCE THE ORIGINAL STUDIES OF PA'lT7 AND S Bacg et a1.2 demonstrated the radioprotective effect of aminothiols against whole body lethality in a number of species, there has been hope that these compounds could be applied in radiation therapy. Because conventional radiation therapy is limited by normal tissue tolerance, it has been felt that a compound which would protect the normal tissue would also allow an increase in dose. The steep relationship between dose and cure of many tumors indicates that small increases in a tolerable dose would result in significant increases in local control rate. Thus, any compound which would allow a differential Presented at the 14th Annual Meeting of the American Society of Therapeutic Radiologists, Phoenix, Ariz., NOV. 1-5, From the Section of Radiation Oncology and Laboratory of Radiobiology, Department of Radiology, University of California, San Francisco, Calif. Supported by the Atomic Energy Commission through the Laboratory of Radiobiology and, in part, by National Cancer Institute Research Training Grant in Radiotherapy, #CA Professor of Radiology. t Staff Research Associate. t Instructor in Radiology. Address for reprints: T. L. Phillips, MD. Department of Radiology, University of California, San Francisco, Cali Received for publication December 21, protection of normal tissue as compared to tumor, even if this differential were small, would be of significant benefit. Early attempts to investigate the potential for aminothiols in this regard were not successful because of the relatively low degree of protection of normal tissues by compounds such as cysteamine and because of their high toxicity in larger mammals and humans.12 The development of effective thiophosphate derivatives of cysteamine has led to a new look at these compounds and their potential in radiotherapy.'" Initial investigations by Yuhas and Storer indicated a high degree of radioprotection of normal tissues by these compounds.17 They demonstrated that one of the compounds, S-2-[3-aminopropylamino] ethyl phosphorothioate (WR2721) gave excellent protection of normal tissues (D.M.F ) while there was little protection of a transplanted mouse mammary carcinoma (D.M.F. = 1.15). They also showed an apparent increase in the therapeutic ratio using a lung tumor system.l6 Our preliminary studies with thiophosphate compounds indicated that they protect aerated cells far more than hypoxic cells with a resultant decrease'in the oxygen enhancement ratio.5 Although there was no significant difference in the ability of

2 No. 9 THIOPHOSPHATE RADIOPROTECTION Phillips et al. 529 tumor and normal tissue cells to dephosphorylate the compound, the difference in effect on euoxic and hypoxic cells and the slow diffusion of the compound into cells suggested that differential protection of normal vs. tumor tissues might be observed in intact solid tumors. The present studies were designed to determine the degree of protection obtainable with two of the thiophosphate compounds in as wide a range of normal and tumor tissues as possible using single doses. The two compounds studied in detail are S-2-[3- aminopropylamino] ethyl phosphorothioate (WR2721) and sodium hydrogen S- (2-aminoethyl) phosphorothioate (WR638). MATERIALS AND METHODS Comfiounds: The thiophosphate compounds were supplied to us by Dr. W. Rothe, Division of Medicinal Chemistry, Walter Reed Institute of Medical Research. The code numbers assigned to these compounds by Walter Reed are used throughout this paper for purposes of brevity. All of the experiments presented here were conducted with a single batch of compound obtained in 1971 from Dr. Rothe. The compound was tested for toxicity, free sulphydryl content, and protective efficacy. The compound thus checked for purity was divided into 25-g samples, sealed in evacuated vials, and frozen until use. Prior to use, the compound was thawed, dissolved in distilled water, and injected intraperitoneally in a volume of 0.2 ml 30 min. prior to irradiation. The dose used was 500 to 600 mg/kg of animal weight, approximately two-thirds the lethal dose. Statistical analysis: The values for the Do of colony-forming unit cells (CFU s) were calculated using a standard linear regression method. In many experiments in which dose response curves were derived, a number of experiments were combined using the logarithmic mean of surviving fractions. The 50% lethal dose values were calculated using a computer method based on probit analysis as developed by Finney,s and by Aitchison and Brown. This same program was used for the calculation of LD/50 values for drug toxicity and for tumor control dose values (TCD/50). Zt-radiaticm techniques: The experiments were carried out using either 300 Kvp x-rays, 20 ma, half-value layer 1.6 mm of copper, target surface distance cm or with a selfcontained 2000 Curie 137 Cesium irradiator. Dose rates for the 300 Kvp x-rays ranged from 350 to 600 rads/min. Dose rates for the 137 Cesium irradiator ranged from 150 to 350 rads/min. All doses were measured in dummy animals or dead animals using TLD dosimeters appropriately calibrated for the energy tested. Skin damage: Skin damage was measured by the reaction grading system developed by Fowler et al.4 A modified version of the scoring system was used (Table 1). The left foot and lower leg of Swiss Webster mice were irradiated using a small skin treatment cone on the 300 Kvp machine. A range of doses was given to control mice and to mice 30 min. after intraperitoneal injection of 500 mg/kg WR2721 or WR638. The degree of reaction observed on the foot was scored every 2 days for 160 days. The grade of reaction observed over days 16 through 40 was averaged and plotted vs. the dose given. From the resulting dose-response curves, the D.M.F. values for the two drugs were obtained. Zntestinal damage: Protection against intestinal damage was measured using the intestinal mucosa microcolony assay system of Withers and Elkind.14 Swiss Webster female mice were given whole body irradiation using 137 Cesium. Controls and those given 500 mg/kg of WR638 or WR2721 were sacrified 3y2 days after irradiation. The central third of small intestine was removed, pinned on wax boards, and fixed. Eight sections were taken from the fixed intestine and embedded vertically so they could be sectioned on end. Four mice were included in each dose group so that 32 sections were available for scoring of the number of surviving microcolonies at each dose level. Bone marrow colony-forming units (CFU): Score TABLE 1. Mouse Skin Reaction Grades Description doubtful if different from normal. 2.0 Definite abnormal reaction, erythema, edema, hair loss. 3.0 Severe erythema, scaling, small area of moist desquamation. 4.0 Several small areas of moistness or one medium area of moistness (one-fourth foot). 5.0 Considerable area of moist desquamation; toes stuck together. 6.0 Moist desquamation of one-half of foot. 7.0 Moist desquamation of three-fourths of foot. 8.0 Severe moist desquamation of entire foot. 9.0 Severe crust and slough on foot or leg or both.

3 530 CANCER September 1973 Vol. 32 The marrow CFU technique as described by Till and McCulloch was used.13 LAF, female mice were treated with 600 mg/kg of W721 or WR63S and graded doses of 300 Kvp whole body x-rays. Following irradiation, femoral marrow was removed and injected into recip ient mice previously given 950 rads whole body radiation. Spleen colonies were counted 8 days following intravenous injection of the marrow cells. Whole body lethality: Whole body lethality secondary to bone marrow damage was measured by exposing DBA/2 mice in the 137 Cesium irradiator. Control and protected mice were then checked for deaths for 30 days. No early deaths were observed, and all deaths had occurred by 30 days. The dose vs. per cent responding data were then used to determine the LD50/30. Esophageal damage: Esophageal damage was measured by the esophageal LD50/28 technique. LAFl male mice were exposed to 300 Kvp x-rays administered to the thoracic region with shielding over the head and body. A 2.5- cm strip was given graded doses of radiation and animals were observed for death. All early deaths were noted to occur within 28 days. Histologic preparations of the entire thoracic contents revealed that the only damage linked to the death of the animals was denudation, perforation, and infection in the esophageal region. Pulmonary damage: Pulmonary damage was measured by the pulmonary LD50/160 technique previously reported.0 Animals were exposed to the thorax in a manner similar to that used for the esophagus. In both cases, mice were exposed while breathing air. When lower doses were used, the early deaths due to esophageal damage were not observed. Mice were found to die between days 100 and 160 of what appeared histologically to be a typical radiation pneumonitis. Renal damage: Histologic observations had revealed that renal injury following local irradiation appears between 4 and 8 months after irradiation.8 Based on this, a technique was devised in which the right kidney was excised from LAF, male mice. Four weeks later, after completion of hypertrophy, the left kidney was enlarged and easily palpable. It was irradiated through the abdominal wall using a 2-cm applicator on the 300 Kvp machine. Following the administration of graded doses, deaths were noted to occur between 6 and 12 months after irradiation. Histologic sections revealed severe renal damage as the cause of death. Early death due to bowel damage was not observed. The data were used to establish renal LD50 values at the 12-month (365 day) period. P-388 leukemia system: The response of a hematologic malignancy was measured using the P-388 leukemia in DBA/2 mice. Leukemia cell stock was maintained as an ascites tumor and transferred weekly. Cell suspensions were prepared from the ascitic fluid, and 106 cells were injected intravenously through a tail vein. Deaths from leukemia occurred uniformly with doses as small as a single cell. Survival times varied from 13 days for 10 cells to 7 days for 106 cells. Animals to be treated were given 106 cells intravenously, followed 24 hours later with 500 mg/kg of WR2721 or saline. Thirty minutes following this, they were given graded doses of whole body radiation using 137 Cesium. Mouse mammary carcinoma culture assay: In order to assay tumor cell response in vivo, the EMT-6 tumor developed by Rockwell et al. was employed.11 The tumor was carried alternately in Balb/c female mice and in tissue culture using fortified Eagle's medium containing 15% fetal calf serum. Cell suspensions were prepared by trypsinization of stock cultures and injected into the flank of Balb/c mice subcutaneously. When the tumors had attained an average size of 1000 mm3, the mice were given graded doses of whole body radiation. The experimental mice were given 500 mg/kg of WR2721 intraperitoneally 30 min. prior to irradiation. Following exposure, the tumors were removed, minced, and trypsinized, and single cell suspensions were prepared. These suspensions were plated in plastic flasks. The number of colonies resulting from control and irradiated tumors 11 days following exposure were compared and surviving fractions determined. The control plating efficiency averaged 25 f 5%. Tumor control system: The EMT-6 mouse mammary carcinoma in the Balb/c mouse was also used for a tumor cure experiment. Tumor cell suspensions were prepared from tissue culture as described above; 106 cells were injected subcutaneously on the dorsum of the foot. When the tumors had reached a volume of mm3, they were irradiated using a collimated 137 Cesium beam which delivered less than 1% of the tumor dose to the body of the mouse. The experimental animals were given 500 mg/kg of WR

4 No. 3 - THIOPHOSPHATE RADIOPROTECTION Phillips et L i 2Dt I A m.t - A rn A r n A SINGLE DOSE, AIR, S/w 0 Air, Do=156 rods 0 Air, 2721, Do-415 rods 638, Do-174 rods 1.o REACTION GRADE FIG. 1. Dose-modifying factors for mouse foot skin reaction in the Swiss Webster mouse with WR638 and WR2721 as a function of the severity of the average reaction grade over days after irradiation. min. prior to irradiation. Following exposure, the mice were observed for tumor regression with measurements three times per week. Tumor cure value (TCD 50/60) at 60 days was determined by using the ratio of animals alive without the tumor to the total of those dead with tumor, alive with tumor, and alive without tumor. Animals dying prior to the 60-day point free of tumor for over 2 weeks were considered intercurrent deaths and eliminated from the analysis. RESULTS Normal tissue protection: Dose-modifying factors for skin damage ranging from slightly more than two down to slightly more than one were observed for both WR638 and WR2721 (Fig. 1). There appeared to be a gradually decreasing D.M.F. as a function of increasing reaction severity and increasing dose. The protection observed using WR2721 and WR638 in intestinal microcolonies (Fig. 2) was seen as a shift of the dose-response curve for WR638 with only a small Do change but as a shift and Do change for WR2721. If one considers the dose required to yield 10 microcolonies as the endpoint, the dose-modifying factor is 2.1 for WR2721 and 1.5 for WR638. The degree of protection afforded by WR2721 was measured in two different bone marrow systems. In the CFU system treated prior to transplantation, a dose-modifying factor of 3.0 was observed (Table 2). When the bone marrow cells were made hypoxic by exposure to 5% oxygen, the dose-modifying factor was reduced to When DBA/2-0 lo RADS FJC. 2. Survival curves for intestinal crypts in the Swiss Webster mouse using the microcolony assay tcchnique. female mice were exposed to whole body ratliation with and without WR2721, a significant difference in the LD50/30 was observed, yielding a D.M.F. of 2.2 (Table 3). The esophageal lethality tletermiiia ti0115 with WR2721 yielded a somewhat lower D.M.F. of 1.4 (Table 4). It should be noted that this endpoint occurs at a relatively high total dose. Thus, animals may survive high doses due to a relatively resistant residual cell population which is less amenable to radioprotection. The results for whole lung irrndiation using the pulmonary lethality endpoint indicate an even lower D.M.F. of 1.2 (Table 5). This experiment has now been re- Air TABLE 2. Bone Marrow CFU's in LXFI hlice Mode Do( rads) D.il1.F. Air + WR Hypoxia 183 Hypoxia + WR a3

5 ~~ ~ 532 CANCER September 1973 Vol. 32 TABLE 3. Whole Body Lethality in DBA/2 Mice LD60/30 (r+) Mode (95% Conf. limits) D.M.F. Air 907 ( ) Air + WR ( ) 2.2 TABLE 5. Pulmonary Lethality in LAFI Mice LD50/180 (rads) Mode (95y0 Conf. limits) D.M.F. Air 1339( ) Air + WR ( ) 1.2 TABLE 4. Esophageal Lethality in LAFi Mice TABLE 6. Renal Lethalitv in LAFI Mice LDSO/ZII (rads) Mode (95% Conf. limits) D.M.F. Air 2682( ) Air + WR ( ) 1.4 LD5Ol3.35 (rads) Mode (95% Conf. limits) D.M.F. Air 1791 ( ) Air + WR ( ) 1.5 peated twice with large numbers of mice, and the lung appears to be a significant exception to the generally high degree of protection offered by this compound. The renal lethality studies were successful with similar patterns of death observed in normal and protected mice. Although the total doses used were relatively high, a dose-modifying factor of 1.5 was observed at 12 months following irradiation (Table 6). Tumor protection: The P-388 leukemia system was used as a model for leukemia. In general, such tumors are thought to be well vascularized and to have small fractions of hypoxic cells.1 The experiments summarized in Table 7 indicate that whole body irradiation of mice given lo6 leukemic cells intravenously 24 hours prior to irradiation increases mean surviva1 time from 7 to 10 days. Although the total body dose deliverable in mice protected by WR2721 can be increased by a factor of 2.2, the D.M.F. observed in DBA/Z mice for whole body lethality, there is no resultant increase in the mean survival time of tumorbearing animals. Thus, one must conclude that there is a similar degree of protection of the tumor and the normal bone marrow cells as measured by the LD50/30. The results of in vivo irradiation with in vitro culture of the EMT-6 mouse tumor are shown in Fig. 3. The survival of tumor cells in control animals appears to follow a twocomponent type of relationship. This has been interpreted by Powers and Tolmachlo and Kallmans to be due to an anoxic fraction of tumor cells. When animals bearing similar tumors are irradiated after receiving WR2721, a significant change in the response is noted with marked protection occurring in the lower dose range. At doses of 1200 rads and higher, the curve for tumors protected with WR2721 parallels the control curve but is shifted above it. This suggests that the compound is protecting the aerated and, likewise, well vascularized portion of the tumor but is not protecting or is not reaching the more resistant part of the tumor whose cells survive doses above 1200 rads. When the same tumor cells are irradiated in the foot of the mouse and the tumor cure at 60 days is used as an endpoint (Table 8), a small D.M.F. is observed. Because of numerous deaths in tumor-free animals due to infection, only a 60-day endpoint was possible. The TCD50/60 values for control and protected animals do not differ significantly since their 95% confidence limits overlap. In order to compare the D.M.F. values obtained for normal tissue and tumor at different dose levels and in different systems, the Controls TABLE 7. P-388 Leukemia in DBA/2 Mice WR2721 MST* MST-Tumort MST* MST-Tumor+ Dose (rads) (days) (days) Dose (rads) (days) (days) 0 > > > > * Mean survival time without tumor. t Mean survival time of mice given 106 tumor cells.

6 No. 3 THIOPHOSPHATE RADIOPROTECTION * Phillips et al. 533 EMT.6 TUMOR Bdb/C MICE 1.o IN VlVO IRRADIATION-IN VlTRO CULTURE CONTROLS.AIR 272l.AIR TABLE 8. EMT-6 Carcinoma in Balb/c Mice TCDW~O (rads) Mode (95%..- Conf. limits) D.M.F. Air 4896 ( ) Air + WR ( ) 1.3 DOSE IRodsI FIG. 3. Survival curves for EMTS tumor cells irradiated in situ in the flank of Balb/c mice breathing air with and without WR2721 given 30 min. before irradiation. D.M.F. values obtained are plotted as a function of the dose required for the specific endpoint in control animals (Fig. 4). Although in the 50% lethality experiments only one dose level is available, in the skin reaction grading system, the CFU system, and the EMT-6 tumor culture system a number of D.M.F. values as a function of dose may be extra-polated from the dose-response curves. The general trend of results as displayed in this plot indicates a decreasing D.M.F value as a function of increasing dose. It also indicates rather small differences in D.M.F. values between tumor and normal tissue with small individual doses and with cells not expected to be hypoxic or to have impaired blood supply. DISCUSSION for lung found by Yuhas.15 His mice and radiation technique were quite different, however. There seems to be some correlation between the proliferative capacity of the tissue and its D.M.F. value. The relatively rapidly proliferating bone marrow, gut, and skin showed D.M.F. values at lower doses of 2 or greater. The value for the esophagus, which is lined with squamous epithelium, is somewhat lower but when corrections are made for the relatively high dose at the endpoint, it differs little from skin. The D.M.F. values obtained for organs containing slowly proliferating cells, such as kidney and lung, are definitely smaller and suggest that caution should be used in assuming the degree of radioprotection of such tissues. It should be noted that in our skin damage experiments and in those of Yuhas and Storer,lBJT late skin damage does not appear to be greater than the early reaction grade, thus suggesting that the subcutaneous vasculature is as well protected as the proliferating cells of the skin itself. Thus, one cannot explain the decreased protection observed in lung and, to a certain extent, in kidney, on the basis of the lack of protection of vascular tissues. It should be noted that all of these studies were done with drug dose of approximately DM I VALUES 0 NORMAL TISSUE A TUMOR These experiments have confirmed the relatively outstanding efficacy of thiophosphates as radioprotective compounds in a wide range of normal tissues. Our results agree quite well I L I I I I X with those previously presented using differ- loo0 Zoo ent endpoints by Yuhas and Storer. 16,17 Dose lor Endpoint. rods Because the systems used here have in many cases utilized local irradiation and specific tissue endpoints, they are in some cases more ap- Plicable to radiation therapy* With the exception of the lung there would appear to be good evidence to assume a D.M.F. varying from 1.5 to 3 in normal tissues. The low value for lung does not agree with a value close to 2 FIG. 4. D.M.F. values for normal tissue and tumor as a function of dose size for the given endpoint in control animals. cfa = aerated CFU's. cfh = hypoxic CFU's. emt = EMTS carcinoma. Pa = P-388 leukemia. i = intestinal crypts. 1 = lung LD50/160. k = kidney LD50/365. = &in reaction grade days = esophageal LD m = LD50/3O in DBAP mice.

7 534 CANCER September 1973 Vol. 32 two thirds the 50% lethal dose in the mouse studied. The LD50 values for the radioprotective compound were observed to range from 751 mg/kg for Balb/c mice to 1140 mg/kg for LAF, mice. Thus, the doses used were quite large, and one cannot expect that doses yielding D.M.F.'s in these ranges could be delivered in humans without severe toxicity, Although no mice die from drug administration when given two thirds the LD50 dose, they do appear extremely agitated, disoriented, and somewhat cyanotic for several hours after administration of the drug. However, significant doses may be tolerated in humans (W. Rothe, personal communication). It appears clear from these experiments that one cannot expect a beneficial differential protection using thiophosphate compounds in hematologic malignancies. The absence of any differential prolongation of life in spite of a dose-modifying factor for bone marrow death of 2.2 seems to rule out the potential for using these agents in conjunction with total body radiation for the treatment of leukemias and lymphomas. The results with the solid EMT-6 carcinoma are somewhat more encouraging. The original report by Yuhas and Storerl? indicated that they could find a D.M.F. no greater than 1.15 for a mouse mammary carcinoma system. Although the endpoint was not specifically cell survival or tumor cure, it did relate to transplantability following relatively high single doses. Thus, one would expect that the transplantation would depend on a survival of resistant or hypoxic cells within the tumor. It is, thus, analogous to tumor cure experiments. The tumor cure experiment reported here with the EMT-6 carcinoma of the foot of Balb/c mice does indicate a relatively low D.M.F. of 1.3. In addition, it should be noted that the TCD50/60 values are not significantly different for control and protected tumors. This is due, in part, to difficulties with the tumor cure system due to the death of tumor-free mice after loss of their foot secondaty to tumor and radiation damage It should be noted that the opposite foot, which received the same dose, was not lost in these mice and thus the loss of the foot must be due to combined damage by tumor and irradiation. In spite of these complications, it would appear that for the cure of this tumor with a single dose there is little protective effect, placing us in agreement with Yuhas and Storer Such a result can be predicted from the tumor dose-response curves obtained by irradiating flank tumors in vivo and then determining cell survival in vitro. The appearance of an initial increase in slope as compared to controls and then a response above but parallel to control response suggests that at relatively high doses there would be very little dose modification. It is not possible to conclude from the information presented here and by other workers whether WR2721 and similar efficacious thiophosphate compounds would be of benefit in clinical radiotherapy. Although the D.M.F. obtained for the EMT-6 carcinoma at doses of 500 rads or higher is certainly lower than for many normal tissue endpoints, doses of this size are rarely used clinically. At very low doses, the data presented here suggest that a D.M.F. of 2 might result in this particular tumor. Since the reduced D.M.F. obtained in solid tumors appears to relate to perfusion and oxygenation, the problem in predicting an increased therapeutic ratio secondary to the use of these compounds is similar to that for hyperbaric oxygen and high LET radiations. It is clear that single dose endpoints, because of the high total dose used and the dominance of hypoxic and poorly vascularized tumor segments in determining failure, are not good endpoints for comparison to clinical conditions. Tumor cure experiments using a number of tumor types and using a minimum of 10 fractions must be used and compared to the reaction of normal tissues using the same number of fractions and preferably in the same mouse. Such experiments are now under way and must prove an increase in the therapeutic ratio before any thought is given to clinical evaluation or trial of these compounds. REFERENCES 1. Aitchison, J.. and Brown, J. A. C.: The Log Norma1 Distribution. London, Cambridge University Press, 1957; pp Bacg, 2. M., et al.: Protection against x-rays and therapy of radiation sickness with b-meraptoethylamine. Science 117: , Finney, D. J.: A statistical treatment of the sigmoid response curve. In Probit Analysis, London, Cambridge University, 1947; pp Fowler, J. F., et al.: The effect of divided doses of 15 MeV electrons on the skin response of mice. Znt. J. Radiat. Bid. 9: , 1965.

8 No. 3 THIOPHOSPHATE RADIOPROTECTION Phillips et al Harris, J. W., and Phillips, T. L.: Radiobiological and biochemical studies of radibprotective corn ounds related to cysteamine. Radial. Res. 46: , I&. 6. Kallman, R. F.: The phenomenon of reoxygenation and its implications for fractionated radiotherapy. Radiology 105: , Patt, H. M.: Protective mechanisms in ionizing radiation injury. Physiol. Rev. 33:35-76, Phillips, T. L., Benak, S.. and Ross, G.: Ultrastructural and cellular effects of ionizing radiation. Front. Radiat. Ther. Oncol. 6:21-43, Phillips, T. L., and Margolis, L.: Radiation - thology and the clinical response of lung and esope:- gus. Front. Radiat. Ther. Oncol , ' 10. Powers, W. E., and Tolmach, L. J.: A multicomponent x-ray survival curve for mouse lymphosarcoma cells irradiated in vivo. Nature 197:71&711, Rockwell, S. C., Kallman, R. F., and Fajardo, L. F.: Characteristics of a serially transplanted mouse mammary tumor and its tissue culture adapted derivative. 1. Natl. Cancer Znst , Shapiro, B., Schwartz, E., and Kollman, G.: The distribution and chemical forms of the radioprotective A.E.T. in mammary tumor bearing mice. Cancer Res. 23: , Till, J. E., and McCulloch, E. A.: A direct measurement of the radiation sensitivity of normal mouse bone marrow cells. Radiat. Res. 14: , Withers, H. R., and Elkind, M. M.: Microcolony survival assay for cells of mouse intestinal mucosa exposed to irradiation. Znt. J. Rndiat. Biol. 3: , Yuhas, J. M.: Improvement in lung tumor radiotherapy through differential chemoprotection of normal and malignant tissues. 1. Natl. Cancer Znst. 48: , Yuhas, J. M., and Storer, J. B.: Chemoprotection against three modes of radiation death in the mouse. Znt. J. Radiat. Biol. 15: , , and -. Differential chemoprotection of normal and malignant tissues. 1. Natl. Cancer Znst. 42: