b. radiosensitizers/radioprotectors working group

Inr. J. Rod&ion Oncology Biol. Phys. Vol. 5, pp. 651-676, 1979. Pergamon Press Ltd. Printed in the U.S.A.

B. RadiosensitizerslRadioprotectors Working Group

I. RESEARCH LOGIC

1.0 Logic Model

Overview

1.1 Modality Phase

1.2 Exnerimental Phase

1.3 Clinical Phase

2.0. Research Matrix

II. PROGRESS REPORT AND LONG RANGE PLAN

Overview

1.0 Modality Studies: Drug Development

1.1 Review of Structures Already Synthesized

1.2 New Analogues and New Types of Radiosensitizers and Radioprotectors

1.3 Development of Standard In Vitro Screening Technique

1.4 Research Recommendations

2.0 Experimental Studies: Animal Testing of Radiosensitizers and Radioprotectors

2.1

2.2

2.3

2.4

2.5

Evaluation In Vivo of Newly Identified Hypoxic Cell Sensitizers from Drug Development Programs

Screening of Compounds for Radiosensitization in Tumors In Vivo

Screening for Radioprotection in a Range of Normal Tissues

Development of In Vivo Systems for Testing Newly Identified Compounds

Research Recommendations

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3.0 Clinical Studies

3.1 Phase I Studies

3.2 Phase II Studies

3.3 Phase III Studies


4.0 Summary of Specific Proposals: Tasks

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RADIOSENSITIZERS/RADIOPROTECTORS WORKING GROUP

Theodore L. Phillips, M.D. University of California Coordinator San Francisco, California

John M. Yuhas, Ph.D. Co-Coordinator

University of New Mexico Albuquerque, New Mexico

Gedd E. Adams, Ph.D Institute of Cancer Research Royal Cancer Hospital, England

J. Martin Brown, Ph.D. Stanford University Stanford, California

J. Donald Chapman, Ph.D University of Alberta Edmonton, Alberta, Canada

Michael J. Cory, Ph.D. Burroughs Wellcome Co. North Carolina

Richard .I. Johnson, M.B. Roswell Park Memorial Institute Buffalo, New York

Thomas P. Johnston, Ph.D. Southern Research Institute Birmingham, Alabama

Kenneth Kinnamon, D.V.M. Uniformed Services U. of the Health Sciences

Bethesda, Maryland

John S. Penta, Ph.D. NC1 Representative

National Cancer Institute

James G. Schwade, M.D. NC1 Representative


Helen *B. Stone, Ph.D. Colorado State University Fort Collins, Colorado

Raul C. Urtasun, M.D. University of Alberta Edmonton, Alberta, Canada

Joella F. Utley, M.D. University of California San Diego, California

William W. Lee, Ph.D. Ad-Hoc Consultant

Stanford Research Institute Stanford, California

Harry Wood, Ph.D. Ad-Hoc Consultant


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RADIOSENSITIZERS/RADIOPROTECTORS

I. RESEARCH LOGIC

1.0 Logic Model

RADIOSENSITIZERS/RADlOPROTECTORS

MODALITY EXPERIMENTAL LARGE ANIMAL CLIN. PHASE CLIN. PHASE DEVELOPMENT SYSTEMS TOXICOLOGY l/II Ill

+ Key Interface 0 lntertace 4 Recommendations by Steering I Subcommittee

f Recommendations by Planning Group

The radiosensitizers program is modeled after the research logic outlined in the DCT treatment linear array which delineates the criteria for moving potential chemotherapeutic compounds into clinical trials. The radiosensitizer/radioprotector program is particularly suited to adopt this research logic since the electron-affinic hypoxic cell radiosensitizers, the nitroheterocyclic compounds, have specific characteristics that allow for testing both in vitro and in vivo against a specific panel of tumors before it is advancedin the linear array. A specialized advisory subcommittee of the DCT Decision Network Committee, i.e., the Data Review Subcommittee (DRS) for Radiosensitizers, consisting of active investigators, reviews the progress and data of analogue development and recommends which drugs deserve extensive biological and clinical testing. Detailed research logic arrays have been developed for both radiosensitizers and radioprotectors as modifications of the DCT treatment linear array.

1.1 Drug Development

STAGE 1: Evaluation.

Selection and Acquisition of Electron Affinic Agents for

The analogue drug development program in radiosensitizers/radioprotectors is underway with numerous new compounds being synthesized and characterized. The electron affinity of the compound determines its radiation sensitivity and the lipophilicity determines its access to CNS and tumors. A recommendation to further evaluate the literature and supply existing agents from foreign countries has been made. In addition to the two contractors synthesizing new agents, an additional two are proposed. The third recommendation is to develop an in vitro animal testing. -

test for toxicity and activity prior to

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1.2 Experimental Phase

STAGE IIA: Determination of Radiosensitizer/Radioprotector Activity and Toxicity of New Agents.

One of the unique aspects of the radiosensitizer program is the selection of a panel of solid animal tumors which have a known fraction of hypoxic cells and can easily be assayed. Thus, unlike other tumor panels, the mechanism of action, named hypoxic cell sensitization, can be specifically tested. The requirements for further development and testing are that the new agents should be active in three tumor systems selected with three endpoints and be superior to Ro-07-0582 (misonidazole) in tumor effects with accept- able or lower toxicity (LDso). A recommendation has been made for more support of biologic testing both in tumors and in the development of small animal models for neurotoxicity assessment.

STAGE IIB: Drug Evaluation, Production and Formulation.

When the agent enters this stage, it enters secondary evaluation to determine schedule dependency and optimal route of administration. Studies are also conducted to determine feasibility of large scale production for clinical trials.

STAGE III: Large Animal Toxicology.

Large animal toxicology research is ‘very costly and the DCT toxicology program is not currently organized to study radiation toxicology . Fortunately, the DCT is able and willing to test the radiosensitizer/ radioprotector agent in dogs and/or monkeys to meet FDA requirements for the IND. Since nervous tissue toxicity is a major problem with the clinical use of these agents, careful evaluation for seizures, motor or sensory dysfunctions, and peripheral neuro- pathies will be sought. Since most normal tissues are believed not to have a significant number of hypoxic cells, most normal organ and tissue tolerances should be unchanged. If the combination of a radiosensitizer and radiation result in an unusual organ toxicity, further testing in radiation toxicology systems will be required.

1.3 Clinical Stage

STAGE IV: Toxicity in Humans: Phase I Studies.

With the approval and receipt of an IND clinical trials can begin, and have been initiated under the auspices of the RTOG in two institutions. Phase I studies are reaching completion identifying suitable single dose and fractionation dose schedules for misonidazole. A recommendation has been made to increase this capacity and capa- bility, through the existing cooperating group structure, by ex- panding the RTOG effort or adding another group.

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STAGE V: Screening for Clinical Activity: Phase II Studies.

A number of Phase II protocols have been activated in the RTOG and others for different tumor sites are to be activated shortly. Although a panel of ten tumors known as “signal tumors”, represent- ing a wide array of human maiignancies, are commonly tested for new chemotherapeutic agents, tumors believed to have hypoxic components, i.e., brain gliomas, lung cancers, advanced head and neck cancers and large metastatic deposits, are favored for field testing. This activity should be largely supported through radiation oncology investigations in existent cooperating groups.

2.0 Research Matrix

The use of radiosensitizers may selectively improve the therapeutic ratio and provide a therapeutic enhancement ratio. This radiosensitizers program serves as an excellent vehicle through which to become familiar with the much wider chemotherapy drug development program. It provides an insight into the resources, time and costs to move new agents from their discovery through the stages of laboratory tumor testing, in vivo and in vitro production procedures, toxicity testing for FlK q finally, itshtroduction into clinical trials.

The ability to successfully develop radiosensitizers by this highly organized program logic can be compared to the development and introduction of hyperbaric oxygen breathing into clinical trials. Many sound radiobiologic concepts fail to lead to clinical advances since their design and too rapid introduction into clinical studies lead to undue toxicity and high complication* rates with small therapeutic gains. This effort in radiosensitizers is one of the most comprehensive and inte- grated scientific-clinical research programs in radiation oncology, and is being advanced in cooperation with the NC1 drug development program. The use of their large animal toxicology resources is an essential ele- ment in the development of this program since obtaining the IND from the F’DA can be a major obstacle to the introduction of new agents in this country. Also, the basic theses of utilizing radiosensitizers in combination with radioprotectors and with other innovations such as hyperthermia and high LET radiation require even further careful toxicology studies prior to introduction into human studies.

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II. PROGRESS REPORT AND LONG RANGE PLAN

Overview .

The modification of the initial radiochemical events which occur in tissue and in tumors during clinical radiotherapy has significant potential for the improvement of the results of clinical radiotherapy. Two classes of compounds exist. Sensitizer compounds that are electron- affinic and mimic oxygen and thus sensitize cells which are deficient in oxygen up to the maximum oxygen enhancement ratio for the ideal compound. Protective compounds, in general, contain sulfhydryl groups which antag- onize the action of the electron-affinic compounds and induce repair of the initial radiochemical lesions.

Extensive radiobiologic information now exists which indicates that the hypoxic cell sensitizers are extremely effective in animal tumors. Ini- tial clinical trials with two of the compounds, metronidazole and misonidazole, suggest significant sensitization, moderately good tolerance, and the need for extensive clinical trials. On the other hand, the existing compounds have significant neurologic and gastrointestinal toxicity and improved compounds are desperately needed.

The situation is similar for radioprotector compounds. The best current compound has good protective efficacy in bone marrow and intestine, but not in critical tissue such as lung. It also appears to be fairly toxic in animal studies, although probably sufficiently non-toxic to be tolerated in protective doses in humans.

Radiation oncologists are faced with exciting new compounds that have significant potential to protect normal tissues and sensitize tumors, but which have problems in therapeutic ratio. Thus, a large drug development program is indicated which will look for analogues and new compounds with increased sensitization or protective efficacy as a function of compound dose and of concurrent toxicity. This plan outlines the steps which should be taken over the next five to ten years to develop such improved compounds.

1.0 Modality Studies: Drug Development

Drug development may be divided into four major areas:

0 intellectual selection of compound design.

0 new compound synthesis.

0 chemical and radiochemical characterization.

0 in vitro testing in aerated and hypoxic mammalian cells. --

Although the electron-affinic radiosensitizers can be expected to receive the majority of the attention in this development plan, several other compounds which sensitize cells to ionizing radiation by different mechanisms are likely to have increasing importance.

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The United States Army Medical Research and Development Command carried on a screening program for radioprotective drugs which ended in 1969. This program developed highly efficient sulphur containing drugs and identified a number of new classes of protective agents. The best compound to come out of this study was WR-2721. The standards against which new compounds will be compared are misonidazole for hypoxic cell sensitizers and WR-2721 for radioprotectors.

1.1 Review of Structures Already Synthesized

A number of compounds of interest exist in the stores of various drug companies in the United States, Europe and Japan, in the DCT of the NCI, and in the files of the US Army Medical Research and Devel- opment Command at Walter Reed Army Medical Center. In order to adequately review the hypoxic cell sensitizer structures already synthesized, a computer listing of the various nitroheterocyclic and other electron-affinic structures synthesized in various chemotherapeutic programs by different groups should be requested to determine if additional compounds with radiosensitizing potential exist. A review of structural information should be made at least semi-annually by a committee composed of NC1 staff and members of the drug development subcommittee. Criteria for selection would include appropriate water solubility, electrophilicity, lipophilicity and other attributes suggesting potential as a hypoxic cell sensitizer.

Although the US Army effort synthesized over more than 100,000 drugs, the overall analysis was never completed and a systematic search through the data is indicated. It is propocad that an ad hoc committee be created to meet semi-annually with NC1 staff to review the information available on these compounds and should include, as a minimum, a biologist, pharmacologist and chemist.

Since the electron-affinic hypoxic sensitizers and the sulfur containing radioprotectors are approaching theoretical maximum effectiveness, (a tripling of sensitivity of hypoxic cells or a tripling of resistance of aerated cells) the committee should be charged with identifying those agents whose activity. equals or exceeds that of the standards , misonidazole or WR-2721. In addition, it should search for agents whose sensitization or protection occurs at lower fractions of the toxic dose than the standards, and should concentrate on identifying new chemical classes of agents which protect or sensitize.

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Investigations outside the US, i.e., Japan and Russia, have provided new drugs in the protector class, as have the efforts in the United Kingdom and Canada for the sensitizers. In conjunction with the effort directed toward review of all existing compounds, a search of the literature using a computer type vehicle for reports on foreign compounds is indicated, as are inquiries to leading labs in other countries. It may be necessary to request follow-up information from specific authors and to organize an international conference to determine which of the identified agents is worth comparing to the current standards.

Following this review of available information it should be possible to answer some of the following questions:

0 Should the most effective class of agents known be pursued further?

0 Should other common classes of agents be pursued further?

0 Should the newly discovered classes of agents be pursued further?

0 Should the search be continued for additional new classes of agents?

0 Should analogue development be pursued?

0 Should agents which sensitize or protect by other means than electron affinity or other mechanisms be pursued further?

0 Should agents be sought which protect organs not protected by present agents?

1.2 New Analogues and New Types of Radiosensitizers/Radioprotectors

In addition to a complete review of existing compounds, the DCT program should continue to promote and supervise the synthesis of new analogues of proven radiosensitizers and radioprotectors. It is important to establish the chemical rationale upon which such a synthetic program would proceed.

For the hypoxic cell sensitizers, there is no doubt that radiosensitizing effectiveness of the nitroaromatic structures is strongly correlated with electron affinity and/or the one-electron reduction potential. Lipophilicity, however, is not a strong indicator of radiosensitizing effectiveness for cells irradiated in vitro. It may be an extremely important parameter in regard to animanicity and in particular neurotoxicity. A structural feature of effective radiosensitizers appears to be the hydroxyethyl substituent on the adjacent ring position to the electrophilic nitro group. It appears that cellular and whole animal toxicity is lowered by steric shielding of the reactive site on the sensitizer molecule. These and other criteria should be exploited in the design and synthesis of new analogues of proven sensitizers and new chemical classes of

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hypoxic cell sensitizers. Similar approaches will be required for the sulfur containing protective compounds, although the structure activity relationships are not as clearly defined. These criteria will await the review of the existing compounds and their effectiveness so that some basic conclusions can be reached concerning structure activity relationships.

As other classes of radiosensitizers and radioprotectors which operate by different mechanisms become identified, synthesis programs will be required to support such studies as well as those of more conventional compounds. Interesting new approaches include the thio-sugars which have hypoxic cell sensitizing, hypoxic cell killing, and aerated cell protective potential. Certain compounds which selectively modify the shoulder region of tumor cell survival curves are also of great clinical interest, although compounds developed to date do not appear active in vivo. In the future, however, they may become an important portion ynthesis programs. The use of drugs such as cis-platinum in combination with ionizing radiation needs detailed study because of the potential use of this agent in the treatment of human cancer alone and its potential for augmenting radiation effect through several mechanisms including hypoxic cell sensitization. Several analogues of these compounds exist at the DCT for such a study. As the rationale for improved radioprotector and radiosensitizer effectiveness emerges, synthesis contracts will need to be expanded to support these areas.

1.3 Development of Standard In Vitro Screening Technique

The development of new in vitro screening techniques for the purpose of more quickly and lessexpensively determining the clinical potential of specific radiosensitizing/radioprotecting compounds should be supported. In this regard research is necessary for the purpose of defining optimum laboratory procedures whereby radiosensitizing/radioprotecting effectiveness and cytotoxic properties of compounds can be rapidly determined. For example, cellular radiosensitization is measured in various laboratories according to quite different experimental protocols. Some cells are irradiated in the presence of drugs after an extremely short exposure to that drug whereas in other laboratories the exposure time can be up to one hour or longer. Such a pre-incubation of cells with drugs will effect the measured sensitizer enhancement ratio at a specific drug concentration. Factors relating to pre-incubation time, tempera- ture, composition of medium in which cells are bathed or suspended during irradiation, etc. should all be optimized for best predictive value for animal and clinical studies. Such in vitro criteria for sensitizing compounds become even more importantwhen dealing with compounds which operate selectively by modifying the shoulder of the radiation survival curve. This is also true for compounds which inhibit tumor cell respiration and consequently may lead to the reoxygenation of hypoxic regions of tumors at the time of treatment and for the selective cytoxicity of hypoxic cells by certain electron-affinic sensitizers. This optimization of in vitro screening procedures must be accomplished in collaboratioii- withthe animal screening program and the clinical trials.

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The problem of in vitro initial evaluation and screening is even more complex for theradioprotective compounds. Although the sulfhydryl compounds themselves are active in vitro, the thiophos- -- phates are not in most situations because of necessary activation which occurs in vivo. A portion of the research on the development of in vitro screening methods should address this problem and try to determinehow such compounds may be activated in vitro using enzyme substrates, liver extracts, etc.

-- so that a successful in vitro

screening could be used for protective as well as hypoxic x sensitizing compounds. At the present time, the screening of drugs for radioprotective activity must be conducted in vivo for most types of agents. As stated, in vitro studies woulcfailo identify radioprotectors which require ‘- metabolic activation such as the standard WR-2721 and would yield false positive results for agents which are rapidly detoxified in vivo. This is true as well for hypoxic cell sensitizers whichare too toxic or have too short half lives in vivo. There is so much attractiveness to the in vitro screenixg ofuch compounds, in terms of cost and speed,>hat warrants further development.

In addition to studying compounds for efficacy in vitro, it is possible that evaluations of toxicity other than diG%t-%rI killing could be carried out in vitro. For example, certain cultures of neuronal cells might beysed predict neurotoxicity. As part of a planned augmentation of the program for the development of in vitro screening systems a search for systems which predict for invivo toxicity should be carried out.

--

Research Recommendations

It is recommended that an NC1 task force be created to review existing data on known compounds and make an intellectual selection of potentially active compounds. This task would consist of reviewing all available compounds with potential as radiosensitizers and as radioprotectors, including the available information on attributes such as solubility, electron-affinity, hypoxic cell sensitization or radioprotection. A list of structure activity relationships would be finalized for both radiosensitizers and radioprotectors and the best identified compounds selected for in vitro and in vivo testing. There is a need for a wide range of expertise 1% chemistry, pharmacology, toxicology, and radiobiology and it is likely that several man years of effort will be required by staff in addition to the effort of a number of consultants.

It is recommended that the two existing contracts for synthesis of hypoxic cell sensitizer be augmented by the addition of two contracts which would expand the synthesis of analogues and new compounds with hypoxic cell sensitization and radioprotection capabilities. It is possible that each would support a lab with both capabilities, or that one would be devoted to radiosensitizers and one to radioprotectors. The contractor should be required to perform preliminary chemical and structure activity evaluations as well as preliminary in vitro and in vivo screening for activity and toxicity.

-- --

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It is recommended that an RFA be issued for the support of research and development on in vitro testing of activity and toxicity of radiosensitizers andradioprotectors. This should require experi- mentation on the optimum in vitro screen and on optimum techniques for predicting in vivo activityof both radiosensitizers and radioprotectors. Itsh;3?rrd concentrate on developing methods of acti.- vating those radioprotectors requiring activation in vivo and on methods of insuring accurate prediction of in vivo a&v- of both -- radiosensitizers and radioprotectors. It should be expected that 4-6 successful applications would be funded as a result of the RFA. These grants would complement the synthesis contracts and provide a mechanism for the development of radiosensitizers which operate by different basic mechanisms and for the radioprotectors which might be missed in conventional in vivo screens or which might act by different mechanisms. They would also be available for the in vitro evaluation of compounds identified from the review of existing structures conducted previously. These laboratories would augment the contractors in assuring an adequate flow of useful compounds to the animal screening program and to the clinical trials level.

It is highly desirable that the result of these programs be the identification of approximately 50 compounds per year showing in vitro activity at least as good as the standard compounds and thz at least ten of these compounds be sufficiently interesting to move forward toward in vivo testing. Thus it can be seen that several hundred compoundsi&?&fied through a review of existing compounds or through synthesis would reduce to about 50 compounds after in vitro screening and then to about ten compounds to undergo complete invo testing. -- From these, four would be selected to go through large animal toxicology testing and eventual Phase I clinical trial. If the program is sufficiently successful, the ultimate plan is for two radiosensitizers and one radioprotector per year to enter Phase I clinical trials.

2.0 Experimental Studies: Animal Testing of Radiosensitizers and Radioprotectors

2.1 Evaluation In Vivo of Newly Identified Hypoxic Cell Sensitizers from Drug Development Programs

Compounds which emerge from the synthesis contracts, from literature review, or from analysis of promising structures (i.e., from the drug development program) have to be tested in vivo before they can -- be analyzed by the Radiosensitizer/Radioprotector Subcommittee (Data Review Committee) prior to submission into Stage IIA. Prior to this animal testing the compound will have to fulfill criteria 1 through 6 of Stage I (Selection and Acquisition of Electron-Affinic Agents for Evaluation, pp. 9-11 of DCT Treatment Linear Array: Radiosensi- tizers, September 1977). ment of drug LD50,

The animal testing will involve measure- and determination of radiosensitization of a

solid tumor in vivo. Prior to testing for radiosensitization in vivo, it is recommended that determinations of plasma and tumor concentrations be obtained, if possible, as a function of time after

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injection. If plasma clearance is so rapid that it precludes the build up of a reasonable tumor concentration then there is no need for further in vivo testing. --

The sequence of testing of new agents identified in the drug development program will be:

?? Determination of LDso/2d.

0 Determination of plasma and tumor concentrations when an assay is available as function of time after i.p. injection of drug

- - at concentration equal to 2/3 LDso/2d.

0 Determination of radiosensitization of solid tumors a concentration equal to 2/3 LDso as a function of injection of drug, or at the time of maximum tumor tion.

in vivo at timeafter concentra-

Rased on experience, compounds could be rejected at either of these three steps. Agents which show good activity after these preliminary tests in vivo will be presented to the Data Review Subcom- mittee of theRZ&ensitizer/Radioprotector Working Group as can- didates for further testing in Stage IIA.

This initial in vivo testing should be performed on a contractual basis, but itis recommended that the work be handled by the same contractors that are doing synthesis or those performing the tests using the three tumor systems with three endpoints (Stage IIA - see below).

2.2 Screening of Compounds for Radiosensitization in Tumors In Vivo Using at Least Three Endpoints in at Least Three Tumor Systems

Radiosensitizers which are judged by the DRS to be worthy of further testing should be sent to contractors or grantees for testing of radiosensitization with the DCT panel of radiosensitizer/protector tumor screens. At least three of these tumors with three endpoints are to be used for this testing before any drug goes back to the Drug Evaluation Committee (Radiosensitizer/Radioprotector Subcom- mittee).

It is recommended that one contract be funded as soon as possible to do this work and a second contract may be needed depending on the number of new compounds which have to be screened. These contractors will have two main duties:

0 Initial in vivo screening of new drugs not already screened in one in vivosr (see 2.1 above). --

0 Follow-up testing of promising drugs using three solid tumor systems with three different endpoints as requested by the Radiosensitizer/Radioprotector Working Group.

The precise conditions of the testing will be determined by the Radiosensitizer/Radioprotector Working Group. Periodic checking of

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results or further analysis of particular compounds could be performed by others not funded specifically for this screening. This could be done by groups already funded for other aspects of radiosensitizer evaluation.

In addition to the testing of hypoxic cell sensitizers against tumors in vivo, there is a real need for an endpoint in mice or another-&all animal for the study of neurotoxicity. It is important to know whether the LDso/2d bears any relationship to the dose giving neurological damage. If it does, then the comparative evaluation of different radiosensitizers can be based on their comparative LDso/2d values. However, if this is not the case, then the compounds will have to be evaluated on a functional and/or histologic endpoint based on neurological damage rather than on the acute LD50. An RFA is recommended to encourage submission of grants designed to develop neurological endpoints suitable for screening of hypoxic cell sensitizers in small animals.

2.3 Screening for Radioprotection in a Range of Normal Tissues Using Newly Identified or Developed Radioprotective Compounds

The study of radioprotective drugs, as they relate to radiotherapy, is more complicated than the study of radiosensitizers since it is possible that an individual drug might be inappropriate for one site but applicable to another. Therefore decisions must be reached at each step of the process and partial advancement in the decision network is possible. The reaching of these decisions should remain in the standard DCT decision network with the initial data analysis and recommendation being made by the Radiosensitizer/Radioprotector Working Group of the ROCS.

The precise mechanism for doing this will result from the development of the treatment linear array for radioprotectors. By analogy with the radiosensitizers, criteria for a radioprotector passing stage 2A could be that it is shown to be superior than the current standard (WR-2721) in at least three normal tissues in the mouse. To be better than WR-2721 means that the compounds produce greater protection at a given fraction of the LDso than does WR-2721 and/or be more active in more or different tissues. The normal tissues selected must be those for which a qualitative endpoint is available and at least one should be a late endpoint such as for the lung, spinal cord, kidney, leg fibrosis or brain. Good quantitative early endpoints exist for the gut, bone marrow, and skin.

Assuming the drugs are to be tested in vivo, the initial screen should be the one which is most susceptible to known radioprotectants. Three systems would appear most promising: bone marrow CFU survival or the gut clone method.

skin injury, The major ad-

vantages of each are: skin - few animals needed, quantitative data generated, and little skilled handling requiring; bone marrow - most responsive to radioprotectors; and gut clone - assay complete in three days. Decisions regarding the system of choice will be made after using all three in initial studies and after taking into con-

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sideration the number of drugs to be screened and the accuracy requirements of the decision making process (i.e., the number of drugs to be tested may be small enough to allow use of the most accurate, although difficult, test, or the protective activity may need to be known only in general terms before a further decision is made). Further, it would be necessary to determine at this stage whether the drug would protect standard solid tumors.

Assuming that an agent has qualified in the initial screen, it should then be considered relative to a series of representative normal tissues. These would include a more rigorous analysis in the screening system (well protected), in a slowly dividing tissue (kidney, lung, etc.), and in poorly protected tissues (e.g., spinal cord).

Following successful completion of these studies, the agent would be studied in additional normal tissues (dictated by the sites most amenable to sparing through the use of radioprotectants) and at lower drug doses. If the agents equalled or exceeded the activity of WR-2721 at a given site (superior protection at the same fraction of the toxic dose or equal protection at a lower fraction), toxicity and pharmacology studies would be conducted in species other than the mouse in anticipation of clinical application.

The following specific steps would be involved following identification of an agent:

0 Determine toxic LD50 of drug.

0 Determine protective effectiveness in screening system, at maximum tolerated dose (MTD).

0 Determine protection at 50x, 25x, and 10% of MTD.

0 Determine ability of MTD to protect solid tumors (growth delay and cure) and, if necessary, at lower doses as above.

?? Determine protective effectiveness against battery of normal tissues.

?? Determine protection at selected promising sites.

?? Toxicity and pharmacology data in other species.

The radioprotector testing has to be performed in both normal tissues and in tumors. Quantitative endpoints are now in use for studying radiation effects on the mouse brain, esophagus, lung, small intestine, kidneys, bone marrow, testes, leg fibrosis and contraction, and skin reactions. The rabbit can be used for studying radiation effects on the heart. The liver is a prominent exception on this list and a quantitative endpoint is needed in this tissue. However, even with this gap, sufficient normal tissue and tumor endpoints exist for adequate testing of radioprotectors in mice.

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In order to augment the in vivo testing being performed by synthesis contractors, and voluntarilyby CREG and other research supported groups 9 it is essential to support two contractors for the evaluation in vivo of radiosensitizers using at least three tumors with three en=ts. The same contractors could be required to perform in vivo testing of radioprotectors in a series of three or more Gr&rtissues as recommended by the radioprotector development linear array. Since similar radiobiologic expertise is required for sensitizer and protector evaluation, it is highly likely that the same contractor would be capable of performing both types of assay quite adequately.

Because of the current lack of a standardized small animal model for neurotoxicity measurements, it is recommended that a RFA be issued, with its goal the development of neurologic endpoints including both functional and histologic, which will predict neurotoxicity of hypoxic cell sensitizers in both large animal and humans. At this point the exact dose-limiting toxicity of sulfhydryl and thiophosphate radioprotectors is not known, although it may be general toxicity including changes in blood pressure. However, prolonged administration of such compounds has never been carried out and other types of delayed toxicity may be noted which could also be evaluated, with the goal being the development of a small animal model for that type of toxicity.

It is expected that ten compounds will flow through the small animal testing program in the radiosensitizer area and perhaps two to four compounds in the radioprotector area. This will result in approximately four compounds per year which will be recommended for large animal toxicology. It is recommended that the existing DCT Toxi- cology Program be used for the evaluation of these compounds. It should be ascertained whether this program has the capacity to add four new compounds for thorough testing in dogs with 1, 5, and at least 20 drug doses for both radiosensitizers and radioprotectors. The guidelines suggested by the Hypoxic Cell Sensitizer Toxicology Workshop in San Francisco in January 1978 should be used for the establishment of the evaluations in dogs which will be conducted prior to the onset of Phase I testing in humans.

After Phase I testing of WR-2721, it is highly likely that other toxicities will be identified and that a similar toxicology meeting will be indicated to develop the necessary guidelines for large animal testing of the radioprotective compounds. Continued NC1 staff support and funding for the organization and support of such conferences will be required.

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It is expected that the program will develop at least one and probably two new hypoxic cell sensitizers and one radioprotective compound per year which will require Phase I clinical testing. The number of compounds successfully passing through Phase I testing and showing evidence of superiority to the standard compound will of necessity be less than this. Since these are not cancer chemotherapeutic agents and will not have different spectra of anti-tumor activity, but are only desired as radiation modifiers, they must be better than the standard before introduction in clinical trials above the Phase I level.

Phase I studies in the radiotherapy setting are designed primarily to measure the toxicity of the drug and to determine the maximum tolerated dose using a schedule which can be coordinated with some form of fractionated radiotherapy. The goal of Phase I studies will be to determine whether the compound can be used with conventional radiotherapy, i.e., 5 times weekly, but all such Phase I studies should be designed, first, to determine the maximum tolerated dose on a single dose basis, then, on a once weekly basis, and finally, after several steps, on a 5 times weekly basis.

Phase II radiotherapy studies with drug modification are designed to determine the acceptability and efficacy of a combination of drug and a selected radiotherapy fractionation scheme. In these Phase II studies, 30-40 patients are used to determine that a proposed regimen is accept- able and apparently as effective as conventional radiotherapy. The successful Phase II studies will then be used as arms in Phase III evaluations in comparison to conventional treatment.

Phase III trials are designed like other conventional Phase III studies, in which a known successful treatment is compared to the new treatment combining drug and radiation. In the future it may be that one radiosensitizer is compared against another, although it is highly likely that any radiosensitizer at the Phase III level will have already been proven at least as efficacious and certainly less toxic than the standard.

Other goals of clinical trials are to determine the usefulness of radiosensitizers and radioprotectors with conventional radiotherapy with both large and small fractions and with hemibody irradiation. Studies will be designed to determine the cytotoxic effect of hypoxic cell sensitizers on the hypoxic zones of tumors in conjunction with daily fractionated radiotherapy. The Phase III studies will be designed to determine the overall impact of sensitizers in patient management as measured by local control and five year survival.

Studies will also be designed to explore enhancement of tumor cytotoxicity through hyperthermia and in combination with systemic chemotherapy. If laboratory studies support it, these hypoxic cell sensitizers may also be explored with low dose rate radiation, using either external beam or brachytherapy sources. A number of studies may explore combinations of radioprotectors and radiosensitizers in addition to combinations of either with cancer chemotherapeutic agents because of the lack of over- lapping toxicity.

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3.1 Phase I Studies

Phase I studies will be required for all compounds passing through the large animal testing and still demonstrating superiority over the standard compound in terms of efficacy and toxicity as determined by review by the Radiosensitizer/Radioprotector Working Group. If the judgement is made that the compound is superior, it will be presented to the DCT Decision Network for approval for filing an IND and initiation of a Phase I study.

It is recommended that the Phase I studies be carried out by members of cooperative groups who are active in radiotherapy clinical research. Although most new cancer chemotherapeutic agents are given Phase I testing by a limited number of contractors, it is felt that this route is not appropriate for radioprotectors and radiosensitizers because of the special expertise in radiotherapy required to administer these agents.

Members of cooperative groups with expertise in radiobiology, phar- mcology, and in the conduct of clinical trials should be selected to introduce new radiosensitizers and radioprotectors. Those with experience with prior compounds would be preferred, because direct comparison of toxicity would be possible by the same investigators using the same techniques.

Within the next year it is anticipated that Ro-05-9963 and WR-2721 will enter Phase I clinical trials. In the subsequent year it is likely that Ro-07-0741 will enter clinical trials at the Phase I level, as may thio-D-glucose or another thio sugar.

It is recommended that two to four institutions be supported for such Phase I testing, either through a separately announced Phase I contract for hypoxic cell sensitizers and radioprotectors or through supplements to existing cooperating group research grants.

3.2 Phase II Trials

If Phase I testing of new compounds has been carried out by members of cooperative groups, the expertise will already exist in these groups for the design and conduct of Phase II studies. Such expertise will include the ability to: monitor drug levels, accrue patients, and clinically evaluate the toxicity of these compounds. Phase II radiotherapy studies will then be designed to determine the acceptability of the compound in combination with radiation using a range of dose and fractionation schemes and a range of drug administration schedules. These schedules will be selected to maximize the potential for hypoxic cell sensitization, determine the acceptability of any new radiation fractionation schemes and confirm the acceptability of the established maximum tolerated dose from the Phase I studies. The Phase II studies should be designed in a way that they may become arms of Phase III studies as soon as completed successfully.

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Misonidazole is just completing Phase I evaluation by the RTOG and Phase II protocols are in the final stages of development or activation. They are models for future Phase II studies of radiosensitizers and radioprotectors as are listed below:

Gliomas Brain metastases Head and neck A Head and neck B Hemibody radiation for metastases Esophagus Lung Cervix Melanoma/sarcoma Liver metastases Intraoperative radiotherapy

3.3 Phase III Studies

It is highly recommended that Phase III studies be carried out within existing cooperating groups which have a strong radiotherapy expertise, strong radiotherapy committees and good radiotherapy quality control. The RTOG should receive prime consideration for Phase I and II studies because of its strong expertise in the whole area of radiotherapy and in the use of sensitizers, in particular. At the Phase III level it is antrcipated that other groups beside the RTOG will become involved if they have sufficient expertise.

Phase III studies should be designed to cover as wide a range of fractionation schedules as is possible for a specific site. For example, in lung cancer one group might look at once per week fractionation with a radiosensitizer given during every other radiation treatment. If these studies are done by three different cooperative groups and if care is taken to have similar control arms, it can be quickly learned whether any of the proposed regimens with radiosensitizer are more effective than conventional therapy and which regimen is best.


It is recommended that new compounds showing superiority to the standard agents be brought to clinical trial at the rate of approximately two per year including one radiosensitizer and one radioprotector or two radiosensitizers, as the drugs become available. It is highly recommended that these Phase I and II studies be done within an existing cooperative group that could then conduct Phase II and III studies, with prime consideration given to the RTOG. It is not felt that an additional Phase I contract is needed or that these drugs should be tested by the Phase I contractors studying cytotoxic chemotherapeutic agents. Additional support will be required to study -approximately eighty Phase I cases per year with two drugs (40 cases each). This should be supported through supplements to existing cooperative group grants or, if necessary, through a separate radiosensitizer/radioprotector Phase I contract.

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Phase II studies should be conducted in existing clinical cooperative groups. It is anticipated that one new drug per year will be found better than preceding drugs and require Phase II evaluation. If the pattern of the current RTOG studies holds, approximately 10 protocols with 40 patients each or 400 patients will be studied annually in Phase II evaluations of new radiosensitizers or radioprotectors. Although the majority of these evaluations will be within the RTOG, a few additional Phase II studies may be undertaken after review and recommendation for approval by the Radiosensi- tizer/Radioprotector Working Group and the Council of Radiation Oncology Committee Chairpersons.

It is estimated that one new radiosensitizer or radioprotector will become available every other year for Phase III evaluation, and that from 500-1000 patients/year will be entered on Phase III studies involving radioprotectors and radiosensitizers. This number may approach 2000 if the compounds are used by other cooperative groups in addition to the RTOG. Some supplemental funding may be required for certain groups entering radiosensitizer and radioprotector studies because of the requirement for the monitoring of blood levels at the time of irradiation and because of augmented case accession when these protocols become part of the total number of protocols available to a cooperative group. This is particularly true if that group has been primarily a chemotherapy group with a limited number of radiotherapy studies.

4.0 Summary of Specific Proposals: Tasks

4.1 Modality Studies

Task A: In-depth Review of Data Available on Existing Compounds Held by Drug Companies, the Division of Cancer Treatment, and the United States Army

Develop a panel of experts and an in-house DCT staff to conduct a detailed review of existing compounds for their potential as better radiosensitizers or radioprotectors.

Task B: Synthesis of New and Analogue Radiosensitizers and Radio- protectors

It is recommended that four contracts be funded for the development and preliminary testing of radiosensitizers and radioprotectors. Two such contracts exist and two new contracts would provide for the synthesis of both radiosensitizers and radioprotectors or one would be for radiosensitizers and the other for radioprotectors.

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Task C: Development of In Vitro Screening Techniques for Toxicity and for the Elects of Radiosensitizers and Radiopro- tectors

It is recommended that an RFA be issued to stimulate the development and perfection of in vitro techniques. --

4.2 Experimental Studies

Task D: Radiobiologic Testing of Radiosensitizers and Radiopro- tectors in Small Animals

Basic support for the radiobiologic testing of radiosensitizers and radioprotectors in small animals is required in addition to that available from the synthesis contractors and from grant supported investigators. It is recommended that two contractors be funded to test these compounds in vivo as required by the radiosensitizer and the -- radioprotector linear arrays.

Task E: Development of Small Animal Models of Neurologic and Other Toxicity Expected from Radiosensitizers or Radioprotectors

It is recommended that an RFA be issued for a limited number of projects for the in vivo evaluation of neurotoxicity of hypoxic cell se%iGrs and for the future evaluation of any specific toxicity identified or associ- ated with sulfhydryl-type protective compounds.


Task F: The Phase I Clinical Evaluation of Radiosensitizers and Radioprotectors Through Existing Cooperative Groups

It is estimated that two new drugs will be studied per year in approximately 80 patients. Current experience indicates that Phase I radiosensitizer/radioprotector studies are done at less cost than for conventional chemotherapeutic agents.

Task G: Phase II Clinical Studies

Phase II evaluation of drugs in existing cooperative groups with emphasis on the RTOG. It is estimated that one new drug will be evaluated per year in ten sites with 40 patients per site, for a total of 400 patients per year.

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Task H: Phase III Clinical Trials

It is estimated that Phase III trials will be conducted with a new drug introduced every two years. One thousand to two thousand patients will be accessioned annually. The cost will not be an incremental one, since these protocols will replace some protocols now being conducted and funded, but there will be some additional funding required.

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b. radiosensitizers/radioprotectors working group

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