Predicting anticancer drug effects in man from laboratory animal studies

J. chron. Dis. Vol. 15, pp. 223-228. Pergamon Press Ltd. Printed in Great Britain

PREDICTING ANTICANCER DRUG EFFECTS IN MAN FROM LABORATORY ANIMAL STUDIES ALBERT H. OWENS, JR., M.D. Department

of Medicine, Johns Hopkins University,

School of Medicine, Baltimore,

Maryland

(Received 11 Junuary 1962)

CURRENTLY, a potentially useful cancer chemotherapeutic agent is initially identified by its performance in model rodent tumor systems. Usually, prior to clinical trials, additional studies are completed to characterize the agent’s antitumor action in a spectrum of animal systems and to delineate the relative therapeutic effectiveness of various dose schedules and routes of administration against susceptible tumors in rats and mice. When one contemplates the clinical trial of a new agent, its possible antitumor effect is only one source of concern. No drug has a single action. This is particularly true of cancer chemotherapeutic agents as we know them today. The problem of demonstrating each significant biologic effect of a given drug is a formidable one. However, it is abundantly clear that the risk of a human trial is reduced in proportion to the ability of preclinical pharmacologic studies to forecast clinical events and forewarn investigators of possible toxicity. In general, it has been assumed that such predictive value exists. Interestingly, retrospective evaluations of such assumptions have rarely been published. BARNES and DENZ [l] after an extensive review of chronic toxicity testing concluded that extrapolation to the human situation was a matter of guesswork. LITCHFIELD [2] in a retrospective study of six drugs (among which were an antibacterial, a central nervous system depressant, a glucocorticoid and an antialcoholic which had been extensively examined in the rat, dog and man), concluded that significant correlations between species existed and that observations in dogs were more closely related to those in man than were findings in rats. I shall review this situation as it pertains to cancer chemotherapeutic agents. The scope of my presentation has necessarily been limited to a relatively few compounds, twenty-one in number. However, I have tried to include alkylating agents, antimetabolites and drugs with an as yet ill-defined mechanism of action. In general, these drugs are of recent clinical interest and several have been developed in relation to the Cancer Chemotherapy National Service Center (CCNSC) program. Perhaps one might begin by reviewing the scheme for the preclinical toxicologic evaluation of a compound which has been suggested by the CCNSC [3]. One must realize, of course, that these requirements are a minimum-a point of departureand not the specifications of a finished product. These investigations are divided into three parts. First, various dosage regimens and routes of drug administration are studied in order to establish “optical conditions” 223

ALBERT H. OWENS

224

for treating susceptible tumors. These dosage regimens and routes of administration are further studied in the subsequent toxicologic evaluation. Second, single-dose toxicity studies are performed in mice, rats and dogs. Drugs are administered to the rodents orally and parenterally. The characteristics of the LDso are determined as well as the time and mode of dying and the nature of apparently reversible pharmacologic effects. Observations in dogs are designed primarily to demonstrate the acute pharmacologic effects of the agent with particular reference to the cardiovascular, respiratory, and central nervous systems. Third, repeated-dose toxicity studies are completed in rats and dogs for the purpose of delineating the nature of the biologic effects produced by the administration of drug at near-lethal dose levels over a fourweek period. Next, one might consider serially the several organ systems most frequently affected by antitumor agents and attempt some simple interspecies correlations of these observed effects. Table 1 presents a summary of bone-marrow toxicity as it has been encountered in mice, rats, dog, monkey, and man. With respect to the alkylating agents, methotrexate, 6-mercaptopurine, 4-aminopyrazolo (3, 4-D) pyrimidine and vincaleukoblastine, laboratory studies predicted very well the occurrence of marrow depression in man. With carzolamide, actinomycin P2, mithramycin and roseolic acid, clinical investigators frequently encountered marked thrombocytopenia, seemingly more severe in degree than the observed leukopenia. This situation was not forecast accurately by the available preclinical data. TABLE1. Mechlorethamine Cyclophosphamide Myleran I-Sarcolysin 2-Chloroethylmethanesulfonate Methotrexate 6Mercaptopurine 4-Aminopyrazolo (3, 4n)pyrimidine VincaleukobIastine Carzolamide Actinomycin P2 Mithramycin Roseolic acid *predominantly

BONE-MARROW TOXICITY Rodent + + + +

Dog -F t + T

Monkey t L

+ I

+

-

+ i -

+I t

Human : + + ? + i ::

thrombocytopenia

Table 2 summarizes data concerned with gastrointestinal effects. They indicate a high degree of interspecies correlation between rodent, dog, monkey and man. Recorded observations of hepatic and renal damage in the several species are not as numerous as those detailing bone-marrow and gut toxicity. The available data are summarized in Tables 3 and 4 and indicate a rather good correlation between species. With respect to carzolamide, mithramycin, and roseolic acid, the dog seemed to predict hepatic damage in humans more accurately than did the rodent studies performed. One report of apparent renal toxicity in man induced by 2-chloroethylmethanesulfonate has been brought to our attention [4]. No studies of drug-induced hepatic or renal effects in monkeys were available for this analysis.

Predicting Anticancer Drug Effects in Man from Laboratory TABLE2.

225

Animal Studies

GASTRO-INTESTINAL TOXICKY Rodent + + + +

Me&lore&amine Cyclophosphamide Myleran 1-Sarcolysin 2Chloroethylmethanesulfonate Methotrexate 6-Mercaptopurine 4-Aminopyrazolot3, 4-o)pyrimidine Carzolamide 6-Azauracil Vincaleukoblastine Actinomycin P2 Mithramycin

Monkey + + + + + +

Dog + + + + -I-

+ + + -

I -t

+ +

Human + + + + I + + + +

_L

7 +

t + -

t

-

TABLE 3. HEPATIC TOXICITY

Rodent T + 1. i: Z!Z -

2-Chloroethylmethanesulfonate 6-Mercaptopurine 4Aminopyrazolo(3, 4-o)pyrimidine Carzolamide Mithramycin Roseolic acid TABLET.

2-Chloroethylmethanesulfonate Aminoiminomethanesulfinic acid Puromycin nucleoside

RENAL

i

Human + + + T + +

Dog -I+ 4

Human (+) + +

Dois 4 + + +

TOXICITY

Rodent + + +

TABLE 5. NERVOUS-SYSTEM TOXICRY

Mechlorethamine Chloroquine mustard Carzolamide 6-Azauracil Vincaleukoblastine Nitrofurazone NSC 38280

Rodent + 5 -

Dog + + -

Monkey + + -

Human + + + + ;: +t

*Peripheral neuropathy. tExtraocular palsies.

Table 5 summarizes the reported studies of nervous system toxicity. In general, central nervous system toxicity in man from the alkylating agents was predicted by preclinical studies. On the other hand, the occurrence of central nervous system dysfunction induced in humans by carzolamide and 6-azauracil was not anticipated seen in humans from prior observations in lower animals. The peripheral neuropathy following the administration of vincaleukoblastine or nitrofurazone does not seem to have its counterpart in laboratory animals. The extraocular palsies seen in humans treated with NSC 38280 (tereththalanilide; 4’, 4”-di-Zimidazolin,2-yl2-chlor, _diHCl) were totally unexpected [S].

ALBERT H. OWENS

226

Table 6 contains data relative to effects noted on the skin and its appendages and refers in particular to dose-related skin rashes, dermatitis and alopecia. These drug effects in humans seem totally unpredictable by existing laboratory techniques. Mention might be made with profit of an additional biologic effect of roseolic acid. It was observed to produce fever in a majority of the patients in which it was first studied. This would have been overlooked in the usual preclinical pharmacologic studies mentioned above. It was noted, however, during the routine tests performed to detect the possible presence of pyrogenic materials in drug-containing ampoules prepared for human trials. It is thought that this pyrogenic capability is possibly related to the polysaccharide nature of this compound. Methylglyoxal-bis-guanylhydrazone has been mentioned previously in relation to the dermatitis it has produced. In addition, it has been responsible for the induction of hypoglycemia in man. MIHICH has studied this phenomenon in man and animals [6] and demonstrated the hepatic toxicity which is responsible for its development. TABLE

6.

SKIN

AND

APPENDAGES

TOXICITY

(Dermatitis and alopecia) Rodent -

Cyclophosphamide Methotrexate Vincaleukoblastine 84zaguanine Actinomycin P2 Methylglyoxal-bis-guanylhydrazone

Dog -

CONCLUDING

Monkey -

Human + + + + + +

COMMENTS

Having thus disposed of several pounds of pharmacologic reports, one might be However, I would like to stress the fact that tempted to draw a set of conclusions. only a small number of compounds has been reviewed and only the more commonly encountered evidences of drug action have been tabulated. Further, one must remember that in laboratory animal studies, drugs are purposely given in intolerable doses in order that the spectrum of their biologic effects may be elicited. In man, on the other hand, antitumor drugs are administered cautiously so that the fewest Thus, data on which to base a number of undesirable biologic actions are manifested. complete comparison of the several biologic effects produced by a drug in lower animals with their possible counterparts in man, are necessarily incomplete. Instead of conclusions, perhaps I might be permitted a few cautious comments. First, with regard to the most commonly encountered toxic manifestations produced by the currently available cancer chemotherapeutic agents, it would seem that the presently performed preclinical animal studies provide adequate warning. There is a generally good correlation between laboratory species and man as far as drug-induced effects on the bone marrow, the gastrointestinal tract, the liver and kidney (Table 7) are concerned. Second, with regard to potential nervous system toxicity, the usual animal experiments leave much to be desired. In part, one might attribute some of the observed discrepancies to man’s ability to communicate his feelings and to manifest impairment of intellectual function more easily. On the other hand, there exists the probability

Predicting Anticancer

Drug Effects in Man from Laboratory

TABLE 7. PREDICTIVEVALUE-PRECLINICALTOXICITY 1. Good

Bone marrow,

2.

Questionable

Nervous system

3.

None

Skin and appendages

Animal Studies

227

STUDIES

G.I. Tract, liver, kidney

that animal study techniques can be improved in this regard. There has been much recent study of the effects of various drugs on the function of the nervous system, a portion of which may well be applicable to the preclinical evaluation of anticancer drugs. Third, with regard to the demonstration of unusual drug action, I can merely echo the sentiments of others far more experienced than I. New agents suggested for clinical trial in the future are likely to be of varying chemical types and productive of varying biologic effects. Preclinical investigators must alter their pharmacologic studies accordingly. Fourth, with regard to the animal species studied, one might comment that within the framework of this limited review, the monkey has demonstrated no drug effects that were not equally apparent in the dog. Fifth, although the data seemed insufficient to make extensive comparisons of the quantitative aspects of drug action in man and animals, one comment seems appropriate. It is a common practice to begin a human trial at one-tenth of the maximum tolerable dose in the most susceptible animal studied. In general, this seems a safe procedure. It has seemed “so safe” to some as to be regarded as a nuisance. With this in mind, two instances might be cited involving drugs considered in this survey. In the repeated dose toxicity studies of 6-azauracil performed in dogs and monkeys, a dose of 90 mg/kg daily was tolerated. In man, however, 4-5 mg/kg daily produced marked central nervous system toxicity in a few days. Fortunately, this was rapidly recognized and readily reversible [7, 81. Studies with dichloromethotrexate revealed that rodents were able to tolerate a dose at least fifty times greater than the maximum tolerable dose of methotrexate similarly administered. Dogs were able to tolerate doses ten to fifteen times greater. In this instance, again, man proved to be “the more susceptible species”, being able to tolerate the dichlorinated congener in doses only about three times those of methotrexate. Lastly, one might venture a comment on the large volume of histologic studies completed in conjunction with the usual preclinical evaluation of a new compound. In two instances in this review there was a suggestion from histologic study that an agent manifested some degree of selective toxicity. Mithramycin produced pronounced changes in the testis and aminoiminomethanesulfinic acid produced marked changes in the kidney. This calls to mind the fact that 2-o-chlorophenyl-2-p-chlorophenyl-I, I-dichloroethane was selected for clinical trial in adrenal cortical cancer because it had been shown to damage rather selectively the zona fasciculata and reticularis of the canine adrenal cortex [9]. We may be able to uncover promising agents by more imaginative use of such selective toxicity.

228

ALBERT

H. OWENS

Acknowledgments-In conclusion, I would like to express my gratitude for the assistance which Dr. WAALKESand the stalf of the CCNSC provided in obtaining the clinical and preclinical data from which the major portion of this review is derived. The investigators responsible for this original material are too numerous to cite individually. I am also indebted to Dr. E. K. MARSHALL,Jr. for his invaluable advice and criticism.

REFERENCES 1. BARNES,J. M. and DENZ, F. A.: Experimental methods used in determining chronic toxicity; critical review, Pharmacol. Rev. 6, 191, 1954. 2. LITCHFIELD,J. T. : Forecasting drug effects in man from studies in laboratory animals, J. Amer. med. Ass. 177, 34, 1961.

3. Specifications for preliminary toxicological evaluation of experimental cancer chemotherapeutic agents, Cancer Chemother. Rep. 1,89,1959. 4. KRANT, M. J.: Personal Communication. 5. FREI, E. T. : Personal Communication. 6. MWCH, E.: Toxic hypoglycemic effects caused in animals and humans by methylglyoxal-bisguanylhydrazone. Second National Cancer Chemotherapy Conference, Cancer Chemother. Rep. In press. 7. SHNLDER,B. I. et al. Clinical studies of 6-Azauracil, Cancer Res. 20,28, 1960. and neurological changes induced in man by the 8. WELLS, C. E. et al. Electroencephalographic administration of 1, 2,4-Triazine-3, 5 (2H, 4H)dione (6-Azauracil), EEG Clin. Neurophysiol. 9, 325, 1957.

9. NELSON, A. A. and W~~DARQ G.: Severe adrenal cortical atrophy (cytotoxic) and hepatic damage produced in dogs by feeding 2, 2-bis(parachlorophenyl)-1, I-dichloroethane (DDD or TDE), A.M.A. Arch. Pathol. 48,387, 1949.