Evaluation of drug candidates in a battery of short-term genetic toxicology assays: Overview

Mutation Research, 223 (1989) 105-109 Elsevier

105

MTR 01408

Evaluation of drug candidates in a battery of short-term genetic toxicology assays: Overview C.S. Aaron The Upjohn Company, Kalamazoo, MI 49001 (U.S.A.) (Received 2 January 1989) (Accepted 3 January 1989)

Keywords: Drug candidates, evaluation

Introductory comments Genetic toxicology tests have become an important factor in the safety assessment of new drugs and chemicals. This paper is a brief introduction for a series of 6 papers (Aaron et al., 1989a-f) giving genetic toxicology test results obtained with drug candidates at the Upjohn Company. The basic battery of 4 tests used in assessing the mutagenicity of drug candidates and intermediates at Upjohn includes the Salmonella/microsome (Ames) test, an in vitro unscheduled DNA synthesis (UDS) assay, a mammalian cell mutation assay and the micronucleus test. The choice of these tests was made in an attempt to evaluate a diversity of genetic and biological endpoints. The first two (Ames assay and the UDS assay) are carried out at an early stage of the development process; as the drug candidate is further developed the other assays are conducted. The ideal outcome of safety assessment, from a drug development perspective, is that genetic toxicology assays produce negative responses. Unfortunately, this is not always the case and therefore additional studies (Aaron et al., 1989g) may be carried out in order to develop mechanistic understanding and to aid in risk assessment. The assessment usually involves evaluation of the type

Correspondence: Dr. C.S. Aaron, The Upjohn Company, Kalamazoo, MI 49001 (U.S.A.).

of assay which is positive (gene mutations versus chromosomal aberrations), the intensity of the positive response, whether addition of metabolic activation increases or decreases the activity and whether in vivo responses confirm the results of in vitro experiments. However, thorough evaluation also involves assessment of probable dose levels (high dose versus low dose) and schedule in people (single dose versus chronic dosing), bioavailability (comparison of oral versus injected drug concentration), likely serum levels and drug metabolism. In short, the occurrence of a single positive response in a genetic toxicology assay requires additional evaluation before the decision to halt drug development is made. Most of the data included in the 6 accompanying manuscripts is negative. Publication of such negative findings is neither scientifically interesting nor exciting under most conditions. However, since much recent scientific publicity has been devoted to studies in which a few compounds of extremely diverse structure (Tennant et al., 1987) have been presented and discussed, publication of these negative results takes on added importance. The experience described by the Tennant et al. paper is not consistent with the wealth of knowledge available in the drug industry in several respects. First o f all, the frequency of positive responses in a routine genetic toxicology battery of randomly chosen biologically active materials such as drug candidates is not likely to be greater than 5%. Obviously, specific series of analogs of

0165-1218/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)

106 TABLE 1 SUMMARY OF RESULTS OF GENETIC TOXICOLOGY TESTING OF DRUG CANDIDATES Upjohn "U-number" code for the compounds

Aaron et al. (1989) paper in which data appears a

U-10,149A U-10,136 U-27,182

NEG NEG

U-41,123F U -42,126 U-42,842

NEG N EG NEG

U-46,785B U-47,931E U-52,047

NEG NEG

U-53,059 U-53,996H U-54,461

b

c

d

NEG NEG

NEG

e

NEG

NEG NEG NEG

POS NEG

NEG NEG NEG

NEG

NEG

NEG NEG NEG

NEG NEG NEG

NEG NEG

U-54,669F U-56,769 U-58,797E

NEG NEG NEG

NEG NEG

NEG NEG

POS NEG

U-60,257B U-61,431F U-62,066E

NEG NEG NEG

NEG NEG NEG

NEG NEG NEG

NEG NEG

U-63,196E U-63,366F U-63,557A

NEG NEG NEG

NEG NEG NEG

NEG NEG NEG

NEG NEG NEG

U-64,417 U-67,590A U-68,553B

NEG NEG

NEG NEG NEG

NEG NEG NEG

NEG NEG NEG

U-69,167 U-69,725 U-70,138

NEG NEG NEG

NEG NEG NEG

NEG NEG NEG

NEG NEG

U-71,038 U-72,107 U-72,791A

NEG NEG NEG

NEG NEG NEG

U-72,996 U-73,975 U-74,006F

NEG

NEG

NEG

NEG

U-78,586 Furoic acid

f

NEG NEG

POS

POS

NEG

NEG

NEG NEG NEG

NEG POS NEG

POS NEG

NEG NEG

compounds with demonstrated genotoxicity do not fit i n t o this e s t i m a t e . F u r t h e r m o r e , t h e f r e q u e n c y o f m u l t i p l e p o s i t i v e r e s p o n s e s in t h e b a t t e r y s h o w n in t h e a c c o m p a n y i n g p a p e r s is also low. O u r e x p e r i e n c e has b e e n t h a t f o l l o w - u p t e s t i n g w i t h m a t e r i als g i v i n g p o s i t i v e r e s p o n s e s is a l w a y s r e q u i r e d to

m a k e d e c i s i o n s a b o u t t h e likely m e c h a n i s m s a n d r e l e v a n c e o f a p o s i t i v e r e s p o n s e . T h e r e a s o n for p u b l i s h i n g this t y p e o f e s s e n t i a l l y n e g a t i v e d a t a is to call a t t e n t i o n to t h e f a c t t h a t m a n y c o m p o u n d s a r e t e s t e d for e a c h o n e t h a t is p o s i t i v e a n d therefore may represent a real human hazard.

107

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Fig. 1. Chemical structures of some drug candidates tested in a battery of genetic toxicology assays.

108

U-61431F

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Fig. 1 (continued).

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109 I n this b r i e f i n t r o d u c t i o n , I have p r o v i d e d a figure c o n t a i n i n g the c h e m i c a l structures of the tested substances (Fig. 1) a n d a s u m m a r y t a b l e of the results of the b a t t e r y ( T a b l e 1). A s will b e a p p a r e n t , results for s o m e c o m p o u n d s in some tests are n o t i n c l u d e d in this d a t a set. Several reasons for the o m i s s i o n include the fact that these results were o b t a i n e d over a p e r i o d of years. Particularly, in the case of the A m e s test d a t a the evolution of the p r o t o c o l s for the c o n d u c t of the studies resulted in significant differences in the choice of strains over time a n d some technical aspects of the assay. The results p r e s e n t e d in each m a n u s c r i p t were o b t a i n e d using as n e a r l y identical p r o t o c o l s as possible. In some cases, the results with p a r t i c u l a r c o m p o u n d s were n o t o b t a i n e d because drug d e v e l o p m e n t was s u s p e n d e d p r i o r to c a r r y i n g out the studies. This latter p o i n t is imp o r t a n t in u n d e r s t a n d i n g the n o n - a c a d e m i c aspects of genetic toxicology testing in the d r u g industry, n a m e l y these tests are s e l d o m the only results which can l e a d to t e r m i n a t i o n of d e v e l o p m e n t of a drug. F u r t h e r m o r e , a clean bill of h e a l t h in genetic toxicology alone does n o t spell success in a thera p e u t i c sense. F u t u r e p a p e r s f r o m this l a b o r a t o r y will p r o v i d e results of other assay with some of these m a t e r i a l s as well as m o r e c o m p l e t e b a t t e r y results with a variety of o t h e r c o m p o u n d s u n d e r test. This set of d a t a will a d d significantly to the available d a t a for inter-test s t r u c t u r e - a c t i v i t y analysis in the a r e a of genetic toxicology.

References Aaron, C.S., J. Mazurek, D.M. Zimmer and D.H. Swenson, (1989a) The Salmonella mutagenicity test: Evaluation of 29 drug candidates, Mutation Res., 223, 171-182. Aaron, C.S., P.R. Harbach and S.K. Wiser (1989b) The in vitro unscheduled DNA synthesis (UDS), Assay in rat primary hepatocytes: Evaluation of 2-furoic acid and 7 drug candidates, Mutation Res., 223, 163-169. Aaron, C.S., P.R. Harbach, K.L. Steinmetz, J.P. Bakke and J.C. Mirsalis (1989c) The in vitro unscheduled DNA synthesis (UDS), Assay in rat primary hepatocytes: Evaluation of 24 drug candidates, Mutation Res., 223, 141-151. Aaron, C.S., L. Stankowski and D. Zimmer (1989d) The CHO/HPRT Assay: Evaluation of 19 drug candidates, Mutation Res., 223, 153-161. Aaron, C.S., R. Sorg and D. Zimmer (1989e). The mouse bone marrow micronucleus test: Evaluation of 21 drug candidates, Mutation Res., 223, 129-140. Aaron, C.S. and L.F. Stankowski Jr. (1989f) Comparison of the AS52/XPRT and the CHO/HPRT assays: evaluation of six drug candidates, Mutation Res., 223, 121-128. Aaron, C.S., L.F. Stankowski Jr., P.R. Harbach, R. Valencia, J.K. Mayo, J. Mirsalis, J.H. Mazurek, K.L. Steinmetz, S.K. Wiser, D.M. Zimmer and R.J. Trzos (1989g) Comparative mutagenicity testing of a drug candidate, U-48,753E: Mechanism of induction of gene mutations in mammalian cells and quantitation of potential hazard, Mutation Res., 223, 111-120 Tennant, R.W., B.WH. Margolin and M.D. Shelby (1987) Prediction of chemical carcinogenicity in rodents from in vitro genetic toxicity assays, Science, 236, 933-941.