EUROPEAN JOURNAL OF DRUG METABOLISM AND PHARMACOKINETICS, 1978, No 2, p.61-66

DRUG METABOLISM IN "EXOTIC" ANIMALS

John CALDWELL*, R, Tecwyn WILLIAMS **, Olumbe BASSIR *** and Michael R. FRENCH **** Department ofBiochemical and Experimental Pharmacology, St. Mary's Hospital Medical School, London W2 lPG, England

*** Department ofBiochemistry, University of Ibadan, Nigeria

Received for publication: December 12, 1977

Key words: drug metabolism, species differences, lion, civet, genet, hyena, fox, bats, elephant, reptiles

SUMMARY

Most studies of foreign compound metabolism have been performed in a restricted number of common laboratory animal species. Theexamination of animals not commonly found in the laboratory may aid in understanding the effects of environmental chemicals on wildanimals and in the search for better animal models for human drug metabolism. We have performed a number of studies in "exotic" animals,such as the lion, civet, genet, hyena, elephant, various bats and reptiles, and this review compares the findings with those in commonlaboratory animals, to explore the possibility that drug metabolism studies may aid in the zoological classification of animals.

There are a very large number of species of animals inthe world and these are divided into some 17 phyla, butthe study of the metabolism of xenobiotics has beenconfined mainly to two of these phyla, namely theArthropoda, which include insects and spiders, and theChordata in which fish, amphibia, reptiles, birds andmammals occur. Even amongst the mammals there aremore than 4,000 species, only a few of which have beenexamined from the point of view of drug metabolism.

The term "exotic" used in the title of this paper isdefined for present purposes simply to mean those ani­mals which are not normally used in the laboratory.Those which are used in the laboratory, frequently orless frequently, for drug metabolism studies includerodents such as rats, mice, guinea pigs or hamsters,domestic carnivores such as cats, dogs and ferrets, herbi­vores such as rabbits and sometimes horses and cattle,birds, usually the domestic hen and pigeons, and pri­mates such as the various species of monkeys. Discus­sions of interspecies variations in drug metabolism of themore common laboratory animals have been publishedfrom this laboratory (1,2,3,4). However, some studieshave also been made in this laboratory of the fate ofcertain compounds in animals which are hardly everexamined from this point of view. These include lionsand related felines, hyenas, elephants, bats and reptiles

** Emeritus Professor of Biochemistry in the University ofLondon.

Send reprint requests to: Dr. John CALDWELL, St. Mary'sHospital Medical School, London W2 lPG, England.

and it is this type of animal for which we wish to use theterm "exotic". The object of such studies was to com­pare these animals with related common laboratory ani­mals, e.g. how does the lion compare with the domesticcat, and to find out whether drug metabolism studiescan be of any value in the zoological classification ofanimals i.e. taxonomy. So far, however, these studieshave been few but it is hoped such investigations will becarried out more frequently in the future. Such studiesmay also be of value in understanding the effects ofchemical pollution of the environment on wild life. Ithas already been noted that animal species in the wildvary greatly in their sensitivity to tranquillizing drugs (5)and this could be related to species variations in themetabolism of these drugs.

When a xenobiotic .enters an animal, one of threethings could happen to it. In the first place it could bemetabolized by enzymes and this, in fact, is the fate ofthe large majority of foreign compounds, In the secondplace, the compound could undergo spontaneous trans­formations without the intervention of enzymes, as inthe case of thalidomide which is unstable in solution atphysiological pH values and therefore breaks down inthe body to some 12 products (6, 7). Then in the thirdplace, the compound may be metabolically stable anddoes not change at all in the body and is eliminatedunchanged as in the case of saccharin (8). In this paperwe shall be concerned only with compounds which aremetabolized by enzymes.

The basic pattern of drug metabolism is similar in allanimal species in that compounds are usually metabol-

62 European Journal ofDrug Metabolism and Pharmacokinetics, 1978, No 2

ized in two phases (9). In the first phase the compoundundergoes reactions which can be classified as oxida­tions, reductions and hydrolyses and which are enzymecatalysed. During this phase the compound usuallyacquires OH, COOH, NH2 or SH groups through whichit can undergo the second phase, the reactions of whichare syntheses, usually referred to as conjugations, whichare also enzyme catalysed. This concept can be illus­trated by the main metabolic reactions of benzene, forwhich the basic pattern is as follows (omitting details) :

O oxidation

Phase I •

OH

o OS03H

synthesis. APhase II V

zoic acid (16), phenylacetic acid (17) and o-naphthyla­cetic acid (18). Glucuronic acid conjugation, however, isnot entirely absent from the domestic cat, since hydra­tropic and diphenylacetic acids are extensively conju­gated with glucuronic acid in the cat (19, 20) (seebelow).

1. Acids not conjugated with glucuronic acid in the cat

Benzoic acid Phenylacetic acid 1- Naphthylacetic acid

Benzene Phenol Phenyl sulphate2. Acids conjugated with glucuronic acid in the cat

* The animals used were African lion cub (Panthera leo), lynx(Lynx caracal), African civet (Viverra civetta), African forestgenet (Genetta pardina) and spotted hyena (crocuta crocuta).

The dog, whilst it does not have the above defect inglucuronic acid conjugation, has a defect in N-acetyl­ation in that it appears to be unable to acetylate thearomatic amino group as found in, for example, sul­phanilamide and sulphadimethoxine. The defect alsoextends to hydrazines such as isonicotinic hydrazide, butprobably not to other types of amino groups. In fact,whilst N4-acetylsulphanilamide is not formed in the dog,Nl-acetylsulphanilamide is a known metabolite of sul­phanilamide in this animal.

Diphenylacetic acid

c.cICOOH

SulphadimethoxineSutphanllamide

oCH.COOHI

CH3Hydratropic acid

Some of the above compounds have now been exa­mined in the "exotic" species, lion *, civet *, genet *, andhyena *.These animals belong to the super-family Feloideaincluding the hyena, which the uninitiated might thinkwas more like a dog than a cat.

Phenol. The main reaction of phenol in the body isconjugation with sulphate and/or glucuronic acid and inthe domestic cat, the conjugation is almost entirely withsulphate.

Table 1 shows that the lion, lynx, civet, genet andhyena are similar to the cat in this respect. The othercarnivores, the dog and ferret, excrete glucuronide aswell as sulphate which is also true for the rat, a rodent.The domestic pig has been added to this table, since it

Not all compounds undergo the two phases since itcan be seen from the above pattern for benzene, phenolis mainly metabolized in phase II by synthesis to phenylsulphate (and to phenylglucuronide depending on doseand species). It should be added, however, that there is aminor pathway of metabolism of phenol involving aphase I oxidation to quinol which is then conjugated.

Within the above basic pattern which is common toall species, there are tremendous species variations andthese are largely due to species variations in the occur­rence and the activity of the enzymes catalysing the twophases of drug metabolism. These variations in the morecommon species of animals have been described(Williams (1); see Parke and Smith (10) where plants andbacteria are also discussed). This paper will be largelyconcerned with species variation in the conjugation reac­tions of a limited number of vertebrates. Comprehensivereviews of the metabolism of xenobiotics in fish (11)and invertebrates (12) have recently been published.

Conjugation Reactions in Carnivores

The order Carnivora contains two super-families ofanimals called the Canoidea and Feloidea (13). Thedomestic dog belongs to the former and the domestic catto the latter. Both the dog and the cat have characteris­tic defects in certain conjugation reactions and it was ofinterest to find out whether these defects also occurred inrelated "exotic" species such as the lion and the fox.Some of the results quoted were obtained from experi­ments on one or two animals only, and in some of theseonly a small amount of the administered dose was reco­vered. Although this could be due to slow excretion ofthe compounds, it is probably due to incomplete reco­very of excreta. Since there are difficulties in workingwith large, dangerous animals, the rather crude tech­niques employed are necessary under the circumstances(14).

The domestic cat has a defect in the glucuronic acidconjugation mechanism since it is hardly able to glucuro­nidate simple phenols such as phenol itself and 1- and2-naphthols (15) and simple aromatic acids such as ben-

J. Caldwell et al.• Drug Metabolism in "exotic" animals 63

appears to have the opposite defect to the cat that is inthe sulphate conjugation of phenol.

Table 1Conjugation of phenol in various species

Proportion (%) of excreted material as

Species Sulphate Glucuronide Reference

Cat 97 0 21Lion 97 0 22Lynx 37 0 23Civet 99 0 22Genet 97 0 22Hyena 90 0 14

Dog 82 18 21Ferret 58 40 21

Rat 54 42 21

Pig 6 94 21

OH OS03H oHA0 •o and/or

Phenol Phenyl sulphate Phenyl glucuronide

l-Naphthylacetic acid

This acid was of interest since in carnivores it appear­ed to be excreted partly as a conjugate with taurine,whilst in herbivores (rabbit, Indian fruit bat) and pri­mates, taurine conjugation was either not found oroccurred at a low level (18). Table 2 shows results withsome "exotic" felines. Taurine conjugation was found inthe civet, genet and hyena at a lower level than in thedomestic cat, but it was not detected in the lion. Thehyena, however, differs from the cat, lion, civet andgenet in that it shows a substantial glucuronic acidconjugation of l-naphthylacetic acid. The hyena there­fore is like the ferret in this respect, but like the cat asfar as glycine conjugation is concerned.

Table 2Conjugation of I-Naphthylacetic Acid

in various species

Proportion (%) of excreted material conjugated with

Species Glycine Taurine Glucuroni c Acid Reference

Cat 59 39 0 18Lion 94 0 0 22Civet 74 6 0 22Genet 70 18 0 22Hyena 46 11 40 14

Dog 7 65 26 24Ferret 5 49 20 25

Rat 23 tr 51 18

Rabbit 6 0 86 18

Sulphadimethoxine. This long acting sulphonamidedrug shows an interesting species difference in its meta­bolism, in that in man and eight species of sub-humanprimates its main metabolite is sulphadimethoxineNl-glucuronide (see below) whereas in nine species ofnon-primate laboratory animals this glucuronide waseither not formed at all or only to a minor extent (26).The other metabolite of this drug is the N4-acetyl deriva­tive (see below).

Table 3Conjugation of Sulphadimethoxine

in various species

Proportion (%) of excreted material as

Species N4.Acetyl N1.Glucuronidp Reference

Cat 18 0 26Lion 48 0 22Civet 66 0 22Genet 50 0 22Hyena 0 4 11+

Dog 0 19 26Ferret 27 0 26

Rat 47 8 26

Man and Sub- ca 25 >50 26Human primates

CH30

)=N 0~NHS02 NHCOCH3

CH30N4 - Acetylsulphadimethoxine

The extent of formation of these two metabolites isshown in Table 3. It is clear that the lion, civet, andgenet are like the domestic cat in that they acetylate thedrug. The hyena, however, does not acetylate the drugand in this case is like the dog. It may well be that thehyena has a defect in being unable to acetylate thearomatic amino group like the dog. The ferret and thedog, both carnivores, are also different and in this casethe ferret is like the cat and the lion.

64 European Journal of Drug Metabolism and Pharmacokinetics, 1978, No 2

The English fox is not an animal used in laboratoriesand can be regarded as "exotic" for present purposes. Itis related to the dog and could have the dog's defect inacetylation. This has been shown to be the case in thislaboratory with a fox (27). When given sulphanilamidethe fox does not excrete N4-acetylsulphanilamide, but itexcretes a small amount of Nl-acetylsulphanilamide(10% of dose) and is thus like the dog which excretes9% of the dose in this form. Most other animal speciesexcrete sulphanilamide mainly as the N4-acetyl deriva-

tive (8-56% of dose) together with small amounts ofNl-acetyl- (1-10%) and Nl-N4-diacetyl-sulphanilamide(1-7%) (Bridges and Williams, 1963).

Studies on the ElephantThe elephant is a large herbivorous animal which

probably needs an external source of ascorbic acid asdoes man, monkeys, the guinea pig and the Indian fruitbat. little is known, however, about foreign compoundmetabolism in the elephant. like other herbivorous ani-

Table 4The Conjugationof Benzoic Acid in Various Species

Proportion (%) of excreted material conjugated with

Animal Dose Glycine Glucuronic Ornithine Glutamic Ref.mg/kg Acid Acid

Lion 75 84 22Civet 75 81 22Genet 75 78 22

Elephant (African) 100 90 14

Turtle (side-necked) 50 16 2 72 16Gecko 19 6 6 75 16

Indian fruit bat 100 0 90 0 10 22,31African fruit bat 100 31 32 0 11 29Pipistrelle bat 2 75 0 0 0 30

Compare with

Cat 50 100 0 0 22Rabbit 50 100 0 0 22Hen 50 21 3 54 22Pigeon 50 84 1 0 22

Table 5Conjugation of Phenylacetic Acid in Various Species

Proportion (%) of excreted material conjugated with

Species Dose Glycine Ornithine Taurine Glutamine Glucuronic Ref.mg/kg Acid

Hyena 25 87Elephant (Afri can) 100 100Indian fruit bat 25 76Vampire bat 80 100African hammerheaded fruit bat 100 66Pipistrelle bat 1 80

Compare with

Man 1 tr 6 93Ferret 100 43 32 7Hen 100 tr 68 11Pigeon 100 55 0 42

1414

Ca 12 3217

3330

1717

o 17tr 17,34

J. Caldwell et al., Drug Metabolism in "exotic" animals 65

mals, e.g. cattle, horses, rabbits, it might be expectedthat the elephant would have efficient glycine and glucu­ronic acid conjugation mechanisms. It is known thatnormal elephant urine contains benzoylglycine (hippuricacid) and cyclohexylcarbonylglycine (hexahydrohip­puric acid) (28). We have found that both 14C-labelledbenzoic acid and phenylacetic acid fed at a level of 100mg/kg to a young female African elephant are excretedin the urine mainly as hippuric acid (90% of excreted14C) and phenaceturic acid (87%), respectively (seeTables 4 and 5). On administration of [l4C] phenol tothe same animal, the urinar-y metabolites were found tobe phenylsulphate (73% of excreted 14C), phenylglucu­ronide (25%) and quinol sulphate (1%) (14). Theselimited studies indicate nothing unusual in the meta­bolism of these substances by the African elephant.

Some Observations on Bats and Small Reptiles

Tables 4 and 5 contain metabolic data on the conju­gation of benzoic acid in the gecko and the side-neckedturtle, and in three species of bats. The gecko and theturtle convert benzoic acid mainly to ornithuric acid andare therefore like the domestic hen. It is to be noted thatthe domestic hen differs sharply from the pigeon in theconjugation of both benzoic acid and phenylacetic acid.

A very striking difference in the conjugation of theseacids is to be seen in the four species of bats which wehave studied, namely, the Indian fruit bat (Pteropusgiganteus), the African fruit bat (Epomops [ranqueti),the African hammer-headed fruit bat (Hypsignathusmonstrosus) and the British pipistrelle bat (Pipistrelluspipistre/lus). The Indian fruit bat is peculiar in that it isthe only species that we have examined which does notform hippuric acid (Table 4) and has thus a defect inglycine conjugation. But this defect does not extend tothe homologue of benzoic acid, phenylacetic acid, whichthe Indian fruit bat conjugates with glycine to a majorextent (Table 5), as do the vampire bat, the Africanhammer-headed fruit bat and the pipistrelle bat. Thisdefect in the glycine conjugation of benzoic acid, howev­er, does not occur in the African fruit bat or the pipis­trelle bat. In fact in the pipistrelle, hippuric acid is theonly metabolite of benzoic acid. In the Indian fruit batthe main metabolite of benzoic acid is benzoylglucu­ronide, (900/0) but there is also formed a minor meta­bolite (lOo/c) containing an amino acid, glutamic acid,which has not yet been recorded as a conjugating agentin other species. Benzoylglutamic acid is also a meta­bolite in the African fruit bat, but not in the pipistrellebat. The significance of these differences is not at allclear and further work on other species of bats is neces­sary.

The authors hope that more studies of drug meta­bolism in "exotic" animals will be made in the futureand that some of the large scientific zoos of the worldwill eventually have active departments of drug meta­bolism.

ACKNOWLEDGEMENTS

We are grateful to Mr. R.R. Golding, Director of theUniversity of Ibadan Zoo, Nigeria, for providing facilities at theZoo for studies on the elephant, lion, lynx, civet, genet andhyena. Studies in Ibadan were supported by the U.K. Inter­University Council for Higher Education Overseas.

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66 European Journal ofDrug Metabolism and Pharmacokinetics, 1978, No 2

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(23) Caldwell J. and French M.R. : Unpublished data.

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(33) Renwick A.G., Idle J.R. and Williams R.T. : Unpublisheddata.

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