Uses and Regulation of Veterinary Drugs - ACS Publications

Chapter 1 Uses and Regulation of Veterinary Drugs Introduction

Downloaded by NORTHERN ILLINOIS UNIV on August 18, 2016 | http://pubs.acs.org Publication Date: August 24, 1992 | doi: 10.1021/bk-1992-0503.ch001

Κ.N.Woodward Veterinary Medicines Directorate, Woodham Lane, New Haw, Addlestone, Surrey KT153NB,United Kingdom

There are a variety of veterinary drugs a v a i l a b l e and at the disposal of the veterinarian and the safety of these to the consumer must be assured i n most countries of the world before marketing authorizations can be granted. A large body of pharmacological, toxicological and residues data i s generated and assessed so that the t o x i c o l o g i c a l p r o f i l e of the drug can be established and a maximum residue limit elaborated. This, along with the residues depletion p r o f i l e , allows a withdrawal period to be defined so that the consumer is protected from exceeding the acceptable daily intake for the drug in question. Worker safety is also of paramount importance when assessing a drug and its formulations p r i o r to marketing authorisation. Veterinary medicines take many forms and a few of the more important groups are described in this article. The range of veterinary medicines now available r e f l e c t s both the diseases they are intended to combat and the range of species they are intended to treat. In addition, there are a number of drugs a v a i l a b l e for so-called zootechnical treatment (eg the use of s t e r o i d hormones i n synchronization of oestrus) as opposed to disease treatment or prophylaxis. These aspects w i l l be b r i e f l y discussed i n t h i s a r t i c l e , but a c e r t a i n amount of l i b e r t y has been taken with the t i t l e so that the word "use" i s interpreted i n i t s widest sense to include a description of what occurs before "use" i s allowed! Consequently, a large part of t h i s work w i l l deal with the general requirements of marketing authorization; the regulatory requirements that are applied before a veterinary medicine may be marketed. UB&U

o fVeterinary

Medicines

Veterinary medicines, l i k e t h e i r human counterparts come i n many forms (1). Some would be i n s t a n t l y recognizable as tablets, pills

0097-6156/92/0503-00002$06.00/0 © 1992 American Chemical Society Hutson et al.; Xenobiotics and Food-Producing Animals ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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and i n j e c t a b l e s b u t o t h e r s , because o f c l i n i c a l need, t h e s p e c i e s i n v o l v e d o r t h e needs o f h e r d o r f l o c k s c a l e treatment, differ m a r k e d l y from anything g i v e n t o humans. The major groups o f m e d i c i n e s however a r e r o u g h l y t h e same and t h e s e w i l l be briefly d e s c r i b e d below, w i t h t h e emphasis b e i n g on t h o s e used i n food producing animals. A n t i m i c r o b i a l and a n t i b i o t i c a g e n t s . A l a r g e range of s y n t h e t i c , s e m i s y n t h e t i c and n a t u r a l a n t i b i o t i c s a r e used f o r the treatment of infectious diseases i n food producing animals. In the mid-1950s p e n i c i l l i n was seen as a g e n e r a l tool of s a l v a t i o n i n b o t h v e t e r i n a r y and human m e d i c i n e , b u t now an a r r a y o f B - l a c t a m a n t i b i o t i c s i s a v a i l a b l e f o r a l a r g e range o f i n d i c a t i o n s . The o t h e r major c a t e g o r i e s a r e t h e t e t r a c y c l i n e s , t h e a m i n o g l y c o s i d e s , t h e m a c r o l i d e s and p o l y m i x i n s ( 2 - 5 ) . S u l f o n a m i d e s a r e t h e major c l a s s o f a n t i m i c r o b i a l a g e n t (as opposed t o a n t i b i o t i c s ) and t h e s e c a n be viewed a s d e r i v a t i v e s o f s u l f a n i l a m i d e , the archetypal sulfonamide. A r a n g e o f compounds has been s y n t h e s i s e d for fast, medium o r l o n g a c t i n g abilities (6-7). They a r e o f t e n f o r m u l a t e d w i t h f o l a t e a n t a g o n i s t s s u c h as t r i m e t h o p r i m t o o b t a i n what i s c l a i m e d t o be a s y n e r g i s t i c a c t i o n . S u l f a m e t h a z i n e ( s u l p h a d i m i d i n e ) i s p e r h a p s t h e most w i d e l y used sulfonamide i n v e t e r i n a r y medicine, especially i n p i g production where i t i s u s e d to prevent r e p i r a t o r y disease ( 6 ) . The u s e o f t h i s drug i n p i g production has g i v e n rise t o problems o f r e s i d u e s , p a r t i c u l a r l y i n t h e k i d n e y , i n b o t h Europe and t h e USA. The r e a s o n s for this remain u n c l e a r f o r although failure to observe withdrawal periods plays a part, other c o n t r i b u t i n g factors include i n g e s t i o n o f f a e c e s from t r e a t e d animals and contamination of untreated feed with medicated feed ( 8 ) . Ectoparasiticides. C a t t l e and sheep a r e p a r t i c u l a r l y vulnerable to ectoparasites. I n C e n t r a l and South A m e r i c a , t h e USA and Europe, c a t t l e are attacked by c a t t l e grubs. In the Northern Hemisphere t h e s e "grubs" a r e t h e l a r v a l s t a g e s o f t h e w a r b l e f l y (Hypoderma spp) w h i l e i n South America, l a r v a e o f t r o p i c a l w a r b l e s (Dermatobia h o m i n i s ) a r e t h e c u l p r i t s ( 6 ) . They a r e g e n e r a l l y treated with pour-on formulations which tend t o be viscous p r e p a r a t i o n s o f t e n c o n t a i n i n g organophosphorus compounds. Sheep scab i s a n o t i f i a b l e disease i n both t h e USA and t h e U n i t e d Kingdom, which r e s u l t s i n t h e l o s s o f t h e f l e e c e . Although i t i s n o t g e n e r a l l y r e g a r d e d as a l e t h a l d i s e a s e , i t p o s e s s e v e r e a n i m a l w e l f a r e and economic p r o b l e m s . I t i s almost u n i v e r s a l l y treated by dipping i n aqueous solutions containing organophosphorus insecticides or synthetic pyrethroids (5). Anthelmintics. T h e r e i s i n s u f f i c i e n t scope i n a p a p e r o f this kind to discuss t h e more interesting points of veterinary parasitology. S u f f i c e i t t o say t h a t food producing animals of a l l s p e c i e s a r e l i t e r a l l y plagued by a l a r g e r a n g e o f internal parasites resulting i n distressing diseases and substantial economic l o s s e s ( 6 , 7 ) . The i n t e r e s t e d r e a d e r i s r e f e r r e d t o t h e c h a p t e r s by Roberson i n t h e e x c e l l e n t work e d i t e d by Booth and

Hutson et al.; Xenobiotics and Food-Producing Animals ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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McDonald (31) which w i l l serve as useful introductions. A v a r i e t y of a n t i - p a r a s i t i c drugs have been developed and three examples, the benzimidazoles, levamisole and ivermectin w i l l be mentioned b r i e f l y here. Thiabendazole was the f i r s t benzimidazole to achieve wide use. It i s indicated f o r a range of infestations including those caused by Haemonchus, Trichostrongylus and Strongyloides species i n c a t t l e and sheep. Newer compounds include albendazole, oxfendazole, fenbendazole and mebendazole ( 9 , 1 0 ) . In addition pro-drugs, exemplified by febantel, which undergo c y c l i s a t i o n i n vivo to y i e l d benzimidazoles have now been developed. Most of these drugs have some degree of teratogenic p o t e n t i a l leading to concern about t h e i r residues, but a more p r a c t i c a l concern has arisen over e f f e c t s on the developing fetus i n treated animals. Albendazole, cambendazole and parbendazole are teratogenic i n sheep leading to s p e c i f i c contra-indications i n pregnant animals, while other benzimidazoles are inactive i n t h i s respect. Levamisole i s highly e f f e c t i v e against gastrointestinal nematodes and i s widely used i n c a t t l e , sheep and pigs i n addition to numerous other species. Levamisole i s the 1-isomer of dl-tetramisole. It appears to be the active isomer of the racemic mixture which i s i t s e l f marketed as an antinematodal agent. Concerns have been expressed by Joint FAO/WHO Expert Committee on Food Additives (JECFA) over i t s apparent a b i l i t y to induce agranulocytosis and neutropenia i n humans given the drug f o r therapeutic purposes. This concern led the Committee to set a temporary acceptable d a i l y intake (ADI) of 0 - 0 . 0 0 3 mg/kg body weight pending the r e s u l t s of further research on t h i s phenomenon and i t s relevance to the safety assessment of levamisole residues (11). Ivermectin i s a macrolide compound derived from abamectin, a metabolite produced by Streptomyces a v e r m i t i l i s . More p r e c i s e l y , i t i s a mixture of two compounds, 22,23-dihydroavermectin Β (80%) and 22,23-dihydroavermectin B (20%) (12). I t i s widely and successfully used f o r onchocerciasis treatment i n human medicine and has found widespread use as a nematocidal and cestodocdal agent i n veterinary medicine (12). Although i t i s claimed that i t i n h i b i t s m o t i l i t y of the parasite by acting on ^-aminobutyric acid (GABA) receptors and by blocking chloride ions, the f u l l mechanism i s as yet not f u l l y understood. 1 b

Antifungal agents. Several drugs are a v a i l a b l e as t o p i c a l antifungal agents including thiabendazole, ketoconazole and a number of a l i p h a t i c acids such as undecylenic acid (4,13). Perhaps the two best known systemic drugs are nystatin and g r i s e o f u l v i n . Nystatin i s remarkably low i n t o x i c i t y when given o r a l l y but i s much more toxic a f t e r parenteral administration. As i t i s poorly absorbed a f t e r o r a l administration i t i s useful f o r g a s t r o i n t e s t i n a l t r a c t i n f e c t i o n s . Griseofulvin, a f t e r absorption from the g a s t r o i n t e s t i n a l t r a c t i s deposited i n skin, h a i r and n a i l s and i s useful i n the treatment of dermatomycoses. I t i s , however, teratogenic at high doses, at least i n the cat, and i t s


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use i n p r e g n a n c y i s t h e r e f o r e species.

c o n t r a - i n d i c a t e d i n t h i s and

other


S t e r o i d Hormones. Anabolic hormones l i k e t e s t o s t e r o n e and i t s s y n t h e t i c analogues such a s t r e n b o l o n e have been w i d e l y u s e d i n beef p r o d u c t i o n f o r several years. The n o n - s t e r o i d a l anabolic agent z e r a n o l has a l s o been w i d e l y u s e d f o r t h i s p u r p o s e ( 1 4 ) . Growth p r o m o t i n g u s e s o f a l l s t e r o i d hormones were r e c e n t l y banned by the European Community, but various zootechnical (eg synchronization of estrus) and t h e r a p e u t i c (eg p r e v e n t i o n o f a b o r t i o n ) uses o f t h e endogenous hormones and t h e i r synthetic e s t e r s a r e p e r m i t t e d (15,16). Somatotropins. Somatotropins a r e n a t u r a l l y o c c u r i n g polypeptides found i n a l l s p e c i e s although those intended f o r use i n food production, l a r g e l y to increase milk y i e l d s , may be synthesized using recombinant technology (17-19). T h e r e h a s been much controversy over the use of these materials ranging from t h e q u e s t i o n o f economic need t o f o o d s a f e t y c o n c e r n s (20-22). The European Community's Committee f o rVeterinary Medicinal Products c o n s i d e r e d t h a t a t l e a s t one o f t h e s e p r o d u c t s d i d n o t p r e s e n t a r i s k t o human h e a l t h , and gave a p o s i t i v e o p i n i o n on t h e a g e n t (23). I t i s i m p r a c t i c a l i n an a r t i c l e o f t h i s t y p e t o l i s t a l l t h e arguments f o r and a g a i n s t t h e s e drugs but the c o n t r o v e r s i e s i n v o l v e d seem s e t t o rumble on f o r some t i m e t o come! Currently, the somatotropins a r e not a u t h o r i s e d as milk y i e l d enhancers i n t h e USA o r i n t h e U n i t e d Kingdom. F i s h farming. Although not a " t h e r a p e u t i c use", f i s h farming or a q u a c u l t u r e as i t i s now o f t e n c a l l e d , i s worthy o f m e n t i o n a s i t r e p r e s e n t s a new a r e a o f a n i m a l p r o d u c t i o n and i t p o s e s i t s own problems. One o f t h e major growth a r e a s i n f i s h f a r m i n g i s salmon culture. This i s p a r t i c u l a r l y s u i t e d t o areas of the world where l a r g e i n l a n d expanses o f b o t h s a l t and f r e s h water a r e a v a i l a b l e , i n r e l a t i v e l y s h e l t e r e d environments. These c o n d i t i o n s a l l o w t h e salmon farmer t o grow t h e f i s h i n t h e i r n a t u r a l m a r i n e and f r e s h water e n v i r o n m e n t s thus r e f l e c t i n g t h e i r n a t u r a l h a b i t a t s . Salmon farming i s associated with two major diseases. F u r u n c u l o s i s , a b a c t e r i a l d i s e a s e c a u s e d by Aeromonas s a l m o n i c i d a , and s e a - l i c e , an a r t h r o p o d and an e x t e r n a l p a r a s i t e w h i c h attacks t h e s u r f a c e o f t h e f i s h (24,25). Both d i s e a s e s a r e r a p i d l y fatal and c a u s e s e r i o u s economic l o s s e s . I n t h e U n i t e d Kingdom, t h e o n l y l i c e n s e d medicine used t o c o n t r o l s e a - l i c e i s a formulation c o n t a i n i n g t h e organophosphorus compound d i c h l o r v o s . B e f o r e this c o u l d be a u t h o r i s e d , an enormous amount o f e c o t o x i c o l o g y d a t a had t o be r e v i e w e d a l l o w i n g f o r a r i s k assessment of dangers t o the immediate a q u a t i c environment, i n c l u d i n g the hazards t o other arthropods. Furunculosis i s prevented with the use of v a r i o u s a n t i b i o t i c s i n c l u d i n g a m o x y c i l l i n and o x o l i n i c a c i d ( 2 6 ) . Again, e n v i r o n m e n t a l a s s e s s m e n t s were n e c e s s a r y b e f o r e t h e s e d r u g s could be a u t h o r i z e d . Although i t i s not the t o p i c under d i s c u s s i o n here, i t i s worth n o t i n g that there i s also considerable


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XENOBIOTICS AND FOOD-PRODUCING ANIMALS environmental concern over the "natural" e f f l u e n t s from hatcheries (27). Marketing Authorization It would be impracticable to t r y to describe the various schemes established by regulatory agencies throughout the world to authorize users of veterinary medicines. Instead, the general requirements w i l l be discussed. Usually, three main areas, pharmaceutical quality, e f f i c a c y and drug safety are examined and these must be s a t i s f a c t o r y before marketing authorization i s granted. The corner-stone of these three areas i s product safety. A drug which i s not of the correct q u a l i t y because i t contains toxic contaminants or because i t i s not s t e r i l e i s not safe. S i m i l a r l y , a drug which does not perform i n the way described i n the product l i t e r a t u r e i s also not safe. The term "safety" speaks for i t s e l f : the product i t s e l f must be "safe" f o r the intended animal, i t must not be hazardous to the person using i t , i t must not harm the environment and i t must not leave p o t e n t i a l l y harmful residues i n food intended for human consumption. In the public mind and i n the corporate regulatory mind, i t i s drug safety which i s of paramount importance when food producing animals are being considered. Food safety issues are currently the subject of intense debate i n many areas of the world and many see p e s t i c i d e and veterinary drug residues as posing potential threats to public health. The safety of a veterinary drug to humans depends on a number of factors, some i n t r i n s i c to the animal being treated, some to the properties of the drug i t s e l f , and some to the method of use - or abuse of the veterinary medicine. These can conveniently considered under two main headings: safety to the consumer safety to the operator


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Safety to the Consumer For any chemical agent to exert a toxic e f f e c t i n humans or i n animals there are two important considerations - the toxic properties of the substance and the dose received. For veterinary drug residues, an assessment of safety involves an investigation of the t o x i c i t y of the drug and a quantitative study of the residues present i n animal tissues. Over the l a s t two decades, a broadly accepted package of t o x i c i t y tests has emerged f o r assessing the t o x i c i t y of chemicals whatever t h e i r intended purpose. The tests are conducted i n laboratory animals, usually rats and mice, and t h e i r objective i s the i d e n t i f i c a t i o n of a dose l e v e l at which a toxic e f f e c t does not occur, the no-effect or no-observed e f f e c t l e v e l (NEL or NOEL) (28,29). From the r e s u l t s of these tests also help to e s t a b l i s h the t o x i c o l o g i c a l p r o f i l e of the chemical can also be established - i s i t a teratogen, a genotoxic carcinogen or an uncoupler of oxidative phosphorylation f o r instance? Different regulatory a u t h o r i t i e s have d i f f e r i n g requirements therefore i t i s impossible



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to compose a precise l i s t of studies required f o r drug approval. Sometimes the requirement to conduct one test depends on the r e s u l t s of another. However, most a u t h o r i t i e s demand more or less the following: a study of acute t o x i c i t y i n rodents a study of short-term t o x i c i t y - 28 or 90 days a battery^ of tests f o r genotoxicity an i n v e s t i g a t i o n of carcinogenic a c t i v i t y an i n v e s t i g a t i o n of teratogenic a c t i v i t y studies of reproductive performance. Current s c i e n t i f i c dogma claims that there i s no safe l e v e l for a genotoxic carcinogen, an agent which causes cancer by a d i r e c t e f f e c t on the genetic material of a c e l l . T h e o r e t i c a l l y a single molecule could give r i s e to a mutation r e s u l t i n g i n a cancer c e l l and then a cancer (30). Whatever the merits of t h i s argument, i t i s widely regarded as unacceptable to be faced with the p o s s i b i l i t y of residues of a genotoxic carcinogen i n food of animal (or any other) o r i g i n . Similar sentiments would apply f o r s i m i l a r reasons, to genotoxic materials with the a b i l i t y to a f f e c t the germ-line c e l l s . Of course, i f a genotoxic carcinogen i s metabolized i n the target animal to non-active residues, then an a l t e r n a t i v e r i s k assessment i s possible and the drug w i l l be viewed as more acceptable. This was i n fact part of the evaluation of the drug carbadox, a growth promoter f o r pigs, by JECFA. I t noted that the drug i t s e l f was both mutagenic and carcinogenic i n laboratory studies but i t s residues were i n a c t i v e and hence acceptable ( 1 1 ) . Assuming that there are no adverse manifestations such as genotoxic carcinogenicity, and assuming again that some t o x i c e f f e c t s have been noted, an NEL should be i d e n t i f i a b l e providing that a suitable range of doses has been chosen f o r the toxicology studies. Once an NEL has been i d e n t i f i e d an acceptable daily intake (ADI) can be calculated using a s u i t a b l e safety factor ( 2 , 3 1 - 3 3 ) . There i s considerable debate over the magnitude of t h i s safety factor but the one usually chosen i s 100. If the drug produces no t o x i c e f f e c t s i n laboratory species but some minor adverse reaction has been noted i n humans, f o r example during use as a human medicine, a smaller factor, usually 10, may be employed. I f the range of tests was l i m i t e d and the r e s u l t s of dubious s i g n i f i c a n c e , or i f the studies were poorly performed, a larger safety factor may be applied and a temporary ADI adopted. The ADI therefore as NEL/100 i s usually quoted i n terms of mg drug/kg body weight/day or mg/kg per day ( 3 2 - 3 6 ) . An ADI can also be calculated f o r a non-genotoxic carcinogen (one which operates v i a an epigenetic mechanism) providing that the mechanism of carcinogenicity i s known. A good example i s provided by the JECFA assessment of the sulfonamide drug sulfamethazine (sulphadimidine). This was shown to be a thyroid carcinogen i n rodents but was accepted to be a non-genotoxic compound. Moreover, i t was concluded that the mechanism of carcinogenicity involved perturbations of the thyroid-pituitary-hypothalamus axis, changes i n thyroid hormone l e v e l s and a r e s u l t i n g hyperplasia of the thyroid. Although the



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f u l l mechanism i s s t i l l p o o r l y u n d e r s t o o d , i t was a c c e p t e d t h a t an NEL c o u l d be b a s e d on t h e t h y r o i d e f f e c t s and an ADI calculated (37). S i m i l a r a p p r o a c h e s have been t a k e n w i t h s t e r o i d hormones where no-hormonal effect levels can be d e t e r m i n e d in suitable e x p e r i m e n t a l models (38,39). H a v i n g d e t e r m i n e d an ADI, i t i s e s s e n t i a l t h a t consumption of food of animal o r i g i n by humans w i l l not r e s u l t in this value b e i n g exceeded. T h i s upper l i m i t i s known as t h e maximum residue l i m i t or MRL (28,31). I t s e l a b o r a t i o n depends on a number of f a c t o r s i n c l u d i n g the l i k e l y degree o f consumption of the food commodity o r commodities i n q u e s t i o n and t h e g e n e r a l q u a n t i t i e s o r d i s p o s i t i o n o f r e s i d u e s i n each t i s s u e . JECFA has p r o p o s e d daily i n t a k e s o f f o o d commodities; 300g muscle, 100g l i v e r , 50g kidney, 50g f a t and 1.51 o f m i l k , which i t u s e s t o e l a b o r a t e t h e MRL and e n s u r e t h a t t h e ADI i s u n l i k e l y t o be exceeded (11,35). There is c u r r e n t l y much d e b a t e over whether these values represent a r e a l i s t i c f o o d i n t a k e f o r t h e commodities i n v o l v e d . For example, do t h e y t a k e i n t o a c c o u n t t h e s o - c a l l e d extreme consumer who might as an instance eat l a r g e d a i l y q u a n t i t i e s of l i v e r ? Do they r e p r e s e n t i n t e r n a t i o n a l f o o d consumption? Or, t o put t h e q u e s t i o n more p l a i n l y , does 300g muscle c o v e r t h e USA consumption o f beef and t h e t h i r d world consumption o f beef? The answer i s q u i t e evidently "no". Nevertheless JECFA, through the Codex A l i m e n t a r i u s system a t t e m p t s t o recommend MRL v a l u e s w h i c h w i l l be u n i v e r s a l l y a p p l i c a b l e and f o r these reasons, some might say l i m i t a t i o n s , what might be seen as average values for food consumption must be a d o p t e d i f a practical s o l u t i o n i s to be found. Having a r r i v e d at an MRL or MRLs for a commodity or commodities, i t i s n e x t e s s e n t i a l to ensure t h a t the t i s s u e s of animals t r e a t e d with v e t e r i n a r y medicines do n o t exceed these values. In p r a c t i c e t h i s i s n o t as s i m p l e as i t might a t first appear. Even f o r a s i n g l e a c t i v e i n g r e d i e n t , t h e r e q u i r e m e n t s of therapy (and marketing advantages) d i c t a t e that numerous f o r m u l a t i o n s a d m i n i s t e r e d by v a r i o u s r o u t e s must be d e v e l o p e d and made a v a i l a b l e f o r t h e v e t e r i n a r i a n and farmer. Consequently, r e s i d u e s d e p l e t i o n i n the l i v i n g a n i m a l w i l l n o t be c o n s t a n t but w i l l vary according to the f o r m u l a t i o n g i v e n and the r o u t e of administration. I t goes a l m o s t w i t h o u t s a y i n g t h a t the species too, or more c o r r e c t l y the metabolism i n that species, will d e t e r m i n e t h e r a t e o f r e s i d u e s d e p l e t i o n f o r any g i v e n f o r m u l a t i o n a d m i n i s t e r e d by a specific route. This means t h a t residues s t u d i e s a r e g e n e r a l l y r e q u i r e d by r e g u l a t o r y a u t h o r i t i e s f o r each species using each formulation and route of administration. Thus, r e s i d u e s d e p l e t i o n below t h e MRL f o r each situation is e n s u r e d (28,29). Drug r e s i d u e s studies usually involve treating the animal i n q u e s t i o n with the drug u s i n g the intended route of a d m i n i s t r a t i o n , the intended formulations, u s u a l l y at the highest recommended dose and the maximum d u r a t i o n of administration. Animals are then s e r i a l l y s l a u g h t e r e d so t h a t r e s i d u e s depletion can be s t u d i e d and t h e time t a k e n t o a c h i e v e l e v e l s below t h e MRL e s t a b l i s h e d (31).



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9

These studies should, i f conducted properly, show the residues depletion p r o f i l e of the formulation under study and w i l l reveal for example any re-emergence of residues because of entero-hepatic r e c i r c u l a t i o n . The time taken for the residues to be depleted to below the MRL for each of the tissues of interest i s then usually chosen as the withdrawal period or withholding period (or times) for that formulation. Usually the studies must be conducted i n each of the indicated species although simpler and cheaper bi©equivalence studies where pharmacokinetic p r o f i l e s are examined and compared, may be used to evaluate the withdrawal period i n other food-producing species. Withdrawal periods can be a cause f o r dispute between companies and regulatory authorities, often f o r competitive marketing reasons. If two s i m i l a r products are a v a i l a b l e f o r a p a r t i c u l a r therapeutic purpose, the v e t e r i n a r i a n or farmer w i l l usually choose the one with the shorter withdrawal periods so that i f necessary the animal can be sent to slaughter at the e a r l i e s t possible time a f t e r recovery. These considerations are extremely important for milk because i t cannot be kept u n t i l residues have depleted to below the MRL and milking cannot be postponed. Contaminated milk has to be discarded, thus a t t r a c t i n g financial penalties these being a l l the more important the longer the withdrawal period. Drugs with shorter withdrawal periods o f f e r an obvious advantage. Similar considerations can be applied to honey. When bees are treated for disease conditions, the drug accumulates i n the honey (40-42). Here, i t may slowly change to non-biologically active residues (43-45) but i t may p e r s i s t and the honey w i l l need to be discarded u n t i l treatment has f i n i s h e d . Treating f i s h poses d i f f e r e n t technical and therapeutic problems some of which w i l l be mentioned i n a l a t e r part of t h i s Chapter. From a residues point of view, a p a r t i c u l a r problem arises from the general p h y s i o l o g i c a l processes p e c u l i a r to poikilotherms (46). Their metabolic rates are p a r t l y governed by t h e i r body temperature which i s dependent upon the ambient temperature of the water i n which they l i v e ; the cooler the water, the longer residues depletion takes. For t h i s reason, residues depletion studies i n f i s h are usually conducted at several temperatures chosen to represent the range of temperatures to which they w i l l be exposed under farming conditions. Withdrawal periods are then quoted i n degree days these being a function of both time and temperature. S p e c i f i c problems are raised by bound residues. After an animal has been treated with a medicine, i t s residues are present i n plasma and tissues as parent drug and metabolite or metabolites. The residues may be present as free drug and/or metabolites or as covalently bound residues. This then raises the question of the degree of b i o a v a i l a b i l i t y of these bound residues and t h e i r b i o l o g i c a l a c t i v i t y (47,48). In most cases, this question has no simple answers. Of course a drug may be metabolized to carbon dioxide and hence bicarbonate ion or some other simple precursor of normal endogenous biochemicals. If these a r i s e from a r a d i o l a b e l l e d portion of the molecule, measurements of residues simply as incorporated radiolabel will



XENOBIOTICS AND


lead the investigator to suspect bound residues when i n fact there are only normal bodily constituents containing incorporated isotope. If bound residues are found, t h e i r impact on the ADI must be assessed; can they be ignored or are they of t o x i c o l o g i c a l significance? JECFA has addressed t h i s issue and has recommended a systematic approach to the problem (11,37). JECFA suggests use of a mild extraction procedure to determine those residues which are c l e a r l y b i o a v a i l a b l e . This i s followed by a more vigorous extraction using acids or enzymic techniques to assess whether p o t e n t i a l l y b i o l o g i c a l l y active compounds may be released i n vivo. These studies can be backed-up by relay methodologies (residues transfer studies) whereby tissues from treated animals are fed to laboratory species and the release of drug-related moitiés measured, e.g. i n plasma ( 4 7 , 4 9 , 5 0 ) . The Committee stressed the need to treat each drug on a case-by-case basis (rather than laying down s t r i c t protocols to deal with bound residues as a common issue). The complexity of the problem can e a s i l y be seen by reference to the work of Lu and colleagues i n the United States (51). They investigated bound residues a r i s i n g from use of ronidazole, a nitroimidazole drug structurally related to metronidazole. Among other discoveries, they found that ronidazole covalently bound to proteins i n the pig forming an adduct which they investigated a f t e r acid hydrolysis. Ronidazole i t s e l f i s mutagenic but Lu and h i s co-workers demonstrated that the bound residues were devoid of genotoxic p o t e n t i a l and so did not o f f e r a r i s k to the consumer. A s i m i l a r analysis has been made for residues of furazolidone (52). The current FDA Guidelines outline a series of short-term and i n v i t r o tests for the safety assessment of bound residues together with t h e i r chemical characterisation (53). A study of r e v e r s i b i l i t y of adduct formation i s also included and as with the JECFA recommendation, drugs are investigated i n an i n d i v i d u a l manner. It seems l i k e l y that the JECFA and FDA approaches, at least i n general terms, w i l l become widely adopted i n t h i s p a r t i c u l a r area of hazard and r i s k assessment. Microbiological Risk Assessment Microbiological r i s k i n t h i s context r e f e r s to the possible e f f e c t s of residues of antimicrobial drugs on the gut f l o r a i n humans exerting a s e l e c t i v e pressure favoring e i t h e r the growth of microorganisms with natural resistance to the drug i n question, or the growth of microorganisms with acquired resistance. This i s a very controversial area of r i s k assessment, for while the phenomenon of induction of resistance i s well known (54-58), there i s no evidence for the supposed e f f e c t s i n humans i n vivo as a r e s u l t of ingestion of food containing veterinary drug residues (59-61). While i t i s widely accepted that i t would be virtually impossible to detect a toxic e f f e c t i n humans due to drug residues because of the wide background of disease and the difficulties involved i n a t t r i b u t i n g any e f f e c t to residues, emerging drug resistance should be more evident. Many antimicrobial and



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11

a n t i b i o t i c s u b s t a n c e s a r e o f low t o x i c i t y and MRLs t h e r e f o r e a r e o f t e n q u i t e generous. However, a p p l y i n g t h e r e s u l t s o f t e s t i n g f o r a n t i m i c r o b i a l r e s i s t a n c e may r e s u l t i n v e r y low MRLs and long withdrawal periods. T h e r e a r e t h r e e b a s i c t y p e s o f s t u d y a v a i l a b l e (11,62,63): s t u d i e s i n human v o l u n t e e r s s t u d i e s i n germ-free (holoxenic) rodents i n v i t r o studies with b a c t e r i a l populations The f i r s t o f t h e s e i n v o l v e s t h e e x a m i n a t i o n o f t h e human fecal f l o r a b e f o r e and a f t e r t r e a t m e n t w i t h a n t i b i o t i c s . Colonisation of the g a s t r o i n t e s t i n a l tract, e.g. t h e oral cavity, by a d v e n t i t i o u s microorganisms i s also investigated. The u s e o f g e r m - f r e e r o d e n t s models t h e human s i t u a t i o n . These a n i m a l s a r e i n o c u l a t e d w i t h human g u t f l o r a and t h e e f f e c t s o f a n t i b i o t i c s and a n t i m i c r o b i a l s c a n t h e n be s t u d i e d . In v i t r o investigations examine t h e e f f e c t s o f v a r y i n g c o n c e n t r a t i o n s o f t h e d r u g o r d r u g s o f i n t e r e s t on c u l t u r e s o f i n d i c a t o r o r g a n i s m s . A l l these studies c a n be u s e d t o d e r i v e n o - e f f e c t l e v e l s f o r t o x i c i t y towards t h e b a c t e r i a employed. More s p e c i f i c a l l y , t h e minimum i n h i b i t o r y c o n c e n t r a t i o n (MIC) v a l u e s c a n be d e t e r m i n e d . An example o f t h i s t y p e o f assessment i s p r o v i d e d by t h e JECFA d e l i b e r a t i o n s on o x y t e t r a c y l i n e a t i t s m e e t i n g i n Rome i n 1990 (11). The Committee n o t e d that the t o x i c o l o g i c a l p o t e n t i a l of oxytetracyline was low b u t studies were a v a i l a b l e on i t s a n t i m i c r o b i a l e f f e c t s i n dogs and human v o l u n t e e r s and a n o - e f f e c t dose o f 2mg p e r day was i d e n t i f i e d from t h e v o l u n t e e r e x p e r i m e n t s . T h i s l e d t o an ADI o f 0-0.003mg/kg body w e i g h t using a safety f a c t o r o f 10. The t o x i c o l o g i c a l s t u d i e s would have g i v e n an ADI o f a r o u n d 0.18 mg/kg body weight u s i n g a s a f e t y f a c t o r o f 100. The i n d u c t i o n o f a n t i b i o t i c r e s i s t a n c e has been r e c o g n i s e d f o r many y e a r s , and t h e r e i s some e v i d e n c e t o s u g g e s t t h a t antibiotic r e s i s t a n c e may d e v e l o p i n pathogens i n a n i m a l s g i v e n antibiotics (58,60). These r e s i s t a n t pathogens may t h e n be t r a n s m i t t e d to humans. There i s however no e v i d e n c e a t t h e moment that a n t i m i c r o b i a l r e s i d u e s i n meat o r o t h e r a n i m a l p r o d u c t s may lead t o e f f e c t s on t h e human g u t f l o r a and more r e s e a r c h i s o b v i o u s l y r e q u i r e d i n t h i s a r e a b e f o r e major r e g u l a t o r y d e c i s i o n s a r e made. I n t h e meantime, many w i l l s e e t h e c a l c u l a t i o n o f ADIs b a s e d upon m i c r o b i o l o g i c a l data a s an i n t e r i m and added safety measure, w h i l s t others w i l l regard i t as dubious s c i e n c e (59). Operator

Safety

A l t h o u g h t h i s a s p e c t i s i r r e l e v a n t t o t h e assessment o f residues and t h e i r i m p l i c a t i o n s f o r consumer s a f e t y , no a c c o u n t o f t h e u s e o f v e t e r i n a r y d r u g s would be complete w i t h o u t some p a s s i n g mention which w i l l r e s t r i c t i t s e l f t o c o n s i d e r a t i o n s o f s a f e t y i n use of medicines rather than safety i n their manufacture. Many f o r m u l a t i o n s used i n v e t e r i n a r y medicine o f f e r very l i t t l e scope f o r s i g n i f i c a n t o c c u p a t i o n a l exposure. T a b l e t s and c a p s u l e s f o r example e n s u r e that occupational exposure i s minimal, i f not non-existent. However, o p e r a t o r e x p o s u r e does o c c u r and means need t o be t a k e n t o r e d u c e t h i s . Organophosphorus compounds a r e



12

XENOBIOTICS AND


used in veterinary medicines as ectoparasiticides in food-producing and companion animals; i n several countries, including the UK, these have replaced organochlorine compounds. Companion animal products are exemplified by slow release c o l l a r s used for the control of fleas and other parasites i n cats and dogs. Here, i t i s important to ensure that the product does not allow the rapid release of large quantities of the active ingredient which would otherwise pose a serious r i s k to the owners. Sheep-dips and warblicide formulations contain such ingredients as chlorfenvinphos, diazinon and propetamphos. Apart from the obvious toxic e f f e c t s of anti-cholinesterases (64) concern has been expressed over possible long-term effects following occupational exposure to organophosphorus compounds (65-71). In the United Kingdom, there i s a comprehensive adverse reactions reporting system which covers suspected adverse reactions i n both the animal patient and i n humans using the medicines (72). This has revealed a small series of suspected adverse reactions to sheep dips which some have attributed to the organophosphorus component. These suspected adverse reactions have included wheezing, coughs, i n f l u e n z a - l i k e e f f e c t s and headaches (72). At the present time i t i s unclear i f these are due to the active ingredients, to other excipients such as organic solvent or other, unknown factors. In the United Kingdom and other European Community Member States, a review of veterinary medicines i s currently taking place under European Community l e g i s l a t i o n . This requires that many products currently on the market are assessed for safety, q u a l i t y and e f f i c a c y as i f they were new marketing authorisation applications (73) and as part of t h i s review, the products containing organophosphorus compounds w i l l be rigorously s c r u t i n i z e d to determine whether new precautions or adjustments to the formulations can be made to reduce the frequency of adverse reactions. Another example of occupational problems associated with veterinary medicines i s that of s e l f - i n j e c t i o n of oil-based vaccines (72). This can give r i s e to vascular compression, ischaemia and tissue damage, p a r t i c u l a r l y i f i n j e c t i o n into the tendon sheath occurs (74). In the United Kingdom, t h i s has resulted i n advice being provided to hospital emergency departments so that prompt and appropriate s u r g i c a l treatment can be given (75). Many a n t i b i o t i c formulations are given i n feed and as many a n t i b i o t i c s have a l l e r g e n i c properties, problems can a r i s e i n t h e i r use. Regulatory a u t h o r i t i e s now insist that such formulations are rendered dust-free by admixture with i n e r t oils or by the production of granular formulations so that occupational exposure i s minimized (76). In general, sensible occupational precautions, can be taken so that occupational exposure to veterinary medicines i s reduced or excluded. The use of impervious gloves f o r spreading pour-on formulations, overalls for dealing with sheep-dips and face-masks or r e s p i r a t o r s for handling dusty products can a l l reduce human exposure. I t i s also essential for basic occupational hygiene advice to be given on the product or i n the product l i t e r a t u r e so


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WOODWARD


that informed users can take the necessary precautions extreme s i t u a t i o n s , u s e a s u i t a b l e a l t e r n a t i v e ( 7 6 ) .

13 or i n


Summary I t would n o t be e f f e c t i v e i n any way t o r e v i e w a l l t h e d i f f e r e n t t y p e s o f m e d i c i n e s a v a i l a b l e t o t h e v e t e r i n a r y surgeon, n o r i n d e e d the various dosage forms available and t h e methods of administration. The r e a d e r i s r e f e r r e d t o o t h e r works f o r t h a t information. Suffice i t t o say t h a t a variety of active i n g r e d i e n t s and i n c r e a s i n g l y s o p h i s t i c a t e d f o r m u l a t i o n s a r e now s u p p l i e d by p h a r m a c e u t i c a l companies. The onus i s on their t o x i c o l o g i s t s , p h a r m a c o l o g i s t s and r e s i d u e s e x p e r t s t o e n s u r e t h a t t h e i r l a b o u r s g u a r a n t e e s o f a r as i s r e a s o n a b l y p r a c t i c a b l e , that t h e s e p r o d u c t s a r e s a f e f o r t h e consumer and f o r t h o s e otherwise e x p o s e d t o them, i n c l u d i n g t h e v e t e r i n a r i a n and f a r m e r .

Literature cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11 12.

Blodinger, J. In Formulation of Veterinary Dosage Forms; Blodinger, J., Ed.; Marcel Dekker: New York, 1983; pp 135-173. Huber, W.G. In Veterinary Pharmacology and Therapeutics; Booth, N.H.; McDonald, L.E., Eds; Iowa State University Press:Ames, Iowa, 1988; pp 813-821. Huber, W.G. In Veterinary Pharmacology and Therapeutics, Booth, N.H.; McDonald LE. Eds.; Iowa State University Press:Ames, Iowa, 1988; 822-848. Brander, G.C. In Chemicals for Animal Health Control; Taylor and Francis : London, 1986 and references therein. Brander, G.C.; Pugh, D.M.; Bywater, R.J.; In Veterinary Applied Pharmacology and Therapeutics; Balliere Tindall: London, 1982; pp 356-433. Anon. In The Merck Veterinary Manual; 6th Ed.; Merck and Co. Inc.: Rahway, 1986. Bevill, R.F. In Veterinary Pharmacology and Therapeutics; Booth N.H.; McDonald L.E. Eds.; Iowa State University Press:Ames; Iowa, 1988; pp 785-795. McCaughey, W.J.; Elliott C.T.; Crooks S.R.H. Vet. Rec., 1990, 126, 351-354. Roberson, E.L. In Veterinary Pharmaology and Therapeutics; Booth, N.H.; McDonald, L.E. Eds.; Iowa State University Press:Ames; Iowa, 1988 pp 877-881. Roberson, E.L. In Veterinary Pharmacology and Therapeutics; Booth, N.H.; McDonald, L.E. Eds.; Iowa State University Press:Ames: Iowa; 1988 pp 928-949. Joint FAO/WHO Expert-Committee on Food Additives. Evaluation of Certain Veterinary Drug Residues in Food. Technical Report Series 799, WHO:Geneva; 1990. Fink, D.W.; Porras A.G. In Ivermectin and Abamectin; Campbell, W.C.; Ed.; Springer-Verlag, London, 1989; pp 113-130 and references therein.


14

XENOBIOTICS AND FOOD-PRODUCING ANIMALS 13. 14. 15. 16. 17.


18.

19.

20. 21. 22. 23. 24. 25. 26. 27. 28

29. 30. 31. 32.

33.

34. 35.

Huber, W.G. In Veterinary Pharmacology and Therapeutics; Booth, N.H.; McDonald, L.E. Eds.; Iowa State University Press:Ames, Iowa, 1988; pp 849-860. Lindsay, D.G. Food Chem. Toxicol, 1985, 23, 767-774. Editorial. Vet. Rec., 1986, 118, 522-524. Anon - Council Directive of 7 March 1988; 88/146/EEC. Offic J. Europ Commun., 1988, No L70, 16-18. Vernon R.G.; Flint D.J. In Biotechnology in Growth Regulaton; Heap, R.B.; Prosser, C.G.; Lamming, G.E., Eds.; Butterworths: London, pp 57-71 and references therein. Chilliard, Y. In Use of Somatropin in Livestock Production; Sejrsen, K., Vestergaard, M.; Neimann-Sorensen, Α., Eds; Elsevier Applied Science: London, pp 61-87 and references therein. Burrenich, C.; Vandeputte-van Messom, G.; Roets, E; Fabry, J.; Massart-Leen, A.-M. In Use of Somatotropin in Livestock Production Sejrsen, K.; Vestergaard, M.; Neimann-Sorensen, A. Eds.; Elsevier Applied Science: London, pp 377-280. Hardin P. The Milkweed, February 1990. Epstein S.S. Ecologist, 1989, 19, 191-195. Juskevich J.C.; Guyer C.G. Science, 1990, 249, 875-884 Anon. Animal Pharm 12th April 1991, 1. Roberts, R.J.; Shepherd, C.J. In Handbook of Trout and Salmon Diseases, Fishing News Books: Oxford; 1986, pp. 46-87. Stuart, N. In Practice, March 1988, 47-53. Liao, P.B. Water and Sewage Works, 1970, 117, 291-297. National Office of Animal Health (NOAH). In Compendium of Data Sheets for Veterinary Products 1990-91, Datapharm Publications Limited: London, 1990. Woodward, Κ. N. In Food Contaminants: Sources and Surveillance; Creaser, C.; Purchase, R., Eds.; Royal Society of Chemistry: London, 1991, pp 99-108, and references therein. Woodward, Κ. N. Biologist, 1991, 38, 105-108, and references therein. Somogyi, A. In Carcinogenic Risks. Strategies for Intervention, IARC Scientific Publications, No. 25, IARC: Lyon, 1979; pp 123-127. Booth, Ν. H. In Veterinary Pharmacology and Therapeutics; Booth, N.H.; McDonald, L. E. Eds.; Iowa State University Press: Ames; Iowa, 1988; pp 1149-1205. International Programme on Chemical Safety. In Principles for the Safety Assessment of Food Additives and Contaminants in Food, Environmental Health Criteria 70, WHO: Geneva, 1987, pp.75-86. International Programme on Chemical Safety. In Principles for the Toxicological Assessment of Pesticide Residues in Food, Environmental Health Criteria 104, WHO: Geneva, 1990, pp.76-83. Bigwood, E. J. CRC Crit.Rev. Toxicol., 1973, 2, 41-93. Perez, M. K. J. Toxicol. Environ. Health, 1977, 3, 837-857.


1. WOODWARD 36.

37. 38.


39. 40. 41. 42. 43. 44. 45. 46. 47. 48.

49. 50. 51. 52.

53.

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