OXYTETRACYCLINE 1. EXPLANATION Oxytetracycline (OTC) was evaluated at the twelfth meeting of the Joint FAO/WHO Expert Committee (Annex 1, Reference 17), at which a temporary ADI of 0-0.15 mg/kg b.w. was established. Since that time additional data have become available; they are summarized and discussed in the following monograph. The previously- published monograph has been expanded and is incorporated into this monograph. 2. BIOLOGICAL DATA 2.1 Biochemical aspects 2.1.1 Absorption, distribution and excretion 2.1.1.1 Mice After oral administration of 47.6 mg 14C-labelled OTC-HCl/kg b.w. to mice, 72% of the applied dose was found in the large intestine after 2 hours; only 5% was absorbed, of which the major portion (3.6%) was excreted in the urine. In the liver 1.9% and 1.1% of the dose applied was recovered after 1 and 2 hours, respectively (Snell et al., 1957). 2.1.1.2 Dogs Dogs received 10, 50 or 100 mg OTC/kg b.w. as a single oral dose or 2 oral doses 12 hours apart of 10 or 50 mg OTC/kg b.w. OTC concentrations in plasma were determined by a fluorometric method. A single administration resulted in peak blood levels 2 hours after dosing of 0.88, 1.01 and 2.51 µg/ml, respectively. These levels dropped to about 60% after 12 hours. Slightly higher blood levels were attained after administration of a second dose (Immelman, 1977). 2.1.1.3 Pigs Six and 4 pigs received 20 mg/kg b.w. i.m. of OTC as long-acting (OTC-LA) formulation or as conventional formulation (OTC-C), respectively. Blood and urine samples were taken and OTC concentrations were determined spectrofluorometrically. The levels of sensitivity were 0.1 µg/ml (plasma) and 0.2 µg/ml (urine). OTC-C was distributed slowly. The maximum plasma concentration (609 µg/ml) was obtained about 4 hours after dosing. About 60% of the administered dose was excreted in the urine during the first 24 hours and a total of 69% was recovered in the urine within 1 week. After injection with OTC-LA the initial absorption rate was faster and the maximum plasma concentration was reached within 1 hour after dosing. Although the excretion rate was lower with OTC-LA than after administration of OTC-C, the total amounts excreted in urine were comparable. In 3 days 60-75% of the total dose was excreted in the urine (Xia et al., 1983). Oral administration of 50 mg OTC-HCl/kg b.w. to 21 Yorkshire swine produced detectable OTC residues in the kidney (highest amounts), liver, lung, adrenal, heart, bile, fat, lymph node, spleen, thyroid and urine. The highest residue levels (441 µg/ml) were observed in the urine 3 hours after dosing and were still detectable at 48 hours. Mean peak plasma concentrations of 6.3 (range 4.2-8.7) were observed after 3 hours (Black & Gentry, 1984). Weaned piglets were given a single oral dose of 20 mg OTC/kg b.w. as a drench or were given a diet with 400 mg OTC/kg feed during 3 consecutive days. The drench route of administration revealed a maximum plasma concentration 6x higher than that of the feed route (1.27 versus 0.2 µg/ml). A peak plasma concentration was reached after 3 ± 2 hours by the drench route, while the feed route revealed a steady state concentration of 0.2 µg/ml beyond 30 hours after the onset of administration until administration stopped. Within 48 hours after cessation plasma OTC levels were below the detection limit (0.06 µg/ml). Estimated OTC bioavailabilities were low: 9.0% and 3.7% after the drench and the feed route, respectively (Mevius, et al., 1986a). After i.v. administration of 20 mg OTC/kg b.w., OTC was well distributed in the body (distribution volume 1.62 ± 0.83 l/kg). The elimination half-life ranged between 11.6 and 17.2 hours and the mean overall body clearance was estimated to be 0.249 l/kg/hour. Urinary recovery of OTC within 72 hours post injection ranged between 42 and 62% of the administered dose (Mevius, et al., 1986a). 2.1.1.4 Cattle Three groups of calves (3, 12, or 14 weeks old) received i.v. doses of 7.54, 6.88 or 17.00 mg OTC/kg b.w., respectively, and 2 groups of cows (lactating and non-lactating) received 3.32 and 7.94 mg OTC kg b.w., respectively. Blood samples were collected for determination of OTC concentrations by the agar-plate diffusion method (detection limit not reported). Distribution volume in 3-week old calves was 2.48 1/kg which was 2 to 3 times higher than in the cows. Half-life was 13.5 ± 3.6 hours and 8.8 ± 0.52 for the 3- and 12-week old calves, respectively. The dose and state of lactation did not affect the distribution volume or the half-life time in cows (Nouws et al., 1983). Dairy cows were injected i.v. and i.m. with 3 different 10% OTC- formulations (dose rates about 5 mg/kg b.w.). Serial blood and urine samples were collected. Distribution volume was 1.00 ± 0.18 1/kg and did not differ for the various formulations. Peak plasma concentrations of 2.28 ± 0.15 µg/ml were reached at 7 hours after i.m. administration. Plasma half life was 9.02 ± 0.88 hours. Most of the OTC was excreted by the kidney (85-86%) and a very small portion (2%) via the bile (Nouws et al., 1985). Five dairy cows were treated with single i.m. injections of 5 different 20% OTC formulations at a dose rate of 10 mg/kg b.w. OTC concentrations in plasma and the renal clearance of OTC and creatinine were determined (sensitivity of determination by microbiological assay: 0.05 mg/l). Maximum plasma concentrations were achieved 5 to 10 hours after treatment and varied between 4.6 and 6.8 µg/ml depending on the formulation involved. Plasma concentrations exceeding 0.5 µg/ml were maintained for 48 to 72 hours depending on the formulation involved. Mean renal clearance was 0.062 l/kg/hour. The urinary recovery of OTC within 72 hours after treatment ranged between 61.7 and 88% of the dose administered (Mevius et al., 1986b). Newborn calves (aged from 1 to 42 days) and older calves (250 days) were administered OTC at an i.v. dose rate of 10 mg/kg b.w. on day 2 and at weeks 1, 2, 4 and 6 of the study. Blood samples were collected for determination of OTC concentrations (detection limit not reported). The elimination of OTC was significantly slower in newborn calves. The half-life of elimination decreased from 11.2 ± 1.7 hours in newborn calves to 6.4 ± 1.3 hour at 6 weeks of age, to 6.3 ± 0.7 hours in the 250-day old calves (Burrows et al., 1987). Five Jersey cows received single i.m. doses of 5 mg OTC/kg b.w. Peak concentrations in plasma (1.67 ± 0.66 µg/ml) and milk (1.38 ± 0.46 µg/ml) were attained after 6 and 12 hours, respectively. The elimination half-life was 7.99 ± 2.20 hours (Prasad et al., 1987). 2.1.1.5 Humans OTC is incompletely absorbed from the gastrointestinal tract of humans. After oral administration about 60% of the ingested dose is absorbed (Fabre et al., 1971). After a single oral dose to humans, peak plasma concentrations are attained within 2 to 4 hours and within 2.5 hours after repeated dosing (Sande & Mandell, 1985). In humans given 7 daily oral doses of 500 mg OTC the volume of distribution appeared to be 4.07 1/kg (Green et al., 1979). Absorption of oxytetracycline is impaired by milk products, aluminum hydroxide gels, sodium bicarbonate, calcium and magnesium salts and iron preparations due to chelation and an increase in gastric pH (Sande & Mandell, 1985). 2.1.2 Effects on enzymes and other biochemical parameters 2.1.2.1 Rats In three trials, groups of 6 male Sprague-Dawley rats were treated daily for 14 days with i.p. injections of 0, 20, 40 and 100 mg OTC/kg b.w. in a sterile physiological saline solution. In the first trial, rats were killed after 2, 4, 6, 8, 10 and 14 days of treatment. In trials 2 and 3, rats were killed after 1, 2, 4, 6, 8, 10 and 14 days of treatment. At 100 mg/kg b.w. weight gain was significantly reduced and rats showed a decreased activity in microsomal O dealkylation and in epoxidation. Gross pathology revealed enlarged pale kidneys and at histopathology focal interstitial nephritis with mixed infiltration of neutrophils and lymphocytes was observed (Tarara et al., 1976). 2.1.3 Interactions with bones and teeth 2.1.3.1 Rats Fifteen Hebrew University Sabra rats (15 days old) received 6 consecutive injections of 100 mg OTC-HCl/kg b.w. every 12 hours during a period of 72 hours. Five rats served as controls. Rats were sacrificed 4 hours after the last injection and tibial bones were removed and epiphyseal plates were examined by either transmission or scanning microscopy to establish the influence of OTC-HCl on matrix vesicle production and initial calcification of epiphyseal cartilage. Degeneration of chondrocytes in the proliferating and hypertrophic zones was observed. Chondrocytes had short processes with only few matrix vesicles covering their surface. There were fewer matrix vesicles in the hypertrophic and calcifying cartilage as compared to controls and their ability to aggregate and form mineralized calcospherites was impaired. In ashed bones, minerals containing calcospherites were hardly seen (Levy et al., 1980). 2.2 Toxicological studies 2.2.1 Acute toxicity studies Table 1 summarizes the results of acute toxicity studies with OTC. 2.2.2 Short-term studies 2.2.2.1 Mice In a range finding study groups of B6C3F1 mice (10/sex/group) were fed diets containing 0, 3100, 6300, 12500, 25000 or 50000 ppm OTC-HCl for 13 weeks. These dose levels are equal to an intake of 392, 741, 1845, 3821 or 8300 mg/kg of body weight for males and 459, 845, 1850, 3860 or 7990 mg/kg body weight for females. No dose related effects were observed on mortality, food consumption, macroscopy and histology. Body weights were decreased from 3 to 15% at 25000 ppm and at 50000 ppm. OTC concentrations in bone were measurable fluorometrically in high-dosed females (NTP, 1987). Table 1: Acute toxicity of Oxytetracycline Species Sex Route Chemical LD50 Reference form (mg pure OTC/ kg b.w.) Mouse M&F oral pure >5200 P'An et al., 1950 M&F oral OTC-HCl 7200 P'An et al., 1950 M&F oral OTC-HCl 3600-4400 Bacharach et al., 1959 M&F i.p. OTC-HCl 285-420 Bacharach et al., 1959 M&F i.v. OTC-HCl 192 P'An et al., 1950 M&F oral OTC-HCl 154-189 Bacharach et al., 1959 M&F s.c. pure >3500 P'An et al., 1950 M&F s.c. OTC-HCl 892 P'An et al., 1950 M&F s.c. OTC-HCl 243-330 Bacharach et al., 1959 Rat M&F i.v. OTC-HCl 280 P'An et al., 1950 Rabbit M&F i.v. OTC-HCl 75-112 P'An et al., 1950 Dog M&F i.v. OTC-HCl 150 P'An et al., 1950 2.2.2.2 Rats In a range finding study, groups of F344/N rats (10/sex/group) were fed diets containing 0, 3100, 6300, 12500, 25000 or 50000 ppm OTC-HCl for 13 weeks. These dose levels were equal to intakes of 198, 394, 778, 1576 or 3352 mg/kg of body weight for males and 210, 431, 854, 1780 or 3494 mg/kg of body weight for females. No dose related effects were observed on mortality, food consumption, body weight or macroscopy. Minimal periacinar fatty metamorphosis in the liver of male rats was observed at all dose levels (no dose relation, control values not given). Measurable OTC concentrations in bones were detected in both sexes and increased with the dose. The OTC concentration in bone was significantly increased in females from 12500 ppm and up and in males at 50000 ppm only (NTP, 1987). 2.2.2.3 Dogs Groups of mongrel dogs (2/sex/group) were fed diets containing 0, 5000 or 10000 ppm OTC-HCl for 12 months. Observations included clinical signs, mortality, body weight, food consumption, haematology, organ weights, macroscopy and histopathology. No dose related effects were observed except for a degenerating germinal epithelium in the testicular tubules in high-dosed male dogs. The NOAEL in this study was 5000 ppm in the diet, equivalent to 125 mg/kg b.w. Groups of 8 male dogs, four beagle dogs and four mongrel dogs per group, were fed diets containing 0, 1000, 3000 or 10000 ppm OTC-HCl for 24 months. An interim sacrifice of 1 beagle and 1 mongrel dog/group was performed after 12 months. Observations included clinical signs, mortality, body weight, food consumption, haematology, alkaline phosphatase (ALP), bromosulphophthalein (BSP) clearance, urea nitrogen determinations, organ weight macroscopy, histopathology and semen examination. Two dogs died after 12 and 24 months, respectively (1 because of filaria and 1 because of gastroenteritis). No dose- related effects were observed. Atrophy of testes and epididymus occurred more frequently in control dogs than in treated ones. The NOAEL was 10000 ppm in the diet (the highest dose tested), equivalent to 250 mg/kg b.w. (Deichmann et al., 1964). 2.2.3 Long-term/carcinogenicity studies 2.2.3.1 Mice Groups of B6C3F1 mice (50/sex/group) were fed diets containing 0, 6300 or 12500 ppm OTC-HCl (purity 98.8%) for 103 weeks. Observations included clinical signs, mortality, body weight, food consumption, macroscopy and histopathology. Mean body weights of high dosed mice were 5-9% lower than those in the control group only after the first half year of the study. The tumour incidence was not significantly increased in either sex. The NOAEL in this study was 12500 ppm in the diet (the highest dose tested), equal to 1372 mg/kg b.w. (NTP, 1987). 2.2.3.2 Rats Groups of Osborne-Mendel male rats were fed diets containing 0 (180 rats), 100 (100 rats), 1000 (130 rats) or 3000 ppm (100 rats) OTC-HCl for 24 months. Observations included clinical signs, mortality, food consumption, body weight, haematology, macroscopy, and histopathology. After 24 months the mortality rates were 43, 23, 23 and 13% for the control and experimental groups, respectively. Treated rats gained weight more rapidly than controls. Body weight and haematology were not affected. At macroscopy pale kidneys were observed in 4, 7, 16 and 16% in the control and treated groups, respectively. A slight to moderate brownish pigmentation of the thyroid gland was seen in treated rats, but it was not dose-related. Tumour incidences were not enhanced. The NOAEL in this study was 3000 ppm (highest dose tested), equivalent to 150 mg/kg b.w. (Diechmann et al., 1964). Groups of F344/N rats (50/sex/group) were fed diets containing 0, 25000, or 50000 ppm OTC-HCl (purity 98.8%) for 103 weeks. Observations included clinical signs, mortality, body weight, food consumption, macroscopy and histopathology. Mean male body weights were 5-8% lower during the first year of the study at 50000 ppm. Histological examination showed a dose related increase in the incidence of benign phaeochromocytomas in the adrenal gland of male rats. In females an increase in the incidence of adenomas of the pituitary gland was found in the highest dose group (see Tables 2 and 3, respectively) (NTP, 1987). Table 2: Adrenal gland lesions in male rats Oxytetracycline in the diet 0 ppm 25000 ppm 50000 ppm Adrenal medullary 7/50 14/50 9/50 hyperplasia Phaeochromocytoma 10/50 18/50 25/50 (benign) Phaeochromocytoma 2/50 1/50 0/50 (malignant) Table 3: Pituitary gland lesions in female rats Oxytetracycline in the diet 0 ppm 25000 ppm 50000 ppm Hyperplasia 16/50 10/50 11/50 Adenoma 19/50 17/50 30/50 Adenocarcinoma 2/50 7/50 3/50 2.2.4 Reproduction studies 2.2.4.1 Rats Groups of 30 female and 10 male Wistar rats were fed diets containing 0 or 360 ppm OTC-HCl beginning at weaning (23 days of age). Animals were first mated at 120 days of age. A second mating was performed one month after the weaning of the first litter. One male and one female of these second litters were mated and effects on reproduction and lactation were determined in the second generation. Growth rate was not significantly affected. Reproductive parameters such as litter size, litter and pup weight, and the number and percent of live or dead fetuses did not show significant differences in the first and second generations. Dosed pups of both generations gained significantly more weight from days 3-21 post partum compared to controls. The NOAEL in this study was 360 ppm OTC-HCl, equivalent to 18 mg/kg b.w. (Uram, et al., 1954) 2.2.4.2 Cattle Nine beef bulls were treated with OTC-HCl by a single dose of 26.4 mg/kg b.w. administered subcutaneously followed by 6 doses of 17.6 mg/kg b.w. each (12 hours between the doses). Another 9 bulls were kept as controls. Semen was collected by electroejaculation twice daily beginning on day 3 and then every 4 days. None of the dosed bulls obtained palpable penile engorgement or protrusion during electroejaculation on day 3. However, there were no adverse effects on spermatogenesis, seminal pH, ejaculate volume, percentage of motile spermatozoa, rate of spermatozoal motility or concentration of spermatozoa in ejaculates harvested on days 3 or 7 (Abbitt et al., 1984). 2.2.5 Special studies on cardiovascular effects 2.2.5.1 Rabbits Six anaesthetized male rabbits were injected i.v. with 2 or 5 OTC mg/kg b.w. dissolved in 0.5 ml saline. The injections were performed in 3, 10, 20 or 60 seconds. One to 3 minutes after rapid injection, slowing of the heart rate was observed (normal between 270 and 310 per min.) to 100 per min. or less. This effect increased with higher doses and shorter injection time. No effect on arterial blood pressure was found. A depressive effect on respiration was seen which led, at high doses, to respiratory arrest of up to 60 seconds and otherwise to a slow, shallow respiration for 1-2 minutes (Gyrd-Hansen, 1980). 2.2.6 Special studies on embryotoxicity and/or teratogenicity 2.2.6.1 Mice Pregnant CD-1 mice (42/group) were orally dosed at 0, 1325, 1670 or 2100 mg OTC-HCl in corn oil/kg b.w. from days 6-15 of gestation. On day 17 all animals were sacrificed. Mortalities were 0/42, 1/42, 3/41 and 3/39, respectively. Gravid uterine weight and maternal absolute liver weight were significantly reduced at 2100 mg/kg b.w. There were no significant differences among treated and control groups with respect to maternal body weight, number of implantations, resorptions, dead and live fetuses, fetus weight and gross external, visceral and skeletal abnormalities. The NOAEL for maternal toxicity was 1670 mg/kg b.w. (Morrissey et al., 1986). 2.2.6.2 Rats Groups of pregnant Sprague-Dawley rats received a diet containing 0, 250, 1000 or 2000 ppm OTC and all rats were given intragastrically 1.0 ml of a solution containing 0.7 µCi calcium-45 and 20 µg calcium twice daily from days 1 to 20 of gestation. Fetuses were delivered by Caesarean section on day 21 and weighed. Maternal femurs and 3 individual fetuses of each litter were incinerated and analyzed for radiocalcium. No compound-related effects were observed on food consumption, body weight, number of fetuses/litter and mean fetal weight. All fetuses were viable at delivery and showed no external developmental anomalies. Maternal as well as fetal radiocalcium uptake in the bones increased in a dose-related manner with the OTC dose in the diet (Likins & Pakis, 1965). In a limited study 17 pregnant rats received daily i.m. injections of 41.5 mg OTC/kg b.w. from days 7 to 18 of gestation. No effects were observed on the number of implantations, the number of live and normal fetuses, the number and percentage of resorptions or fetal body weight; no macroscopic malformations were observed (Savini et al., 1968). Pregnant Wistar rats were orally dosed with 48, 240 or 480 mg OTC/kg b.w. from days 1 to 21 of gestation. On day 21 all rats were sacrificed. Fetuses were removed and skeletal anomalies were examined by staining their bones with alizarin red. Compared to the control group, ossification in the anterior extremities of fetuses was reduced and an increase in fetal resorptions was observed in all dose groups. The disturbances were more frequently observed in rats at the highest dose (Szumigowska-Szrajber & Jeske, 1970). Pregnant CD rats (36/group) were orally dosed with 0, 1200, 1350, or 1500 mg OTC-HCl in corn oil/kg b.w. by gavage from days 6-15 of gestation. On day 20 all animals were sacrificed. A dose-related increase in mortality rate was observed (0, 5.6, 15.2 and 24.2% for groups treated with 0, 1200, 1350 or 1500 mg/kg b.w., respectively). Clinical signs such as respiratory difficulties and rough coat occurred with increased incidence in treated dams. Maternal body weight gain and maternal absolute liver weight and fetal body weight were significantly reduced in all treated groups. No teratogenic effects were observed (Morrissey et al., 1986). 2.2.6.3 Rabbits In a limited study 12 pregnant rabbits received daily i.m. injections with 41.5 mg OTC/kg b.w. from days 10 to 28 of gestation. The number and percentage of partial and total resorptions were significantly increased compared to the controls (54% to 25%, respectively). No effects on fetal body weight were observed. No abnormalities were found at macroscopy (Savini et al., 1968). 2.2.6.4 Dogs In a limited study, 10 pregnant dogs of unspecified origin, received daily i.m. injections of 20.75 mg OTC/kg b.w. from days 18 to 48 of gestation. Laparotomy was performed on day 18 and hysterectomy was performed on day 58. Eight dogs served as controls (4 of these delivered spontaneously, the other 4 dogs were operated in the same way as the experimental animals). No resorptions or abnormalities were observed in 8/8 control dogs or in 1/10 treated dogs. The number of resorptions in treated dogs was high (9/10 dogs); from a total of 69 observed implantations 30 were resorbed. Malformations were found in 5/10 treated dogs; 12/39 treated pups showed skeletal malformations; displacia of the hind paws (5), angled tail (4), monstrum with general oedema and cranio-faciale anomaly (2), omphalocele (1) and 1 macerated fetus were observed. Visceral malformations included shortened digestive tract, enlarged stomach with thin walls and dilated and shortened intestinal tract (1 dog), polycystic kidneys (1 dog) and absence of pulmonary development (2 dogs) (Savini et al., 1968). 2.2.7 Special studies on genotoxicity Table 4 summarizes the results of genotoxicity studies that have been performed on OTC. 2.2.8 Special studies on the effect of combined treatment of OTC and nitrite Groups of 15 male and 15 female Sprague-Dawley rats received 0.1% OTC or 0.1% OTC + 0.1% sodium nitrite in the drinking water for 60 weeks. Surviving animals were killed at 130 weeks. No differences were observed in the mortality rates of the two groups. No liver tumours were observed in the OTC group. In the group receiving combined treatment, 3 hepatocellular tumours and 1 hepatic cholangioma were found (Taylor & Lijinsky, 1975). Table 4: Results of genotoxicity assays on oxytetracycline-HCl Test system Test Object Concentration Results Reference In vitro Ames test S. typhimurium 1 µg/pl in DMSO negative Andrews et. al., TA1535, TA1537, (+/- act) 1980 TA1538, TA98 and TA100 Ames test S. typhimurium 0-1.0 µg pl in DMSO negative1 NTP, 1987 TA100, TA1535, (+/- act) TA1537, and TA98 Mouse lymphoma forward L5178Y/TK+/- 12.5-800 µg/ml toxic negative NTP, 1987 mutation assay cells >400 µg/ml2 25-400 (- act) µg/ml toxic >200 positive µg/ml3 (+ act) Chromosome aberration Chinese hamster 80-200 µg/ml4 negative NTP, 1987 assay ovary cells (- act) 700-900 µg/ml5 negative (+ act) Sister chromatid Chinese hamster 60, 70 and 80 µg/ml4 negative NTP, 1987 exchange assay ovary cells (- act) 400, 500 and 700 negative µg/ml5 (+ act) In vivo Micronucleus assay Mouse 2 times 50, 250 and positive6 Blitek et. al., 500 mg/kg b.w., 1983 24 hours apart Host mediated assay Mouse S. typhimurium 100 mg/kg b.w. negative7 Blitek et. al., G46 1983 Table 4 (continued) 1 Both with and without rat and hamster liver S9 fraction. 2 Ethylmethanesulfonate was used as a positive control. 3 Methylcholanthrene was used as a positive control. 4 Mitomycin C was used as a positive control. 5 Cyclophosphamide was used as a positive control. 6 There was no dose relationship, increases in micronuclei were 5.5, 46.2 and 5.6 times, respectively. DMNA was used as a positive control. 7 DMNA was used as a positive control. The nitrosation product of OTC and sodium nitrite was tested for mutagenicity in the Ames Salmonella assay. Positive results were obtained in Salmonella typhimurium strains TA1537, TA1538, TA98 and A100 with and without metabolic activation (Andrews et al., 1980). OTC simultaneously administered with potassium nitrite (150 mg/kg b.w.) was positive in a micronucleus assay and in a host mediated assay with mouse/S. typhimurium G46 (Blitek et al., 1983). 2.2.9 Special studies on microbiological effects 2.2.9.1 Mice Three clones of E. coli K-12 strains were introduced into germ- free mice. The organisms were allowed to multiply and establish a stable population. OTC was then administered through the drinking water and it was shown that the susceptible strains remained dominant in number throughout. The minimal selecting dose of OTC in this mouse model was 8 to 12 µg/ml. These OTC-concentrations were higher than the MIC of the susceptible strain used (MIC=0.5 µg/ml) (Corpet & Lumeau, 1987). In vitro findings with the same clones and OTC showed a minimal selecting dose of OTC of 1/10 of the MIC, 0.05 µg/ml (Lebek & Egger, 1983). 2.2.9.2 Rats Mature albino rats (3 controls, 9 treated) were fed a diet containing 0 or 10 mg OTC/kg diet for 6 weeks. After that period the OTC concentration was raised to 50 mg/kg diet and administration was continued for 2 weeks. No evidence was found for the development of OTC-resistant organisms in the faeces of treated rats (Rollins et al., 1975). 2.2.9.3 Dogs Mature beagles (5/group) were fed a diet containing 0, 2 or 10 mg OTC/kg diet for 44 days. Faecal samples from individual animals were collected during the experimental period and examined for resistant coliforms by a comparative plate counting technique. A shift to drug resistant organisms was observed at 10 mg OTC/kg diet. No effect was found at 2 mg/kg diet, equivalent to 50 µg/kg b.w. (Rollins et al., 1975). 2.2.9.4 Turkeys Groups of Nicholas strain male poults received 0, 50, or 100 mg OTC/kg feed for 18 weeks. The turkeys were sacrificed after 8, 16 or 18 weeks and bacteria were isolated from blood and liver tissue. The isolates were tested for resistance against 8 antibiotics. The antibiotic resistance increased with increasing OTC levels (Swezey et al., 1981). 2.3 Observations in humans 2.3.1 Effects after medical treatment A variety of toxic and irritative effects in humans have been reported from the use of OTC. OTC may cause gastrointestinal irritation. Epigastric burning and distress, abdominal discomfort, nausea, vomiting, and diarrhea may occur. I.V. administration may produce thrombophlebitis. Long-term therapy may produce changes in the peripheral blood. Leukocytosis, atypical lymphocytes, toxic granulation of granulocytes and thrombocytopenia purpura may occur. A phytotoxic reaction may occur, sometimes accompanied by oncholysis and pigmentation of the nails. Liver injury and delayed blood coagulation may occur. Children under 7 years of age may develop a brown discoloration of the teeth. Infants of mothers treated with OTC during pregnancy may develop discoloration of the teeth. OTC is deposited in the skeleton in fetuses and children which can produce depression of bone growth (which is readily reversible when the period of exposure to OTC is short) (EPA, 1988). 2.3.2 Hypersensitivity In a 4 year old girl and a 6 year old boy sensitization was observed after treatment with OTC for otitis and a urinary tract infection, respectively (Walczynski & Stengel, 1968). The presence of eczematous contact allergy to OTC was established in 3 patients by patch testing, which was negative in all controls (Bojs & Moller, 1974). In a patch test 31 patients were used as negative controls and 10 patients were sensitized with 3% OTC. In 7/10 treated patients a strong reaction was observed and in 2/10 patients a weak reaction was observed; negative reactions were seen in 30 control patients and in 1/10 treated patients (Moller, 1976). 2.3.3 Special studies on microbiological effects The ecological impact of low doses of OTC on faecal microflora was studied in normal adult volunteers. Thirty subjects were controlled once a week during 4 consecutive weeks for the total population of faecal Enterobacteriaceae and for OTC-resistant Enterobacteriaceae. Fourteen other volunteers received OTC orally during 7 consecutive days: 2 received 2 x 1 g/day, 6 received 2 x 10 mg/day and another 6 volunteers received 2 x 1 mg/day. Faecal OTC concentrations, total count of anaerobes, and morphologic and physiologic characterization of the dominant strains of anaerobes were described. Determination of the MIC of OTC on these dominant anaerobes and the count of total and OTC resistant Enterobacteriaceae were measured before treatment, on day 7 of treatment and 7 days after the end of treatment. At 2 g OTC/day the dominant anaerobes and the OTC-susceptible Enterobacteriaceae were effectively eliminated while an overwhelming growth of OTC-resistant Enterobacteriaceae was observed and colonization by yeasts occurred. At 20 mg OTC/day the composition of the dominant anaerobic flora was not affected and no colonization by exogenous microorganisms was observed. However, if the OTC susceptible Enterobacteriaceae were not eliminated, the most OTC susceptible anaerobes disappeared, indicating that OTC at this dosage may have an ecological impact in the digestive tract. At 2 mg OTC/day the composition and the OTC susceptibility of faecal flora were not modified. The no-effect dose in this study was 2 mg OTC/day (Tancrede & Barakat, 1987). 3. COMMENTS The Committee considered pharmacokinetic data and results from short-term studies in rats, mice, and dogs, a multigeneration study in rats, teratogenicity studies in mice, rats, rabbits, and dogs, long- term/carcinogenicity studies in mice and rats, mutagenicity tests and studies on microbiological effects in laboratory animals and humans. Pharmacokinetic studies demonstrated that about 60% of ingested oxytetracycline was absorbed from the gastrointestinal tract in humans, compared to 4-9% in mice and swine. Following absorption by various routes of administration, oxytetracycline was widely distributed in the body, particularly in the liver, kidney, bones, and teeth. Systemically available oxytetracycline was primarily excreted in the urine, as parent drug. In the short-term toxicity studies, oxytetracycline was incorporated into the diets of mice and rats at levels up to the equivalent of 7500 and 2500 mg/kg b.w./day, respectively. Decreased body weights were observed in mice at 3750 and 7500 mg/kg b.w./day and a non-dose-related incidence of minimal periacinar fatty metamorphosis of the liver was observed in male rats at all dose levels. In a study in dogs that received oxytetracycline at 0, 5 or 10 g/kg in the diet degenerative change in the germinal epithelium of the testes were noted at 10 g/kg. However, these findings were not confirmed in a second study. The no-observed-effect level was equivalent to 250 mg/kg b.w./day. No effects on reproductive performance were observed in a two-generation study in rats in which the compound was incorporated at a level of 360 mg/kg in the diet, equivalent to 18 mg/kg b.w./day. In studies in rats which received the compound orally at 48, 240, and 480 mg/kg b.w./day on days 1 to 21 of gestation, teratogenic effects were observed. However, an increase in mortality rate and number of fetal resorptions and a decrease in fetal ossification were noted at all dose levels tested. In a study in mice in which the compound was administered orally, the highest dose of 2100 mg/kg b.w./day caused maternal toxicity. There was no evidence of a teratogenic effect. In teratogenicity studies in rats and rabbits in which oxytetracycline was administered intramuscularly at 41 mg per kg of body weight, there was no evidence of a teratogenic effect. However, an increased number of fetal resorptions was noted in rabbits. Intramuscular administration of the compound in dogs, at approximately 20 mg/kg b.w./day, caused skeletal and visceral malformation in the pups. The Committee noted that this study was difficult to evaluate since it was poorly reported. In a carcinogenicity study in mice and a similar study in rats, in which oxytetracycline was administered in the diet at doses up to 1372 and 150 mg/kg b.w./day respectively, there was no evidence of an increase in the incidence of tumours. In a second study, rats were fed diets containing up to 2000 mg of oxytetracycline per kg of body weight per day for 103 weeks. A dose-related increase in the incidence of benign phaeochromocytomas was observed in males, but because the survival rate of male rats in the control group was low, this increase was not considered to be significant. Although there was an increase in the incidence of benign neoplasms of the pituitary gland in female rats in the highest-dose group, there was a lower incidence of pituitary-gland hyperplasia than in controls. The Committee concluded that there was no evidence of a carcinogenic effect in rats or mice. The mutagenic potential of oxytetracycline was investigated in a range of studies. Negative results were recorded in bacterial tests, a chromosomal aberration test, a sister chromatid exchange test (with and without metabolic activation), and in a mouse lymphoma test without metabolic activation. The Committee noted that a positive effect in the mouse lymphoma test with metabolic activation was obtained using dose levels close to toxic concentrations and that the positive effect in the in vivo micronucleus assay in mice was not dose-related. In assessing the microbiological effects of oxytetracycline, the Committee considered the results of studies on the induction of drug- resistant organisms in dogs and humans. In a 6-week study in dogs, which received oxytetracycline, there was no increase in the level of resistant faecal coliforms at 2 mg/kg in the diet (equivalent to 50 µg per kg of body weight per day). In humans receiving oral treatment with oxytetracycline at 2 g, 20 mg, or 2 mg per day for 7 consecutive days, there was no evidence of resistant bacteria of the family enterobacteriaceae in the faeces at the lowest dose. The data on the induction of bacterial resistance in dogs, when recalculated on the basis of a 60-kg person, yielded a similar no-effect dose of 3 mg per day. 4. EVALUATION In view of the results of studies on the toxicological and microbiological effects of oxytetracycline, the Committee concluded that information about the induction of resistant coliforms in the human intestine was most appropriate for the safety assessment of oxytetracycline. The Committee adopted this conservative approach, although it recognized that the no-effect levels in toxicological studies were 18 mg/kg b.w./day or higher. An ADI of 0-0.003 mg per kg of body weight was established for oxytetracycline, based on a no-observed-effect level of 2 mg per person per day from the study with human volunteers and a safety factor of 10. It should be noted that the next dose tested was 20 mg per person per day, so the true no-effect level may be significantly higher than suggested by this study. 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See Also: Toxicological Abbreviations OXYTETRACYCLINE (JECFA Evaluation)