Sorption and mobility of metronidazole, olaquindox, oxytetracycline and tylosin in soil

Laboratory studies were conducted to characterise four different antibiotic compounds with regard to sorption and mobility in various soil types. Distribution coefficients (Kd values) determined by a batch equilibrium method varied between 0.5 and 0.7 for metronidazole, 0.7 and 1.7 for olaquindox and 8 and 128 for tylosin. Tylosin sorption seems to correlate positively with the soil clay content. No other significant interactions between soil characteristics and sorption were observed. Oxytetracycline was particularly strongly sorbed in all soils investigated, with Kd values between 417 in sand soil and 1026 in sandy loam, and no significant desorption was observed. Soil column leaching experiments indicated large differences in the mobility of the four antibiotic substances, corresponding to their respective sorption capabilities. For the weakly adsorbed substances metronidazole and olaquindox the total amounts added were recovered in the leachate of both sandy loam and sand soils. For the strongly adsorbed oxytetracyline and tylosin nothing was detected in the leachate of any of the soil types, indicating a much lower mobility. Results from defractionation and extraction of the columns (30 cm length) showed that 60-80% of the tylosin added had been leached to a depth of 5 cm in the sandy loam soil and 25 cm in the sand soil.

Volume effect on biological properties of a calcium phosphate hydraulic cement: experimental study in sheep

Injectable calcium phosphate hydraulic cements (CPHC) are a new family of bone substitutes within the class of bone reconstruction biomaterials. In this work, CPHC were tested in two consistencies (preset blocks or liquid paste) in an experimental model of cancellous bone defect in sheep. The defects were eight times larger than those investigated previously in rabbits. Three delays (12, 24, and 52 weeks) were used. Before death, a double label of oxytetracycline and alizarine was made intravenously. The distribution of implants was randomized, histomorphometric evaluation was performed and compared with micrographic observation, and optical microscopy of stained sections was performed either under visible, ultraviolet, or polarized light. The results were compared with spontaneous healing of empty defects and with a control group of normal cancellous bone from sheeps of the same age. No significant difference has been observed between premolded and injected implants. In the sheep model, the degradation and new bone formation rates are three times slower, compared with those observed previously in rabbits. New bone formation increased from 5.9% (12 weeks) up to 11.0% (24 weeks) in the empty defect group. In the cement groups, 28.3% new bone was obtained at 12 weeks, which seemed then to level off (27.8% new bone at 24 weeks). Cement residues appear as radio-opaque cylinders on microradiographs. In all cases, a radiolucent layer was observed at the cement/bone interface at 24 weeks. Stained sections showed the formation of a fibroconnective capsule around the residual cement, which presumably slows down new bone formation. Nevertheless, quantitative bone remodeling was accelerated in the cement group; mineral apposition as well as adjusted apposition rates were higher, and the formation period as well as the mineralization of osteoid tissue were faster compared with empty cavities and controls. These results point to higher osteoblast activity and better exchange with surrounding tissues in the defects filled with cement.

Oxytetracycline transfer into chicken egg yolk or albumen

This study was conducted to determine whether the approved doses of oxytetracycline (OTC) for breeder hens and meat-type poultry would produce drug residue transfer into egg components when fed to laying hens. Twenty hens were assigned to equal groups (n = 10) and fed either 50 or 200 g/ton OTC for 5 d. Oxytetracycline concentrations in egg components were determined daily during a 2-d pretreatment control period, the 5-d dosing period, and following drug withdrawal. The stability and drug content of the medicated feed were determined the day dosing was started and the day of withdrawal. Residues of OTC were not detectable during the predosing, dosing, or withdrawal period in egg yolks. Oxytetracycline residues were detectable, however, in egg albumen during the 5th d of treatment and the 1st d of medicated feed withdrawal. These concentrations were close to the limit of the assay's sensitivity (117 ppb). These data indicate that illegal or unintentional dosing of laying hens with feed medicated at the doses allowed for breeder hens or meat-type poultry should not produce consistently detectable levels of residues of OTC in eggs.

A new hybrid strain of an oxytetracycline-producing organism, Streptomyces rimosus

A new strain, Streptomyces rimosus LS-T hybrid, characterized by a lower level of foaming and higher antibiotic activity as compared to the other active strains of S. rimosus, is obtained as a result of selection among prototrophic recombinant forms of strains LS-T293 and BS-21. The studies on strain LS-T hybrid show the possibilities of combining properties of different strains of S. rimosus in the hybrid forms. The occurrence of new properties in the hybrid forms is detected.

Liquid chromatographic determination of oxytetracycline in swine tissues

A simple and rapid method was developed for determination of oxytetracycline (OTC) in swine muscle and kidney by liquid chromatography (LC). The method involved homogenization of sample in acetonitrile-1M imidazole buffer containing 10 mM disodium ethylenediaminetetraacetic acid (Na2.EDTA) and 50 mM magnesium acetate (15 + 85) with added hexane, centrifugation, removal of the hexane phase, and ultrafiltration of the supernatant. L-column ODS (150 x 4.6 mm) with a mobile phase of acetonitrile-1M imidazole buffer containing 50 mM magnesium acetate and 10 mM Na2.EDTA (10 + 90) was used for the LC separation. A fluorescence detector was used at an excitation wavelength of 380 nm and an emission wavelength of 520 nm. The calibration graph was linear from 1.25 to 200 ng OTC. Recoveries of OTC from swine tissue fortified at levels of 0.05-1.0 microgram/g ranged from 58.0 to 67.3%. The quantitation and detection limits were 0.05 and 0.04 microgram/g, respectively.

Concentration-time profiles of oxytetracycline in blood, kidney and liver of tench (Tinca tinca L) after intramuscular administration

Oxytetracycline (OTC) is a widely-used antibiotic in several animal species. The Food and Drug Administration allows OTC to be used in fish intended for human food, but there is limited kinetics data available. We studied OTC concentrations in plasma, kidney and liver in tenches (Tinca tinca L) after im administration using HPLC. Concentrations were fit to a monocompartment open model by extended least squares regression analysis using the MULTI (ELS) computer program. Peaks of OTC concentrations (Cmax) occurred at 4 h for blood and kidney and 72 h for liver and were 134.1 micrograms/mL, 129.8 micrograms/g and 333.4 micrograms/g, respectively. There was a high correlation (r = 0.9448) between blood and kidney concentrations and less between blood and liver. Concentrations were statistically different for each system. The blood OTC concentrations were higher than renal concentrations 92% of the time and were higher than hepatic concentrations 29% of the time. The plasma OTC half-life (21.2 h) was longer than in homeothermic species. The tench liver maintains considerable OTC residues and may affect food products derived from that organ.

Morphogenesis, ATP content and oxytetracycline production by Streptomyces rimosus in solid substrate cultivation

On agar plates, mycelia of Streptomyces rimosus TM-55 had a slender and smooth surface at 24 h, developed a shorter form and rough surface at 116 h, and lysed after 504 h incubation. Oxytetracycline potency reached a maximum at 94 h. During cultivation on solid substrate, mycelia rarely occurred on the surface at 24 h. The organism appeared as short rods at 166 h, and formed spores at 504 h. The ATP content was maximal at 166 h, while energy charge varied between 0.63 and 0.77 during cultivation. The oxytetracycline potency was maximal at 166 h and each gram of substrate produced 11 mg of antibiotic. ATP content and energy charge of mycelia showed a positive correlation with cell activity and oxytetracycline production.

Comparison of a radioimmunoassay (Charm II) test with high-performance liquid chromatography for detection of oxytetracycline residues in milk samples from lactating cattle

A radioimmunoassay test for tetracyclines (Charm II) was compared with high-pressure liquid chromatography (HPLC) for detection of oxytetracycline (OTC) residues in milk samples from individual lactating cows. Oxytetracycline was administered by 1 of 3 routes (IV, IM, or intrauterine) to 21 lactating dairy cows. A total of 292 duplicate milk samples were collected from milkings before and through 156 hours after OTC administration. Concentration of OTC in these samples was determined by use of the Charm II test and an HPLC method with a lower limit of quantitation, approximately 2 ng of OTC/ml. Samples were also classified with respect to presence of OTC residues relative to the FDA safe concentration (< or = 30 ng/ml), using the Charm II (by control point determination) and HPLC methods. There was a significant (P < or = 0.05) difference between test methods in classification of milk samples with respect to presence or absence of OTC at the FDA safe concentration. A total of 48 of the 292 test results (16.4%) did not agree. Using the HPLC test results as the standard with which Charm II test results were compared, 47 false presumptive-violative test results and 1 false presumptive-nonviolative Charm II test result (a sample containing 31 ng of OTC/ml, as evaluated by HPLC) were obtained. The samples with false presumptive-violative Charm II results contained < 30 ng of OTC/ml, as evaluated by HPLC. In some respects, the Charm II test performed appropriately as a screening test to detect OTC residues in milk samples from individual cows.(ABSTRACT TRUNCATED AT 250 WORDS)

Stability of penicillin G, ampicillin, amikacin and oxytetracycline and their interactions with food in in vitro simulated equine gastrointestinal contents

Penicillin G was extensively (84.7 per cent) and amikacin moderately (14.4 per cent) degraded when incubated for one hour in a chloride buffer at pH 1.9 designed to mimic the equine gastric pH. Ampicillin and oxytetracycline were stable at pH 1.9. Penicillin and ampicillin were moderately stable (more than 90 per cent) when incubated in equine caecal liquor for three hours but were degraded by about 65 per cent after 24 hours. More than 80 per cent of the initial concentrations of amikacin and oxytetracycline were recovered after 24 hours' incubation in equine caecal liquor. The concentrations of short chain fatty acids in equine caecal liquor were not affected by incubation with penicillin G, ampicillin, amikacin or oxytetracycline. More than 84 per cent of penicillin G and amikacin became bound to hay in buffers at pH 1.9 and pH 7.0. Ampicillin did not become bound to hay at pH 1.9, but more than 60 per cent became bound at pH 7.0.

Effect of temperature and diet composition on residue depletion of oxytetracycline in cultured channel catfish

Oxytetracycline is an antibacterial agent widely used in fish farming. The normal method of administration of oxytetracycline to the fish is to mix the drug into the feed. As a consequence, the concentration of the drug in feed, together with the preparation and the composition of feed, can influence the disposition of the drug itself. An experimental study was carried out to evaluate the residue depletion of oxytetracycline from muscle tissue of channel catfish (Ictalurus punctatus) fed different medicated diets. Three hundred channel catfish were randomly divided into six tanks (50 fish per tank), maintained at water temperatures of 18 degrees C (three tanks) and 23 degrees C (three tanks). The animals were fed with three diets, differing in their energy content and composition, for the duration of the experiment oxytetracycline was added to the diets at a level of 7500 mg kg-1 for 7 d. After cessation of the treatment, five fish from each tank were killed on days 1, 3, 8, 13, 18, 24, 30, 35 and 40. Oxytetracycline residues in muscle tissue were determined by high-performance liquid chromatography. The results indicate that the energy level and chemical composition of the medicated diets administered to channel catfish influence oxytetracycline disposition in fish, and that temperature is an important factor in conditioning the reported dietary effects. Therefore, formulation of specific diets to administer drugs to farmed fish could assure better bioavailability of the chemotherapeutant and shorter withdrawal times.

Characterization of an oxytetracycline-resistance gene, otrA, of Streptomyces rimosus

The sequence of a 2657 bp DNA fragment containing the coding and regulatory regions of the oxytetracycline (OTC)-resistance gene, otrA, from the OTC producer Streptomyces rimosus was determined. The predicted amino acid sequence of OtrA had extensive identity with tetracycline-resistance genes from other bacteria which mediate resistance via non-covalent ribosomal modification. The N-terminal domain had extremely high identity with the GTP-binding sites of elongation factors, such as EF-G and EF-Tu, suggesting that binding and hydrolysis of GTP is important to the function of the protein. Significant identity with EF-G was present throughout the polypeptide. Transcriptional activity upstream of the otrA coding region was investigated. An Escherichia coli-type promoter, otrAp1, was identified. Transcriptional readthrough of otrA from the upstream gene (otcZ) was also detected in S. rimosus cultures. A divergent promoter activity was identified with subclones of the OtrA fragment in promoter probe vectors analysed in Streptomyces lividans. However, this activity was not identified in a subclone containing more than half of the otrA coding sequence in S. lividans or at all in S. rimosus, indicating that OtrA negatively regulates the expression of the divergent transcript. The data are consistent with regulation of antibiotic production by OtrA to prevent 'suicide'.

Tetracycline as a marker in hard tissue research: effects on enamel formation in rat maxillary incisors

Tetracycline in low dose is often used as a marker in hard tissue research. A low dose effect on enamel formation has never been proven despite well known toxic reactions in enamel after higher doses. The purpose of the investigation was to evaluate enamel formation in maxillary incisors of rats given a single low dose of a tetracycline with low toxicity, commonly used in research. Eight young rats received an intraperitoneal injection of 2 mg oxytetracycline per 100 g. The rats were terminated after periods between 1 h until 5 days. Undemineralized ground sections were prepared and studied under fluorescence microscopy. Microradiograms were taken to study the degree of mineralization. A diffuse uptake was seen within enamel in areas corresponding to late secretory stage and incisally in maturing enamel with a low mineralization degree. Tendencies of enamel aberrations were seen in all rats. These appeared as splittings within the formed enamel. In conclusion, low toxicity tetracycline disturbs normal amelogenesis, even if administered in low doses. It should therefore be avoided in hard tissue research where enamel formation is studied.

Molecular cloning of resistance genes and architecture of a linked gene cluster involved in biosynthesis of oxytetracycline by Streptomyces rimosus

The isolation of mutants of Streptomyces rimosus which were blocked in oxytetracycline (OTC) production was described previously. The genes for the early steps of antibiotic biosynthesis mapped together. Genomic DNA fragments of S. rimosus which conferred resistance to OTC and complemented all of these non-producing mutants have been cloned. The cloned DNA is physically linked within approximately 30 kb of the genome of S. rimosus. The gene cluster is flanked at each end by a resistance gene each of which, independently, can confer resistance to the antibiotic. In OTC-sensitive strains of S. rimosus, the entire gene cluster including both resistance genes has been deleted. Complementation of blocked mutants by cloned DNA fragments in multi-copy vectors was often masked by a secondary effect of switching off antibiotic production in strains otherwise competent to produce OTC. This adverse effect on OTC production was not observed with recombinants using low copy-number vectors.

Pharmacokinetics of oxytetracycline hydrochloride in rabbits

Pharmacokinetics of oxytetracycline HCl (OTC) was studied in rabbits. After 10 mg of OTC/kg of body weight was administered IV, the distribution half-life was 0.06 hour, terminal half-life was 1.32 hours, volume of distribution area was 0.861 L/kg, and total body clearance was 0.434 L/kg/h. After 10 mg of OTC/kg was given IM, the absorption half-life was 2.09 hours, extent of absorption was 71.4%, and total body clearance of the absorbed fraction was 0.576 L/kg/h. Based on these kinetic data, a dosage of 15 mg of OTC/kg, every 8 hours was developed. This dose given IM for 7 consecutive days resulted in observed steady-state maximum and minimum concentrations (mean +/- SD) of 4.7 +/- 0.3 micrograms/ml and 3.2 +/- 0.6 micrograms/ml, respectively. Twice this dose (30 mg of OTC/kg, every 8 hours) given IM caused anorexia and diarrhea.

Comparative pharmacokinetics of gentamicin, neomycin and oxytetracycline in newborn calves

The pharmacokinetics of three antibiotics--gentamicin, neomycin and oxytetracycline were determined in newborn calves. The kinetic determinations, using two-compartment open models, were made at increasing ages from 1 day to 42 days and compared with those made from older calves (250+ days). Although all three antibiotics are eliminated unchanged primarily by glomerular filtration, there were marked differences in the development of elimination processes for individual drugs. The pharmacokinetics of neomycin were not influenced by age. Although the elimination half-life of gentamicin appeared to decrease with age, the changes were not significant and were due to an increased elimination rate in only one calf. There was no change with age in the remaining three calves. Oxytetracycline elimination was significantly reduced in newborn calves. This was exemplified by a decrease in the half-life of elimination t1/2 (beta) from 672.5 +/- 99.4 in the newborn to 385.6 +/- 76.8 at 6 weeks of age, and 377.3 +/- 40.8 min in the 250-day-old calf. These changes were consistent in all four calves. The rate of elimination remained low for the first 4 weeks of life. The volume of distribution Vd, area was not changed after the first week of life. Based on pharmacokinetic changes, an adjustment of dosage is indicated for oxytetracycline in the newborn calf as compared to the older calf or adult.

Pharmacokinetics and renal clearance of oxytetracycline in piglets following intravenous and oral administration

The pharmacokinetics of oxytetracycline (OTC) in three weaned piglets was studied following three routes of administration: intravenously, orally as drench, both at a dose of 20 mg/kg, and orally as medicated (400 ppm OTC) pelleted feed administered during 3 consecutive days. Analysis of the intravenous data according to the three compartment pharmacokinetic model revealed that OTC was well distributed in the body (Vf: 1.62 l/kg), had an overall body clearance of 0.25 litre/kg/h, and the elimination half-lives were in the range between 11.6 and 17.2 hrs. The mean OTC binding to plasma proteins was 75.5 +/- 4%. Following the drench route of administration the maximum plasma OTC concentration was achieved between 1 and 5 h post application and ranged between 1.18 and 1.41 micrograms/ml. The mean maximum plasma OTC concentration during medicated feed administration was 0.20 +/- 0.06 microgram/ml, which was achieved approximately 30 hours after the onset of the administration. A steady state OTC plasma level (approximately 0.2 microgram/ml) was maintained till the end of the trial. Within 48 hours after cessation of medicated feed administration the plasma OTC levels were beneath 0.06 microgram/ml. The mean OTC bioavailabilities of the oral routes were low: after the drench route of administration 9.0 +/- 0.67%, and after medicated pelleted feed administration 3.69 +/- 0.8%. The mean OTC renal clearances of each piglet ranged between 10.1 and 13.9 ml/min/kg (based on free OTC plasma fractions). The renal OTC clearance values were urine flow dependent in all piglets and significantly correlated with the renal creatinine clearance (P less than 0.005), being 3-5 times higher than the latter. It is concluded that in piglets OTC is excreted mainly by glomerular filtration and partly by tubular secretion. The potential clinical efficacy of 400 ppm OTC as medicated feed with respect to treatment, e.g. atrophic rhinitis, is discussed.

Comparative pharmacokinetics, bioavailability and renal clearance of five parenteral oxytetracycline-20% formulations in dairy cows

Oxytetracycline (OTC) concentrations on plasma and milk of dairy cows were determined following a single intramuscular injection of five oxytetracycline-20% formulations at a dosage of approximately 10 mg/kg. For obtaining pharmacokinetic reference parameters, one 10% OTC formulation was administered intravenously. The five 20% formulations were compared and evaluated pharmacokinetically with respect to absorption rate, peak plasma and milk OTC concentrations, biological half-life, and relative bioavailability. The mean maximum plasma OTC concentrations varied between 4.5 and 6.8 micrograms/ml and were achieved between 5 and 10 h p.i., depending on the formulation involved. The mean maximum milk concentrations, ranging from 1.12 to 1.92 micrograms/ml, were achieved 12 to 24 h p.i. A plasma OTC concentration exceeding 0.5 microgram/ml was maintained for 48 h to 70 h, and in milk for 33 to 49 h, depending on the formulation involved. Formulations exhibiting the lowest clinically noticeable irritation showed the highest peak plasma OTC concentrations and the best bioavailability. Among the formulations the calculated withholding periods for milk were in the range of 3 to 4 days and for edible tissues of 9 to 14 days. The OTC and creatinine clearances were significantly correlated to each other and to the urinary flow. OTC was excreted predominantly by glomerular filtration, partly by tubular secretion minus urogenital (distal renal tubuli and bladder) reabsorption.

Chloramphenicol, lincomycin and oxytetracycline disposition in calves with experimental pneumonic pasteurellosis

The effects of pneumonia on the pharmacokinetics of chloramphenicol, lincomycin, and oxytetracycline were evaluated in two-month-old calves. Pneumonia was induced by injection of Pasteurella haemolytica cultures directly through the thoracic wall into each lung. Six days prior to induction of pneumonia, the antibiotics were administered in a single i.v. dose. The antibiotics were administered again 48 (i.v.), 60 and 72 h (i.m.) following injection of P. haemolytica. The pharmacokinetics of chloramphenicol (25 mg/kg) and lincomycin (10 mg/kg) were not significantly different in calves with pneumonia. The hybrid rate constant beta for oxytetracycline was increased in calves with pneumonia from 0.0034 +/- 0.0003/min to 0.0048 +/- 0.0007/min between 2 h and 8 h. Thus the elimination half-life in serum was shortened from 212.4 +/- 20.3 min to 149.3 +/- 19.5 min. In addition, there was an apparent but not statistically significant decrease in K12 with pneumonia. These findings accentuate the need for observance of 12-h dose intervals with oxytetracycline.

Penetration of oxytetracycline into tissue-cages in calves

Concentrations of oxytetracycline (OTC) in serum and tissue-cage fluid (TCF) from subcutaneous tissue-cages were determined after single and repeated intravenous and intramuscular doses of 10 mg/kg to calves. Intravenous administration resulted in higher levels, and greater area under curve (AUC) in TCF, than did intramuscular administration. However, the penetration measured as the ratio of AUC in TCF to AUC in serum was equal, and therefore independent of the route of administration. A linear relationship between AUC in serum and AUC in TCF could be demonstrated. Half-lives of OTC in serum were 4.9 +/- 3.1 h after intravenous, and 6.1 +/- 2.0 h after intramuscular administration. In TCF the half-lives were 21.5 +/- 4.4 h and 24.5 +/- 11.5 h after intravenous and intramuscular administration, respectively. Repeated dosing resulted in accumulation of OTC in TCF. Lesser accumulation in older cages indicated altered characteristics of the cages with the passage of time. In serum, no substantial accumulation was seen after repeated i.v. dosing until the dosing interval was shortened to 6 h.

A comparison of the oxytetracycline preparations Aquacycline and Terramycin 100 with regard to absorption characteristics, local tissue reactions and residues following dewlap injections in calves

In an experiment with 12 calves, Aquacycline® in a 5 % (OTC-A5) and a 10 % (OTC-A10) solution, was compared with Terramycin®-100 (OTC-C) by injecting 20 mg OTC/kg bwt. of these preparations in the dewlap and monitoring serum concentrations as well as tissue reactions and residues at the site of injection. All 3 preparations resulted in oxytetracycline (OTC) serum concentrations above 0.5 µg/ml of approximately 60 h. During this period, OTC-A5 resulted in a 39 % and OTC-A10 in a 20 % larger area under the serum concentration-time curve, as compared to OTC-C (P < 0.05). The recorded tissue reaction in the form of swelling during the first week following injection of OTC-A5 averaged 72 % of that after OTC-C (P < 0.01), while the mean swelling after OTC-A10 was 81 % of the corresponding value after OTC-C (P < 0.05). The OTC residue levels at the sites of injection were lower after OTC-A5, but none of the preparations resulted in OTC residues exceeding 0.3 mg at 28 days and about 0.15 mg at 42 days after injection. The pathological changes at the site of injection were somewhat more pronounced in those calves which received OTC-C. Accordingly, these results give some support to the claims that Aquacycline® offers advantages with respect to absorption characteristics and tissue tolerance.

Oxytetracycline concentrations in plasma and lung of healthy and pneumonic calves, using two oxytetracycline preparations

Drug concentrations in lung tissue and plasma pharmacokinetics of a long-acting preparation of oxytetracycline were compared with those of a standard preparation of oxytetracycline given to healthy and pneumonic calves. Both preparations achieved higher concentrations in the pneumonic lung when compared with those in the normal-appearing lung of the same group. A difference in concentrations in lung tissue was not found between preparations at either 24 or 48 hours. The plasma pharmacokinetic values did differ between preparations. Comparatively, the long-acting oxytetracycline had a smaller rate constant for disposition, a larger volume of distribution divided by bioavailability constant, and a longer half-life. A difference in pharmacokinetic values was not observed between healthy and pneumonic calves.

Metal ion-tetracycline interactions in biological fluids. Part 5. Formation of zinc complexes with tetracycline and some of its derivatives and assessment of their biological significance

A series of studies was previously devoted to the dependence of the bioavailability of various tetracyclines on their coordination with calcium and magnesium ions. Several clinical investigations have also shown zinc to interfere with the gastrointestinal absorption of the drug in humans. On the other hand, the administration of tetracycline to rats was reported to result in the increase of the elimination rate of zinc, which could originate in zinc-tetracycline interactions in blood plasma. Formation constants for zinc complexes with tetracycline, oxytetracycline, doxycycline, minocycline, chlortetracycline and demethylchlortetracycline were thus determined at 37 degrees C in NaCl 0.15 mol. dm-3 aqueous medium. Computer simulations were then carried out to investigate the drug influence on the distribution of the low-molecular-weight fraction of zinc in human blood plasma. Zinc-tetracycline interactions in the gastrointestinal fluid were also simulated, using clinical data relative to fasting subjects as taken from the literature. No significant effect can be expected from tetracyclines on the distribution of zinc in plasma at the usual therapeutic levels. However, zinc-tetracycline interactions have been found to be determining factors for the bioavailabilities of the metal as well as of the antibiotic in the gastrointestinal fluid.