J. vet. Pharmacol. Therap. doi: /j x. E. A. ABU-BASHA* R. GEHRING T. M. HANTASH* A. F. AL-SHUNNAQ* & N. M.

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1 J. vet. Pharmacol. Therap. doi: /j x Pharmacokinetics and bioavailability of sulfadiazine and trimethoprim following intravenous, intramuscular and oral administration in ostriches (Struthio camelus) E. A. ABU-BASHA* R. GEHRING T. M. HANTASH* A. F. AL-SHUNNAQ* & N. M. IDKAIDEK à *Department of Veterinary Basic Medical Sciences, Faculty of Veterinary Medicine, Jordan University of Science and Technology, Irbid, Jordan; Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University, Manhattan, KS, USA; à Department of Pharmaceutics, College of Pharmacy, Petra University, Amman, Jordan Abu-Basha, E. A., Gehring, R., Hantash, T. M., Al-Shunnaq, A. F., Idkaidek, N. M. Pharmacokinetics and bioavailability of sulfadiazine and trimethoprim following intravenous, intramuscular and oral administration in ostriches (Struthio camelus). J. vet. Pharmacol. Therap. doi: j x. A pharmacokinetic and bioavailability study of sulfadiazine combined with trimethoprim (sulfadiazine trimethoprim) was carried out in fifteen healthy young ostriches after intravenous (i.v.), intramuscular (i.m.) and oral administration at a total dose of 30 mg kg body weight (bw) (25 and 5 mg kg bw of sulfadiazine and trimethoprim, respectively). The study followed a single dose, three periods, cross-over randomized design. The sulfadiazine trimethoprim combination was administered to ostriches after an overnight fasting on three treatment days, each separated by a 2-week washout period. Blood samples were collected at 0 (pretreatment), 0.08, 0.25, 0.50, 1, 2, 4, 6, 8, 12, 24 and 48 h after drug administration. Following i.v. administration, the elimination half-life (t 1 2b ), the mean residence time (MRT), volume of distribution at steady-state (V d(ss) ), volume of distribution based on terminal phase (V d(z) ), and the total body clearance (Cl B ) were (13.23 ± 2.24 and 1.95 ± 0.19 h), (10.06 ± 0.33 and 2.17 ± 0.20 h), (0.60 ± 0.08, and 2.35 ± 0.14 L kg), (0.79 ± 0.12 and 2.49 ± 0.14 L kg) and (0.69 ± 0.03 and ± 1.38 ml min kg), for sulfadiazine and trimethoprim, respectively. No significant difference in C max (35.47 ± 2.52 and ± 3.39 lg ml), t max (2.47 ± 0.31 and 2.47 ± 0.36 h), t ½b (11.79 ± 0.79 and ± 0.56 h), V d(z) F (0.77 ± 0.06 and 0.89 ± 0.07 L kg), Cl B F (0.76 ± 0.04 and 0.89 ± 0.07) and MRT (12.39 ± 0.40 and ± 0.36 h) were found in sulfadiazine after i.m. and oral dosing, respectively. There were also no differences in C max (0.71 ± 0.06 and 0.78 ± 0.10 lg ml), t max (2.07 ± 0.28 and 3.27 ± 0.28 h), t ½b (3.30 ± 0.25 and 3.83 ± 0.33 h), V d(z) F (6.2 ± 0.56 and 6.27 ± 0.77 L kg), Cl B F (21.9 ± 1.46 and ± 1.72) and MRT (3.68 ± 0.19 and 4.34 ± 0.14 h) for trimethoprim after i.m. and oral dosing, respectively. The absolute bioavailability (F) was 95.41% and 86.20% for sulfadiazine and 70.02% and 79.58% for trimethoprim after i.m. and oral administration, respectively. (Paper received 1 August 2008; accepted for publication 8 October 2008) Ronette Gehring, Department of Clinical Sciences, College of Veterinary Medicine, Kansas, State University, 2005 Research Park Circle, Manhattan, KS 66502, USA. rgehring@vet.ksu.edu INTRODUCTION Classical combinations of sulfonamides and trimethoprim have been extensively prescribed against serious infectious diseases of bacterial or protozoal origin, especially against respiratory and alimentary tract infections in different animal species (Nielsen & Gyrd-Hansen, 1994; Prescott et al., 2000). Sulfonamides and trimethoprim are bacteriostatic agents when each drug used individually, whereas they exert a bactericidal effect when used in combination (Bushby, 1980). Each drug inhibits a different 1

2 2 E. A. Abu-Basha et al. step in bacterial folic acid biosynthesis pathway, and thus results in a synergistic action (Batzias et al., 2005). This synergism lowers the minimum inhibitory concentration (MIC) of both drugs, broadens the bacterial spectrum and decreases bacterial resistance (Bushby, 1980; Plumb, 2002). In veterinary practice, a combination of sulphadiazine trimethoprim is widely used at a 5:1 ratio (Riviere & Spoo, 2001; Batzias et al., 2005). The ostrich (Struthio camelus) belongs to the ratite family that includes several other flightless birds like rhea, emu, cassowary and kiwi (McGowan, 1984). Not only do ostriches provide meat, feathers and hides, but they are also used for racing and zoo exhibits in certain countries (Jensen, 1998; Shane, 1998; Verwoerd, 2000). To our knowledge, there is no information currently available about the pharmacokinetic behavior of sulfadiazine and trimethoprim in the ostrich. Full knowledge about the pharmacokinetics and pharmacodynamics of sulfadiazine trimethoprim in ostriches is important to optimize dosing regimens in this species. Several pharmacokinetic studies of sulfadiazine and or trimethoprim have been conducted in chickens (Baert et al., 2003), cattle and calves (Shoaf et al., 1989; Kaartinen et al., 1999), ponies and horses (Van Duijkeren et al., 1994, 2002), dogs and cats (Sigel et al., 1981), sheep (Batzias et al., 2005) and pigs (Baert et al., 2001), but not in ostriches. Accordingly, the current study was designed to investigate the disposition kinetics and bioavailability of sulfadiazine and trimethoprim combination following i.v., i.m. and oral administration in this species. MATERIAL AND METHODS Ostriches Fifteen healthy ostriches (Struthio camelus), months old, weighing kg, were used for this study. These ostriches were obtained from local farm (Albekerat Ostrich Ranch; Amman, Jordan). The animals were housed in an isolated open system pen and were monitored for two weeks for any apparent clinical signs of disease before drug administration. The ostriches had free access to water and antibacterial-free food. Drug Trimethoprim sulfadiazine injectable solution (Norodine Ò, 200 mg sulfadiazine + 40 mg trimethoprim ml, Norbrook, UK) and oral solution (Sulfasol Ò, 200 mg sulfadiazine + 40 mg trimethoprim ml, Medmac, Jordan) were used in the current experiment. The two formulations were analyzed by HPLC to determine the concentration of sulfadiazine and trimethoprim, which was found to match the values provided by manufacturers. Experimental design The ostriches were allocated into three groups (five ostriches per group). The ostriches were given a single total dose of 30 mg kg bw of trimethoprim sulfadiazine (5 and 25 mg kg bw of trimethoprim and sulfadiazine, respectively) by i.v., i.m. and oral administration according to a randomized three period cross-over design, separated by a 2-week washout period. Norodine Ò (200 mg sulfadiazine + 40 mg trimethoprim ml, Norbrook, UK) was given in the right brachial vein and pectoral muscle for i.v. and i.m. administration, respectively. Sulfasol Ò (200 mg sulfadiazine + 40 mg trimethoprim ml, Medmac, Jordan) was given directly into the stomach using stomach tube for oral administration. Food was withheld from 12 h before through 6 h after drug dosing. Water was provided ad libitum. Blood samples (2 3 ml) were collected from the left brachial vein into heparinized tubes at 0 (pretreatment), 0.08, 0.16, 0.25, 0.5, 1, 2, 4, 6, 8, 10, 24, and 48 h after drug administration. All blood samples were centrifuged directly at 1000 g for 5 min to obtain clean plasma and stored at )20 C until analysis. Analytical method The plasma concentrations of trimethoprim and sulfadiazine were determined by a reversed phase high performance liquid chromatography (HPLC) method that has been previously described (Batzias et al., 2002). All solvents used were of HPLC grade; acetonitrile and water (Frutarom, UK), and zinc sulphate (ApliChem, Germany). The HPLC system (Shimadzu, Japan) consisted of an LC-10A DVP pump, SPD-10 AVP UV-vis detector, SIL-10A DVP auto injector, DGV-12 A degasser and Shimadzu class-vp software Ver 6.12 SP4. Chromatographic separation was performed using a Purospher star RP-18e (5 lm, 125 mm 4.6 mm) column (Merck, Germany) with isocratic mobile phase acetonitrile:water:0.15% triehylamine:0.15% phosphoric acid (11:87:1:1, v v). The mobile phase was filtered using a 0.45 lm membrane and degassed. The mobile phase was eluted at a flow rate of 2.0 ml min with UV detector was set at a wavelength of 220 nm. The volume of injection was 100 ll. Sample preparation The plasma protein was precipitated by adding 200 ll zinc sulphate (16%) to 100 ll of ostrich plasma or a standard sample. The mixture was vortexed for 30 s and centrifuged for 5 min at 1000 g. The clear supernatant was injected directly into the HPLC system using special glass vials. Calibration curve Daily fresh calibration curves were prepared by dissolving standard trimethoprim and sulfadiazine powders in HPLC-grade water to obtain a concentration of 1 mg ml. This solution was used to prepare standards of 0.1, 0.5, 1, 5, 10, 25, 50, 100 and 200 lg ml in HPLC-grade water or drug-free ostrich plasma. The calibration curves were obtained by plotting the peak area as a function of the respective trimethoprim and sulfadiazine concentrations and the linear regression was calculated.

3 Pharmacokinetics of SDZ TMP in ostriches 3 The HPLC method was validated for both trimethoprim and sulfadiazine by assessing the inter- and intra-day reproducibility at a concentration of 2, 40 and 120 lg ml, and the extraction efficiency. The calculated limit of detection and quantitation were 0.03 and 0.1 mg ml for both trimethoprim and sulfadiazine. Standard curves were linear over the range lg ml (r 2 > 0.998) for both Drugs. The mean percentage analytical recoveries in plasma were 96.4% and 94.2% for trimethoprim and sulfadiazine, respectively. The intra-day and inter-day assay coefficient of variations were % and %, respectively. Concentration (µg/ml) SDZ (i.v) SDZ (i.m) SDZ (oral) TMP (i.v) TMP (i.m) TMP (oral) Pharmacokinetic and statistical analysis Pharmacokinetic analysis of the data was performed using noncompartmental methods based on statistical moment theory according to Gibaldi and Perrier (1982), with the help of the commercially available software program WinNonlin Ò (Pharsight Corporation, Carry, NC, USA). The maximum concentration (C max ) and the corresponding peak time (t max ) were determined by the inspection of the individual drug plasma concentration-time profiles. The slope of the terminal phase of the semi-log-time-concentration curve (k z ) was determined by linear regression and converted to an elimination half-life (T ½k ) by multiplying the reciprocal by The area under plasma concentration-time curve (AUC) was calculated using the trapezoidal rule and extrapolated to infinity by adding the term C last k z, where C last is the last measured plasma drug concentration. The absolute bioavailability (F) was calculated as (AUC ev AUC iv ) 100. The other pharmacokinetic parameters [total body clearance (Cl B ), mean residence time (MRT), mean absorption time (MAT), volume of distribution based on the terminal phase (V d(z) ), volume of distribution at steady-state (V ss )] were calculated according to well-established equations (Baggot, 1977). The data were statistically analyzed (ANOVA) using commercially available software (SPSS, SPSS Inc., Chicago, IL, USA). The differences were considered significant when P < All values were expressed as mean ± SE. RESULTS All tested ostriches were clinically healthy throughout the study. There were no adverse reactions seen after administration of the drug by all routes. The mean concentration-time profiles for sulfadiazine trimethoprim combination at a total single dose of 30 mg kg bw, after i.v., i.m. and oral administration are shown in Fig. 1. Trimethoprim was not detected at 24 h post drug administration, whereas sulphadiazine was measurable at the last sampling point (48 h) for all routes in all ostriches. The ratios of sulfadiazine to trimethoprim concentrations after i.v., i.m. and oral administration is shown in Table 1. Trimethoprim concentrations declined more rapidly than the respective value of sulfadiazine, therefore the ratio increased with time after all administered routes Time (h) Fig. 1. Semilograthimic plot showing the plasma concentration-time profile of sulfadiazine and trimethoprim after i.v., i.m. and oral administration at a total dose of 30 mg kg (25 mg sulfadiazine + 5 mg trimethoprim kg) bw, as determined by HPLC method. Values are mean ± SE (n = 15). Table 1. Concentration ratios (mean ± SD) of SDZ vs. TMP as a function of time after i.v., i.m. and oral administration of the SDZ TMP combination at a total dose of 30 mg kg (25 mg SDZ + 5 mg TMP kg) bw in ostrich (n = 15) Time post administration (h) SDZ TMP concentration ratio (mean) i.v. SDZ TMP concentration ratio (mean) i.m. SDZ TMP concentration ratio (mean) oral The pharmacokinetic parameters are shown in Table 2. The mean absorption time for trimethoprim following intramuscular administration was significantly faster than for sulfadiazine (1.51 ± 0.23 and 2.33 ± 0.43 h, respectively). It was also faster than for sulfadiazine and trimethoprim administered orally (2.02 ± 0.37 and 2.17 ± 0.21 h, respectively). The plasma sulfadiazine concentration reached a peak (C max ) of ± 2.52 and ± 3.39 lg ml at 2.47 ± 0.31 and 2.47 ± 0.36 h, respectively after a single i.m. and oral administration. Whereas, plasma trimethoprim reached a peak of 0.71 ± 0.06 and 0.78 ± 0.10 lg ml at 2.07 ± 0.28 and 3.27 ± 0.28 h, respectively after a single i.m. and oral administration. The absolute bioavailability (F) of sulfadiazine and trimethoprim after i.m. administration was 95.41% and 70.02%, respectively. Whereas, oral bioavailability was 86.2% and 79.58% for sulfadiazine and trimethoprim, respectively.

4 4 E. A. Abu-Basha et al. Table 2. Pharmacokinetic parameters of sulfadiazine trimethoprim combination after i.v., i.m. and oral administration at a total dose of 30 mg kg (25 mg sulfadiazine + 5 mg trimethoprim kg) bw in ostrich. Values are mean ± SE (n = 15) Route of administration i.v. i.m. oral Parameters Units SDZ TMP SDZ TMP SDZ TMP t 1 2b h ± ± ± ± ± ± 0.33 MRT h ± ± ± ± ± ± 0.14* MAT h 2.33 ± ± 0.23* 2.02 ± ± 0.21 V d(z) F L kg 0.79 ± ± ± ± ± ± 0.77 V d(ss) L kg 0.60 ± ± 0.14 Cl B F ml min kg 0.70 ± ± ± ± ± ± 1.72 C max lg ml ± ± ± ± 0.10 t max h 2.47 ± ± ± ± 0.28* AUC 0-t lgæh ml ± ± ± ± ± ± 0.49 AUC 0- lgæh ml ± ± ± ± ± ± 0.49 F % ± ± ± ± 8.17 *Values are significantly different (P < 0.05). t 1 2b, elimination phase half-life; MRT, mean residence time; MAT, mean absorption time; V d(z) F, volume of distribution F; V d(ss), volume of distribution at steady-state; Cl B F, total body clearance F; C max, maximum plasma concentration; t max, time to peak concentration; AUC 0-t, area under plasma concentration-time curve from zero to the last time post drug administration; AUC 0-, area under plasma concentration-time curve from zero to infinity; F, absolute bioavailability. DISCUSSION Despite the great ongoing worldwide interest in the ostrich industry, there have been few pharmacokinetic pharmacodynamic (PK PD) studies of antimicrobials in this species. Drugs for which pharmacokinetic data in ostriches are available are: sodium salicylate, flunixin meglumine, meloxicam, penicillin G, antipyrine, enrofloxacin and marbofloxacin (Clarke et al., 2001; Baert et al., 2002; de Lucas et al., 2004, 2005). The wide use of sulfadiazine trimethoprim combination in production animal medicine and the lack of pharmacokinetic data for this combination in ostriches were the reasons to conduct the current study. The present experiment determined the pharmacokinetics and bioavailability of sulfadiazine trimethoprim administered to healthy ostriches as a combination (5:1) in a single i.v., i.m. and oral dose of 30 mg kg (25 mg sulfadiazine + 5 mg trimethoprim kg). After i.v. administration, the elimination half-life (t 1 2b ) of trimethoprim (1.95 h) was shorter than for sulfadiazine (13.23 h), which indicates the tendency of ostriches to eliminate trimethoprim more rapidly than sulfadiazine. The t 1 2b of trimethoprim and sulfadiazine obtained in this study was longer than has been reported for chickens (1.2 and 3.2 h) (Baert et al., 2003) and (1.0 and 2.7 h) (Loscher et al., 1990). This is most likely due to differences in the total body clearance (Cl B ) between these two avian species (Table 3). Higher values for Cl B in chickens compared to ostriches could be ascribed to a higher metabolic rate in the birds of lower bodyweight. There were also differences in Cl B and t 1 2b calculated for ostriches in the current study compared to reported values for various mammalian species. This could be attributable to physiological, anatomical or biochemical variations (Cho et al., 1984; Bezuidenhout, 1986). The t 1 2b of sulfadiazine and Table 3. Comparison of values for Cl B for various species, including the ostrich calculated in the current study Species Sulfadiazine Cl B (ml min kg) Trimethoprim Cl B (ml min kg) Reference Ostriches Current study Chickens Baert et al., 2003 Sheep Batzias et al., 2005 Pigs Baert et al., 2001 trimethoprim were both calculated to be longer in ostriches compared to values reported for sheep (4.1 and 0.59 h) (Batzias et al., 2005), most likely due to higher Cl B in the latter species (Table 3). In contrast, the t 1 2b of trimethoprim has been reported to be longer in pigs (2.66 h) (Baert et al., 2001), with a comparatively lower Cl B than in ostriches (Table 3). The apparent volume of distribution (V d(z) ) provides an estimate of the extent of drug distribution in the body. A value of V d(z) >1L kg implies that a drug is widely distributed (Baggot, 2001). The V d(z) values of (2.49 and 0.79 L kg) indicate that trimethoprim is more widely distributed than sufladiazine in ostriches. These values are in agreement with those reported in chickens (0.43 and 2.21 L kg) (Baert et al., 2003), (0.96 and 3.3 L kg) (Loscher et al., 1990), sheep (0.71 and 2.81 L kg) and pigs (0.55 and 2.02 L kg) (Baert et al., 2001) for trimethoprim and sulfadiazine, respectively. Trimethoprim is a lipid-soluble weak organic base that is widely distributed and therefore, the concentration of trimethoprim in plasma is expected to be lower than those in tissues (Prescott et al., 2000; Baggot, 2001). Following single i.m. dose, sulfadiazine was rapidly absorbed with a C max of lg ml achieved at 2.47 h (t max ). Of interest in the current study, trimethoprim concentration was

5 Pharmacokinetics of SDZ TMP in ostriches 5 measured in ostrich plasma with a C max of 0.71 lg ml achieved at 2.07 h post injection. This is different from what was reported in sheep, where trimethoprim was not detected at any time point after i.m. administration (Batzias et al., 2005). Moreover, trimethoprim was not detected after s.c. injection of a sulfadiazine trimethoprim combination in calves (Shoaf et al., 1987). Whereas, trimethoprim was absorbed slowly after i.m. injection in calves and cattle (White et al., 1981; Kaartinen et al., 1999). Bioavailability is the fraction of a drug administered by any nonvascular route that gains access to the systemic circulation (Toutain & Bousquet-Melou, 2004). The absolute bioavailability (F) was calculated in the current study to be 95.4% and 70.02% for sulfadiazine and trimethoprim, respectively, following intramuscular administration. The F value of sulfadiazine was higher than those reported in sheep (69%) (Batzias et al., 2005) and dairy cows (55%) (Kaartinen et al., 1999). After oral administration, the observed C max (37.5, 0.8 lg ml) was attained at a time post injection (t max ) of 2.5 and 3.3 h for sulfadiazine and trimethoprim, respectively. Our recorded C max for both sulfadiazine and trimethoprim was similar to those reported in chickens (39.32, 1.13 lg ml) (Baert et al., 2003) and pigs (29.5, 1.2 lg ml) (Baert et al., 2001). The t max of sulfadiazine was similar to the value reported in pigs (2.19 h) (Baert et al., 2001) and longer than in chickens (1.64 h) (Baert et al., 2003). On the other hand, the t max of trimethoprim was longer than those observed in chicken (1.42 h) (Baert et al., 2003) and pig (1.8 h) (Baert et al., 2001). The oral bioavailability of sulfadiazine and trimethoprim was 86.2% and 79.6%, respectively. The calculated SDZ bioavailability was in agreement with those reported in chickens (80%) (Baert et al., 2003) and lower from those reported in pigs (106%). The oral bioavailability of trimethoprim was similar to those reported in chickens (79%) (Baert et al., 2003) and pig (73%) (Baert et al., 2001). There were significant differences (P < 0.05) in the t max and MRT of trimethoprim between intramuscular and oral routes, suggesting that trimethoprim is more slowly absorbed following oral administration. However, none of the other pharmacokinetic parameters were found to be significantly different were found in the other pharmacokinetic parameters for sulfadiazine or trimethoprim after i.m. and oral administration. In vitro studies suggest that a 16:1 to 20:1 ratio of sulfadiazine:trimethoprim should be maintained to ensure synergistic action against susceptible bacteria (Prescott et al., 2000). However, microorganisms with MICs lower than lg ml for sulfadiazine trimethoprim, respectively, are considered susceptible (e.g. Salmonella species, E. coli, Haemophilus species and Actinobacillus pleuropneumoniae) (Bushby, 1980; Shoaf et al., 1987; Mengelres et al., 1995, 2000; Prescott et al., 2000). Synergism has been observed over a wide range of ratios, where both drugs are present at levels above their MICs (Bushby, 1980; Greko et al., 2002). It is therefore possible for synergistic activity to occur at ratios ranging from 1:1 to 40:1. The plasma concentration of trimethoprim is considered more important than the sulfadiazine concentration (Plumb, 2002). Nevertheless, as the in vivo ratio between sulfadiazine trimethoprim is not always optimal, a bactericidal action may not be guaranteed. Trimethoprim and sulfadiazine are both bacteriostatic agents and therefore plasma concentrations of each agent should be maintained above their respective MIC values for susceptible bacterial species throughout the recommended dosage interval (McKinnon & Davis, 2004). No information is currently available about the MIC values of different bacterial strains in ostriches. Following i.v., i.m. and oral administration of sulfadiazine trimethoprim combination, the plasma concentration of SDZ remained above 9.5 lg ml for 12 h, whereas trimethoprim concentrations fell below 0.5 lg ml at 4 h post i.v. dose and remained above 0.5 lg ml for 4 h after i.m. and oral administration. The relatively rapid elimination of trimethoprim may be a limiting factor for the synergism between sulfadiazine and trimethoprim. The peak plasma concentrations of sulfadiazine trimethoprim were , , and lg ml, after i.v., i.m. and oral administration, respectively. These peak concentrations were above the MIC ( lg ml, for sulfadiazine trimethoprim) for most susceptible microorganisms. However, the optimal ratio of sulfadiazine:trimethoprim of 20:1 was not achieved in the present experiment. Following i.v. administration, a near optimal ratio of about 40:1 was only obtained between 0.08 to 2 h post injections. Our results was in agreement with those reported in sheep (Baert et al., 2003).Whereas, following intramuscular and oral administration, the sulfadiazine:trimethoprim ratio was >45:1 at all time points. Similar ratios of 60 85:1 was found in broiler chicken throughout the period of treatment (Loscher et al., 1990; Dagorn et al., 1991; De Baere et al., 2000). In conclusion, sulfadiazine trimethoprim was detected in ostrich plasma at concentrations higher than the MICs for most susceptible microorganisms but not for a reasonable period. No significant difference was found in the sulfadiazine trimethoprim kinetic parameters between i.m. and oral route except in the t max and MRT for trimethoprim. The optimal ratio of sulfadiazine trimethoprim of 20:1 was not achieved in the present experiment after administration by different routes. Therefore, further pharmacokinetic pharmacodynamics (PK PD) studies are warranted to launch an effective plasma level during a satisfied period and to extrapolate a suitable therapeutic dose of sufadiazine trimethoprim in ostriches. ACKNOWLEDGMENTS This work was supported by Deanship of Research, Jordan University of Science and Technology (grant no ). The authors wish to thank Mr. Farook Al-Zghoul and Mr. Eyad Hamzah for technical support in sample analysis. REFERENCES Baert, K., De Baere, A., Croubles, D., Gasthuys, F. & De Backer, C. (2001) Pharmacokinetics and bioavailability of sulfadiazine and trimethoprim (trimazin 30%) after oral adminstration in non-fasted young pigs. 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