Lung microdialysis study of florfenicol in pigs after single intramuscular administration

For most bacterial lung infections, the concentration of unbound antimicrobial agent in lung interstitial fluid has been considered as the gold standard for estimating the antibacterial efficacy. In this study, the pharmacokinetics of florfenicol (FF) in porcine lung interstitial fluid was investigated after single intramuscular administration at two different doses (20 and 50 mg/kg). Twelve pigs underwent thoracotomy under general anesthesia. Then, the CMA/30 probe was implanted into the lung and perfused at 1 μL/min. The microdialysis (MD) samples were collected on a preset schedule and analyzed by high‐performance liquid chromatography (HPLC). Noncompartmental pharmacokinetic analysis was performed. FF exhibited rapid distribution and slow elimination in porcine lung interstitial fluid. The main pharmacokinetic parameters at 20 and 50 mg/kg were 4.88 ± 0.54 and 10.36 ± 2.52 μg/mL for the maximum concentration (C_max), 3.25 ± 0.32 and 3.50 ± 0.27 h for the time to C_max (T_max), 9.47 ± 6.84 and 7.75 ± 3.23 h for the half‐life (t_1/2), 0.10 ± 0.06 and 0.10 ± 0.04 1/h for the terminal elimination rate constant (λ_z), 13.85 ± 7.97 and 11.42 ± 2.79 h for the mean residence time (MRT), 37.77 ± 8.13 and 71.15 ± 16.99 h·μg/mL for the area under the curve from time 0 to 18.25 h (AUC_0–18.25), and 51.18 ± 20.11 and 88.78 ± 27.58 h·μg/mL for the area under the curve from time 0 to infinity (AUC_0–∞), respectively.     

A physiologically based pharmacokinetic model for the prediction of the depletion of methyl‐3‐quinoxaline‐2‐carboxylic acid,the marker residue of olaquindox,in the edible tissues of pigs

To estimate the consumer exposure to olaquindox (OLA) residues in porcine edible tissues, a physiologically based pharmacokinetic (PBPK) model for methyl‐3‐quinoxaline‐2‐carboxylic acid (MQCA), the marker residue of OLA, was developed in pigs based on the assumptions of the flow‐limited distribution, hepatic metabolism, and renal excretion. The model included separate compartments corresponding to blood, muscle, liver, kidney, adipose, and an extra compartment representing the remaining carcass. Physiological parameters were determined from literatures. Plasma protein binding, partition coefficients, and renal clearance for MQCA were determined in in vitro and in vivo studies. The metabolic conversion of OLA to MQCA was assumed as a simple, one‐step process, and an apparent first‐order rate constant (k) was employed to describe this metabolic process. The PBPK model was optimized and validated with plasma and tissue data from literatures and our study. Sensitivity analysis and Monte Carlo simulation were also implemented to estimate the influence of model parameters on the goodness of fit. When compared with the observed data, the PBPK model underestimated the MQCA level in all compartments at the early time points, whereas gave excellent predictions of MQCA concentration in porcine edible tissues at later time points. The correlation coefficients between the predicted and observed values were over 0.88. The consistency between the model predictions and the real residues of OLA in pigs proved the good applicability of our model in food safety risk assessment.     

Development of a multiroute physiologically based pharmacokinetic model for orbifloxacin in rabbits

To predict the orbifloxacin concentrations in rabbits after multiple routes of administration, a flow‐limited multiroute physiologically based pharmacokinetic (PBPK) model was developed. Three routes of administration (IV, IM, and PO) were incorporated into this model. Physiological parameters including tissue weights and blood flows through different tissues were obtained from the literature. The tissue/plasma partition coefficients (P_Xs) for noneliminating tissues were calculated according to the area method, while the P_Xs for kidney and the rest of the body compartment, together with other parameters for absorption and elimination, were optimized based on the published concentrations. The comparisons between predicted and observed orbifloxacin concentrations proved its validity, and the present model predicted available concentration data well, including those in liver, kidney, muscle, lung, heart, and plasma after oral, intravenous, or intramuscular administration. A local sensitivity analysis was also performed, which showed that the parameters for oral absorption were most influential on the orbifloxacin concentrations. This model was used to predict plasma and tissue concentrations after multiple oral or intramuscular administration. This study demonstrated the feasibility of predicting drug residues in minor species after multiple routes of administration in the extra‐label manner using the PBPK modeling.     

Stereoselective biotransformation of ketamine in equine liver and lung microsomes

Stereoselectivity has to be considered for pharmacodynamic and pharmacokinetic features of ketamine. Stereoselective biotransformation of ketamine was investigated in equine microsomes in vitro. Concentration curves were constructed over time, and enzyme activity was determined for different substrate concentrations using equine liver and lung microsomes. The concentrations of R/S‐ketamine and R/S‐norketamine were determined by enantioselective capillary electrophoresis. A two‐phase model based on Hill kinetics was used to analyze the biotransformation of R/S‐ketamine into R/S‐norketamine and, in a second step, into R/S‐downstream metabolites. In liver and lung microsomes, levels of R‐ketamine exceeded those of S‐ketamine at all time points and S‐norketamine exceeded R‐norketamine at time points below the maximum concentration. In liver and lung microsomes, significant differences in the enzyme velocity (V_max) were observed between S‐ and R‐norketamine formation and between V_max of S‐norketamine formation when S‐ketamine was compared to S‐ketamine of the racemate. Our investigations in microsomal reactions in vitro suggest that stereoselective ketamine biotransformation in horses occurs in the liver and the lung with a slower elimination of S‐ketamine in the presence of R‐ketamine. Scaling of the in vitro parameters to liver and lung organ clearances provided an excellent fit with previously published in vivo data and confirmed a lung first‐pass effect.     

Pharmacokinetics of florfenicol and its metabolite,florfenicol amine,in rice field eel (Monopterus albus) after a single‐dose intramuscular or oral administration

The pharmacokinetics of florfenicol (FF) and its metabolite, florfenicol amine (FFA), were studied in rice field eel (Monopterus albus) after a single dose (20 mg/kg) by intramuscular (i.m.) or oral gavage (p.o.) dose at 25 °C. The elimination half‐lives (t_1/2β), peak concentration of FF (C_max), and time to reach FF peak concentration (T_max) in plasma were estimated as 18.39 h, 10.83 μg/mL, and 7.00 h, respectively, after i.m. injection and 13.46 h, 8.37 μg/mL, and 5 h, respectively, after p.o. administration. The T_max values of FF in tissues (i.e., kidney, muscle, and liver) were larger for i.m. injection compared with those for p.o. administration. The t_1/2β had the following order kidney > muscle > liver for i.m. administrated and kidney > liver > muscle for p.o. administrated. The largest area under the concentration–time curve (AUC) was calculated to be 384.29 mg · h/kg after i.m. dosing, and the mean residence time (MRT) was 42.46 h by oral administration in kidney. FFA was also found in all tissues with a lower concentration than FF for both i.m. and p.o. administrations throughout the study. The elimination of FFA was slow with a t_1/2β between 18.19 and 47.80 h in plasma and tissues. The mean metabolic rate of FFA for i.m. and p.o. administrations was >23.30%.     

Serum and tissue concentrations of erythromycin in calves with induced pneumonic pasteurellosis

The effects of pneumonia on the pharmacokinetics of erythromycin administered IM and the tissue concentration changes with time were evaluated in 2-month-old calves. Pneumonia was induced by injection of Pasteurella haemolytica cultures through the thoracic wall into each lung. Six days prior to induction of pneumonia, erythromycin (15 mg/kg) was administered in a single IM dose. Erythromycin was administered again 48, 72, and 96 hours after injection of P haemolytica. On the third day of erythromycin administration (96 hours), the calves were serially euthanatized in groups of 4 calves each at 2, 5, 8, 12, 18, and 24 hours after the final dose was given. Tissue concentrations of erythromycin in kidney, liver, lung, muscle, CSF, and serum were determined. Neither the serum concentrations nor the overall pharmacokinetic values were significantly (P less than or equal to 0.05) changed by pneumonia. The concentrations of erythromycin were maximal at 5 hours for liver, muscle, and serum and at 8 hours for CSF, kidney, and lung. Serum and muscle concentrations were similar, whereas concentrations in CSF were lower than in serum and higher in kidney, liver, and lung. The lung/serum ratios were approximately 2.5 to 3 at 8 through 24 hours after IM administration. The peak concentration in lung was approximately 6 micrograms/g at 8 hours.     

A physiologically based pharmacokinetic model for quinoxaline‐2‐carboxylic acid in rats,extrapolation to pigs

A multi‐compartment physiologically based pharmacokinetic (PBPK) model to describe the disposition of cyadox (CYX) and its metabolite quinoxaline‐2‐carboxylic acid (QCA) after a single oral administration was developed in rats (200 mg/kg b.w. of CYX). Considering interspecies differences in physiology and physiochemistry, the model efficiency was validated by pharmacokinetic data set in swine. The model included six compartments that were blood, muscle, liver, kidney, adipose, and a combined compartment for the rest of tissues. The model was parameterized using rat plasma and tissue concentration data that were generated from this study. Model simulations were achieved using a commercially available software program (ACSLX_Libero version 3.0.2.1). Results supported the validity of the model with simulated tissue concentrations within the range of the observations. The correlation coefficients of the predicted and experimentally determined values for plasma, liver, kidney, adipose, and muscles in rats were 0.98, 0.98, 0.98, 0.99, and 0.95, respectively. The rat model parameters were then extrapolated to pigs to estimate QCA disposition in tissues and validated by tissue concentration of QCA in swine. The correlation coefficients between the predicted and observed values were over 0.90. This model could provide a foundation for developing more reliable pig models once more data are available.     

Comparative pharmacokinetics of florfenicol in healthy and Pasteurella multocida‐infected Gaoyou ducks

Pasteurella multocida is the causative agent of fowl cholera, and florfenicol (FF) has potent antibacterial activity against P. multocida and is widely used in the poultry industry. In this study, we established a P. multocida infection model in ducks and studied the pharmacokinetics of FF in serum and lung tissues after oral administration of 30 mg/kg bodyweight. The maximum concentrations reached (C_max) were lower in infected ducks (13.88 ± 2.70 μg/ml) vs. healthy control animals (17.86 ± 1.57 μg/ml). In contrast, the mean residence time (MRT: 2.35 ± 0.13 vs. 2.27 ± 0.18 hr) and elimination half‐life (T_½β: 1.63 ± 0.08 vs. 1.57 ± 0.12 hr) were similar for healthy and diseased animals, respectively. As a result, the area under the concentration curve for 0–12 hr (AUC_0–12 hr) for FF in healthy ducks was significantly greater than that in infected ducks (49.47 ± 5.31 vs. 34.52 ± 8.29 μg hr/ml). The pharmacokinetic differences of FF in lung tissues between the two groups correlated with the serum pharmacokinetic differences. The C_max and AUC_0–12 hr values of lung tissue in healthy ducks were higher than those in diseased ducks. The concentration of FF in lung tissues was approximately 1.2‐fold higher than that in serum both in infected and healthy ducks indicating that FF is effective in treating respiratory tract infections in ducks.     

Depletion study and withdrawal period calculation of florfenicol in the crucian carp (Carassius auratus) following multiple intramuscular injections

The previously adopted marker residue for florfenicol (FF) in China was only florfenicol amine (FFA); however, the marker residue has been changed to FF plus FFA since the end of 2017. The previous official withdrawal period determined based on the only concentration of FFA may no longer be suitable. Therefore, the present study aimed to determine the depletion profiles of FF and FFA and further calculate the withdrawal period in the crucian carp (Carassius auratus) based on the new marker residues. Florfenicol was intramuscularly administered at 10 mg/kg bodyweight daily for five consecutive days to crucian carps reared in freshwater at 10°C. After the last dose, plasma and tissue samples were randomly collected from 10 fish at different time points. The FF and FFA concentrations were simultaneously determined by high-performance liquid chromatography (HPLC) with a fluorescence detector and further subjected to noncompartmental analysis. The elimination half-life (h) of FF in different tissues decreased as follows: liver (39.1) > kidney (36.3) > skin plus muscle (34.6) > plasma (31.7), whereas that of FFA decreased as follows: kidney (41.4) > skin plus muscle (39.4) > liver (39.3) > plasma (35.7). Considering a maximum residue limit of 1 μg/g for the total concentration of FF and FFA in the skin plus muscle, a withdrawal period of 6 days was calculated based on the upper limit of the one-sided 95% confidence interval.     

A PBPK model for midazolam in four avian species

A physiologically based pharmacokinetic (PBPK) model was developed for midazolam in the chicken and extended to three other species. Physiological parameters included organ weights obtained from 10 birds of each species and blood flows obtained from the literature. Partition coefficients for midazolam in tissues vs. plasma were estimated from drug residue data obtained at slaughter. The avian models include separate compartments for venous plasma, liver, kidney, muscle, fat and all other tissues. An estimate of total body clearance from an earlier in vitro study was used as a starting value in the model, assuming almost complete removal of the parent compound by liver metabolism. The model was optimized for the chicken with plasma and tissue data from a pharmacokinetic study after intravenous midazolam (5 mg/kg) dose. To determine which parameters had the most influence on the goodness of fit, a sensitivity analysis was performed. The optimized chicken model was then modified for the turkey, pheasant and quail. The models were validated with midazolam plasma and tissue residue data in the turkey, pheasant and quail. The PBPK models in the turkey, pheasant and quail provided good predictions of the observed tissue residues in each species, in particular for liver and kidney.     

Pharmacokinetic/pharmacodynamic relationship of cefquinome against Pasteurella multocida in a tissue‐cage model in yellow cattle

The cephalosporin antimicrobial drug cefquinome was administered to yellow cattle intravenously (i.v.) and intramuscularly (i.m.) at a dose of 1 mg/kg of body weight in a two‐period crossover study. The pharmacokinetic (PK) properties of cefquinome in serum, inflamed tissue‐cage fluid (exudate), and noninflamed tissue‐cage fluid (transudate) were studied using a tissue‐cage model. The in vitro and ex vivo activities of cefquinome in serum, exudate, and transudate against a pathogenic strain of Pasteurella multocida (P. multocida) were determined. A concentration‐independent antimicrobial activity of cefquinome was confirmed for levels lower than 4 × MIC. Integration of in vivo pharmacokinetic data with the in vitro MIC provided mean values for the time that drug levels remain above the MIC (T > MIC) in serum was 14.10 h after intravenous and 14.46 h after intramuscular dosing, indicating a likely high level of effectiveness in clinical infections caused by P. multocida of MIC 0.04 μg/mL or less. These data may be used as a rational basis for setting dosing schedules, which optimize clinical efficacy and minimize the opportunities for emergence of resistant organisms.     

Liver Condition Affects Bovine Oocyte Qualities by Changing the Characteristics of Follicular Fluid and Plasma

The liver is an important organ that contributes to milk production in dairy cows. The aim of this study was to examine whether liver conditions affect the characteristics of blood plasma and follicular fluid (FF) and whether supplementing in vitro maturation medium with FF from either cows with damaged livers (DL) or those with healthy livers (HL) affects oocyte developmental competence. Biochemical characteristics of FF were significantly correlated with those in plasma. As such, the characteristics of both plasma and FF were similarly affected by liver conditions in that the concentrations of total protein and inorganic phosphorus were higher for the DL cow group than for the HL cow group, whereas the concentrations of albumin, lactate dehydrogenase and calcium were lower for DL cows than for HL cows. In addition, supplementing the medium with DL‐FF retarded the progression of the nuclear maturation of oocytes collected from the HL cows. On culturing oocytes in maturation medium containing HL‐FF, DL‐FF or foetal calf serum, the highest developmental rate to the blastocyst stage was observed in the HL‐FF group, while the lowest developmental ratio was observed in the DL‐FF group. The growth factor array of the FFs revealed that 10 growth factors were significantly downregulated in the DL‐FF compared with those in HL‐FF. In conclusion, the characteristics of plasma and FF are affected by liver conditions in a similar way. Concentrations of several growth factors were low in DL‐FF, as was the ability of DL‐FF to support oocyte maturation compared with that of HL‐FF.     

A physiologically based pharmacokinetics model for florfenicol in crucian carp and oral‐to‐intramuscular extrapolation

Yang, F., Sun, N., Sun, Y. X., Shan, Q., Zhao, H. Y., Zeng, D. P., Zeng, Z. L. A physiologically based pharmacokinetics model for florfenicol in crucian carp and oral‐to‐intramuscular extrapolation. J. vet. Pharmacol. Therap.  36 , 192–200. In this study, an oral physiologically based pharmacokinetics (PBPK) model was developed for florfenicol in crucian carp (Carassius auratus). Subsequently, oral‐to‐intramuscular extrapolation was performed and the two models were used to predict florfenicol concentrations in the edible tissues of crucian carp. The oral model gave good predictions in most tissues, except for kidney and liver in which the florfenicol concentrations were underestimated at the later time points. In contrast, using the intramuscular model, the concentrations in the kidney were overestimated at the later time points. Both models had the best predictive ability in the main edible tissue, the muscle. The oral model also accurately predicted the florfenicol concentrations in the muscle after multiple doses. The present study demonstrated the feasibility of predicting florfenicol concentrations in the edible tissues of crucian carp using a route‐to‐route extrapolation method.     

Pharmacokinetics of florfenicol and its metabolite florfenicol amine in crucian carp (Carassius auratus) at three temperatures after one single intramuscular injection

The pharmacokinetic profiles of florfenicol (FF) or florfenicol amine (FFA) in crucian carp were compared at different water temperatures after single intramuscular administration of FF at 10 mg/kg bodyweight. The concentrations of FF and FFA were determined by a high‐performance liquid chromatography method, and then, the concentration versus time data were subjected to compartmental analysis using a one‐compartment open model. At the water temperatures of 10, 20, and 25°C, the peak concentrations (C_maxs) of FF were 2.28, 2.29, and 2.34 μg/ml, respectively, while those of FFA were 0.42, 0.71, and 0.82 μg/ml, respectively. And the absorption half‐life (t_1/2ka) of FF was 0.21, 0.19, and 0.21 hr, while the elimination half‐life (t_1/2kel) was 31.66, 24.77, and 21.48 hr, respectively. For FFA, the formation half‐life (t_1/2kf) was 3.85, 8.97, and 12.43 hr, while the t_1/2kel was 58.34, 30.27, and 21.22 hr, respectively. The results presented here demonstrated that the water temperature had effects on the elimination of both FF and FFA and the formation of FFA. Based on the T > MIC values calculated here, to treat the infections of bacterial with MIC value ≤ 0.5 μg/ml, FF intramuscularly given at 10 mg/kg bodyweight with a 72‐hr interval is sufficient at the water temperature of 10°C, while the intervals of 60 and 48 hr were needed at 20 and 25°C, respectively. But to treat bacterial with higher MIC values, more FF or FF at 10 mg/kg BW but with shorter intervals should be intramuscularly given to the infected fish.     

Tissue/fluid correlation study for the depletion of sulfadimethoxine in bovine kidney, liver, plasma, urine, and oral fluid

Sulfonamides are among the oldest, but still effective, antimicrobial veterinary medicines. In steers and dairy cows, the sulfonamides are effective in the treatment of respiratory disease and general infections. Sulfadimethoxine (SDM) has been approved by US Food and Drug Administration (FDA) for use in steers and dairy cows with a tolerance of 100 ng/g (ppb) in edible tissues and 10 ppb in milk. The detection of SDM residue above tolerance in the animal slaughtered for food process will result in the whole carcass being discarded. This report describes a comprehensive depletion study of SDM (and its main metabolite) in plasma, urine, oral fluid, kidney, and liver. In this study, nine steers were injected intravenously with the approved dose of SDM; the loading dose was 55 mg/kg, followed by 27.5 mg/kg dose at 24 h and again at 48 h. Fluids (blood, urine, and saliva) and tissue (liver and kidney) samples were collected at intervals after the last dose of SMD. The combination of laparoscopic serial sampling technique with the liquid chromatography/mass spectrometry method provided the data to establish the tissue/fluid correlation in the depletion of SMD. A strong correlation and linearity of the log-scale concentration over time in the depletion stage has been confirmed for kidney, liver, and plasma.     

A physiologically based pharmacokinetic model for valnemulin in rats and extrapolation to pigs

A flow-limited, physiologically based pharmacokinetic (PBPK) model for predicting the plasma and tissue concentrations of valnemulin after a single oral administration to rats was developed, and then the data were extrapolated to pigs so as to predict withdrawal interval in edible tissues. Blood/tissue pharmacokinetic data and blood/tissue partition coefficients for valnemulin in rats and pigs were collected experimentally. Absorption, distribution and elimination of the drug were characterized by a set of mass-balance equations. Model simulations were achieved using a commercially available software program. The rat PBPK model better predicted plasma and tissue concentrations. The correlation coefficients of the predicted and experimentally determined values for plasma, liver, kidney, lung and muscle were 0.96, 0.94, 0.96, 0.91 and 0.91, respectively. The rat model parameters were extrapolated to pigs to estimate valnemulin residue withdrawal interval in edible tissues. Correlation (R(2) ) between predicted and observed liver, kidney and muscle were 0.95, 0.97 and 0.99, respectively. Based on liver tissue residue profiles, the pig model estimated a withdrawal interval of 10 h under a multiple oral dosing schedule (5.0 mg/kg, twice daily for 7.5 days). PBPK models, such as this one, provide evidence of the usefulness in interspecies PK data extrapolation over a range of dosing scenarios and can be used to predict withdrawal interval in pigs.     

Tulathromycin assay validation and tissue residues after single and multiple subcutaneous injections in domestic goats (Capra aegagrus hircus)

Clothier, K. A., Leavens, T., Griffith, R. W., Wetzlich, S. E., Baynes, R. E., Riviere, J. E., Tell, L. A. Tulathromycin assay validation and tissue residues after single and multiple subcutaneous injections in domestic goats (Capra aegagrus hircus). J. vet. Pharmacol. Therap.  35 , 113–120. Tulathromycin is a macrolide antimicrobial labeled for treatment of bacterial pneumonia in cattle and swine. The purpose of the present research was to evaluate tissue concentrations of tulathromycin in the caprine species. A tandem mass spectrometry regulatory analytical method that detects the common fragment of tulathromycin in cattle and swine was validated with goat tissues. The method was used to study tulathromycin depletion in goat tissues (liver, kidney, muscle, fat, injection site, and lung) over time. In two different studies, six juvenile and 25 market‐age goats received a single injection of 2.5 mg/kg of tulathromycin subcutaneously; in a third study, 18 juvenile goats were treated with 2.5, 7.5, or 12.5 mg/kg tulathromycin weekly with three subcutaneous injections. Mean tulathromycin tissue concentrations were highest at injection site samples in all studies and all doses. Lung tissue concentrations were greatest at day 5 in market‐age goats while in the multi‐dose animals concentrations demonstrated dose‐dependent increases. Concentrations were below limit of quantification in injection site and lung by day 18 and in liver, kidney, muscle, and fat at all time points. This study demonstrated that tissue levels in goats are very similar to those seen in swine and cattle.     

Modelling the concentration-time relationship in milk from cattle administered an intramammary drug

Whittem, T., Whittem, J. H., Constable, P. D. Modelling the concentration–time relationship in milk from cattle administered an intramammary drug. J. vet. Pharmacol. Therap.  35 , 460–471. Antimicrobial drugs are often infused directly through the streak canal into the bovine udder for the treatment or prevention of mastitis. These infusions have two major problems: drug residues in milk and variable antimicrobial efficacy. Both problems are influenced by the pharmacokinetics of intramammary delivery and elimination of drugs. This pharmacokinetics does not conform to the assumptions of traditional first‐order mamillary pharmacokinetic models. To help understand drug delivery into and elimination from the udder, a new approach to pharmacokinetic modelling of the udder is proposed. This new model was used to predict the movement of drug within the udder and the concentrations of drug achieved within physiological compartments of the udder. These predictions were examined using computer modelling. The model was evaluated using data from in vivo intramammary infusion of cefuroxime. The model predicts that changes in milking efficiency (residual volume), milk productivity and milking frequency can impact both the drug residue persistence and the time that milk drug concentrations exceed the minimum inhibitory concentrations for pathogens. The model provides a new tool for future evaluation of intramammary dosing studies.     

Subcutaneously implanted tissue chambers — a pharmacokinetic study

The purpose of this study was to characterize the pharmacokinetics of a subcutaneously implanted tissue-chamber model. Thermoplastic tissue chambers were implanted in the paralumbar fossae of six steers. Starting 30 days after implantation, the distribution of intravenously administered antipyrine and phenylbutazone into the tissue chambers was studied. These pharmacokinetic experiments were repeated 10 days later to determine the effect of time after implantation on tissue-chamber distribution. Fifty days after implantation, tissue chambers were drained of transudate, refilled with sterile saline and the rate of influx of endogenous urea, creatinine and albumin was measured. Delayed diffusion of antipyrine and phenylbutazone into tissue chambers was well described using a compartmental model in which tissue-chamber fluid represented the third of three compartments arranged in series. The distribution of antipyrine into tissue chambers was greater than that of phenylbutazone; an observation which is well correlated with the high degree of protein binding of phenylbutazone. There was no effect of time on the penetration of the two agents. Rapid diffusion of urea and creatinine and extremely slow influx of albumin into chambers showed that these chambers formed true interstitial compartments.