Pharmacokinetics and tissue penetration of tazobactam and piperacillin in patients undergoing colorectal surgery.

The pharmacokinetics of tazobactam and piperacillin in plasma and different tissues after a 30-min intravenous infusion of 4 g of piperacillin and 0.5 g of tazobactam were investigated in 18 patients who underwent elective colorectal surgery. Serial blood samples were collected for up to 6 h after the initiation of the infusion. The types of tissue collected were fatty tissue, muscle, skin, appendix, and intestinal mucosa (proximal and distal). On the basis of concentrations in plasma, the following pharmacokinetic parameter values were obtained (values are means +/- standard deviations): maximum concentration of drug in serum, tazobactam, 27.9 +/- 7.67 micrograms/ml; piperacillin, 259 +/- 81.8 micrograms/ml; time to maximum concentration of drug in serum, tazobactam, 0.51 +/- 0.03 h; piperacillin, 0.51 +/- 0.03 h; area under the concentration-time curve, tazobactam, 47.6 +/- 13.3 micrograms.h/ml; piperacillin, 361 +/- 80.3 micrograms.h/ml; clearance, tazobactam, 188 +/- 52.3 ml/min; piperacillin, 194 +/- 42.9 ml/min; half-life, tazobactam, 1.42 +/- 0.32 h; piperacillin, 1.27 +/- 0.24 h; apparent volume of distribution, tazobactam, 0.31 +/- 0.07 liter/kg of body weight; piperacillin, 0.29 +/- 0.06 liter/kg; volume of distribution at steady state, tazobactam, 0.28 +/- 0.04 liter/kg; piperacillin, 0.25 +/- 0.05 liter/kg. The concentrations of tazobactam and piperacillin in fatty tissue and muscle tissue were 10 to 13 and 18 to 30% of the levels in plasma, respectively. In skin, the concentrations of piperacillin were 60 to 95% of the levels in plasma, whereas the concentrations of tazobactam in plasma were 49 to 93% of the levels in skin tissue. The mean concentration of tazobactam in the investigated gastrointestinal tissues (appendix, proximal and distal mucosa) exceeded levels in plasma after 1 h, while piperacillin showed a mean penetration into these tissues of 43 and 53%. The mechanisms that can be used to explain the extent of penetration of piperacillin and tazobactam are discussed. Simple diffusion may take place in fatty and muscle tissue, while penetration into skin and gastrointestinal tissue is governed by more complex mechanisms which lead to differences in penetration between piperacillin and tazobactam. For all tissues investigated (except fatty tissue), the time course of the concentrations of both compounds was similar, with a peak in concentration at between 1 and 2 h after the start of infusion followed by a decline of concentrations that were almost parallel to the curves of the drug concentrations in plasma. In plasma and in all investigated tissues, piperacillin as well as tazobactam reached or exceeded the concentrations found to be effective in vitro.

the levels in plasma, whereas the concentrations of tazobactam in plasma were 49 to 93% of the levels in skin tissue. The mean concentrations of tazobactam in the investigated gastrointestinal tissues (appendix, proximal and distal mucosa) exceeded levels in plasma after 1 h, while piperacillin showed a mean penetration into these tissues of 43 to 53%. The mechanisms that can be used to explain the extent of penetration of piperacillin and tazobactam are discussed. Simple diffusion may take place in fatty and muscle tissue, while penetration into skin and gastrointestinal tissue is governed by more complex mechanisms which lead to differences in penetration between piperacillin and tazobactam. For all tissues investigated (except fatty tissue), the time course of the concentrations of both compounds was similar, with a peak concentration at between 1 and 2 h after the start of infusion followed by a decline of concentrations that were almost parallel to the curves of the drug concentrations in plasma. In plasma and in all investigated tissues, piperacillin as well as tazobactam reached or exceeded the concentrations found to be effective in vitro.
Tazobactam, a new penicillanic acid sulphone derivative which acts as an irreversible inhibitor of bacterial P-lactamases, has been developed for coadministration with piperacillin. In studies in vitro, it has been found that 4 p,g or less of tazobactam per ml is required to reduce the MICs of piperacillin from the resistant to the susceptible category for 90% of the Escherichia coli, Bacteroides fragilis, and Proteus strains tested; for Staphylococcus aureus, Haemophilus influenzae, and Branhamella catarrhalis, 1 p.g or less of tazobactam per ml is adequate.
Pharmacokinetic studies in healthy male volunteers indicated that 4 p.g or more of tazobactam per ml is maintained in plasma for 3 h after a 30-min infusion of 0.5 g of tazobactam in combination with 4 g of piperacillin (20).
To evaluate the therapeutic value of this new combination, it is necessary to study concentrations in plasma and tissue to confirm the presence of an effective antibacterial concentration at the site of infection. Six tissues were selected for this study so that the extent of penetration into tissue could be determined. The development of a new and highly sensitive dual-column high-pressure liquid chromatography (HPLC) method allowed, for the first time, a study of the * Corresponding author. tissue penetration of a ,B-lactamase inhibitor by measurement of concentrations based on a chromatographic separation.
In the study described here, we characterized the pharmacokinetic profiles of tazobactam and piperacillin and the penetration of these two agents into the tissues of patients undergoing colorectal surgery following a 30-min intravenous infusion of 4 g of piperacillin and 0.5 g of tazobactam.

MATERUILS AND METHODS
Patients. Eighteen patients (nine females, nine males) were enrolled in the study. The mean age was 66.8 + 12.0 years (range, 29 to 77 years; 17 of the 18 patients were 54 years or older), the mean height was 170 + 9.6 cm (range, 159 to 190 cm), and the mean body weight was 72.3 + 11.4 kg (range, 53 to 93 kg). Based on the calculation of creatinine clearance (CLCR) by the equation CLCR = [body weight. (140age)]/(0.814-concentration of creatinine in plasma) F, where F is 1.0 for males and 0.85 for females (2), all patients were found to have normal kidney function in relation to their age. Of the 18 patients studied, there were 6 patients with rectal cancers, 5 with colon cancers, 3 with sigmoid cancers, 2 with tubovillous adenomas, 1 with temporary transverse colostomy, and 1 with temporary ileostomy. All ANTIMICROB. AGENTS CHEMOTHER. patients underwent bowel resections; and the anesthesia was induced with morphine-scopolamine, atropine, midazolam, thiopental, fentanyl, and celocurin. Selected nontumorous healthy tissues (subcutaneous fatty tissue, muscle and skin from the abdomen, mucosa from the large intestine, and appendix) were taken from the patients if the particular tissue could be obtained.
The intravenous dosage forms containing piperacillin and tazobactam in a single syringe were prepared following the instructions provided with the clinical supplies. Intravenous doses were administered by using a syringe pump over a 30-min period. All infusion lines were then flushed with sterile saline after the termination of infusion to ensure that the entire dose had been administered.
The drugs that were coadministered because of the surgical procedure or because the patient took them chronically were mostly beta-blockers and drugs for the treatment of diabetes. None of those drugs is known to cause a potential pharmacokinetic interaction with either tazobactam or piperacillin or their combination. This assumption was made on the basis of the knowledge of the pharmacokinetics of both agents in the body (19,20).
Specimen collection. Blood samples (5 ml each) were collected predose (0 h) and 0.5 (end of infusion), 1, 2, 3, 4, 5, and 6 h after the initiation of the 30-min infusion. All blood samples were collected from the arm opposite the arm used for the intravenous infusion.
Blood samples were kept on ice and were centrifuged in a refrigerated centrifuge within 1 h of collection. The heparinized plasma was separated and placed into polypropylene tubes and immediately frozen and stored at -80°C at the study site.
Several samples of the selected type of tissue were collected during surgery, which was between 0.5 and 4 h after the initiation of the infusion. The exact time of sample collection was registered, and a corresponding blood sample was taken. The size of the tissue samples was at least 1 cm3 or larger. All samples were trimmed of connective tissue, attached blood was removed from the tissues by cleaning with a dry, sterile gauze, and the sample was weighed. The tissues were placed into sterile polypropylene tubes and stored at -80°C at the study site.
Storage of samples. The samples were stored at -80°C until analysis. The samples were analyzed within 16 to 167 days of collection. During this period, spiked plasma and tissue samples stored at -80°C showed no signs of degradation.
Sample preparation procedures. To control variations in drug preparations as well as variations in sample injection, an internal standard (for tazobactam, cefpodoxime at 10 ,ug/ml; for piperacillin, mezlocillin at 25 ,ug/ml) was used.
For tazobactam measurement in plasma, 250 RI of the sample was deproteinized by the addition of 500 ,ul of acetonitrile containing the internal standard. After mixing and centrifugation at 15,000 rpm (Sorvall centrifuge), the acetonitrile was removed by extraction with 1 ml of dichloromethane. The aqueous phase (120 ,u) was injected onto the HPLC system. For determination of piperacillin in plasma, 100 RI of the sample was stabilized with 100 RI of 0.1 M potassium dihydrogen phosphate buffer (pH 5.5). After mixing, the sample was deproteinized with 400 RI of acetonitrile containing the internal standard. The sample was then treated in the same way as the tazobactam samples were. The aqueous phase (40 RI) was injected onto the HPLC system.
Adhesive blood from the tissue samples, was dabbed with a small piece of blotting paper. The tissue was weighed, and a double amount of Milli-Q water was added. Then, the tissue samples were homogenized with an Ultra-Turrax homogenizer (IKA-Ultra-Turrax T 25; IKA-Labortechnik, Staufen, Germany) at 4°C for 30 s. The stability of the compounds in the tissue homogenates at room temperature was determined over 4 h. The processed samples then were stable over 48 h in the autosampler. We did not make any correction for blood contamination, because in preliminary investigationg in the laboratory, we found that when tissue samples are very carefully taken, blood contamination does not play a significant role. Centrifugation (15,000 rpm, 10 min; Sorvall centrifuge) of the homogenate yielded a clear supernatant, which was then treated like plasma.
HPLC systems. The tazobactam concentrations were measured by using a dual-column HPLC system with UV detection. The precolumn (RP-2, 10 ,um, 40 by 4.6 mm [inner diameter]; Bischoff Chromatography, Leonberg, Germany) was connected to the analytical column (Spherisorb ODS II, 5 ,m, 250 by 4.6 mm [inner diameter]; Bischoff Chromatography) via an automatic switching valve. The mobile phases consisted of 0.1 M sodium dihydrogen phosphate, 5 mM tetrabutylammonium hydrogen sulfate per liter, and 5% (precolumn) or 10% (analytical column) acetonitrile (pH 6.5). The flow rates of the precolumn and the analytical column were 1.0 and 1.5 ml/min, respectively. The temperature of the column bath was set at 25°C. The detection of tazobactam was obtained at 210 nm and that of cefpodoxime was obtained at 300 nm by using a GAT-LCD 502 detector (Gamma Analysen Technik GmbH, Bremerhaven, Germany). The retention times of the compounds were 18.6 and 24.9 min, respectively.
The plasma samples were measured against a plasma calibration row. Plasma samples in the calibration row were prepared by diluting a tested drugfree plasma sample 10:1 with a stock solution to obtain the highest calibration level. The other calibration levels were obtained by 1:1 dilution of the highest calibration level or a level of higher concentration with plasma. Samples with drug concentrations above the quantification limits were prediluted with tested drugfree plasma.
For control of interassay variation, spiked quality controls in plasma were prepared by adding defined amounts of the stock solution or the spiked control of higher concentration to defined amounts of tested drugfree plasma.
The tissue samples were measured against tissue calibration rows. Calibration standards and spiked quality control standards were prepared in the same manner as described above for sample preparation procedures. To ensure the similarity of the matrix composition of calibration standards and spiked quality control standards (ratio tissue:water), the calibration and quality control standards were prepared by adding twofold amounts of aqueous calibration solutions to tested drugfree tissues. Then, the tissue was homogenized and the resulting homogenate was centrifuged at 10,000 rpm for 10 min (Sorvall centrifuge). The supernatant was used as the calibration level or as the spiked quality control standard.
PHARMACOKINETICS OF TAZOBACTAM AND PIPERACILLIN 1999 No interferences were observed in plasma or tissues for tazobactam, piperacillin, or the internal standards. Calibration was performed by weighted (1/concentration) linear regression. The linearities of tazobactam and piperacillin calibration curves in plasma were proven between 0.096 and 52.1 ,.g/ml and between 0.386 and 100 p,g/ml, respectively. In tissues, linearity in the following concentration ranges were found: tazobactam, 0.076 to 32.5 ,ug/g; piperacillin, 0.10 to 68 ,ug/g. The quantification limits were identical with the lowest calibration levels.
The accuracies of the spiked quality controls in different tissues ranged from 102.4 to 105.5% for high tazobactam concentrations, from 94.3 to 105.2% for middle tazobactam concentrations, and from 99.1 to 106.7% for low tazobactam concentrations. For piperacillin the following values were found: 92.2 to 103.6% for high concentrations, 96.5 to 98.0% for middle concentrations, and 95.7 to 100.7% for low concentrations.
Pharmacokinetic calculations. The pharmacokinetic parameters of piperacillin and tazobactam were estimated by noncompartmental methods (6). All pharmacokinetic parameters were derived individually for each subject from the drug concentrations in plasma. Arithmetic means and standard deviations were calculated for all parameters.
Evaluation of tissue penetration. For characterizing the penetration of the study drugs into a specific tissue, tissue: plasma concentration ratios were calculated by dividing the study drug concentration in the tissue sample by the con-centration in a plasma sample collected at the time of tissue sampling.
Samples of a specific tissue were divided into groups according to collection time. In each group, all samples from this tissue type obtained in a defined interval after initiation of the infusion were included. For tazobactam and piperacillin, mean + standard deviation (SD) collection times, mean + SD concentrations in tissues, and mean + SD tissue:plasma concentration ratios were calculated for each sample group formed as described above.
For characterizing differences in the pharmacokinetic behaviors of tazobactam and piperacillin, the ratio of the concentrations of both compounds (piperacillin/tazobactam) was calculated for each tissue sample and for the corresponding plasma sample. The means + SDs of those ratios were calculated for each defined group.

RESULTS
Tazobactam pharmacokinetics. All concentrations of tazobactam measured in plasma are plotted against time in Fig.  1A. The mean values of the pharmacokinetic parameters are given in Table 1 Penetration of tazobactam and piperacillin into tissue. The mean concentrations and the tissue:plasma concentration ratios of tazobactam and piperacillin in the various tissues measured are shown in Fig. 2A and B. To assess and to compare the penetration of tazobactam and piperacillin into tissues, four different groups (with the exception of intestinal mucosa [proximal and distal] and appendix, which were only subdivided into two or three groups, respectively) were formed on the basis of the blood and/or tissue sampling time relative to the drug infusion. Mean + SD concentrations and mean + SD tissue:plasma concentration ratios in each group are given in Tables 2 and 3. The mean + SD piperacillin: tazobactam concentration ratios in plasma and tissues are given in Table 4.
The number of data available for calculations of the mean ratios in Tables 3 and 4 was, in many cases, smaller than the number of actual samples measured. This was because not more than 10% of tissue samples collected before or after plasma sampling were included in the calculations of the ratios. Fatty tissues. Tazobactam concentrations were between 1.46 0.667 ,ug/g for samples taken at 30 to 60 min after the start of infusion and 0.695 + 0.407 ,ug/g for samples taken at 151 to 270 min after the start of infusion. The concentrations of piperacillin were between 10.1 + 5.06 ,ug/g for samples taken at 30 to 60 min after the start of infusion and 3.95 ± 2.98 ,ug/g for samples taken at 151 to 270 min after the start of infusion. The mean tissue:plasma concentration ratios of tazobactam and piperacillin in fatty tissue ranged between 0.097 ± 0.053 and 0.128 ± 0.080 (tazobactam) and 0.088 ± 0.038 and 0.115 ± 0.070 (piperacillin). The piperacillin-to- tazobactam concentration ratios in fatty tissue increased from 7.35 + 2.43 at the 30to 60-min collection period to 7.82 + 1.08 at the 61to 90-min collection period and then decreased to 5.47 + 1.49 at the 151to 270-mmn collection period.
Muscle tissues. Tazobactam concentrations were between  2.43 + 0.917 ,ug/g for samples taken at 30 to 60 min after the start of infusion and 1.38 + 0.722 ,ug/g for samples taken at 151 to 270 min after the start of infusion. The concentrations of piperacillin were between 19.9 + 8.98 ,ug/g for samples taken 30 to 60 min after the start of infusion and 9.35 + 4.94 ,ug/g for samples taken at 151 to 270 min after the start of infusion. The mean tissue:plasma concentration ratios of tazobactam and piperacillin tended to show time dependency. They increased from the period 30 to 60 min after the start of infusion, when they were 0.180 + 0.069 (tazobactam) and 0.183 ± 0.068 (piperacillin), to the period 91 to 150 min after the start of infusion, when they were 0.295 ± 0.159 (tazobactam) and 0.288 ± 0.109 (piperacillin). In the last collection period (151 to 270 min), the tissue:plasma concentration ratios of tazobactam (0.248 ± 0.120) and piperacillin (0.284 ± 0.115) began to decline. After the mean piperacillin-totazobactam concentration ratios in muscle tissue increased from 8.05 ± 1.18 (30 to 60 min) to 8.64 ± 1.97 (61 to 90 min), they decreased to 7.82 ± 4.51 at the 151to 270-min collection period.
Skin tissues. Tazobactam concentrations were between 6.63 ± 3.06 ,ug/g for samples taken at 30 to 60 min after the start of infusion and 3.99 ± 2.61 ,ug/g for samples taken at 151 to 270 min after the start of infusion. The concentrations of piperacillin were between 65.0 ± 32.3 ,ug/g for samples taken at 30 to 60 min after the start of infusion and 34.8 -22.1 ,ug/g for samples taken at 151 to 270 min after the start of infusion. The tissue:plasma concentration ratios of tazobactam and piperacillin tended to show time dependency. The increase between 30 to 60 and 91 to 150 min after the start of infusion was from 0.492 ± 0.257 to 0.933 ± 0.867 (tazobactam) and from 0.603 ± 0.268 to 1.11 ± 0.360 (piperacillin). At 151 to 270 min after the start of infusion, the tissue:plasma concentration ratios of tazobactam (0.626 + 0.215) and piperacillin (0.952 ± 0.412) began to decline. In skin tissue, the piperacillin-to-tazobactam concentration ratios were between 12.7 ± 10.4 for samples taken at 30 to 60 min after the start of infusion and 10.5 ± 7J16 for samples taken at 151 to 270 min after the start of infusion.
Appendix. The fact that a sufficient number of samples were taken from the appendix allowed analysis of timedependent tissue penetration. For both tazobactam and piperacillin, clear peaks in levels in the appendix at the 61-to 90-min period over those in the preceding and following periods were observed.

DISCUSSION
This study addressed the pharmacokinetics and tissue penetration of a new beta-lactam-,-lactamase inhibitor combination. Previous studies in healthy volunteers (1,20) have shown that the pharmacokinetic parameters of piperacillin are unaltered when it is administered together with tazobactam. On the other hand, the pharmacokinetics of tazobactam were significantly affected by the coadministration of piperacillin. The mechanism of this interaction is most likely a competition of piperacillin and tazobactam for tubular transport in the kidney. Alterations in the volume of distribution of tazobactam following combined administration with piperacillin were also observed, but the mechanism for this remains obscure.
Only the fixed combination of piperacillin-tazobactam could be tested in this study of drug penetration in patient tissues. Therefore, we cannot speculate whether the extent of the pharmacokinetic interaction between piperacillin and tazobactam is different in this population of mostly older patients undergoing elective colorectal surgery. A comparison of the pharmacokinetic parameters calculated for our patients, for whom the mean creatinine clearance was 72.4 ± 21.3 ml/min, with those of healthy volunteers of Cheung et al. (1), who had normal creatinine clearances, shows the expected difference in the pharmacokinetic parameters. Unfortunately, in the work of Wise et al. (20), typographic errors prevented our use of those data for comparison. In addition, the total clearance of piperacillin in the young and healthy subjects in that study (20) was far below what is usually reported in the literature for the 4-g dose in volunteers (9,(16)(17)(18)(19)(20). The total clearance of tazobactam obtained by Wise et al. (20) was almost 40% less than that obtained by Cheung et al. (1) and was also less than that obtained in this study (extrapolated to normal kidney function). This makes that report (20) unusable for comparison of the basic pharmacokinetics but also for blister fluid penetration since the penetration into blister fluid is related to the plasma AUC, which reflects total clearance. In tissue penetration studies, the absolute concentration of drug in tissue, the time course of those concentrations, as well as the extent of penetration are of interest. The extent of penetration of piperacillin and tazobactam into tissues was based on the tissue-to-plasma concentration ratio at specific collection times.
The data for piperacillin found in our study are in close agreement with results reported previously (7). Although there were specific differences in penetration between tissues and the two compounds, which need interpretation, the overall penetration of piperacillin and tazobactam into nonepithelial tissues was grossly similar. The extent of penetration into fat (-10% of the levels in plasma) and muscle tissue (-20 to 30% of the levels in plasma) was almost identical for both compounds. Although the blood flow to fatty tissue when the body is at rest is higher than that to muscle (13), the penetration of both piperacillin and tazobactam into muscle tissue was higher. This may be explained by the eight times higher water content of muscle tissue compared with that of fatty tissue (11), into which water-soluble compounds like beta-lactam antibiotics may preferably diffuse. Also, the time course of drug levels in tissue may reflect the differences in blood flow and water content in those tissues. While levels of drug in fatty tissue were in equilibrium within 1 h following drug administration because of the low water content, it took more time for the concentrations of piper-acillin and tazobactam to peak and reach an equilibrium in muscle tissue.
Penetration of piperacillin and tazobactam into skin was 5 to 10 times higher than that into fatty tissue and about three times higher than that into muscle tissue. Skin has about the same water content as muscle tissue, so other mechanisms must contribute to this finding. Skin tissue, as it was collected in our study, is made up of several cell types and interstitial fluid and may thus be a biophase which is entirely different from muscle and fat, which each consist of only one cell type. Although both compounds share the beta-lactam structure, piperacillin, with its bulky substituent at position 6, may have a different affinity than tazobactam to tissues like skin when the affinities are compared with those to fatty tissue. Similar findings of high penetration of beta-lactams into skin were also reported by other investigators (7). The exact mechanism that causes this important finding needs further investigations.
The penetrations of piperacillin-tazobactam into three sites of the gastrointestinal (GI) tract were also investigated. The blood flow to the GI tract is about 10 times higher than that to muscle at a comparable water content. Based on this fact, the penetration of drug into the GI tract should be the highest of those of all tissues studied. This was, in fact, the case for tazobactam. The concentrations of tazobactam in the distal mucosa were up to two times higher than those in plasma, but this was not the case for piperacillin, which had a very homogeneous penetration of about 50% into all three segments of the GI tract studied. The understanding of the penetration of beta-lactam antibiotics into the GI tract is, however, further complicated by the fact that, compared with skin, muscle, and fat, the GI tract is lined by epithelial cells which can actively take up and secrete acids or bases (12,15). According to the results of this study, the uptake of a drug into epithelial cells is governed by the structural and physicochemical aspects of the compound. Blood flow to the organ would more indirectly support the possible uptake of piperacillin and tazobactam by supplying enough oxygen and nutrients to the cells. In addition, the results of a basic pharmacokinetic interaction study (1) suggest very strongly that piperacillin inhibits the process of elimination of tazobactam from the GI tract. We found no difference in tissue penetration between the three sites (proximal and distal intestine, appendix) for piperacillin, although these sites differ significantly in their mucosal fine structures. A difference was found for tazobactam, with the highest level of penetration being into the distal intestinal mucosa. These findings may suggest that both tazobactam and piperacillin are actively taken up by the serosal site of mucosal cells and that piperacillin does not inhibit this process. Piperacillin may, however, inhibit the secretion of tazobactam at the luminal site of the cell, which could, consequently, lead to the accumulation of tazobactam in the mucosa. Summarizing all the results from our investigation of penetration of drugs into tissue, it has become evident that no single mechanism can explain the extent of penetration of drugs into tissue. Further studies will have to establish quantitative relationships among blood flow, water content, the biochemical composition of the tissue, and the physicochemical properties of the agent.
While the preceding discussion was based on the mechanisms of penetration, it is the absolute concentrations of tazobactam and piperacillin and the ratio between the two concentrations which are important for the prediction of antibacterial activity. When the bias of any tissue penetration study of beta-lactam antibiotics caused by the work-up VOL. 36,1992 procedures that cause dilution of the extracellular concentration by cell lysis is considered, the data demonstrate again that the free piperacillin concentrations in all tissues exceeded the MICs for most microorganisms that are susceptible to the agent. The addition of tazobactam when the microorganisms produce ,3-lactamases has led in vitro to drastic reductions in otherwise elevated MICs (1,3,4,8,10). Our data show that the concentration ratios used to determine the effects in vitro are also achieved or even exceeded at the tissue site of infection. The exact relationship between levels of antibiotics at infection sites and their effects is not yet fully established for beta-lactam antibiotics alone (5,14), and thus, the relationship for a combination may be far from established.
In conclusion, the introduction of new antimicrobial agents requires information on not only clinical efficacy but also the basic pharmacokinetics of the agents. Since pharmacokinetic data from prospective studies by validated assays are usually less variable than clinical efficacy data and since they are less affected by the complex clinical situation, there is a great deal of trust in the prediction of pharmacodynamic effects. In the present investigation, one of the agents, piperacillin, has a long and proven role in the treatment of severe infections. This was confirmed in this study again by levels in blood and tissue that exceeded the MICs for virtually all pathogenic microorganisms. These levels ensure clinical efficacy in infections in which susceptible microorganisms are involved. Clinical situations that require the addition of a ,-lactamase inhibitor are those in which microorganisms are resistant to piperacillin because they produce a ,B-lactamase. The high levels of piperacillin alone in tissue are therefore not high enough to eradicate those microorganisms. The levels of tazobactam necessary to obtain anti-,B-lactamase activity at the site of infection were achieved in the present investigation. Therefore, the data on the pharmacokinetics of piperacillin and tazobactam presented here support their clinical efficacies seen in clinical trials.