Diastereomeric beta-lactam antibiotics. Analytical methods, isomerization and stereoselective pharmacokinetics

Exploiting pH mismatch in preparative high-performance liquid chromatographic recovery of ertapenem from mother liquor streams

Preparative chromatography was successfully employed to recover ertapenem from mother liquor streams. The recovery process involved concentration of mother liquor stream by evaporation, purification by reversed-phase preparative high-performance liquid chromatography (HPLC), and removal of chromatographic solvents in the recovered fractions by evaporation. HPLC feed was prepared by stripping off the organic solvents from the mother liquor using a wiped-film evaporator. Purification was first carried out on a 25 cm x 0.46 cm analytical column packed with 10-microm Kromasil C8 particles and then scaled up to a 25 cm x 5 cm preparative column. Gram-level recovery of ertapenem with high purity was achieved by exploiting a novel approach based on pH mismatch between the feed and the eluent. Purified ertapenem streams from preparative HPLC runs were combined, evaporated and recycled into the crystallizer for ertapenem isolation.

Stereoselective binding and degradation of sulbenicillin in the presence of human serum albumin

Binding of sulbenicillin (SBPC) isomers to human serum albumin (HSA) was stereoselective. There were at least two classes of binding sites on HSA for SBPC isomers. At the stereoselective high affinity site, binding was in favor of R-SBPC, the binding constant of R-SBPC being approximately 2.3-fold greater than that of S-SBPC. By using site marker ligands, it was revealed that the stereoselective site was Site I (warfarin binding site). Affinity for the low affinity (nonstereoselective) site was similar for the diastereomers, approximately 7--30-fold lower than for the stereoselective site. R-SBPC and S-SBPC appeared to displace each other competitively at both binding sites. On the other hand, R-SBPC was degraded much faster than S-SBPC in the presence of HSA, with a degradation rate constant approximately 7-fold greater for R-SBPC than for S-SBPC. The degradation of R-SBPC was inhibited in the presence of warfarin and dependent on the concentration of R-SBPC bound to Site I. The results demonstrate that Site I is responsible for the stereoselective degradation.

Role of the monocarboxylic acid transport system in the intestinal absorption of an orally active beta-lactam prodrug: carindacillin as a model

Transport of carbenicillin (CBPC) and its orally active prodrug (carindacillin, CIPC) was studied with rat intestinal brush border membrane vesicles (BBMV). CIPC was transported uphill into BBMV in the presence of a H(+) gradient, indicating that CIPC absorption is carrier-mediated. Indeed, CIPC was predominantly transported by the monocarboxylic acid transport system, although it might be possible that CIPC possesses some affinity to the oligopeptide transporter. In contrast, CBPC exhibited no affinity to either the oligopeptide or the monocarboxylic acid transport system. Apparent uptake clearance of CIPC was approximately 70-fold greater than that of CBPC. It was clarified that the modification of the chemical structure of CBPC (a dicarboxylic acid) to CIPC (a monocarboxylic acid) by ester formation may have resulted in the increased affinity to the monocarboxylic acid transport system, which, in turn, led to improved absorption of the prodrug.

In vivo efficacies of combinations of beta-lactams, beta-lactamase inhibitors, and rifampin against Acinetobacter baumannii in a mouse pneumonia model

The effects of various regimens containing combinations of beta-lactams, beta-lactam inhibitor(s), and rifampin were assessed in a recently described mouse model of Acinetobacter baumannii pneumonia (M. L. Joly-Guillou, M. Wolff, J. J. Pocidalo, F. Walker, and C. Carbon, Antimicrob. Agents Chemother. 41:345-351, 1997). Two aspects of the therapeutic response were studied: the kinetics of the bactericidal effect (treatment was initiated 3 h after intratracheal inoculation, and bacterial counts were determined over a 24-h period) and survival (treatment was initiated 8 h after inoculation, and the cumulative mortality rate was assessed on day 5). Two clinical strains were used: a cephalosporinase-producing strain (SAN-94040) and a multiresistant strain (RCH-69). For SAN-94040 and RCH-69, MICs and MBCs (milligrams per liter) were as follows: ticarcillin, 32, 64, 256, and >256, respectively; ticarcillin-clavulanate, 32, 64, and 512, and >512, respectively; imipenem, 0.5, 0.5, 8, and 32, respectively; sulbactam, 0.5, 0.5, 8, and 8, respectively; and rifampin, 8, 8, 4, and 4, respectively. Against SAN-94040, four regimens, i.e., imipenem, sulbactam, imipenem-rifampin, and ticarcillin-clavulanate (at a 25/1 ratio)-sulbactam produced a true bactericidal effect (>/=3-log10 reduction of CFU/g of lung). The best survival rate (i.e., 93%) was obtained with the combination of ticarcillin-clavulanate-sulbactam, and regimens containing rifampin provided a survival rate of >/=65%. Against RCH-69, only regimens containing rifampin and the combination of imipenem-sulbactam had a true bactericidal effect. The best survival rates (>/=80%) were obtained with regimens containing rifampin and sulbactam. These results suggest that nonclassical combinations of beta-lactams, beta-lactamase inhibitors, and rifampin should be considered for the treatment of nosocomial pneumonia due to A. baumannii.

Epimerization of moxalactam by albumin and simulation of in vivo epimerization by a physiologically based pharmacokinetic model

We investigated the mechanism of epimerization (R to S or S to R) of moxalactam in serum of rats, dogs, and humans. The epimerization of moxalactam occurred in the serum of these animals, but not in the serum filtrate. The albumin fraction of human serum purified by gel filtration catalysed the epimerization of moxalactam at an identical rate to serum, but other fractions (i.e., lipoproteins and globulins) showed slower epimerization. alpha 1-acid glycoprotein, which was eluted in the same fraction with albumin by G-200 gel filtration, did not epimerize moxalactam. The presence of 2 mM warfarin decreased the binding of R- and S-moxalactam and decreased the epimerization of moxalactam in human serum. These results demonstrate moxalactam was epimerized on the warfarin binding site on albumin in serum. Additionally, a physiologically based pharmacokinetic model shows that the epimerization of moxalactam after administration in dogs is simulated by the epimerization in serum.

Simple and rapid analytical method for carbapenems using capillary zone electrophoresis

A simple and rapid analytical method for carbapenems using high-performance capillary electrophoresis is described. All therapeutic carbapenem injections in Japan (imipenem, panipenem and meropenem) and four other beta-lactams (piperacillin, cefotiam, cefotaxime, latamoxef) were separated and determined with good repeatability in about 10 min using simple free zone capillary electrophoresis. The electrophoresis buffer was 100 mM phosphate buffer of pH 8.0, and a fused-silica capillary of 25 microm I.D. and 47 cm length was adopted. The present method was successfully applied to monitor the degradation of carbapenems under various conditions (at various temperatures or in coexistence with other drugs prescribed in the case of methicillin-resistant Staphylococcus aureus).

Stereoselective renal secretion of carbenicillin in rabbits: role of the organic anion transporter at the renal brush border membrane

Stereoselectivity in the renal secretion of carbenicillin (CBPC) was studied in rabbits. Significant renal secretion of CBPC was observed in vivo, with the secretion of the S-epimer being greater than that of the R-epimer. Stereoselective transport of CBPC was further studied in vitro using basolateral and brush border membrane vesicles prepared from rabbit kidneys. The transport of CBPC by the organic anion transporter into the basolateral membrane vesicles (BLMV) was not stereoselective. In contrast, a distinct stereoselectivity was observed in the transport of CBPC by the organic anion transporter into the brush border membrane vesicles (BBMV), with the transport of the S-epimer being more favorable. Significant epimer-epimer interactions were also observed in the transport into BBMV. The stereoselectivity of the transport of CBPC was calculated from the kinetic parameters with consideration of epimer-epimer interactions and was similar to that observed in vivo. It was concluded that the observed stereoselectivity in the renal secretion of CBPC in vivo reflected that of transport via the organic anion transporter located at the brush border membrane.

Stereoselective disposition of sulbenicillin in humans

Stereoselective disposition of sulbenicillin (SBPC) epimers in healthy human volunteers was studied in order to clarify the differences in pharmacokinetic behavior between the epimers. Stereospecific high-performance liquid chromatography was used for the determination of SBPC epimers. Plasma protein binding was measured in vitro with an ultrafiltration method. The binding was stereoselective, with the unbound fraction (fu) of the R-epimer being approximately 1.3-fold greater than that of the S-epimer. SBPC was administered intravenously to human volunteers, and concentrations of SBPC in plasma and urinary excretion rates were measured. Renal clearance (CLR) for the unbound drug (approximately 400 ml/min) was greater than the glomerular filtration rate (GFR) (approximately 109 ml/min) for both epimers, suggesting that both epimers are secreted at the renal tubules. Renal tubular secretion appeared to be greater for the S-epimer. When probenecid was coadministered, the CLR values of both epimers were significantly reduced and were approximately equal to the GFR values. CLR was greater for the S-epimer (37.5 and 49.8 ml/min for R-SBPC and S-SBPC, respectively), which was simply due to the greater fu of the S-epimer in plasma. In contrast, total body clearance was greater for the R-epimer (67.8 and 56.3 ml/min for R-SBPC and S-SBPC, respectively) because of the stereoselective degradation of the R-epimer in plasma. It was revealed that stereoselective degradation in the body had significant influence on the disposition of SBPC epimers.