Pharmacology & Pharmacy, 2012, 3, 201-206 201 http://dx.doi.org/10.4236/pp.2012.32027 Published Online April 2012 (http://www.SciRP.org/journal/pp) Possible Factors Involved in Oral Inactivity of Meropenem, a Carbapenem Antibiotic Toshihide Saito, Rinako Sawazaki, Kaori Ujiie, Masako Oda, Hiroshi Saitoh* Department of Pharmaceutics, School of Pharmaceutical Sciences, Health Sciences University of Hokkaido, Ishikari-Tobetsu, Hok- kaido, Japan. Email: *[email protected] Received January 15th, 2012; revised February 12th, 2012; accepted March 5th, 2012 ABSTRACT Meropenem, a carbapenem antibiotic, is inactive after oral administration and administered exclusively by injection. In this study, in order to address the factors involved in the oral inactivity of meropenem, in vitro permeation characteris- tics across rat ileal segments was investigated using diffusion cells. Moreover, stability of meropenem was evaluated in the Japanese Pharmacopoeia (JP) 1st and 2nd fluid for disintegration test. Cefotaxime, ceftibuten, and faropenem were used for comparison. The permeation of meropenem across rat ileal segments was approximately 5-fold greater in sec- retory direction than in absorptive direction. The secretory-oriented transport of meropenem markedly diminished by replacement of D-glucose in the experimental medium with unmetabolizing 3-O-methyl-D-glucose, suggesting that the secretory transport of meropenem was an energy-dependent process. Cefotaxime exhibited extensively secretory-ori- ented permeation. On the other hand, much weaker directionalities were observed in ceftibuten and faropenem. While meropenem as well as other three β-lactam antibiotics was stable in JP 2nd fluid (pH 6.8), it declined rapidly in JP 1st fluid (pH 1.2). These results suggest that, in addition to the hydrophilic property of meropenem, its instability at gastric pH and secretory transport in the small intestine are possible factors involved in the inactivity of meropenem after oral administration. Keywords: Meropenem; Oral Inactivity; Secretory Transport; Degradation; Gastric pH 1. Introduction that a secretory transporter different from P-glycoprotein (MDR1, ABCB1) was involved in limiting absorption of Recently, various carbapenem antibiotics have been de- -lactam antibiotics such as cefazolin, cefoperazone, and veloped to overcome microbial resistance to cephems cefaloridine [8,9]. It is, therefore, feasible that similar se- and penicillins [1-3]. However, currently available carba- cretory transport system potentially interferes with the penems such as imipenem, meropenem, panipenem, bia- penem and doripenem are orally inactive and, thus, ad- translocation of carbapenems from intestinal lumen into ministered exclusively by injection. The lack of oral ac- portal vein. Until now, however, little information has been tivity of carbapenems is attributed mainly to their hy- available about involvement of these factors in the oral drophilic properties, which disadvantage their passive inactivity of carbapenems. Also, some factors besides low diffusion across the intestinal epithelium. Development lipophilicity may be responsible for the poor availability of oral dosage forms of carbapenems is considered to be of carbapenems following oral administration. For ex- beneficial to the treatment of outpatients suffering from ample, degradation in the gastrointestinal lumen or intes- infectious diseases. Therefore, prodrug approaches have tinal epithelium often becomes a critical reason for poor been undertaken to overcome the poor oral availability of oral availability. It has long since been shown that some carbapenems [4]. Some prodrugs have been reported to -lactam antibiotics are unstable in aqueous solution [10]. afford increases in the oral availability of parent carba- Meropenem was introduced into clinical use world- penems [5,6]. wide in the 1990s as a parenteral carbapenem [11]. This There are many reports demonstrating that various ef- carbapenem exerts an excellent intrinsic stability to hu- flux transporters, which locate on the apical membrane of man renal dehydropeptidase I (DHP-I), which is known enterocytes, behave as potent absorption barriers against to extensively inactivate imipenem and panipenem [12]. clinically relevant drugs [7]. Previously, it was shown Although some carbapenemases have been reported to *Corresponding author. compromise the efficacy of meropenem [13], the rela- Copyright © 2012 SciRes. PP 202 Possible Factors Involved in Oral Inactivity of Meropenem, a Carbapenem Antibiotic tively low toxicity of meropenem has led to its increasing pre-warmed to approximately 37˚C. The diffusion cells use in the treatment of serious infections in adult and were fixed on exclusive heating blocks and their tem- pediatric patients [14]. In this study, using meropenem as perature was maintained at around 37˚C during the ex- a model carbapenem, we addressed factors restricting periment. The fluid in each compartment was circulated oral availability of carbapenems, with special interest in by gas lift with 95% O2 - 5% CO2 and 0.5 mL receiver its permeation manner across the rat intestinal segments solution was taken at designated time points until 120 and its stability at the gastrointestinal pH. min for the measurement of drug that permeated either in the mucosal-to-serosal (M-to-S, absorptive) or serosal-to- 2. Experimental mucosal (S-to-M, secretory) directions. After each sam- 2.1. Materials pling, drug-free Tyrode’s solution (0.5 mL) was added to receiver compartment. The pH of mucosal and serosal Meropenem trihydrate (Meropen® for intravenous drip solution was set at 6.5 and 7.4, respectively. infusion) was purchased from Dainippon Sumitomo Pha- The cumulative permeation in either direction was rma Co. (Osaka, Japan). Cefotaxime sodium and glycyl- plotted vs time for each permeation experiment. The tryptophan were obtained from Wako Pure Chem. Ind. permeation rate was calculated from the slope of the li- (Osaka, Japan) and Sigma (St. Louis, MO, USA), respec- near portion of the cumulative permeation vs time plot. tively. Faropenem sodium and ceftibuten were kindly The tissue mounting process was completed within 30 supplied by Daiichi Suntory Pharma Co. (Tokyo, Japan) min after resection of the small intestine. and Shionogi & Co. (Osaka, Japan). Other reagents were of the highest grade available. 2.3. Stability Experiments Using the Japanese Pharmacopoeia (JP) 1st and 2nd Fluid for 2.2. In Vitro Permeation Experiments Using Rat Disintegration Test Ileal Segments The JP 1st and 2nd fluid for disintegration test were pre- In the present study, principles of good laboratory animal pared according to JP XV. To prepare the 1st fluids, 2 g care were followed and animal experimentation was per- of sodium chloride and 7 mL of hydrochloric acid were formed in compliance with the Guidelines for the Care first dissolved in 100 mL of purified water and then di- and Use of Laboratory Animals in the Health Sciences luted to 1000 mL with purified water. To prepare the 2nd University of Hokkaido. fluid, 118 mL of 0.2 M sodium hydroxide solution was Unless otherwise mentioned, Tyrode’s solution com- added to 250 mL of 0.2 M potassium dihydrogen phos- prising of 137 mM NaCl, 3 mM KCl, 2 mM CaCl2, 2 mM phate (KH2PO4) solution and then diluted to 1000 mL MgCl2, 12 mM NaHCO3, 0.4 mM NaH2PO4 and 6 mM with purified water. The pH of the 1st and 2nd fluid was D-glucose was used as experimental medium. In vitro 1.2 and 6.8, respectively. Drugs were dissolved in the 1st permeation experiments using rat ileal segments were and 2nd fluid at a concentration of 10 µM and incubated performed using diffusion cells (Corning Corstar, Acton, at 37˚C for 6 h. At designed time points, the drug con- MA, USA) as described previously [9]. The surface area centration in each solution was determined. available for permeation was 1.78 cm2. After overnight fasting, male Wistar rats (Hokudo, Sapporo, Japan) 2.4. Degradation Experiments Using Supernatant weighing 300 - 400 g were anesthetized with ether and Fluid Obtained by Centrifugation of the entire small intestine was quickly removed and rinsed Mucosal Homogenates in ice-cold saline. The 10 cm portion immediately distal to the pylorus was discarded and the remaining small Under ether anesthesia, the entire small intestine was qui- intestine was cut in two. The contents of the lower part of ckly isolated. The mucosa was scraped off with a spatula the small intestine were flushed out, and it was kept in and homogenized in 2 mL of Tyrode’s solution (pH 7.4). ice-cold saline as ileal segment. The ileal segment was The homogenates obtained were diluted to 10 mL with trimmed to approximately 4 cm in length without includ- Tyrode’s solution (pH 7.4) and centrifuged at 27,000× g ing Peyer’s patches and replaced in ice-cold Tyrode’s for 30 min at 2˚C. An aliquot (0.5 mL) of the resultant solution, which had been fully gassed with 95% O2 - 5% supernatant fluid was taken and mixed with an equal CO2, until use. Each piece was cut open longitudinally volume of meropenem solution (10 µM) and the mixture and its mucosal surface was gently washed with ice-cold was then stood at 37˚C. At designated time points, the saline. The piece was then mounted onto the pins of the mixture was mingled with 1 mL of methanol, stood for 5 diffusion cells, and the cell halves were clamped together. min in ice-cold water, and then centrifuged at 27,000× g To donor compartment was added 7 mL of drug solution for 30 min at 2˚C. The resultant supernatant fluid was (1 mM), and to receiving compartment was added 7 mL used for meropenem assay. For time 0 sample, methanol of drug-free Tyrode’s solution. Both solutions had been was added immediately after mixing the supernatant fluid Copyright © 2012 SciRes. PP Possible Factors Involved in Oral Inactivity of Meropenem, a Carbapenem Antibiotic 203 40 and meropenem solution. Ileum (M to S) 2.5. Determination of Drugs by HPLC Ileum (S to M) 30 All drugs were assayed using a Shimadzu LC-10A HPLC system (Shimadzu, Kyoto, Japan) equipped with a Shi- madzu SPD-10A spectrophotometer.