Environ. Sci. Technol. 2006, 40, 721-726
Stereoselective Degradation Kinetics of Theta-Cypermethrin in Rats QIUXIA WANG, JING QIU, WENTAO ZHU, GUIFANG JIA, JUNLING LI, CHENGLU BI, AND ZHIQIANG ZHOU* Department of Applied Chemistry, China Agricultural University, Beijing 100094, China
The enantioselective degradation and chiral conversion of theta-cypermethrin (TCYM) in rats have been studied via intravenous (iv) injection. The degradation kinetics and the enantiomer fraction (EF) were determined by means of normal-phase high-performance liquid chromatography (HPLC) with diode array detection (DAD) and a cellulosetris-(3,5-dimethylphenylcarbamate)-based chiral stationary phase (CDMPC-CSP). The degradation followed pseudofirst-order kinetics. The degradation of the (+)-TCYM was much faster than that of the (-)-TCYM in plasma, heart, liver, kidney, and fat after administration of racemic TCYM (rac-TCYM). The EFs were over 0.500 in these tissues and muscle. The results showed the conversion of (+)enantiomer to (-)-enantiomer in plasma after injection of (-)- and (+)-TCYM separately. The results for the major differences in the degradation of the enantiomers may have some implication for the environmental and ecological risks assessment for chiral pesticides.
Introduction Many pesticides in use are chiral compounds containing stereoisomers. The biological transformation of chiral compounds can be stereoselective, and the uptake, metabolism, and excretion of enantiomers may also be quite different (1, 2). Such enantioselectivity has been shown to result in different distribution patterns (3-5) and bioaccumulation potentials between enantiomers in the environment (6, 7). Achiral analysis gives only partial information, and the enantioselective analysis is often required for a full understanding of the biological behavior of such compounds. Information on the stereoselective degradation kinetics and bioaccumulation of chiral pesticides will help improve our understanding of the pesticide’s safety to humans, animals, and the environment. Cypermethrin (CYM) [(RS)-R-cyano-3-phenoxybenzyl (1RS)-cis-trans-3-(2, 2-dichlorovinyl)-2,2-dimethyl-cyclopropanecarboxylate] contains two chiral carbons in the cyclopropyl ring and one chiral position at the R-cyano carbon. In CYM, it is known that (S)-(1R)-cis and (S)-(1R)-trans are the only isomers with insecticidal activity (8). Isomer selective degradation of CYM has been previously reported in soil (9) and sediment (10). However, isomer selectivity in other environmental processes, e.g., degradation in animals, has been largely ignored so far in the risk assessment for CYM. Theta-Cypermethrin (TCYM) [CAS Registry No. 7169759-1] is a racemic mixture of (-)-(R)-(1S)-trans-[(-)-TCYM] * Corresponding author phone: +8610-62731294; fax: +861062732937; e-mail:
[email protected]. 10.1021/es052025+ CCC: $33.50 Published on Web 01/04/2006
2006 American Chemical Society
and (+)-(S)-(1R)-trans-[(+)-TCYM] enantiomers (Figure 1). Enantioselective degradation and chiral conversion of TCYM have not been reported yet. Thus, this research was conducted to determine the stereoselectivity of two TCYM enantiomers in rats. Findings from this study may be used to better understand the stereoselective biodegradation of CYM isomers in animals. The results may have some implication for the environmental and ecological risks assessment for chiral pesticides.
Experimental Section Chemicals and Reagents. rac-TCYM (>98% purity) was provided by the China Ministry of Agriculture’s Institute for Control of Agrichemicals. (-)-TCYM and (+)-TCYM were prepared on an Agilent HPLC with a preparatory chiral column (CDMPC-CSP, provided by the Department of Applied Chemistry, China Agricultural University, Beijing). Water was purified by a Milli-Q system. Acetonitrile, nhexane, and 2-propanol (HPLC grade) were from Fisher Scientific (Fair Lawn, NJ), and n-hexane (analytical grade) was from VAS (Beijing, China). HPLC-DAD Analysis. Chromatography was performed using an Agilent 1100 series HPLC (Agilent Technology, U.S.A.) equipped with a G1311A pump, G1322A degasser, G1328A injector, a 20-µL sample loop, and G1315A DAD. The signal was received and processed by an Agilent chemstation for 3D LC. Enantiomers were separated on CDMPC-CSP (provided by the Department of Applied Chemistry, China Agricultural University, Beijing) using a mobile phase of 99% hexane and a modifier of 1% 2-propanol with a flow rate of 0.8 mL/min. The capacity factor (k), selectivity (R), and resolution (Rs) were calculated from the formulas k ) (t - t0)/t0, R ) k2/k1, Rs ) 2(t2 - t1)/(W1 + W2), where t was the retention time and t0 was the void time at given conditions, and W was the baseline peak width. The CSP was prepared according to the procedure described in the literature (11, 12). CSP was packed into a 250 mm × 4.6-mm (i.d.) stainless steel column. Chromatographic separation was conducted at 25 °C and DAD detection at 230 nm. HPLC-Chiroptical Detection Analysis. The optical rotations of the TCYM enantiomers were not determined before and were distinguished by an HPLC system (Jasco, Tokyo, Japan), equipped with a PU-2089 quaternary gradient pump, manual injector, a 20-µL sample loop, and a CD-2595 circular dichroism detector. The separation conditions were the same as described for the Agilent 1100 series HPLC. Sample Preparations. Analysis of TCYM was performed using the methods for CYM in bovine tissues (13). Aliquots (1 mL) of the rat plasma or 1 g of homogenized tissue matrix was weighed into a 50 mL polypropylene centrifuge tube. About 10 mL of acetone/n-hexane (1:1) was added, and the sample was extracted with a shaker for 30 min. After centrifugation at 4000 rpm for 15 min, the clear solution was decanted into a 250 mL separatory funnel. The extraction and centrifuge steps were repeated with another 10 mL of acetone/hexane (1:1). The combined extracts were partitioned in a separatory funnel with 20 mL of n-hexane and 20 mL of aqueous sodium chloride solution (40 g/L). The n-hexane layer was collected, concentrated to 4 mL, and partitioned with 3 × 4 mL of acetonitrile. The sample solution in acetonitrile was then evaporated to near dryness under reduced pressure at 40 °C and reconstituted in 2 mL of hexane. The samples were cleaned up by solid-phase extraction on a silica cartridge (SI 1g). The SI cartridge was conditioned with 5 mL of 60% ethyl acetate in hexane and 5 mL of hexane. VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Chemical structures of theta-cypermethrin enantiomers.
FIGURE 2. Representative HPLC chromatograms of (A) extract from untreated rat plasma, (B) extract from untreated rat plasma spiked with rac-TCYM (0.5 µg/mL), (C) extract from a plasma sample collected from a rat 1 min after iv treatment with (+)-TCYM at 20 mg/kg bd wt, (D) extract from a plasma sample collected from a rat 1 min after iv treatment with rac-TCYM at 20 mg/kg bd wt, (E) extract from a fat sample collected from a rat 30 min after iv treatment with rac-TCYM at 20 mg/kg bd wt (n-hexane/2-propanol ) 99:1, flow rate ) 0.8 mL/min). After sample loading, the cartridge was washed with 6 mL of 2.5% ethyl acetate in hexane, and the residue was eluted with 6 mL of 5% ethyl acetate in hexane, evaporated, and reconstituted in 0.25 mL of hexane. A 20-µL aliquot was injected into the HPLC. Calibration Curves and Assay Validation. Plasma (1 mL) obtained from untreated rats was spiked with working 722
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standard rac-CYTM solutions to generate calibration samples ranging from 0.1 to 50 µg/mL for both (-)- and (+)-TCYM. Calibration samples were prepared as described above. Calibration curves were generated by plotting the peak area of each enantiomer versus the concentration of the enantiomer in the spiked samples. Linear regression analysis was performed using Microsoft Excel. The precision and accuracy
of the method were obtained by comparing the predicted concentration (obtained from the calibration curve) to the found concentration of each enantiomer spiked in blank plasma. The standard deviation (SD) and the coefficient of variation (CV ) SD/mean) were calculated over the entire calibration range. The within-day precision was determined in six replicates at concentration of 1, 5, and 30 µg/mL on the same day. The between-day precision was evaluated in six replicates at the above concentrations on six different days. The recoveries of each enantiomer of TCYM from plasma and tissue samples were determined analyzing quality control samples at three different concentrations. The peakarea ratios of six extracted samples at each concentration were compared with those of six injections of standard solutions to derive a percent recovery. The limit of quantification (LOQ) was defined as the lowest concentration in the calibration curve with acceptable precision and accuracy (The acceptance criteria for the LOQ were that the precision and accuracy for extracted samples were under 20% variability). Degradation Studies. Male Wistar rats weighing 200250 g (provided by the Experimental Animal Research Institute of China Agriculture University) were housed under a 12-h light/dark cycle at 22 °C. Twelve hours before experiments, the rats were fasted but had free access to water. Forty rats were divided into four groups of ten each; TCYM was dissolved in ethanol and then diluted to 20 mg/mL with normal saline, and the percentage of ethanol in the whole system was 85%. Racemic TCYM was administered at 20 mg/ kg body weight (bd wt) by iv injection in the tail vein. The groups were sacrificed at 0 (blank), 1, 5, 15, 20, 30, 60, 120, 180, and 240 min after treatment, and one time point corresponds to one animal. Four mL of blood was collected and centrifuged at 4000 rpm for 5 min, and the plasma was transferred to a new tube. The heart, kidney, liver, lung, fat, muscle, spleen, and brain of each rat were excised and weighed separately. Plasma and tissue samples were stored at -20 °C for later analysis. To determine the chiral conversion of the two TCYM enantiomers in plasma, two groups of four male rats were administered 20 mg/kg bd wt of (-)- and (+)-TCYM in the same manner as mentioned above separately, and blood samples were taken at 1 and 15 min after injection. The degradation experiments were all performed in duplicate on two occasions. Degradation Kinetics Analysis. The degradation of both TCYM enantiomers appeared to follow a pseudo-first-order kinetic reaction, and the degradation rate constants were derived from “ln(C0/C) versus t” plots by regression analysis for experiment (Excel 2000, Microsoft, Inc.). The starting point was the maximum value of the concentration. The half-life (T1/2, min) was estimated from equation I.
T1/2 ) 0.693/k
(I)
The enantiomer fraction (EF) which is considered superior to the enantiomer ratio (ER) (15-17) was used as a measure of the enantioselectivity of the TCYM enantiomers in rats. EF is defined by equation II.
EF ) peak areas of the (-)/[(+) + (-)]
(II)
A racemic EF ) 0.500, whereas preferential degradation of the (-) or (+) yields EF 0.500, respectively. The peak area corresponding with the limit of detection will be used if the enantiomer is not found.
Results Chiroptical Detection. On CDMPC the levorotatory enantiomer in the n-hexane/2-propanol mobile phase elutes first. The elution order of the two enantiomers was reported
TABLE 1. Accuracy (%) and Precision (CV%) of the Chiral HPLC Method for Measurement of TCYM Enantiomers (n ) 6) theoret concn (mg/mL)
(-)-TCYM concn found
accuracy
within- 1 0.94 ( 0.03 day 5 4.87 ( 0.29 30 30.57 ( 0.44 day-to- 1 0.91 ( 0.04 day 5 4.81 ( 0.29 30 29.93 ( 0.54
101.06 102.87 98.50 101.10 103.53 97.49
(+)-TCYM CV
concn found
3.19 0.90 ( 0.02 5.95 4.91 ( 0.31 1.44 30.09 ( 0.65 4.40 0.89 ( 0.05 6.03 4.79 ( 0.33 1.80 30.07 ( A Å 0.47
accuracy
CV
101.11 100.20 99.93 103.37 101.88 99.73
2.22 6.31 2.16 5.62 6.89 1.56
TABLE 2. Summary of Method Recovery Data for Both TCYM Enantiomers from Fortified Rat Plasma and Tissues (n ) 6)a recovery
rac-TCYM fortification (µg/g)
(-)-TCYM
(+)-TCYM
plasma
10 5 0.5
91.51 ( 2.75 91.87 ( 2.36 80.87 ( 5.34
90.89 ( 3.24 91.05 ( 3.13 81.57 ( 5.67
liver
30 10 5
94.37 ( 2.43 90.45 ( 2.93 89.45 ( 2.23
94.41 ( 3.17 91.75 ( 3.28 88.32 ( 3.68
lung
150 100 50
93.14 ( 5.55 91.74 ( 4.95 92.48 ( 4.48
92.86 ( 5.09 90.86 ( 5.15 91.91 ( 3.86
91.17 ( 3.64 90.32 ( 2.85 81.38 ( 5.69
91.78 ( 2.96 90.24 ( 2.77 81.70 ( 7.06
matrix
kidney
5 1 0.5
heart
15 5 1
90.62 ( 2.98 90.84 ( 4.15 89.71 ( 3.54
91.71 ( 2.88 89.47 ( 5.87 88.35 ( 2.86
spleen
15 10 1
91.35 ( 4.39 90.43 ( 3.84 84.38 ( 3.11
90.69 ( 3.86 91.44 ( 2.94 86.15 ( 2.69
fat
15 5 0.5
93.03 ( 3.15 90.68 ( 2.87 84.15 ( 3.91
92.43 ( 2.96 89.71 ( 1.98 82.75 ( 3.65
5 1 0.5
89.26 ( 4.91 88.75 ( 3.78 80.06 ( 2.74
90.27 ( 3.25 88.16 ( 2.84 80.40 ( 3.13
muscle
a
Values represent the means ( SD.
previously as (R)-(1S)-trans-TCYM first (18). Thus, the first eluted stereoisomer was confirmed as (-)-(R)-(1S)-transTCYM, while the second one was (+)-(S)-(1R)-trans-TCYM in our study. Assay Validation. There were no endogenous interference peaks eluted at retention times equal to (-)- (14 min) and (+)-TCYM (18 min) in blank plasma and tissue samples. Representative HPLC chromatograms of extracts from untreated rat plasma, extract from untreated rat plasma spiked with rac-TCYM (0.5 µg/mL), treated plasma, and fat sample are shown in Figure 2. As shown, (-)- and (+)-TCYM were baseline separated. The R and Rs were 1.31 and 2.01. Linear calibration curves were obtained over the concentration range of 0.1-50 µg/mL in plasma for both (-)TCYM (y ) 69.214x - 73.429, R2 ) 0.9995) and (+)-TCYM (y ) 67.199x - 92.218, R2 ) 0.9992). The accuracy and precision of the assay, for both enantiomers, ranged from 1% to 7% (CV) and 97% to 104% (accuracy) over the entire calibration range (Table 1). Method recovery data for fortified plasma and tissues are presented in Table 2. The lowest recovery was over 80%. The LOQ was 0.1 µg/mL plasma. At this concentration, the precision and accuracy were satisfactory ((-)-TCYM, CV ) 3.65%, accuracy ) 106.39%; (+)-TCYM, CV ) 4.71%, accuracy ) 102.7%). VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Plasma and tissues concentration-time curves of TCYM enantiomers in rats following rac-TCYM administration at 20 mg/kg bd wt (9 ) (-)-TCYM, 4 ) (+)-TCYM; values represent the means ( SD). Kinetic Degradation in Plasma. Plasma concentrationtime curves of (-)- and (+)-TCYM after iv treatment of rats with rac-TCYM at 20 mg/kg bd wt are shown in Figure 3A. The (-)-TCYM was prevailing in plasma. The concentration of (+)-enantiomer declined rapidly and was below the LOQ after only 30-60 min. The T1/2 of (-)-TCYM and (+)-TCYM were 61 and 7 min, separately. The result indicated that the (+)-TCYM degraded faster than its antipode. Chiral Conversion in Plasma. The (-)-TCYM was detected in rat plasma after injecting the single (+)-enantiomer, and a representative HPLC chromatogram of treated plasma is shown in Figure 2C. The ER was 0.742 at 1 min and 0.884 at 15 min after treatment. But the (+)-isomer was not found in plasma after administration of (-)-TCYM. The results clearly indicated that there was chiral conversion from the (+)-enantiomer to (-)-TCYM but no conversion from (-)to (+)-enantiomer. Kinetic Degradation in Tissues. Both enantiomers increased initially and reached the maximum at 20-60 min, then decreased with time in the liver, kidney, lung, spleen, fat, and muscle. But they decreased with time in the heart (Figure 3). Brain residue concentrations were generally less 724
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TABLE 3. Half-Lives (T1/2, min) for the Degradation of Both TCYM Enantiomers in Plasma and Tissues Following iv Injection of rac-TCYM at 20 mg/kg bd wt in Rats plasma liver lung kidney heart spleen (-)-TCYM (+)-TCYM
61 7
102 248 90 248
122 120
71 60
126 147
fat
muscle
239 198
161
than the LOQ. Only a small amount of (+)-enantiomer was found in muscle, and it was below the LOQ after 30-60 min. The T1/2 for degradation of both TCYM enantiomers is shown in Table 3. The TCYM was persistent in the lung. (-)-TCYM degraded faster than the (+)-enantiomer in the liver, kidney, heart, and fat but slower in the spleen and almost the same in the lung. The (-)-enatiomer concentrations were in the following order at 240 min: lung > liver > spleen > fat > heart > kidney > muscle. For the (+)-enantiomer, the distribution pattern was the following: lung > liver > spleen > fat > kidney > heart > muscle. The (-)-TCYM was enantio-enriched in plasma, fat, muscle, liver, heart, and kidney. The EFs not only exceeded 0.50 but also increased with time in these samples (Table 4).
TABLE 4. EFs in Plasma and Tissues at Different Time Points (min) Following iv Injection of rac-TCYM at 20 mg/kg bd wt in Ratsa plasma heart liver kidney lung spleen fat muscle a
1
5
15
20
30
60
120
180
240
0.801 ( 0.032 0.555 ( 0.014 0.514 ( 0.011 0.528 ( 0.019 0.511 ( 0.007 0.485 ( 0.045 0.684 ( 0.069 0.705 ( 0.036
0.883 ( 0.025 0.559 ( 0.017 0.515 ( 0.009 0.532 ( 0.021 0.503 ( 0.011 0.499 ( 0.027 0.718 ( 0.040 0.759 ( 0.054
0.901 ( 0.011 0.582 ( 0.034 0.526 ( 0.014 0.538 ( 0.026 0.499 ( 0.023 0.537 ( 0.033 0.720 ( 0.018 0.756 ( 0.064
0.937 ( 0.019 0.618 ( 0.011 0.543 ( 0.032 0.583 ( 0.017 0.508 ( 0.014 0.549 ( 0.037 0.746 ( 0.019 0.750 ( 0.027
0.928 ( 0.054 0.630 ( 0.017 0.538 ( 0.017 0.587 ( 0.051 0.505 ( 0.014 0.558 ( 0.024 0.805 ( 0.028 0.983 ( 0.104
0.976 ( 0.041 0.646 ( 0.025 0.535 ( 0.024 0.612 ( 0.033 0.510 ( 0.031 0.513 ( 0.017 0.828 ( 0.046 0.975 ( 0.086
0.966 ( 0.032 0.650 ( 0.037 0.565 ( 0.031 0.610 ( 0.018 0.501 ( 0.043 0.507 ( 0.021 0.828 ( 0.022 0.964 ( 0.047
0.952 ( 0.027 0.661 ( 0.019 0.580 ( 0.044 0.603 ( 0.014 0.501 ( 0.014 0.507 ( 0.009 0.827 ( 0.034 0.955 ( 0.063
0.918 ( 0.048 0.676 ( 0.011 0.576 ( 0.015 0.597 ( 0.028 0.514 ( 0.036 0.504 ( 0.011 0.832 ( 0.013 0.959 ( 0.027
Values represent the means ( SD.
The higher EFs were measured in plasma, fat, and muscle. The concentrations of the two enantiomers were almost the same in lung and spleen (Table 4). These results indicated there was substantial stereoselectivity on degradation of TCYM enantiomers in rats.
Discussion In previous reports (14, 18), the racemate of TCYM yielded a good resolution on a CDMPC-CSP by normal-phase HPLC. The selectivity factor increased when the percentage of 2-propanol in the mobile phase (2-propanol + n-hexane) decreased. In the present work, the HPLC method was developed to separate enantiomers of TCYM in plasma and tissue samples. CDMPC-CSP effectively resolved TCYM and separated the enantiomers from impurities in samples under normal-phase HPLC. Studies using 14C-labled isomers showed that the rate of soil degradation was isomer-specific for CYM (9). The trans diastereomers degraded significantly faster than the corresponding cis diastereomers, and in the same diastereomers, the (S)-enantiomer declined faster than the corresponding (R)-enantiomer. Liu. et al. evaluated isomer selectivity in biodegradation of CYM in sediment (10). Bacteria strains isolated from sediment selectively degraded CYM diastereomers and enantiomers. The trans diastereomers were preferentially degraded over the cis diastereomers. Of the two active enantiomers, (S)-(1R)-cis was degraded slower, whereas it was degraded faster than the other stereoisomers. Similar isomer selectivity was observed during CYM degradation in sediment. In the present study, we found that (+)TCYM ((S)-enantiomer) degraded faster than the antipode. The declined trend was similar with the above study. In this study, the results had documented in vivo chiral conversion of TCYM where the (+)-enantiomer converted to the (-)-enantiomer and the chiral conversion of TCYM in plasma was unidirectional. The plasma level of the (-)enantiomer was over twice that of the (+)-TCYM at 1 min after (+)-enantiomer administration. Liu W. et al. found stereoisomers of CYM were unstable under heat or in water. But isomer conversion occurred only at the R-C in a slow rate (19). Chiral conversion was not caused by the above factors because the conversion rate was very fast in plasma. Chiral conversion of TCYM in plasma was likely the result of biotic interactions. In the present study, an acute administration of rac-TCYM to rat resulted in stereoselective degradation of (+)- over (-)-enantiomer in some tissues. There were several possible factors involved in the different degradation behaviors of TCYM enantiomers in tissues. The most possible factor was chiral inversion of the two enantiomers in plasma. The second factor was likely stereoselective distribution of (+)- and (-)-TCYM in tissues. Generally, the drug flowed into the heart, then into the lung, finally into the arteries and different tissues after iv injection. The drug was easily
metabolized and stockpiled by the lung (lung first-pass effect) (20). In the study, the concentration of TCYM was very high in the lung, so there was a first-pass effect in the lung. TCYM may be stockpiled in the lung initially and then distributed to the other tissues. The distribution from the lung to other tissues was likely enantioselective because the maximum concentration values of the two enantiomers were different in tissues. Because the TCYM flowed into the heart first without the experience of the first-pass effect in the lung, the change trend of TCYM in the heart was different from that in other tissues. In this context, it is noteworthy that these in vivo studies showed a higher degradation T1/2 of (+)- than (-)-TCYM in the liver (Figure 3) and the EFs of the two enantiomers increased with time in the liver (Table 4). These results suggested that the (+)-enantiomer was metabolized faster than the (-)-enantiomer in rat liver. Stereoselective xenobiotic disposition could also be attributed to excretion, especially in the kidneys. In this study, the concentration of (+)-TCYM in rat kidney was lower than that of (-)-TCYM (Figure 3), and EFs increased with time (Table 4). Thus renal excretion of rac-TCYM may have been stereoselective, with (+)-TCYM excreted more rapidly than its antipode. Several, perhaps all, of the distribution and elimination factors or the chiral conversion mentioned above may have contributed to the observed stereoselective degradation of TCYM. Consistent with the lipophilic nature of TCYM, both TCYM enantiomers were found to accumulate in fat to a great degree with time, and the (-)-enantiomer was more prevalent in fat (Figure 3).
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(15) De Geus, H. J.; Wester, P. G.; De Boer, J.; Brinkman, U. A. Enantiomer fractions instead of enantiomer ratios. Chemosphere 2000, 41, 725-727. (16) Harner, T.; Wiberg, K.; Norstrom, R. Enantiomer fractions are preferred to enantiomer ratios for describing chiral signatures in environmental analysis. Environ. Sci. Technol. 2000, 34, 218220. (17) Ulrich, E. M.; Helsel, D. R.; Foreman, W. T. Complications with using ratios for environmental data: Comparing enantiomeric ratios (ERs) and enantiomer fractions (EFs). Chemosphere 2003, 53, 531-538. (18) Wang, P.; Zhou, Z. Q.; Jiang, S. R.; Yang, L. Chiral resolution of cypermethrin on cellulose-tris(3,5-dimethylphenyl-carbamate) chiral stationary phase. Chromatographia 2004, 59, 625629. (19) Liu, W.; Qin, S.; Gan, J. Chiral stability of synthetic pyrethroid insecticides. J. Agric. Food Chem. 2005, 53, 3814-3820. (20) Liang, W. Q. Biopharmaceutics and Pharmacokinetics; People’s Medical Publishing House: Beijing, 2003.
Received for review October 13, 2005. Revised manuscript received November 22, 2005. Accepted November 30, 2005. ES052025+
