Enzyme-selective effects of nitric oxide on affinity and maximum velocity of various rat cytochromes P450.

Nitric oxide (NO) has recently been shown to decrease cytochrome P450 (P450) enzyme activity rapidly (< or =30 min), concentration dependently, and enzyme-selectively in the rat liver. Interestingly, among all the studied P450 enzymes, only CYP2D1 was not affected by NO donors. However, these studies were conducted using only a single concentration of the substrates, thus lacking information about the possible simultaneous changes in both maximum velocity (Vmax) and affinity (Km) of the enzymes. In the present study, we systematically evaluated the effects of NO on the enzyme kinetic parameters of marker substrates for a range of P450 enzymes, including 2D1. Livers were perfused (1 h) in the absence (control) or presence of two NO donors with different mechanisms of NO release. At the end of the perfusion, microsomes were prepared and used for kinetic analysis. Except for 2D1, NO reduced the Vmax of all the model reactions studied, although to a varying degree. However, the effects of NO donors on Km were more diverse. Whereas the Km values for testosterone 6beta-hydroxylation (3A2) and 16alpha-hydroxylation (2C11) significantly decreased, the values for chlorzoxazone 6-hydroxylation (2E1), dextromethorphan N-demethylation (3A2), and high affinity ethoxyresorufin O-dealkylation (1A1/2) significantly increased in the presence of NO donors. Furthermore, the Km values for the high-affinity component of dextromethorphan O-demethylation and benzyloxyresorufin O-dealkylation remained unchanged. These results indicate that NO can potentially change both the Vmax and Km of various substrates selectively and confirm our previous findings that the activity of CYP2D1 is not affected by NO donors.     

Effects of Hepatic Ischemia-Reperfusion Injury on the P-Glycoprotein Activity at the Liver Canalicular Membrane and Blood–Brain Barrier Determined by In Vivo Administration of Rhodamine 123 in Rats

Purpose

To investigate the effects of normothermic hepatic ischemia-reperfusion (IR) injury on the activity of P-glycoprotein (P-gp) in the liver and at the blood–brain barrier (BBB) of rats using rhodamine 123 (RH-123) as an in vivo marker.

Methods

Rats were subjected to 90 min of partial ischemia or sham surgery, followed by 12 or 24 h of reperfusion. Following intravenous injection, the concentrations of RH-123 in blood, bile, brain, and liver were used for pharmacokinetic calculations. The protein levels of P-gp and some other transporters in the liver and brain were also determined by Western blot analysis.

Results

P-gp protein levels at the liver canalicular membrane were increased by twofold after 24 h of reperfusion. However, the biliary excretion of RH-123 was reduced in these rats by 26%, presumably due to IR-induced reductions in the liver uptake of the marker and hepatic ATP concentrations. At the BBB, a 24% overexpression of P-gp in the 24-h IR animals was associated with a 30% decrease in the apparent brain uptake clearance of RH-123. The pharmacokinetics or brain distribution of RH-123 was not affected by the 12-h IR injury.

Conclusions

Hepatic IR injury may alter the peripheral pharmacokinetics and brain distribution of drugs that are transported by P-gp and possibly other transporters.     

Development and Application of an Isolated Perfused Rat Liver Model to Study the Stimulation and Inhibition of Tumor Necrosis Factor-α Production ex Vivo

Purpose. To develop an isolated perfused rat liver (IPRL) model with low baseline levels of tumor necrosis factor (TNF)- in the outlet perfusate to study the effects of immunostimulants and immunosuppressants on the release of TNF- from this organ. Methods. Isolated rat livers were perfused with a buffer containing no albumin or three different bovine serum albumin (BSA) preparations. Using the no-albumin perfusate, the inhibitory effects of methylprednisolone (MP) on lipopolysaccharide (LPS)-stimulated release of TNF- were studied in livers isolated 1 or 5 h after the intravenous administration (5 mg/kg) of MP. The concentrations of TNF- in the outlet perfusates were measured using enzyme-linked immunosorbent assay. Results. In the absence of albumin, the perfusate levels of TNF- were close to zero. However, when the perfusate contained BSA, the TNF- levels in the perfusate reached as high as 1200 pg/ml at steady state. An injection of LPS into IPRLs perfused with a no-albumin perfusate resulted in mean (± SD) TNF- steady-state concentrations of 825 ± 125 pg/ml. The pretreatment of rats with MP before liver harvest attenuated the LPS-induced TNF- release in the livers. However, the attenuation was substantial (>60%) and was statistically significant only 5 h after pretreatment with MP. Conclusions. Perfusates containing BSA may result in nonphysiologically high levels of TNF-. An IPRL with a no-albumin perfusate is more suitable for studies of the stimulation and inhibition of TNF- production by this organ.     

Hepatic disposition and effects of nitric oxide donors: rapid and concentration-dependent reduction in the cytochrome P450-mediated drug metabolism in isolated perfused rat livers

Various mechanisms, including high levels of cytokines and nitric oxide (NO), have been proposed as mediators for inflammation-induced cytochrome 450 down-regulation. However, the contribution of each of these mediators to the observed effects is controversial. We used an isolated perfused rat liver (IPRL) model to test the direct effects of NO donors on CYP450 down-regulation in the absence of cytokines or other confounding in vivo factors. Our hypothesis was that NO rapidly and concentration-dependently decreases CYP450 activities in IPRL. Livers were perfused (60 min) with 50 to 500 microM sodium nitroprusside (SNP) or 100 to 500 microM isosorbide dinitrate (ISDN) as NO donors, and the perfusate and biliary disposition of SNP, ISDN, and generated nitrate/nitrite (NO(x)) were determined. Additionally, at the end of perfusion, catalytic activities and protein levels of various cytochrome isoenzymes were measured. Both SNP and ISDN exhibited linear hepatic disposition with extraction ratios of approximately 0.30 and 0.50, respectively. Furthermore, although in small amounts, both NO donors and NO(x) were found in the bile. Except for CYP2D1, the catalytic activities of all the studied isoenzymes were substantially (up to 85%) decreased by both NO donors. However, the apoprotein levels of isoenzymes remained largely unchanged. Additionally, the inhibitory effects of NO donors were concentration-dependent, with the concentrations of SNP producing one-half of maximum inhibition being in the order of 2C11 > 2B1/2 > 2E1 = 3A2 > 1A1/2. These studies indicate that the effects of NO on the down-regulation of cytochrome 450 catalytic activity are rapid, concentration-dependent, and isoenzyme-selective.     

Stereoselective pharmacokinetics and pharmacodynamics of anti-asthma agents

OBJECTIVE: To review the previously published studies on pharmacokinetics and pharmacodynamics of chiral drugs used in the treatment of asthma. DATA SOURCES: Primary and review articles were identified with a MEDLINE search (1980-May 2001) and through secondary sources. STUDY SELECTION AND DATA EXTRACTION: All English-language studies and reviews obtained from the MEDLINE search pertaining to stereoselective pharmacokinetics and pharmacodynamics of chiral anti-asthma drugs were assessed. DATA SYNTHESIS: Several anti-asthma drugs (e.g., beta(2)-adrenergic agonists, leukotriene modifiers) are chiral and marketed as racemates, which consist of equal proportions of 2 enantiomers. Significant stereoselectivity has also been reported in pharmacodynamics and pharmacokinetics of the beta(2)-agonists. The enantiomers of beta(2)-agonists in the R configuration are primarily responsible for the bronchodilating effects of the racemate. The plasma concentrations of the enantiomers of anti-asthma drugs may differ as a reflection of stereoselectivity in clearance, volume of distribution, and route of administration. CONCLUSIONS: Stereoselectivity in the pharmacokinetics of anti-asthma drugs may complicate the relationship between dose and/or plasma concentration of racemic drug versus effect relationship. An appreciation of the stereoselective pharmacokinetics and pharmacodynamics of chiral anti-asthma drugs may optimize the use of these agents in asthmatic patients.     

Dextran-Methylprednisolone Succinate as a Prodrug of Methylprednisolone: Local Immunosuppressive Effects in Liver After Systemic Administration to Rats

Purpose. The purpose of this work was to study the local immunosuppressive effects of systemically administered methylprednisolone (MP) and its prodrug, dextran-methylprednisolone (DMP), in rat livers. Methods. Single 5 mg/kg (MP equivalent) doses of MP or DMP were injected intravenously to rats, and livers were isolated at different time points (0-72 h; n = 4/time point). Isolated livers were stimulated ex vivo with bacterial lipopolysaccharide, and outlet perfusate and bile samples were analyzed for their concentrations of tumor necrosis factor (TNF)- by enzyme-linked immunosorbent assay. The area under the perfusate TNF- concentration-time curve (AUC) was used as a measure of immune response. Hepatic concentrations of MP and DMP were also measured by high-performance liquid chromatography. Results. Both MP and DMP resulted in a decrease in lipopolysaccharide-induced increase in TNF- AUC. MP injection 8 h before liver isolation resulted in a maximum of 50% decrease in TNF- AUC. Compared with MP, the maximum effect of the prodrug (DMP) was both more intense (80% reduction in TNF- AUC) and delayed (maximum inhibition at 24 h). Overall, the area under the effect (% inhibition of TNF-)-time (%inhibitionh) for DMP (3680 ± 406) was approximately four times more than that for the parent drug (846 ± 114). Whereas the MP concentrations in the liver were not quantifiable after the injection of the parent drug, relatively large concentrations of DMP and regenerated MP were found in the liver of DMP-injected rats. Conclusions. After systemic administration to rats, both MP and DMP exhibit local immunosuppressive effects in the liver. The local effects of the prodrug (DMP), however, appear to be more intense and sustained than those of the parent drug (MP).     

Relationship of Apparent Systemic Clearance to Individual Organ Clearances: Effect of Pulmonary Clearance and Site of Drug Administration and Measurement

The relationships between apparent total-body clearance (CL) and individual organ clearances were mathematically defined with respect to the site of drug administration and measurement. The derived equations can be applied to drugs undergoing different pathways of elimination, including pulmonary clearance. A physiological pharmacokinetic model was used to test the validity of the equations. The apparent systemic clearance values obtained through the equations, using the individual organ clearance values, were identical to those calculated utilizing the model-generated data, indicating the validity of the equations. Furthermore, it was shown that the conventional estimation of CL of drugs subject to pulmonary clearance is highly dependent upon the site of drug administration and measurement. The relationships were further utilized to explain the reported CL values which are higher than the cardiac output. The equations developed here may be used to predict the contribution of different organs, such as the lungs, to the apparent systemic clearance of drugs.     

Kinetics of hydrolysis of dextran-methylprednisolone succinate, a macromolecular prodrug of methylprednisolone, in rat blood and liver lysosomes.

A macromolecular prodrug of methylprednisolone (MP) was synthesized by conjugating MP with dextran with a M(W) of 70000 through a succinic acid linker. It has been shown previously that the dextran-MP conjugate (DMP) releases MP directly or indirectly through formation of methylprednisolone succinate (MPS) which is further hydrolyzed to MP. To investigate the suitability of DMP conjugate as a prodrug of MP for systemic administration, the kinetics of hydrolysis of the conjugate was studied in vitro in rat blood and liver lysosomes. In blood, the hydrolysis of MPS to MP was approximately ten-fold faster than that in buffer. However, the hydrolysis rate constants of DMP conjugate to MP or MPS in blood were not different from those in buffer. Overall, the hydrolysis of DMP in the rat blood occurred with a half life of approximately 25 h. Hydrolysis of MPS to MP also occurred in the liver lysosomal fraction, but not in the control samples lacking lysosomes. However, the rate constants for the hydrolysis of DMP conjugate to MP and MPS in the lysosomal fraction were not significantly different from those in the control samples. These data suggest that the slow hydrolysis of DMP conjugate to MP or MPS in both rat blood and liver lysosomes occurs mostly, if not completely, via chemical hydrolysis. However, the conversion of MPS to MP is apparently enzymatic. The data may have significant implications for systemic administration of the prodrug.     

Molecular-weight-dependent pharmacokinetics of fluorescein-labeled dextrans in rats.

The dependency of the pharmacokinetics of fluorescein-labeled dextrans on M(r) was studied in adult Sprague-Dawley rats. Single intravenous doses (5 mg/kg) of the dextrans with M(r) of 4000, 20,000, 40,000, 70,000, or 150,000 and single oral doses (50 mg/kg) of the dextrans with M(r) of 4000, 20,000, or 40,000 were administered to different groups of rats. A specific and sensitive high-performance, size-exclusion chromatographic method was used to measure the concentrations of the dextrans in serial serum and urine samples, and the relevant kinetic parameters were calculated. After intravenous administration, the profiles of the concentration of dextran in serum versus time for all the studied dextrans exhibited an apparent biexponential decline, with all the calculated kinetic parameters being dependent on M(r). However, the degree of dependency varied for different parameters. Among the calculated kinetic parameters, renal clearance and volume of distribution values were affected the most and least, respectively, by the differences in M(r). Furthermore, we observed a clear separation between the labeled dextrans with M(r) less than or equal to 20,000 and those with M(r) greater than or equal to 40,000 with respect to their concentrations in serum and kinetic parameters. After oral administration, the dextrans could not be detected in serum, and on the basis of urine data, only negligible amounts of the macromolecules were absorbed into the systemic circulation (less than 0.4% of the dose). The data presented here may be used in the future design of microvascular and drug delivery studies with dextrans.