Recommendations for bioequivalence testing of cyclosporine generics revisited

The immunosuppressant cyclosporine is generally considered a critical-dose drug. The validity of standard criteria to establish bioequivalence between cyclosporine formulations has recently been challenged. Recommendations included establishment of individual bioequivalence rather than average bioequivalence, establishment of bioequivalence in transplant patients and in subgroups known to be poor absorbers, as well as long-term efficacy and safety studies in transplant patients. However, at the moment individual bioequivalence is a theoretical concept, the practical benefits of which have not statistically been proven. The proposed patient pharmacodynamic studies can be expected to require an unrealistically high number of subjects to achieve sufficient statistical power. It is well established that the common practice of blood-concentration-guided dosing of cyclosporine efficiently compensates for interindividual and intraindividual variability and allows for safely switching cyclosporine formulations as bioinequivalent as Sandimmune and Neoral. Recent studies comparing the generic cyclosporine formulation SangCya with Neoral, including individual bioequivalence, bioequivalence in transplant patients, and long-term safety after switching from Sandimmune to SangCya, confirmed that it was valid to conclude bioequivalence of both cyclosporine formulations based on standard average bioequivalence criteria. Present FDA guidelines for approving bioequivalence can be considered adequate and sufficient for generic cyclosporine formulations.

Active transport of the angiotensin-II antagonist losartan and its main metabolite EXP 3174 across MDCK-MDR1 and caco-2 cell monolayers

1. We studied the functional interaction between transport and metabolism by comparing the transport of losartan and its active metabolite EXP 3174 (EXP) across cell monolayers. 2. Epithelial layers of Caco-2 cells as well as MDR1, MRP-1 and MRP-2 overexpressing cells, in comparison to the respective wildtypes, were used to characterize the transcellular transport of losartan and EXP. 3. Losartan transport in MDCK-MDR1 and Caco-2 cells was saturable and energy-dependent with a significantly greater basolateral-to-apical (B/A) than apical-to-basolateral (A/B) flux (ratio=31+/-1 in MDCK-MDR1 and ratio 4+/-1 in Caco-2 cells). The B/A flux of losartan was inhibited by cyclosporine and vinblastine, inhibitors of P-glycoprotein and MRP. In contrast, no active losartan transport was observed in MRP-1 or MRP-2 overexpressing cells. 4. The metabolite was only transported in Caco-2 cells with a B/A-to-A/B ratio of 5+/-1, while lacking active transport in the MDR1, MRP-1 or MRP-2 overexpressing cells. The B/A flux of EXP was significantly inhibited by cyclosporine and vinblastine. 5. In conclusion, losartan is transported by P-glycoprotein and other intestinal transporters, that do not include MRP-1 and MRP-2. In contrast, the carboxylic acid metabolite is not a P-glycoprotein substrate, but displays considerably higher affinity for other transporters than losartan, that again most probably do not include MRP-1 and MRP-2.

The novel immunosuppressant SDZ-RAD protects rat brain slices from cyclosporine-induced reduction of high-energy phosphates

1. SDZ-RAD, 40-O-(2-hydroxyethyl)-rapamycin, is a novel macrolide immunosuppressant. Because of its synergistic interaction, SDZ-RAD is under clinical investigation as immunosuppressant in combination with cyclosporine after organ transplantation. Neurotoxicity is a critical side-effect of cyclosporine. 2. We studied the effect of SDZ-RAD and its combination with cyclosporine on high-energy phosphates, phosphocreatine (PCr) and nucleoside triphosphates (NTP), in brain slices using 31P-magnetic resonance spectroscopy (MRS). 3. Cyclosporine significantly reduced high-energy phosphates after 2 h in a dose-dependent manner (100 micrograms l-1: 93 +/- 3% of control (NTP), 91 +/- 3% (PCr); 500 micrograms l-1: 84 +/- 2% (NTP), 73 +/- 2 (PCr); 5000 micrograms l-1: 68 +/- 3% (NTP), 55 +/- 5% (PCr); n = 6; P < 0.02). 4. In contrast, after perfusion for 2 h, SDZ-RAD (500 micrograms l-1 and 5000 micrograms l-1) significantly increased high-energy phosphate concentrations in the brain slices (P < 0.02). Even at the lowest concentration, SDZ-RAD protected brain energy metabolism against cyclosporine toxicity: 100 micrograms l-1 SDZ-RAD + 5000 micrograms l-1 cyclosporine: 86 +/- 3% (NTP), 83 +/- 7% (PCr), n = 3, P < 0.03 compared to cyclosporine alone. 5. As evaluated using an algorithm based on Loewe isobolograms, the effects of SDZ-RAD/cyclosporine combinations on brain energy reduction were antagonistic. Both drugs were found in mitochondria using h.p.l.c-MS analysis. 6. We conclude that cyclosporine inhibits mitochondrial high-energy phosphate metabolism, which can be antagonized by SDZ-RAD.

Combined immunosuppression with cyclosporine (neoral) and SDZ RAD in non-human primate lung transplantation: systematic pharmacokinetic-based trials to improve efficacy and tolerability

BACKGROUND

We studied the efficacy and tolerability of combined immunosuppressive therapy with cyclosporine A microemulsion (Neoral) plus the macrolide SDZ RAD 40-0 (2-hydroxyethyl) rapamycin (RAD) in a stringent cynomolgus monkey lung graft model in comparison with cyclosporine or SDZ RAD monotherapy.

METHODS

Thirty-nine cynomolgus monkeys received mixed lymphocyte reaction (MLR) mismatched unilateral lung transplants. Immunosuppressants were administered orally as single daily doses. The observation period was 28 days and follow-up included serial trough blood drug concentrations measured by high performance liquid chromatography/mass spectrometry, blood analyses, chest radiographs, open lung biopsies, as well as tissue drug concentrations and graft histology at necropsy.

RESULTS

Graft biopsies in monkeys treated with vehicle (n=4), Neoral (day 1-7: 150 mg/kg/day; day 8-28: 100 mg/kg/day; n=6; mean +/- SE trough level (MTL): 292+/-17 ng/ml) or SDZ RAD monotherapy (1.5 mg/kg/day; n=6; MTL: 15+/-1 ng/ml) showed severe rejection. Coadministration in two transplant monkeys of Neoral (150/100 mg/kg/day) and SDZ RAD (1.5 mg/kg/day) caused their early death. In both animals, SDZ RAD blood levels were more than 5-fold higher than under monotherapy (MTL: 82+/-18 ng/ml). Simultaneous administration (n=6) of Neoral (150/100 mg/kg/day; MTL: 217+/-16 ng/ml) and SDZ RAD (0.3 mg/kg/day; MTL: 24+/-2 ng/ml) improved graft outcome (mild rejection). Side effects included renal failure (n=2) and seizures (n=1). Three monkeys survived to day 28. In this group the MTL for cyclosporin was 143+/-13 and for RAD 38+/-3. Staggered treatment completely prevented rejection in four of six grafts. However, five of six monkeys had moderate to severe diarrhea. In a concentration-controlled trial of simultaneously administered Neoral and SDZ RAD in transplant monkeys (target SDZ RAD MTL: 20-40 ng/ml; cyclosporine MTL: 100-200 ng/ml) all six monkeys survived with improved drug tolerability and an average biopsy score of mild rejection.

CONCLUSION

Combination of orally administered SDZ RAD and Neoral showed excellent immunosuppressive efficacy in a stringent lung transplant model. The drug interaction and the narrow therapeutic index of this drug combination required careful dose adjustments to optimize tolerability and efficacy.

Ethynylestradiol-mediated induction of hepatic CYP3A9 in female rats: implication for cyclosporine metabolism

Repeated treatment of female rats with the synthetic estrogen ethynylestradiol (EE(2)) increases the formation of the cyclosporine A (CyA) metabolites AM1c and AM9 by 3-fold, whereas the formation of AM1 and AM4N is not significantly enhanced. The formation of all four CyA metabolites was inhibited by greater than 80% by the CYP3A-selective substrate midazolam or polyclonal anti-rat CYP3A IgGs in liver microsomes of untreated and EE(2)-induced rats. In contrast, anti-rat CYP2C6 IgGs had little effect, indicating the involvement of a CYP3A but not 2C6 in this EE(2)-stimulated CyA metabolism. Semiquantitative reverse-transcriptase polymerase chain reaction was used to determine the mRNA content for four CYP3A genes (CYP3A2, CYP3A9, CYP3A18, and CYP3A23) in livers of control and EE(2)-treated female rats. EE(2) selectively induced CYP3A9 by 3.3-fold whereas the expression of CYP3A18 and CYP3A23 was slightly decreased; neither CYP3A2 mRNA nor CYP3A1 mRNA was detectable in these EE(2)-treated livers. To determine whether rat liver microsomal CYP3A9 was indeed responsible for the EE(2)-stimulated CyA metabolism, a recombinant CYP3A9 was heterologously expressed in Escherichia coli. When functionally reconstituted, this enzyme was active in metabolizing CyA preferentially to its AM9 and AM1c metabolites as compared with CYP3A4. These findings thus support the notion that the increased CyA-metabolizing capacity of EE(2)-treated female rat liver microsomes is due to the induction of the CYP3A9 enzyme.

Intestinal MDR transport proteins and P-450 enzymes as barriers to oral drug delivery

Cytochrome P-450 3A4 (CYP3A4), the major phase I drug metabolizing enzyme in humans, and the multidrug efflux pump, MDR or P-glycoprotein (P-gp), are present at high levels in the villus tip enterocytes of the small intestine, the primary site of absorption for orally administered drugs. These proteins are induced or inhibited by many of the same compounds and demonstrate a broad overlap in substrate and inhibitor specificities, suggesting that they act as a concerted barrier to drug absorption. A series of studies from our laboratory of cyclosporine and tacrolimus in humans and a novel cysteine protease inhibitor in rats, dosed concomitantly with inhibitors and inducers of CYP3A4 and P-gp, suggest that gut extraction can be modeled using measures of intestinal metabolism and absorption rate, the latter reflecting changes in P-gp. Results evaluating a preliminary model applied to the CYP3A substrate drugs midazolam, indinavir, saquinavir, and rifabutin suggest that the model may be useful for predicting in vivo intestinal metabolism from in vitro data.

Small intestinal metabolism of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor lovastatin and comparison with pravastatin

We compared the intestinal metabolism of the structurally related 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors lovastatin and pravastatin in vitro. Human small intestinal microsomes metabolized lovastatin to its major metabolites 6'beta-hydroxy (apparent K(m) = 11.2 +/- 3.3 microM) and 6'-exomethylene (apparent K(m) = 22.7 +/- 9.0 microM) lovastatin. The apparent K(m) values were similar for lovastatin metabolism by human liver microsomes. 6'beta-Hydroxylovastatin formation by pig small intestinal microsomes was inhibited with the following inhibition K(i) values: cyclosporine, 3.3 +/- 1.2 microM; ketoconazole, 0.4 +/- 0.1 microM; and troleandomycin, 0.8 +/- 0.9 microM. K(i) values for 6'-exomethylene lovastatin were similar. Incubation of pravastatin with human small intestinal microsomes resulted in the generation of 3'alpha,5'beta, 6'beta-trihydroxypravastatin (apparent K(m) = 4560 +/- 1410 microM) and hydroxypravastatin (apparent K(m) = 5290 +/- 1740 microM). In addition, as in the liver, pravastatin was metabolized in the small intestine by sulfation and subsequent degradation to its main metabolite 3'alpha-iso-pravastatin. It was concluded that lovastatin is metabolized by cytochrome P-450 3A enzymes in the small intestine. Compared with lovastatin, the cytochrome P-450-dependent intestinal intrinsic clearance of pravastatin was >5000-fold lower and cannot be expected to significantly affect its oral bioavailability or to be a significant site of drug interactions.

Photoinduced covalent binding of frusemide and frusemide glucuronide to human serum albumin

AIMS

To study reaction of photoactivated frusemide (F) and F glucuronide (Fgnd metabolite) with human serum albumin in order to find a clue to clarify a mechanism of phototoxic blisters from high frusemide dosage.

METHODS

F was exposed to light in the presence of human serum albumin (HSA). HSA treated with this method (TR-HSA) was characterized by fluorescence spectroscopic experiment, alkali treatment and reversible binding experiment.

RESULTS

Less 4-hydroxyl-N-furfuryl-5-sulphamoylanthranilic acid (4HFSA, a photodegradation product of F) was formed in the presence of HSA than in the absence of HSA. A new fluorescence spectrum excited at 320 nm was observed for TR-HSA. Alkali treatment of TR-HSA released 4HFSA. Quenching of the fluorescence due to the lone tryptophan near the warfarin-binding site of HSA was observed in TR-HSA. The reversible binding of F or naproxen to the warfarin-binding site of TR-HSA was less than to that of native HSA. These results indicate the photoactivated F was covalently bound to the warfarin-binding site of HSA. The covalent binding of Fgnd, which is also reversibly bound to the warfarin-binding site of HSA, was also induced by exposure to sunlight. Fgnd was more photoactive than F, indicating that F could be activated by glucuronidation to become a more photoactive compound.

CONCLUSIONS

The reactivity of photoactivated F and Fgnd to HSA and/or to other endogenous compounds may cause the phototoxic blisters that result at high F dosage.

Evaluation of individual and combined neurotoxicity of the immunosuppressants cyclosporine and sirolimus by in vitro multinuclear NMR spectroscopy

Neurotoxicity, a crucial side effect of immunosuppressive therapy with cyclosporine, also has been demonstrated in vitro for sirolimus, a novel macrolide immunosuppressant, which is under clinical investigation in combination with cyclosporine. NMR spectroscopy was used to study the separate and combined effects of cyclosporine and sirolimus on cerebral metabolism, both in brain cells and in perfused rat brain slices. The high-energy phosphate metabolism was already affected significantly at cyclosporine concentrations as low as 100 micrograms/liter: phosphocreatine was reduced by 10 +/- 2% [half-maximal inhibition concentration (IC50) = 1850 +/- 600 micrograms/liter], and nucleoside triphosphate was reduced by 11 +/- 5% (IC50 = 1110 +/- 420 micrograms/liter; n = 4, P <.05). At 500 micrograms/liter cyclosporine, N-acetylaspartate and glutamate were decreased by 13 +/- 7% (IC50 = 1100 +/- 330 micrograms/liter) and 22 +/- 9% (IC50 = 360 +/- 220 micrograms/liter; n = 4, P <.05), respectively. As evaluated using an algorithm based on Loewe isobolograms, combination of cyclosporine and sirolimus resulted in a synergetic reduction of high-energy phosphate metabolites. Addition of sirolimus to the perfusion medium increased brain slice concentrations of cyclosporine. It is concluded that cyclosporine significantly reduced high-energy phosphate metabolism in brain tissue at in vivo relevant concentrations. Combination with sirolimus resulted in synergism, which, in part, is explained by a greater distribution of cyclosporine into the brain tissue in the presence of sirolimus.

Coadministration of neoral and the novel rapamycin analog, SDZ RAD, to rat lung allograft recipients: potentiation of immunosuppressive efficacy and improvement of tolerability of staggered versus simultaneous treatment

BACKGROUND

Neoral and rapamycin derivative (RAD) have complementary mechanisms for inhibition of lymphocyte activation and are substrates for the same pathways of drug metabolism. Therefore, we investigated treatment regimens designed to minimize pharmacokinetic interactions and to potentiate immunosuppressive efficacy in a highly stringent rat lung allograft model.

METHODS

Lewis recipients of Brown Norway lungs received the following daily oral doses: (A) RAD at 2.5 mg/kg (n=9); (B) Neoral at 7.5 mg/kg (n=8); (C) RAD at 2.5 mg/kg + Neoral at 7.5 mg/kg simultaneously (n=8); or (D) RAD at 2.5 mg/kg + Neoral at 7.5 mg/kg (n=6) staggered 6 hr apart. Rats were assessed by daily weights, chest radiographs, drug trough levels (high-performance liquid chromatography/mass spectrometry), and blinded scoring of graft histology at death (day 21).

RESULTS

Radiographs were completely opacified in all grafts of control and RAD monotherapy groups on days 7 and 14, respectively. Grafts were mildly opacified (Neoral monotherapy) and completely clear (both RAD + Neoral groups) on day 21. Simultaneous or staggered combined treatment dramatically reduced histologic rejection compared with treatment with either drug alone. Simultaneous treatment caused poor tolerability (poor grooming, lethargy) and significantly higher day-14 RAD and cyclosporine (CsA) trough levels (49+/-5 and 638+/-106 ng/ml; P<0.04) than in the staggered group (28+/-3 and 318+/-25 ng/ml) in which all animals were clinically normal. RAD and CsA day-14 trough levels in the staggered group were the same or lower than trough levels in animals treated with either drug alone (RAD 27+/-3/Neoral 815+/-67 ng/ml).

CONCLUSIONS

(1) Administration of RAD + Neoral suppressed lung rejection more effectively than treatment with either drug alone. (2) Trough levels did not differ between monotherapy and staggered combination therapy for RAD but were lower for CsA. These results suggested that pharmacological, rather than pharmacokinetic, interactions between the parent drugs were responsible for the potentiation of immunosuppression when these drugs were coadministered. 3) Staggered administration of RAD+Neoral avoided the pharmacokinetic interactions that caused the elevated drug blood levels and poor tolerability caused by simultaneous administration. Thus, we could potentiate efficacy and improve tolerability by staggering administration of RAD and Neoral.

Interaction of human serum albumin with furosemide glucuronide: a role of albumin in isomerization, hydrolysis, reversible binding and irreversible binding of a 1-O-acyl glucuronide metabolite

Furosemide 1-O-acyl glucuronide (Fgnd) was reversibly bound to a single class of binding sites on human serum albumin (HSA), and the binding of Fgnd decreased with increasing F concentrations, suggesting that Fgnd binds to the same warfarin binding sites on HSA as F binds. The rate of Fgnd degradation (hydrolysis and acyl migration) decreased in the presence of HSA. Although the formation of acyl migration isomers of Fgnd was slower in the presence of HSA than in its absence, hydrolysis of Fgnd to F was faster in the presence of HSA. Rapid minor irreversible binding of Fgnd to HSA within 30 min was followed by slow major irreversible binding. Slow irreversible binding of Fgnd to HSA was decreased by F, though not significantly. This suggests that major irreversible binding may proceed via reversible binding. It has been reported that acyl migration is a prerequisite for irreversible binding. Therefore, these results indicate that HSA decreases irreversible binding of Fgnd to protein by suppressing acyl migration. Furthermore, these results suggest that HSA may prevent irreversible binding of Fgnd to other proteins in the body by decreasing the concentration of reactive Fgnd in the unbound form. HSA eliminates reactive Fgnd by hydrolysis to F. Therefore, it is concluded that HSA works as a scavenger to decrease reactive compounds by reversible binding or eliminates reactive compounds by irreversible binding.

Grapefruit juice activates P-glycoprotein-mediated drug transport

PURPOSE

Grapefruit juice (GJ) is known to increase the oral bioavailability of many CYP3A-substrates by inhibiting intestinal phase-I metabolism. However, the magnitude of AUC increase is often insignificant and highly variable. Since we earlier suggested that CYP3A and P-glycoprotein (P-gp) form a concerted barrier to drug absorption, we investigated the role of P-gp in GJ-drug interactions.

METHODS

The transcellular bidirectional flux of drugs that are (i) CYP3A-and/or P-gp substrates (Vinblastine, Cyclosporine, Digoxin, Fexofenadine, Losartan) or that are (ii) primary CYP3A-substrates (Felodipine, Nifedipine) was evaluated across MDCK-MDR1 cell monolayers with or without GJ, verifying monolayer integrity at all times.

RESULTS

While both apical-to-basal (A-B) and basal-to-apical (B-A) fluxes of all CYP3A/P-gp substrates tested were increased in the presence of GJ, the resulting net efflux (B-A/A-B) was in all cases significantly greater with GJ than control (Vin, 28.0 vs. 5.1; CsA, 9.9 vs. 2.8; Dig, 22. 9 vs. 14.7, Fex, 22.3 vs. 11.1, Los, 39.6 vs. 26). In contrast, no such GJ flux effect was observed with Fel and Nif, substrates of CYP3A only (2 vs. 1.7 and 1.2 vs. 1.3).

CONCLUSIONS

GJ significantly activates P-gp-mediated efflux of drugs that are substrates of P-gp, potentially partially counteracting the CYP3A-inhibitory effects of GJ.

In vitro biotransformation of a novel antimalarial cysteine protease inhibitor in human liver microsomes

4-Dimethylamino-4'-(imidazol-1-yl)chalcone (RL3142) is a newly developed antimalarial cysteine protease inhibitor. Four metabolites (M1-M4) were found in human liver microsomes and their structures were identified by LC/MS/MS. Two primary metabolites, M2 (minor) and M4 (major), were determined to be the N-demethylated product (M2) and the product (M4) resulting from 1,2-hydrogenation of the alpha, beta-unsaturated ketone moiety of the parent compound. A combined approach utilizing selective P450 inhibitors, immunoinhibition with CYP3A and NADPH P450 reductase antibodies, and cDNA expressed human CYP3A4 and NADPH P450 reductase, was used for identification of enzymes responsible for the biotransformation. For formation of M2, both a rabbit CYP3A polyclonal antibody (110 microliter/mg microsomal protein) and ketoconazole (2 micromol/l), a CYP3A inhibitor, showed about 50% inhibitory effects; other specific inhibitors of CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP2D6 and CYP2E1 showed no significant effects. For formation of M4, neither CYP3A antibody nor the above mentioned CYP inhibitors exhibited inhibitory effects. Anti-rat NADPH P450 reductase serum (50 microliter/100 microgram microsomal protein) exhibited 70 and 58% inhibitory effects on M2 and M4 formation, respectively. Incubation of RL3142 with cDNA expressed human NADPH P450 reductase yielded formation of M4, but not M2. Carbon monoxide inhibited formation of M2 and M1 (the reduced product of M2), but had no effect on M4 and M3 (the reduced product of M4) formation. Collectively, NADPH P450 reductase solely catalyzed reduction of RL3142 to M4, whereas CYP3A contributed in part to formation of M2.

Net secretion of furosemide is subject to indomethacin inhibition, as observed in Caco-2 monolayers and excised rat jejunum

PURPOSE

To determine if intestinal secretion occurs for the poorly bioavailable diuretic, furosemide.

METHODS

Jejunal segments of male Sprague-Dawley rats were mounted on diffusion chambers, and the permeation of furosemide was measured across the excised tissue in both directions. Studies were repeated using cultured epithelia from adenocarcinoma cells (Caco-2) grown on filter inserts mounted in 6-well plates. Temperature-dependence and chemical inhibition by indomethacin was also tested using the cell culture model.

RESULTS

Net secretion from rat intestine of over 3-fold was observed for 20 microM furosemide. Net secretion of furosemide by Caco-2 cells was over 300% greater than for intestinal segments (10-fold vs. 3-fold). For both models, a decrease in furosemide transport in the direction of secretion was observed in the presence of indomethacin (100 microM), although only results using the Caco-2 cells showed in increase in the absorptive transport. Furosemide secretion from Caco-2 cells decreased with decrease in temperature from 37 degrees C to 4 degrees C, suggesting a carrier-mediated process.

CONCLUSIONS

Furosemide appears to be secreted in the small intestine. These preliminary results indicate that furosemide bioavailability may be limited by an intestinal transporter.

Comparison of cytochrome P-450-dependent metabolism and drug interactions of the 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors lovastatin and pravastatin in the liver

In an in vitro study, the cytochrome P-450 3A (CYP3A)-dependent metabolism and drug interactions of the 3-hydroxy-3-methylglutaryl-Co A reductase inhibitors lovastatin and pravastatin were compared. Lovastatin was metabolized by human liver microsomes to two major metabolites: 6'beta-hydroxy [Michaelis-Menten constant (Km): 7.8 +/- 2.7 microM] and 6'-exomethylene lovastatin (Km,10.3 +/- 2.6 microM). 6'beta-Hydroxylovastatin formation in the liver was inhibited by the specific CYP3A inhibitors cyclosporine (Ki, 7.6 +/- 2.3 microM), ketoconazole (Ki, 0.25 +/- 0.2 microM), and troleandomycin (Ki, 26.6 +/- 18.5 microM). Incubation of pravastatin with human liver microsomes resulted in the generation of 3'alpha,5'beta, 6'beta-trihydroxy pravastatin (Km, 4,887 +/- 2,185 microM) and hydroxy pravastatin (Km, 20,987 +/- 9,389 microM). The formation rates of 3'alpha,5'beta,6'beta-trihydroxy pravastatin by reconstituted CYP3A enzymes were (1,000 microM pravastatin) 1.9 +/- 0.6 pmol.min-1.pmol CYP3A4 and 0.06 +/- 0.04 pmol.min-1.pmol CYP3A5, and the formation rates of hydroxy pravastatin were 0.12 +/- 0.02 pmol.min-1.pmol CYP3A4 and 0.02 +/- 0.004 pmol.min-1.pmol CYP3A5. The specific CYP3A inhibitors cyclosporine, ketoconazole, and troleandomycin significantly inhibited hydroxy pravastatin formation by human liver microsomes, but only ketoconazole inhibited 3'alpha, 5'beta,6'beta-trihydroxy pravastatin formation, suggesting that other CYP enzymes are involved in its formation. It is concluded that, compared with lovastatin [CLint formation 6'beta-hydroxylovastatin (microl.min-1.mg-1): 199 +/- 248, 6'-exomethylene lovastatin: 138 +/- 104)], CYP3A-dependent metabolism of pravastatin [CLint formation 3'alpha,5'beta, 6'beta-trihydroxy pravastatin (microl.min-1.mg-1): 0.03 +/- 0.03 and hydroxy pravastatin: 0.02 +/- 0.02] is a minor elimination pathway. In contrast to lovastatin, drug interactions with pravastatin CYP3A-catalyzed metabolism cannot be expected to have a clinically significant effect on its pharmacokinetics.

Metabolism of digoxin and digoxigenin digitoxosides in rat liver microsomes: involvement of cytochrome P4503A

1. The sequential metabolism of digoxin (Dg3) to digoxigenin bis-digitoxoside (Dg2), digoxigenin mono-digitoxoside (Dg1) and digoxigenin (Dg0) was investigated in rat liver microsomes. 2. Kinetic studies produced results consistent with a single enzyme mechanism describing the successive oxidative cleavages. Formation of Dg2 was catalysed with mean (+/-SD) Km and Vmax of 125 +/- 22 microM and 362 +/- 37 pmol/min/mg protein, respectively. The corresponding values for the formation of Dg1 were 61 +/- 5 microM and 7 +/-1 pmol/min/mg protein. Dg0 formation was catalysed with the apparent values of 30 +/- 9 microM and 310 +/- 30 pmol/min/mg protein. 3. Chemical inhibition of cytochrome P450 (CYP) 3A subfamily with ketoconazole and triacetyoleandomycin decreased the formation of Dg2 and Dg1 by up to 90%. Antibodies specific to rat CYP3A2 lowered the rate of oxidative cleavage of Dg3 and Dg2 by up to 85%. Inhibition of CYP2E1, CYP2C subfamily and CYP1A2 by chemical and immuno-inhibition did not affect initial rates of metabolism of Dg3 and Dg2. In contrast, Dg1 metabolism was not affected by triacetyloleandomycin as well as by antibodies to CYP3A2, CYP2C11, CYP2E1, CYP2B1/2B2 and CYP1A2. It was however inhibited by >80% by gestodene and 17alpha-ethynylestradiol (selective inhibitors of human CYP3A). 4. Collectively, these data support the involvement of CYP3A in the cleavage of Dg3 and Dg2 in rat liver microsomes. The enzyme-metabolizing Dg1 remains to be identified.

Molecular and physical mechanisms of first-pass extraction

This is a report of a symposium held at the March 1997 meeting of the American Society for Pharmacology and Therapeutics in San Diego. Our understanding of the events that control first-pass drug elimination in humans has increased tremendously by two sequential discoveries. First, cytochrome P-450s 3A4 and 5 are expressed at high concentrations in both hepatocytes and upper intestinal enterocytes, and therefore limit the systemic availability of many drugs. Second, P-glycoprotein is expressed at the lumenal surface of the intestinal epithelium and therefore also acts to oppose the absorption of unchanged drug. The following discussion brings together our current understandings of these interrelated phenomena to aid a more complete picture of how they may contribute both qualitatively and quantitatively to first-pass elimination.

Development of a high-performance liquid chromatographic-electrospray mass spectrometric assay for the specific and sensitive quantification of the novel immunosuppressive macrolide 40-O-(2-hydroxyethyl)rapamycin

It was our objective to develop a rapid, sensitive and specific assay to quantify the immunosuppressive macrolide 40-O-(2-hydroxyethyl)rapamycin (SDZ-RAD) in blood of transplant patients. SDZ-RAD was extracted from blood by solid-liquid extraction. SDZ-RAD and its internal standard 28,40-diacetyl rapamycin were quantified using HPLC-electrospray MS. The assay was linear from 0.1 to 100 microg/l (r2 = 0.99). The mean recovery was 83% for SDZ-RAD and 80.5% for the internal standard. The mean day-to-day precision was 8.0%. Extracted samples were stable at 20 degrees C for at least 48 h and SDZ-RAD blood samples at -80 degrees C for at least six months.

Preliminary evaluation of progestins as inducers of cytochrome P450 3A4 activity in postmenopausal women

The effects of intramuscularly and orally administered medroxyprogesterone acetate on cytochrome P450 3A4 (CYP3A4) activity were investigated in twelve postmenopausal women in a randomized, crossover study. Unbound prednisolone clearance and the erythromycin breath test were used as markers of CYP3A4 activity. After 2 months of intramuscular progestin therapy, unbound prednisolone clearance increased by 25% in five of six subjects. Similarly, after intramuscular progestin therapy, results from the erythromycin breath test showed a 23% mean increase in CYP3A4 activity. In contrast, 2 months of oral progestin therapy had no effect on prednisolone pharmacokinetics or erythromycin metabolism. These results suggest that parenterally but not orally administered progestins may induce or activate the CYP3A4 enzyme system, leading to an increased metabolism of many CYP3A4 substrates.

Role of P-glycoprotein and cytochrome P450 3A in limiting oral absorption of peptides and peptidomimetics

Cytochrome P450 3A4 (CYP3A4), the major phase I drug metabolizing enzyme in humans, and the MDR1 gene product P-glycoprotein (P-gp) are present at high concentrations in villus tip enterocytes of the small intestine and share a significant overlap in substrate specificity. A large body of research both in vitro and in vivo has established metabolism by intestinal CYP3A4 as a major determinant of the systemic bioavailability of orally administered drugs. More recently it has been recognized that drug extrusion by intestinal P-gp can both reduce drug absorption and modulate the effects of inhibitors and inducers of CYP3A-mediated metabolism. There is relatively little data regarding the effects of CYP3A and P-gp on peptide drugs; however, studies with the cyclic peptide immunosuppresant cyclosporine as well as peptidomimetics such as the HIV-protease inhibitor saquinavir (Invirase) and a new cysteine protease inhibitor K02 (Morpholine-Urea-Phe-Hphe-Vinyl sulfone; Axys Pharmaceuticals) provide some insight into the impact of these systems on the oral absorption of peptides.

High-performance liquid chromatographic determination and identification of acyl migration and photodegradation products of furosemide 1-O-acyl glucuronide

Stability of furosemide glucuronide, the major metabolite of furosemide, was studied in order to accurately assess the glucuronidation of furosemide. Furosemide glucuronide was purified by high-performance liquid chromatography, and the mass spectrum of furosemide glucuronide showed the molecular ion peaks [M-H]- at 505 and 507 (m/z). Furosemide glucuronide was photodegraded to the compound, which was shown more hydrophilic than furosemide glucuronide by high-performance liquid chromatography assay. The photodegradation product of furosemide glucuronide was hydrolyzed to one of the photodegradation products of furosemide by beta-glucuronidase, indicating that the photodegradation product of furosemide glucuronide possessed a glucuronic acid moiety. Furthermore, the mass spectrum of the photodegradation product of furosemide glucuronide exhibited molecular ion peaks [M-H]- at 487 and [M-2H+2Na]- at 509, indicating the chlorine displacement of furosemide glucuronide by a hydroxyl group. Furosemide glucuronide was unstable in an aqueous solution (pH=7.4), and presumed acyl migration isomers of furosemide glucuronide (furosemide glucuronide-isomers) were detected by high-performance liquid chromatography equipped with photodiode array UV detector. The UV spectra of seven furosemide glucuronide-isomers were closely similar to that of furosemide glucuronide but not furosemide. Exposing a mixture of furosemide glucuronide and furosemide glucuronide-isomers to light resulted in the production of new compounds. UV spectra of photodegradation products of furosemide glucuronide-isomers were closely similar to those of photodegradation product of furosemide glucuronide. These results suggested that furosemide glucuronide-isomers were also photodegraded, resulting in the displacement of chlorine by a hydroxyl group as in furosemide glucuronide.

Characterization of P-glycoprotein mediated transport of K02, a novel vinylsulfone peptidomimetic cysteine protease inhibitor, across MDR1-MDCK and Caco-2 cell monolayers

PURPOSE

Here we characterized the transport properties of morpholine-urea-phenylalanine-homophenylalanine-vinylsulfone-phenyl (K02), a newly developed peptidomimetic cysteine protease inhibitor, across monolayers of P-gp-expressed MDRI transfected MDCK cells (MDR1-MDCK) and Caco-2 cells.

METHODS

MDR1-MDCK, MDCK and Caco-2 cells, grown to confluence on Transwell insert membranes, were used to investigate transcellular transport of [14C]-K02.

RESULTS

The basolateral to apical (B-A) flux of 10 microM [14C]-K02 across MDR1-MDCK cells was markedly greater than its apical to basolateral (A-B) flux (ratio = 39). This specific B-A transport was temperature dependent and saturable, with an apparent Michaelis-Menten constant and maximum velocity of 69.1 +/- 19.5 microM and 148.9 +/- 16.3 pmol/min/cm2, respectively. This B-A flux was significantly inhibited by cyclosporine (IC50 = 17.1 +/- 0.7 microM), vinblastine (IC50 = 75.9 +/- 13.0 microM) and verapamil (IC50 = 236 +/- 63 microM). In Caco-2 cell monolayers, the B-A flux was reduced about 50% compared to that in MDR1-MDCK and the A-B flux was increased about 8-fold. The apparent Michaelis-Menten constant and maximum velocity values for the B-A transport were 71.8 +/- 45.9 microM and 35.3 +/- 9.0 pmol/min/ cm2. This B-A flux was also significantly inhibited by P-gp substrates/ inhibitors. Western blots showed that the P-gp expression in MDR1-MDCK cells was about 10-fold that in Caco-2 cells.

CONCLUSIONS

K02 is transported by P-gp in both MDR1-MDCK and Caco-2 cells, and the in vitro interactions between K02 and various P-gp substrates may provide strategies to overcome the bioavailability barrier by intestinal P-gp.

Effects of ketoconazole on the intestinal metabolism, transport and oral bioavailability of K02, a novel vinylsulfone peptidomimetic cysteine protease inhibitor and a P450 3A, P-glycoprotein dual substrate, in male Sprague-Dawley rats

We investigated the effects of ketoconazole on the oral bioavailability of morpholine-urea-phenylalanine-homophenylalanine-vinylsulfone-phenyl (K02), a vinylsulfone peptidomimetic cysteine protease inhibitor, and a P450 3A (CYP3A) and P-glycoprotein dual substrate, in male Sprague-Dawley rats, so as to evaluate the roles of CYP3A and P-gp in K02 disposition. Male Sprague-Dawley rats (8-10 wk old, n = 3-6) were administered a single dose of K02 (10 mg/kg) i.v. or (30 mg/kg) p.o. with or without a concomitant oral dose of ketoconazole (20 mg/kg). Blood samples were collected from 2 min to 8 h after administration through a implanted jugular vein cannula. K02 plasma concentrations were determined by liquid chromatography/mass spectrometer/mass spectrometer analysis. Ketoconazole markedly raised the area under the curve of orally administered K02 from 9.4 +/- 4.4 to 102 +/- 24 mg . min/liter and decreased K02 oral plasma clearance from 3810 +/- 1620 to 306 +/- 60 ml/min/kg. With concomitant ketoconazole dosing, the changes of AUC of i.v. administered K02 (from 94 +/- 17 to 107 +/- 14 mg . min/liter) and clearance (from 110 +/- 22 to 95 +/- 13 ml/min/kg) were not significant, although K02 oral bioavailability increased from 2.9 +/- 1.4 to 31.0 +/- 7.5% (P < .001). In summary, ketoconazole, a dual inhibitor of CYP3A and P-glycoprotein, can effectively increase K02 oral bioavailability by inhibiting the CYP3A/P-gp absorption barrier in the small intestine.

Identification of the hepatic protein targets of reactive metabolites of acetaminophen in vivo in mice using two-dimensional gel electrophoresis and mass spectrometry

Liver toxicity following an overdose of acetaminophen is frequently considered a model for drug-induced hepatotoxicity. Extensive studies over many years have established that such toxicity is well correlated with liver protein arylation by acetaminophen metabolites. Identification of protein targets for covalent modifications is a challenging but necessary step in understanding how covalent binding could lead to liver toxicity. Previous approaches suffered from technical limitations, and thus over the last 10 years heroic efforts were required to determine the identity of only a few target proteins. We present a new mass spectrometry-based strategy for identification of all target proteins that now provides a comprehensive survey of the suite of liver proteins modified. After administration of radiolabeled acetaminophen to mice, the proteins in the liver tissue lysate were separated by two-dimensional polyacrylamide gel electrophoresis. In-gel digestion of the radiolabeled gel spots gave a set of tryptic peptides, which were analyzed by matrix-assisted laser desorption ionization mass spectrometry. Interrogation of data bases based on experimentally determined molecular weights of peptides and product ion tags from postsource decay mass spectra was employed for the determination of the identities of modified liver proteins. Using this method, more than 20 new drug-labeled proteins have been identified.

Effects of ketoconazole on digoxin absorption and disposition in rat

Digoxin, a cardiac glycoside, is a substrate of the multidrug transporter P-glycoprotein (Pgp), and in rats has also been identified as a substrate for cytochrome P450 3A (CYP3A). Ketoconazole, an antifungal agent, was shown to inhibit Pgp in a multidrug-resistant cell line, and is known to be a potent inhibitor of CYP3A. Here, we determined the effects of ketoconazole on digoxin absorption and disposition in rats. Digoxin was administered intravenously or orally with or without a concomitant oral dose of ketoconazole. When given intravenously, digoxin AUC increased from 93 +/- 22 to 486 +/- 26 microg x h/l with ketoconazole administration. Similarly, ketoconazole raised the AUC of orally administered digoxin from 63 +/- 17 to 411 +/- 50 microg x h/l. Concomitant ketoconazole administration prolonged digoxin elimination, yielding a nonlinear pharmacokinetic profile. Using time-averaged values, digoxin bioavailability increased from 0.68 +/- 0.18 to 0.84 +/- 0.10, while mean absorption time was reduced from 1.1 +/- 0.4 to 0.3 +/- 0.1 h. Thus, in rats, ketoconazole increases digoxin plasma concentrations, rate of absorption and bioavailability. Although the effects of ketoconazole on AUC could be explained by inhibition of both CYP3A and Pgp, which cannot be differentiated in this study, the decreased mean absorption time can only be explained by inhibition of Pgp in the intestine.

Metabolism and transport of the macrolide immunosuppressant sirolimus in the small intestine

Small intestinal metabolism and transport of sirolimus, a macrolide immunosuppressant with a low and highly variable oral bioavailability, were investigated using small intestinal microsomes and intestinal mucosa in the Ussing chamber. After incubation of sirolimus with human and pig small intestinal microsomes, five metabolites were detected using high performance liquid chromatography/electrospray-mass spectrometry: hydroxy, dihydroxy, trihydroxy, desmethyl and didesmethyl sirolimus. The same metabolites were generated by human liver microsomes and pig small intestinal mucosa in the Ussing chamber. Anti-CYP3A antibodies, as well as the specific CYP3A inhibitors troleandomycin and erythromycin, inhibited small intestinal metabolism of sirolimus, confirming that, as in the liver, CYP3A enzymes are responsible for sirolimus metabolism in the small intestine. Of 32 drugs tested, only known CYP3A substrates inhibited sirolimus intestinal metabolism with inhibitor constants (Ki) equal to those in human liver microsomes. The formation of hydroxy sirolimus by small intestinal microsomes isolated from 14 different patients ranged from 28 to 220 pmol.min-1.mg-1 microsomal protein. In the Ussing chamber, >99% of the sirolimus metabolites reentered the mucosa chamber against a sirolimus gradient, indicating active countertransport. Intestinal drug metabolism and countertransport into the gut lumen, drug interactions with CYP3A substrates and inhibitors in the small intestine and an 8-fold interindividual variability of the intestinal metabolite formation rate significantly contribute to the low and highly variable bioavailability of sirolimus.

Overlapping substrate specificities of cytochrome P450 3A and P-glycoprotein for a novel cysteine protease inhibitor

K02 (morpholine-urea-Phe-Hphe-vinylsulfone), a newly developed peptidomimetic, acts as a potent cysteine protease inhibitor, especially of cathepsins B and L (which are associated with cancer progression) and cruzain (a cysteine protease of Trypanosoma cruzi, which is responsible for Chagas' disease). Here we investigated features of the disposition of K02 using in vitro systems, characterizing the interaction of the drug with human cytochrome P450 (CYP) 3A and P-glycoprotein (P-gp), a mediator of multidrug resistance (MDR) to cancer chemotherapy and a countertransporter in the intestine that limits oral drug bioavailability. P-gp functions as an ATP-dependent drug efflux pump to reduce intracellular cytotoxic concentrations. An HPLC assay was developed to analyze K02 and its metabolites formed in human liver microsomes. Three major primary metabolites were determined by LC/MS/MS to be hydroxylated products of the parent compound. A rabbit anti-CYP3A polyclonal antibody (200 microl antibody/mg microsomal protein) produced 75-94% inhibition of the formation of these three hydroxylated metabolites. Ketoconazole (5 microM), a selective CYP3A inhibitor, produced up to 75% inhibition, whereas other CYP-specific inhibitors, i.e. quinidine (CYP2D6), 7,8-benzoflavone (CYP1A2), and sulfaphenazole (CYP2C9), showed no significant effects. An identical metabolite formation profile for K02 was observed with cDNA-expressed human CYP3A4 (Gentest). These data demonstrate that K02 is a substrate for CYP3A. Formation of 1'-hydroxymidazolam, the primary human midazolam metabolite, was markedly inhibited by K02 via competitive processes, which suggests the potential for drug-drug interactions of K02 with other CYP3A substrates. K02 significantly inhibited the photoaffinity labeling of P-gp with azidopine and LU-49888, a photoaffinity analogue of verapamil. Transport studies with [14C]K02, using MDR1-transfected Madin-Darby canine kidney cell monolayers in the Transwell system, demonstrated that the basolateral-to-apical flux of K02 across MDR1-transfected Madin-Darby canine kidney cells was markedly greater than the apical-to-basolateral flux (ratio of 63 with 10 microM [14C]K02). This suggests that K02 is also a P-gp substrate. These studies are important for formulating strategies to increase the absorption and/or decrease the elimination of K02 and to optimize its delivery to malignant cells and parasite-infected host cells.

Stereoselective metabolism of benoxaprofen in rats. Biliary excretion of benoxaprofen taurine conjugate and glucuronide

Benoxaprofen (BOP) was administered iv to bile duct-cannulated rats at a dose of 10 mg/kg. BOP and its metabolites in plasma, urine, and bile were quantified using HPLC. A previously unidentified BOP metabolite was found in HPLC chromatograms of rat bile, and the metabolite was isolated chromatographically. Positive-ion fast-atom bombardment (FAB) MS analysis of the compound showed [M+H]+ at m/z 409, i.e. 108 mass units greater than the molecular weight of BOP (301 mass units). In the 1H NMR spectrum of the compound, two signals assigned to two methylene groups appeared at 2.53 ppm and 3. 30 ppm, in addition to BOP signals. Analysis of FAB mass spectra and 1H-1H and 1H-13C correlated NMR spectra of the isolated metabolite suggested that the new metabolite was a BOP taurine conjugate (BOP-T). A BOP-T standard was chemically synthesized, and physicochemical data were compared with those for the isolated metabolite. Identical results, i.e. RF values from TLC, RT values from HPLC, and FAB MS and 1H-13C correlated NMR findings, were obtained, establishing that the new metabolite found in rat bile was BOP-T. In five rats, mean values for per cent excretion of the dose in bile over 12 hr for BOP glucuronide (BOP-G), BOP-T, and unchanged BOP were 13.2 +/- 2.3, 2.54 +/- 0.80, and 0.33 +/- 0.09%, respectively. Furthermore, the optical isomers of BOP and its metabolites in plasma and bile were analyzed using a chiral HPLC column. (R)-BOP showed rapid plasma elimination, whereas the plasma elimination of (S)-BOP was very slow. The amounts of BOP, BOP-G, and BOP-T enantiomers excreted into the bile were as follows: (S)-BOP-G and (R)-BOP-G, 12.5 +/- 1.8 and 2.1 +/- 0.6% of the dose; (R)-BOP-T and (S)-BOP-T, 2.0 +/- 0.6 and 0.3 +/- 0.05% of the dose; (R)-BOP and (S)-BOP, 0.02 +/- 0.03 and 0.2 +/- 0.1% of the dose, respectively. (S)-BOP was metabolized mainly to BOP-G, and BOP-T excreted into the bile was produced mainly from (R)-BOP.

Mechanisms for covalent binding of benoxaprofen glucuronide to human serum albumin. Studies By tandem mass spectrometry

Tandem MS has been used to establish the structure and specific binding sites of covalent protein adducts formed upon incubation of the acyl glucuronide of the propionic acid nonsteroidal anti-inflammatory drug benoxaprofen with human serum albumin in vitro. Benoxaprofen 1-O-beta-glucuronide was enzymatically synthesized in vitro and incubated with human serum albumin both in the presence and in the absence of NaCNBH3. The modified human serum albumins were digested with trypsin and separated by HPLC. The modified peptides were detected using HPLC-electrospray MS (with selected-ion monitoring) and were structurally characterized by tandem MS using matrix-assisted laser desorption ionization in both the post-source decay and high-energy collision-induced dissociation modes. These studies established that benoxaprofen glucuronide forms covalent adducts with protein nucleophiles both by nucleophilic displacement of glucuronic acid at the anomeric center and by condensation of the rearranged acyl glucuronic acid isomers with epsilon-amino functions of lysine residues after acyl migration of the aglycone from the anomeric center. Lys-159 was identified as the major binding site. Thus, we have established that members of the less reactive propionic acid class of acyl glucuronides, such as the glucuronide of benoxaprofen, are also capable of reacting with protein nucleophiles to form covalent adducts analogous to those of tolmetin glucuronide (tolmetin is an acetic acid nonsteroidal anti-inflammatory drug), via the mechanisms previously reported from this laboratory, and that the specific covalent binding site profile appears to be drug dependent.

Antiprogestin-mediated inactivation of cytochrome P450 3A4

Based on previous observations of very short periods of linearity for antiprogestin metabolite formation and the presence of a common tertiary amine moiety in each compound as the principal site of their metabolism, we hypothesized that mifepristone, lilopristone and onapristone are oxidized by cytochrome P450 (CYP) 3A4 to reactive nitroso species that complex the heme of the enzyme, thereby inactivating it. Upon preincubation with human liver microsomes in the presence (but not the absence) of NADPH, mifepristone inhibited midazolam 1'-hydroxylation, a marker of CYP3A4 catalytic activity, very potently (IC50 approximately 3.5 mumol/l) and extensively (by approximately 87%). Lilopristone and onapristone also displayed NADPH and time-dependent inactivation of CYP3A4 with characteristics very similar to mifepristone. These data support antiprogestin-mediated inactivation of CYP3A4 and suggest the potential for drug-drug interactions and time-dependent nonlinearities in pharmacokinetics upon their long-term administration.

Modulation of P-glycoprotein expression by cytochrome P450 3A inducers in male and female rat livers

A strong overlap between P-glycoprotein (Pgp) and cytochrome P450 3A (CYP3A) substrates and modulators has been reported. To test the hypothesis that CYP3A and Pgp are coordinately regulated, we examined the effects of known inducers of CYP3A (triacetyloleandomycin, rifampicin, dexamethasone, pregnenolone 16alpha-carbonitrile) on Pgp expression in rat liver. We also investigated the gender-specific expression of Pgp and compared its response to dexamethasone between male and female rats. In male rats, western blot analyses showed that rifampicin and dexamethasone caused 50% and 5-fold increases in Pgp levels, respectively. RNase protection assays using gene-specific probes for the three Pgp isoforms revealed a 3-fold increase in mdr2 mRNA levels after dexamethasone administration and a 2-fold increase following rifampicin treatment. Triacetyloleandomycin and pregnenolone 16alpha-carbonitrile had no effect on Pgp expression and mRNA levels. We also observed that the basal level of Pgp was 40% lower in male rats than in females and that mdr2 mRNA levels in male rats were one-half those in females. As opposed to the results in male rats, dexamethasone reduced Pgp expression by approximately 60% and caused a 30% decrease in mdr2 mRNA levels in female rats. Mdr1a was not affected and mdr1b was not detected in female or male rats. We conclude that, at the dosage regimen used, CYP3A and Pgp responses to CYP3A inducers are regulated independently in rat liver. In addition, this study shows that Pgp expression and regulation are gender specific.

Preparative chromatography of furosemide 1-O-acyl-glucuronide from urine using micronized amberiite XAD-2 and its application to other 1-O-acyl-glucuronides

Furosemide 1-O-acyl glucuronide (Fgnd) was extracted from the urine following oral administration of furosemide. The crude Fgnd was applied to micronized Amberlite XAD-2 column (2.5 cm i.d. x 90 cm length, 75-500 microns particle size). The purified Fgnd was identified by mass spectrometry and beta-glucuronidase treatment. This method was also applicable to the purification of glucuronide of tolmetin (nonsteroidal anti-inflammatory drug, NSAID), suggesting that it was applicable to the other NSAIDs, most of which were known to be metabolized to acyl-glucuronides.

Antiprogestin pharmacodynamics, pharmacokinetics, and metabolism: implications for their long-term use

Antiprogestins represent a relatively new and promising class of therapeutic agents that could have significant impact on human health and reproduction. In the present work, the pharmacodynamics, pharmacokinetics, and metabolism of mifepristone (MIF), lilopristone (LIL), and onapristone (ONA) in humans are reviewed, and characteristics bearing important clinical implications are discussed. Although MIF has gained notoriety as an "abortion pill," antiprogestins may more importantly prove effective in the treatment of endometriosis, uterine leiomyoma, meningioma, cancers of the breast and prostate, and as contraceptive agents. MIF pharmacokinetics display nonlinearities associated with saturable plasma protein (alpha 1-acid glycoprotein, AAG) binding and characterized by lack of dose dependency for parent drug plasma concentrations (for doses greater than 100 mg) and a zero-order phase of elimination. LIL and ONA pharmacokinetics are less well characterized but ONA does not appear to bind AAG and displays a much shorter t1/2 than the other agents. The three antiprogestins are substrates of cytochrome P450 (CYP) 3A4, an enzyme exceedingly important in human xenobiotic metabolism. Even more implicative of likely drug-drug interactions subsequent to their long-term administration are recent data from our laboratory indicating that they inactivate CYP3A4 in a cofactor- and time-dependent manner, suggesting that complexation and induction of the enzyme may occur in vivo via protein stabilization. Moreover, it has been demonstrated that MIF increases CYP3A4 mRNA levels in human hepatocytes in primary culture, indicative of message stabilization and/or transcriptional activation of CYP3A4 expression. Finally, MIF has also been shown to inhibit P-glycoprotein function. Whether LIL and ONA share these latter two characteristics with MIF has not yet been determined but they illustrate properties that, in addition to diminished antiglucocorticoid activities and altered pharmacokinetic characteristics, warrant consideration during the development of these and never antiprogestational agents.

Cytochrome P4503A4-mediated N-demethylation of the antiprogestins lilopristone and onapristone

The metabolism of two newer antiprogestational agents, lilopristone and onapristone, was investigated using human liver microsomes, and evidence was obtained supporting a principal role of cytochrome P450 (CYP) 3A4 in their N-demethylations. Kinetic studies with microsomes from three organ donors indicated lack of biphasic kinetics at substrate concentrations up to 200 microM, consistent with a single enzyme mediating the oxidations. Selective chemical inhibitors of CYP1A2 (furafylline), CYP2C9 (sulfaphenazole), CYP2D6 (quinidine), and CYP2A6/2E1 (diethyldithiocarbamic acid) did not affect initial rates of metabolism of either steroid. Gestodene and triacetyloleandomycin (selective for CYP3A enzymes) inhibited the demethylations of both antiprogestins by up to 77%. Rabbit polyclonal antibodies to CYP3A4 decreased initial rates of N-demethylation of the antihormones by up to 82%, whereas antibodies to CYP2C9 were not inhibitory. Collectively, these data thus suggest potential drug-drug interactions of these promising new therapeutic agents with concomitantly administered CYP3A4 substrates.

Role of intestinal P-glycoprotein (mdr1) in interpatient variation in the oral bioavailability of cyclosporine

Interpatient differences in the oral clearance of cyclosporine (INN, ciclosporin) have been partially attributed to variation in the activity of a single liver enzyme termed CYP3A4. Recently it has been shown that small bowel also contains CYP3A4, as well as P-glycoprotein, a protein able to transport cyclosporine. To assess the importance of these intestinal proteins, the oral pharmacokinetics of cyclosporine were measured in 25 kidney transplant recipients who each had their liver CYP3A4 activity quantitated by the intravenous [14C-N-methyl]-erythromycin breath test and who underwent small bowel biopsy for measurement of CYP3A4 and P-glycoprotein. Forward multiple regression revealed that 56% (i.e., r2 = 0.56) and 17% of the variability in apparent oral clearance [log (dose/area under the curve)] were accounted for by variation in liver CYP3A4 activity (p < 0.0001) and intestinal P-glycoprotein concentration (p = 0.0059), respectively. For peak blood concentration, liver CYP3A4 activity accounted for 32% (p = 0.0002) and P-glycoprotein accounted for an additional 30% (p = 0.0024) of the variability. Intestinal levels of CYP3A4, which varied tenfold, did not appear to influence any cyclosporine pharmacokinetic parameter examined. We conclude that intestinal P-glycoprotein plays a significant role in the first-pass elimination of cyclosporine, presumably by being a rate-limiting step in absorption. Drug interactions with cyclosporine previously ascribed to intestinal CYP3A4 may instead be mediated by interactions with intestinal P-glycoprotein.

Tacrolimus oral bioavailability doubles with coadministration of ketoconazole

OBJECTIVE

To quantitate the effect of ketoconazole, an azole antifungal agent and potent inhibitor of CYP3A4 and P-glycoprotein, on the bioavailability of tacrolimus, a substrate of the CYP3A system and of P-glycoprotein.

SUBJECTS AND METHODS

The pharmacokinetics of tacrolimus were studied in six healthy volunteers (two women and four men) in a four-dose study after each received single doses of tacrolimus alone (0.1 mg/kg orally and 0.025 mg/kg intravenously) and with coadministered ketoconazole (200 mg orally at bedtime for 12 days). The dose of tacrolimus was reduced during the ketoconazole phase (0.04 mg/kg orally; 0.01 mg/kg intravenously). Ketoconazole and tacrolimus doses were separated by approximately 10 hours. Whole blood tacrolimus concentrations were determined by enzyme-linked immunosorbent assay. Estimated pharmacokinetic parameters in whole blood (mean +/- SD) before and with ketoconazole were calculated with noncompartmental techniques.

RESULTS

Coadministration of ketoconazole did not consistently affect tacrolimus clearance (55.6 +/- 16.7 ml/hr/kg versus 42.5 +/- 7.6 ml/hr/kg), and steady-state volume of distribution was unchanged (0.99 +/- 0.26 L/kg versus 0.93 +/- 0.25 L/kg). However, a significant increase in tacrolimus bioavailability (14% +/- 5% versus 30% +/- 8%; p < 0.01) was observed with coadministered ketoconazole. Hepatic bioavailability was unchanged by the presence of ketoconazole (96% +/- 1% versus 97% +/- 1%).

CONCLUSIONS

Because ketoconazole did not alter hepatic bioavailability and because 10 hours separated administration times of the drugs, it appears that the marked increase in tacrolimus bioavailability can be explained by ketoconazole having a local inhibitory effect on tacrolimus gut metabolism or on intestinal P-glycoprotein activity.

Investigation of aortic CYP3A bioactivation of nitroglycerin in vivo

Nitroglycerin (GTN) has been used to treat heart disease for many years. It is generally believed that GTN is a prodrug; however, the mechanism for GTN bioactivation remains unknown. Recent studies, using hepatic microsomes, have suggested the involvement of cytochrome P450 3A (CYP3A) in GTN biotransformation. Here, we used an animal model to test the hypothesis that aortic CYP3A plays a role in the bioactivation of GTN in vivo. Ketoconazole (KCZ), a potent CYP3A inhibitor, was given to rats (50 mg/kg i.p.) 1 hr before a bolus dose of GTN (2 mg/rat i.v.). KCZ decreased GTN-induced cGMP (cyclic guanosine monophosphate) levels by 20 to 30% (P < .05), without affecting basal or S-nitroso, N-acetyl penicillamine-induced levels of cGMP. When rats received dexamethasone (DEX, 30 mg/kg, 4 days i.p.), a strong CYP3A inducer, they exhibited a significant (approximately 50%) higher cGMP response to GTN than the control group. When rats received the combination treatment of both DEX and KCZ, they responded to GTN to the same extent as control rats. Although the effect of KCZ on aortic CYP3A activity cannot be detected (activity in control rats is below the detection limit), KCZ markedly inhibited CYP3A activity in rat livers (2.02 +/- 0.04 vs. 0.31 +/- 0.04 nmol/mg prot/min, P < .05, in control vs. KCZ-treated rats, respectively) and in DEX pretreated rat aorta (0.145 +/- 0.036 vs. 0.042 +/- 0.037 nmol/mg prot/min, P < .05, in rats treated with DEX alone vs. rats treated with both DEX and KCZ, respectively). KCZ did not elicit an effect on aortic glutathione S-transferases, another major metabolic enzyme responsible for GTN biotransformation. DEX enhanced the aortic GST mu activity by 3-fold. However, the activity of GST in aorta did not correlate with the cGMP response to GTN. In conclusion, our results demonstrate that CYP3A activity in aorta is correlated with GTN bioactivation in vivo, but the contribution of this enzyme to overall GTN bioactivation is limited.

Stereoselective high-performance liquid chromatography determination of propranolol and 4-hydroxypropranolol in human plasma after pre-column derivatization

A stereoselective reversed-phase HPLC assay to quantify S-(-) and R-(+) enantiomers of propranolol and 4-hydroxypropranolol in human plasma was developed. The method involved liquid-liquid extraction for sample clean-up and employed 2,3,4,6-tetra-O-acetyl-beta-glucopyranosyl isothiocyanate as a pre-column chiral derivatization reagent. The internal standard used was 4-methylpropranolol. The derivatized products were separated on an Altex C18 column using a mixture of acetonitrile-water-phosphoric acid-triethylamine (58:42:0.1:0.06 and 50:50:0.15:0.06, v/v, for propranolol and 4-hydroxypropranolol, respectively) as mobile phase. The detection of propranolol derivatives was made at lambda(ex)=280 nm and lambda(em)=325 nm, and the corresponding 325 and 400 nm were used for 4-hydroxypropranolol derivatives. The assay was linear from 1 to 100 ng/ml and from 2 to 50 ng/ml using 0.5 ml of human plasma for propranolol and 4-hydroxypropranolol enantiomers, respectively. The present assay is used to quantify the enantiomers of propranolol and 4-hydroxypropranolol, respectively, in human plasma for pharmacokinetic studies.

Determination of amiprilose in human plasma by high-performance liquid chromatography with fluorimetric detection

A reversed-phase HPLC method to quantify amiprilose in human plasma is described. The method involves liquid-liquid extraction of amiprilose and the internal standard from plasma. The extracted compounds are derivatized with 1,8-naphthalic dicarboxylic acid using 2-chloro-1-methylpyridinium iodide as a coupling reagent. The derivatized products are separated on a reversed-phase column and monitored fluorimetrically using 280 nm and 340 nm as excitation and emission wavelengths, respectively. The derivatized products which exhibit two peaks on chromatogram, are shown to be the interconvertible isomers. This assay has been used in pharmacokinetic studies of amiprilose in humans.

Pharmacokinetics and pharmacodynamics of metformin in healthy subjects and patients with noninsulin-dependent diabetes mellitus

This study was conducted to assess the effect of noninsulin-dependent diabetes mellitus (NIDDM) and gender on the pharmacokinetics of metformin and to investigate whether or not metformin exhibits dose-dependent pharmacokinetics. The pharmacodynamic effects (on plasma glucose and insulin) of metformin in patients with NIDDM and in healthy subjects also were assessed. Nine patients with NIDDM and 9 healthy subjects received 4 single-blind single-dose treatments of metformin HCL (850 mg, 1,700 mg, 2,550 mg, and placebo) and a multiple-dose treatment of 850 mg metformin HCL (3 times daily for 19 doses). After each single-dose treatment and the final dose of the multiple-dose phase, multiple plasma and urine samples were collected for 48 hours and assayed for metformin levels. Plasma samples were also assayed for glucose and insulin levels. There were no significant differences in metformin kinetics in patients with NIDDM compared with healthy subjects, in men compared with women, or during multiple-dose treatment versus single-dose treatment. Plasma concentrations of metformin increase less than proportionally to dose, most likely due to a decrease in percent absorbed. In patients with NIDDM, single doses of 1,700-mg or higher of metformin significantly decrease postprandial, but not preprandial, glucose concentrations and do not influence insulin concentrations. With multiple doses, both preprandial and postprandial glucose concentrations and preprandial insulin concentrations were significantly lower than with placebo. The effect of metformin on glucose level is correlated with the average fasting plasma glucose level without drug. In healthy subjects, single and multiple doses of metformin showed no effect on plasma glucose, but significantly attenuated the rise in immediate postprandial insulin levels.

Food intake and dosage level, but not tablet vs solution dosage form, affect the absorption of metformin HCl in man

The pharmacokinetics of four single-dose treatments of the metformin administered orally (as the HCl salt) were compared in 24 healthy subjects: 500 mg and 850 mg tablets and 850 mg solution fasting and 850 mg tablet with food. Solution and tablet formulations are bioequivalent. Bioavailability of a 500 mg tablet is 14% greater than that of an 850 mg tablet. Compared with the fasting state, bioavailability is 24% lower, and the peak concentration delayed about 37 min when an 850 mg tablet is administered with food.

Identification of CYP3A4 as the principal enzyme catalyzing mifepristone (RU 486) oxidation in human liver microsomes

Various complementary approaches were used to elucidate the major cytochrome P450 (CYP) enzyme responsible for mifepristone (RU 486) demethylation and hydroxylation in human liver microsomes: chemical and immunoinhibition of specific CYPs; correlation analyses between initial rates of mifepristone metabolism and relative immunodetectable CYP levels and rates of CYP marker substrate metabolism; and evaluation of metabolism by cDNA-expressed CYP3A4. Human liver microsomes catalyzed the demethylation of mifepristone with mean (+/-SD) apparent K(m) and Vmax values of 10.6 +/- 3.8 microM and 4920 +/- 1340 pmol/min/mg protein, respectively; the corresponding values for hydroxylation of the compound were 9.9 +/- 3.5 microM and 610 +/- 260 pmol/min/mg protein. Progesterone and midazolam (CYP3A4 substrates) inhibited metabolite formation by up to 77%. The CYP3A inhibitors gestodene, triacetyloleandomycin, and 17 alpha-ethynylestradiol inhibited mifepristone demethylation and hydroxylation by 70-80%; antibodies to CYP3A4 inhibited these reactions by approximately 82 and 65%, respectively. In a bank of human liver microsomes from 14 donors, rates of mifepristone metabolism correlated significantly with relative immunodetectable CYP3A levels, rates of midazolam 1'-and 4-hydroxylation and rates of erythromycin N-demethylation, marker CYP3A catalytic activities (all r2 > or = 0.85 and P < 0.001). No significant correlations were observed for analyses with relative immunoreactive levels or marker catalytic activities of CYP1A2, CYP2C9, CYP2C19, CYP2D6, or CYP2E1. Recombinant CYP3A4 catalyzed mifepristone demethylation and hydroxylation with apparent K(m) values 7.4 and 4.1 microM, respectively. Collectively, these data clearly support CYP3A4 as the enzyme primarily responsible for mifepristone demethylation and hydroxylation in human liver microsomes.