Metabolic profiles of montelukast sodium (Singulair), a potent cysteinyl leukotriene1 receptor antagonist, in human plasma and bile

Montelukast sodium [1-([(1(R)-(3-(2-(7-chloro-2-quinolinyl)-(E)- ethenyl)phenyl)-3-(2-(1-hydroxy-1-methylethyl)phenyl)propyl)thio]methyl)cyclopropylacetic acid sodium salt] (MK-476, Singulair) is a potent and selective antagonist of the cysteinyl leukotriene (Cys-LT1) receptor and is under investigation for the treatment of bronchial asthma. To assess the metabolism and excretion of montelukast, six healthy subjects received single oral doses of 102 mg of [14C]montelukast, and the urine and feces were collected. Most of the radioactivity was recovered in feces, with </=0.2% appearing in urine. Based on these results and the reported modestly high oral bioavailability of montelukast, it could be concluded that a major part of the radioactivity was excreted via bile. A second clinical study was conducted to identify biliary metabolites of montelukast. The bile was aspirated using a modified procedure involving a nasogastric tube placed fluoroscopically near the ampulla of Vater, after an oral dose of 54.8 mg of [14C]montelukast. This technique appears to be a new application for drug metabolism studies. The study was conducted with fasted and nonfasted subjects, with the bile being aspirated continuously under suction over periods of 2-8 hr and 8-12 hr after the dose, respectively. Two hours before the end of the collection procedure, cholecystokinin carboxyl-terminal octapeptide was administered iv to stimulate gallbladder contraction. Plasma samples also were collected periodically over 10 hr. Due to the nature of the collection procedure and the limited sampling time, recovery of radioactivity in bile was incomplete and varied from 3 to 20% of the dose. Radiochromatographic and LC-MS/MS analyses of bile showed the presence of one major and several minor metabolites, along with small amounts of unchanged parent drug. The minor metabolites were identified, by LC-MS/MS comparison with synthetic standards or by NMR, as acyl glucuronide (M1), sulfoxide (M2), 25-hydroxy (a phenol, M3), 21-hydroxy (diastereomers of a benzylic alcohol, M5a and M5b), and 36-hydroxy (diastereomers of a methyl alcohol, M6a and M6b) analogs of montelukast. The major metabolite was characterized as a dicarboxylic acid (M4), a product of further oxidation of the hydroxymethyl metabolite M6. Chiral LC-MS/MS analyses of M4 revealed that this diacid, like M5 and M6, was formed in both diastereomeric forms. The levels of metabolites in the systemic circulation were low in the fed as well as fasted subjects, with <2% of the circulating radioactivity being due to metabolites M5a, M5b, M6a, and M6b. Overall, this bile aspiration technique, which is less invasive than either T-tube drainage or fine-needle percutaneous puncture, provided a convenient and expedient means of identifying the biliary metabolites of montelukast, relatively free of contributions from colonic microflora.

In vitro metabolism of indinavir in the human fetal liver microsomes

In vitro microsomal formation of primary metabolites of indinavir (CRIXIVAN, MK-0639, L-735,524), an HIV protease inhibitor, were qualitatively similar among the different developmental stages in humans, although the fetal liver had a lower capability to form the metabolites than the pediatric and adult liver. The lower activity of fetal liver was mainly owing to a decrease in the Vmax values. The Vmax value in the fetus was about one-third of that in the adult human, while no significant difference was found in Km values between groups. The liver microsomes were also characterized using P450 markers to examine the development-associated alteration in P450 functional activities. Debrisoquine 4-hydroxylase activity was comparable among the three age groups. In contrast, tolbutamide methyl hydroxylase activity, as well as the CYP3A marker, testosterone 6beta-hydroxylase activity, in the fetal liver microsomes was much lower than in the pediatric and adult by more than 40-fold. However, the difference in testosterone 2beta-hydroxylase and nifedipine N-oxidase activities between fetus and adult was markedly smaller. The ratio of indinavir metabolism in pediatric or adult liver to fetus was 1.7 for pediatric and 3.6 for adult liver microsomes. Similarly, testosterone 2beta-hydroxylase and nifedipine N-oxidase activities showed smaller differences between adult (or pediatric) and fetal liver microsomes than testosterone 6beta-hydroxylase activity. The reason for the observed marked differences in the development-associated alteration may lie in the differences of substrate specificities between CYP3A isoforms.

Structural characterization of pulsed laser-deposited hydroxyapatite film on titanium substrate

Pure, crystalline hydroxyapatite (HA) films with thicknesses of roughly 10 microns have been deposited on titanium substrate using the pulsed laser deposition (PLD) technique. Experimental results indicate that the structure and properties of the PLD-HA films varied with deposition parameters. The PLD process used in the present study did not induce significant amounts of calcium phosphate phases other than apatite, or significant changes in the behaviour of hydroxyl or phosphate functional groups. Broad face scanning electron microscopy showed that HA coating was comprised of numerous essentially spheroidal-shaped particles of different sizes, while the lateral morphology indicated that columnar and dome-shaped structures both existed in the film. Many pinholes and crevices observed on coating surfaces were linked to the original substrate surface crevices/craters. The adhesion strength of the coating, mostly in the range of 30-40 MPa, was found to be closely related to the fractography of the tested specimen. The fracture surfaces of specimens with higher bond strengths were usually accompanied by a higher degree of deformation and coating-substrate debonding, while the fracture of specimens with lower bond strengths occurred more frequently within HA coatings in a more brittle manner. The energy dispersive spectroscopy-determined Ca/P ratios of raw HA powder (1.78) and sintered HA target for PLD (1.79) were very close, indicating that the sintering process used in the present study essentially did not change the Ca/P ratio of HA. After the PLD process, the Ca/P ratio of the HA film increased to 1.99. Cross-sectional scanning electron microscopy-energy dispersive spectroscopy point analysis indicated that the value of the Ca/P ratio was significantly higher in the region near the surface, particularly near the coating-substrate interface, than in the coating interior.

Intramural coronary artery disease in swine with naturally occurring hypertrophic cardiomyopathy

Intramural coronary artery disease (ICAD) has been reported in myocardium affected with hypertrophic cardiomyopathy (HCM), but has never been studied in detail with respect to the cell type or lipid infiltration involved in the wall-thickening. The lack of heart samples may be one of the rationales to hamper the progress in investigating this disease. Recently, the discovery of naturally occurring HCM in swine has provided an excellent opportunity for the study of ICAD because of the high prevalence of ICAD in this animal. The present study provides a detailed structure feature in the thickened arterial wall of ICAD by both histologic and electron microscopic means. Morphologically, the feature of ICAD is due primarily to the neointimal thickening. Smooth muscle cells (SMC) and extracellular matrix (collagen and elastic fibers) are the major components responsible for the thickened neointima. Fragmentation of the internal elastic membrane is associated with the migration and proliferation of SMC from the media to the intima. Therefore, pigs with HCM may be a potential animal model not only for the study of the mechanism by which SMC migrate and proliferate into intima, but also for the future investigation of interventions in coronary artery occlusion.

Congenital short bowel syndrome with left acheiria: report of one case

A case of congenital short bowel syndrome with left acheiria, hemivertebra and dextrocardia is described. Dilatation of the fetal bowel was observed at the 24th week of gestation during a routine ultrasonic scan of a healthy 23-year-old primigravida from a non-consanguineous marriage. Amniocentesis and chorionic villus sampling were denied. The baby was delivered at 38 weeks of gestational age. After delivery, multiple anomalies were noted: left acheiria, congenital short bowel syndrome (15 cm in length of the ileum), pseudo-obstruction of intestine, dextrocardia, and hemivertebrae. We suspected these abnormalities might be due to a vascular accident or failure of lateralization during the early gestational period. To our knowledge, these combinations have not been reported previously in English literature.

Hepatic microsomal metabolism of montelukast, a potent leukotriene D4 receptor antagonist, in humans

Montelukast (L-706,631, MK-0476, SINGULAIR), a potent and selective leukotriene D4 (CysLT1) receptor antagonist, is currently under development for the treatment of asthma. In vitro studies were conducted using human liver microsomes to evaluate: 1) the difference in the metabolic kinetics of montelukast between adult and pediatric subjects; 2) the relative contribution of flavin-containing monooxygenase and cytochrome P450 (P450) to the sulfoxidation; and 3) the P450 isoforms responsible for montelukast oxidation. No statistically significant difference was observed in the in vitro kinetics for acyl glucuronidation and oxidative metabolism between the two age groups. Results from studies on heat inactivation of flavin-containing monooxygenase and immunochemical inhibition by an anti-rat NADPH P450 reductase antibody on montelukast oxidation indicated that all oxidative metabolism of montelukast-including diastereomeric sulfoxidations, as well as 21- and methyl-hydroxylations-are catalyzed exclusively by P450. Five in vitro approaches have been used to identify the P450 isoforms responsible for the human liver microsomal oxidation of montelukast. The experimental results consistently indicated that CYP3A4 catalyzes sulfoxidation and 21-hydroxylation, whereas CYP2C9 selectively mediates methyl-hydroxylation.

Dexamethasone administered into organum vasculosum laminae terminalis of rabbits induced antipyresis via inhibiting nitric oxide pathway in situ

Direct administration of lipopolysaccharide (LPS) into the organum vasculosum laminae terminalis (OVLT) increased the amount of nitric oxide (NO) release and inducible NO synthase expression. These increases paralleled the increase in deep body temperature in unanesthetized rabbits. Pretreatment with dexamethasone, a synthetic glucocorticoid, not only reduced the fever but also attenuated the NO release and the inducible NO synthase expression in the OVLT following an intra-OVLT dose of LPS. The data suggest that steroids such as dexamethasone exert their antipyresis by inhibiting the NO pathway in the OVLT of rabbit brain.

RNA-protein interactions: involvement of NS3, NS5, and 3' noncoding regions of Japanese encephalitis virus genomic RNA

The mechanism of replication of the flavivirus Japanese encephalitis virus (JEV) is not well known. The structures at the 3' end of the viral genome are highly conserved among divergent flaviviruses, suggesting that they may function as cis-acting signals for RNA replication and, as such, might specifically bind to cellular or viral proteins. UV cross-linking experiments were performed to identify the proteins that bind with the JEV plus-strand 3' noncoding region (NCR). Two proteins, p71 and p110, from JEV-infected but not from uninfected cell extracts were shown to bind specifically to the plus-strand 3' NCR. The quantities of these binding proteins increased during the course of JEV infection and correlated with the levels of JEV RNA synthesis in cell extracts. UV cross-linking coupled with Western blot and immunoprecipitation analysis showed that the p110 and p71 proteins were JEV NS5 and NS3, respectively, which are proposed as components of the RNA replicase. The putative stem-loop structure present within the plus-strand 3' NCR was required for the binding of these proteins. Furthermore, both proteins could interact with each other and form a protein-protein complex in vivo. These findings suggest that the 3' NCR of JEV genomic RNA may form a replication complex together with NS3 and NS5; this complex may be involved in JEV minus-strand RNA synthesis.

Hepatic and intestinal metabolism of indinavir, an HIV protease inhibitor, in rat and human microsomes. Major role of CYP3A

The metabolism of indinavir, a human immune deficiency virus (HIV) protease inhibitor, has been characterized extensively in rats and humans. All oxidative metabolites found in vivo were formed when indinavir was incubated with NADPH-fortified hepatic and intestinal microsomes obtained from rats and humans. In vitro kinetic studies revealed that Vmax/Km values (microL/min/mg protein) in rat and human liver microsomes were approximately 8- and 2-fold greater than those in the intestinal microsomes of the corresponding species (55.8 and 6.7 for the liver and intestine, respectively, in rats; 16.5 and 7.7 for the liver and intestine, respectively, in humans). However, when Vmax/Km was scaled up to intrinsic clearance (mL/min/kg body weight), hepatic intrinsic clearance was much greater than the intestinal clearance by 50- to 200-fold. These results suggest that the liver plays a much greater role in first-pass metabolism of indinavir than the intestine in both species. Consistently, ketoconazole, a selective inhibitor for CYP3A, and an anti-rat CYP3A1 antibody strongly inhibited hepatic and intestinal metabolism of indinavir in both rats and humans, suggesting the involvement of CYP3A isoforms in both organs. Oral treatment of rats with dexamethasone (50 mg/kg/day for 4 days), a potent CYP3A inducer, increased both hepatic and intestinal metabolism of indinavir by a factor of 7 and 3, respectively. Furthermore, indinavir selectively inhibited 6beta-hydroxylase activity of testosterone, a CYP3A marker activity, in rat and human liver microsomes; the interactions between testosterone and indinavir were competitive with Ki values of < 1.0 microM.

Variant complex translocations involving chromosomes 1, 9, 9, 15 and 17 in acute promyelocytic leukemia without RAR alpha/PML gene fusion rearrangement

Acute promyelocytic leukemia (APL;M3) is specifically characterized by a predominance of malignant promyelocytes having atypical reciprocal translocation involving chromosome 15 and 17 [t(15;17)(q22;q11)] resulting in the fusion of retinoic acid receptor alpha (RAR alpha) on chromosome 17 and the putative transcription factor gene PML, ie the translocation generates two fusion transcripts, PML/RAR alpha and RAR alpha/PML. We describe a patient with clinical and morphologic characteristics of atypical APL but with a previously undescribed variant translocation. A 35-year-old Hispanic having atypical APL was referred for cytogenetic evaluation. The cytogenetic findings with GTG-banding coupled with FISH analysis revealed the following karyotype: 46,XX,der(9)t(1;9)(q25;q34)der(9)t(9;?)(q34;?), t(15;17)(q22;q11)ish. der(9)t(1;9)(q25;q34)(WCP1+,WCP9+),t(9;17;15)(q34;q11;q22) (WCP9+,WCP15+,PML+;WCP17+,RAR alpha +;WCP15+,WCP17+,PML-)[20]/46,XX[5]. The chromosome 17q was translocated to the chromosome 15q. However, chromosome 15q including the PML gene normally translocating to 17q and creating the RAR alpha/PML fusion gene, translocated to chromosome 9q. Does this patient have another subset of APL? Or is the genetics of APL different in cases with variant translocations as opposed to those with atypical t(15;17) translocation, though in the majority of the cases their clinical presentation remains the same.

Activation of ICAM-1 promoter by lysophosphatidylcholine: possible involvement of protein tyrosine kinases

Lysophosphatidylcholine (lyso-PC) selectively upregulates the mRNA level of intercellular adhesion molecule-1 (ICAM-1) but not that of vascular cell adhesion molecule-1 (VCAM-1) in cultured human umbilical vein endothelial cells. Transfection studies show that lyso-PC activates the ICAM-1 promoter but not the VCAM-1 promoter. Gel mobility shift assays document an increase in NF-kappa B binding in cells treated with lyso-PC. The increases of ICAM-1 mRNA and NF-kappa B binding were inhibited by the protein tyrosine kinase inhibitors, genistein and lavendustin A, but not by inhibitors for cyclic AMP-dependent protein kinases or protein kinase C. Our results suggest that lyso-PC induces ICAM-1 expression most likely by activating NF-kappa B, and that the effect appears to be protein tyrosine kinase-dependent.

A complex structure in the mRNA of Tf1 is recognized and cleaved to generate the primer of reverse transcription

All retroviruses and LTR-containing retrotransposons are thought to require specific tRNA molecules to serve as primers of reverse transcription. An exception is the LTR-containing retrotransposon Tf1, isolated from Schizosaccharomyces pombe. Instead of requiring a tRNA, the reverse transcriptase of Tf1 uses the first 11 bases of the Tf1 transcript as the primer for reverse transcription. The primer is generated by a cleavage that occurs between bases 11 and 12 of the Tf1 mRNA. Sequence analysis of the 5' untranslated region of the Tf1 mRNA resulted in the identification of a region with the potential to form an RNA structure of 89 bases that included the primer binding site and the first 11 bases of the Tf1 mRNA. Systematic mutagenesis of this region revealed 34 single-point mutants in the structure that resulted in reduced transposition activity. The defects in transposition correlated with reduced level of Tf1 reverse transcripts as determined by DNA blot analysis. Evidence that the RNA structure did form in vivo included the result that strains with second site mutations that restored complementarity resulted in increased levels of reverse transcripts and Tf1 transposition. The majority of the mutants defective for reverse transcription were unable to cleave the Tf1 mRNA between bases 11 and 12. These data indicate that formation of an extensive RNA structure was required for the cleavage reaction that generated the primer for Tf1 reverse transcription.

Comparison of defibrillation efficacy using biphasic waveforms delivered from various capacitances/pulse widths

The efficacy of the biphasic waveform shock for the defibrillation of the ventricular myocardium has been reported by researchers and physicians. Although many authors have suggested that biphasic waveforms delivered from lower capacitances and shorter pulse widths could result in the reduction of the energy required for successful defibrillation, no report has described the smallest capacitance and pulse width yielding the lowest DFT. In this study, we compared efficacies of the biphasic waveform shocks and DFT safety margins among five different capacitances (175 mu f, 125 mu f. 100 mu f. 75 mu f, and 50 mu f) combined with 1-3 pulse widths. These experiments performed in six dogs used an endocardial lead/subcutaneous patch defibrillation electrode system. The average DFTs at E50 for 175 mu f (6.5/3.5 ms), 125 mu f (6.5/3.5 ms), 100 mu f (6.0/3.0 ms), 75 mu f (4.0/2.0) ms, and 50 mu f (3.0/2.0 ms) were 8.5, 10.0, 11.0, 14.0, and 16.5), respectively. These results indicate that a biphasic waveform delivered from a larger capacitance with a proper pulse width could achieve a higher defibrillation efficacy. All DFTs at E50 for all waveforms were compared to their deliverable energies and maximum stored energies. This comparison indicated a narrow DFT safety margin with capacitances below 100 mu f. Therefore, it is concluded that higher energy and higher leading edge voltage are required for a biphasic waveform delivered from a smaller capacitance with a shorter pulse width. Since the current capacitor technology provides a maximum voltage of 750 V using two capacitors in series, with the electrode impedance system used in this study, smaller capacitors appear to have a decreased probability of defibrillation success at a given energy.

Comparison between huperzine A, tacrine, and E2020 on cholinergic transmission at mouse neuromuscular junction in vitro

AIM

To compare the effects of huperzine A (Hup A), tacrine, and E2020 on cholinergic transmission at mouse neuromuscular junction in vitro.

METHODS

The isolated mouse phrenic nerve-hemidiaphragm preparations were used with the conventional intracellular recording technique. The miniature end-plate potentials (MEPP), the mean quantal content of end-plate potentials (EPP), and the resting membrane potentials of muscle fiber were recorded.

RESULTS

Hup A, tacrine, and E2020 at the concentration of 1.0 mumol.L-1 increased the amplitude, time-to-peak, and half-decay time of MEPP in the potencies of E2020 > Hup A > tacrine. Hup A did not significantly change the frequency of MEPP, the appearance of giant MEPP or slow MEPP, the resting membrane potentials, and the mean quantal content of EPP.

CONCLUSION

Hup A is a selective and potent cholinesterase inhibitor, by which activity it facilitates the cholinergic transmission at mouse neuromuscular junction, and devoid of pre- and post-synaptic actions.

Induction of vascular cell adhesion molecule-1 by low-density lipoprotein

Low-density lipoprotein (LDL) is a well-established risk factor for atherosclerosis. When endothelial cells are incubated with this lipoprotein in pathophysiologic amounts, the cells are activated. Among the documented cellular responses to LDL is increased recruitment of monocytes, which are believed to play a major role in promoting intimal plaque formation. The findings presented here link an atheogenic lipoprotein, LDL, with the induction of an adhesion molecule important in atherogenesis Human LDL induces the vascular cell adhesion molecule-1 (VCAM-1) transcriptionally with an increase in mRNA levels through activation of the VCAM promoter. This effect is blocked by anti-VCAM antibodies. After a 2-day incubation in LDL, the binding of NF-kappa B, which is believed to be a key oxidative-stress sensor for VCAM regulation, remains at basal level. In contrast, the binding activities of AP-1 and GATA, on the other hand, are increased by LDL. Thus, a component of LDL-enhanced endothelial recruitment of monocytes is attributed to VCAM-1 expression, which appears to be mediated through AP-1 and GATA. These data identify LDL as a VCAM-inducer possibly distinct from cytokines and endotoxin.

Sex-dependent pharmacokinetics of indinavir: in vivo and in vitro evidence

Indinavir, a potent and specific inhibitor of human immunodeficiency virus protease, is used for the treatment of AIDS. This study was designed to investigate the sex-related differences in kinetics and metabolism of indinavir in rats, dogs, and monkeys to support the toxicity studies. When given intravenously, indinavir was cleared rapidly in a polyphasic manner in all species. Indinavir exhibited significant differences in elimination kinetics among species. The rat had the highest plasma clearance (CLp; 41-89 ml/min/kg), and the dog had the lowest CLp (15-26 ml/min/kg), with the monkey exhibiting an intermediate value (36-39 ml/min/kg). Furthermore, marked sex-related differences in CLp were observed in rats and dogs, but not in monkeys. The CLp was 89 ml/min/kg for male rats and 41 ml/min/kg for female rats. In contrast to rats, female dogs cleared indinavir more rapidly than male dogs; the CLp was 26 ml/min/kg for female dogs and 15 ml/min/kg for male dogs. Consistent with the in vivo observations, hepatic microsomes from male rats had a substantially higher metabolizing activity toward indinavir than that from females, whereas liver microsomes from female dogs catalyzed the drug at a higher rate than that from male dogs. Qualitatively, in vitro metabolic profiles of indinavir were similar among species and between male and female animals. Studies with an anti-rat cytochrome P450 (CYP) 3A1 antibody pointed to the probable involvement of isoforms in the CYP3A subfamily in the oxidative metabolism of indinavir in both males and females of all species. The functional activity of CYP3A measured by the formation of testosterone 6beta-hydroxylation and immunoblot analysis of the level of CYP3A proteins strongly suggested that gender differences in the levels of CYP3A isoforms may contribute to the observed sex-related differences in indinavir metabolism in rats and dogs.

Disposition of indinavir, a potent HIV-1 protease inhibitor, after an oral dose in humans

Indinavir, N-[2(R)-hydroxy-1(S)-indanyl]-5-[2(S)-tertiary- butylaminocarbonyl-4-(3-pyridylmethyl)piperazino]-4(S)- hydroxy-2(R)-phenylmethylpentanamide (L-735,524,MK-639, ayl-4- Crixivan), is a potent and specific inhibitor of the HIV-1(3 protease for the treatment of AIDS. Disposition of [14C]indinavir was investigated in six healthy subjects after single oral administration of 400 mg. AUC, Cmax, and Tmax values for indinavir were 492 microM x min, 4.7 microM, and 50 min, respectively. The AUC value for the total radioactivity in plasma was 1.9 times higher than that of indinavir, indicating the presence of metabolites. The major excretory route was through feces, and the minor through urine. Mean recovery of radioactivity in the feces was 83.4%. In the urine, mean recoveries of the total radioactivity and unchanged indinavir were 18.7% and 11.0% of the dose, respectively. HPLC radioactivity and LC-MS/MS analyses of urine showed the presence of indinavir and low levels of quaternary pyridine N-glucuronide (M1), 2',3'-trans-dihydroxyindanylpyridine N-oxide (M2), 2',3'-trans-dihydroxyindan (M3) and pyridine N-oxide (M4a) analogs, and despyridylmethyl analogs of M3 (M5) and indinavir (M6). M5 and M6 were the major metabolites in urine. The metabolic profile in plasma was similar to that in urine. Quantitatively, the metabolites in feces accounted for >47% of the dose, which along with the urinary excretion of approximately 19%, suggested that the absorption of the drug was appreciable. In the feces, radioactivity was predominantly due to M3, M5, M6, and the parent compound. Thus, in urine and feces, the prominent metabolic pathways were oxidations and oxidative N-dealkylations. Excretion of the quaternary N-glucuronide metabolite in the urine, which is a minor metabolite in human, was specific to primates.

Species and organ differences in first-pass metabolism of the ester prodrug L-751,164 in dogs and monkeys. In vivo and in vitro studies

The pharmacokinetics and bioavailability of L-751,164, an ethyl ester prodrug of a potent fibrinogen receptor antagonist, L-742,998, were studied in beagle dogs and rhesus monkeys. In both species, L-751,164 exhibited high clearance. After an intravenous dose, L-751,164 was converted to the parent L-742,998 to the extent of approximately 20% in dogs and 90% in monkeys. After oral administration of the prodrug, however, the bioavailability, measured either as the prodrug or as the active parent, was < 5% in both species. Several experiments were conducted subsequently to investigate possible causes for the observed similarities in the low oral bioavailability of the prodrug between species despite its differences in the in vivo conversion. In vitro metabolism studies using dog liver subcellular fractions indicated extensive metabolism of L-751,164 to metabolites other than L-742,998. Kinetically, L-742,998 formation accounted only for approximately 25% of the prodrug disappearance. In contrast, monkey liver preparations converted L-751,164 exclusively and rapidly to L-742,998. Good agreement between the in vitro hepatic metabolism and the in vivo observations suggests that liver was the major eliminating organ after intravenous administration of the prodrug in both species. In dogs, this suggestion was further supported by low bioavailability of the prodrug (20%) and the parent (below detection limit) after intraportal administration of the prodrug. In vitro metabolism of L-751,164 using intestinal S9 fractions revealed substantial metabolism in monkeys, but not in dogs. Several NADPH-dependent metabolites were observed with monkey intestinal preparation, with the parent L-742,998 being the minor product (approximately 25-30%). Furthermore, L-751,164 was shown, by means of an in vitro Caco-2 cell, and in situ rat intestinal loop models, to be highly permeable to intestinal barriers. Collectively, these results suggest that the apparent species differences in the prodrug conversion observed in vivo likely were due to species differences in the hepatic metabolism of the prodrug. In both species, the high first-pass metabolism of the prodrug, and the extensive conversion of the prodrug to metabolic products other than the parent contributed, at least in part, to the low bioavailability of the prodrug and active parent, respectively, obtained after an oral dose of the prodrug. The latter process was species-dependent, involving primarily the hepatic first-pass elimination in dogs and the intestinal first-pass metabolism in monkeys.

Species differences in the pharmacokinetics and metabolism of indinavir, a potent human immunodeficiency virus protease inhibitor

Indinavir, a potent and specific inhibitor of human immunodeficiency virus protease, is undergoing clinical investigation for the treatment of acquired immunodeficiency syndrome. The studies described herein were designed to characterize the absorption, distribution, metabolism, and excretion of the drug in rats, dogs, and monkeys. Indinavir exhibited marked species differences in elimination kinetics. The plasma clearance was in the rank order: rat (107 ml/min/kg) > monkey (36 ml/min/kg) > dog (16 ml/min/kg). Significant differences in the bioavailability of indinavir also were observed. When given orally as a solution in 0.05 M citric acid, the bioavailability varied significantly from 72% in the dog to 19% in the monkey, and 24% in the rat. These differences in bioavailability were attributed mainly to species differences in the magnitude of hepatic first-pass metabolism. The distribution of indinavir was studied only in rats, both intravenously and orally. Intravenously, indinavir was distributed widely throughout the body. Brain uptake studies showed that indinavir penetrated the blood-brain barrier, but that the penetration was limited. After oral administration, indinavir was distributed rapidly into and out of the lymphatic system. The rapid lymph transfer is of clinical relevance, because a primary clinical hallmark of acquired immunodeficiency syndrome is the depletion of CD4 lymphocytes. Biliary and urinary recovery studies revealed that metabolism was the major route of indinavir elimination in all species, and N-dealkylation, N-oxidation, and hydroxylation seemed to be the major pathways. Although limited to qualitative aspects, the metabolite profile obtained from in vitro microsomal studies generally reflected the in vivo oxidative metabolism of indinavir in all species studies. Results from the chemical and immunochemical inhibition studies indicated the possible involvement of isoforms of the CYP3A subfamily in the oxidative metabolism of indinavir in rats, dogs, and monkeys. This is consistent with our previous studies, which have shown that CYP3A4 is the isoform responsible for the oxidative metabolism of indinavir in human liver microsomes. Furthermore, the in vivo oxidative metabolism of indinavir in rats, dogs, and monkeys was qualitatively similar to that in humans. The high degree of similarity in the metabolite profiles of drug metabolism between animals and humans validates the use of these animal models for toxicity studies of indinavir. Attempts were made to quantitatively extrapolate in vitro metabolic data to in vivo metabolism. With the application of the well-stirred and parallel-tube models, the hepatic clearance and hepatic extraction ratio were calculated using the in vitro Vmax/Km values. In rats, the predicted hepatic clearance (31 ml/ min/kg) and hepatic extraction ratio (0.47) agreed well with the observed in vivo hepatic clearance (43 ml/min/kg) and hepatic extraction ratio (0.68). In addition, the hepatic clearance of indinavir was predicted reasonably well in dogs and monkeys. Based on the in vitro intrinsic clearance of human liver microsomes, a small but significant hepatic first-pass metabolism (ca. 25%) is expected in humans.

Characterization of the NTPase activity of Japanese encephalitis virus NS3 protein

Japanese encephalitis (JE) virus NS3 protein and two N-terminally truncated (delta 1-148 and delta 1-323) forms of NS3 were engineered and expressed in E. coli as fusion proteins with a histidine tag at the N terminus. The purified recombinant proteins his-NS3 and his-NS3(delta 1-148) were found to possess NTPase activity which was stimulated by single-stranded RNA, whereas NS3(delta 1-323) did not. The requirements for MgCl2 and MnCl2 and the salt and pH ranges necessary for optimal activity of the enzyme were determined and shown to be slightly different from those of the NTPases of other flaviviruses. Poly(U) and poly(C) were better than poly(A) at stimulating the NTPase activities, in contrast to other flaviviral NTPases. The substrate preference was in the order GTP > ATP >> UTP > CTP. Interestingly, we found that Ca2+ could not substitute for Mg2+; on the contrary, it inhibited NTPase activity. The removal of the N-terminal 148 amino acids enhanced NTPase activity, but further deletion of the region (amino acids 148-323) completely abolished the activity. Therefore, amino acids 148-323 contain a critical region required for NTPase activity.

In vitro studies on the metabolic activation of the furanopyridine L-754,394, a highly potent and selective mechanism-based inhibitor of cytochrome P450 3A4

L-754,394, a furanopyridine derivative, is an experimental anti-HIV agent which has been shown to be an unusually potent and selective inhibitor of cytochrome P450 3A enzymes in a number of mammalian species. In the present studies, L-754,394 was demonstrated to undergo NADPH-dependent metabolic activation in hepatic microsomal preparations from rats, dogs, rhesus monkeys, and humans to electrophilic intermediates which became bound covalently to cellular proteins. The extent of binding was species-dependent, the highest levels being observed with liver microsomes from rhesus monkeys. Inclusion in incubation media of the nucleophilic trapping agents glutathione, cysteine, or methoxyamine led to a modest (15-25%) decrease in the covalent binding, while trichloropropylene oxide, an inhibitor of epoxide hydrolase, had no effect. When L-754,394 was incubated with monkey liver microsomes, the corresponding dihydrofurandiol was identified as a metabolite by liquid chromatography-tandem mass spectrometry. In contrast, when incubations were carried out in the presence of methoxyamine, the O-methyloxime derivative of the ring-opened dihydrodiol tautomer was formed, while inclusion of glutathione or N-acetylcysteine led to the formation of S-linked conjugates of a putative furan epoxide. Collectively, these results are taken to indicate that L-754,394 undergoes cytochrome P450-dependent oxidation of the fused furan ring system, leading to the formation of chemically-reactive intermediates. One or more of these electrophilic species may be responsible for the autocatalytic destruction of cytochrome P450 enzymes which accompanies L-754,394 metabolism in vitro and in vivo.

A modified procedure for caseinophosphopeptide analysis

A modified procedure is established for analyzing caseinophosphopeptides. The sodium caseinate hydrolysate is first treated by immobilized metal ion affinity chromatography to enrich the phosphopeptides. Because of the formation of Fe(3+)-peptide complexes, ethylenediaminetetraacetic acid is added to the bound fraction eluted with the immobilized metal ion affinity chromatography to disintegrate the complexes. Thus, the subsequent high-performance liquid chromatographic analysis is facilitated. A stepwise gradient elution is also suggested to enhance the resolution of caseinophosphopeptides during high-performance liquid chromatographic analysis.