The duration of fetal antenatal steroid exposure determines the durability of preterm ovine lung maturation

BACKGROUND

Antenatal corticosteroids (ACS) are the standard of care for maturing the fetal lung and improving outcomes for preterm infants. Antenatal corticosteroid dosing remains nonoptimized, and there is little understanding of how different treatment-to-delivery intervals may affect treatment efficacy. The durability of a lung maturational response is important because the majority of women treated with antenatal corticosteroids do not deliver within the widely accepted 1- to 7-day window of treatment efficacy.

OBJECTIVE

We used a sheep model to test the duration of fetal exposures for efficacy at delivery intervals from 1 to 10 days.

MATERIALS AND METHODS

For infusion studies, ewes with single fetuses were randomized to receive an intravenous bolus and maintenance infusion of betamethasone phosphate to target 1-4 ng/mL fetal plasma betamethasone for 36 hours, with delivery at 2, 4 ,or 7 days posttreatment or sterile saline solution as control. Animals receiving the clinical treatment were randomised to receive either a single injection of 0.25 mg/kg with a 1:1 mixture of betamethasone phosphate + betamethasone acetate with delivery at either 1 or 7 days posttreatment, or 2 treatments of 0.25 mg/kg betamethasone phosphate + betamethasone acetate spaced at 24 hours (giving ∼48 hours of fetal steroid exposure) with delivery at 2, 5, 7, or 10 days posttreatment. Negative control animals were treated with saline solution. All lambs were delivered at 121 ± 3 days gestational age and ventilated for 30 minutes to assess lung function.

RESULTS

Preterm lambs delivered at 1 or 2 days post-antenatal corticosteroid treatment had significant improvements in lung maturation for both intravenous and single-dose intramuscular treatments. After 2 days, the efficacy of 36-hour betamethasone phosphate infusions was lost. The single dose of 1:1 betamethasone phosphate + betamethasone acetate also was ineffective at 7 days. In contrast, animals treated with 2 doses had significant improvements in lung maturation at 2, 5, and 7 days, with treatment efficacy reduced by 10 days.

CONCLUSION

In preterm lambs, the durability of antenatal corticosteroids treatment depends on the duration of fetal exposure and is independent of the intravenous or intramuscular maternal route of administration. For acute 24- to 48-hour posttreatment deliveries, a 24-hour fetal antenatal corticosteroids exposure was sufficient for lung maturation. A fetal exposure duration of at least 48 hours was necessary to maintain long-term treatment durability. A single-dose ACS treatment should be sufficient for women delivering within <48 hours of antenatal corticosteroids treatment.

The Differential Absorption of a Series of P-Glycoprotein Substrates in Isolated Perfused Lungs from Mdr1a/1b Genetic Knockout Mice can be Attributed to Distinct Physico-Chemical Properties: an Insight into Predicting Transporter-Mediated, Pulmonary Specific Disposition

PURPOSE

To examine if pulmonary P-glycoprotein (P-gp) is functional in an intact lung; impeding the pulmonary absorption and increasing lung retention of P-gp substrates administered into the airways. Using calculated physico-chemical properties alone build a predictive Quantitative Structure-Activity Relationship (QSAR) model distinguishing whether a substrate's pulmonary absorption would be limited by P-gp or not.

METHODS

A panel of 18 P-gp substrates were administered into the airways of an isolated perfused mouse lung (IPML) model derived from Mdr1a/Mdr1b knockout mice. Parallel intestinal absorption studies were performed. Substrate physico-chemical profiling was undertaken. Using multivariate analysis a QSAR model was established.

RESULTS

A subset of P-gp substrates (10/18) displayed pulmonary kinetics influenced by lung P-gp. These substrates possessed distinct physico-chemical properties to those P-gp substrates unaffected by P-gp (8/18). Differential outcomes were not related to different intrinsic P-gp transporter kinetics. In the lung, in contrast to intestine, a higher degree of non-polar character is required of a P-gp substrate before the net effects of efflux become evident. The QSAR predictive model was applied to 129 substrates including eight marketed inhaled drugs, all these inhaled drugs were predicted to display P-gp dependent pulmonary disposition.

CONCLUSIONS

Lung P-gp can affect the pulmonary kinetics of a subset of P-gp substrates. Physico-chemical relationships determining the significance of P-gp to absorption in the lung are different to those operative in the intestine. Our QSAR framework may assist profiling of inhaled drug discovery candidates that are also P-gp substrates. The potential for P-gp mediated pulmonary disposition exists in the clinic.

ADME SARfari: comparative genomics of drug metabolizing systems

MOTIVATION

ADME SARfari is a freely available web resource that enables comparative analyses of drug-disposition genes. It does so by integrating a number of publicly available data sources, which have subsequently been used to build data mining services, predictive tools and visualizations for drug metabolism researchers. The data include the interactions of small molecules with ADME (absorption, distribution, metabolism and excretion) proteins responsible for the metabolism and transport of molecules; available pharmacokinetic (PK) data; protein sequences of ADME-related molecular targets for pre-clinical model species and human; alignments of the orthologues including information on known SNPs (Single Nucleotide Polymorphism) and information on the tissue distribution of these proteins. In addition, in silico models have been developed, which enable users to predict which ADME relevant protein targets a novel compound is likely to interact with.

An evaluation of ondansetron binding interactions with human cytochrome P450 enzymes CYP3A4 and CYP2D6

The results of an evaluation study of ondansetron binding to human cytochromes P450 CYP3A4 and CYP2D6 is reported. The methodology includes NMR spectroscopic measurements of substrate to heme iron distances together with molecular modelling of the enzyme-substrate interactions. It is shown that there is a generally good agreement between the experimental and calculated binding affinities for ondansetron towards CYP2D6 and CYP3A4 enzymes, based on interactive docking studies. Moreover, the modelled binding orientations for ondansetron in CYP2D6 and CYP3A4 are largely consistent with the NMR data and with the known routes for P450-mediated metabolism of this compound.

Discovery of potent, orally bioavailable, selective 5-HT1A/B/D receptor antagonists

5-HT1 receptor antagonists have been discovered with good selectivity over the 5-HT transporter. This is the first report of highly potent, selective ligands for the 5-HT1A/B/D receptors with low intrinsic activity, which represent a useful set of molecules for further understanding the roles of the 5-HT1 receptor subtypes and providing new approaches for the treatment of depression.

SB-656104-A, a novel selective 5-HT7 receptor antagonist, modulates REM sleep in rats

1 (6-((R)-2-[2-[4-(4-Chloro-phenoxy)-piperidin-1-yl]-ethyl]-pyrrolidine-1-sulphonyl)-1H-indole hydrochloride) (SB-656104-A), a novel 5-hydroxytryptamine (5-HT(7)) receptor antagonist, potently inhibited [(3)H]-SB-269970 binding to the human cloned 5-HT(7(a)) (pK(i) 8.7+/-0.1) and 5-HT(7(b)) (pK(i) 8.5+/-0.2) receptor variants and the rat native receptor (pK(i) 8.8+/-0.2). The compound displayed at least 30-fold selectivity for the human 5-HT(7(a)) receptor versus other human cloned 5-HT receptors apart from the 5-HT(1D) receptor ( approximately 10-fold selective). 2 SB-656104-A antagonised competitively the 5-carboxamidotryptamine (5-CT)-induced accumulation of cyclic AMP in h5-HT(7(a))/HEK293 cells with a pA(2) of 8.5. 3 Following a constant rate iv infusion to steady state in rats, SB-656104 had a blood clearance (CL(b)) of 58+/-6 ml min(-1) kg(-1) and was CNS penetrant with a steady-state brain : blood ratio of 0.9 : 1. Following i.p. administration to rats (10 mg kg(-1)), the compound displayed a t(1/2) of 1.4 h with mean brain and blood concentrations (at 1 h after dosing) of 0.80 and 1.0 micro M, respectively. 4 SB-656104-A produced a significant reversal of the 5-CT-induced hypothermic effect in guinea pigs, a pharmacodynamic model of 5-HT(7) receptor interaction in vivo (ED(50) 2 mg kg(-1)). 5 SB-656104-A, administered to rats at the beginning of the sleep period (CT 0), significantly increased the latency to onset of rapid eye movement (REM) sleep at 30 mg kg(-1) i.p. (+93%) and reduced the total amount of REM sleep at 10 and 30 mg kg(-1) i.p. with no significant effect on the latency to, or amount of, non-REM sleep. SB-269970-A produced qualitatively similar effects in the same study. 6 In summary, SB-656104-A is a novel 5-HT(7) receptor antagonist which has been utilised in the present study to provide further evidence for a role for 5-HT(7) receptors in the modulation of REM sleep.

Evaluation of human intestinal absorption data and subsequent derivation of a quantitative structure–activity relationship (QSAR) with the Abraham descriptors

The human intestinal absorption of 241 drugs was evaluated. Three main methods were used to determine the human intestinal absorption: bioavailability, percentage of urinary excretion of drug-related material following oral administration, and the ratio of cumulative urinary excretion of drug-related material following oral and intravenous administration. The general solvation equation developed by Abraham's group was used to model the human intestinal absorption data of 169 drugs we considered to have reliable data. The model contains five Abraham descriptors calculated by the ABSOLV program. The results show that Abraham descriptors can successfully predict human intestinal absorption if the human absorption data is carefully classified based on solubility and administration dose to humans.

Cytochrome P450 substrate specificities, substrate structural templates and enzyme active site geometries

The structural characteristics of human cytochrome P450 substrates are outlined in the light of extensive studies on P450 substrate specificity. Templates of superimposed substrates for individual P450 isozymes are shown to fit the corresponding enzyme active sites, where contacts with specific amino acid residues appear to be involved in the interaction with each structural template. Procedures leading to the evaluation of likely P450 specificity, binding affinity and rate of metabolism are described in the context of key examples in which molecular modelling appears to rationalize experimentally observed findings.

Molecular modelling of human CYP2E1 by homology with the CYP102 haemoprotein domain: investigation of the interactions of substrates and inhibitors within the putative active site of the human CYP2E1 isoform

1. The construction of a three-dimensional model of human CYP2E1 is reported. It is based on homology with the haemoprotein domain of the unusual bacterial P450, CYP102, which is of known crystal structure. 2. Interactive docking of a number of human CYP2E1 substrates is consistent with their known positions of CYP2E1-mediated metabolism, where specific interactions with key active site amino acid side-chains appear to rationalize the binding and orientation of substrate molecules. 3. Amino acid residues within the putative active site of human CYP2E1, including those associated with the binding of substrates and inhibitors, are shown to correspond with those identified by site-directed mutagenesis experiments conducted on CYP2 family isoforms, and they are known to affect substrate metabolism regioselectivity. 4. Consequently, it was found that the CYP2E1 active site exhibits complementarity with the structural characteristics of known substrates and inhibitors of this enzyme, including their relatively low molecular weights and disposition of hydrogen bond-forming groups.

Molecular modelling of CYP1 family enzymes CYP1A1, CYP1A2, CYP1A6 and CYP1B1 based on sequence homology with CYP102

Molecular modelling of a number of CYP1 family enzymes from rat, plaice and human is described based on amino acid sequence homology with the haemoprotein domain of CYP102, a unique bacterial P450 of known structure. The interaction of various substrates and inhibitors within the putative active sites of rat CYP1A1, human CYP1A2, a fish CYP1 enzyme CYP1A6 (from plaice) and human CYP1B1, is shown to be consistent with P450-mediated oxidation in each example or, in the case of inhibitors, mechanism of inhibition. It is reported that relatively small changes between the enzymes' active site regions assist in the rationalization of CYP1 enzyme preferences for particular substrate types, and a template of superimposed CYP1A2 substrates is shown to fit the putative active site of the human CYP1A2 enzyme.

First principles investigation of singly reduced cytochrome P450

1. The application of novel ab initio quantum mechanical methods to the states in the catalytic cycle of cytochrome P450 following the first reduction step is described. 2. A good correlation was found between the calculated energy of reduction and the experimentally determined redox potential for a range of substrate- and substrate analogue-bound systems. 3. On reduction of the haem system, the ground state of Fe remains Fe3+. On binding of a CO molecule, Fe adopts a low-spin Fe2+ state, in agreement with experiment. However, on binding of an O2 molecule, calculations indicate that the system adopts a ferric superoxide ground state, in which the Fe is in a low-spin Fe3+ state.

Molecular modelling of CYP2B6, the human CYP2B isoform, by homology with the substrate-bound CYP102 crystal structure: evaluation of CYP2B6 substrate characteristics, the cytochrome b5 binding site and comparisons with CYP2B1 and CYP2B4

1. Molecular modelling studies of CYP2B isoforms from rat (CYP2B1), rabbit (CYP2B4) and man (CYP2B6) are reported, with particular emphasis on substrate interactions with the human CYP2B isoform, CYP2B6. 2. The findings represent an advance on our previous study that focused primarily on the rat CYP2B isoform, CYP2B1, and involved homology modelling with substrate-free CYP102. 3. The current work utilizes the recently published substrate-bound CYP102 crystal structure as a template for construction of the CYP2B subfamily isoforms and shows, in particular, that known CYP2B6 substrate specificity and regioselectivity can be rationalized by putative active site interactions.

Molecular modelling of the human cytochrome P450 isoform CYP2A6 and investigations of CYP2A substrate selectivity

(1) The generation of a homology model of CYP2A6, the major catalyst of human hepatic coumarin 7-hydroxylase activity, involves the use of the recently published substrate-bound CYP102 crystal structure as a template. (2) A substantial number of structurally diverse CYP2A6 substrates are found to dock satisfactorily within the putative active site of the enzyme, leading to the formulation of a structural template (or pharmacophore) for CYP2A6 specificity/selectivity. (3) The CYP2A6 model is consistent with available evidence from site-directed mutagenesis studies carried out on CYP2A subfamily isoforms, and enables some explanation of species differences in CYP2A-mediated metabolism of certain substrates. (4) Quantitative structure-activity relationship (QSAR) analysis of CYP2A5 (the mouse orthologue) mutants yields statistically significant correlations between various properties of amino acid residues and coumarin 7-hydroxylase activity.

Advances in drug metabolism screening

Developments in automation, analytical technologies and molecular biology are being exploited by drug metabolism scientists in order to provide enhanced in vitro systems for the study of the metabolic disposition of potential drug candidates. Routine investigation of factors such as metabolic stability and induction and inhibition of drug metabolizing enzymes is now preferred in the early stages of drug discovery. This, in turn, should provide a greater understanding of the underlying principles governing these processes and allow a greater role for drug metabolism in the design of new drug molecules.

Structural determinants of cytochrome P450 substrate specificity, binding affinity and catalytic rate

The structural characteristics of cytochrome P450 substrates are summarised, showing that molecular descriptors can discriminate between chemicals of differing P450 isozyme specificity. Procedures for the estimation of P450 substrate binding interaction energies and rates of metabolism are described, providing specific examples in both individual compounds binding to P450s, including those of known crystal structure, and within series of structurally related chemicals. It is demonstrated that binding energy components are primarily hydrophobic/desolvation and electrostatic/hydrogen-bonded in nature, whereas electronic factors are of importance in determining variations in reaction rates. It is thus shown that the prediction of P450 substrate binding affinities and catalytic rates may be feasible, provided that sufficient structural information is available for the relevant enzyme-substrate complex.

Molecular modelling of human CYP2C subfamily enzymes CYP2C9 and CYP2C19: rationalization of substrate specificity and site-directed mutagenesis experiments in the CYP2C subfamily

1. The results of molecular modelling of human CYP2C isozymes, CYP2C9 and CYP2C19, are reported based on an alignment with a bacterial form of the enzyme, CYP102. 2. The three-dimensional structures of the CYP2C enzymes are consistent with known experimental evidence from site-directed mutagenesis, antibody recognition and regiospecificity of substrate metabolism. 3. The variations in substrate specificity between CYP2C9 and CYP2C19 can be rationalized in terms of single amino acid residue changes within the putative active site region, of which I99H appears to be the most significant.

Molecular modelling of cytochrome P4502D6 (CYP2D6) based on an alignment with CYP102: structural studies on specific CYP2D6 substrate metabolism

1. A molecular model of CYP2D6 has been constructed from the bacterial form CYP102 via a homology alignment between the CYP2D subfamily and CYP102 protein sequences. 2. A number of typical CYP2D6 substrates are shown to fit the putative active site of the enzyme, as can the specific inhibitor quinidine. 3. Some of the allelic variants in CYP2D6, which give rise to genetic polymorphisms in 2D6-mediated metabolism, can be rationalized in terms of their position within the active site region. 4. The results of site-directed mutagenesis experiments are consistent with the CYP2D6 model generated from the CYP102 crystal structure. 5. The possibility of an alternative orientation within the active site may explain the CYP2D6-mediated metabolism of relatively large-sized substrates.

Molecular modelling of CYP3A4 from an alignment with CYP102: identification of key interactions between putative active site residues and CYP3A-specific chemicals

1. A structural model of CYP3A4 is reported on the basis of a novel amino acid sequence alignment between the CYP3 family and CYP102, a bacterial P450 of known crystal structure. 2. Construction of the CYP3A4 model from CYP102 is facilitated by the relatively high sequence homology between the two protein (52% homology; 27% identity) with many conservative amino acid changes, yielding a structure of low internal energy. 3. A considerable number of specific substrates, and some specific inhibitors, are shown to occupy the putative CYP3A4 active site via interactions with the same amino acid residues in almost all cases investigated. 4. The CYP3A4 model rationalizes the known positions of metabolism for many substrates of this major human P450 such that the route of metabolism in novel development compounds can be predicted.

Absorption and disposition of ranitidine hydrochloride in rat and dog

1. The pharmacokinetics of ranitidine were studied in the male beagle dog at a dose level of 50 mg (intravenous) or 5 mg/kg (oral). 2. After intravenous administration, Clp was moderate (10.4 ml/min/kg) with Clr accounting for approximately 30% of total clearance. Vdarea was 3.5 l/kg, resulting in a t1/2 of approximately 4 h. 3. After oral administration, F was good (73%) with peak plasma concentrations of ranitidine (2 micrograms/ml) achieved within 0.5-1 h hour after dosing. t1/2 (4.1 h) was similar to that observed after intravenous administration. 4. The absorption, metabolism and excretion of [14C]-ranitidine were studied in rat and dog after oral administration at a dose level of 50 mg/kg. 5. Urinary excretion was the major elimination pathway for radioactive drug-related material in both species (62-75% of the dose). Unchanged ranitidine was the major radioactive component in both rat and dog urine (0-24 h), accounting for approximately 40% of the dose in each case. 6. In dog, ranitidine undergoes N-oxidation (approximately 30% of dose) whereas in rat, N-oxidation, S-oxidation, N-demethylation and oxidative deamination are all evident, with each metabolite accounting for < 6% of the dose. 7. Two previously unreported metabolites of ranitidine were identified in rat urine using newly developed hplc and lc/ms methods. These metabolites result from single and di-N-demethylation of ranitidine and accounted for 4 and 1% of the dose respectively.

The aliphatic oxidation of salmeterol to alpha-hydroxysalmeterol in human liver microsomes is catalyzed by CYP3A

Salmeterol xinafoate (Serevent) is a long-acting beta2-adrenoceptor agonist, used in the treatment of asthma, that has bronchodilator and anti-inflammatory action. Salmeterol is extensively metabolized by aliphatic oxidation in humans, with the major metabolite being alpha-hydroxysalmeterol. The aim of this investigation was to identify the specific cytochrome P450 (P450) isoform or isoforms involved in the formation of alpha-hydroxysalmeterol in human liver microsomes. [14C]Salmeterol was incubated with a pooled sample (N = 19) of human liver microsomes in the absence or presence of selective chemical inhibitors of the major human P450 isoforms. One microM ketoconazole, a selective inhibitor of CYP3A, substantially inhibited the metabolism of salmeterol to alpha-hydroxysalmeterol. Disulfiram caused a small but consistent decrease in the amount of alpha-hydroxysalmeterol formed, possibly reflecting less than total selectivity for CYP2E1 under the conditions used. Other selective inhibitors had no significant effect on the metabolism of salmeterol. The rates of formation of alpha-hydroxysalmeterol in 10 individual liver microsomal samples showed an approximately 10-fold variation and were found to be highly correlated (r2 = 0.94; p < 0.001) with rates of metabolism of midazolam to 1'-hydroxymidazolam, a marker of CYP3A activity, in the same microsomal samples. No significant correlation was evident for the metabolism of salmeterol with levels of total P450 or other markers of human P450 activities in the same microsomal samples, thus indicating that the formation of alpha-hydroxysalmeterol is catalyzed predominantly by CYP3A. Insect cell microsomes that coexpressed human CYP3A and NADPH-P450 reductase were able to metabolize [14C]salmeterol to alpha-hydroxysalmeterol, thus confirming the role of CYP3A in catalyzing this reaction. The therapeutic dose of salmeterol is very low, so it is unlikely that any clinically relevant interactions will be observed as a consequence of the coadministration of salmeterol and other pharmaceutical agents that are metabolized by CYP3A.

Microbial biotransformation of the angiotensin II antagonist GR117289 by Streptomyces rimosus to identify a mammalian metabolite

Screening a range of microorganisms incubated with the angiotensin II antagonist GR117289 resulted in the use of Streptomyces rimosus to generate five related biotransformation products. These comprised three compounds hydroxylated on the aliphatic side chain, one further oxidized to a ketone, and one hydroxylated on the phenyl ring. These microbial metabolites were used as standards to identify a human metabolite detected in plasma and urine, but present in insufficient quantities for full structural characterisation. This further demonstrates how the use of microbial biotransformation systems at an early stage of drug metabolism studies can act as a valuable tool in facilitating identification of minor human metabolites.

Characterization of glucuronic acid conjugates of a novel angiotensin receptor antagonist

Nanogram quantities of glucuronic acid conjugates of GR117289 in rat and dog bile have been analysed by semi-microbore high-performance liquid chromatography (HPLC)/ionspray mass spectrometry with on-line UV diode array detection. The determination of drug metabolites in bile has often proved problematical due to the large number of endogenous components in this biological matrix, in particular the bile acids. Semi-microbore HPLC is useful for concentrating small quantities of material and, in combination with an on-line diode array detector, for distinguishing between drug related and endogenous components. A novel angiotensin II receptor antagonist, GR117289, had proved difficult to analyse by thermospray mass spectrometry because of its thermal lability. The use of the less thermally dependent technique of ionspray mass spectrometry allowed the characterization of nanogram quantities of glucuronic acid metabolites of GR117289 in bile.