Drug release from starch-acetate microparticles and films with and without incorporated α-amylase

Acetylation of starch considerably decreases its swelling and enzymatic degradation. Thus, starch-acetate (SA) based delivery systems may be suitable for controlled drug delivery. The aim of the present study was to evaluate drug release from the SA microparticles (SA mps) and SA films. The average degree of acetyl substitution (DS) per glucose residue in the starch was either 1.9 (SA DS 1.9) or 2.6 (SA DS 2.6). Timolol (mw 332), calcein (mw 623) and bovine serum albumin (BSA, mw 68,000) were used as model drugs. A continuous timolol release from the both SA mps was observed in phosphate buffer solution (PBS) pH 7.4 (50-days incubation). The release of timolol was faster from the SA DS 1.9 mps than from the SA DS 2.6 mps. Calcein release from both SA mps was continuous in PBS pH 7.4 (5-days incubation). But, calcein release profile from the SA DS 2.6 film in PBS pH 7.4 showed discontinuities. However, the release of calcein from both SA films was continuous in human serum in vitro during the 7-day incubation, i.e. enzymes enhanced calcein release. Thus, alpha-amylase was incorporated into the SA films in order to enhance drug release from the films. However, the effects of incorporation of alpha-amylase on the model macromolecule (BSA) release from the SA films were modest. In conclusion, this study demonstrates the achievement of slow release of different molecular weight model drugs from the SA mps and films as compared to fast release from the native starch preparations. DS of SA, physicochemical properties of a drug and the presence of enzymes can all affect drug release profiles from SA based preparations.

Targeting the cannabinoid CB2 receptor: mutations, modeling and development of CB2 selective ligands

Since the discovery of the cannabinoid CB2 receptor in 1993, there has been a growing interest to clarify the importance of this G-protein coupled receptor (GPCR) for human physiology, and to investigate it as a possible target for current and future drug development. Several mutation studies have examined the receptor activation and structure of the receptor binding cavity. Additionally, 3D models for the CB2 receptor have been constructed to aid in perceiving important ligand-receptor interactions. In recent years, many research groups have succeeded in synthesizing new CB2 selective ligands. This review focuses on (i) important features for ligand recognition and/or receptor activation at CB2, derived from mutation and modeling studies, and (ii) recent advances in the field of CB2 selective ligands.

Predicting plasticization efficiency from three-dimensional molecular structure of a polymer plasticizer

PURPOSE

In polymeric coatings, plasticizers are used to improve the film-forming characteristic of the polymers. In this study, a computerized method (VolSurf with GRID) was used as a novel tool for the prediction plasticization efficiency (beta) of test compounds, and for determining the critical molecular properties needed for polymer plasticization.

METHODS

The film-former, starch acetate DS 2.8 (SA), was plasticized with each of 24 tested compounds. A decrease in glass transition temperature of the plasticized free films (determined by differential scanning calorimeter (DSC)) was used as an indicator for beta. Partial least squares discriminant analysis was used to correlate the experimental data with the theoretical molecular properties of the plasticizers.

RESULTS

A good correlation (r2 = 0.77, q2 = 0.58) between the molecular modeling results and the experimental data demonstrated that beta can be predicted from the three-dimensional molecular structure of a compound. Favorable structural properties identified for the potent SA plasticizer were strong hydrogen bonding capacity and a definitive hydrophobic region on the molecule.

CONCLUSIONS

The VolSurf method is a valuable tool for predicting the plasticization efficiency of a compound. The correlation between experimental and calculated glass transition temperature values verifies that physicochemical properties are primary factors influencing plasticization efficiency of a compound.

A rhodopsin-based model for melatonin recognition at its G protein-coupled receptor

The recent elucidation of the primary structures of different melatonin receptors as well as the deduction of the secondary structure of rhodopsin has allowed us to construct a model for melatonin recognition at its G protein-coupled receptor. To achieve this, we have used the quantum mechanics method Austin model 1 to fully optimize the structures of melatonin and several analogs. We also synthesized three compounds and used the three-dimensional analysis comparative molecular field analysis (CoMFA) to generate a model for the structure-activity relationships of melatonin and 27 melatonin-like compounds. This model predicted with good accuracy the affinities of the synthesized compounds for the melatonin receptor. We propose that recognition of the functional moieties of melatonin occurs through specific interaction of these moieties with fully conserved amino acid residues present in transmembrane helices V, VI and VII of the melatonin receptor. These residues are not found in other members of the G protein-coupled receptor family. The rhodopsin-based model can explain the importance of some structural features of melatonin and related active compounds.

An empirical and theoretical study on mechanisms of mutagenic activity of hydrazine compounds

Hydrazine compounds are important industrial and laboratory chemicals. Many of them are carcinogenic in animal tests. Although the carcinogenicity is well established, the results of mutagenicity tests performed on alkylhydrazines vary greatly in different studies. In an attempt to clarify the situation we have applied Salmonella typhimurium TA102 tests to hydrazine and its mono- and dimethyl derivatives. These compounds were also tested by an Escherichia coli DNA repair-assay. The results of the repair tests indicate that unsymmetrically alkylated hydrazines can cause DNA-lesions which are lethal in repair-deficient strains. Finally QSAR (Quantitative Structure Activity Relationships) was used to develop a model to describe the genotoxic mechanism of hydrazine compounds, taking advantage of the results of previous mutagenicity studies. Energy of the lowest unoccupied molecular orbital together with octanol-water partition coefficient explains nearly completely the mutagenic activity of alkylated hydrazine compounds included in the analysis. The mutagenic activity of unsubstituted hydrazine is apparently based on different mechanisms.

New potent and selective cytochrome P450 2B6 (CYP2B6) inhibitors based on three-dimensional quantitative structure-activity relationship (3D-QSAR) analysis

BACKGROUND AND PURPOSE

The cytochrome P450 2B6 (CYP2B6) enzyme metabolises a number of clinically important drugs. Drug-drug interactions resulting from inhibition or induction of CYP2B6 activity may cause serious adverse effects. The aims of this study were to construct a three-dimensional structure-activity relationship (3D-QSAR) model of the CYP2B6 protein and to identify novel potent and selective inhibitors of CYP2B6 for in vitro research purposes.

EXPERIMENTAL APPROACH

The inhibition potencies (IC(50) values) of structurally diverse chemicals were determined with recombinant human CYP2B6 enzyme. Two successive models were constructed using Comparative Molecular Field Analysis (CoMFA).

KEY RESULTS

Three compounds proved to be very potent and selective competitive inhibitors of CYP2B6 in vitro (IC(50)<1 microM): 4-(4-chlorobenzyl)pyridine (CBP), 4-(4-nitrobenzyl)pyridine (NBP), and 4-benzylpyridine (BP). A complete inhibition of CYP2B6 activity was achieved with 0.1 microM CBP, whereas other CYP-related activities were not affected. Forty-one compounds were selected for further testing and construction of the final CoMFA model. The created CoMFA model was of high quality and predicted accurately the inhibition potency of a test set (n=7) of structurally diverse compounds.

CONCLUSIONS AND IMPLICATIONS

Two CoMFA models were created which revealed the key molecular characteristics of inhibitors of the CYP2B6 enzyme. The final model accurately predicted the inhibitory potencies of several structurally unrelated compounds. CBP, BP and NBP were identified as novel potent and selective inhibitors of CYP2B6 and CBP especially is a suitable inhibitor for in vitro screening studies.

A comparative molecular field analysis of cytochrome P450 2A5 and 2A6 inhibitors

Structure-activity relationships of 23 P450 2A5 and 2A6 inhibitors were analysed using the CoMFA and GOLPE/GRID with smart region definition (SRD). The predictive power of the resulting models was validated using five compounds not belonging to the model set. All models have high internal and external predictive power and resulting 3D-QSAR models are supporting each other. Both Sybyl and GOLPE highlight properties near lactone moiety to be important for 2A5 and 2A6 inhibition. Another important feature for pIC50 was the size of the substituent in the 7-positon of coumarin. The models suggest that the 2A5 binding site is larger that that of 2A6 due to larger steric regions in the CoMFA coefficient maps and corresponding GOLPE maps. In addition, the maps reveal that 2A6 disfavours negative charge near the lactone moiety of coumarin.

The effect of powder blend and tablet structure on drug release mechanisms of hydrophobic starch acetate matrix tablets

This study investigates the release mechanism of a hydrophilic drug (caffeine) from hydrophobic matrix tablets composed of starch acetate. Different particle size fractions of starch acetate were mixed with caffeine (22% V/V) to obtain various mixture organisations in the powder, as well as in the final tablet. The organisation of powder mixtures was calculated by the carrier payload of starch acetate particles, while the pore size distributions in tablets were measured by mercury intrusion porosimetry. A carrier payload below 1 indicated the existence of a free starch acetate particle surface, while numbers greater than 1 pointed to a complete occupation of the starch acetate particle surface area by caffeine particles. The carrier payload calculations gave a good prediction for the existence of a starch acetate matrix in the tablet structures. Caffeine matrices in tablets compressed from the mixtures could be detected by mercury intrusion porosimetry measurements. The existence of different matrices, as well as different pore networks, determined the physical changes of the tablets and the release mechanism of caffeine during dissolution tests. When a tablet contained only a caffeine matrix, rapid tablet disintegration and immediate release of the total amount of caffeine occurred. A single matrix of starch acetate resulted in tablets that remained intact, although cracks were formed. The co-existence of matrices of both materials created surface erosion of the tablet. The caffeine release profiles of tablets that remained intact or showed erosion were fitted by an equation containing both diffusional and relaxational factors to describe the effect of tablet porosity on drug release.

More potent inhibition of human CYP2A6 than mouse CYP2A5 enzyme activities by derivatives of phenylethylamine and benzaldehyde

1. A rapid 96-well plate assay method was developed and validated to measure liver microsomal coumarin 7-hydroxylation in vitro. 2. The method was used to test inhibition of human and mouse CYP2A enzymes by three phenylethylamine derivatives 2-(p-tolyl)-ethylamine, amphetamine, 2-phenylethylamine and benzaldehyde, and two of its derivatives, 4-methylbenzaldehyde and 4-methoxybenzaldehyde. 3. The benzaldehyde derivatives were more potent inhibitors of CYP2A5 than the phenylethylamines. The K(ic) value of 4-methylbenzaldehyde was 3.4 micro M and for 4-methoxybenzaldehyde it was 0.86 micro M for CYP2A5. 4. Amphetamine is a weak inhibitor of CYP2A6, whereas benzaldehyde is a suicide inhibitor with K(inact) = 0.16 min(-1) and K(I) = 18 micro M. The K(ic) values of 2-phenylethylamine, 2-(p-tolyl)-ethylamine, 4-methylbenzaldehyde and 4-methoxybenzaldehyde were 1.13, 0.23, 0.36 and 0.73 micro M for CYP2A6, respectively. 5. Novel potent inhibitors were found for CYP2A6 and, except for 4-methoxybenzaldehyde, all the compounds inhibited CYP2A5 and CYP2A6 enzymes differentially. These data add to the refinement of CYP2A enzyme active sites and provide chemical leads for developing novel chemical inhibitors of the CYP2A6 enzyme.

Competitive inhibition of coumarin 7-hydroxylation by pilocarpine and its interaction with mouse CYP 2A5 and human CYP 2A6

1. We have shown earlier that pilocarpine strongly inhibits mouse and human liver coumarin 7-hydroxylase activity of CYP 2A and pentoxyresorufin O-deethylase activity of CYP 2B in vitro. Since pilocarpine, like coumarin, contains a lactone structure we have studied in more detail its inhibitory potency on mouse and human liver coumarin 7-hydroxylation. 2. Pilocarpine was a competitive inhibitor of coumarin 7-hydroxylase in vitro both in mouse and human liver microsomes although it was not a substrate for CYP 2A5. Ki values were similar, 0.52 +/- 0.22 microM in mice and 1.21 +/- 0.51 microM in human liver microsomes. 3. Pilocarpine induced a type II difference spectrum in mouse, human and recombinant CYP 2A5 yeast cell microsomes, with Ka values of 3.7 +/- 1.6, 1.6 +/- 1.1 and 1.5 +/- 0.1 microM, respectively. 4. Increase in pH of the incubation medium from pH 6 to 7.5 increased the potency of inhibition of coumarin 7-hydroxylation by pilocarpine. 5. Superimposition of pilocarpine and coumarin in such a way that their carbonyls, ring oxygens and the H-7' of coumarin and N-3 of pilocarpine overlap yielded a common molecular volume of 82%. 6. The results indicate that pilocarpine is a competitive inhibitor and has a high affinity for mouse CYP 2A5 and human CYP 2A6. In addition the immunotype nitrogen of pilocarpine is coordinated towards the haem iron in these P450s.

Ultrasound transmission technique as a potential tool for physical evaluation of monolithic matrix tablets

The aim of this study was to investigate the effects of tablet porosity and particle size fraction of compacted Starch acetate powders, with and without model drug caffeine, on acoustic properties of tablets. The ultrasound velocity was determined from the transmission measurements. Tablets of starch acetate (SA DS 2.7) powder with two particle size fractions of 0-53 and 0-710 microm were compressed with a compaction simulator. Porosities of tablets varied in the range from 12% to 43% for both particle size fractions. Strong associations were found between the ultrasound velocity and physical properties of the tablets such as porosity and particle size fraction. Interestingly, ultrasound velocity was practically insensitive to inclusion of the model drug caffeine with the concentrations used. Based on this study ultrasound transmission method is a potential non-destructive tool for studying structural changes of tablets and other solid dosage forms.

Pronounced differences in inhibition potency of lactone and non-lactone compounds for mouse and human coumarin 7-hydroxylases (CYP2A5 and CYP2A6)

1. The structural requirements for a compound to be a potent inhibitor for mouse CYP2A5 and human CYP2A6 enzymes catalysing coumarin 7-hydroxylase activity have been studied. 2. The IC50 of 28 compounds for the pyrazole-treated male DBA/2 mouse and human liver microsomal coumarin 7-hydroxylation were determined at 10 microm coumarin concentration 15 times over Km of coumarin. 3. The three most potent inhibitors for CYP2A5 were gamma-nonanoic lactone, gamma-decanolactone and gamma-phenyl-gamma-butyrolactone with an IC50 = 1.9+/-0.4, 2.1+/-0.2 and 2.4+/-0.3 microM and for CYP2A67-methylcoumarin, butylcyclohexane and indan with an IC50. = 30+/-3.2, 43+/-9 and 50+/-11 microM. 4. Among the 28 compounds studied, only 2-benzoxazolinone, 2-indanone and gamma-valerolactone showed similar inhibitory activity in both species. Indan had a lower IC50 for human than for mouse coumarin 7-hydroxylation, whereas the IC50 of 24 other compounds was higher for human than for mouse coumarin 7-hydroxylation. 5. The largest difference in IC50 between mouse and human activity was observed with 5-substituted phenyl, pentyl, hexyl, heptyl or octyl gamma-lactones or 6-substituted delta-lactones. IC50 of gamma-undecanolactone and gamma-decanolactone was 500 times lower for mouse than human coumarin 7-hydroxylation. 6. The difference in the IC50 between human and mouse coumarin 7-hydroxylation decreased substantially with the corresponding compounds without the lactone ring. 7. It is concluded that certain 5- or 6-position substituted gamma- and delta-lactones are potent inhibitors for mouse CYP2A5 but not for the orthologous human CYP2A6 and that the active site of CYP2A6 could be smaller than the active site of CYP2A5.

Identification of inhibitors of the nicotine metabolising CYP2A6 enzyme--an in silico approach

The compulsive nature of tobacco use is attributable to nicotine addiction. Nicotine is eliminated by metabolism through the cytochrome P450 2A6 (CYP2A6) enzyme in liver. Inhibition of CYP2A6 by chemical compounds may represent a potential supplement to anti-smoking therapy. The purpose of this study was to rationally design potent inhibitors of CYP2A6. 3D-QSAR models were constructed to find out which structural characteristics are important for inhibition potency. Specifically located hydrophobic and hydrogen donor features were found to affect inhibition potency. These features were used in virtual screening of over 60,000 compounds in the Maybridge chemical database. A total of 22 candidate molecules were selected and tested for inhibition potency. Four of these were potent and selective CYP2A6 inhibitors with IC(50) values lower than 1 muM. They represent novel structures of CYP2A6 inhibitors, especially N1-(4-fluorophenyl)cyclopropane-1-carboxamide. This compound can be used as a lead in the design of CYP2A6 inhibitor drugs to combat nicotine addiction.

Changes in solid-state structure of cyclophosphamide monohydrate induced by mechanical treatment and storage

The effects of mechanical treatment and various storage conditions on the structure of cyclophosphamide monohydrate were evaluated by thermal and X-ray analyses and molecular modeling. The monohydrate form of cyclophosphamide was found to convert to the anhydrous form through a metastable phase. Metastable forms were produced by mechanical treatment and by desiccation. These forms could be detected in differential scanning calometric thermograms as endothermic peaks, at approximately 39 degrees C, and X-ray powder diffractometric analysis, e.g.; by a characteristic reflection at 15.3 degrees (2 theta). Molecular modeling was used to study molecular interactions and putative metastable structures. The dehydration enthalpies of the cyclophosphamide monohydrate obtained from quantum chemical calculations and DSC analysis were 51.6 and 36.1 J/g, respectively. In a unit cell of the stable monohydrate, a water molecule is held by O(7) of the cyclophosphamide molecule and N(6)H of a neighboring cyclophosphamide molecule, with hydrogen bonds enabling existence of a water tunnel. The metastable form of cyclophosphamide is detected when a sterically formed block in the possible tunnel is removed, and the water molecules are allowed to leave the system one by one.

Deformation behaviors of tolbutamide, hydroxypropyl-beta-cyclodextrin, and their dispersions

PURPOSE

The deformation behaviors of compressed freeze-dried and spray-dried tolbutamide/hydroxypropyl-beta-cyclodextrin molecular dispersions were evaluated and compared with similarly prepared tolbutamides (TBM), hydroxypropyl-beta-cyclodextrins (HP-beta-CD) and as their physical dispersions.

METHODS

TBM, HP-beta-CD, and their 1:1 molecular dispersions were prepared by freeze-drying and spray-drying, and physical dispersions of TBM and HP-beta-CD were blended. Deformation properties of the prepared materials were evaluated by using a compaction simulator and constants derived from Heckel plots. Molecular dynamics (MD) simulations were performed in order to gain a molecular-level view on the deformation behavior of TBM-HP-beta-CD inclusion complex.

RESULTS

The freeze-dried TBM polymorphic form II was less prone to overall particle deformation than the spray-dried stable form I. Formation of molecular dispersions decreased the plastic and elastic behaviors of these materials. Also, the MD simulations showed a reduced molecular flexibility of the TBM-HP-beta-CD inclusion complex, as compared to HP-beta-CD.

CONCLUSIONS

The formation of TBM and HP-beta-CD molecular dispersion resulted in more rigid molecular arrangements, which were less prone to deformation than either HP-beta-CDs or physical dispersions. The results showed how differing molecular, solid, particle, and powder state properties affect the deformation properties of the materials studied.