Predicting pharmacokinetics of drugs using physiologically based modeling--application to food effects

Our knowledge of the major mechanisms underlying the effect of food on drug absorption allows reliable qualitative prediction based on biopharmaceutical properties, which can be assessed during the pre-clinical phase of drug discovery. Furthermore, several recent examples have shown that physiologically based absorption models incorporating biorelevant drug solubility measurements can provide quite accurate quantitative prediction of food effect. However, many molecules currently in development have distinctly sub-optimal biopharmaceutical properties, making the quantitative prediction of food effect for different formulations from in vitro data very challenging. If such drugs reach clinical development and show undesirable variability when dosed with food, improved formulation can help to reduce the food effect and carefully designed in vivo studies in dogs can be a useful guide to clinical formulation development. Even so, such in vivo studies provide limited throughput for screening, and food effects seen in dog cannot always be directly translated to human. This paper describes how physiologically based absorption modeling can play a role in the prediction of food effect by integrating the data generated during pre-clinical and clinical research and development. Such data include physicochemical and in vitro drug properties, biorelevant solubility and dissolution, and in vivo pre-clinical and clinical pharmacokinetic data. Some background to current physiological absorption models of human and dog is given, and refinements to models of in vivo drug solubility and dissolution are described. These are illustrated with examples using GastroPlus to simulate the food effect in dog and human for different formulations of two marketed drugs.

Predicting the impact of physiological and biochemical processes on oral drug bioavailability

Recent advances in computational methods applied to the fields of drug delivery and biopharmaceutics will be reviewed with a focus on prediction of the impact of physiological and biochemical factors on simulation of gastrointestinal absorption and bioavailability. Our application of a gastrointestinal simulation for the prediction of oral drug absorption and bioavailability will be described. First, we collected literature data or we estimated biopharmaceutical properties by application of statistical methods to a set of 2D and 3D molecular descriptors. Second, we integrated the differential equations for an advanced compartmental absorption and transit (ACAT) model in order to determine the rate, extent, and approximate gastrointestinal location of drug liberation (for controlled release), dissolution, passive and carrier-mediated absorption, and saturable metabolism and efflux. We predict fraction absorbed, bioavailability, and C(p) vs. time profiles for common drugs and compare those estimates to literature data. We illustrate the simulated impact of physiological and biochemical processes on oral drug bioavailability.

Pressure-sensitive and -insensitive coupling in gamma-aminobutyric acid(A) receptors

RATIONALE

Previous behavioral and biochemical studies suggest that allosteric coupling processes initiated by benzodiazepines, barbiturates and neuroactive steroids can be sub-categorized on the basis of their sensitivities to antagonism by increased atmospheric pressure. However, biochemical evidence supporting this hypothesis was limited to single concentration studies in long sleep (LS) mice.

OBJECTIVE

The present paper addresses these issues by extending biochemical investigation of pressure effects on allosteric modulators across a range of concentrations that allosterically enhance gamma-aminobutyric acid (GABA)A receptor function and alter behavior using two mouse genotypes. In addition, the effects of pressure on ligand binding were explored to further investigate the mechanism of pressure antagonism of allosteric modulation.

METHODS

The effects of 12 times normal atmospheric pressure (ATA) of helium-oxygen gas (heliox) on allosteric modulation of GABA(A) receptor function and [3H]flunitrazepam binding was tested in LS and C57BL mouse brain membranes (microsacs) using chloride flux and high-affinity binding assays.

RESULTS

In both genotypes, exposure to 12 ATA heliox antagonized the allosteric enhancement of GABA(A) receptor function by flunitrazepam (0.1-10 microM) and pentobarbital (0.1-50 microM) but did not affect allosteric modulation by 3alpha-hydroxy-5beta-pregnan-20-one (0.1-1 microM). Pressure did not affect benzodiazepine receptor affinity (Kd) or the number of benzodiazepine receptors (Bmax).

THE RESULTS

(1) confirm that there are differences in sensitivity to pressure antagonism of allosteric coupling among GABA(A) allosteric modulators; (2) demonstrate that these differences are not concentration or genotype dependent; (3) add evidence that pressure antagonizes allosteric modulation by uncoupling the receptor and (4) support the hypothesis that allosteric modulation of receptor function can be sub-categorized on the basis of sensitivity to pressure antagonism.

Three-dimensional structure of fibrolase, the fibrinolytic enzyme from southern copperhead venom, modeled from the X-ray structure of adamalysin II and atrolysin C

The fibrinolytic enzyme from southern copperhead snake venom, fibrolase, contains 1 mole of zinc per mole of protein, belongs to the major family of metalloproteinases known as the metzincins, and has been shown to degrade fibrin clots in vitro and in vivo. The purpose of this study was to develop a 3-dimensional model of fibrolase to investigate the geometry of conserved and variable sequences between members of the snake venom metalloproteinases. When compared to atrolysin C (form D) or adamalysin II (metzincins with completely different substrate specificity), fibrolase has approximately 60% overall sequence identity and nearly 100% sequence similarity in the active site. We used the crystal structure of adamalysin II to build a 3-dimensional homology model of fibrolase. Three disulfide bonds were constructed (the highly conserved disulfide bond [118-198] was maintained from the adamalysin II structure and 2 new disulfide bonds were introduced between residues 158-182 and 160-165). We used Sculpt 2.5 and HyperChem 5.0 to "dock" a substrate fragment octapeptide (HTEKLVTS), and a water molecule into the active site cleft. We calculated the differential average homology profile for fibrolase compared to 8 hemorrhagic and 5 nonhemorrhagic metzincins. We then determined the sequence regions that might be responsible for their substrate specificity. Our 3-dimensional homology model shows that the variable sequences lie on the periphery of the identified active site region containing the His triangle; this indicates that substrate specificity may depend on surface residues that are not directly associated with the active site.

Biopharmaceutics of transmucosal peptide and protein drug administration: role of transport mechanisms with a focus on the involvement of PepT1

Non-invasive delivery of peptide and protein drugs will soon become a reality. This is due partly to a better understanding of the endogenous transport mechanisms, including paracellular transport, endocytosis, and carrier-mediated transport of mucosal routes of peptide and protein drug administration. This paper focuses on work related to the elucidation of structure-function, intracellular trafficking, and regulation of the intestinal dipeptide transporter, PepT1.

In vivo and in vitro hyperbaric studies in mice suggest novel sites of action for ethanol

The present study uses increased atmospheric pressure as an ethanol antagonist to test the hypothesis that allosteric coupling pathways in the GABA(A) receptor complex represent initial sites of action for ethanol. This was accomplished using behavioral and in vitro measures to determine the effects of pressure on ethanol and other GABAergic drugs in C57BL/6 and LS mice. Behaviorally, exposure to 12 times normal atmospheric pressure (ATA) of a helium-oxygen gas mixture (heliox) antagonized loss of righting reflex (LORR) induced by the allosteric modulators ethanol and pentobarbital, but did not antagonize LORR induced by the direct GABA agonist 4,5,6,7-tetrahydroisoxazolo-pyridin-3-ol (THIP). Similarly, exposure to 12 ATA heliox antagonized the anticonvulsant effects verses isoniazid of ethanol, diazepam and pentobarbital. Biochemically, exposure to 12 ATA heliox antagonized potentiation of GABA-activated 36Cl-uptake by ethanol, flunitrazepam and pentobarbital in LS mouse brain preparations, but did not alter GABA-activated 36Cl- uptake per se. In contrast to its antagonist effect versus other allosteric modulators, pressure did not antagonize these behavioral or in vitro effects induced by the neuroactive steroid, 3alpha-hydroxy-5beta-pregnan-20-one (3alpha,5beta-P). These findings add to evidence that pressure directly and selectively antagonizes drug effects mediated through allosteric coupling pathways. The results fit predictions, and thus support the hypothesis that allosteric coupling pathways in GABA(A) receptors represent initial sites of action for ethanol. Collectively, the results suggest that there may be common physicochemical and underlying structural characteristics that define ethanol sensitive regions of receptor proteins and/or their associated membranes that can be identified by pressure within (e.g., GABA(A)) and possibly across (e.g., GABA(A), NMDA, 5HT3) receptors.

Proton-driven dipeptide uptake in primary cultured rabbit conjunctival epithelial cells

PURPOSE

To characterize proton-driven carrier-mediated dipeptide uptake in primary cultured conjunctival epithelial cells of the pigmented rabbit using beta-alanyl-L-histidine (L-carnosine) as a model dipeptide substrate.

METHODS

Uptake of tritiated L-carnosine was monitored using conjunctival epithelial cells on days 6 through 8 in culture on a filter support. The structural features of dileucine stereoisomers and cephalexin contributing to interaction with the dipeptide transporter were evaluated by computer modeling and inhibition of tritiated L-carnosine uptake.

RESULTS

Uptake of L-carnosine by primary cultured conjunctival epithelial cells in the presence of an inwardly directed proton gradient showed directional asymmetry (favoring apical uptake by a factor of five), temperature dependence, and saturability correlated with substrate concentration, with a Michaelis-Menten constant (Km) of 0.3 +/- 0.03 mM and a maximum uptake rate (Vmax) of 22.0 +/- 1.0 picomoles per milligram protein per minute. L-Carnosine uptake was optimal at pH 6.0 and was reduced by 60% and 35%, respectively, by 50 microM p-trifluoromethoxyphenylhydrazone (a proton ionophore) and by acid preloading with 50 mM NH4Cl. The constituent amino acids did not inhibit L-carnosine uptake. L-Carnosine uptake was inhibited, however, from 50% to 80% by other dipeptides and structurally similar drugs such as bestatin, beta-lactam antibiotics, and angiotensin-converting enzyme inhibitors. The LL, LD, or DL forms of the dipeptide Leu-Leu inhibited tritiated L-carnosine uptake by approximately 60%, 40%, and 70%, respectively. By contrast, the DD form did not inhibit uptake. Results from computer modeling suggest that an appropriate dipeptide N-terminal to C-terminal distance and a favorable orientation of the side chains may be important for substrate interaction with the conjunctival dipeptide transporter.

CONCLUSIONS

Uptake of the dipeptide L-carnosine in primary cultured pigmented rabbit conjunctival epithelial cells is probably mediated by a proton-driven dipeptide transporter. This transporter may be used for optimizing the uptake of structurally similar peptidomimetic drugs.

Structure, function, and molecular modeling approaches to the study of the intestinal dipeptide transporter PepT1

The proton-coupled intestinal dipeptide transporter, PepT1, has 707 amino acids, 12 putative transmembrane domains (TMD), and is of importance in the transport of nutritional di- and tripeptides and structurally related drugs, such as penicillins and cephalosporins. By using a combination of molecular modeling and site-directed mutagenesis, we have identified several key amino acid residues that effect catalytic transport properties of PepT1. Our molecular model of the transporter was examined by dividing it into four sections, parallel to the membrane, starting from the extracellular side. The molecular model revealed a putative transport channel and the approximate locations of several aromatic and charged amino acid residues that were selected as targets for mutagenesis. Wild type or mutagenized human PepT1 cDNA was transfected into human embryonic kidney (HEK293) cells, and the uptake of tritiated glycylsarcosine [3H]-(Gly-Sar) was measured. Michaelis-Menton analysis of the wild-type and mutated transporters revealed the following results for site-directed mutagenesis. Mutation of Tyr-12 or Arg-282 into alanine has only a very modest effect on Gly-Sar uptake. By contrast, mutation of Trp-294 or Glu-595 into alanine reduced Gly-Sar uptake by 80 and 95%, respectively, and mutation of Tyr-167 reduced Gly-Sar uptake to the level of mock-transfected cells. In addition, preliminary data from fluorescence microscopy following the expression of N-terminal-GFP-labeled PepT1Y167A in HEK cells indicates that the Y167A mutation was properly inserted into the plasma membrane but has a greatly reduced Vmax.

Molecular identification of a role for tyrosine 167 in the function of the human intestinal proton- coupled dipeptide transporter (hPepT1)

hPepT1 is a proton-coupled peptide transporter that mediates the absorption of di- and tripeptides. Here we show that tyrosine 167 (Y167) in transmembrane domain 5 (TMD5) of this 12-transmembrane spanning protein contributes to its transport function. We identified this particular amino acid by a computer model of the arrangement of the TMDs of hPepT1 and investigated its role by site-directed mutagenesis and dipeptide uptake studies. [3H]Gly-sar uptake in cells transiently transfected with Y167A-hPepT1 was abolished completely, even though the level of Y167A-hPepT1 expression by Western blot analysis and cell surface expression by immunofluorescence microscopy was similar to those of the wild type. Therefore, mutation affected transport function, but apparently not the steady-state protein level or trafficking of the transporter to the plasma membrane. Moreover, mutation of Y167 into phenylalanine, serine, or histidine all abolished gly-sar uptake in transfected HEK 293 cells. Taken together, these findings suggest that Y167 plays an essential role in hPepT1 function, perhaps due to the unique chemistry of its phenolic side chain.

Characterization of the anticonvulsant properties of ganaxolone (CCD 1042; 3alpha-hydroxy-3beta-methyl-5alpha-pregnan-20-one), a selective, high-affinity, steroid modulator of the gamma-aminobutyric acid(A) receptor

Ganaxolone (CCD 1042) is a 3beta-methyl-substituted analog of the endogenous neuroactive steroid 3alpha-hydroxy-5alpha-pregnan-20-one. Ganaxolone inhibited binding of the gamma-aminobutyric acid (GABA)A receptor-chloride channel ligand t-[35S]butylbicyclophosphorothionate (IC50 of 80 nM) and enhanced binding of the benzodiazepine site ligand [3H]flunitrazepam (EC50 of 125 nM) and the GABA site ligand [3H]muscimol (EC50 of 86 nM), consistent with activity as a positive allosteric modulator of the GABA(A) receptor. Electrophysiological recordings showed that, whereas nanomolar concentrations of ganaxolone potentiated GABA-evoked chloride currents in Xenopus oocytes expressing the human GABA(A) receptor subunits alpha1beta1gamma2L, alpha2beta1gamma2L or alpha3beta1gamma2L, direct activation of chloride flux occurred to a limited extent only at micromolar concentrations. Ganaxolone was effective in nontoxic doses against clonic convulsions induced by s.c. pentylenetetrazol administration in mice and rats (ED50 values of 4.3 and 7.8 mg/kg i.p., respectively). Ganaxolone also exhibited potent anticonvulsant activity against seizures induced by s.c. bicuculline (ED50 of 4.6 mg/kg i.p.), i.p. TBPS (ED50 of 11.7 mg/kg i.p.) and i.p. aminophylline (ED50 of 11.5 mg/kg i.p.) in mice. Although ganaxolone effectively blocked tonic seizures induced by maximal electroshock in mice (ED50 of 29.7 mg/kg i.p.), it did so only at doses that produced ataxia on the Rotorod (TD50 of 33.4 mg/kg i.p.). Conversely, ganaxolone was a potent anticonvulsant against fully kindled stage 5 seizures induced by corneal kindling in rats (ED50 of 4.5 mg/kg i.p.), producing these effects at doses well below those that resulted in ataxia (TD50 of 14.2 mg/kg i.p.). The seizure threshold, as determined by an increase in the dose of i.v. infused pentylenetetrazol required to induce clonus, was also significantly elevated by nontoxic doses of ganaxolone in mice. In summary, these data indicate that ganaxolone is a high-affinity, stereoselective, positive allosteric modulator of the GABA(A) receptor complex that exhibits potent anticonvulsant activity across a range of animal procedures. The profile of anticonvulsant activity obtained for ganaxolone supports clinical evaluation of this drug as an antiepileptic therapy with potential utility in the treatment of generalized absence seizures as well as simple and complex partial seizures.

Synthesis and in vitro activity of 3 beta-substituted-3 alpha-hydroxypregnan-20-ones: allosteric modulators of the GABAA receptor

Two naturally occurring metabolites of progesterone, 3 alpha-hydroxy-5 alpha- and 5 beta-pregnan-20-one (1 and 2), are potent allosteric modulators of the GABAA receptor. Their therapeutic potential as anxiolytics, anticonvulsants, and sedative/hypnotics is limited by rapid metabolism. To avoid these shortcomings, a series of 3 beta-substituted derivatives of 1 and 2 was prepared. Small lipophilic groups generally maintain potency in both the 5 alpha- and 5 beta-series as determined by inhibition of [35S]TBPS binding. In the 5 alpha-series, 3 beta-ethyl, -propyl, -trifluoromethyl and -(benzyloxy)methyl, as well as substituents of the form 3 beta-XCH2, where X is Cl, Br, or I or contains unsaturation, show limited efficacy in inhibiting [35S]TBPS binding. In the 5 beta-series, the unsubstituted parent 2 is a two-component inhibitor, whereas all of the 3 beta-substituted derivatives of 2 inhibit TBPS via a single class of binding sites. In addition, all of the 3-substituted 5 beta-sterols tested are full inhibitors of [35S]TBPS binding. Electrophysiological measurements using alpha 1 beta 2 gamma 2L receptors expressed in oocytes show that 3 beta-methyl- and 3 beta-(azidomethyl)-3 alpha-hydroxy-5 alpha-pregnan-20-one (6 and 22, respectively) are potent full efficacy modulators and that 3 alpha-hydroxy-3 beta-(trifluoromethyl)-5 alpha-pregnan -20-one (24) is a low-efficacy modulator, confirming the results obtained from [35S]TBPS binding. These results indicate that modification of the 3 beta-position in 1 and 2 maintains activity at the neuroactive steroid site on the GABAA receptor. In animal studies, compound 6 (CCD 1042) is an orally active anticonvulsant, while the naturally occurring progesterone metabolites 1 and 2 are inactive when administered orally, suggesting that 3 beta-substitution slows metabolism of the 3-hydroxyl, resulting in orally bioavailable steroid modulators of the GABAA receptor.

In vitro and in vivo activity of 16,17-dehydro-epipregnanolones: 17,20-bond torsional energy analysis and D-ring conformation

PURPOSE

Certain neuroactive pregnane steroids (also known as "epalons") are allosteric modulators of the GABA, receptor and have been shown to be potent anticonvulsants, anxiolytics, sedative/hypnotics, and anesthetic agents. The purpose of this study was to calculate the structural consequences of introduction of a double bond in the 16,17-position and to determine if this modification would selectively reduce sedative activity, but maintain the potent anticonvulsant activity of neuroactive steroids.

METHODS

We have studied the biochemical and behavioral effects of introducing a 16,17 double bond into the naturally occurring neuroactive steroids, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) and 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alpha,5 beta-P) and three synthetic neuroactive steroid derivatives, 3 alpha-hydroxy-3 beta-methyl-5 alpha-pregnan-20-one (3 alpha,3 beta Me,5 alpha-P), 3 alpha-hydroxy-5 alpha-androstane (3 alpha, 5 alpha-A), and alphaxalone (3 alpha,5 alpha-11-one-P).

RESULTS

The 16-ene analogs of most of these neuroactive steroids were found to be 7- and 16-fold less potent in inhibiting [35S]TBPS binding to GABAA receptors and in a similar fashion, had reduced anticonvulsant and sedative potency in proportional amounts. The exception was the androstane (3 alpha,5 alpha-A) without a 17-acetyl group, that had virtually identical IC50 and ED50 values for the saturated and unsaturated derivatives. Calculation of the torsional energy profile for each of the 17-acetyl side chain conformations showed that the conformational energy minima found in the alpha,beta-unsaturated keto systems, produce an orientation of the 20-keto group that is rotated by 165 degrees when compared to the non-conjugated acetyl group (determined by X-ray crystallography and its minimum energy conformation).

CONCLUSIONS

The modified orientation of the 20-keto group of neuroactive steroids containing a 16-ene, provides an explanation for their decreased biological activity overall, but did not lead to an enhanced protective index.

Low level hyperbaric antagonism of diazepam's locomotor depressant and anticonvulsant properties in mice

Exposure to 12 atmospheres absolute (12 ATA) helium oxygen gas (heliox) (low level hyperbaric exposure) antagonizes the behavioral effects of ethanol and n-propanol, but not morphine. These and other results indicate that the mechanism of the antagonism is direct (pharmacodynamic) and selective. Our study further investigates the selectivity of low level hyperbaric antagonism by testing its effectiveness against diazepam, a high affinity binding drug that acts via allosteric modulation of GABAA receptors. C57BL/6J mice received injections i.p. of vehicle or diazepam, and were then exposed to 1 ATA air, 1 ATA heliox or 12 ATA heliox. Exposure to 12 ATA heliox antagonized the locomotor depressant effect of 4 and 6 mg/kg, but not 8 mg/kg diazepam. Hyperbaric exposure also antagonized the anticonvulsant effect of 8 and 24 mg/kg, but not 4 mg/kg, diazepam vs. 300 mg/kg isoniazid. Exposure to 12 ATA heliox did not significantly affect blood concentrations of diazepam or its metabolite n-desmethyl diazepam. The pharmacological characteristics of the antagonism (direct, surmountable, rightward shift in diazepam's dose-response curve) closely matched those seen in previous studies for hyperbaric antagonism of ethanol. The results add to the evidence that low level hyperbaric exposure is a direct, mechanistic antagonist that selectively antagonizes drugs that act via perturbation or allosteric modulation of receptor function. Moreover, the results suggest that allosteric coupling pathways, which transduce binding events on ligand-gated ion channels, may represent initial sites of action for ethanol.

The neuroactive steroid 3 alpha-hydroxy-5 beta-pregnan-20-one is a two-component modulator of ligand binding to the GABAA receptor

Neuroactive steroids allosterically inhibit [35S]t-butylbicyclophosphorothionate ([35S]TBPS) and enhance [3H]flunitrazepam binding to the GABAA receptor complex. In the presence of 5 microM GABA, 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alpha, 5 beta-P) inhibits [35S]TBPS binding with high- (IC50 21-32 nM) and low- (IC50 24-63 microM) affinity components in bovine cortical, cerebellar, and hippocampal membranes. The percentage of high-affinity sites ranges from 53% in cortex to 65% in cerebellum and hippocampus. However, 3 alpha, 5 beta-P is a single-site inhibitor in thalamus (IC50 43 nM). In the absence of GABA, similar affinities for the high- and low-affinity components were detected, although the percentages of high-affinity sites were reduced. Similarly, 3 alpha, 5 beta-P enhances [3H]flunitrazepam binding with high- (EC50 44-58 nM) and low- (EC50 2-13 microM) affinity components which account for 71-77% and 23-29% of the sites, respectively, in cortex, cerebellum and hippocampus. 3 alpha, 5 beta-P is a single-site enhancer in thalamus (EC50 80 nM). In contrast to 3 alpha,5 beta-P, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) is a single site modulator of [35S]TBPS and [3H]flunitrazepam binding in all regions examined. These data provide pharmacological evidence consistent with receptor heterogeneity for neuroactive steroids.

Calculations of antibody-antigen interactions: microscopic and semi-microscopic evaluation of the free energies of binding of phosphorylcholine analogs to McPC603

The study of antibody-antigen interactions should greatly benefit from the development of quantitative models for the evaluation of binding free energies in proteins. The present work addresses this challenge by considering the test case of the binding free energies of phosphorylcholine analogs to the murine myeloma protein McPC603. This includes the evaluation of the differential binding energy as well as the absolute binding energies and their corresponding electrostatic contributions. Four different approaches are examined: the Protein Dipoles Langevin Dipoles (PDLD) method, the semi-microscopic PDLD (PDLD/S) method, a free energy perturbation (FEP) method based on an adiabatic charging procedure and a linear response approximation that accelerates the FEP calculation. The PDLD electrostatic calculations are augmented by estimates of the relevant hydrophobic and steric contributions. The determination of the hydrophobic energy involves an approach which considers the modification of the effective surface area of the solute by local field effects. The steric contributions are analyzed in terms of the corresponding reorganization energies. This treatment, which considers the protein as a harmonic system, views the steric forces as the restoring forces for the electrostatic interactions. The FEP method is found to give unreliable results with regular cut-off radii and starts to give quantitative results only in very expensive treatment with very large cut-off radii. The PDLD and PDLD/S methods are much faster than the FEP approach and give reasonable results for both the relative and absolute binding energies. The speed and simplicity of the PDLD/S method make it an effective strategy for interactive docking studies and indeed such an option is incorporated in the program MOLARIS. A component analysis of the different energy contributions of the FEP treatment and a similar PDLD analysis indicate that electrostatic effects provide the largest contribution to the differential binding energy, while the hydrophobic and steric contributions are much smaller. This finding lends further support to the idea that electrostatic interactions play a major role in determining the antigen specificity of McPC603.

Differential responses of expressed recombinant human gamma-aminobutyric acidA receptors to neurosteroids

Neuroactive steroids, in particular 3 alpha-hydroxypregnanes, are allosteric modulators of the gamma-aminobutyric acidA (GABAA) receptor. Regionally selective expression of receptor subunit subtypes may account for differential responsiveness of tissues to GABAergic inhibition and neurosteroid modulatory effects. The effect of 5 alpha-pregnan-3 alpha-ol-20-one (epiallopregnanolone) on heterotropic cooperativity on the GABAA receptor complex has been studied in three subtypes of expressed recombinant human receptors and in rat brain and spinal cord. Steroid potentiation of [3H]flunitrazepam binding was greatest for the alpha 3 beta 1 gamma 2 receptor complex, whereas alpha 1 beta 1 gamma 2 and alpha 2 beta 1 gamma 2 complexes showed less than 100% enhancement in binding. Previous studies suggest that the spinal cord is devoid of alpha 1, whereas cerebellum is rich in alpha 1 subunits. Correspondingly, a differential enhancement of [3H]flunitrazepam binding in spinal cord (51%) versus cerebellum (28%) was also observed. The structure of neuroactive steroids is important in determinikng the extent of neuromodulatory activity. The 5 beta-pregnanes,5 beta-pregnan-3 alpha-ol-20-one (epipregnanolone) and 5 beta-pregnan-3 alpha,21-diol-20-one (5 beta-tetrahydrodeoxycorticosterone), were both less potent than their corresponding 5 alpha derivatives. A 3 alpha hydroxyl group is essential for neuromodulatory activity in the expressed receptors, as demonstrated by the observation that 5 alpha-pregnan-3 beta-ol-20-one (allopregnanolone) and 4-pregnen-3, 20-dione (progesterone) were both inactive.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of yeast transcription factor IIIC with dimeric Schizosaccharomyces pombe tRNA(Ser)-tRNA(Met) genes

A unique tRNA(Ser)-tRNA(Met) tandem gene arrangement was characterized previously from Schizosaccharomyces pombe. Three alleles exist in which a tRNA(Ser) gene is separated by 7 base pairs from an initiator tRNA(Met) gene. Promotion of transcription occurs only within the tRNA(Ser) gene, yielding a dimeric precursor transcript. Using nuclease protection and gel retardation assays, we have analyzed how the Saccharomyces cerevisiae RNA polymerase III transcription factor C (TFIIIC) interacts with this dimeric gene template. The primary interaction site of TFIIIC with the tRNA(Ser) gene is at the 3'-internal control region (ICR), which can be distinguished kinetically from its weaker interaction with the 5'-ICR of the gene. We examined a variety of point mutations and double mutations within the tRNA(Ser) gene which reduce transcription. We found that changes in highly conserved nucleotides within the ICRs reduce TFIIIC binding up to 7-fold compared with the parent suppressor gene. The interaction of TFIIIC with the tRNA(Ser) gene does not sterically prevent stable binding of TFIIIC to the 3'-ICR of the tRNA(Met) gene. However, the affinity of binding of TFIIIC to the dimeric template is 7-fold higher than to the tRNA(Met) gene, alone, demonstrating that the tRNA(Met) gene contains intrinsically weak promoter elements. This may contribute to the inability of the tRNA(Met) gene to independently direct transcription from its ICR elements.

Anticonvulsant profile of the progesterone metabolite 5 alpha-pregnan-3 alpha-ol-20-one

5 alpha-Pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) is a naturally occurring metabolite of progesterone that can modulate brain excitability through a specific steroid recognition site on the GABA/benzodiazepine receptor-chloride ionophore complex. The anticonvulsant properties of 3 alpha-OH-DHP were determined using standardized anticonvulsant screening tests in mice. This steroid was found to be effective against metrazol-, (+)-bicuculline- and picrotoxin-induced seizures. The steroid has maximum potency against (+)-bicuculline-induced convulsions and no activity against maximal electroshock and strychnine-induced seizures. These findings support the hypothesis that therapeutically useful anticonvulsant steroids active at the putative steroid recognition site associated with the GABA/benzodiazepine receptor-chloride ionophore complex can be identified.

Thyroid hormone analog inhibition of hepatic 5'-iodothyronine deiodinase activity

Studies using thyroid hormone analogs have provided insight into the structural requirements for thyromimetic activity and for thyroid hormone binding to thyroxine-binding globulin, thyroxine-binding prealbumin, and nuclear T3 receptors. To determine the structural specifications for iodothyronine interaction with 5'-iodothyronine deiodinase (5'-ITD), we examined the ability of 35 thyroid hormone analogs to inhibit hepatic T4 5'-deiodination in vitro. The compounds were incubated in concentrations of 0.1-500 microM with rat liver homogenates, and concentrations producing 50% inhibition of T3 production were calculated. Those iodothyronine analogs which likely serve as substrate for 5'-ITD, e.g.rT3 and 3',5'-T2, and those which have one tyrosyl iodide were the most potent inhibitors of 5'-ITD activity. The presence of tyrosyl iodides enhanced inhibition by compounds with alkyl and halogen substitutions. Inhibition was likely due to direct interaction with the enzyme, since it was readily reversed by DTT. The terminal amino and phenolic hydroxyl groups, as well as the ether linkage, do not appear to be essential components of enzyme interaction.

Analysis of idiotypic and anti-idiotypic antibodies as models of receptor and ligand

Antibodies to small bioactive ligands and peptides may mimic the binding characteristics of the natural receptor; in turn, the anti-idiotypic antibodies generated against the binding sites of such anti-ligand antibodies may mimic some aspects of small bioactive ligands and peptides. Among the several levels of investigation of such antibody-receptor networks are (a) the quantitative structure-activity relationships of ligand binding to antibody as compared with natural receptor; (b) the molecular modeling of antibody-receptor binding sites and the genomic basis for such structures; and (c) the characteristics of the molecular mimicry exhibited by "mimetopes" on anti-idiotypic antibodies. To illustrate the analysis encountered at each of these levels, we discuss here antibody and anti-idiotypic systems that are directed to small neuroactive ligands and their receptors.

Analysis of idiotypic and anti-idiotypic antibodies as models of receptor and ligand

Antibodies to small bioactive ligands and peptides may mimic the binding characteristics of the natural receptor; in turn, the anti-idiotypic antibodies generated against the binding sites of such anti-ligand antibodies may mimic some aspects of small bioactive ligands and peptides. Among the several levels of investigation of such antibody-receptor networks are (a) the quantitative structure-activity relationships of ligand binding to antibody as compared with natural receptor; (b) the molecular modeling of antibody-receptor binding sites and the genomic basis for such structures; and (c) the characteristics of the molecular mimicry exhibited by "mimetopes" on anti-idiotypic antibodies. To illustrate the analysis encountered at each of these levels, we discuss here antibody and anti-idiotypic systems that are directed to small neuroactive ligands and their receptors.

Steroid modulation of the chloride ionophore in rat brain: structure-activity requirements, regional dependence and mechanism of action

Further in vitro studies of steroids active at the gamma-aminobutyric acidA (GABAA) receptor regulated Cl- channel labeled by [35S]-t-butylbicyclophosphorothionate ([35S]TBPS) reveal additional structural requirements necessary for activity. Evaluation of selected steroids for activity against TBPS-induced convulsions show similar requirements for activity. Interestingly, steroids (e.g., 5 alpha-pregnan-3 alpha, 20 alpha-diol) were identified that have high potency but limited efficacy as modulators of [35S]TBPS binding. These characteristics are reminiscent of the clinically useful benzodiazepines (BZs) such as clonazepam. However, interactions between the prototypical anesthetic-barbiturate, sodium pentobarbital, and steroids active at the Cl- channel suggest that they do not share a common site of action as allosteric modulators of [35S]TBPS and BZ receptor binding. The most potent steroid evaluated, 5 alpha-pregnan-3 alpha-ol-20-one, modulates [35S]TBPS binding at low concentrations (IC50 approximately 17 nM) in a regionally dependent manner. All [35S]TBPS binding sites appear to be functionally coupled to a steroid "modulatory site." Because several of the active steroids are metabolites of progesterone, their ability to inhibit the binding of [3H]promegestrone to the cytosolic progestin receptor in rat uterus was evaluated. Those steroids showing potent activity at the GABAA receptor-Cl- ionophore were inactive at the intracellular progestin receptor. Such specificity coupled with their high potency provide additional support for the hypothesis that some of these steroids may be involved in the homeostatic regulation of brain excitability via the GABAA-BZ receptor complex.

Haloperidol binding to monoclonal antibodies. Predictions of three-dimensional combining site structure via computer modeling

The amino acid sequences of five monoclonal antibodies (designated mAbs A-E) which bind to the dopaminergic D-2 antagonist, haloperidol, with a variety of affinities (Kd = 4-810 nM), have been used to build theoretical, three-dimensional, computer models of the variable region combining sites. Physiocochemical interactions which have been previously determined from in vitro binding data have been used to orient the drug molecule within the combining site model. The results indicate that hydrophobic, aromatic, and ionic amino acids are involved in specific interactions with the antagonist molecule. For example, fluorescence quenching data suggests that a tryptophan residue is intimately involved in the binding of haloperidol by mAb A. Examination of the modeled structure reveals five tryptophans within the variable fragment, only one of which (H-50) is within the classical beta-barrel binding pocket and is readily accessible to the antigen. Haloperidol's relatively electron poor fluorophenyl ring system stacks with the electron-rich tryptophan ring system at a distance of 3.3 A and in so doing, places haloperidol's positively charged piperidinyl nitrogen atom within hydrogen bond distance of the negatively charged Glu-95 and Asp-100A residues of the H3 loop (Glu-H-95 and Asp-H-100A). This type of analysis for each antibody provides an interesting profile of changes in amino acid composition and hypervariable loop length which markedly effect binding affinity and specificity for a series of proteins which have similar combining site.

Haloperidol binding to monoclonal antibodies. Hypervariable region amino acid sequence determination

The primary sequences of five monoclonal antibodies (mAbs A-E) which bind with various affinities (Kd = 4-810 nM) to the D-2 dopaminergic antagonist, haloperidol, have been determined. Immunoglobulin light and heavy chain mRNA was isolated and gene sequence determined by primer extension in the presence of dideoxynucleotides. The pattern of insertions and deletions found within the hypervariable regions produce loops which differ in length from one antibody to another, and are directly responsible for establishing the gross architecture of the combining site. Two of the anti-haloperidol mAbs have long hypervariable loops which form a pocket-shaped combining site. Three other mAbs have deletions of 3 or 4 amino acids in the third heavy chain complementarity producing region which result in a groove-like combining site as determined by computer based molecular modeling. A discussion of the probable mechanism by which the given sequences were generated from various gene segments is also presented.

Effects of 2-substituted-4-phenylquinolines on uptake of serotonin and norepinephrine by isolated brain synaptosomes

In this present communication, the in vitro inhibition of the uptake of [3H]-L-norepinephrine ([3H] NE) and [3H]-Serotonin ([3H] 5-HT) by eleven synthesized 2-substituted-4-phenyl quinolines were studied using rat brain synaptosomal preparations. Compounds with an open side chain were relatively weak inhibitors of the synaptosomal uptake of [3H] NE and [3H] 5HT. Compounds having a distance of three atoms between the terminal basic nitrogen of the side chain and the quinoline ring were better inhibitors of serotonin uptake than those compounds having a four-atom distance. The replacement of the side chain with a piperazine ring produced compounds which were more potent and selective inhibitors of the uptake of either [3H] 5-HT or [3H] NE. Further structure-activity relationships are also discussed.

Effect of thyroid hormone analogs on the activity of choline acetyltransferase in cultures of dissociated septal cells

Thyroid hormones influence the expression of transmitter-specific enzymes by central cholinergic neurons. Based on the fact that these cholinergic neurons degenerate selectively in human Alzheimer's disease, it was hypothesized that thyroid hormones might be beneficial in its treatment. However, since thyroid hormones influence the function of most peripheral organs, derivatives selective for central cholinergic neurons are necessary. The structural requirements of the receptor mediating the effects of the thyroid hormones on central cholinergic neurons were therefore compared with those of the receptors mediating actions on peripheral organs. Cultures were prepared of dissociated neurons from the septal area of fetal rat brains, and the differentiation of cholinergic neurons was assessed by measuring the activity of choline acetyltransferase (ChAT). Triiodothyronine (T3) was found to stimulate ChAT activity in a dose-dependent manner. The effect of T3 was additive to that of nerve growth factor. The potency of derivatives of T3 in elevating ChAT activity in the cultures was compared with their known anti-goiter activity determined in vivo and their binding affinity to the hepatic nuclear receptor measured in vitro. The findings indicate that the structural requirements of central and peripheral receptors are similar and that it therefore appears unlikely that analogs of thyroid hormones can be developed which selectively affect cholinergic neurons.

Haloperidol binding to monoclonal antibodies: conformational analysis and relationships to D-2 receptor binding

A library of 22 monoclonal antibodies to the D-2 dopaminergic receptor antagonist haloperidol has been developed by immunizing BALB/c mice with two conformationally distinct immunogens. The two immunogens were prepared by coupling haloperidol to bovine serum albumin through the tertiary alcohol at the 4-position of the piperidine ring with a succinic acid linkage and by coupling to bovine serum albumin through the ketone group of the butyrophenone with a carboxymethyl oxime linkage. Seventeen monoclonal antibodies displayed specific, saturable, high affinity binding of [3H]haloperidol which could be inhibited by a variety of neuroleptic drugs. Three monoclonal antibodies raised against the succinic acid conjugate and two monoclonal antibodies raised against the oxime conjugate were selected for detailed analysis of the molecular characteristics of binding specificity and for relationships to bovine striatal D-2 dopaminergic receptor binding. The monoclonal antibody with highest affinity for haloperidol, 185(2)-1 (raised against the succinic acid conjugate and herein referred to as mAb A), had a Kd of 3.3 (+/- 0.06) nM and primarily recognized chemical determinants in the butyrophenone ring (ring 1) and side chain. Inhibition of [3H]haloperidol binding to mAb A by 16 unlabeled haloperidol analogs displayed a good correlation [r = 0.82, n = 16, m = 1.06(+/- 0.38)] with D-2 receptor binding affinity, suggesting that the parts of the D-2 receptor combining site which recognize butyrophenone antagonists may have molecular characteristics which are similar to those of the monoclonal antibody. Other dopaminergic ligands such as dopamine and the D-1 antagonist SCH-23390 were not recognized by monoclonal antibodies raised against the succinic acid conjugate. Monoclonal antibodies raised against the oxime conjugate such as 258(2)-1 (herein referred to as mAb D) primarily recognized chemical determinants in ring 2 and the tertiary amino group of the piperidine ring. Although the D-2 receptor and mAb D both prefer electron-withdrawing substituents in the para position of ring 2, the antibody was more sensitive than the receptor to changes and displayed affinities that were much lower for substituents attached to the p-chlorophenyl ring (ring 2), which were electron donating. In addition, dopamine was able to completely displace 4 nM [3H]haloperidol from mAb D at a concentration of 6 mM.

Preparation and characterization of antisera and monoclonal antibodies to haloperidol

For the first time a library, of monoclonal antibodies (MoAbs) to the butyrophenone haloperidol (D-2 antagonist) has been prepared. Synthesis of a haloperidol derivative suitable for chemical coupling to a protein carrier via oxobutyric acid produced an immunogen which was used to develop two polyclonal antisera and twelve MoAbs specific for the hapten. Our library of MoAbs can be grouped into three classes; 1) high affinity and specificity for free 3H-haloperidol, 2) moderate affinity with significant cross-reactivity to other butyrophenone ligands, and 3) a group which binds poorly to free 3H-haloperidol but instead recognizes the ligand only when it is coupled to carrier protein. Clone (189(2)-6) was found to have the highest equilibrium binding affinity (Kd = 4 nM) and is far more specific than the currently available antisera to haloperidol. This MoAb has significantly lower affinity for all of the common metabolites of haloperidol. This capability makes 189(2)-6 a candidate for further development with regard to use in clinical radioimmuno-assays of therapeutic drug levels. In addition, one of the anti-haloperidol Moabs (190(2)-6) binds more tightly to spiperone than to haloperidol and displays a qualitative correlation in the rank order of neuroleptic binding affinity for a limited series of analogs when compared to membrane bound D-2 receptor binding.

Photoinactivation of serotonin uptake by an arylazido derivative of 5-hydroxytryptamine

A potential photoaffinity probe for the substrate-binding polypeptide of the neuronal serotonin uptake system has been synthesized. Under dark conditions, 3-(beta-(4-azidobenzamidino)ethyl-5-hydroxyindole (serotonin azidobenzamidine (SABA) was found to inhibit competitively [3H]5-hydroxytryptamine uptake by rat cortical synaptosomes with a K1 of 130 nM. The selectivity of this action was indicated by SABA's much lower potency as an inhibitor of synaptosomal [3H]norepinephrine uptake (K1 = 7 microM). When synaptosomes were irradiated in the presence of SABA, serotonin uptake was irreversibly inhibited in a concentration-dependent fashion with the maximum effect occurring at 1 microM SABA. At this concentration, approximately 40% of the serotonin uptake activity could not be recovered upon repeated washing of the synaptosomes. This inhibition was determined not to result from the production of a potent inhibitory photolysis product of SABA. The photoinactivation of serotonin transport by SABA was found to depend on the time of irradiation and could be prevented by the presence of agents that interact with the uptake system. Serotonin, p-chloroamphetamine, fenfluramine, and alaproclate protected the serotonin carrier against SABA's irreversible effects in a concentration-dependent manner. The presence of high concentrations of Tris or p-aminobenzoic acid, two nitrene-scavenging agents, did not reduce the level of photoinactivation of serotonin uptake by SABA, indicating that the irreversible inhibition is a result of true photoaffinity labeling of the carrier.

Preparation and characterization of antisera and monoclonal antibodies to serotonergic and dopaminergic ligands

In an attempt to produce polyclonal antisera and monoclonal antibodies to serotonin, SKF 38393 (D-1 agonist), dopamine, and haloperidol (D-2 antagonist) several procedures for the preparation of immunogenic ligand-protein carrier conjugates were investigated. The Mannich reaction utilizing formaldehyde as the chemical linker was used to prepare serotonin-protein conjugates; antibodies raised to this conjugate reacted specifically to the conjugated serotonin moiety but did not react to native serotonin. Chemical conjugations involving dimethylpimelylimidate or N-carboxymethyl derivatives for the coupling of serotonin, dopamine and SKF 38393 to carrier proteins produced antibodies primarily directed against the 'chemical coupling arm' and very little antibody activity against the ligand itself could be detected. Synthesis of a haloperidol derivative suitable for chemical coupling to a protein carrier via oxobutyric acid produced an immunogen which was capable of eliciting both polyclonal and monoclonal antibodies specific for the hapten. The pitfalls of the various chemical conjugation procedures and the difficulties of producing antibodies to free ligands are discussed.

Interaction of a fluorescent acyldicholine with the nicotinic acetylcholine receptor and acetylcholinesterase

A fluorescent acyldicholine, bis-(choline)N-[4-nitrobenzo-2-oxa-1,3-diazol-7-yl]-iminodiprop ionate (BCNI), was synthesized and its capacity to associate with acetylcholinesterase and the nicotinic acetylcholine receptor examined. The fluorescent bisquaternary diester competitively inhibits acetylcholinesterase with a Ki of 0.46 microM. Binding is accompanied by a large decrease in BCNI fluorescence and a 40% reduction in enzyme tryptophanyl fluorescence due to spectral overlap between BCNI absorption and the fluorescence emission of tryptophanyl residues on the enzyme. BCNI titrations show a stoichiometry of one site per subunit and a dissociation constant of 0.2 microM. BCNI also inhibits the initial rate of alpha-toxin binding to the membrane-associated nicotinic acetylcholine receptor and yields a protection constant (Kp) of 0.26 microM. Prior exposure of BCNI to the receptor increases the affinity of the complex, and after equilibration Kp is found to be 0.11 microM. Fluorescence titrations reveal that BCNI binds with 1:1 stoichiometry to alpha-toxin sites on the receptor with a dissociation constant of 0.22 microM. Agonists and antagonists, but not local anesthetics, compete with BCNI binding. BCNI behaves as a competitive antagonist on receptors from the snake neuromuscular junction and from BC3H-1 cells. The 4-nitrobenzo-2-oxa-1,3-diazole fluorophore in BCNI shows a hypsochromatic shift and an enhancement of quantum yield when bound to the receptor but is quenched when associated with acetylcholinesterase. Thus, despite the similarity in dissociation constants, the fluorophore exists in very different environments when bound to the two proteins.

Thyroxine analogues. 23. Quantitative structure-activity correlation studies of in vivo and in vitro thyromimetic activities

Quantitative structure-activity correlation studies of thyroid hormone analogues have been utilized to examine (1) in vivo rat antigoiter activities; (2) in vitro binding affinities to intact rat hepatic nuclei, solubilized rat hepatic nuclear protein receptors, and the plasma protein thyroxine binding globulin; and (3) correlations between in vivo antigoiter activities and in vitro binding to nuclear receptors. These studies provide a more precise elucidation of the relative importance of the physiochemical factors which influence thyromimetic activities. In particular, they (1) provide the first systematic QSAR examination of drug-receptor interactions and of the dependence of in vivo activity on such interactions; (2) demonstrate the importance of the interactive effects of the 3' and 5' substituents and of the 4'-OH with each other as well as with nuclear receptors in influencing binding affinity; (3) support the hypothesis that binding to nuclear receptors is the first step in initiating the events which lead to subsequent hormonal expression; (4) show that the free energy of binding to nuclear receptors can be factored into the contributing physicochemical properties of the substituents; and (5) suggest factors that need to be considered in designing new analogues.