Ionization and surface properties of verapamil and several verapamil analogues

Supramolecular β-Sheets Stabilized Protein Nanocarriers for Drug Delivery and Gene Transfection

Suckerin proteins, recently discovered in the sucker ring teeth of squids, represent a family of promising structural biomacromolecules that can form supramolecular networks stabilized by nanoconfined β-sheets. Exploiting this feature as well as their specific amino acid composition, we demonstrate that artificial suckerin-19 (S-19) can be engineered into nanocarriers for efficient drug delivery and gene transfection in vitro and in vivo. First, we demonstrate that S-19 self-assembles into β-sheet stabilized nanoparticles with controlled particle sizes of 100-200 nm that are able to encapsulate hydrophobic drugs for pH-dependent release in vitro, and that can effectively inhibit tumor growth in vivo. We also show that S-19 can complex and stabilize plasmid DNA, with the complexes stabilized by hydrophobic interactions of the β-sheet domains as opposed to electrostatic interactions commonly achieved with cationic polymers, thus lowering cytotoxicity. The elevated Histidine content of S-19 appears critical to trigger endosomal escape by the proton sponge effect, thereby ensuring efficient gene transfection both in vitro and in vivo. Our study demonstrates that S-19 represents a promising functional protein nanocarrier that could be used for various drug and gene delivery applications.

Active transport inhibition in rat small intestine by amphiphilic amines: an in vitro study with various local anaesthetics

In the present investigation with rings of everted rat small intestine, amphiphilic amines such as local anaesthetics (e.g. lidocaine, procaine, tolycaine) were employed to study their effects on intestinal absorption of methyl alpha-D-glucoside, L-leucine, D-fructose, and 2-deoxy-D-glucose. All the amphiphilic amines tested, except for benzocaine, significantly inhibited Na(+)-dependent active uptake of methyl alpha-D-glucoside and L-leucine while leaving uptake of D-fructose (facilitated diffusion) and 2-deoxy-D-glucose (passive diffusion) unaffected. Increasing concentrations of lidocaine in the incubation medium inhibited the uptake of methyl alpha-D-glucoside (IC(50) approximately 3.5 mmol/L) and L-leucine (IC(50) approximately 6 mmol/L) in a dose-dependent manner. Complete reversibility of the inhibitory effect could only be achieved at short-term incubations (</=2 min) and low lidocaine concentrations (</=3 mmol/L), otherwise inhibition became partially irreversible. Uptake kinetics of methyl alpha-D-glucoside and L-leucine in the presence of lidocaine revealed a significant increase in the apparent Michaelis constant, leaving the maximal transport capacity essentially unaltered. Reducing the Na(+) concentration in the incubation medium aggravated inhibition by lidocaine of the uptake of methyl alpha-D-glucoside. Analysis of the inhibition kinetics by Dixon plots revealed a competitive interaction between Na(+) and the amphiphiles. However, phlorizin binding was not affected by lidocaine. Changing the pH of the incubation medium from 5.6 to 8.0 increased the inhibitory effect of the amphiphiles, which indicated that the non-ionised and, thus, more lipophilic form participates in the mechanism of inhibition. However, benzocaine, a rather lipophilic local anaesthetic with no aliphatic amino group, did not impair active uptake of methyl alpha-D-glucoside. Whether the amphiphilic amines act by their partition into the membrane matrix or directly interact with sodium binding sites remains to be elucidated, however.

The effect of deep pore mutations on the action of phenylalkylamines on the Kv1.3 potassium channel

We investigated the action of the phenylalkylamines verapamil and N-methyl-verapamil on the Kv1.3 potassium channel using the whole-cell configuration of the patch-clamp technique. Our goal was to identify their binding as a prerequisite for using the phenylalkylamines as small, well-defined molecular probes, not only to expand the structural findings made with peptide toxins or by crystallization, but also to use them as lead compounds for the generation of more potent and therefore more specific K+ channel modulators. Competition experiments with charybdotoxin, known to interact with external residues of Kv1.3, showed no interaction with verapamil. The internal application of quarternary N-methyl-verapamil in combination with verapamil suggested competition for the same internal binding site. Verapamil affinity was decreased 6 fold by a mutation (M395V) in a region of the internal pore which forms part of the internal tetraethylammonium (TEA+) binding site, although mutations at neighbouring residues (T396 and T397) were without effect. Modification of C-type inactivation by mutations in the internal pore suggest that this region participates in the inactivation process. The action of phenylalkylamines and local anaesthetics on L-type Ca2+ channels and Na channels, respectively, and verapamil on Kv1.3 indicate very similar blocking mechanisms. This might allow the use of these compounds as molecular probes to map the internal vestibule of all three channel types.

Mechanism of verapamil block of a neuronal delayed rectifier K channel: active form of the blocker and location of its binding domain

1. The mechanism of verapamil block of the delayed rectifier K currents (I K(DR)) in chick dorsal root ganglion (DRG) neurons was investigated using the whole-cell patch clamp configuration. In particular we focused on the location of the blocking site, and the active form (neutral or charged) of verapamil using the permanently charged verapamil analogue D890. 2. Block by D890 displayed similar characteristics to that of verapamil, indicating the same state-dependent nature of block. In contrast with verapamil, D890 was effective only when applied internally, and its block was voltage dependent (136 mV/e-fold change of the on rate). Given that verapamil block is insensitive to voltage (Trequattrini et al., 1998), these observations indicate that verapamil reaches its binding site in the uncharged form, and accesses the binding domain from the cytoplasm. 3. In external K and saturating verapamil we recorded tail currents that did not decay monotonically but showed an initial increase (hook). As these currents can only be observed if verapamil unblock is significantly voltage dependent, it has been suggested (DeCoursey, 1995) that neutral drug is protonated upon binding. We tested this hypothesis by assessing the voltage dependence of the unblock rate from the hooked tail currents for verapamil and D890. 4. The voltage dependence of the off rate of D890, but not of verapamil, was well described by adopting the classical Woodhull (1973) model for ionic blockage of Na channels. The voltage dependence of verapamil off rate was consistent with a kinetic scheme where the bound drug can be protonated with rapid equilibrium, and both charged and neutral verapamil can unbind from the site, but with distinct kinetics and voltage dependencies.

Surface activity and concentration dependent intestinal permeability in the rat

PURPOSE

To investigate the relation between intestinal effective permeability (P(eff)) and surface activity of fluvastatin and verapamil.

METHODS

P(eff)-values were determined for fluvastatin, antipyrine and D-glucose following colon perfusions in the rat in situ. The perfusion solitions differed regarding concentrations of fluvastatin (0-2500 microM) and surface tension (58.9-43.7 mN/m). A cellulose derivative, ethyl-(hydroxyethyl) cellulose (EHEC), was added to lower the surface tension of one of the perfusion solutions. The surface tension of perfusion solutions containing R/S-verapamil (8-814 microM) and R/S-verapamil + chlorpromazine (814 microM + 10 mM) were related to the corresponding P(eff)-values from the literature.

RESULTS

The P(eff)of fluvastatin correlated inversely (r2 = 0.985, p < 0.05) with the surface tension of the perfusion solutions below the critical micelle concentration (CMC, 1 mM). Decreasing the surface tension with EHEC increased the P(eff) of fluvastatin by 36% (p < 0.001), but not to the extent anticipated from the correlation between the P(eff) and the surface tension. EHEC also increased the P(eff) of antipyrine by 49% (p < 0.01 ) but not for D-glucose. The P(eff) of R/S-verapamil correlated inversely with the surface tension (r2 = 0.980, p < 0.001).

CONCLUSIONS

The ability of fluvastatin to decrease the surface tension at the membrane surface can partly explain the concentration dependent colonic P(eff) of fluvastatin. This study shows that the surface activity of the drug molecule itself is an important physicochemical factor that should be taken into consideration when evaluating drug absorption studies performed in vitro or in situ.

Ionization constants and distribution coefficients of phenothiazines and calcium channel antagonists determined by a pH-metric method and correlation with calculated partition coefficients

The pH-metric technique was used to determine the ionization constants and distribution coefficients of 10 phenothiazines and five ionizable calcium channel antagonists. Because the studied compounds were poorly water soluble and quite lipophilic with partition coefficients in the range of 3.5 to 5.5, organic cosolvents had to be added for the determination of the ionization constants to avoid precipitation of the free bases. The effect of the cosolvents dioxane and methanol on the extrapolation to pure water was compared. For both cosolvents a very good agreement with accessible published ionization constants was obtained, however the slope of the regression line was much smaller for dioxane, yielding more reliable estimates according to the standard deviation of the extrapolated values. Thus, dioxane might be preferable to methanol as a cosolvent for the determination of ionization constants of sparingly water soluble bases. Also the n-octanol/water partition coefficients were determined and compared with published data and values calculated with the ClogP, ACD, and HINT programs. Although the obtained values were approximate in conformity with the published data, the calculated partition coefficients differed from the experimental ones considerably for the majority of the investigated compounds. Furthermore, the ion pair partitioning and the distribution coefficients at physiological pH 7.4 were determined. The pH-dependent distribution profiles showed the strong influence of the ionization constants and of the distribution of the ion pairs on the overall distribution. This result strongly suggests that greater use should be made of measured distribution coefficients in quantitative structure-activity relationship studies. The potentiometric method is a convenient way to determine the distribution properties of drug molecules at pH values relevant for the biological system under investigation.

Dihydropyridines and verapamil inhibit voltage-dependent K+ current in isolated outer hair cells of the guinea pig

Dihydropyridines and verapamil are widely used as blockers of voltage-dependent Ca++ channels. In this work we show that these compounds can have a direct blocking action on a class of voltage-activated potassium channels. Voltage-dependent whole-cell currents were recorded from isolated guinea-pig outer hair cells (OHCs) under conditions such that the free Ca++ concentration in both the internal and external solutions was minimized. A substantial Ca(++)-independent K+ current was revealed by this procedure. Both conventional K+ and Ca++ channel ligands inhibited this current. The order of potency (in terms of the half inhibitory concentrations (IC50) of channel inhibitors) was: nimodipine (6 microM) > Bay K 8644 (8 microM) > verapamil (11 microM) > 4-aminopyridine (22 microM) > nifedipine (32 microM) > quinine (49 microM) > TEA (10236 microM). Except for verapamil, these channel ligands reduced the size of the K+ currents without much alteration of the time course of the currents. In contrast, verapamil caused a more than 10-fold increase in the apparent inactivation rate of the K+ currents without significantly altering the activation of the currents. The observation that relatively low concentrations of calcium channel ligands can directly inhibit potassium currents in isolated OHCs indicates that caution should be taken when these pharmacological agents are used as tools for studying cochlear hair cell physiology.

Self-association of dexverapamil in aqueous solution

The pKa and intrinsic solubility of monomeric dexverapamil were determined from its pH-solubility profile to be 8.90 and 6.6 x 10(-5) M, respectively. The solubility of dexverapamil below pH 7.0 was higher than expected on the basis of the aforementioned values. This unusually high solubility is believed to be due to the self-association of cationic dexverapamil. The apparent pKa of the self-associated drug is estimated to be approximately 7.99. The self-association of dexverapamil hydrochloride is supported by the fact that it is surface active and that it increases the solubility of both naphthalene and anthracene in aqueous solutions. The dependence of the drug solubility on pH and the solubilization of naphthalene and anthracene as a function of ionized drug concentration suggest that the self-associated dexverapamil is a cationic dimer.

Semi-rigid analogues of the calcium antagonist verapamil: a molecular modelling study

In this work the rigid-analogue approach has been used to obtain information on the active conformation(s) of the calcium antagonist verapamil. A series of semi-rigid analogues of verapamil were synthesized and their biological activities evaluated on guinea-pig heart and aorta. These molecules were analysed by means of molecular modelling techniques. On the basis of the pharmacological profile and conformational analysis of these compounds, two different models for negative inotropic and negative chronotropic activity are proposed. The two actions seem to be due to conformations of the molecules which differ in the orientation of their phenylethylamino groups.

Discontinuous alterations of platelet structure and function by bound, ionizable verapamil

We have investigated several effects of verapamil (5-[(3,4-dimethoxyphenethyl)methylamino]-2-(3,4-dimethoxyphenyl)-2- isopropylvaleronitrile) on platelet structure and function. At concentrations below a threshold of approximately 4.2 x 10(7) molecules.cell-1, verapamil binds to platelets according to a typical Langmuir adsorption isotherm (i.e., binding is saturable and noncooperative). By extrapolation, we calculate that saturation would occur at 6.8 x 10(7) +/- 1.9 x 10(7) molecules.platelet-1, with one bound verapamil molecule per two membrane phospholipids. Saturation is never achieved, however, because past the threshold surface concentration, the adsorption isotherm becomes discontinuous and further adsorption becomes a linear function of the concentration of drug in solution. We attribute this discontinuity to disorganization of the membrane bilayer which is stretched beyond cohesion by insertion of too many amphiphilic molecules of a length shorter than that of the phospholipid. The partitioning of verapamil between the bulk aqueous phase and the newly created lipid phase would then account for the linear portion of the adsorption isotherm. The discontinuity of binding is accompanied by discontinuities in the verapamil-dependent swelling of platelets and the verapamil-dependent inhibition of both ADP-inducible binding of fibrinogen to, and aggregation of, platelets. In contrast, N-methylverapamil (5-[(3,4-dimethoxyphenyl)- methylamino]-2-(3,4-dimethoxyphenyl)-2-isopropylvaleronitrile), a hydrophilic quaternary amine derivative of verapamil, neither swells platelets nor inhibits the ADP-dependent processes that we investigated. From this we conclude that deprotonated verapamil is the operative species of the drug. Collectively, these data suggest that verapamil alters platelet structure and function by mechanisms involving disorganization of the platelet plasma membrane.

Calcium fluxes caused by bile salts in rat pancreatic acini

The effect of bile salt on Ca2+ uptake, Ca2+ efflux, and cytosolic free Ca2+ concentration ([Ca2+]i) in rat pancreatic acini has been studied. The dihydroxy bile salts, taurodeoxycholate (TDC) and taurochenodeoxycholate (TCDC), were found to accelerate Ca2+ uptake. Dihydroxy bile salt and the trihydroxy bile salt taurocholate (TC) stimulated Ca2+ efflux from the acini. Verapamil increased the Ca2+ efflux induced by dihydroxy bile salts but did not influence the Ca2+ uptake. Under calcium-equilibrated conditions TDC and TCDC caused a quick net Ca2+ efflux, followed by an increase in Ca2+ uptake, whereas TC only caused a net Ca2+ efflux. TDC and TCDC, but not TC, increased the [Ca2+]i. This effect of TDC and TCDC was abolished in Ca2+-free EDTA-containing (0.2 mM) medium. The amylase release caused by bile salts was related in time with the Ca2+ fluxes. In conclusion, bile salts may change the Ca2+ homeostasis of pancreatic acini in different ways, depending on the type of bile salt. This change of Ca2+ homeostasis may play an important role in the bile salt-stimulated amylase release.