Favorable amphiphilicity of nimodipine facilitates its interactions with brain membranes

Nimodipine Pharmacokinetic Variability in Various Patient Populations

Nimodipine has been shown to improve outcomes following aneurysmal subarachnoid hemorrhage. Guidelines recommend that all patients receive a fixed dose of oral nimodipine for 21 days. However, pharmacokinetic studies have suggested variability of nimodipine pharmacokinetics in subarachnoid hemorrhage and in other patient populations. The clinical relevance of such variability is unknown. Therefore, the objective of the present review is, first, to conduct a literature review and summarize nimodipine pharmacokinetic data and sources of variability in various patient groups. Second, to determine if there is any evidence reporting an association between nimodipine exposure and clinical outcomes in patients with subarachnoid hemorrhage. A systematic literature search was performed in MEDLINE and EMBASE. The following keywords were used: ("nimodipine" OR "nymalize" OR "nimotop") AND ("pharmacokinetic*", OR "PK"). The search results were limited to English language and human studies. A large interpatient variability in nimodipine pharmacokinetics has been reported. Patient-specific factors that had an influence on pharmacokinetic parameters are age, comorbidities, variabilities in metabolism due to genetic polymorphism and co-administered medications, as well as nimodipine administration technique. The association between nimodipine exposure and clinical outcomes remains unclear and data available are too scarce to reach a firm conclusion. Here, we present a narrative review with a systematic literature search discussing nimodipine pharmacokinetic variability in various patient populations. It is not clear if minimal or lack of systemic exposure to nimodipine denies its benefit and contributes to worsening outcomes in patients with subarachnoid hemorrhage. Further studies are needed to determine if such an association exists.

Effect of natural borneol on the pharmacokinetics and distribution of nimodipine in mice

The purpose of this study was to investigate the effect of natural borneol (NB) on the pharmacokinetics and distribution of nimodipine in mice. A single dose of nimodipine was administered intravenously (2 mg/kg) to mice pretreated with NB (250 mg/kg) or vehicle. Blood as well as brain, liver, and kidney tissue samples were collected at 5, 10, 20, 40, and 60 min post-dose nimodipine. The concentrations of nimodipine in plasma and tissues were determined by ultra performance liquid chromatography (UPLC) coupled with UV detection, and the pharmacokinetic parameters were calculated based on non-compartmental analysis. NB increased the plasma AUC5-60 min by 26 % compared to the vehicle. In addition, brain concentrations of nimodipine in NB-treated mice were significantly higher than those in control mice with the increased AUC5-60 min by 30 %. In liver and kidney, NB also caused 26 and 47 % increase in AUC5-60 min, respectively. These results implicated that NB may inhibit the metabolism or elimination of nimodipine and enhance its distribution in brain and kidney tissue.

Dihydropyridine inhibition of the glycine receptor: subunit selectivity and a molecular determinant of inhibition

The dihydropyridines (DHPs), nifedipine and nicardipine, modulate native glycine receptors (GlyRs) at micromolar concentrations. Nicardipine has a biphasic potentiating and inhibitory effect, whereas nifedipine causes inhibition only. The present study sought to investigate (1) the molecular mechanism by which these compounds inhibit recombinant GlyRs, and (2) their potential utility as subunit-selective inhibitors of alpha1, alpha1beta, alpha3 and alpha3beta GlyRs. The rate of onset of inhibition in the open state was accelerated by pre-application of DHP in the closed state, with the degree of acceleration proportional to the concentration of pre-applied DHP. This implies a non-inhibitory binding site close to the DHP inhibitory site. DHP inhibition was use-dependent and independent of glycine concentration, consistent with a pore-blocking mode of action. DHP sensitivity was abolished by the G2'A mutation, providing a strong case for a DHP binding site in the pore. Nifedipine exhibited an approximately 10-fold higher inhibitory potency at alpha1-containing relative to alpha3-containing receptors, whereas nicardipine was only weakly selective for alpha1-containing GlyRs. The differential sensitivities of nifedipine and nicardipine for different GlyR isoforms suggest that DHPs may be a useful resource to screen as pharmacological tools for selectively inhibiting different synaptic GlyR isoforms.

Tetracaine-membrane interactions: effects of lipid composition and phase on drug partitioning, location, and ionization

Interactions of the local anesthetic tetracaine with unilamellar vesicles made of dimyristoyl or dipalmitoyl phosphatidylcholine (DMPC or DPPC), the latter without or with cholesterol, were examined by following changes in the drug's fluorescent properties. Tetracaine's location within the membrane (as indicated by the equivalent dielectric constant around the aromatic fluorophore), its membrane:buffer partition coefficients for protonated and base forms, and its apparent pK(a) when adsorbed to the membrane were determined by measuring, respectively, the saturating blue shifts of fluorescence emission at high lipid:tetracaine, the corresponding increases in fluorescence intensity at this lower wavelength with increasing lipid, and the dependence of fluorescence intensity of membrane-bound tetracaine (TTC) on solution pH. Results show that partition coefficients were greater for liquid-crystalline than solid-gel phase membranes, whether the phase was set by temperature or lipid composition, and were decreased by cholesterol; neutral TTC partitioned into membranes more strongly than the protonated species (TTCH(+)). Tetracaine's location in the membrane placed the drug's tertiary amine near the phosphate of the headgroup, its ester bond in the region of the lipids' ester bonds, and associated dipole field and the aromatic moiety near fatty acyl carbons 2-5; importantly, this location was unaffected by cholesterol and was the same for neutral and protonated tetracaine, showing that the dipole-dipole and hydrophobic interactions are the critical determinants of tetracaine's location. Tetracaine's effective pK(a) was reduced by 0.3-0.4 pH units from the solution pK(a) upon adsorption to these neutral bilayers, regardless of physical state or composition. We propose that the partitioning of tetracaine into solid-gel membranes is determined primarily by its steric accommodation between lipids, whereas in the liquid-crystalline membrane, in which the distance between lipid molecules is larger and steric hindrance is less important, hydrophobic and ionic interactions between tetracaine and lipid molecules predominate.

Plasma concentration and CNS effects of Ca antagonists darodipine and nimodipine after single-dose oral administration to healthy volunteers

The dynamics at the brain level (quantitative EEG), plasma kinetics and effects on blood pressure and heart rate of the Ca antagonists, darodipine (slow-release, 50- 200 mg) and nimodipine (30 mg), were compared in a double-blind cross-over study on healthy volunteers during a 9-hour period following single drug/placebo administration. Increased EEG total power was observed after 100 and 200 mg daropidine; a concomitant decrease of 14.5-32.0 Hz relative power was observed at 100 mg. The 50-mg dose proved ineffective. These effects were correlated with the darodipine plasma concentration only at the 100-mg dose, with indications of an active concentration interval at approximately 5-10 ng/ml; a reduction in diastolic blood pressure and increased heart rate proved to be linearly correlated with the drug plasma concentration throughout the entire concentration range. Comparable EEG effects were observed after nimodipine, but they did not correlate with the plasma concentration. Implications of the predictability of the brain effect from the drug plasma concentration and differential thresholds for the brain action and effects on (peripheral) circulation are suggested.

Competitive, reversible inhibition of cytosolic phospholipase A2 at the lipid-water interface by choline derivatives that partially partition into the phospholipid bilayer

Cytosolic phospholipase A2 (cPLA2) catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. When assaying the human recombinant cPLA2 using membranes isolated from [3H]arachidonate-labeled U937 cells as substrate, 2-(2'-benzyl-4-chlorophenoxy)ethyl-dimethyl-n-octadecyl-ammonium chloride (compound 1) was found to inhibit the enzyme in a dose-dependent manner (IC50 = 5 microM). It was over 70 times more selective for the cPLA2 as compared with the human nonpancreatic secreted phospholipase A2, and it did not inhibit other phospholipases. Additionally, it inhibited arachidonate production in N-formyl-methionyl-leucyl-phenylalanine-stimulated U937 cells. To further characterize the mechanism of inhibition, an assay in which the enzyme is bound to vesicles of 1,2-dimyristoyl-sn -glycero-3-phosphomethanol containing 6-10 mol % of 1-palmitoyl-2-[1-14C]arachidonoyl-sn-glycero-3-phosphocholine was employed. With this substrate system, the dose-dependent inhibition could be defined by kinetic equations describing competitive inhibition at the lipid-water interface. The apparent equilibrium dissociation constant for the inhibitor bound to the enzyme at the interface (KI*app) was determined to be 0.097 +/- 0.032 mol % versus an apparent dissociation constant for the arachidonate-containing phospholipid of 0.3 +/- 0.1 mol %. Thus, compound 1 represents a novel structural class of inhibitor of cPLA2 that partitions into the phospholipid bilayer and competes with the phospholipid substrate for the active site. Shorter n-alkyl-chained (C-4, C-6, C-8) derivatives of compound 1 were shown to have even smaller KI*app values. However, these short-chained analogs were less potent in terms of bulk inhibitor concentration needed for inhibition when using the [3H]arachidonate-labeled U937 membranes as substrate. This discrepancy was reconciled by showing that these shorter-chained analogs did not partition into the [3H]arachidonate-labeled U937 membranes as effectively as compound 1. The implications for in vivo efficacy that result from these findings are discussed.

The effect of Phoneutria nigriventer (armed spider) venom on arterial blood pressure of anaesthetised rats

The changes induced in the mean arterial blood pressure of anaesthetised rats following the administration of armed spider (Phoneutria nigriventer) venom have been investigated. The intravenous injection of Phoneutria nigriventer venom (0.1 mg/kg) evoked a brief and reversible decrease in the mean arterial blood pressure whereas a higher dose of venom (0.3 mg/kg) caused a biphasic response characterized by a short-lasting hypotension followed by a sustained and prolonged hypertension (40-50 min). These changes were accompanied by tachycardia, salivation, fasciculations, defecation and respiratory disturbances. Pretreatment of the animals with atropine (10 mg/kg), propranolol (100 mg/kg), phenoxybenzamine (100 mg/kg) and indomethacin (4 mg/kg) did not significantly affect the mean arterial blood pressure changes induced by Phoneutria nigriventer venom. Similarly, the bradykinin B2 receptor antagonist Hoe 140 (D-Arg-[Hyp3,Thi5,DTic7,Oic8]-bradykinin) (0.6 mg/kg), the PAF receptor antagonist WEB 2086 (3-(4-(2-chlorophenyl)-9-methyl-6H-thieno-(3,2f) (1,2,4)-triazolo-(4,3-a) (1,4)-diazepine-2-yl)-(4-morpholinyl)-1-propanone) (20 mg/kg), the tachykinin NK1 receptor antagonist SR 140333 ((S)1-(2-[3-(3,4-dichlorophenyl)-1-(3-iso-propoxyphenyl acetyl) piperidin-3-yl] ethyl)-4-phenyl-1-azoniabicyclo[2.2.2] octane, chloride) (0.5 mg/kg), the tachykinin NK2 receptor antagonist SR 48968 ((S)-N-methyl-N[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophen yl) butyl]benzamide) (0.5 mg/kg) and the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (10 mg/kg) had no significant effect on the mean arterial blood pressure changes induced by Phoneutria nigriventer venom. The increase in the blood pressure induced by Phoneutria nigriventer venom was also not significantly affected by either the angiotensin II receptor antagonist losartan (10 mg/kg) or the endothelin ETA receptor antagonist FR 139317 ((R)2-[(R)-2-[[1-(hexahydro-1H-azepinyl]carbonyl]amino-4-methyl- pentanoyl]amino-3-[3-(1-methyl-1H-indoyl)]propionyl] amino-3-(2-pyridyl) propionic acid) (30 mg/kg). The ATP-dependent K+ channel antagonist glibenclamide (50 mg/kg) reduced by 40% the hypotension induced by Phoneutria nigriventer venom without affecting the hypertensive response. Pretreatment of the animals with L-type Ca2+ channel antagonists such as verapamil (10-100 micrograms/kg/min), diltiazem (40-120 micrograms/kg/min) and nifedipine (0.3-10 mg/kg) markedly attenuated the hypertension induced by Phoneutria nigriventer venom. Verapamil (30 micrograms/kg/min) and diltiazem (120 micrograms/kg/min) also promptly reversed the established hypertension induced by Phoneutria nigriventer venom when infused 8 min after venom injection. Our results indicate that the brief decrease of blood pressure induced by Phoneutria nigriventer venom is partially due to ATP-dependent K+ channel activation. The prolonged hypertension seems to result from direct Ca2+ entry into vascular and/or cardiac muscles.