Reinhard Oertel

Induction of P‐glycoprotein by rifampin increases intestinal secretion of talinolol in human beings: A new type of drug/drug interaction

P‐Glycoprotein is an efflux pump in many epithelial cells with excretory function. It has been demonstrated that rifampin (INN, rifampicin) induces P‐glycoprotein, particularly in the gut wall. We therefore hypothesized that rifampin affects pharmacokinetics of the P‐glycoprotein substrate talinolol, a β1‐blocker without appreciable metabolic disposition but intense intestinal secretion in human beings.

Pharmacokinetics and pharmacodynamics in clinical use of scopolamine

The alkaloid L-(−)-scopolamine [L-(−)-hyoscine] competitively inhibits muscarinic receptors for acetylcholine and acts as a nonselective muscarinic antagonist, producing both peripheral antimuscarinic properties and central sedative, antiemetic, and amnestic effects. The parasympatholytic scopolamine, structurally very similar to atropine (racemate of hyoscyamine), is used in conditions requiring decreased parasympathetic activity, primarily for its effect on the eye, gastrointestinal tract, heart, and salivary and bronchial secretion glands, and in special circumstances for a CNS action. Therefore, scopolamine is most suitable for premedication before anesthesia and for antiemetic effects. This alkaloid is the most effective single agent to prevent motion sickness. Scopolamine was the first drug to be made commercially available in a transdermal therapeutic system (TTS-patch) delivering alkaloid. Recently, pharmacokinetic data on scopolamine in different biozlogic matrices were obtained most efficiently using liquid chromatographic-tandem mass spectrometric (LC-MS/MS) or gas chromatography online coupled to mass spectrometry. Pharmacokinetic parameters are dependent on the dosage form (oral dose, tablets; parenteral application; IV infusion; SC and IM injection). Scopolamine has a limited bioavailability if orally administered. The maximum drug concentration occurs approximately 0.5 hours after oral administration. Because only 2.6% of nonmetabolized L-(−)-scopolamine is excreted in urine, a first-pass metabolism is suggested to occur after oral administration of scopolamine. Because of its short half-life in plasma and dose-dependent adverse effects (in particular hallucinations and the less serious reactions, eg, vertigo, dry mouth, drowsiness), the clinical use of scopolamine administered orally or parenterally is limited. To minimize the relatively high incidence of side effects, the transdermal dosage form has been developed. The commercially available TTS-patch contains a 1.5-mg drug reservoir and a priming dose (140 μg) to reach the steady-state concentration of scopolamine quickly. The patch releases 0.5 mg alkaloid over a period of 3 days (releasing rate 5 μg/h). Following the transdermal application of scopolamine, the plasma concentrations of the drug indicate major interindividual variations. Peak plasma concentrations (Cmax) of approximately 100 pg/mL (range 11-240 pg/mL) of the alkaloid are reached after about 8 hours and achieve steady state. During a period of 72 hours the plaster releases scopolamine, so constantly high plasma levels (concentration range 56-245 pg/mL) are obtained, followed by a plateau of urinary scopolamine excretion. Although scopolamine has been used in clinical practice for many years, data concerning its metabolism and the renal excretion in man are limited. After incubation with β-glucuronidase and sulfatase, the recovery of scopolamine in human urine increased from 3% to approximately 30% of the drug dose (intravenously administered). According to these results from enzymatic hydrolysis of scopolamine metabolites, the glucuronide conjugation of scopolamine could be the relevant pathway in healthy volunteers. However, scopolamine metabolism in man has not been verified stringently. An elucidation of the chemical structures of the metabolites extracted from human urine is still lacking. Scopolamine has been shown to undergo an oxidative demethylation during incubation with CYP3A (cytochrome P-450 subfamily). To inhibit the CYP3A located in the intestinal mucosa, components of grapefruit juice are very suitable. When scopolamine was administered together with 150 mL grapefruit juice, the alkaloid concentrations continued to increase, resulting in an evident prolongation of tmax (59.5 ± 25.0 minutes; P < 0.001). The AUC0-24h values of scopolamine were higher during the grapefruit juice period. They reached approximately 142% of the values associated with the control group (P < 0.005). Consequently, the related absolute bioavailabilities (range 6% to 37%) were significantly higher than the corresponding values of the drug orally administered together with water (range 3% to 27%). The effect of the alkaloid on quantitative electroencephalogram (qEEG) and cognitive performance correlated with pharmacokinetics was shown in studies with healthy volunteers. From pharmacokinetic-pharmacodynamic modeling techniques, a direct correlation between serum concentrations of scopolamine and changes in total power in α-frequency band (EEG) in healthy volunteers was provided. The alkaloid readily crosses the placenta. Therefore, scopolamine should be administered to pregnant women only under observation. The drug is compatible with nursing and is considered to be nonteratogenic. In conclusion, scopolamine is used for premedication in anesthesia and for the prevention of nausea and vomiting associated with motion sickness. Pharmacokinetics and pharmacodynamics of scopolamine depend on the dosage form. Effects on different cognitive functions have been extensively documented.

Oral bioavailability of digoxin is enhanced by talinolol: Evidence for involvement of intestinal P‐glycoprotein

Recent data indicated that disposition of oral digoxin is modulated by intestinal P‐glycoprotein. The cardioselective β‐blocker talinolol has been described to be secreted by way of P‐glycoprotein into the lumen of the gastrointestinal tract after oral and intravenous administration. We therefore hypothesized that coadministration of digoxin and talinolol may lead to a drug‐drug interaction based on a competition for intestinal P‐glycoprotein.

Clinical Pharmacokinetics of Articaine

SummaryArticaine is the most widely used local anaesthetic agent in dentistry in a number of European countries. The amide structure of articaine is similar to that of other local anaesthetics, but it contains an additional ester group which is quickly hydrolysed by esterases.High performance liquid chromatography has been used to determine the concentrations of articaine and its metabolite articainic acid in serum. Rapid sample preparation is critical in the accurate determination of articaine serum concentrations, since blood and serum are the sites of metabolism. The time to maximum drug concentrations of articaine occurs about 10 to 15 minutes after submucosal injection of articaine 4% 80mg, irrespective of epinephrine (adrenaline). The mean maximum plasma drug concentration is about 400 μg/L for articaine with epinephrine 1: 200 000 and 580 μg/L for articaine without epinephrine. The elimination half-time of articaine is about 20 minutes.The rapid breakdown of articaine to the inactive metabolite articainic acid is related to a very low systemic toxicity and consequently to the possibility of repeated injections. Equal analgesic efficacy along with lower systemic toxicity (i.e. a wide therapeutic range) permits the use of articaine in higher concentrations than other amide-type local anaesthetics. Complete anaesthesia can be observed in nearly 90% of all cases, using articaine 4% 60 to 80mg with epinephrine 1: 200 000. Articaine is better able to diffuse through soft tissue and bone than other local anaesthetics.The concentration of articaine in the alveolus of a tooth in the upper jaw after extraction was about 100 times higher than that in systemic circulation. The plasma protein binding rate of articaine and articainic acid is 70%. It has been concluded that an unintentional intravascular injection of articaine 80mg does not cause toxic effects in healthy individuals.

The Effects of Lipid Infusion on Myocardial Function and Bioenergetics in L-Bupivacaine Toxicity in the Isolated Rat Heart

BACKGROUND:It is unclear whether improved metabolism or a “lipid sink” effect of lipid infusion is responsible for the positive effects in local anesthetic-induced myocardial depression. METHODS:We used an isolated rat heart, constant-pressure perfused, nonrecirculating Langendorff preparation and exposed hearts to 5 &mgr;g/mL l-bupivacaine and 9 &mgr;L/mL lipid emulsion. Hearts were freeze-clamped and energy was charge measured by HPLC. In a second experiment the effects of pacing hearts was evaluated. The effects of lipid addition on local anesthetic concentrations in Krebs–Henseleit buffer and human plasma were examined by using a mass spectrometer. RESULTS:With spontaneously beating hearts l-bupivacaine led to a significant decrease in heart rate (to 74% ± 7% of baseline), +dP/dt (69% ± 7%), systolic pressure (78% ± 6%), coronary flow (61% ± 8%), and to an increase in PR (177% ± 52%) and QRS intervals (166% ± 36%). Lipid infusion exerted a positive inotropic effect, significantly augmenting +dP/dt and systolic pressure back to 94% ± 11% and 102% ± 16% of baseline in l-bupivacaine-treated hearts. Heart rate, coronary flow, PR, and QRS intervals remained unchanged after lipid intervention. Lipid infusion in paced hearts had a significant effect on +dP/dt, systolic pressure, and Mvo2. Neither l-bupivacaine nor lipids had an effect on energy charge. A lipid concentration of 500 &mgr;L/mL plasma was necessary to effect changes in the plasma concentration of local anesthetics. CONCLUSION:Lipid application in l-bupivacaine-induced cardiac depression had a significant positive inotropic effect, which we would attribute to a direct inotropic effect. However, in an isolated heart model, indirect, local anesthetic plasma-binding effect of lipids cannot be excluded.

Grapefruit juice ingestion significantly reduces talinolol bioavailability

Our objectives were to evaluate the effect of single and repeated grapefruit juice ingestion relative to water on the oral pharmacokinetics of the nonmetabolized and P‐glycoprotein‐transported drug talinolol in humans and to assess the potential impact of grapefruit juice ingestion on P‐glycoprotein and intestinal uptake transporters.

P-glycoprotein inhibitor erythromycin increases oral bioavailability of talinolol in humans.

OBJECTIVE Increased bioavailability of the P-glycoprotein (Pgp) substrates digoxin and cyclosporin due to erythromycin has been observed in vivo. The aim of the present study was to investigate the effect of orally administered erythromycin on the oral bioavailability of the beta-blocker talinolol. Talinolol is a suitable model compound for Pgp drug-drug interaction studies due to its Pgp-related active intestinal secretion and lack of any significant metabolism. METHODS In a randomized crossover study, the oral pharmacokinetics of talinolol (50 mg) after a concomitant single oral dose of erythromycin (2 g) or placebo were investigated in 9 healthy men. Concentrations of talinolol were measured in serum and urine by HPLC. RESULTS The area under the curve of talinolol serum concentrations from 0 to 24 h (AUC(0-24)) and the maximum serum concentrations (Cmax) were significantly increased after administration of erythromycin compared to placebo. t(max) values were significantly reduced. The renal clearance (CLR) of talinolol was unchanged after co-administration of erythromycin and there was a small but statistically significant decrease in elimination half-life (t1/2). Serum pharmacokinetics correlate with the results derived from urine concentration measurement. One subject suffered from moderate diarrhea after erythromycin and was excluded from the analysis. CONCLUSION We suggest that the increase in oral bioavailability of talinolol after concomitant erythromycin is caused by increased intestinal net absorption due to Pgp inhibition by erythromycin.

Direct demonstration of small intestinal secretion and site‐dependent absorption of the β‐blocker talinolol in humans

To examine the relevance of site‐dependent small intestinal absorption for incomplete intestinal absorption of the poorly metabolized β1‐adrenergic receptor antagonist talinolol.

Induction of intestinal P-glycoprotein by St John's wort reduces the oral bioavailability of talinolol.

St Johns wort (SJW) is known to induce cytochrome P450 (CYP) 3A4 and P‐glycoprotein through pregnane X‐receptor activation. Our study evaluated the effects of long‐term SJW administration on oral and intravenous pharmacokinetics of the nonmetabolized in vivo probe of P‐glycoprotein, talinolol, in relation to intestinal P‐glycoprotein expression. In a controlled, randomized study (N=9), the pharmacokinetics of oral (50 mg) and intravenous talinolol (30 mg) was determined before and after 12 days SJW (900 mg daily, Jarsin 300®). Duodenal biopsies were taken and MDR1 genotypes assessed. SJW reduced the oral talinolol bioavailability by 25% (P=0.049) compared with water control. A 93% increase in oral clearance (P=0.177) and a 31% reduction in area under the serum concentration time curve (AUC; P=0.030) were observed. Renal and nonrenal clearance (CLNR), elimination half‐life, peak serum drug concentration (Cmax), and time to reach Cmax were not significantly altered. After intravenous talinolol, SJW affected only CLNR (35% increase compared with water, P=0.006). SJW increased MDR1 messenger ribonucleic acid (mRNA) as well as P‐glycoprotein levels in the duodenal mucosa. Subjects with the combined MDR1 genotype comprising 1236C>T, 2677G>T/A, and 3435C>T polymorphisms had lower intestinal MDR1 mRNA levels and displayed an attenuated inductive response to SJW as assessed by talinolol disposition. Long‐term SJW decreased talinolol AUC with a corresponding increase in intestinal MDR1 expression, suggesting that SJW has a major inductive effect on intestinal P‐glycoprotein. Interestingly, the magnitude of induction appeared to be affected by MDR1 genotype.

Intestinal secretion of intravenous talinolol is inhibited by luminal R‐verapamil

To examine the secretion of the β1‐adrenergic receptor antagonist talinolol into the small intestine during its intravenous administration and to show the relevance of the P‐glycoprotein–modulating drug verapamil for this secretory transport mechanism in humans.