Gifford L. Hoyer

Human experience with amiodarone in the embryonic period

Reported use of amiodarone in pregnancy is rare, despite increased use of the drug in women of child-bearing age. Use of amiodarone in late pregnancy has been reported and a lithium-like fetopathy discovered1–5 consisting of hypothyroidism and goiter (due to its lithium-like antithyroid actions). There are only 2 reports of amiodarone use during the embryonic period and organogenesis.6,7 Both gravidae had relatively low drug exposure. No congenital malformation was observed. Studies in rodents suggest the possibility of additional embryonic and fetal toxicities.8,9 We report 3 cases of human amiodarone use during the first trimester at a substantially higher dose than in the previous cases. We detail one major malformation due to abnormal organogenesis, and we report fetal amiodarone and N-desethyl-amiodarone levels for the first time.

Pharmacokinetic interaction between intravenous phenytoin and amiodarone in healthy volunteers.

To determine the mechanism of the amiodarone‐phenytoin interaction, seven healthy male subjects were given intravenous phenytoin, 5 mg/kg, before (phase I) and after (phase II) 3 weeks of oral amiodarone, 200 mg/day. Serum AUC increased from 245 ± 37.6 to 342 ± 87.3 mg · hr/L (p = 0.007); area under the first moment curve increased from 5666 ± 1003 to 11,632 ± 4198 mg · hr2/L (p = 0.008); the time‐averaged total body clearance decreased from 1.57 ± 0.3 to 1.17 ± 0.33 L/hr (p = 0.0004); and the apparent elimination half‐life increased from 16.1 ± 1.32 to 22.6 ± 3.8 hours (p = 0.001) for phenytoin during phase II. The volume of distribution at steady state and the unbound fraction for phenytoin remained unchanged. However, the formation of p‐hydroxyphenytoin as a function of serum phenytoin concentration decreased during phase II. These findings suggest that amiodarone inhibits phenytoin metabolism. These observations also suggest that phenytoin doses will need to be reduced when coadministered with amiodarone. The magnitude of this reduction is difficult to predict because of the saturable pharmacokinetics of phenytoin, and therapeutic monitoring is recommended if amiodarone is added to the phenytoin regimen.

Steady-state interaction between amiodarone and phenytoin in normal subjects☆

Amiodarone has been reported to increase phenytoin levels. This study was designed to evaluate the pharmacokinetic basis of this interaction at steady-state. Pharmacokinetic parameters for phenytoin were determined after 14 days of oral phenytoin, 2 to 4 mg/kg/day, before and after oral amiodarone, 200 mg daily for 6 weeks in 7 healthy male subjects. During amiodarone therapy, area under the serum concentration time curve for phenytoin was increased from 208 +/- 82.8 (mean +/- standard deviation) to 292 +/- 108 mg.hr/liter (p = 0.015). Both the maximum and 24-hour phenytoin concentrations were increased from 10.75 +/- 3.75 and 6.67 +/- 3.51 micrograms/ml to 14.26 +/- 3.97 (p = 0.016) and 10.27 +/- 4.67 micrograms/ml (p = 0.012), respectively, during concomitant amiodarone treatment. Amiodarone caused a decrease in the oral clearance of phenytoin from 1.29 +/- 0.30 to 0.93 +/- 0.25 liters/hr (p = 0.002). These results were due to a reduction in phenytoin metabolism by amiodarone as evidenced by a decrease in the urinary excretion of the principal metabolite of phenytoin, 5-(p-hydroxyphenyl)-5-phenylhydantoin, 149 +/- 39.7 to 99.3 +/- 40.0 mg (p = 0.041) and no change in the unbound fraction of the total phenytoin concentration expressed as a percentage, 10.3 +/- 2.7 versus 10.7 +/- 2.1% (p = 0.28) during coadministration of amiodarone. The alterations in phenytoin pharmacokinetics suggest that steady-state doses of phenytoin of 2 to 4 mg/kg/day should be reduced at least 25% when amiodarone is concurrently administered. All dosage reductions should be guided by clinical and therapeutic drug monitoring.

Effects of Coadministration of Propafenone on the Pharmacokinetics of Digoxin in Healthy Volunteer Subjects

Previous reports have suggested an interaction between propafenone and digoxin. We investigated the pharmacokinetics of IV digoxin when given alone (Phase I), after pretreatment with propafenone 150 mg every 8 hours for seven days (Phase II), and after propafenone 300 mg every 8 hours for 7 days (Phase III). The total body clearance of digoxin during Phase I was 2.45 ml/min/kg and was 2.17 ml/min/kg during Phase II (NS) and decreased to 1.92 ml/min/kg during Phase III (P < 0.05). The renal clearance and half‐life of digoxin were not significantly altered by propafenone. There was a trend towards a decrease in the volume of distribution of digoxin from 9.43 L/kg in Phase I, to 9.33 L/kg in Phase II, and 8.02 L/kg in Phase III. Similarly there was a trend towards a decreased nonrenal clearance of digoxin from 1.21 ml/min/kg during Phase I to 1.01 ml/min/kg during Phase II and to 0.75 ml/min/kg during Phase III. The changes in volume of distribution and nonrenal clearance parallel each other resulting in no change in the elimination half‐life of digoxin. It is postulated that the mechanism of this interaction is due to decreases in the volume of distribution and nonrenal elimination of digoxin by propafenone. The degree of this interaction was related to the dose of propafenone. The magnitude of this interaction may be greater in patients and, thus, may require a reduction in the digoxin dose.

Selective stability-indicating high-performance liquid chromatographic assay for recombinant human regular insulin

This report presents a selective HPLC assay capable of separating recombinant human regular insulin from insulin decomposition and transformation products. The assay utilizes an isocratic delivery of mobile phase, a C18 peptide column, UV detection and is performed at ambient temperature. The standard curve ranges from 0.2 to 2.5 U/ml. The inter-day and intra-day variabilities are less than 7 and 5%, respectively, at the concentrations studied. The accuracy and precision are within 5% over the range of the standard curve.

Effect of Phenytoin on the Clinical Pharmacokinetics of Amiodarone

Five healthy male volunteers were given oral amiodarone hydrochloride, 200 mg per day for 6½ weeks, to determine its effects on the pharmacokinetics of both intravenous and oral phenytoin. Predose amiodarone and N‐desethylamiodarone serum concentrations were obtained weekly during weeks 2–6. Amiodarone serum concentrations (ASC) increased during weeks 2–4 and then decreased sharply during weeks 5–6 when oral phenytoin, 2–4 mg/kg/day, was co‐administered. In addition, N‐desethylamiodarone serum concentrations (DEASC) exceeded corresponding ASC during weeks 5–6 whereas during weeks 2–4, DEASC were less than ASC. Because of the long elimination half‐life for amiodarone previously reported in healthy volunteers after single doses of amiodarone and the frequent administration of amiodarone associated with this half‐life, a modified equation for a continuous infusion was used to describe each subjects ASC versus time data. Pre‐phenytoin ASC were fitted to an appropriate function to predict ASC during weeks 5–6 assuming no interaction. Observed versus predicted ASC were compared for weeks 5 and 6. Observed ASC during weeks 5 and 6 were (mean ± SD) 0.25 ± 0.09 μg/mL and 0.19 ± 0.07 μg/mL, respectively. Corresponding predicted ASC were 0.36 ± 0.12 μg/mL (P = .011) and 0.38 ± 0.13 μg/mL (P = .004). These represented percent differences of 32.2 ± 12.5% and 49.3 ± 5.6% for weeks 5 and 6, respectively. Assuming there were no changes in the bioavailability of amiodarone during continuous administration, these findings strongly suggest induction of amiodarone metabolism by phenytoin. The clinical significance of this interaction remains to be determined.

Proenkephalin A-derived peptide E and its fragments alter opioid contractility in the small intestine

The human and canine small intestine exhibit increased contractility when exposed to exogenous or endogenous opioid peptides. The response of the canine small intestine to the proenkephalin A-derived peptide, peptide E and related processing fragments [Met5]enkephalin, BAM-12P, BAM-18P and BAM-22P was investigated by administering each peptide to isolated, small intestine segments which causes a significant increase in intraluminal pressure. Concentration-response curves from intraarterial bolus administration of peptide E, [Met5]enkephalin, BAM-12P, BAM-18P and BAM-22P showed decreasing efficacy with decreasing amino acid chain length while naloxone (305 nM) significantly antagonized the response. Results using the classical guinea pig ileum/myenteric plexus longitudinal muscle and mouse vas deferens bioassays with specific opioid receptor antagonists provide evidence that peptide E and BAM-18P are relatively specific to the mu opioid receptor, [Met5]enkephalin is more delta specific, BAM-22P is both mu and kappa specific and BAM-12P is kappa opioid receptor specific. These studies demonstrate that locally released (and possibly circulating) peptide E and related processing fragments increase contractility in the small intestine and may be active through more than a single receptor mechanism, particularly the mu receptor.

A Sensitive Stability Indicating Assay for the H2 Blocker Ranitidine

Abstract A stability indicating high performance liquid chromatography assay for ranitidine has been developed for the study of ranitidine in intravenous solutions. The assay is based on an isocratic mobile phase, a C18 reverse phase column, UV detection and utilizes an internal standard. The assay is shown to be accurate and precise over a range of ranitidine concentration commonly found in IV solutions. The mean inter- and intra-assay variability is low and retention times are stable. The assay separates ranitidine form ranitidine decomposition products produced at acidic and basic pHs.

Changes in opioid receptor selectivity following processing of peptide E: Effect on gut motility

Peptide E is a mu-selective opioid peptide derived from proenkephalin A which contains [Met5]-enkephalin at the amino end and [Leu5]-enkephalin at the carboxyl end. Peptide E is further processed both centrally and peripherally to a [Leu5]-enkephalin-containing fragment which was investigated to determine if processing leads to alterations in receptor selectivity. Peptide E-(15-25) inhibited electrically stimulated contractions in both the mouse vas deferens, longitudinal muscle, myenteric (IC50 = 459 nmol/L), and guinea pig ileum (IC50 = 2630 nmol/L), indicating a sixfold delta-receptor selectivity. When administered intracerebroventricularly to mice, peptide E-(15-25) also produced potent analgesia which was completely antagonized by naloxone pretreatment, but the peptide had no effect on intestinal transit as measured by the radiochromium geometric center method. This is consistent with earlier findings that intracerebroventricular delta-opioid-selective agents are analgesic but do not inhibit intestinal transit. In vitro radioligand binding assays were performed using male Sprague-Dawley rat whole brain homogenates. The IC50 for peptide E against [3H]naloxone was 1.8 nmol/L compared with the delta-opioid ligand, [3H] [D-Pen2, D-Pen5]-enkephalin of 38.8 nmol/L. The IC50 for peptide E-(15-25) against [3H]naloxone was 497 nmol/L, but for [3H] [D-Pen2, D-Pen5]-enkephalin it was 50.6 nmol/L. Therefore, peptide E loses mu-opioid receptor affinity (1.8-497 nmol/L) after proteolytic processing and the loss of the amino terminal tyrosine but maintains a high delta-opioid affinity (38.8-50.6 nmol/L). These studies demonstrate that enzymatic peptide processing of peptide E to peptide E-(15-25) leads to a shift from mu- to delta-receptor selectivity and a different spectrum of biological effects on gut motility.

High-performance liquid chromatographic method for the quantitation of quinidine and selected quinidine metabolites.

A specific and sensitive assay for the separation and quantitation of quinidine, 3-hydroxyquinidine, quinidine-N-oxide, O-desmethylquinidine and dihydroquinidine is presented. The assay is shown to be sensitive to concentrations of 0.1 microgram/ml for all the above compounds when using a serum sample of 0.1 ml. The standard curve demonstrates linearity at concentrations from 0.1 to 5 micrograms/ml. The extraction procedure consists of adjusting the serum to an alkaline pH and extracting once with a mixture of methanol-dichloromethane (15:85). The organic extract is dried and the residue is solubilized in mobile phase. The chromatographic conditions are an isocratic delivery of the mobile phase 0.01 M K2HPO4-acetonitrile (96:4) through a C18 column at ambient temperature. Detection of the compounds of interest is by ultraviolet absorption at a wavelength of 210 nm. For each compound the inter-assay variation is less than 10% and the intra-assay variation is less than 15%. No interfering compounds were detected when a commercially prepared serum spiked with 28 commonly used therapeutic compounds was assayed by this method. The analytical method presented here for the isolation and quantitation of quinidine, several active metabolites, and dihydroquinidine has adequate sensitivity and specificity for monitoring the concentration of quinidine and quinidine metabolites in patient samples.