Effect of novel motilide ABT-229 versus erythromycin and cisapride on gastric emptying in dogs.

ABT-229 (8,9-anhydro-4"-deoxy-3'-N-desmethyl-3'-N-ethylerythromycin B-6,9-hemiacetal), a synthetic derivative of erythromycin (ERY) with no antibiotic activity, has been shown to bind to motilin receptors and stimulate contractile activity of the antrum and small intestine. The objective of this study was to determine the effect of ABT-229 on canine gastric emptying (GE) and contractile activity of the antrum and duodenum in response to a solid meal. Six beagles were used to determine GE of a solid meal and contractile activity in response to either vehicle, ABT-229 (0.17, 0.83, 2.5, or 5.0 microg/kg/min), ERY (33.3 microg/kg/min), or cisapride (CIS) (10 microg/kg/min). Lag (t(lag)), half-emptying (t(1/2)), and complete emptying (t(full)) times were determined. Contractile data were analyzed for motility index and gastroduodenal coordination. Compared with vehicle, ABT-229 dose dependently accelerated GE, t(lag) was decreased at the two highest doses, t(1/2) was decreased compared with vehicle at the three highest doses, and t(full) was decreased at all doses compared with vehicle. ERY also decreased t(1/2) and t(full), whereas CIS decreased all GE parameters. The slopes of the linear phase of GE curves for all drugs and doses were greater than those for vehicle. ABT-229 dose dependently increased the motility index as well as gastroduodenal coordination. ABT-229 (two highest doses) and CIS accelerated GE of a solid meal by decreasing the lag phase and increasing the rate of GE, whereas ERY only increased the rate of GE. The data suggest that ABT-229 is 7- to 40-fold more potent than ERY in accelerating GE.

Which form of erythromycin should be used to treat gastroparesis? A pharmacokinetic analysis.

BACKGROUND: Erythromycin is a macrolide antibiotic that exhibits prokinetic effects. It has been shown to enhance antral contractility and accelerates gastric emptying rates, primarily by stimulating motilin receptors. AIM: To determine the optimal dosage form of erythromycin for use as a prokinetic agent. METHODS: Eight normal volunteers and three patients with documented gastroparesis ingested 250 mg erythromycin in tablet. suspension and intravenous forms. Serum erythromycin levels were determined at frequent intervals. These data were plotted vs. time and analysed for lag time, time to maximum concentration (tmax), maximum concentration (Cmax) and bioavailability (F). RESULTS: The absorption kinetics of the erythromycin suspension was notable for short lag times and early tmax, while lag times and tmax were delayed with the tablet form. Median lag time was 15 min for the suspension vs. 90 min for the tablet (P < 0.005). Median tmax for the suspension was 45 min vs. 180 min for the tablet (P < 0.005). A non-significant decrease in F was seen with the suspension compared to the tablet (P = 0.12). CONCLUSION: Based on the kinetic data from this study, erythromycin suspension is the ideal dosage form for administration of this drug as a prokinetic agent.

Synthesis of 9-deoxo-4''-deoxy-6,9-epoxyerythromycin derivatives: novel and acid-stable motilides.

In our quest toward the discovery of highly potent and acid-stable motilides, novel 4"-deoxy derivatives of 9-deoxo-6, 9-epoxyerythromycin were designed, synthesized, and evaluated for their gastrointestinal prokinetic activities. These compounds, in their 9R configuration, were more potent than their 6,9-enol ether homologues in inducing well-coordinated smooth muscle contractions in an in vitro rabbit duodenal assay: e.g., (9R), (8S)-9-deoxo-4"-deoxy-3'-N-desmethyl-3'-N-ethyl-6, 9-epoxyerythromycin A (10) and (9R), (8S)-9-deoxo-4"-deoxy-3'-N-desmethyl-3'-N-ethanol-6, 9-epoxyerythromycin A (15) had a pED50 of 8.54 and 8.11 compared to a pED50 of 7.22 for compound 2 (ABT-229). Reduction of the 6,9-enol ether, which was aimed at improving the acid stability, afforded the most stable motilides to date with t1/2 of 5.5 h for 10 and 15. Compounds 10 and 15 bind specifically to rabbit antral smooth muscle motilin receptors with pIC50 values of 8.52 and 8.70.

Preparation of 9-deoxo-4"-deoxy-6,9-epoxyerythromycin lactams "motilactides": potent and orally active prokinetic agents.

A series of new, highly potent and orally active "motilactides", 9-deoxo-4"-deoxy-6,9-epoxyerythromycin lactams was designed, synthesized, and evaluated for their gastrointestinal motor stimulating activity. These compounds were acid stable and showed good oral efficacy.

Azole endothelin antagonists. 3. Using delta log P as a tool to improve absorption.

The oral absorption profile of a family of azole-based ET(A)-selective antagonists has been improved through a rational series of structural modifications which were suggested by analysis of the physicochemical parameter delta log P. Comparison of urea 2 with a series of well-absorbed compounds using delta log P analysis suggested that 2 has an excess capacity for forming hydrogen bonds with solvent. A series of urea modifications were explored as a means of reducing H-bonding capacity while maintaining affinity for the ET(A)-receptor. The correlation between delta log P values and absorption in an intraduodenal (id) bioavailability model was good; this strategy uncovered replacements for each of the urea NH groups which simultaneously improve both potency and drug absorption. A combination of these optimized modifications produces carbamate 16h, a highly-selective ET(A) antagonist with a potency/bioavailability profile consistent with an oral route of administration.

Method to estimate the rate and extent of intestinal absorption in conscious rats using an absorption probe and portal blood sampling.

PURPOSE: A variety of methods exist which determine the rate and extent of intestinal absorption. The method described here employs an internal absorption reference probe and portal blood sampling in unanesthetized rat. METHODS: Theophylline and tritiated water were selected as absorption reference probes since they are quantitatively absorbed in conscious rat. The fraction of an intestinal dose which reaches portal blood was determined from the resulting portal-systemic blood concentration gradients of the drug relative to the absorption probe. The absorption probes provide a means to calculate the drug mass reaching portal blood without the need of measuring the portal blood flow rate. The technique was evaluated with verapamil and a well-absorbed 5-lipoxygenase inhibitor, A-79035. RESULTS: The fraction of an intrajejunal dose of A-79035 reaching the portal vein (FG) was 0.86 using theophylline as the absorption probe. Verapamil, which is susceptible to extensive hepatic first-pass elimination, was completely absorbed (FG = 0.98) within 1 hour, but was only 21.4% bioavailable. Absorption rate constants, estimated from initial appearance rates in portal blood, were used to monitor factors that affect drug absorption. For example, with a dose solution containing 30% PEG-400, the absorption rate constants of theophylline and A-79035 were significantly reduced. Anesthesia reduced the absorption rate constant for theophylline in rats by 40% compared to conscious animals. CONCLUSIONS: The technique detailed here allows reliable, direct measurement of intestinal absorption which may assist in characterizing oral dosing for novel therapeutic agents.

Synthesis of 4"-deoxy motilides: identification of a potent and orally active prokinetic drug candidate.

As an approach to discovering highly potent motilides with oral activity, novel 4"-deoxy derivatives of 8,9-anhydroerythromycin 6,9-hemiacetal were designed, synthesized, and evaluated for their gastrointestinal prokinetic activities. These compounds were orders of magnitude more potent than their 4"-hydroxy analogs in inducing smooth muscle contractions in an in vitro rabbit duodenal assay. Removal of the 12-hydroxy group, which was aimed at improving oral bioavailability, also afforded further potentiation in in vitro activity, leading to the identification of 8,9-anhydro-4"-deoxy-3'-N-desmethyl-3'-N-ethylerythromycin B 6,9-hemiacetal (ABT-229) as a potential prokinetic drug. ABT-229 was > 300,000 times more potent than erythromycin in vitro and had 39% oral bioavailability in dog compared to its 4",12-dihydroxy congener (EM-523), which was only 400 times more potent than erythromycin and had relatively low (1.4%) oral bioavailability.

Stereoselective glucuronidation of zileuton isomers by human hepatic microsomes.

The glucuronidation of the R-isomer and S-isomer of the 5-lipoxygenase inhibitor zileuton was examined using human hepatic microsomes. The glucuronidation of both isomers followed Michaelis-Menten kinetics, but glucuronidation rates were between 3.6- and 4.3-fold greater for the S-isomer. The apparent Km's (microM) for the R-isomer (392.9 +/- 35.9) and S-isomer (322.5 +/- 22.0) glucuronidation were similar, whereas the apparent Vmax (nmol/mg protein/min) was 3.4-fold greater for the S-isomer (5.2 +/- 0.7). In combination, each isomer competitively inhibited the glucuronidation of its antipode. The average Ki (microM) determined for S-isomer inhibition of R-isomer glucuronidation (197.8 +/- 61.3) was 2.4-fold lower than the Ki for the reciprocal interaction. These data indicate that the glucuronidation of the zileuton isomers in human hepatic microsomes is stereoselective. This stereoselective glucuronidation may be the basis for the more rapid clearance of the S-isomer observed in humans receiving zileuton.

Effects of oral erythromycin on esophageal pH and pressure profiles in patients with gastroesophageal reflux disease.

Erythromycin, a possible motilin agonist, is a potent gastrokinetic agent that may increase the lower esophageal sphincter pressure. Therefore, we assessed the effects of erythromycin in two dosages (250 and 500 mg per os four times a day) on esophageal pH and pressure profiles in reflux patients using prolonged ambulatory monitoring systems. Studies were blinded, placebo-controlled with randomized crossover design. Patients took each drug for three days prior to studies, with erythromycin serum levels obtained the day of esophageal studies. Erythromycin 250 mg four times a day had no effect on esophageal contraction pressures or peristalsis during the day or meal periods. In the supine position, however, erythromycin significantly (P = 0.012) decreased esophageal contraction velocity and showed a strong trend (P = 0.059) towards increasing the percentage of peristaltic waves. Despite these potentially beneficial effects on esophageal clearance, no significant difference in acid exposure times during 24-hr pH studies were observed between placebo and low-dose erythromycin. High-dose erythromycin (500 mg four times a day) was associated with drug levels in the typical antibiotic efficacy range (normal 1-3 micrograms/ml; patients 1.7-7.0 micrograms/ml), but, here again, there was no significant difference in all acid reflux parameters between placebo and erythromycin phases. Therefore, "standard" doses of erythromycin have no important clinical effects on esophageal pressures or acid reflux parameters.

Dose-dependent stimulation of gallbladder contraction by intravenous erythromycin in man.

We have previously shown that a single oral dose of 500 mg erythromycin causes gallbladder contraction. The effect of intravenous erythromycin on antroduodenal motility is dose-dependent; < 3 mg/kg body weight stimulates propagated contractions in a fashion similar to motilin while doses > 7 mg/kg cause giant non-propagated antral contractions not seen with motilin. Using ultrasound, we have examined the effect of differing doses of intravenous erythromycin on gallbladder motility in man. Erythromycin (1 mg/kg) caused fasting gallbladder contraction to 52% of basal gallbladder volume (P < 0.001), and increased gallbladder emptying following a liquid meal (maximal percentage emptied 75 +/- 6.8% vs. 58 +/- 9.0% following saline, P < 0.05). Erythromycin (7 mg/kg) however, had no effect on gallbladder fasting or post-prandial motor activity. We conclude that the effect of erythromycin on gallbladder motility is dose-dependent, with higher doses having no effect. It is possible that at higher doses erythromycin stimulates other receptors in addition to the motilin receptor, and that the combined effect is different to the stimulation of the motilin receptor alone.

Stabilization of the N-terminal residues of luteinizing hormone-releasing hormone agonists and the effect on pharmacokinetics.

To stabilize leuprolide (1) against chymotrypsin and intestinal degradation several agonists of LHRH (2-12), modified at position 1, 2, or 3 and/or containing N-alpha-methyl at positions 1, 2, or 4, were synthesized by SPPS. These agonists were tested in vitro for (a) rat pituitary LHRH receptor binding, (b) LH release from rat pituitary cells, (c) stability against chymotrypsin, and (d) stability against rat intestinal degradation. The clearances of the compounds in the rat were determined using a RIA. Complete stabilization against chymotrypsin (t1/2) and lumenal degradation (T1/2) was achieved with substitution of NMe-Ser4 in leuprolide; however, with an increase in clearance. Substitution with 1-Nal3 increased both t1/2 and T1/2, while substitution with NAc-Sar1 increased only T1/2. [NAcSar1,NMeSer4,D-Trp6,Pro9NHEt]LHRH (12), the doubly stabilized analogue, was tested in the rat by both iv and id administrations, and its bioavailabilities were measured. No significant improvement in id absorption over leuprolide was observed.

Enantiomeric activation of glucuronidation in dog hepatic microsomes.

Isomer-specific mechanisms of conjugation were investigated by evaluating the hepatic glucuronidation of the enantiomers of the 5-lipoxygenase inhibitor zileuton. The glucuronidation of the individual isomers was stereoselective, as dog hepatic microsomes glucuronidated the S-isomer but failed to generate a glucuronide conjugate of the R-isomer. In combination, the nonglucuronidated R-isomer caused a concentration-dependent increase in the rate of glucuronidation of its enantiomorph. Kinetic analysis of this interaction indicated that the R-isomer affected rates of glucuronidation by decreasing the Km of the S-isomer for this process. This effect appeared enantioselective as the achiral analogue A-65838 had no effect on the Vmax and Km of S-isomer glucuronidation. The data were modeled using Michaelis-Menten kinetics in which the Km of S-isomer glucuronidation was reduced in a saturable manner by the concentration of the R-isomer. These data indicate that the nonconjugated R-isomer competitively activates the glucuronidation of its enantiomorph. To our knowledge, these data represent the first demonstration of enantiomeric activation of an enzyme involved in hepatic drug metabolism.

Gastrointestinal motor effects of erythromycin in humans.

The effects of an antibacterially effective IV dose of erythromycin on gastrointestinal motor activity were investigated in eight normal healthy human volunteers in the fasted state and the fed state. Motor activity was recorded by a multilumen manometric tube. Data were analyzed visually and by a computer method. Blood samples were obtained for erythromycin and motilin assays. In the gastric antrum, erythromycin significantly increased the total duration, amplitude, and area under contractions from 0 to 60 minutes and frequency of contractions from 0 to 30 minutes from the start of its infusion in the fasted state. A similar response in the fed state occurred mostly from 0 to 30 minutes after the start of erythromycin infusion. By contrast, erythromycin inhibited the frequency and decreased the duration of small intestinal contractions in the fed state but had no effect in the fasted state. The gastric motor response was related to the plasma concentration of erythromycin, but not to plasma motilin. Erythromycin significantly shortened the duration of migrating motor complex disruption by a meal. Erythromycin also induced symptoms of upper abdominal pain, bloating, and nausea. Abdominal pain was related to strong antral contractions in both fasted and fed states; bloating occurred only in the fed state. Nausea occurred in both fasted and fed states, but it was not related to any specific pattern of motor activity. It is concluded that the strong antral contractions induced by erythromycin may accelerate the rate of gastric emptying, but they may also be responsible for causing the sensations of upper abdominal pain and bloating. The motor response to erythromycin is less during the fed than during the fasted state. The strong antral contractions induced by erythromycin are not mediated by the release of motilin.

Intestinal calcium absorption. Interplay of paracellular and cellular pathways.

Intestinal absorption of calcium as characterized by studies in the rat follows both mediated mechanisms through enterocytes as well as fluxes between epithelial cells. Data summarized in this review suggests that the extracellular or paracellular route in distal small intestine plays a dominant role in calcium absorption at luminal calcium concentrations above 1 mM; proximal small intestine and cecum/colon show a much greater dependence on cellular translocation processes. The wide spectrum of paracellular:cellular transport ratios found in the intestine suggests a well-adapted mechanism to minimize energy expenditure for calcium absorption in the presence of adequate dietary calcium with capacity for efficient calcium scavenging when dietary supplies are limited. No evidence to date supports a role for vitamin D in the control of paracellular calcium absorption. Other mechanisms regulating paracellular calcium absorption will undoubtedly become a significant new area for future inquiry.

Effects of quinacrine on calcium active transport by rat intestinal epithelium.

To determine the possible role of acidic lysosomal vesicles in the transcellular transport of Ca, bidirectional Ca fluxes were measured across intestinal segments in vitro in the absence of electrochemical gradients. Mucosal addition of the weak base quinacrine (0.2 mM) caused a 67% decline in the mucosal-to-serosal Ca flux (Jm----s) across duodenum (175 +/- 34 vs. 58 +/- 9 nmol.cm-2.h-1, P less than 0.007) and reduced cecal Ca Jm----s (177 +/- 15 vs. 45 +/- 4, P less than 0.0001). Higher concentrations of up to 2.0 mM caused no further decline in cecal Ca Jm----s. Inhibition of cecal Ca Jm----s by mucosal chloroquine (0.1 mM) or ammonium chloride (10 mM) varied from 37 to 50%. Addition in vitro of quinacrine to enterocyte basolateral membrane vesicles failed to inhibit ATP-dependent Ca uptake. The present studies demonstrate that agents that collapse lysosomal pH gradients inhibit transcellular Ca transport. These observations are consistent with the hypothesis that Ca destined for transcellular transport is functionally associated with acidic lysosomes and that these organelles play an important role in transepithelial Ca translocation.

Suppression by cathepsin L inhibitors of the invasion of amnion membranes by murine cancer cells.

Cysteine proteinases, particularly cathepsins B and L, have been strongly implicated in fostering metastasis in mice. In this work four different inhibitors of cysteine proteinases have been shown to inhibit the invasion of the human amnion by murine melanoma and mammary carcinoma cells in vitro. Two of the inhibitors are synthetic peptides [ZPhePheCHN2 (benzyloxycarbonyl-L-phenylalanyl-L-phenylalanyldiazomethane) and ZPheAlaCH2F [3-(N-benzyloxycarbonylphenylalanylamido)-DL-1-fluoro-2-butanone]] and two are thiol protease inhibitors (TPIn, TPId) isolated from the skeletal muscle of the hind limbs of normal and dystrophic mice, respectively. The inhibitors (ZPhePheCHN2, TPId), with apparent selectivity for cathepsin L, blocked invasion as effectively as inhibitors (ZPheAlaCH2F, TPIn) effective on both cathepsins. The data reveal that in these cell lines the cysteine proteinases contribute significantly to the invasive capacity of the cells, but to a lesser extent than do the metalloproteinases. We suggest that the cysteine proteinases facilitate the action of metalloproteinases (collagenase, gelatinase, and stromelysin), possibly by activating them, by inactivating the tissue inhibitor of metalloproteinases, and/or by making basement membrane matrix more accessible.

Azido glycols: potent, low molecular weight renin inhibitors containing an unusual post scissile site residue.

Azidomethyl-substituted 1,2- and 1,3-diols were prepared from Boc-cyclohexylalanal and evaluated as transition state analogue renin inhibitors, leading to the development of a small (MW less than 600), nanomolar inhibitor. Remarkable aqueous solubility enhancement followed the incorporation of an N-terminal urea functionality. Evaluation of selected compounds both in vivo and in vitro demonstrated that while transport across the intestine occurred upon id administration, extensive liver extraction resulted in low systemic levels.