LY353433, a potent, orally effective, long-acting 5-HT(4) receptor antagonist: comparison to cisapride and RS23597-190

Although many 5-HT (serotonin, 5-hydroxytryptamine)(4) receptor antagonists have been described, none possess the requisite oral activity and duration of action for a clinically effective therapeutic agent. The present report identifies LY353433 (1-(1-methylethyl)-N-[2-[4-[tricyclo[3.3.1.1(3,7)]dec-1-ylcarbo nyl) amino]-1-piperidinyl]ethyl]-1H-indazole-3-carboxamide), an indazole amide, as a high affinity antagonist at the 5-HT(4) receptor in the rat esophagus. LY353433 (10(-8), 3 x 10(-8), 10(-7) M) inhibited 5-HT-induced relaxation of carbamylcholine-contracted esophagus with greater potency than cisapride or RS23597-190, a known 5-HT(4) receptor ligand. Furthermore, RS23597-190 possessed marked agonist activity as did cisapride, whereas LY353433 did not relax the rat esophagus in concentrations up to 10(-5) M. LY353433 (up to 10(-5) M) did not possess appreciable affinity for adrenergic, dopaminergic, histaminergic, muscarinic or GABAergic receptors and, thus, was a highly selective 5-HT(4) receptor antagonist. In addition, LY353433 only slowly associated with an dissociated from the 5-HT(4) receptor, an attribute that conferred long-lasting 5-HT(4) receptor antagonist activity, in contrast to RS23597-190, which rapidly dissociated from the 5-HT(4) receptor. LY353433 dose-dependently inhibited the 5-HT(4) receptor-mediated ex vivo relaxation in the rat esophagus after either i.v. (0.1, 0.3 and 1.0 mg/kg) or p.o. (0.1, 0.3, 1.0 and 3.0 mg/kg) administration. Furthermore, the p.o. to i.v. dose ratio was approximately one, suggesting that LY353433 was well absorbed with excellent pharmacodynamics in the rat. LY353433 (0.3 mg/kg p.o.) blocked esophageal 5-HT(4) receptors ex vivo through 6 hr after p.o. dosing with responses returning to control by 16 hr, indicative of long duration receptor blockade. Lastly, in rats LY353433 was exceptionally safe because acute doses up to 300 mg/kg p.o. did not result in either symptoms or deaths. Thus, LY353433 is a potent, selective, orally effective, long-acting and safe 5-HT(4) receptor antagonist that is highly suitable for clinical use.

Antagonism of serotonin 5-HT1A receptors potentiates the increases in extracellular monoamines induced by duloxetine in rat hypothalamus

In the current study we examined the effects of coadministration of a serotonin 5-HT1A antagonist, (+-)-1-(1H-indol-4-yloxy)-3-(cyclohexylamino)-2-propanol maleate (LY 206130), and a dual 5-HT and norepinephrine (NE) uptake inhibitor, duloxetine, on extracellular levels of NE, 5-HT, dopamine (DA), 5-hydroxyindoleacetic acid, and 3,4-dihydroxyphenylacetic acid in rat hypothalamus microdialysates. LY 206130 (3.0 mg/kg, s.c.) alone significantly increased NE and DA levels by 60 and 34%, respectively, without affecting 5-HT levels. Duloxetine administration at 4.0 mg/kg, i.p. alone produced no significant changes in levels of 5-HT, NE, or DA. In contrast, when LY 206130 and duloxetine were coadministered at 3.0 mg/kg, s.c. and 4.0 mg/kg, i.p., respectively, 5-HT, NE, and DA levels increased to 5.7-, 4.8-, and threefold over their respective basal levels. These data demonstrate that antagonism of somatodendritic 5-HT1A autoreceptors and concomitant inhibition of 5-HT and NE uptake with duloxetine may promote synergistic increases in levels of extracellular 5-HT, NE, and DA in hypothalamus of conscious, freely moving rats.

Inhibition of NAD(H)/NADP(H)--requiring enzymes by aurintricarboxylic acid

Aurintricarboxylic acid (ATA), an inhibitor of Ca(2+)-dependent endonuclease activity, is often used to implicate a role for increased intracellular calcium in mechanistic toxicology studies. We report here on the ability of ATA to inhibit the activity of several NAD(H)/NADP(H)-requiring enzymes (purified or cellular homogenates), including lactic dehydrogenase, alcohol dehydrogenase, cytochrome c reductase, ethoxycoumarin o-dealkylase, isocitric dehydrogenase, glutathione reductase and glucose-6-phosphate dehydrogenase. These results were compared with the ability of ATA to inhibit micrococcal nuclease and rat liver Ca(2+)-dependent endonuclease activity in similar incubations. With the exception of alcohol dehydrogenase, ATA was a potent inhibitor of each of the purified enzymes, with IC50s ranging from 0.5 to 82 microM. In cell homogenates, however, ATA was from 10 to 100-fold less potent at inhibiting these enzymes. When exogenous protein was added to purified enzyme incubations, the effect of ATA was similarly diminished. Our results demonstrate that ATA inhibits a wide range of NAD(H)/NADP(H)-requiring enzymes in in vitro incubations using purified enzymes, but that the inhibitory effects are markedly reduced in incubations which more closely resemble a cellular milieu.

HPLC and 31P NMR characterization of the reaction between antitumor platinum agents and the phosphorothioate chemoprotective agent S-2-(3-aminopropylamino)ethylphosphorothioic acid (WR-2721)

In prior studies, we examined the effects of the radioprotective and chemoprotective agent WR-2721 [S-2-(3-aminopropylamino)ethylphosphorothioic acid] on the in vivo biotransformation of the cisplatin [cis-diamminedichloroplatinum(II)] analog ormaplatin [(d,I)trans-1,2-diaminocyclohexanetetrachloroplatinum(IV), Pt(dach)Cl4, (formerly called tetraplatin)]. Those data suggested that a direct interaction between WR-2721 and ormaplatin and/or the corresponding Pt(II) drug, Pt(dach)Cl2, may be occurring in vivo. This would be in contrast to the generally accepted hypothesis that WR-2721 is a prodrug that must first be converted by alkaline phosphatase to a free thiol compound, WR-1065, before any appreciable reactivity would be evident. However, the major biotransformation product observed in the peritoneal fluid, plasma, and all tissues was Pt(dach)(WR-1065). We report here on further investigations into the in vitro reactivity of Pt(dach) compounds with WR-2721 and WR-1065. Separation of reaction products resulting from incubation of Pt(dach)(malonato) with either WR-2721 or WR-1065 under physiological conditions gave profiles that were indistinguishable by reverse phase HPLC and cation exchange HPLC at two different pHs. 31P NMR characterization of the dephosphorylation of WR-2721 revealed essentially no loss of inorganic phosphate for up to 24 hr when incubated in unbuffered water at 30 degrees. In contrast, when incubated with a 1:1 molar ratio of cisplatin under the same conditions, the WR-2721 signal was decreased markedly in the first 5 min, and had disappeared almost completely by 1 hr. The signal corresponding to inorganic phosphate increased in parallel to the decrease in the WR-2721 signal. No intermediate formation of a complex containing both platinum and phosphate could be detected at any time. These data suggest that the reaction between WR-2721 and platinum complexes results in rapid dephosphorylation of WR-2721, and, consequently, that the reaction products formed with either WR-2721 or WR-1065 and Pt(II) complexes are identical.

Effect of the chemoprotective agent WR-2721 on disposition and biotransformations of ormaplatin in the Fischer 344 rat bearing a fibrosarcoma

The effects of the phosphorothioate agent, WR-2721, have been investigated with respect to the biotransformations of ormaplatin in the Fischer 344 rat bearing a transplanted fibrosarcoma. A number of different paradigms of dosing route and schedule for the administration of the two agents have been investigated. In the first group of experiments, WR-2721 (200 mg/kg, i.p.) was administered 30 min before ormaplatin (12.5 mg/kg, i.p.), and then peritoneal fluid, plasma, and tissues were harvested at 30 min after the ormaplatin administration. Our results suggest that a significant interaction between WR-2721 and ormaplatin is occurring in the peritoneal cavity. The interaction was evident in terms of both effects on distribution and disposition of total platinum and in alterations of the profiles of biotransformation products formed in the various tissues and fluids. Plasma protein binding of ormaplatin was decreased by 50% in the presence of WR-2721. Total platinum in the spleen was decreased by 66% and in the liver by 50%. There were no trends among the findings that would indicate any selectivity between tumor and nontumor tissue with respect to the effects of WR-2721 on the parameters measured. Subsequent investigations examined the effects of dosing the WR-2721 by the i.v. route while continuing with the i.p. administration of the ormaplatin. WR-2721 was administered either 30 or 5 min before the ormaplatin, and the plasma and tissues were harvested at 15, 30, or 60 min after ormaplatin administration. The reverse-phase HPLC peak, which behaved chromatographically as a Pt(dach)(WR-1065) standard, was less prominent after the i.v. administration of WR-2721 than it was after i.p. administration under any of the paradigms tested. There was again no evidence for selectivity between tumor and nontumor tissue in the findings from any of the paradigms. It is concluded that if WR-2721 is capable of selectively protecting nontumor tissue from the toxicities of platinum-based chemotherapy, it is doing so by some mechanism other than its selective uptake into normal tissue and subsequent nonspecific inactivation of any reactive cytosolic platinum species formed. Other possible mechanisms are briefly discussed.

Inhibition of mitochondrial respiration by a para-quinone methide

A relatively stable para quinone methide was prepared from 4-allyl-2,6-dimethoxyphenol. In aqueous solution the quinone methide had a half-life of 52 min, yet reacted rapidly with thiols such as glutathione or cysteine. The unusual stability of this quinone methide allowed us to directly test its effects on mitochondrial respiration. The quinone methide was a potent inhibitor of succinate-dependent mitochondrial respiration (IC50 = 47 microM). The inhibition appeared to be due to the depletion of protein thiols, since its effects were comparable to 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, Ellman's reagent). This quinone methide may prove a useful tool in the investigation of the specific effects of quinone methides on cells which lead to cytotoxicity.

o-Methoxy-4-alkylphenols that form quinone methides of intermediate reactivity are the most toxic in rat liver slices

The effects of p-alkyl substituents on the relative cytotoxicity of 4-alkyl-2-methoxyphenols were investigated in isolated rat liver slices. The derivatives of 4-alkyl-2-methoxyphenol studied were 4-methyl- (creosol), 4-ethyl-, 4-propyl-, 4-isopropyl-, 4-allyl-2-methoxyphenol (eugenol), as well as 4-allyl-2,6-dimethoxyphenol. The data were correlated with previous microsomal experiments which showed that all of the 4-alkyl-2-methoxyphenols were converted to quinone methides (QMs; 4-methylene-2,5-cyclohexadien-1-ones) via a cytochrome P450-catalyzed process [Bolton, J. L. Comeau, E., and Vukomanovic, V. (1995) Chem.-Biol. Interact., in press]. The present investigation showed little correlation between the rate of QM formation in microsomes and the relative toxicities of the alkylphenols, unless the QMs formed were of similar reactivity. In contrast, a plot of alkylphenol toxicity versus the relative hydrolysis rates of QMs derived from these phenols fit a parabolic equation with a minimum at the data for 4-isopropyl-2-methoxyphenol. These data suggest that in vivo oxidation of phenols to QMs which have lifetimes in the 10 s-10 min range results in cytotoxicity. QMs with reactivities outside this window are less toxic since the electrophile is either too stable for reaction with cellular nucleophiles or too reactive for nucleophilic cellular macromolecules to compete with solvent. These data suggest that a reactivity window exists for QMs which is a primary determinant of the extent of cytotoxic injury caused by these reactive electrophiles.

Quinone methide formation from para isomers of methylphenol (cresol), ethylphenol, and isopropylphenol: relationship to toxicity

The oxidative metabolism and toxicity of the para isomers of methylphenol (cresol), ethylphenol, and isopropylphenol were studied using male Sprague-Dawley rat liver microsomes and precision-cut liver slices. Reactive intermediates from each compound were trapped using radiolabeled glutathione and were detected and quantified by HPLC. Conjugates were collected and their structures determined by fast atom bombardment mass spectrometry and proton nuclear magnetic resonance. During microsomal incubations each test compound formed monoglutathione conjugates with structures which are consistent with the formation of quinone methide intermediates. In each case the glutathione moiety was attached to the benzylic carbon on the alkyl side chain of the phenol. With ethylphenol, which has a prochiral benzylic carbon, two isomeric conjugates were detected. The rate of formation of the glutathione conjugates in liver slice incubations was 4-isopropylphenol > 4-ethylphenol > 4-methylphenol. This correlated with the toxicity of the three compounds in liver slices. At equimolar concentrations 4-isopropylphenol was the most toxic while 4-methylphenol was the least toxic. Depletion of intracellular glutathione was observed in the presence of each test compound which preceded cell death. Enhancement of cellular thiol levels with N-acetylcysteine protected cells from the toxic effects of all three compounds as did inhibition of cytochrome P450 activity with metyrapone. These results suggest the formation of quinone methide intermediates from three alkylphenols during oxidative metabolism and demonstrate a correlation between the amount of reactive intermediate formed and toxicity observed in liver slices.

Organ-specific biotransformation of ormaplatin in the Fischer 344 rat

We examined the intracellular biotransformation products of ormaplatin [(d,l-trans)1,2-diaminocyclohexanetetrachloroplatinum(IV)] (formerly called tetraplatin) in liver, kidney, spleen, small intestine, and plasma of the adult male Fischer 344 rat. Previous studies have established that the rank order of ormaplatin toxicity in Fischer 344 rats is spleen approximately gastrointestinal tract > kidney >> liver. Animals were given tritium-labelled drug i.v. at 12.5 mg/kg, and tissues were harvested 30 min later. The kidney was found to concentrate total and cytosolic platinum to a greater extent than any of the other tissues. The absolute amount of cytosolic platinum, in micrograms per gram tissue, that was irreversibly bound to protein and/or other macromolecules was also greatest in the kidney. However, when the amount bound was expressed as a percentage of the total cytosolic platinum, the kidney was significantly lower than any other tissue. Of the various low molecular mass platinum biotransformation species characterized, by far the most abundant were complexes of platinum with the sulfur-containing molecules cysteine, methionine, and glutathione (GSH). There was more of the methionine complex in the blood plasma than in any of the tissues except for the spleen. No significant differences among the tissues were detected for the dichloro, cysteine, methionine, or the GSH complexes. The tritium-labelled diaminocyclohexane (DACH) carrier ligand appeared to remain stably bound to the platinum while in the plasma, as there was less free DACH ligand detected in plasma ultrafiltrate than in any tissue ultrafiltrate. Among the tissues, the free DACH levels were in the range of 20% of the radioactivity recovered from the HPLC column and were not significantly different. Consequently, neither biodistribution nor tissue-specific biotransformation of ormaplatin provides a ready explanation for the tissue specificity of ormaplatin toxicity in Fischer 344 rats. However, in kidney there was much less of the reactive PtCl2(DACH) species than has previously been reported for the corresponding Pt(NH3)2Cl2 species in cisplatin-treated rats. Thus, these data suggest a possible explanation for differences in nephrotoxicity induced by cisplatin versus that by ormaplatin.

Characterization of acetylcholinesterase in rabbit intrapulmonary arteries

Acetylcholine (ACh) acts on the pulmonary vasculature to evoke vasodilation and, in some species, vasoconstriction. The actions of ACh are terminated by its rapid hydrolysis by cholinesterases. Aside from histochemical localization studies, there is little information on cholinesterase enzymes in pulmonary blood vessels. The present study addresses the hypothesis that pulmonary blood vessels contain sufficient cholinesterase activity to regulate the action of ACh in these tissues. Accordingly, studies were undertaken to characterize and quantify cholinesterase activities in pulmonary arteries and veins, quantify inhibition of enzyme activity, and investigate functional physiological consequences of cholinesterase inhibition. Cholinesterase activities in aorta and trachea also were examined for comparison. Kinetic studies showed that the lobar pulmonary arterial enzyme has a Michaelis constant of 55.3 +/- 17.0 microM and a maximum velocity of 8.6 +/- 2.7 nmol/min/mg protein similar to cholinesterases found in other peripheral tissues. Studies with selective inhibitors revealed that > 98% of total enzyme activity was attributable to acetylcholinesterase. Similar levels of enzyme activity were found in homogenates of lobar branch intrapulmonary arteries, intrapulmonary veins, and aorta. The majority of enzyme activity was localized to the membrane fraction, although a moderate amount was found in the cytosol. Studies in intact intrapulmonary lobar arteries showed that these vessels had cholinesterase activity comparable with that found in intact trachealis muscle and that neostigmine (10 nM to 10 microM) caused concentration-dependent inhibition of enzyme activity. In isolated intrapulmonary lobar arteries, functional studies showed that 1 and 10 microM neostigmine significantly potentiated ACh-induced contractions.(ABSTRACT TRUNCATED AT 250 WORDS)