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The cardiovascular effects with bronchodilating plasma concentration of theophylline (3.8–12.6 mg/l) and enprofylline (0.8–3.0 mg/l) were studied in six healthy male subjects by means of non-invasive procedures. With these plasma concentrations only minor effects were noted with regard to heart rate, blood pressure and systolic time intervals. However, both xanthines seem to have a vasodilating ability and a weak positive inotropic effect on the heart.

Adenosine is a powerful bronchoconstrictor of asthmatic, but not normal, airways. In vitro studies on isolated human mast cells and basophils revealed that adenosine and selective analogues augmented inflammatory mediator release from mast cells by stimulating A2 receptors. Pharmacological blockade of mast cell mediator release in vivo also attenuated adenosine-induced bronchoconstriction, as did theophylline, by adenosine A2 receptor antagonism. Further in vitro studies revealed that the asthmatic response to adenosine is likely to be mediated via the A2B subtype which is selectively antagonised by enprofylline. Studies in animal models, especially mice, have shown a close synergistic interaction between adenosine, Th2 and airway remodelling responses. The recent description of A2B receptors on human airway smooth muscle cells that mediate cytokine and chemokine release and induce differentiation of fibroblasts into myofibroblasts strengthens the view that adenosine maybe more than an inflammatory mediator in asthma but also participates in airway wall remodelling in this disease. These data have provided a firm basis for developing adenosine A2B receptor antagonists as a new therapeutic approach to this disease.

1. Adenosine produced a biphasic lowering of the mean BP with a drastic bradycardic effect at the highest doses. The first phase hypotensive response was significantly reduced by the nitric oxide (NO) synthase inhibitor L-NAME. 2. The A(2a)/A(2b) agonist NECA produced hypotensive and bradycardic responses similar to those elicited by adenosine, which were not significantly modified by the A(2b) antagonist enprofylline. 3. The A(2a) agonist CGS 21680 did not significantly influence basal HR while induced a hypotensive response antagonized by the A(2a) selective antagonist ZM 241385, and reduced by both L-NAME and the guanylate cyclase inhibitor methylene blue. 4. The A(1) agonist R-PIA showed a dose-dependent decrease in BP with a drastic decrease in HR at the highest doses. The A(1) selective antagonist DPCPX significantly reduced the bradycardic activity and also the hypotensive responses obtained with the lowest doses while it increased those obtained with the highest ones. 5. The A(1)/A(3) agonist APNEA, in the presence of the xanthinic non-selective antagonist 8-pSPT, maintained a significant hypotensive, but not bradycardic, activity, not abolished by the histamine antagonist diphenhydramine. 6. The selective A(3) agonist IB-MECA revealed a weak hypotensive and bradycardic effect, but only at the highest doses. 7. In conclusion, in the systemic cardiovascular response to adenosine two major components may be relevant: an A(2a)- and NO-mediated hypotension, and a bradycardic effect with a consequent hypotension, via atypical A(1) receptors. Finally, an 8-pSPT-resistant hypotensive response not attributable to A(3) receptor-stimulation or to release of histamine by mastocytes or other immune cells was observed.

This study examined the effects of thyroid status on the lipolytic responses of rat white adipocytes to β-adrenoceptor (β-AR) stimulation. The β1- and β3-AR mRNAs and proteins were measured by Northern and saturation analyses, respectively. Glycerol production and adenyl cyclase (AC) activity induced by various non-selective and selective β1/β3-AR agonists and drugs which act distal to the receptor in the signalling cascade were measured in cells from untreated, tri-iodothyronine (T3)-treated and thyroidectomized rats. The β3-AR density was enhanced (72%) by T3-treatment and reduced (50%) by introduction of a hypothyroid state while β1-AR number remained unaffected. The β1- and β3-AR density was correlated with the specific mRNA level in all thyroid status. The lipolytic responses to isoprenaline, noradrenaline (β1/β3/β3-AR agonists) and BRL 37344 (β3-AR agonist) were potentiated by 48, 58 and 48%, respectively in hyperthyroidism and reduced by about 80% in hypothyroidism. T3-treatment increased the maximal lipolytic response to the partial β3-AR (CGP 12177) and β1-AR (xamoterol) agonists by 234 and 260%, respectively, increasing their efficacy (intrinsic activity: 0.95 versus 0.43 and 1.02 versus 0.42). The maximal AC response to these agonists was increased by 84 and 58%, respectively, without changing their efficacy. In the hypothyroid state, the maximal lipolytic and AC responses were decreased with CGP (0.17±0.03 versus 0.41±0.08 μmol glycerol/106 adipocytes; 0.048±0.005 versus 0.114±0.006 pmol cyclic AMP min−1 mg−1) but not changed with xamoterol. The changes in lipolytic responses to postreceptor-acting agents (forskolin, enprofylline and dibutenyl cyclic AMP, (Bu)2cAMP) suggest the modifications on receptor coupling and phosphodiesterase levels in both thyroid states. Thyroid status affects lipolysis by modifying β3-AR density and postreceptor events without changes in the β1-AR functionality. British Journal of Pharmacology (2000) 129, 448–456; doi:10.1038/sj.bjp.0703008

A potentiating effect by adenosine on the bronchocontractile response to histamine has been previously described in a guinea-pig model (Breschi et al. [1994] Pharmacology 49:42-51). In the present study, the subtype(s) of purinergic receptors involved in this modulatory effect of the nucleoside have been investigated. A possible role for P2 receptors or for the putative P3 receptor subtype was excluded because of the very poor modulatory effect of the P2 agonist α,β-methylene ATP when compared with the effects of all the P1 agonists tested, with the exception of 5'-N-Methyl-N6-(3-iodobenzyl)adenosine. The order of potency revealed for the P1 agonists was R-N 6 -[2-phenylisopropyl]adenosine (R-PIA) = 5'-N-methylcarboxamidoadenosine > N6-2-(4-aminophenyl)ethyladenosine ≥ adenosine and in the presence of R-PIA, a significantly greater maximal effect was observed. Among the adenosine receptor antagonists evaluated, the A 2 antagonist 3,7-dimethyl-1-propargylxanthine even increased the effect of adenosine and no influence was evident with the reported A 2B antagonist enprofylline. A significant reduction of the potentiating activity of adenosine was, on the contrary, obtained with the non selective blocker theophylline or the A 1 antagonist DPCPX. In conclusion, the above data suggest that the potentiation of histamine-induced bronchoconstriction by adenosine is mediated by the stimulation of A 1 receptors.

1 We investigated by intravital microscopy in rats, the in vivo direct effects of theophylline on the diameters of second and third order diaphragm arterioles. 2 Theophylline (1–100 μm) dilated second and third order diaphragm arterioles significantly, and with an amplitude which was not statistically different from the one obtained with adenosine (1–100 μm). Enprofylline (1–100 μm), a theophylline analogue with poor adenosine-receptor antagonism but with similar or higher phosphodiesterases inhibition properties than theophylline, also dilated diaphragm arterioles, causing however, a significantly smaller dilatation than theophylline. 3 Neither the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX, 50 nm), nor the A2 adenosine receptor antagonist 3,7-dimethyl-1-proparglyxanthine (DMPX, 10 μm) reduced significantly theophylline-induced arteriolar dilatation. 4 Theophylline (100 nm) abolished adenosine-induced arteriolar dilatation. 5 The dilatation induced by theophylline was unchanged by the nitric oxide (NO) synthase inhibitor Nω-nitro-l-arginine (NNA, 300 μm). 6 Theophylline-induced arteriolar dilatation was abolished by the prostaglandin synthesis inhibitors mefenamic acid or indomethacin (20 μm). 7 These findings show that theophylline induced a significant dilatation of diaphragm arterioles via the release of prostaglandins. British Journal of Pharmacology (1998) 124, 1355–1362; doi:10.1038/sj.bjp.0701962

On the basis of their structure, we compared the ability of 35 xanthine derivatives to activate the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel stably expressed in chinese hamster ovary (CHO) cells using the cell-attached patch clamp and iodide efflux techniques. Activation of CFTR channels was obtained with 3-mono, 1,3-di or 1,3,7-tri-substituted alkyl xanthine derivatives (enprofylline, theophylline, aminophylline, IBMX, DPMX and pentoxifylline). By contrast, xanthine derivatives substituted at the C8- or N9-position failed to open CFTR channels. The CFTR chloride channel activity was blocked by glibenclamide (100 μM) but not by DIDS (100 μM). Activation of CFTR by xanthines was not mimicked by the calcium ionophore A23187, adenosine, UTP, ATP or the specific phosphodiesterase inhibitors rolipram, Ro 20-1724 and milrinone. In addition, we found no correlation between the effect of xanthines on CFTR and on the cellular cyclic AMP or ATP levels. We then synthesized a series of 3,7-dimethyl-1-alkyl xanthine derivatives; among them, 3,7-dimethyl-1-propyl xanthine and 3,7-dimethyl-1-isobutyl xanthine both activated CFTR channels without increasing the intracellular cyclic AMP level, while the structurally related 3,7-dimethyl-1-(2-propenyl) xanthine and 3,7-dimethyl-1-(oxiranyl methyl) xanthine were inactive. Our findings delineate a novel function for xanthine compounds and identify the molecular features that enable xanthine activation of CFTR. These results may be useful in the development of new molecules for studying the pharmacology of chloride channels. British Journal of Pharmacology (1998) 123, 683–693; doi:10.1038/sj.bjp.0701648

It has previously been shown that caffeine, in a dose-dependent manner, increases the expression of the protooncogene c-fos in the rat brain, predominantly in the caudate-putamen and tuberculum olfactorium. In this study we examined the effect of related xanthines and of selective phosphodiesterase inhibitors on c-fos expression. The effect of caffeine (75 mg kg-1) was mimicked by 3-isobutyl-1-methyl xanthine (IBMX) (25 mg kg-1) and theophylline (100 mg kg-1) but not by 8-p-sulfophenyltheophylline (10 mg kg-1), enprofylline, theobromine or paraxanthine (each at 100 mg kg-1). Moreover, the cyclic AMP-selective phosphodiesterase inhibitors rolipram (10 or 20 mg kg-1) and SQ 20,006 (25 mg kg-1) and the cyclic GMP-selective phosphodiesterase inhibitor zaprinast (10 mg kg-1) failed to induce c-fos in striatum, but caused a clear-cut induction in the overlying cerebral cortex. Thus, c-fos is induced in rat striatum following administration of caffeine and other xanthines that (provided they enter the brain) block adenosine receptors, suggesting an involvement of central adenosine receptors. Inhibition of cyclic nucleotide phosphodiesterase does not appear to play any important role in c-fos induction by the xanthines.

A series of imidazo[4,5-e][1-4]diazepine-5,8-diones were synthesized from hypoxanthines. Certain of these cyclic homologues of caffeine, theophylline, theobromine, 3-isobutyl-1-methylxanthine, and enprofylline were inhibitors of binding of adenosine analogues to rat brain A1 and A2 adenosine receptors and were antagonists of A2 adenosine receptors stimulatory to adenylate cyclase in rat PC12 cell membranes. Activity at adenosine receptors was lower than the corresponding xanthines, perhaps because imidazodiazepinediones contain a boat-shaped seven-membered ring rather than the planar heteroaryl ring system of the xanthines. The imidazodiazepinediones had low affinity for brain benzodiazepine sites.

Summary Repeated airway exposures to toluene diisocyanate (TDI) may cause sensitization and asthma. This study has examined the acute inflammatory response to TDI in guinea-pig tracheobronchial airways, the development of increased sensitivity to TDI and the effects of xanthines and a glucocorticoid on these responses to TDI. A restricted surface area of the tracheobronchial mucosa of Ketalar-Xylazin anaesthetized guinea-pigs was exposed to TDI, dissolved in olive oil, by means of 1 min infusions through an oral catheter. The TDI-induced inflammatory process was quantified by determination of airway luminal entry of plasma. Already 3 nl (∼20 pmol) of TDI produced a significant and sustained exudation response (P < 0.001 to P < 0.01, 5 and 17 hr after exposure). Pretreatment with intravenous enprofylline (25 μmol/kg) reduced (P < 0.05) the acute response measured at 5 hr whereas budesonide (116 μmol/kg intraperitoneally or 26 μmol/kg by tracheal superfusion) was without effect. Two repeated exposures to TDI 3 nl (on days 1 and 8) made the animals hyperresponsive to TDI so that on day 15 a previously subthreshold dose of TDI (0.3 nl) produced significant exudation both at 5 and 17 hr after exposure (P < 0.001 to P < 0.01). Similarly, two repeated dermal exposures to a large dose of TDI (20 μl) lowered the threshold for tracheal provocation with TDI. Budesonide (2.6 μmol/kg orally) given daily during the topical airway ‘sensitization’ regimen (days 1–14) significantly reduced the response to the subsequent 0.3 nl challenge dose of TDI (P < 0.05). The effects of daily treatments with either theophylline (100 μmol/kg) or enprofylline (50 μmol/kg) were not significant. We conclude that two separate airway exposures to a low inflammatory dose of TDI increased about tenfold the airway mucosal sensitivity to this agent. Only the acute inflammatory response was reduced by xanthines. The development of increased responsiveness but not the acute inflammatory response to TDI was reduced by glucocorticoid treatment.

1. The in vitro biological activities and the effect of protein binding on the relaxant effects in vivo of N-3-alkylxanthine and N-3-alkyl-N-1-methylxanthine derivative were investigated in guinea-pigs. 2. A significantly positive correlation was observed among the in vitro muscle relaxant activity, the cyclic adenosine monophosphate (cAMP) phosphodiesterase (PDE) inhibitory activity and the protein-binding potency of xanthine derivatives. However, there was a weak relationship between these activities and affinity for adenosine receptors. 3. When theophylline, enprofylline and 1-methyl-3-propylxanthine (MPX) were injected intravenously in guinea-pigs, their ED50 values were 6.1, 3.3 and 1.0 mg/kg, respectively. Plasma concentrations of these drugs obtained following the intravenous injection of the ED50 approximated the theoretically effective concentration (EC50) predicted from both the relaxant effects in vitro and the protein binding parameters. A good linear correlation was observed between bodyweight in four species (rats, guinea-pigs, rabbits and humans) and certain pharmacokinetic parameters of enprofylline and theophylline. 4. The present study indicates that differences in the relaxant effects of these drugs in vitro and in vivo can be explained in part by protein binding, and that the protein binding of these xanthine bronchodilators is an important determinant for their pharmacological activity. Guinea-pigs provide a useful model for studying pharmacodynamic-pharmacokinetic relationships of new bronchodilators.

1. For the purpose of clarifying the mechanism of the airways smooth muscle relaxant action of xanthines, cyclic guanosine monophosphate (GMP) phosphodiesterase (PDE) from guinea-pig trachealis muscle was purified with diethylaminoethyl ether (DEAE) cellulose column chromatography. 2. Five 3-alkylxanthines (3-methylxanthine, 3-ethylxanthine, 3-n-propylxanthine (enprofylline), 3-n-butylxanthine, and 3-iso-butylxanthine), and five 1-methyl-3-alkylxanthines (1-methyl-3-methyl-xanthine (theophylline), 1-methyl-3-ethylxanthine, 1-methyl-3-n-propylxanthine, 1-methyl-3-n-butylxanthine, and 1-methyl-3-iso-butylxanthine (IBMX] were compared in terms of purified cyclic GMP PDE inhibition. The relationship between the structure and inhibition of cyclic GMP PDE was studied. 3. The -log EC50 values for relaxation of spontaneous tone of isolated guinea-pig trachealis preparations by the 3-alkylxanthines and 1-methyl-3-alkylxanthines were determined. 4. The five 1-methyl-3-alkylxanthines were each more potent in relaxing isolated trachealis smooth muscle than the corresponding 3-alkylxanthines. The 1-methyl-3-alkylxanthines were also more potent than the corresponding 3-alkylxanthines in their cyclic GMP PDE inhibitory effect. There was a strong positive correlation between the concentration of inhibitor which inhibited hydrolysis by 50% (IC50) values for cyclic GMP PDE inhibition by the xanthine derivatives and their EC50 values for trachealis muscle relaxation. 5. It is suggested that the mechanism by which xanthine derivatives relax trachealis smooth muscle involves inhibition of cyclic GMP PDE in addition to inhibition of cyclic adenosine monophosphate PDE.

The efficacy and safety of different regimens of intravenously administered enprofylline, an anti-asthma xanthine, were evaluated in a randomized open study, including 155 patients with acute exacerbation of obstructive lung disease. The regimen 2.5 mg/kg i.v. over 10 min was canceled after seven patients had been included, due to two cases of hypotensive/vasovagal reactions. The regimens 2.0 mg/kg/20 min and 2.5 mg/kg/20 min were significantly more effective with regard to bronchodilation than 2.0 mg/kg/10 min (PEF increase +35%, +30% and +17% respectively). Nausea and headache were the most common side effects (16-33% and 23-33% of the patients respectively on different regimens) with the lowest frequency on 2.0 mg/kg/20 min. Four additional hypotensive reactions occurred; one on each 2.0 mg/kg regimen and two on 2.5 mg/kg/20 min. The regimen 2.0 mg/kg20 min was found to be the most favourable with regard to efficacy and side effects. Enprofylline i.v. was found to be an effective bronchodilating treatment of acute airway obstruction but the frequency of side effects has to be considered.

SUMMARY 1. We investigated effects of theophylline (widely used for the treatment of asthma) and enprofylline (a new xanthine derivative with negligible adenosine antagonism) on O2- production by human neutrophils with n-formyl-methionylleucyl-phenylalanine (FMLP) stimulation. 2. Therapeutic concentrations of theophylline (1–100 μmol/L) enhanced O2- production, maximally by 43.1 ± 24.4% at 30 μmol/L; the same concentrations of enprofylline inhibited O2- production. 3. When each agent was administered after pre-incubation with adenosine deaminase (ADA) (0.1 U/mL), O2- production was inhibited in a concentration-dependent manner in comparison with that under administration of ADA alone. 4. These results suggest that the difference of effects in the two xanthine derivatives at therapeutic concentrations might be due to the presence or absence of adenosine antagonism.

1. The ability of 1,3,8-substituted xanthines (1,3-dipropyl-8-(4-(2-aminoethyl)amino)carbonylmethyloxyphenyl) xan thine (XAC), 1,3-dipropyl-8-(4-carboxymethyloxyphenyl)xanthine (XCC), 1,3-dipropyl-8-(2-amino-4-chlorophenyl)xanthine (PACPX), 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), 1,3-diethyl-8-phenylxanthine (DPX) and 8-phenyltheophylline (8-PT)), of 1,3,7-substituted xanthines (1-propargyl-3,7-dimethylxanthine (PGDMX) and caffeine), and of a 3-substituted xanthine (enprofylline) to antagonize the inhibitory effect of 2-chloroadenosine (CADO) on the amplitude of nerve-evoked twitches was investigated in innervated sartorius muscles of the frog. 2. All the 1,3,8-substituted xanthines, in concentrations virtually devoid of effect on neuromuscular transmission, shifted to the right, in a near parallel manner the log concentration-response curve for CADO. Linear Schild plots with slopes near to unity at concentration-ratios less than 14 were obtained for XAC, XCC, DPCPX, DPX and 8-PT. 3. The order of potency of the 1,3,8-substituted xanthines as antagonists of the effect of CADO was XAC (Ki = 23 nM) greater than or equal to DPCPX (35 nM) greater than 8-PT (200 nM) greater than or equal to DPX (295 nM) greater than XCC (1905 nM) greater than or equal to PACPX (2291 nM). No correlation was found between the potency of these xanthines as antagonists of the adenosine receptor at the frog neuromuscular junction and their reported potency as antagonists of the A1- or A2-adenosine receptors. 4. The 1,3,7-substituted xanthines, PGDMX and caffeine, in concentrations virtually devoid of effect on neuromuscular transmission, also caused parallel shifts to the right of the log concentration-response curves for CADO, but were less potent than the 1,3,8-substituted xanthines. PGDMX was more than 20 times more potent than caffeine. 5. Enprofylline in concentrations up to 100 microM did not antagonize the inhibitory effect of CADO on neuromuscular transmission. 6. It is concluded that the antagonist profile of the adenosine receptor mediating inhibition of transmission at the frog neuromuscular junction is different from the antagonist profile of the A1- and A2-adenosine receptors.

The anticonvulsant potency of phenobarbital (PB) (120 min before the test), phenytoin (PHT) (120 min), carbamazepine (CBZ) (60 min), valproate (VPA) (30 min), and acetazolamide (60 min) alone or in combination with either aminophylline (50 mg/kg, 30 min) or enprofylline (46.2 mg/kg, 30 min) was measured against maximal electroshock-induced convulsions in mice. All drugs were administered intraperitoneally (i.p.), and the protective efficacy of each drug was expressed as ED50 in milligrams per kilogram. Aminophylline decreased anticonvulsant activity of PB, PHT, CBZ, and VPA, increasing the respective ED50 values from 16 to 28 mg/kg, 7.4 to 14 mg/kg, 18 to 26 mg/kg, and 260 to 335 mg/kg. On the contrary, enprofylline in the equimolar dose did not exert such effect. Furthermore, neither aminophylline nor enprofylline affected anticonvulsant action of acetazolamide. The present data favor enprofylline as a preferable drug for treatment of obstructive lung diseases in epilepsy patients. However, accurate pharmacokinetic data in mice and men for both xanthines are necessary if one attempts to compare their cerebral and pulmonary actions.

Six normal male subjects, ages 28-40 years, were studied on separate days during increasing infusions with adenosine, 40-120 micrograms kg-1 min-1, before and during infusions of two xanthine derivatives, theophylline (mean plasma concentration 9 mg l-1) and enprofylline (mean plasma concentration 3 mg l-1). The study was double-blind, randomized, placebo controlled. Cardio-respiratory variables were measured non-invasively. Adenosine by itself increased heart rate (P less than 0.05), skin temperature (P less than 0.05), resting minute ventilation (P less than 0.01) and decreased estimated Pa, CO2 (P less than 0.01). Compared with placebo enprofylline increased heart rate (P less than 0.05) and shifted the heart rate and ventilation dose-response curves of adenosine upwards (P less than 0.05 and P less than 0.02, respectively). Theophylline did not by itself affect heart rate but significantly (P less than 0.05) reduced the heart rate response to adenosine. Compared with placebo theophylline caused a small increase in minute ventilation (P less than 0.05) and flattened the dose-response curves of the effects of adenosine on ventilation (P less than 0.01) and Pa, CO2 (P less than 0.01). Theophylline also reduced abdominal and chest discomfort caused by adenosine permitting significantly (P less than 0.05) higher infusion rates of adenosine. These findings suggest that, with equipotent bronchodilating plasma concentrations, theophylline can inhibit while enprofylline augments some cardio-respiratory stimulant effects of infused adenosine in man.

1. The sucrose-gap technique was used to study pre- and postjunctional actions of P1-purinoceptor and P2-purinoceptor agonists and a range of xanthine derivatives in the guinea-pig caecum circular muscle. 2. Adenosine, 2-chloroadenosine (2-ClAd), ATP and alpha,beta-methylene ATP all caused concentration-dependent hyperpolarization of the smooth muscle membrane with a rank order of potency of 2-ClAd greater than alpha,beta-methylene ATP greater than adenosine. 3. The xanthine derivatives caffeine, theophylline, 8-phenyltheophylline and 1,3-dipropyl-8-(2-amino-4-chlorophenyl) xanthine (PACPX) at submicromolar concentrations evoked depolarization of the smooth muscle membrane. At higher concentrations, all these compounds and enprofylline caused concentration-dependent hyperpolarization. 4. All the purine compounds tested caused a reduction in the amplitude of the non-adrenergic, non-cholinergic inhibitory junction potential (i.j.p.). For the P1-purinoceptor agonists adenosine and 2-ClAd this was almost entirely a prejunctional effect. For the P2-purinoceptor agonists this was mostly a postjunctional effect because both ATP and alpha,beta-methylene ATP caused significantly greater increases in the conductance of the smooth muscle membrane than did adenosine or 2-ClAd. 5. All the xanthine compounds tested (up to 100 microM), except enprofylline, were capable of increasing the amplitude of the i.j.p. At millimolar concentrations both caffeine and theophylline could reduce the i.j.p. amplitude. 6. It is concluded that there are inhibitory prejunctional P1-purinoceptors on the i.j.p.-producing neurones in the guinea-pig caecum circular muscle and that, of the xanthine derivatives tested, none of them would be suitable to use as a P1-purinoceptor antagonist in this preparation because of their own direct effects.

Human lung bronchiolar segments (about 2 mm long and with a diameter of 0.6-1.5 mm) were dissected and circular muscle tension recorded. Airways were identified by histology and in some preparations by relaxant responses to noradrenaline (0.1-10 microM). Adenosine (1-100 microM) produced only very weak contractions, whereas carbachol (EC50 = 0.40 microM), histamine (EC50 = 0.63 microM), prostaglandin D2 (EC50 = 0.50 microM), substance P (EC50 = 4.6 microM) and ATP (1-100 microM) produced much greater ones. The contractions generally developed rapidly and were stable. The mean maximum increase in tension achieved with the most efficient constrictor, carbachol, was 0.5 g. ATP was the least efficient producing only about 40% of carbachol's maximum. Terbutaline, theophylline and enprofylline relaxed carbachol (2.0 microM = EC70)-contracted preparations. Terbutaline (3-3000 nM) relaxed 4 out of 11 bronchioles. Theophylline (10-4000 microM) and enprofylline (1-400 microM) consistently relaxed the bronchiolar preparations including those exhibiting little responsiveness to the beta 2-adrenoceptor agonist. Since enprofylline (which does not block adenosine receptors) was a five times more potent relaxant than theophylline and since adenosine produced only weak contractions, antagonism of adenosine receptors is probably not involved in relaxation of the small airways. It is suggested that the present data, which apparently differ from those obtained with lung parenchymal strips, are of relevance for human small airways responsiveness.

1. Caffeine (10 mM)-induced relaxation of guinea-pig isolated trachealis was attenuated and converted to a small spasmogenic response on cooling to 22 degrees C. The relaxant response was restored on rewarming to 37 degrees C and was abolished by indomethacin (2.8 microM). Cooling to 22 degrees C in the presence of indomethacin revealed spasmogenic responses to caffeine which were abolished on rewarming to 37 degrees C. 2. Trachealis treated with indomethacin (2.8 microM) was repeatedly dosed with acetylcholine (ACh, 10 microM). Caffeine (1 or 10 mM), added as each ACh-induced spasm reached equilibrium, transiently augmented but then suppressed the spasm. On cooling from 37 degrees C to 12 degrees C, the increment in spasm evoked by caffeine increased relative to the spasm evoked by ACh. 3. Trachealis treated with indomethacin (2.8 microM) was repeatedly dosed with caffeine (10 mM). At 37 degrees C caffeine had little effect but it caused spasm when the tissue was cooled to 32 degrees C. Spasm amplitude increased as cooling progressed to 12 degrees C. Similar results were obtained with caffeine (1 mM). 4. At 37 degrees C, caffeine, enprofylline, 1,3,7,9-tetramethylxanthinium (TMX), theobromine, theophylline, xanthine and forskolin each caused concentration-dependent suppression of tracheal tone. Among the xanthine derivatives the rank order of potency was enprofylline greater than theophylline greater than caffeine greater than theobromine greater than xanthine greater than TMX. 5. In trachealis treated with indomethacin (2.8 microM) and maintained at 12 degrees C, the xanthines each caused concentration-dependent spasm. The rank order of potency was theobromine greater than or equal to theophylline greater than or equal to caffeine greater than or equal to enprofylline greater than xanthine greater than TMX. Forskolin was devoid of spasmogenic activity. 6. Trachealis treated with indomethacin (2.8 microM) and maintained at 12 degrees C, was repeatedly dosed with either caffeine (10 mM) or potassium chloride (KCl, 40 mM). Caffeine-induced spasm was attenuated in a Ca2+-free medium containing EGTA (2 mM), modestly at first but subsequently more profoundly. KCl did not evoke spasm at 12 degrees C but at 37 degrees C the KCl-induced spasm was virtually abolished at its first trail in the Ca2+-free, EGTA-containing medium. 7. It is concluded that caffeine, other alkylated xanthines and xanthine itself share a spasmogenic action in guinea-pig isolated trachealis which is best observed when the tissue is treated with indomethacin (2.8 microM) and maintained at 12 degrees C. The spasmogenic action represents the release of Ca2+ from intracellular sites of sequestration and may not depend on the intracellular accumulation of cyclic AMP. The rank order of spasmogenic potency of the xanthine derivatives differs markedly from their rank order of potency in suppressing the spontaneous tone of the trachealis observed at 370C. Since, at 12 degrees C, TMX is spasmogenic at concentrations identical to those causing relaxation at 37 degrees C, it is likely that TMX penetrates the cell. The relaxant effects of TMX do not, therefore, indicate that methylxanthine-induced relaxation is mediated by a receptor located on the external surface of the cell.

The effect of stable adenosine analogues, including adenosine 5'-N-ethylcarboxamide (NECA) and N6-L-phenylisopropyl-adenosine (L-PIA), were studied on cyclic adenosine 3', 5'-monophosphate (cyclic AMP) accumulation in rat and guinea-pig thymocytes. NECA was approximately 10 times more potent than L-PIA, in thymocytes from both species. D-PIA was more potent in guinea-pig than in rat thymocytes. The effect of a number of adenosine analogues followed the order: NECA greater than 2-chloro-adenosine greater than L-PIA greater than N6-cyclohexyl-adenosine (CHA), an order of potency characteristic for adenosine receptors of the A2-subtype. Thymocytes may be used as a model system to study the pharmacology of such receptors. Several xanthines were studied as antagonists of the NECA (1 microM)-induced cyclic AMP accumulation. The order of potency was: 1,3-diethyl-8-phenylxanthine greater than 8-phenyl-theophylline greater than IBMX = 8-p-sulphophenyltheophylline = verrophylline greater than theophylline greater than caffeine greater than enprofylline greater than theobromine greater than pentoxiphylline. The pA2 value for 8-phenyltheophylline was 0.35 microM, and the antagonism was shown to be competitive. The order of potency of the xanthine is virtually identical to that found earlier in several other systems in which the receptors are of the A1-subtype. None of the xanthine derivatives tested thus seem to discriminate between A1 and A2-receptor-mediated adenosine actions.

Bronchodilating effects produced by increasing intravenously administered doses of enprofylline and theophylline compared to placebo were evaluated in 20 asthmatic outpatients. Three mean plasma plateaux of enprofylline of 1.5, 2.9 and 4.0 micrograms/ml produced a mean increase in forced expiratory volume in the first second (FEV1.0) as a percentage of baseline, of 12.8%, 18.8% and 30.1%, respectively. Comparable plasma plateaux of theophylline i.e. 5.5, 10.8 and 15.2 micrograms/ml produced a mean increase of FEV1.0 in percent of basal values of 12.4%, 21.6% and 28.2%, respectively. Enprofylline at plasma concentrations above 2.9 micrograms/ml induced more headache and slightly more nausea than theophylline and placebo. Theophylline infusion produced more tremor (finger oscillation) than enprofylline and placebo. Intravenously administered enprofylline produces bronchodilation comparable to theophylline in a mean dose ratio of 3.8.

The tracheobronchial mucosa of anaesthetized guinea-pigs (normal or sensitized with ovalbumin to produce IgE and IgG antibodies) was superfused (0.02 ml min-1, 5 min) with saline, mediators, and (in sensitized animals) ovalbumin via a catheter atraumatically introduced orally. The intravascular blood pool and amount of macromolecules in excised trachea and adjoining main bronchi were quantified by measuring erythrocytes, that had been labelled in vivo with 99Tcm, and analysing for FITC-dextran, MW = 70,000, that had been given i.v. Extravasation of macromolecules was determined as the analysed total content minus the calculated intravascular content of FITC-dextran. Capsaicin 0.1 nmol extravasated 223 micrograms of FITC-dextran per g wet weight of airway tissue (P less than 0.001). Substance P 0.1 nmol, 41 micrograms g-1 (P greater than 0.05); substance P 0.3 nmol, 142 micrograms g-1 (P less than 0.001); eledoisine 0.1 nmol, 101 micrograms g-1 (P less than 0.01); ovalbumin 0.1 microgram, 179 micrograms g-1 (P less than 0.001); LTC4 0.2 pmol, 180 micrograms g-1 (P less than 0.001); LTD4 0.2 pmol 223 micrograms ml-1 (P less than 0.001). Bronchi and trachea were similarly affected by these agents. Prior superfusion (0.02 ml min-1, 30 min) with terbutaline 0.06 nmol, enprofylline 12 nmol, or lidocaine 6 nmol significantly reduced the effect of capsaicin. Enprofylline also reduced significantly the effect of LTC4. The degree of extravasation in this study was smaller than could be detected by changes in tissue wet to dry weight ratios. The present data support the view that tracheobronchial vascular permeability to macromolecules is subject to physiological and pharmacological control.

The effect of intravenously administered ampicillin on plasma levels and renal clearance of enprofylline was investigated in seven young and six elderly volunteers. On one occasion, each subject received 2 g of ampicillin intravenously, and serum concentrations of ampicillin were followed for 4 hours. On a separate occasion, a loading infusion of enprofylline was administered over 60 min., aiming at a plasma level of 2 micrograms/ml, and followed by a maintenance infusion. During this an infusion of 2 g of ampicillin was given for 10 min., and subsequently, renal clearance and plasma levels of enprofylline were followed for 4 hours. Plasma enprofylline levels increased significantly in the subjects after ampicillin infusion, but the effects on renal clearance of enprofylline were not statistically significant. The magnitude of the effects on enprofylline plasma levels and renal clearance from high doses of ampicillin do not suggest that interaction with beta-lactam antibiotics will be a serious problem for patients on enprofylline treatment.

Isolated sensitized guinea pig tracheal smooth muscle tone was induced by use of egg albumin or SRS-A (slow reacting substance of anaphylaxis). The dose-response relationships of theophylline and enprofylline were studied on these preparations. Enprofylline was more potent than theophylline in relaxing the egg albumin- or SRS-A-induced tracheal muscle tone. The theophylline relaxation-curve was significantly shifted to the left after addition of adenosine-deaminase to the egg albumin-contracted trachea, but this was not observed with the enprofylline relaxation-curve. In SRS-A-contracted tracheas, the addition of adenosine-deaminase did not significantly alter the relaxation curves of theophylline or enprofylline. It is therefore suggested that the relaxing effects of theophylline and enprofylline on SRS-A-induced contractions, at the therapeutically relevant concentrations demonstrated in this study, might be of importance for the antiasthmatic effects of xanthines.

Histamine release from human basophilic leukocytes was induced by increasing concentrations of anti-IgE in the presence of various concentrations of 2-deoxyglucose (2-DOG), the histamine H2-receptor agonist dimaprit, theophylline, or enprofylline (a new antiasthmatic xanthine derivative). The results show that the degree of inhibition produced by each agent differed with the concentration of anti-IgE used and with the nature and concentration of the inhibitor. These data indicate that great care should be used when characterizing an inhibitor of mediator release by simply giving a figure for the per cent inhibition of release observed in its presence.

1. Epithelium removal did not influence the development of spontaneous tone in guinea-pig tracheal smooth muscle mounted as open ring preparations with two adjoining cartilaginous rings in vitro. 2. Epithelium removal did not change the potency of carbachol but tended to reduce the maximal contraction. In the presence of epithelium the EC50 of carbachol was not different in tracheal open ring compared with intact tube preparations (comprising four cartilaginous rings), suggesting that the size of continuous epithelium in vitro was not critical for the potency of carbachol. 3. Substance P produced the same response in intact and rubbed tracheae. The enkephalinase inhibitor thiorphan (0.1 mM) by itself contracted the trachea and appeared to potentiate the substance P response five times more in the absence than in the presence of epithelium. Capsaicin (1 microM)-induced contractions did not differ between intact and rubbed preparations. 4. Arachidonic acid, 22 microM, variably produced small relaxations and contractions in intact as well as in rubbed tracheae. The mean effects of arachidonic acid were not significantly altered by epithelium removal. 5. Adenosine produced small contractions and dose-dependent relaxations in the presence and absence of epithelium. 6. Epithelium removal had no effect on the potency of the relaxant agonists theophylline and enprofylline. The isoprenaline curve was shifted 2 fold to the left and the terbutaline curve 1.5 fold to the right. The maximal relaxations were generally reduced in epithelium-free tissue. The reduction reached statistical significance with theophylline. 7. The present results suggest that epithelium removal is of little consequence for the pharmacology of the guinea-pig tracheal open ring preparation in vitro.

The bronchodilating effect of two doses of peroral enprofylline was compared with placebo in 24 asthmatic patients. Enprofylline produced significantly greater bronchodilatation than placebo. A dose of 2 mg/kg b.wt. and 4 mg/kg b.wt. caused a mean maximal increase in FEV1 of 26% and 35%, respectively. The degree and the incidence of headache and nausea were estimated by means of a scoring system. Dose-related effects on both parameters were observed. Other side effects were negligible. In seven patients the mean plasma half-life of enprofylline was found to be 113 min. It is suggested that enprofylline should be studied further in patients suffering from obstructive lung disease.

In ferret and guinea pig papillary muscles enprofylline (10 microM-10 mM) and theophylline (0.1-2 mM) alone or in combination with terbutaline (0.05 microM-0.1 microM) decreased the action potential duration and increased the plateau height, increased the peak force of contraction and facilitated the depolarization-induced automaticity. In voltage clamp, the xanthines alone or in combination with terbutaline increased second inward currents, ICa,f and ICa,2, but had relatively less effect on the time-dependent outward current. No qualitative differences between enprofylline and theophylline could be detected but the former was about 5 times more potent in increasing ICa,f. In clinically relevant concentrations, enprofylline and theophylline alone, or in combination with terbutaline caused a small (2-5%) shortening of the action potential.

1 The kinetic disposition of a novel xanthine bronchodilator, enprofylline, was determined in young healthy male volunteers in the presence and absence of amoxycillin or erythromycin. These data were compared to those derived from a similar study of theophylline disposition in the presence and absence of erythromycin. 2 Erythromycin inhibited theophylline disposition only in those subjects in whom the control kinetic study was done after antibiotic ingestion, but the effect was modest. Erythromycin had no effect on enprofylline disposition. 3 Amoxycillin reduced the renal clearance of enprofylline, but the change was not statistically significant.

Aminophylline in a low dose has been shown to reverse diazepam sedation. The present investigation was performed as a double-blind study to compare the effects of aminophylline and enprofylline on deep diazepam sedation after surgery. Enprofylline is a xanthine derivative with anti-asthmatic effect but, in contrast to aminophylline, enprofylline has very weak adenosine antagonistic properties. A comparison should make it possible to evaluate if adenosine is involved in the observed effects. Twenty male patients undergoing transurethral surgery in spinal anaesthesia were given diazepam during surgery to maintain a state of deep sedation. Postoperatively aminophylline or enprofylline was given from coded ampoules in equipotent anti-asthmatic doses (4.5 or 1.5 mg/kg). The degree of sedation was assessed prior to and after the injections. A difference between the groups was obvious. Patients given aminophylline showed rapid reversal of sedation, persisting throughout the 2-h observation period. Following enprofylline, a markedly slower reversal of sedation was observed. It is likely that aminophylline antagonises diazepam sedation by blocking adenosine receptors. Some clinical implications are outlined.

Thirty-nine patients with asthma requiring acute treatment were included in a randomized, double-blind, paralle1 study which compared the effect of enprofylline (1.0 mg/kg) and theophylline (3.0 mg/kg). The drugs were given intravenously over a period of 10 min. Peak flow (PEF), heart rate and plasma concentrations of the drugs were determined before and after the injection. Both enprofylline and theophylline significantly increased the PEF 30 min after the injection, the mean increases over the baseline values being 21 % and 23 %, respectively. The clinical effects assessed by the patient and the physician and the significant decreases in heart rate were similar for the two drugs. Side effects were rare. Thus, 1.0 mg/kg of enprofylline was comparable to 3.0 mg/kg of theophylline in the treatment of patients with acute asthma.