Universal adenosine receptor antagonism is neither necessary nor desirable with xanthine antiasthmatics

Differential effects of various xanthines on pentylenetetrazole-induced seizures in rats: an EEG and behavioural study

The present study deals with the EEG (electroencephalogram) and behavioural effects of a subconvulsant dose (30 mg/kg i.p.) of pentylenetetrazole in freely moving rats pretreated (100 mg/kg p.o., 1 h before pentylenetetrazole) with two classic (theophylline and caffeine) and two new (enprofylline and isbufylline) xanthines. In rats treated with vehicle, pentylenetetrazole caused a slight desynchronization of the EEG, characterized by periods of 'wave discharges', and 'spike-and-wave discharge complexes'. In rats pretreated with xanthines (theophylline or caffeine) pentylenetetrazole produced a dramatic increase in ictal seizures with the appearance of continuous spikes; concomitantly animals experienced myoclonic jerks (100%) and in some cases (ca. 20%) the animals died. In contrast, in enprofylline-pretreated rats, pentylenetetrazole induced only brief periods of wave discharges and spike-and-wave discharge complexes whose duration was significantly reduced compared to that of controls, although these discharges were associated with mild epileptic behaviour. When isbufylline-pretreated rats were challenged with pentylenetetrazole, the EEG was characterized by a short run of wave discharges (whose duration was shorter than that of other groups). No enprofylline- or isbufylline-treated rats developed seizures or died. In conclusion, only xanthines with strong adenosine A1 receptor antagonism (theophylline and caffeine) markedly enhance the EEG and behavioural effects of a subconvulsive dose of pentylenetetrazole. The present experimental approach could be used to evaluate the pro-convulsive potential of new xanthine derivatives.

Pre- and postjunctional actions of purine and xanthine compounds in the guinea-pig caecum circular muscle

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.

Effects of enprofylline and theophylline may show the role of adenosine

It is well established that at low and clinically relevant concentrations theophylline (and caffeine) exerts antagonism at cell surface receptor sites for adenosine. However, it is not known which actions of theophylline are due to adenosine antagonism, because theophylline apparently activates other cellular mechanisms at the same low concentrations. Investigations into the actions of xanthines and their structure activity relationships have identified xanthine compounds like enprofylline (3-propylxanthine) that only has some actions in common with theophylline and that has a negligible ability to antagonize adenosine. Enprofylline is a more potent smooth muscle relaxant and antiasthmatic drug than theophylline but does not produce, e.g., theophylline-like diuretic effects, CNS-stimulant behavioural effects (restlessness - seizures), gastric secretory effects and release of free fatty acids. It is proposed that pharmacodynamic dissimilarities between enprofylline and theophylline may indicate physiological roles of adenosine.

Enprofylline and theophylline on small human placental arteries: studies of in vitro effects and mode of action

Vascular effects of theophylline and enprofylline, a novel xanthine derivative lacking adenosine receptor antagonism, were studied comparatively in tubular preparations of small human placental arteries mounted in an isometric myograph. Both xanthines produced concentration-dependent (10(-7)-3 X 10(-3) M) relaxation of arteries contracted by PGF2 alpha or PGE2 in both normal Ca2+-medium and in Ca2+-depleted medium. Enprofylline was about three times more potent than theophylline. Also in vasopressin-contracted arteries enprofylline was a more potent vasodilator in both media. In contrast the xanthines were equally potent in relaxation of the tonic as well as the phasic part of a K+-induced contraction, but less potent than in relaxation of PG-induced contractions. Propranolol, phentolamine, atropine, indomethacin or tetrodotoxin did not influence the xanthine relaxations. It is concluded that the theophylline-induced relaxation of small human placental arteries is not due to adenosine receptor antagonism. A common important mechanism of action, in which enprofylline is more potent than theophylline, seems to be interference with intracellular Ca2+-binding/mobilisation processes. Some decrease in cellmembrane Ca2+-permeability produced by the xanthines seems to take part in the mechanism of relaxation.

Effects of theophylline on small human placental arteries in vitro

Effects of theophylline on contractions induced by relevant physiological agents in tubular segments of small human placental arteries were measured in an isometric myograph. Theophylline 10(-7) - 3 X 10(-3)M caused marked relaxation of steady contractions produced by prostaglandin F2 alpha (PGF2 alpha), prostaglandin E2 (PGE2), vasopressin or potassium. EC50 for theophylline relaxation of the PGF2 alpha - contraction was 1.6 X 10(-4) M. Emax was 91.5% relaxation. Theophylline was less potent in relaxing the potassium induced contraction. Pretreatment experiments with 10(-4) and 10(-3)M theophylline showed a pronounced decrease in contractile Emax-values for PGF2 alpha, PGE2 and vasopressin as a possible indication of non-competitive antagonism. The results motivate further studies to elucidate possible effects of theophylline and other xanthines on the vascular resistance and blood flow in placenta.

Toxic effects of some xanthine derivatives with special emphasis on adverse effects on rat testes

Testicular toxicity and effects on thymus and body weights of 4 xanthine derivatives (D4026: 1,8-dimethyl-3-phenylxanthine, D4152: 8-methyl-3-phenylxanthine, D4160: 1,8-dimethyl-3-(2-methylbutyl)-xanthine, D4173: 8-methyl-3-(2-methylbutyl)-xanthine) were studied in Sprague-Dawley rats and cellular toxicity in human embryonal cells. The effect on toxicity by variation of substituent at positions 1 and 3 was tested. The compounds were administered orally to the rats once a day for 1 month. Mortalities were noted only with D4160. Dose related decreases in body weight gain were found for all substances, but only marginally with D4152. A significant decrease in thymus weight relative to control was observed with all substances, D4152 being the least potent. No effects on testes weights were found with any treatment but histological examination disclosed degeneration of germ producing epithelium of all rats given 100 mumol/kg of D4026 but not at 25 mumol/kg. One rat out of 5 showed testicular damage at 400 mumol/kg of D4173 or D4152. Plasma analysis for unchanged compounds showed significantly higher plasma concentrations at the high dose compared with the low dose with the exception for D4152 showing unexpectedly low levels. In the cellular toxicity test, D4160 was the most potent while D4152 was the least potent. D4026 had a steeper dose-response curve than the others but was less potent than D4160. The 1-methylated xanthine derivatives seemed to be more toxic than the two in position 1 unsubstituted analogues. Mechanisms for testicular toxicity of xanthine derivatives in the rat and clinical relevance of animal data are discussed.

Comparison of oral enprofylline and theophylline in asthmatic patients

The bronchodilator effect and side-effects of a single oral dose of enprofylline were compared with the corresponding actions of a therapeutic dose of theophylline in 20 asthmatic patients, in a randomized cross-over clinical trial. Enprofylline 4 mg/kg and theophylline 8 mg/kg produced mean maximum plasma levels of 4.40 +/- 0.91 micrograms/ml and 16.5 +/- 2.58 micrograms/ml and mean maximum increases in FEV1.0 of 38.5% and 34.8%, respectively. The degree of headache and nausea was estimated by a scoring system. Enprofylline produced significantly higher scores for headache than theophylline, and both drugs produced s light nausea. No other side-effects were seen. Enprofylline seems to have bronchodilating properties comparable to those of theophylline without producing severe side-effects.

Tracheal relaxant and cardiostimulant actions of xanthines can be differentiated from diuretic and CNS-stimulant effects. Role of adenosine antagonism?

Theophylline (1, 3-dimethylxanthine) and enprofylline (3-propylxanthine) have been examined for effects in the rat. Enprofylline was 3.8 times as potent as theophylline as a tracheal relaxant in vitro, and 1.3 times as potent as theophylline to increase the rate of isolated perfused hearts. An oral dose (5 mg/kg) of enprofylline to rats was almost completely recovered in the urine as unchanged drug, showing that this xanthine is well absorbed and negligibly metabolised. Theophylline (10 and 30 mg/kg p.o.) significantly and dose-dependently increased locomotor activity in rats whereas the same doses of enprofylline were without effect on behaviour. Theophylline ( 5-20 mg/kg p.o.) produced significant and dose-dependent natriuretic and volume diuretic effect with little augmentation of potassium excretion. Enprofylline up to 10 mg/kg was without diuretic effects. At the large dose of 20 mg/kg enprofylline decreased sodium excretion and produced some volume diuresis. It is suggested that lack of diuretic and CNS-stimulant behavioural effects by enprofylline is due to its low ability to antagonise adenosine receptor stimulation. Pharmacodynamic differences between enprofylline and the potent adenosine antagonist theophylline may indicate a functional importance of endogenous adenosine.

Xanthine derivatives as adenosine receptor antagonists

The potency of a series of xanthine derivatives as adenosine antagonists was studied in fat cells (A1-receptors) and hippocampal slices (A2-receptors) and on L-[3H]phenylisopropyladenosine (PIA) binding in membranes from rat cortex. The order of potency in all three tests systems was: diethyl-8-phenyl-theophylline greater than 8-phenyltheophylline greater than 8-p-sulfophenyltheophylline greater than verrophylline greater than isobutylmethylxanthine greater than theophylline caffeine greater than theobromine. Enprofylline was about 20 times more potent in the hippocampus system than in the other two systems.