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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.

Whether organic anion and cation transporters are involved in the renal excretion of xanthine derivatives, 3-methylxanthie and enprofylline, remains unclear. In this study, we have investigated the effects of typically predominant substrates for organic anion and cation transporters on the tubular secretion of 3-methylxanthine and enprofylline in rats. In the renal clearance experiments using typical substrates for organic anion transporters, probenecid and p-aminohippurate, probenecid (20 mg/kg), but not p-aminohippurate (100 mg/kg), significantly decreased the renal clearance and clearance ratio of 3-methylxanthine and enprofylline. The typical substrates for organic cation transport systems, tetraethylammonium (30.6 mg/kg) and cimetidine (50 or 100 mg/kg), significantly decreased the renal clearance and clearance ratio of 3-methylxanthine and enprofylline. These results suggest that the renal secretory transport of 3-methylxanthine and enprofylline are mediated by probenecid-, cimetidine- and tetraethylammonium-sensitive transport systems. Uric acid, an organic anion, significantly inhibited the renal secretion of 3-methylxanthine, but not enprofylline, suggesting that the renal tubular transport of 3-methylxanthine is also mediated via uric acid-sensitive transport system. These findings suggest the possibility that both organic anion and cation transporters are, at least, involved in the renal tubular transport of 3-methylxanthine and enprofylline in rats.

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.

Ischemic preconditioning or phosphodiesterase inhibition improves lung protection during prolonged hypothermic storage. In ischemic preconditioning of cat lungs, adenosine A1 receptor antagonism was suggested as a possible mechanism. Some phosphodiesterase inhibitors (such as theophylline) are also adenosine antagonists; we showed theophylline to be particularly effective in protecting lungs. In isolated, perfused and ventilated rat lungs, we examined (1) whether synergy exists between phosphodiesterase inhibition and ischemic preconditioning and (2) whether theophylline acts both to inhibit phosphodiesterase and block adenosine receptors, by comparing its effects with enprofylline (selective phosphodiesterase inhibition) or xanthine amine congener (selective adenosine A1 receptor antagonism). In Study 1, rolipram (added to St Thomas' cardioplegia) or ischemic preconditioning before hypothermic storage (8 h) did not improve lung function during reperfusion (40 min); a combination of these treatments was also ineffective. In Study 2, lungs stored in St Thomas' cardioplegia containing enprofylline or theophylline had improved recovery of function compared to control lungs; however, xanthine amine congener was without effect. Thus, no interaction exists between phosphodiesterase inhibition and ischemic preconditioning. Adenosine A1 receptor antagonism plays no role in protecting rat lungs from the effects of prolonged hypothermic storage by either preconditioning or addition of theophylline to the storage solution.

The antiasthmatic drug enprofylline was the first known selective, though not potent, A(2B) antagonist. On the basis of structure-activity relationships (SARs) of xanthine derivatives, we designed a novel selective adenosine A(2B) receptor antagonist, 3-isobutyl-8-pyrrolidinoxanthine (IPDX), with potency greater than that of enprofylline. IPDX displaced [3H]ZM241385 ([3H]4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a]-[1,3,5]triazin-5-ylamino]ethyl)phenol) from human A(2B) adenosine receptors with a K(i) value of 470 +/- 2 nM and inhibited A(2B)-dependent cyclic AMP (cAMP) accumulation in human erythroleukemia (HEL) cells with a K(B) value of 625 +/- 71 nM. We found that IPDX was more selective than enprofylline toward human A(2B) receptors. It was 38-, 55-, and 82-fold more selective for human A(2B) than for human A(1) (K(i) value of 24 +/- 8 microM), human A(2A) (K(B) value of 36 +/- 8 microM), and human A(3) (K(i) value of 53 +/- 10 microM) adenosine receptors, respectively. IPDX inhibited NECA (5'-N-ethylcarboxamidoadenosine)-induced interleukin-8 secretion in human mast cells (HMC-1) with a potency close to that determined for A(2B)-mediated cAMP accumulation in HEL cells, thus confirming the role of A(2B) adenosine receptors in mediating human mast cell activation. Since adenosine triggers bronchoconstriction in asthmatic patients through human mast cell activation, IPDX may become a basis for the development of new antiasthmatic drugs with improved properties compared with those of enprofylline. Our data demonstrate that IPDX can be used as a tool to differentiate between A(2B) and other adenosine receptor-mediated responses.

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.

Biochemical changes such as intracellular cAMP and Ca(2+) underlying morphine dependence and tolerance have been suggested. Therefore, we investigated the effects of nefiracetam (N-(2, 6-dimethyl-phenyl)-2(2-oxo-1-pyrrolidinyl) acetamide), which increases intracellular cAMP and Ca(2+) levels, on the development of morphine dependence and tolerance. Mice administered morphine (6 or 20 mg kg(-1)) twice daily for 5 days, showed withdrawal symptoms (jumping, diarrhea and body weight loss) after naloxone challenge (5 mg kg(-1)), indicating morphine dependence. Furthermore, tolerance to the analgesic effect of morphine was observed in these mice. Co-administration of nefiracetam (5 or 10 mg kg(-1)) with morphine during the pretreatment period, significantly reduced the signs of withdrawal symptoms, moreover, the tolerance was significantly attenuated. Elevation of cAMP levels in the cortex was observed in morphine-dependent mice, but not in mice co-administered nefiracetam. Acute administration of nefiracetam shows no effect on the withdrawal symptoms and the analgesic effect in morphine-naive mice. Theophylline (3 or 10 mg kg(-1)) tended to attenuate and enprofylline (10 or 30 mg kg(-1)) significantly attenuated the development of morphine dependence and tolerance. These findings suggest that co-administration of nefiracetam or compounds, which increase the cAMP level, may be a useful strategy for attenuating the development of morphine dependence and tolerance in the clinic.

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

Noise-exposure makes non-sensitized guinea pigs hyporesponsive to Acetylcholine (Ach), while in Ovalbumin (OA)-sensitized guinea pigs the responsiveness to the cholinergic mediator is not modified by acoustic stress ( Nieri et al., 1996 ). The occurrence of bronchial hyporesponsiveness after acoustic stress in non-sensitized guinea pigs was verified also with histamine, obtaining a result similar to that observed with Ach. Moreover, the role of adenosine as modulator of the bronchial responsiveness to Ach after noise-exposure was assessed both in normal and in sensitized guinea pigs. In non-sensitized noise-exposed guinea pigs, the hyporesponsiveness to Ach was abolished by pretreatment of the animals with the peripheral A 1 /A 2 antagonist 8- p -(sulfophenyl)theophylline (8-pSPT, 3 mg/kg i.v.) or with the A 2 -selective blocker 3,7-dimethyl-1-propargylxanthine (DMPX, 80 μ g/kg i.v.) but not with the A 1 -selective antagonist Xanthine Amine Congener (XAC, 0.1 mg/kg i.v.). In sensitized guinea pigs, pretreatment with theophylline (25 mg/kg i.v.) makes noise-exposed animals again hyporesponsive to Ach, while no effect was obtained with the selective A 1 and A 2 antagonists employed. Also enprofylline (10 mg/kg i.v.), a phosphodiesterase inhibitor more potent than theophylline, does not modify the responsiveness to Ach in sensitized noise-exposed guinea pigs. The overall data presented suggest the involvement of the peripheral purinergic system in the regulation of airway reactivity after the stressful condition and indicate an altered functionality of this system as a consequence of sensitization. Furthermore, noise-exposure makes it possible to reveal in guinea pigs an opposite influence by theophylline on airway responsiveness to Ach, in sensitized, with respect to normal, animals.

Of the four G protein coupled adenosine receptor (AR) subtypes, the A 1 is best suited for studies of reconstitution with G proteins. Recombinant A 1 receptors extended with hexahistidine and FLAG have been purified to near homogeneity. In reconstitution assays using pure recombinant G protein subunits, the composition of the γ subunit influences coupling to purified A 1 ARs. The least well characterized AR is the A 2B . New data indicate that A 2B ARs can trigger the degranulation of canine and human mast cell lines. Recombinant human A 2B ARs are blocked by the anti-asthma drugs theophylline and enprofylline at concentrations that are used therapeutically to treat asthma. Although A 2B ARs have long been known to stimulate adenylyl cyclase, they also can activate phospholipase C and mobilize Ca 2+ by signaling through Gq/11. There is great potential for new therapies based on compounds that selectively target individual AR subtypes.

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

Previously, we demonstrated that a single intravenous injection of theophylline can induce recovery in a hemidiaphragm paralyzed by cervical (C2) spinal cord hemisection for up to 3 h. The present study contrasts the actions of enprofylline and theophylline on inducing hemidiaphragmatic recovery after cervical spinal cord hemisection. Both drugs are methylxanthines; however, theophylline is an adenosine receptor antagonist while enprofylline is not. To further test the involvement of adenosine receptors, N6 (L-2-phenylisopropyl) adenosine (L-PIA), an analogue of adenosine was used in conjunction with theophylline. Following a left C2 spinal cord hemisection, animals were injected with either enprofylline (2.5-20 mg/kg) or theophylline (15 mg/kg) alone or in combination. Theophylline-injected animals demonstrated robust respiratory-related activity in the previously quiescent left phrenic nerve and hemidiaphragm. No recovery was observed in any of the enprofylline-injected rats. When enprofylline injection was followed later with theophylline, recovery occurred. Prior L-PIA administration blocked theophylline-induced recovery. When given after theophylline, L-PIA attenuated and then blocked the induced activity in both the nerve and hemidiaphragm ipsilateral to spinal cord hemisection. We conclude that adenosine receptor antagonism is implicated in hemidiaphragmatic recovery after hemisection and theophylline may be useful in the treatment of spinal cord injured patients with respiratory deficits.

The pharmacokinetic characteristics, peritoneal permeability and hydrophobicity of three xanthine derivatives, theophylline, enprofylline and 1-methyl-3-propylxanthine (MPX), were investigated in rats. Isotonic saline (30 ml) containing xanthine (2.5, 5 and 10 mg/kg) and blue dextran (0.2%) was administered intraperitoneally. The pharmacokinetic parameters of these xanthines were estimated using concentration-time data obtained from the peritoneal cavity and systemic circulation. Disappearance of these xanthines from the peritoneum declined in almost a monoexponential manner regardless of the dose administered. The volume of distribution (33.9 ml) in the peritoneal cavity was similar to the injection volume, indicating that dialysate was not diluted by the fluid in the peritoneal cavity and the effect of drug adsorption on the peritoneal membrane was minimal. The pharmacokinetics of MPX was dose-dependent, but that of theophylline and enprofylline was not. The fraction of the administered dose absorbed through the peritoneal cavity was 0.71, 0.85, 0.93 for theophylline, enprofylline and MPX, respectively. The peritoneal clearance was significantly different (p < 0.05) among the three xanthines by two-way analysis of variance, and a strong correlation was noted between their peritoneal clearance and hydrophobicity (r = 0.98, p < 0.01). These findings suggest that hydrophobicity is an important determinant in the peritoneal permeation of these xanthines.

Adenosine potentiates mast cell activation, but the receptor type and molecular mechanisms involved have not been defined. We, therefore, investigated the effects of adenosine on the human mast cell line HMC-1. Both the A2a selective agonist CGS21680 and the A2a/A2b nonselective agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased cAMP, but NECA was fourfold more efficacious and had a Hill coefficient of 0.55, suggesting the presence of both A2a and A2b receptors. NECA 10 microM evoked IL-8 release from HMC-1, but CGS21680 10 microM had no effect. In separate studies we found that enprofylline, an antiasthmatic previously thought to lack adenosine antagonistic properties, is as effective as theophylline as an antagonist of A2b receptors at concentrations achieved clinically. Both theophylline and enprofylline 300 micro completely blocked the release of IL-8 by NECA. NECA, but not CGS21680, increases inositol phosphate formation and intracellular calcium mobilization through a cholera and pertussis toxin-insensitive mechanism. In conclusion, both A2a and A2b receptors are present in HMC-1 cells and are coupled to adenylate cyclase. In addition, A2b receptors are coupled to phospholipase C and evoke IL-8 release. This effect is blocked by theophylline and enprofylline, raising the possibility that this mechanism contributes to their antiasthmatic effects.

The effect of various phosphodiesterase (PDE) inhibitiors on anti-CD3 induced interleukin-(IL) -4 and IL-5 production of the murine T helper cell clone of type 2 phenotype D10.G4.1 (D10) has been investigated in vitro. D10 cells were incubated in the presence of drugs and anti-CD3 mAb for 16 h before measurement of cytokines in the cell supernatants by ELISA. Whereas all PDE inhibitors tested exerted minimal effects on anti-CD3 induced IL-4 production, a marked increase in IL-5 production by the non-selective PDE inhibitors IBMX, theophylline and enprofylline was observed. The action of these non-selective PDE inhibitors was mimicked by the PDE IV-selective inhibitor rolipram and in part by the PDE III-selective inhibitors motapizone and milrinone, whereas the PDE V-selective inhibitor zaprinast was inactive. Rolipram and motapizone enhanced IL-5 production in a synergistic fashion. In support of the functional importance of PDE III and IV for IL-5 synthesis in intact murine D10 cells, we have found PDE III and IV to be the predominant isoenzyme activities in corresponding cell lysates. The stimulatory effect of rolipram on IL-5 production was almost totally reversed by the protein kinase A inhibitor KT-5720. In addition, the membrane-permeable cAMP analogue 8-bromo-cAMP mimicked the stimulatory effect of PDE inhibitors on IL-5 production while leaving IL-4 levels unaffected. Both results support the view that the action of the PDE inhibitors on murine D10 cells is mediated via an elevation of intracellular cAMP.

Cyclic nucleotide phosphodiesterases form a group of enzymes that catalyse the breakdown of the intracellular second messengers cyclic AMP and cyclic GMP. Inhibitors of these enzymes, such as theophylline and enprofylline, are standard agents in the therapy of bronchial asthma but are limited in their usefulness by their poor potency and frequent adverse effects.

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.

Time-dependent changes in the pharmacokinetics and renal handling of enprofylline induced by bacterial endotoxin (Klebsiella pneumoniae LPS) were investigated in rats. To evaluate the early effect of LPS on kidney functions and the renal excretion of enprofylline, which is an organic anion drug excreted primarily by an active tubular secretion, LPS (250 micrograms/kg) was infused for 5 min under constant infusion at rates of 2.3 and 23 micrograms/min/kg for inulin and enprofylline, respectively. LPS caused a drop in the glomerular filtration rate (GFR), estimated as the renal clearance of inulin, to 65-75% of that observed in the control rats within 30 min after the LPS treatment. The renal clearance (CLr) of enprofylline decreased in conjunction with GFR, while the percentage of decrease in the CLr was slightly greater than that in GFR. LPS-induced decreases in the CLr for enprofylline and GFR continued over the testing period of 120 min. The time-dependent effect of LPS on the pharmacokinetics of enprofylline was examined by a single injection of enprofylline (2.5 mg/kg) to rats pretreated 2, 10 or 24 h earlier with or without LPS. The pharmacokinetic parameters of enprofylline were determined by a model-independent method. Significant changes in the systemic clearance for enprofylline were observed in rats pretreated 2 and 10 h earlier with LPS, but no such changes were observed in rats pretreated 24 h earlier with LPS. These findings indicate the existence of a time-dependent effect of LPS on the pharmacokinetics of enprofylline, and suggest that LPS at a dose of 250 micrograms/kg, at least, does not induce cytotoxicity to kidney cells.

Background Enprofylline is a new xanthine derivative that shares theophylline's bronchodilator properties but is free of theophylline's adenosine receptor antagonist activity. We compared the long-term efficacy and tolerability of enprofylline and theophylline given over a 1-year period to adults with asthma. Methods Patients were recruited from 18 centers and 4 countries to participate in a 1-month double-blind comparison of enprofylline or theophylline in the treatment of asthma and were subsequently maintained on a regimen with the assigned medication for a further 11 months. The dosage of each xanthine was incremented from 150 mg twice daily at initiation to 300 and later 450 mg twice daily depending on the patient's tolerance and, in the case of theophylline, the rapidly assayed serum theophylline level. Patients kept a diary in which they recorded peak expiratory flow rate (PEFR) measured morning and evening, asthma symptom score, and the number of β 2 -agonists taken. Spirometry was checked at clinic visits at 3, 6, 9, and 12 months following randomization. Results Three-hundred forty-eight patients (174 enprofylline, 174 theophylline) participated in the trial. For both drugs there were significant improvements in PEFR and FEV 1 during the first month of treatment with no significant difference between drugs (0.25 L for enprofylline vs 0.30 L for theophylline). Similarly, there were no differences in clinical outcome such as asthma exacerbations or β 2 -agonist usage between the two groups over follow-up. However, inhaled steroid dosage was more likely to have been incremented in theophylline-treated than enprofylline-treated patients (18% vs 8%, p=0.025). Both drugs produced a modest increase in heart rate throughout the trial (approximately 5 beats/min). In 31 patients (26 enprofylline, 5 theophylline), asymptomatic elevations in aspartate aminotransferase and/or alanine aminotransferase occurred at least once during the study. In five patients (four enprofylline, one theophylline), the increase exceeded three times the upper limit of the normal range. In some subjects receiving enprofylline, serum enprofylline levels rose progressively despite constant oral dosage of the drug. Conclusions: Long-term xanthine therapy is well-tolerated by most adult asthmatics. However, long-term enprofylline administration may be associated with elevation in liver enzyme levels and unpredictable blood levels, thereby limiting its clinical usefulness.

We evaluated a rapid monoclonal antibody theophylline assay for two reasons: (a) to determine its specificity with respect to the possible confounding influence of a structurally related xanthine, enprofylline, and (b) to assess its accuracy relative to high-performance liquid chromatography (HPLC). Blood samples were taken from 233 patients who had been randomized in double-blind fashion to receive either oral theophylline (n = 117) or enprofylline (n = 116) for the treatment of chronic reversible obstructive airways disease. Monoclonal antibody assays (MAAs) were performed in 10 clinical sites by 10 trained paramedical technicians. Three patients, who actually received enprofylline but not theophylline, had MAA theophylline values of > or = 3.2 micrograms/ml, giving a specificity of 97%. HPLC determination of simultaneous blood samples confirmed that theophylline levels were in fact < 3.2 micrograms/ml and that theophylline was not being taken surreptitiously. Good correlation was observed between MAA and HPLC in patients taking theophylline (y = 1.07 x + 0.36; r = 0.93; standard error of the estimate (SEE) = 1.93). However, there was wide variability from technician to technician such that r values for individual sites ranged from 0.67 to 0.99. Based on the overall correlation, the prediction of an individual HPLC value from an individual MAA value had broad 95% confidence limits: when the MAA value was 10 micrograms/ml, the predicted HPLC value was 9.19 +/- 3.32; when MAA = 15 micrograms/ml; HPLC = 13.19 +/- 3.33; and when MAA = 20 micrograms/ml; HPLC = 17.19 +/- 3.36.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of ageing on the pharmacokinetics, renal handling and protein binding of enprofylline were investigated in 6-, 13- and 18-month-old male Fischer 344 rats. Concentrations of enprofylline in plasma and urine were determined by HPLC, and pharmacokinetic parameters were estimated by model-independent methods. No significant differences in the volume of distribution, systemic clearance of enprofylline or urinary recovery of unchanged enprofylline (> 85%) were observed among any of the groups of rats. The dissociation constant and free fatty acid concentration in plasma increased with age. Age-dependent decreases in the systemic clearance for unbound drug were observed, and the volume of distribution for unbound drug tended to decrease with age. The ratio of systemic clearance for unbound drug to the glomerular filtration rate (GFR) decreased with ageing. Ageing was associated with decreases in the apparent maximum capacity of transport (Vmax) (223·33,160·24 and 142·98 μg min−1 kg−1 for 6-, 13- and 18-month-old rats, respectively) and in the tubular secretory intrinsic clearance (Vmax/Km) of enprofylline (75·45, 51·03 and 44·13 mL min−1 kg−1, respectively), while a slight change in the Michaelis-Menten constant (Km) was observed. These results indicate that the mechanism responsible for age-related changes in the disposition and renal handling of enprofylline may be responsible for a decrease in the ability of the tubular anion transport system.

The effects of the new nephroprotective drug N-benzoyl-β-alanine (BA) on the disposition and renal excretion of the bronchodilator enprofylline, which is actively secreted in urine, were investigated in rats. Enprofylline was administered intravenously at a dosage of 2·5 mg kg−1 under three different steady-state plasma BA concentrations (100,200 and 400 μg mL−1) which were achieved by constant infusion rates. Pharmacokinetic parameters for both total and unbound enprofylline were estimated by model-independent methods. The presence of BA (400 μg mL−1) increased the systemic clearance by 25% and the volume of distribution at steady-state by 90%. A significant increase in the dissociation constant, which is the protein binding parameter of enprofylline was observed in the presence of BA (400 μg mL−1), indicating that BA competitively inhibits the protein binding of enprofylline. However, BA significantly decreased the systemic clearance and volume of distribution for unbound enprofylline. These results suggest that BA, the organic anion transport inhibitor, inhibits renal excretion of enprofylline with a high affinity for renal tubular secretion, although the unbound concentration of enprofylline increases with administration of BA. We conclude that BA decreases the renal tubular secretion of enprofylline probably by reducing the affinity of the tubular transport system, and that these changes have marked effects on the pharmacokinetic behaviour of enprofylline.

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.