Methylxanthines and intestinal drug absorption

Enhancement of antinociception by co-administration of ibuprofen and caffeine in arthritic rats

It has been observed that caffeine improves antinociceptive efficacy of some non-steroidal antiinflammatory drugs (NSAIDs) in several experimental models, however, these effects have been questioned in humans. Controversy in clinical studies may be due to the use of different protocols as well as to high interindividual variability in patient response. In addition, the antinociceptive interaction of ibuprofen+caffeine has not been studied. To assess a possible synergistic antinociceptive interaction, the antinociceptive effects of ibuprofen, and caffeine administered either separately or in combinations were determined in a model of arthritic pain: "Pain-induced functional impairment in the rat (PIFIR model). The antinociceptive efficacies were evaluated using several dose-response curves and time courses. The antinociceptive effects from the combination that produced the greater effect were compared with the maximal antinociceptive effect of either morphine or acetylsalicylic acid alone. The animals were administered with 0.05 ml intra-articular (i.a.) of uric acid to induce nociception. Groups of six rats received either acetylsalicylic acid, morphine, ibuprofen or caffeine, or a combination ibuprofen+caffeine (18 combinations). We report here that caffeine (17.8 and 31.6 mg/kg) is able to potentiate the antinociceptive effect of ibuprofen. This investigation showed that six combinations presented effects of potentiation and twelve combinations only showed antinociceptive effects not different from that of ibuprofen alone. The maximum antinociceptive effect was 270.7+/-12.7 area units (au), produced by ibuprofen 100 mg/kg+caffeine 17.8 mg/kg; this effect was greater than the maximum produced by morphine 17.8 mg/kg (244.7+/-22.9 au) in these experimental conditions. The maximum potentiation was 197 % produced with the combination of ibuprofen 17.8 mg/kg+caffeine 17.8 mg/kg. These results suggest that the antinociceptive effect of ibuprofen was significantly potentiated by doses of caffeine that by themselves are ineffective in this model.

Effect of caffeine on antinociceptive action of ketoprofen in rats

BACKGROUND

To assess a possible synergistic antinociceptive interaction, the antinociceptive effects of ketoprofen (KET), and caffeine (CAF) administered either separately or in combinations were determined in a model of arthritic pain.

METHODS

Antinociceptive activity was assayed using "ellipsis pain-induced functional impairment in the rat" (PIFIR model). The antinociceptive efficacies were evaluated using several dose-response curves and time courses. The antinociceptive effects from the combination that produced the greater effect were compared with the maximal antinociceptive effect of either morphine, acetylsalicylic acid (ASA), or KET alone. The animals were administered with 0.05 mL intra-articular (i.a.) of uric acid to induce nociception. Groups of six rats received orally either ASA, morphine (MOR), KET, CAF, or a combination KET + CAF (24 combinations).

RESULTS

ASA (ED(50) 465.2 +/- 1.5 mg/kg), MOR (ED(50) 71.0 +/- 1.6 mg/kg), and KET (ED(50) 7.2 +/- 1.4 mg/kg) alone induced dose-dependent antinociception, whereas CAF alone showed no activity at the assayed doses. Nine combinations showed various degrees of potentiation (p <0.01), while the remainder exhibited the antinociceptive effect of KET only. Combinations of 17.8 mg/kg CAF with either 1.0, 1.8, 3.2, 5.6, or 10.0 mg/kg KET yielded the highest antinociceptive potentiations. For example, antinociceptive effect was 125.6 +/- 21.4 area units (au) with KET (3.2 mg/kg) alone, but the combination with CAF (17.8 mg/kg) showed 309.5 +/- 10.3 au. The median effective dose (ED(50)) of KET alone was 7.2 +/- 1.4 mg/kg, whereas the ED(50) of KET + CAF 17.8 mg/kg was 0.4 +/- 0.6 mg/kg: KET in the presence of CAF was approximately 18 times more potent than the analgesic drug without CAF.

CONCLUSIONS

These results showed that CAF was able to potentiate the analgesia of KET, but only at selected dose combinations: CAF in the doses of 10.0 and 17.8 mg/kg was able to potentiate the analgesic effect of KET, the most efficacious drug combination being CAF 17.8 mg/kg + KET 3.2 mg/kg. The combination of analgesic drugs and CAF can produce better antinociceptive effects than the analgesic drug alone. This knowledge will permit the selection of the therapeutically most effective combination ratio of drugs, employing lower doses of each drug.

Effect of coadministration of caffeine and either adenosine agonists or cyclic nucleotides on ketorolac analgesia

Caffeine potentiation of ketorolac-induced antinociception in the pain-induced functional impairment model in rats was assessed. Caffeine alone was ineffective, but increased the effect of ketorolac without affecting its pharmacokinetics. Intra-articular administration of adenosine and N6-cyclohexyladenosine (CHA, an adenosine A1 receptor agonist), but not 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680, an adenosine A2A receptor agonist), significantly increased ketorolac antinociception. This effect was not local, as contralateral administration was also effective. Ipsilateral and contralateral administration of adenosine and CHA also increased antinociception by ketorolac-caffeine. Intra-articular 8-Bromo-adenosine cyclic 3',5'-hydrogen phosphate sodium or 8-Bromo-guanosine-3',5'-cyclophosphate sodium (cGMP) given ipsilaterally or contralaterally did not affect ketorolac-induced antinociception. Nevertheless, ipsilateral, but not contralateral, administration of 8-Br-cGMP significantly increased antinociception by ketorolac-caffeine, suggesting a local effect. The results suggest that caffeine potentiation of ketorolac antinociception is mediated, at least partially, by a local increase in cGMP and rule out the participation of adenosine receptor blockade.

Effect of caffeine on ibuprofen-induced gastric mucosal damage in rats

During investigations on the effect of caffeine on ibuprofen-induced gastric mucosal lesions in rats, we have found that caffeine (p.o.) inhibits the development of ibuprofen-induced gastric lesions in a dose-dependent manner (ED50 18.4 mg kg(-1)). To investigate this protective effect of caffeine, we have studied the effect of caffeine on HCl-ethanol-induced gastric mucosal lesions with or without indomethacin pretreatment. Caffeine inhibited the development of HCl-ethanol-induced gastric lesions with and without indomethacin pretreatment. These results indicate that caffeine did not act as a mild irritant but, on the contrary, had protective effects. We measured the gastric mucosal prostaglandin E2 (PGE2) concentrations and gastric mucosal blood flow, as representative protective factors for gastric mucosa. Caffeine did not affect the gastric mucosal PGE2 concentrations 4h after administration of ibuprofen. However, topical administration of caffeine resulted in an increase in gastric mucosal blood flow, as measured by laser Doppler flowmetry. We investigated the gastric acid secretion and gastric mucosal myeloperoxidase activity as representative aggressive factors for gastric mucosa. When caffeine was administered intraduodenally in pylorus-ligated rats, gastric acid secretion decreased in a dose-dependent manner, with an ED50 of 44.9 mg kg(-1). Caffeine decreased ibuprofen-induced gastric myeloperoxidase activity in a dose-dependent manner, with an ED50 of 9.1 mg kg(-1). These findings indicate that caffeine, at least in rats, may inhibit the development of acute gastric mucosal injury. The mechanisms underlying the protective actions of caffeine are unclear, but may be related in part to an increase in gastric mucosal blood flow and suppression of neutrophil activation.

The effect of caffeine on the pharmacokinetics of acetaminophen in man

The influence of caffeine (60 mg) was studied on the pharmacokinetic characteristics of acetaminophen (500 mg single dose) in ten healthy male human volunteers in a complete cross-over design. A high-performance liquid chromatography (HPLC) method was used to analyse serum drug concentrations. Caffeine caused a highly significant (p < 0.01) increase in AUC and AUMC, a significant (p < 0.05) increase in Cmax, and a significant (p < 0.05) decrease in clearance (C1/F) of acetaminophen. We conclude that caffeine taken in doses commonly available commercially or in a cup of coffee can significantly potentiate the therapeutic potential of acetaminophen in man.

The phenomenon and cause of the dose-dependent oral absorption of chlorothiazide in rats: extrapolation to human data based on the body surface area concept

The reported incomplete and dose-dependent absorption of chlorothiazide in humans was demonstrated in six rats after five oral solutions at doses of 0.93, 2.55, 9.23, 25.6, and 70.2 mg/kg. Mean 48-hr urinary recoveries of intact drug were 57.3, 50.4, 36.7, 22.8, and 15.3%, respectively. A similar degree of dose dependency in absorption was found in rat, dog, and human when the doses were related to unit body surface area (BSA) but not on unit body weight, indicating similar interspecies absorptive capacity in terms of unit BSA. This finding may be partly rationalized by marked similarities in the reported solution transit time (2-3 hr) in the small intestine as well as in the calculated gross surface area of the small intestine per unit BSA (0.163 for rat and 0.132 for human). Contrary to the previous postulation of a specific absorption site, the drug was absorbed from different regions of the GI tract with apparent 1-hr absorption rates, studied by the in situ closed-loop method, in the following rank order: jejunum (34.6%) greater than duodenum (32.7%) greater than large intestine (20.1%) greater than ileum (18.0%) greater than stomach (12.4%). Different from the commonly assumed first-order absorption process, the intestinal loop absorption was concentration-dependent, suggesting a saturable mechanism. For example, the absorption rate at 0.008 mg/mL was higher than that at 0.2 mg/mL in ileal loops (61%, p less than 0.01) and jejunal loops (22%, p less than 0.1). In addition, the absorption rates at pH 6 and 7.4 were statistically identical, indicating a lack of ionization effect that is important in the passive absorption process. The solubility-limited absorption could probably be ruled out at doses below 2.55 mg/kg for rat and 125 mg for human in view of higher aqueous solubilities at 37 degrees C (e.g., 1.3 mg/mL at pH 7) found in the present study. Contrary to the previous hypothesis of low membrane permeability as a limiting factor for absorption, the "intrinsic" partition coefficient in 1-octanol/aqueous buffer was moderate, 0.6. Furthermore, the absorption in ileal and jejunal loops was enhanced by an apparent increase in mesenteric blood flow by caffeine. The existence of prolonged oral absorption in rats and humans is discussed.

Enhanced gastrointestinal absorption of drugs in rats pretreated with the synthetic immunomodulator, levamisole

The effect of levamisole on drug absorption from the rat small intestine has been investigated by means of an in-situ recirculation technique. The absorption of salicylic acid, sulphanilamide and aminopyrine was significantly increased by the intraperitoneal administration of levamisole (2 mg) 1 day before the absorption studies, but there was no significant effect on absorption from the small intestine of indomethacin, bromthymol blue, sulphafurazole (sulfisoxazole), quinine, sulphanilic acid, phenol red (phenolsulfonphthalein), L-tryptophan and fluorescein isothiocyanate-dextrans. The effect of levamisole on the absorption from the small intestine of salicylic acid was marginally dose- and time-dependent, the maximal effect being observed after pretreatment with 2 mg of levamisole 1 day before the absorption studies. Sulphanilamide, similarly, was better sorbed from the small intestine and also from the stomach in the presence of levamisole. The intraperitoneal administration of levamisole may influence the absorption of some low molecular weight drugs from the gastrointestinal tract.

Influence of caffeine on aspirin pharmacokinetics

The effects of caffeine on the pharmacokinetics and bioavailability of aspirin were studied in 12 healthy adult male volunteers. The subjects received 650 mg of aspirin or 650 mg of aspirin with 120 mg of caffeine citrate orally. It was found that caffeine significantly increased the rate of appearance as well as the maximum concentration of salicylate in the plasma by about 25% and 17%, respectively. Moreover, area under the plasma concentration-time curve of salicylate was significantly higher in the subjects given the drug combination as compared to those given aspirin alone. There was no change in the plasma half-life, volume of distribution and clearance of salicylate.

Effect of systemic anaphylaxis on the absorption of salicylic acid from the rat small intestine

Rats were immunized by intraperitoneal injection of ovalbumin (egg albumin) emulsified in Freund's incomplete adjuvant, and then the effect of an intravenous challenge with ovalbumin on salicylic acid absorption from the small intestine was examined by means of an in situ recirculation technique. The disappearance of salicylic acid from the luminal solution was significantly decreased in rats treated with ovalbumin compared with control groups treated with saline. The decreased absorption of salicylic acid in ovalbumin-immunized rats was related to the anti-ovalbumin antibody responses examined by passive cutaneous anaphylactic reactions. On the other hand, the decreased absorption of salicylic acid was not found in ovalbumin-immunized rats challenged intravenously with bovine gamma-globulin. Similar results were also noted in rats immunized via the footpads with ovalbumin. However, no significant change was observed in the intestinal absorption of salicylic acid in normal (nonimmunized) rats challenged intravenously with ovalbumin. Furthermore, intestinal absorption of sulfadimethoxine and sulfanilamide was significantly decreased during systemic anaphylaxis, whereas no change was observed in the absorption of sulfisoxazole, quinine, sulfanilic acid, and phenolsulfonphthalein. This suggests that the intestinal absorption of rapidly absorbed drugs, including salicylic acid, is more sensitive to systemic anaphylaxis than that of poorly absorbed drugs.

Effect of caffeine on ergotamine absorption from rat small intestine

The effect of caffeine on the absorption of ergotamine from the rat small intestine was studied. The results of a series of experiments showed that caffeine significantly enhanced absorption of ergotamine from solutions of pH 5.0 when both substances were in solution and when an intact blood supply was either absent (in vitro everted sac experiments) or present (in situ experiments). Caffeine did not appear to influence the absorption rate of ergotamine in situ experiments when the solution pH was 3.0. Isosorbide dinitrate, a vasodilator, enhanced ergotamine absorption when both substances were administered simultaneously into intestinal loops in situ. Isosorbide dinitrate probably exerts its effect by increasing blood flow to the intestine. The results are consistent with an hypothesis that the rate-determining step in ergotamine absorption is the transport of the drug from a lipid phase (GI membrane) into an aqueous phase (blood). Caffeine is thought to exert its rate-accelerating effect by increasing the water solubility of ergotamine neutral molecules.

[Reciprocal action between caffeine, other stimulants and drugs]

Caffeine is ingested not only with beverages as coffee, tea, coca-cola but also in form of many analgetic drugs. Therefore interactions of this substance with other biologically active substances and drugs should be expected, and the knowledge of these would be of practical importance. The interactions between caffeine and alcohol, smoking, salicylic acid, phenacetin, barbiturates, and theobromine are described.

Inhibition by morphine of prostaglandin E1-stimulated secretion and cyclic adenosine 3',5'-monophosphate formation in the rat jejunum in vivo

1 The effects were studied of prostaglandin E1 (PGE1), theophylline and morphine on net water flux and mucosal cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels in the jejunum of anaesthetized rats in vivo. 2 Infusion of PGE1 (3.2 micrograms/min, i.a.) caused a reversal from net water absorption to net secretion and enhanced the mucosal cyclic AMP content by 54%. 3 Theophylline (5 mg/ml, intraluminal) similarly produced a reversal from net water absorption to net secretion and increased mucosal cyclic AMP content by 54%. Additional intra-arterial infusion of PGE1 resulted in a massive increase in net water secretion and an increase in mucosal cyclic AMP content by about 200%. 4 Pretreatment with morphine (10 mg/kg, s.c.) reduced the effect of PGE1 on net water flux and completely inhibited its effect on the mucosal cyclic AMP content. Naloxone (10 mg/kg, s.c.) abolished both effects of morphine. 5 A good correlation (r = 0.99) was demonstrated between mucosal cyclic AMP levels and net water flux. 6 The present results demonstrate that PGE1 stimulates intestinal fluid secretion by increasing mucosal cyclic AMP levels. The antidiarrhoeal effect of morphine can be explained by its inhibition of the PGE-mediated increase in cyclic AMP levels, which, in turn, leads to a reduction in intestinal secretion.

PGE-release, blood flow and transmucosal water movement after mechanical stimulation of the rat jejunal mucosa

1. The influence of weak mechanical stimulation of the jejunal mucosa in vivo on PGE-release, on intestinal blood flow and transmucosal water movement was studied in rats. 2. Mechanical stimulation of the mucosa increased PGE-release into the venous outflow and into the gut lumen. This increase is followed by an increase in intestinal blood flow and transmucosal movement of tritiated water in both directions. 3. Pretreatment of the rat with indomethacin reduced the effect of mechanical stimulation on PGE-release. Indomethacin further reduced the increase in blood flow and in secretion of tritiated water. Absorption of tritiated water was not changed in these experiments. 4. Pretreatment of the rat with atropine (1 mg/kg, i.p.) or perfusion of the gut with methysergide (10 microgram/ml) did not influence the increase in intestinal blood flow after mechanical stimulation. 5. It is suggested that enhanced intestinal blood flow and transmucosal movement of tritiated water after mechanical stimulation are mainly provoked by a preceding release of PGE. 6. It is further supposed that such a mechanism may be physiologically involved in regulation of intestinal blood flow and transmucosal water movement during food intake.

Effects of PGE2 and colchicine on the intestinal fluid volume

a) Colchicine, prostaglandin E2 (PGE2) and carbachol increase the intestinal fluid volume in the rat. b) The effect of carbachol and prostaglandin E2 is augmented whereas that of colchicine is diminished in the pithed rat. c) The effect of colchicine is diminished in anaesthesia whereas that of PGE2 remains unchanged. d) The effect of PGE2 and of colchicine are both inhibited by atropine. e) The effects of PGE2 and of colchicine are both enhanced by phentolamine. f) Pretreatment with indomethacin decreased the effect of colchicine whereas that of PGE2 remained unchanged. g) The PGE2-induced increase in intestinal fluid volume may partly be mediated by a peripheral mechanism involving acetylcholine receptors. h) The increase in intestinal fluid volume by colchicine can mainly be explained by a central action of the drug which is mediated by cholinergic neurones.

[Coffee and health]

Coffee as a rule develops stimulating effects on the central nervous system, heart and circulation which are mainly caused by caffeine. In certain cases coffee may also have a sedative effect and sometimes even it is useful to fall asleep quickly. Furthermore coffee may be advantageous in the treatment of some functional disorders caused by lacking of dopamine, because coffee is able to increase the dopamine formation in brain. Concerning the effects of coffee in the gastrointestinal-tract and liver-bile system caffeine is only of secondary importance. Hereby certain roasting substances, possibly also chlorogenic acid or caffeic acid should be responsible for the stimulating effects observed in these organs. These stimulating effects could be caused whether directly or indirect e.g. by liberating gastrin or other gastrointestinal hormones. Vitamin niacin, which is formed in greater amounts from trigonelline during the roasting process, may also be important from the nutritional standpoint. Therefore coffee may be prescribed as a true drug in cases of deficiency in vitamin niacin or also in the pellagra disease. By extensive epidemiological studies performed lately it could be demonstrated that there exists no correlation between coffee consumption and certain risk factors as hypertension, heart infarction, diabetes, gout or cancer diseases. Furthermore there was no evidence that coffee or its caffeine content are able to induce genetic alterations or even malformations.

The function of prostaglandins in transmucosal water movement and blood flow in the rat jejunum

1. Jejunal loops of anaesthetized rats were perfused with isotonic buffer containing PGE1, PGF2alpha or indomethacin. Intestinal blood flow, absorption and secretion of tritiated water were measured. 2. PGE1 at the low concentration of 0.1 microgram ml-1 did not influence intestinal blood flow but increased secretion and decreased absorption of tritiated water. In higher concentrations (0.5 and 6.5 microgram ml-1), blood flow, secretion and absorption were enhanced. 3. PGF2alpha, even in the high concentration of 50 microgram ml-1, did not influence intestinal blood flow but enhanced secretion and decreased absorption of tritiated water. 4. Indomethacin (1 microgram ml-1) decreased intestinal blood flow and secretion but enhanced absorption of tritiated water. 5. The effects of indomethacin on blood flow can be prevented and those on secretion can be even reversed by an additional infusion of PGE1 (0.5 microgram ml-1). 6. PGs appear to play a physiological role in the regulation of intestinal blood flow and transmucosal water movement, since inhibition of endogenous PG synthesis by indomethacin results in effects opposite to those of intraluminally applied PGE1. The results obtained with the low concentration of PGE1 (0.1 microgram ml-1) and with PGF2alpha (50 microgram ml-1) strongly indicate that intestinal water movement can be changed independently of intestinal blood flow.