Loperamide. Survey of studies on mechanism of its antidiarrheal activity.

In castor oil challenged rats, low doses of loperamide inhibit diarrhea and normalize intestinal propulsion. Unlike other opioids, loperamide is devoid of central opiate-like effects, including blockade of intestinal propulsion, up to the highest subtoxic oral dose. Nevertheless, the antidiarrheal action of loperamide can be considered to be mu-opiate receptor mediated, only a few in vitro effects at rather high concentrations being not naloxone-reversible. There is little evidence that interactions with intestinal opiate receptors directly change epithelial cell function. When secretory stimuli increase mucosal tension, however, loperamide may reverse the elevated hydrostatic tissue pressure that opposes normal absorption. This antisecretory effect at the mucosal level is accompanied by motor effects when loperamide reaches the myenteric mu-opiate receptors. At therapeutic doses for the treatment of acute diarrhea, it is likely that the mucosal effect prevails.

Normalization of small intestinal propulsion with loperamide-like antidiarrheals in rats.

Gastrointestinal propulsion and the presence of diarrhea were assessed in rats pretreated with various opioids and challenged orally with either castor or paraffin oil, which both contained phenol red as a marker of gastrointestinal propulsion. In solvent-pretreated rats, diarrhea was always observed within 90 min after castor oil, reflecting a state of hyperpropulsive activity of the gut, but never (up to 8 h) after paraffin oil, reflecting normal intestinal propulsion (which amounted to an average distance of 91% of the total length of the small intestine in 90 min). Paraffin oil propulsion was blocked (to values less than 60%) by all opioids tested with the exception of the gut-selective compounds loperamide, loperamide oxide and fluperamide oxide (ED50s: greater than or equal to 160 mg/kg). Castor oil diarrhea was antagonized by all opioids tested and, at comparable but slightly (1.3-2.6 times) higher doses, propulsion was normalized to values (less than 100%) comparable to those measured in paraffin oil-challenged control rats. Castor oil propulsion was further reduced to subnormal values (less than 60%) by still higher doses of the opioids, comparable to those that blocked propulsion after paraffin oil. However, the required dose increment varied consistently among the opioids tested and ranged, depending on gut selectivity, from a factor 2.3 times the antidiarrheal dose for narcotic analgesics such as pethidine and dextromoramide to greater than 300 for antidiarrheals such as loperamide, loperamide oxide and fluperamide oxide. Protection from diarrhea and normalization of propulsion showed a close correlation; both failed to correlate with central analgesic activity and are thought to be mediated via peripheral opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Is in vivo dissociation between the antipropulsive and antidiarrheal properties of opioids in rats related to gut selectivity?

The antipropulsive activity of a series of opioids in the charcoal test was compared with their antidiarrheal activity in the castor oil test and their analgesic activity in the tail withdrawal test. The obtained antipropulsive/antidiarrheal potency ratios varied from 0.71 to greater than 552 [pethidine (oral ED50's in mg/kg: 21.5/30.2), fentanyl (0.77/0.49), dextromoramide (5.39/2.90), methadone (14.2/6.38), codeine (98.4/10.8), morphine (56.6/5.21), diphenoxylate (8.15/0.54), nufenoxole (74.7/1.72), difenoxin (7.10/0.16), loperamide oxide (greater than 160/0.34) and loperamide (greater than 160/0.29)]. The above ratios correlated with the gut selectivity of the compounds as defined by their analgesic/antidiarrheal potency ratios (r = 0.92, P less than 0.001). Furthermore, inhibition of propulsion was found to correlate with central analgesic activity (r = 0.93, P less than 0.001) but not with protection from diarrhea (r = 0.023, P greater than 0.05). Indeed, gut-selective opioids such as loperamide and loperamide oxide failed to affect propulsion up to doses more than 450 times their antidiarrheal doses. In contrast, alpha 2-adrenoceptor agonists delayed propulsion at doses comparable to their antidiarrheal doses [clonidine (0.085 vs 0.021), lidamidine (2.35 vs 1.66)] and anticholinergics inhibited propulsion even at doses many times below their antidiarrheal doses [atropine (0.26 vs 9.30), dexetimide (0.13 vs 5.03) and isopropamide (0.78 vs 74.6)]. The present results indicate that the in vivo inhibition of gastrointestinal propulsion by opioids in rats is mediated by a central action. Effects on intestinal fluid transport or, alternatively, on motility events distal to the ileocecal junction rather than effects on propulsion through the small intestine, seem to be the primary mechanism of antidiarrheal action of gut-selective opioids such as loperamide and loperamide oxide.

Delay of castor oil diarrhoea in rats: a new way to evaluate inhibitors of prostaglandin biosynthesis.

Forty-four non-steroidal anti-inflammatory compounds were tested for possible effects on castor oil-induced diarrhoea in rats. A small but significant delay of intestinal evacuations was found with all compounds. Quantitatively, the oral doses required to delay diarrhoea beyond the first hour after castor oil challenge reflected the acute anti-inflammatory potency of the tested compounds. Qualitatively, the evolution of the effective doses with increasing delay was linear for potent inhibitors of prostaglandin biosynthesis. The evolution for less potent compounds was markedly different and suggested the earlier occurrence of non-specific drug effects. Suprofen, the most potent of the series of compounds, produced the 1 h delay at an oral dose of 1.11 mg kg-1; the ED50 increased linearly to 115 mg kg-1 for a 4 h delay. Compared with other compounds the activity pattern of suprofen was consistent with that of a very potent, short-acting inhibitor of prostaglandin biosynthesis, which maintains its specific action over a wide dose range. It is concluded that delay of castor oil-induced diarrhoea in rats allows a detailed characterization of aspirin-like compounds, and that inhibition of prostaglandin biosynthesis is insufficient to suppress the intestinal effects of the oil.

Suprofen, a potent antagonist of ultraviolet-erythema in guinea-pigs.

A standardized ultraviolet erythema test in guinea-pigs is described in detail. The erythema reaction to 120 sec of UV irradiation was assessed after 1, 2 and 4 h. A direct quantitative comparison of the anti-erythema effect was made between phenylbutazone, indometacin, tolmetin, ketoprofen and alpha-methyl-4-(2-thienylcarbonyl)benzeneacetic acid (suprofen) in a specially designed experiment. All compounds were given by oral gavage 1 h before irradiation. Among the compounds studied suprofen was the most potent antagonist of ultraviolet-induced erythema in guinea-pigs, i.e. about 45 times tolmetin, 25 times phenylbutazone, 10 times indometacin and 5 times ketoprofen activity.

The activity of suprofen on nystatin-induced paw oedema in rats.

Subplantar injection of nystatin into the rat paw induces acute paw oedema of long duration. In this test the anti-inflammatory activity of alpha-methyl-4-(2-thienylcarbonyl)benzeneacetic acid (suprofen) has been studied using doses from 1.25 up to 160 mg/kg. The lowest dose significantly reducing paw diameter increases was 2.5 mg/kg; more intense and longer lasting reductions appeared progressively with increasing doses. The dose producing 50% inhibition of diameter increase was 2.70 mg/kg 1 h and 1.5 h after administration (lowest ED50). Comparative lowest ED50-values for simultaneously tested reference compounds were 6.2 mg/kg for indometacin, 34 mg/kg for phenylbutazone and 38 mg/kg for acetyl-salicylic acid. Within the first 2 h after administration suprofen exhibited anti-inflammatory activity from 2 to 14 times as potent as did the reference compounds.

The effects of suprofen in rats with implanted cotton pellets.

Implanted cotton pellets with closely adherent granuloma tissue were removed from rats treated orally during 7 consecutive days with solvent or 2.5, 5.0, 10, 20 or 40 mg/kg of alpha-methyl-4-(2-thienylcarbonyl)benzeneacetic acid (suprofen) by gavage. Highly significant decrease (P less than 0.001) of the total cotton pellet dry weight was observed in the groups given 5.0 mg/kg of suprofen or more. The pronounced activity on total cotton pellet weight was obtained without any effect on body weight gain, on food consumption or on the weight of thymus and adrenal glands in the same animals. We conclude that a maximal effect against the proliferative phase of inflammation is obtained with 10 mg/dg of suprofen, in the absence of any steroid-like or toxic side-effect.

The effects of suprofen in rats with Mycobacterium butyricum-induced arthritis.

The activity of alpha-methyl-4-(2-thienylcarbonyl)benzeneacetic acid (suprofen) was studied in rats with established Mycobacterium butyricum-induced arthritis. The arthritic response was evaluated by measuring the diameter changes of the hind paws and tibiotarsal joints, using a new apparatus. Treatment once daily by gavage during 14 days revealed activity at the dose of 1.25 mg/kg and above. The lowest ED50 was 6.2 mg/kg. In comparison with simultaneously studied reference compounds, this corresponded approximately to the activity of phenylbutazone. In the higher dose range, 10 to 40 mg/kg, the reduction of external arthritic symptoms was associated with marked reversal of multiple bone deformations. A treatment schedule with 4 administrations of 2.5 mg/kg/day was significantly more effective than a single daily administration of 10 mg/kg and comparable to that of 10 mg/kg once daily. Mixing of suprofen with the diet, to give the low daily dose of 1.28 mg/kg confirmed the high activity expected from the maintenance of a practically constant compound level in the treated rats. In a comparative study involving suprofen, acetyl-salicylic acid, indometacin, phenylbutazone and tolmetin, all administered over 14 days with the diet in a dose range up to toxic dose levels, dose-related anti-inflammatory and toxic effects were obtained. The potency order and ED50's of the compounds were: indometacin (0.31 mg/kg) greater than suprofen (1.48) greater than tolmetin (18.0) = phenylbutazone (18.2) greater than acetyl-salicylic acid (440). Suprofen had by far the largest safety margin (48), followed by phenylbutazone (20), tolmetin (16), indometacin (8) and acetyl-salicylic acid (2). In conclusion, three major symptoms of adjuvant arthritis: joint inflammation, impaired growth and bone erosion, were markedly reduced by different doses of suprofen, which revealed to be a safe compound when compared to several reference compounds.

Gastrointestinal effects and acute toxicity of suprofen.

The acute effects of orally administered, high doses of alpha-methyl-4-(2-thienylcarbonyl)benzeneacetic acid (suprofen) were studied in various tests, related to gastrointestinal functions. A decrease of the diarrheal stools in the castor oil test in rats was the first effect noted; the ED50 in this test was 40 mg/kg. This dose is 540 times higher than the ED50 of suprofen in the acetic acid-induced writhing test in rats (ED50 = 0.074 mg/kg). Temporarily decreased food consumption in rats was first noted after administration of 80 mg/kg. This is more than 1000 times the ED50 in the rat anti-writhing test. The appearance of gastrointestinal lesions was also studied in rats and a direct quantitative comparison was made with indometacin, acetyl-salicylic acid and ketoprofen. The dose of suprofen that produced lesions in 50% of the rats was 200 mg/kg, viz. 2700 times the ED50 in the rat anti-writhing test. Similarly obtained safety margins are 9.0 for indometacin, 78 for acetyl-salicylic acid and 102 for ketoprofen. The mortality after a single oral administration of suprofen was studied in mice, rats, guinea-pigs and dogs. LD50-values, based on mortality 7 days after administration, were 590 mg/kg, 353 mg/kg, 280 mg/kg and more than 160 mg/kg, respectively. Comparative LD50's in mice and rats were 14 and 19 mg/kg for indometacin, 280 and 70 mg/kg for ketoprofen. Therefore the safety margin in rats, with respect to the ED50 in the acetic acid-induced writhing test, is 4770 for suprofen, 156 for ketoprofen and 17.3 for indometacin. In guinea-pigs the safety margin of suprofen is 1470 with respect to the ED50 in UV-erythema and in dogs more than 250 with respect to the ED50 in urate-induced arthritis. From these data we may conclude that suprofen is comparatively safer than the reference compounds studied and that its effects on the gastrointestinal tract appear at doses far above those required for effectiveness in tests related to pain, fever and inflammation.

Loperamide antagonism of castor oil-induced diarrhea in rats: a quantitative study.

The time-course of castor oil-induced diarrhea in fasted rats was quantified by weighing stools every 15 minutes for 8 hours after the challenge and then after 24 hours. Diarrhea began within 1 hour as a series of rapidly occurring evacuations over 20 to 40 minutes. The mean weight of these stools was 5.7 g; later irregular evacuations increased the weight to 9.4 g at 8 hours. The area of individual time-weight diagrams had a median value of 432 units. Pretreatment with 0.16 mg/kg of loperamide, a new antidiarrheal drug, significantly decreased the area of similarly obtained diagrams; 0.31 mg/kg caused a 50% reduction. This antagonism by loperamide of castor oil-induced diarrhea may involve reduction in the severity of inflammatory changes in the intestinal wall.