Potential roles of P-gp and calcium channels in loperamide and diphenoxylate transport.

This study examined the accumulation and transport of two related systemic opioids used as antidiarrhoeal drugs and compared their rates of transport with known P-glycoprotein (P-gp) substrates used in our in vitro environment. Cellular uptake and efflux and transcellular transport were all determined using Caco-2 cells after exposure to loperamide or diphenoxylate, with or without a range of efflux inhibitors. Bidirectional transport studies of 5 microM loperamide showed efflux to be fivefold higher than influx (42 x 10(-6) compared to 8 x 10(-6) cm/s); however, this decreased to twofold at 10 microM and was abolished using 100 microM loperamide. An uptake pathway was also discovered when P-gp was inhibited which, in the presence of Ca(2+) channel blockers, was amplified, providing a potential mechanism for central nervous system effects to be increased upon blockage of L-type calcium channels, quite separate from any P-gp inhibition. Diphenoxylate transport, however, showed little sign of P-gp-mediated efflux. Diphenoxylate accumulated readily within cells, yet transport through cells was very low. Additionally, efflux inhibitors had little impact on transport or accumulation, suggesting that diphenoxylate was not a substrate for an efflux mechanism.

Do antidiarrhoeal opiates accumulate in the rat intestinal lumen?

The opiate antidiarrhoeal drugs loperamide (0.6 mg kg-1, i.p.) or difenoxin (0.77 mg kg-1, s.c.), were administered in an anaesthetic mixture (pentobarbitone 60 mg kg-1) to rats. A length of jejunum (approx. 30 cm) was cannulated, washed and then perfused with iso-osmotic saline for 20 min. The perfusion commenced 50 min after drug administration and continued for 20 min. The perfusates were collected for analysis of fluid transport rates and antidiarrhoeal drug content. These doses of the antidiarrhoeals caused marked inhibition of intestinal fluid secretion induced by intra-arterial infusion of vasoactive intestinal peptide. However, neither of the antidiarrhoeal drugs were detected in the intestinal perfusates (< 0.5 ng by HPLC). The results indicate that loperamide and difenoxin have a different pharmacokinetic profile compared with that previously found for morphine under the same conditions.

Diphenoxylate-atropine (Lomotil) overdose in children: an update (report of eight cases and review of the literature)

Eight pediatric accidental overdoses of diphenoxylate-atropine (Lomotil) are reported, and 28 literature cases are reviewed. This overdose is primarily an opioid intoxication, occasionally associated with atropine toxicity. Only 6 of 36 children showed signs of atropine overdose (central nervous system excitement, hypertension, fever, flushed dry skin). Contrary to popular belief, atropine effects occur before, during, or after opioid effects. Opioid overdose (central nervous system and respiratory depression with miosis) predominated or occurred without any signs of atropine toxicity in 33 cases (92%). Diphenoxylate-induced hypoxia was the major problem and was associated with slow or fast respirations, hypotonia or rigidity, cardiac arrest, and in 3 cases cerebral edema and death. Respiratory depression recurred 13 to 24 hours after the ingestion in 7 cases and was probably due to accumulation of difenoxine, an active metabolite of diphenoxylate. Recommended treatment is intravenous naloxone for depressed or inadequate respirations, followed by continuous intravenous naloxone infusion, prompt gastric lavage, repeated administration of activated charcoal, and close monitoring for 24 hours.

The evaluation and application of a radioimmunoassay for the measurement of diphenoxylic acid, the major metabolite of diphenoxylate hydrochloride (Lomotil), in human plasma.

Antibodies to diphenoxylic acid, the pharmacologically active metabolite of Lomotil, were successfully used to develop a precise and specific radioimmunoassay for the measurement of diphenoxylic acid in human plasma. The observed cross-reaction of the antiserum with Lomotil (23.5%) and p-hydroxy diphenoxylic acid (2.9%) was not considered to affect significantly the accuracy of the direct determination of diphenoxylic acid in plasma from human volunteers after ingestion of Lomotil tablets. Within-day and between-day coefficients of variation were better than 3 and 6%, respectively, over the concentration range of 3.4 to 255 ng ml-1. Comparable precision could be achieved at 2 ng ml-1 by doubling the volume of sample analyzed. The assay was used to measure plasma concentrations of diphenoxylic acid in 12 human volunteers for up to 24 hr after ingestion of Lomotil (10mg) tablets. Plasma diphenoxyllic acid levels rose to a mean (SE) maximum level of 87.8 (2.7) ng ml-1 3.3 (0.3) hr after dosing. By 24 hr after dosing plasma levls had decreased to 14.26 (1.67) ng ml-1. The appearance and elimination of plasma diphenoxylic acid could be described by a biexponential function. The appearance half-life was calculated to be 0.82 (0.09) hr, and the elimination half-life was 7.24 (0.73) hr.