The greater brain 5-HT synthesis in the female IBS patients than in the controls may be related to the pathological visceral pain processing of the IBS patients, a larger female predominance of the disorder, and the sex difference of the efficacy of the 5-HT3 antagonist in treatment.

SUMMARYBackground: Irritable bowel syndrome patients demonstrate colonic hypersensitivity after duodenal lipid infusion. Aim: To investigate the role of 5-hydroxytryptamine-3 (5-HT 3 ) receptors in this sensory component of the gastrocolonic response in irritable bowel syndrome. Methods: Fifteen female patients with diarrhoea-predominant irritable bowel syndrome completed a trial with the 5-HT 3 receptor antagonist alosetron (1 mg b.d.) or placebo (b.d.) over 15 days, followed by the alternative treatment. Each treatment period was followed by a colonic distension trial before and after duodenal lipids. Changes in colonic thresholds, tone and compliance and viscerosomatic referral pattern after lipids were compared between treatments.

Serotonin (5-HT) plays an important role in the pathophysiology of irritable bowel syndrome (IBS). Using alpha-[(11)C]methyl-L-tryptophan-positron emission tomography (PET), it was demonstrated that brain 5-HT synthesis is increased in patients with IBS, in a gender-specific manner. The aims of the study were to evaluate the effects of alosetron on brain 5-HT synthesis in patients with IBS. Six male and five female non-constipation-predominant IBS patients were enrolled. The subjects received alosetron or a placebo for 14 days, separated by a 2-week washout period. On day 14, rectal distensions commenced just prior to the PET scan (which was performed for 80 min), and continued for 20-min periods. The functional images were analysed with SPM99. Alosetron vs placebo treatments, in a randomized, double-blinded, crossover manner, were studied. 5-HT synthesis was greater in several regions in the males than in the females during the alosetron treatment, whereas there was no region in which the females had greater synthesis. There were significant gender-treatment interactions of synthesis in the cingulate gyrus, caudate nucleus, globus pallidus, and cerebellum. The gender differences in the effect of alosetron on brain 5-HT synthesis may be related to the gender differences in the efficacy of alosetron.

Lotronex (alosetron hydrochloride) is a 5-HT3 receptor antagonist indicated for the treatment of irritable bowel syndrome (IBS) in females whose predominant bowel habit is diarrhea. Alosetron is extensively metabolized by multiple cytochrome P450 (CYP) enzymes, including CYP 2C9 and 3A4. Alprazolam is a short-acting benzodiazepine commonly prescribed for the treatment of anxiety disorders and a potential comedication in patients with IBS. Alprazolam is extensively metabolized by CYP3A4. This clinical study was conducted to assess the potential for a metabolic drug interaction between these two CYP3A4 substrates. This was an open-label, randomized, two-period, crossover study in 12 healthy female and male volunteers to determine the effect of concomitant administration of alosetron at the recommended dose of 1 mg p.o. bid on the pharmacokinetics of alprazolam following a single oral 1 mg dose. The results showed no effect of alosetron on the pharmacokinetics of alprazolam. Mean alprazolam AUC was 210 and 202 ng.h/mL in the absence and the presence of alosetron, respectively. Therefore, alprazolam may be safely coadministered with alosetron without the need for dosage adjustment.

Lotronex (alosetron hydrochloride) is a 5-HT3 receptor antagonist indicated for the treatment of irritable bowel syndrome (IBS) in females whose predominant bowel habit is diarrhea. Alosetron is extensively metabolized by multiple cytochrome P450 (CYP) enzymes, including CYP2C9 and CYP3A4. Fluoxetine is an antidepressant that is administered as a racemic mixture of equipotent R- and S-enantiomers. Fluoxetine metabolism involves CYP2D6 and CYP2C9 in the formation of its major metabolite, norfluoxetine. This metabolite is also present as two enantiomers, of which only the S-enantiomer exhibits comparable antidepressant activity. This study was conducted to assess the potential for an effect of alosetron on the pharmacokinetics of fluoxetine. This was an open-label, two-period, nonrandomized, crossover study in 12 healthy female and male volunteers. The pharmacokinetics for both enantiomers of fluoxetine and norfluoxetine were examined following single oral doses of 20 mg fluoxetine, given alone and in combination with alosetron 1 mg twice daily for 15 days. The results showed small delays in peak concentration but no clinically significant effect of alosetron on the pharmacokinetics of S- and R-fluoxetine or S- and R-norfluoxetine. Coadministration of alosetron and fluoxetine was well tolerated by all subjects.