A novel method for the preclinical assessment of rectal irritation

The purpose of this study concerns a novel method for preclinical assessment of rectal irritation caused by suppositories introduced into the rectum. Rectal irritation was assessed by the balloon method in fasting conscious rats. This method is based on measuring rectal contractions due to possible irritation caused by the presence of drugs and adjuvants in the suppository. In control experiments (vehicle only), significant rectal contractions were not observed in a range of pH 1.5-11.0 and osmotic pressure 70-2000 mOsm kg-1 H2O, respectively. On the other hand, strong contractions were observed after rectal administration of an aqueous solution of 50% glycerin, 100 mM sodium caprate or 25 mM sodium cholate. The intensity of contraction after rectal administration of sodium caprate or sodium cholate was dependent on the concentration in the dosing solution. In addition, the effect of sodium caprate and sodium cholate on rat rectal mucosa was investigated by optical light microscopy. Although slight or moderate alteration such as the presence of mucinous substance in lumen and congestion, oedema and haemorrhage of the rectal membrane 20 min after rectal administration, there was no major damage to the rectal mucosa. There was a correlation between the median score for mucinous substance in lumen and mean intensity of rectal contraction. For comparative purposes, defecating sensations, pain, itch, burning sensations, and awareness of the presence of a foreign body after administration of suppositories containing 0, 1, 2 and 4% sodium caprate were examined in eight healthy volunteers. The defecating sensation in the human subjects correlated with the intensity of rectal contraction in rats. The results suggest that rectal contraction in conscious rats could be a useful index for prediction of a defecating sensation in man.