Physiological Pharmaceutics

Colon and rectal drug delivery 159Table 7.3 Colonic transit of single unit dose formsThere is a large variability in the data, but in general, units administered prior toretiring for the night have a slower colonic transit than those dosed in the morning, whichis in keeping with the suppressed electrical and contractile activity 70–73 , and decreased tonein the colon overnight 74 . There are conflicting values in the literature for the average transittime from caecum to splenic flexure; in one study it was reported to be 14 hours 31 ; howeveranother study reported that 50% of large units reach the splenic flexure within 7 hours ofentering the colon, and the size and density of such units had little effect on the transittimes 75 . Steady state experiments, conducted by repeated daily administrations oftechnetium-99m labelled resin, demonstrate that the transverse colon remains relativelyempty during the day.Studies examining the transit of different sized particles through the colon havesuggested that large objects move more rapidly than smaller ones. In a study on a limitednumber of healthy volunteers (n=6), a pressure sensitive radiotelemetry device (25×9 mm)was seen to move ahead of dispersed pellets (0.5–1.8 mm) in the ascending and transversecolon (Figure 7.6) 76 . Similarly a tendency for transit rates to increase with increasing unitvolume (0.3 to 0.8 to 1.8 cm 3) has also been demonstrated 75 . Differential transit rates havealso been noted between 0.5–1.8 mm pellets and 6 mm plastic markers 77 . In contrast, othershave found no difference in the passage of 0.2mm and 5mm particles 78 or between 0.2mm,5mm, and 8.4mm particles 79 through the ascending colon. Interestingly, in this latter study,there was a trend towards a shorter residence time within the ascending colon for 0.2mmFigure 7.6 Transit of a large non-disintegrating unit and pellets through thegastrointestinal tract—a scintigraphic study