MODULATION BY VERAPAMIL OF VINCRISTINE PHARMACOKINETICS AND SENSITIVITY TO METAPHASE ARREST OF THE NORMAL RAT COLON IN ORGAN-CULTURE

The in-vitro pharmacokinetics of vincristine (VCR) in normal rat colonic mucosa were studied. Two complementary approaches were adopted using an explant organ-culture system. Firstly [G-H-3]vincristine (H-3VCR) accumulation, retention and efflux were characterized under basal conditions and compared with measurements made either under energy-depleted conditions, or in the presence of VRP. Secondly, a histological method-the postmetaphase index (PMI)-was used to compare the sensitivity of explants to VCR in the presence or absence of verapamil (VRP). This latter technique involves the measurement, by counting, of the proportion of mitotic figures escaping from metaphase arrest. The studies yielded the following results: H-3VCR accumulation in colonic mucosa showed no evidence of saturability up to the maximum dose studied (130 nM), at a dose of 52 nM accumulation was enhanced in energy-depleted conditions by a factor of 1.8, and in the presence of VRP (6.6-mu-M) by a factor of 1.4. In the presence of VRP (6.6-mu-M) retention of H-3VCR was increased by a factor of 1.3 and efflux was reduced by a factor of 0.8 after 2 hr. VRP (6.6-mu-M) reduced the PMI of colonic mucosal epithelial cells exposed to 11 nM VCR from 18.8% to 11.4% (i.e. 40% reduction) indicating sensitization of the cells to this property of VCR. These results provide evidence that the sensitivity of normal colonic mucosa to vincristine is, at least in part, regulated by drug transport. Qualitatively our observations resemble those described in multidrug resistance. Given that P-glycoprotein has been demonstrated by several groups in colonic mucosal cells, the results support a normal role for this membrane transport molecule in the protection of intestinal cells from plant alkaloids and other xenobiotic agents ingested in the diet.