The pharmacokinetic disposition of
pyrantel after intravenous (i.v.) and oral (p.o.) administration as the
citrate and p.o. administration as the pamoate
salt was determined in pigs. Following i.v. administration
pyrantel was quickly cleared from the bloodstream, exhibiting a terminal half-life of 1.75 +/- 0.19 h and a residence time (MRT) of 2.54 +/- 0.27 h. After p.o. administration as the
citrate salt, the absorption time (MAT) of
pyrantel was 2.38 +/- 0.25 h and although significant quantities of
pyrantel were absorbed (mean bioavailability of 41%) the rapid clearance resulted in a MRT of only 4.92 +/- 0.36 h. By comparison, the significantly extended MAT of the less soluble pamoate
salt resulted in reduced circulating concentrations and a significantly lower mean bioavailability of 16%. The poor efficacy of
pyrantel citrate against nematodes inhabiting the large intestine of pigs is therefore suggested to result from insufficient quantities of
drug passaging to the site of
infection. When tested against
pyrantel-resistant adult Oesophagostomum dentatum the mean efficacy of
pyrantel citrate was only 23%, whereas the efficacy of the lesser absorbed
pyrantel pamoate was 75%. These results indicate that for maximum activity
pyrantel should be administered to pigs as the pamoate
salt.