Biol. Pharm. Bull. 28(7) 1249—1253 (2005)

absorbed in the intestine mainly through the paracellular pathway. Their oral bioavailability is usually low since this pathway is severely restricted by the presence of the tight junctions. One of the possible strategies for solving this problem would be a controlled and reversible opening of the tight junctions, which could be achieved by compounds called paracellular permeability enhancers (PPEs). However, presently none of the known PPEs (Ca chelators, bile salts, anionic surfactants etc.) is used pharmaceutically since they cause, or might cause, toxicity (mucosal damage) “in vivo”. The main “tools” used in the research of PPEs are “in vitro” models of the intestinal epithelium, such as Caco-2 monolayers while experimental animals can be used for “in vivo” studies. We can only speculate on the possible side effects of substances, which are known to increase the “in vitro” paracellular permeability, while the side effects of registered drugs are well documented. Therefore, instead of clinically testing a substance, which has been proven to increase the paracellular permeability “in vitro”, a therapeutic agent, already used in clinical practice could be tested for its “in vitro” ability to increase the paracellular permeability—an activity similar to that of PPEs. From the undesired gastrointestinal effects of such therapeutic agent or from its gastrointestinal tolerability the possibility for safe use of PPEs could be assessed. Thus, we have evaluated the “in vitro” ability of clodronate (a bisphosphonate) to increase the paracellular permeability of rat jejunum. Bisphosphonates are a group of drugs prescribed for the treatment of osteoporosis, Paget’s disease, tumor-induced hypercalcemia, bone metastases and other diseases characterized by elevated bone resorption. They are capable of chelating divalent metal ions such as Ca , Mg and Fe . Some have been shown to increase the paracellular permeability of Caco-2 monolayers; PEG400 flux was increased by tiludronate at concentrations above 20 mM added to both sides (apical and basolateral) of the Caco-2 monolayers. Pamidronate and zoledronate (also bisphosphonates) altered the mannitol flux for 30% at 10.3 and 40.7 mM respectively, when added only to the apical side of the Caco-2 monolayers. Pamidronate and zoledronate at concentrations of 30 mM, but not at 3 and 10 mM also caused a leakage of the plasma marker raffinose into the luminal perfusate in the perfused rat ileal loop “in vivo”. A sugar absorption test has also shown that pamidronate can increase the intestinal permeability in humans. The presumed mechanism by which these substances increase the “in vitro” epithelial permeability is similar to that of Ca chelators—a group of PPEs (EDTA (ethylene diamine tetraacetic acid), EGTA (ethylene glycol-bis(b-aminoethyl ether)-N,N,N ,N -tetraacetic acid) etc.). However, bisphosphonates are much less potent for binding Ca . Therefore, their “in vitro” effects on the integrity of epithelia can be expected at higher concentrations. Clodronate is a first generation bisphosphonate. Its bioavailability is only 1—2% and it is freely soluble in water. The most commonly used oral dose of clodronate is 1600 mg/d given in a single dose. Therefore its expected gastrointestinal concentration can be estimated to 22 mM if 250 ml is the estimated volume for the dissolution of the given dose. We have decided to compare the ability of clodronate to modulate the paracellular permeability by opening the tight junctions of rat small intestine “in vitro” with that of EDTA. Similar to other Ca chelators, EDTA can deplete the extracellular Ca , which is necessary for the maintenance of the paracellular barrier. Experiments were performed on isolated segments of rat jejunum mounted in side-by-side diffusion chambers. The tissue viability after the July 2005 Biol. Pharm. Bull. 28(7) 1249—1253 (2005) 1249