An in vitro intestinal cell culture model was used to characterize and investigate factors affecting uptake and transport of rapamycin (RAPA), a potent immunosuppressive drug. Studies were performed on three human intestinal cell monolayers (Caco-2, HCT-8, and T84), grown on microporous membrane inserts for 12 days. RAPA transport in all three monolayers was found to be dose dependent. The highest rates of transport were found at the highest tested final RAPA concentration of 10,000 micrograms/L. Apical to basal RAPA transport was linear in Caco-2 cell monolayers for up to 60 min, and in HCT-8 and T84 cell monolayers for up to 120 min. Temperature sensitive RAPA transport was found because incubation at 4 degrees C markedly attenuated transport by 97, 90, and 78% for Caco-2, HCT-8, and T84 monolayers, respectively. In all three monolayers RAPA transport was highly polarized because the apical to basal transport was greater than that in the opposite direction. RAPA uptake and transport across cell monolayers were compared when 10,000 micrograms/L of RAPA (cold) plus 0.05 microCi 14C-RAPA was added in combination with varying final concentrations (1,000, 10,000, and 100,000 micrograms/L) of the immunosuppressive drugs, CsA or RS. Increasing concentrations of CsA resulted in a significant dose-dependent decrease in 14C-RAPA transport across cell monolayers. In contrast, at high (100,000 micrograms/L) RS concentrations, 14C-RAPA transport was significantly increased. Uptake of 14C-RAPA into cell monolayers was significantly decreased only with the 100,000 micrograms/L CsA concentration. These studies suggest that combinations of immunosuppressive drugs given orally have a potential for altering the intestinal transport and uptake of RAPA.(ABSTRACT TRUNCATED AT 250 WORDS)