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Evidence for interactions of exogenous steroids with P-glycoprotein and the breast cancer resistance protein (BCRP)
Prolonged treatment with steroids for the management of chronic inflammatory conditions may be associated with perturbations of the hypothalamic-pituitary-adrenal (HPA) axis (Robinson et al., 2002). Distribution of these glucocorticoids within the body may be affected by efflux transporters located at barrier sites such as the blood-brain barrier, so influencing the responses of the HPA axis to these exogenous steroids. In the present study, six clinically relevant inhaled glucocorticoids ie fluticasone propionate (FP), budesonide (BUD), beclomethasone dipropionate (BDP), triamcinolone acetonide (TAA), mometasone furoate (MF) and ciclesonide active principle (CIC-AP) were investigated for their possible interactions with P-glycoprotein (Pgp, also termed ABCB1) and with the Breast Cancer Resistance Protein (BCRP, also termed ABCG2), two important efflux transporters present at the human blood-brain barrier. The effects of these steroids used at concentrations between 0.1 and 10µM on substrate transport by these membrane proteins were examined using flow cytometry (Cooray et al., 2004). Concentration-dependent inhibition of efflux was evident with BDP, CIC-AP and MF, resulting in increased accumulation of mitoxantrone in BCRP-expressing MCF7/MR breast cancer cells following 40 min exposure to 5µM mitoxantrone (increases with 10µM BDP, CIC-AP and MF of 2.4±0.2, 2.3± 0.3 and 2.3± 0.2 fold respectively, n=3, p<0.01) and of calcein in Pgp-expressing SW620R colon carcinoma cells following 40 min exposure to 0.25µM calcein-AM, a Pgp substrate that is the non-fluorescent but cell-permeant esteric precursor of fluorescent calcein (increases with 10µM BDP, CIC-AP and MF of 33±3, 22± 2 and 22± 5 fold respectively, n=3, p<0.01). Using the MTT cell viability assay, it could be shown that these same three steroids used at 5µM could increase sensitivity of the resistant SW620R to the toxic effects of doxorubicin (IC50 values as means and s.e.m in m M of 9.5±0.27 in the absence of steroid and of 2.5±0.06, 1.5±0.01 and 2.5±0.01 in the presence of BDP, CIC-AP and MF respectively; n=6 in each case). No equivalent effects were seen in the sensitive SW620P cells. In addition these particular steroids were able to stimulate the vanadate–sensitive ATPase activity associated with BCRP in membrane vesicles prepared from Lactococcus.lactis expressing the human BCRP. Human BCRP expressed in L.lactis (Janvalisri et al., 2003) exhibited a basal vanadate-sensitive ATPase activity of 36±11 nmol Pi mg-1 min-1, n=4. This was significantly increased when measured in the presence of 0.1–5µM BDP, CIC-AP and MF (values as means and s.e.m in nmol Pi mg-1 min-1 for 5µM BDP, CIC-AP and MF of 220±25 (p<0.002), 208±33 (p<0.01) and 247±26 (p<0.001) respectively, n=4 in each ca e) but was not altered by TAA or FP. A small increase in ATPase stimulation (p<0.02, n=4) was seen also with BUD but at 5µM only. These data demonstrate that these clinically used glucocorticoids interact to varying degrees with efflux transporters. These interactions may explain differences in both their pharmacokinetics and their systemic effects.
Cooray H.C., et al (2004) Biochem Biophys Res Commun, 317, 269-75. |