Many orally administered drugs must overcome several barriers before reaching their target site. The first major obstacle to cross is the intestinal epithelium. Although lipophilic compounds may readily diffuse across the apical plasma membrane, their subsequent passage across the basolateral membrane and into blood is by no means guaranteed. Efflux proteins located at the apical membrane, which include P-glycoprotein (P-gp, MDR1) and Multidrug Resistance-associated Protein (MRP2), may drive compounds from inside the cell back into the intestinal lumen, preventing their absorption into the blood. Intestinal P-gp is localised to the villus tip enterocytes, i.e. the main site of absorption for orally administered compounds and in close proximity to the lumen. P-gp is therefore ideally positioned to limit the absorption of compounds by driving efflux back into the lumen. Drugs may also be modified by intracellular phase I and phase II metabolizing enzymes. This process may not only render the drug ineffective, but it may also produce metabolites that are themselves substrates for P-gp and/or MRP2. Drugs that reach the blood are then passed to the liver, where they are subjected to further metabolism and biliary excretion, often by a similar system of ATP binding cassette (ABC) transporters and enzymes to that present in the intestine. Thus a synergistic relationship exists between intestinal drug metabolizing enzymes and apical efflux transporters, a partnership that proves to be a critical determinant of oral bioavailability. Aim: The aim of this study was to investigate the effect of selected methoxy flavonoids (3-methoxyflavone, 5-methoxyflavone, 6-methoxyflavone and 7- methoxyflavone) on the mean ratio of Rhodamine123 (Rho 123) transport across rat intestine (jejunum) and to investigate structure activity relationships (SAR) of the selected flavonoids with reference to inhibition of P-gp. Methods: 3-Methoxyflavone, 5- methoxyflavone, 6-methoxyflavone and 7-methoxyflavone were evaluated at a concentration of 10μM and 20μM as modulators of Rho 123 transport across rat jejunum. The Sweetana-Grass diffusion cells were used to determine the transport of Rho 123. Each modulator was studied bidirectionally with two cells measuring transport in the apical to basolateral direction (AP/BL) and two cells measuring transport in the basolateral to apical direction (BUAP). The rate of transport was expressed as the apparent permeability coefficient (Papp)and the extent of active transport was expressed by calculating the ratio of BUAP to AP/BL. Each modulators Papp ratio was then compared with that of the control. Results: 3-Methoxyflavone decreased the Papp ratio from 3.34 (control) to 1.66 (10μM) and 1.33 (20μM) and showed statistical significant differences. 7-Methoxyflavone decreased the Papp ratio to 1.94 (10μM) and 1.55 (20μM) but only showed a statistical significant difference at 10μM. 5- Methoxyflavone decreased the Papp ratio to 2.41 (10μM) and 1.71 (20μM) and 6- methoxyflavone decreased the Papp to 3.03 (10μM) and 2.49 (20μM). Both 5- and 6- methoxyflavone showed no statistical significant differences from the control. The structure activity relationships with reference to P-gp inhibition clearly indicated that the C3 and C7 positioning of the methoxy-group on the A ring played a major role in the inhibition of Rho 123 transport. Conclusion: All the selected modulators showed inhibition of Rho 123 transport across the jejunum. This should affect the bioavailability of the substrates of P-gp and other active transporters. In summary, this study describe the inhibitory interaction of selected flavonoids with P-gp. Structure activity relationships were identified describing the inhibitory potency of the flavonoids based on methoxy groups positioning. The inhibitory potency results were 3-methoxyflavone > 7- methoxyflavone > 5-methoxyflavone> 6-methoxyflavone / Thesis (M.Sc. (Pharmaceutics))--North-West University, Potchefstroom Campus, 2005.

Many orally administered drugs must overcome several barriers before reaching their target site. The first major obstacle to cross is the intestinal epithelium. Although lipophilic compounds may readily diffuse across the apical plasma membrane, their subsequent passage across the basolateral membrane and into blood is by no means guaranteed. Efflux proteins located at the apical membrane, which include P-glycoprotein (P-gp, MDR1) and Multidrug Resistance-associated Protein (MRP2), may drive compounds from inside the cell back into the intestinal lumen, preventing their absorption into the blood. Intestinal P-gp is localised to the villus tip enterocytes, i.e. the main site of absorption for orally administered compounds and in close proximity to the lumen. P-gp is therefore ideally positioned to limit the absorption of compounds by driving efflux back into the lumen. Drugs may also be modified by intracellular phase I and phase II metabolizing enzymes. This process may not only render the drug ineffective, but it may also produce metabolites that are themselves substrates for P-gp and/or MRP2. Drugs that reach the blood are then passed to the liver, where they are subjected to further metabolism and biliary excretion, often by a similar system of ATP binding cassette (ABC) transporters and enzymes to that present in the intestine. Thus a synergistic relationship exists between intestinal drug metabolizing enzymes and apical efflux transporters, a partnership that proves to be a critical determinant of oral bioavailability. Aim: The aim of this study was to investigate the effect of selected methoxy flavonoids (3-methoxyflavone, 5-methoxyflavone, 6-methoxyflavone and 7- methoxyflavone) on the mean ratio of Rhodamine123 (Rho 123) transport across rat intestine (jejunum) and to investigate structure activity relationships (SAR) of the selected flavonoids with reference to inhibition of P-gp. Methods: 3-Methoxyflavone, 5- methoxyflavone, 6-methoxyflavone and 7-methoxyflavone were evaluated at a concentration of 10μM and 20μM as modulators of Rho 123 transport across rat jejunum. The Sweetana-Grass diffusion cells were used to determine the transport of Rho 123. Each modulator was studied bidirectionally with two cells measuring transport in the apical to basolateral direction (AP/BL) and two cells measuring transport in the basolateral to apical direction (BUAP). The rate of transport was expressed as the apparent permeability coefficient (Papp)and the extent of active transport was expressed by calculating the ratio of BUAP to AP/BL. Each modulators Papp ratio was then compared with that of the control. Results: 3-Methoxyflavone decreased the Papp ratio from 3.34 (control) to 1.66 (10μM) and 1.33 (20μM) and showed statistical significant differences. 7-Methoxyflavone decreased the Papp ratio to 1.94 (10μM) and 1.55 (20μM) but only showed a statistical significant difference at 10μM. 5- Methoxyflavone decreased the Papp ratio to 2.41 (10μM) and 1.71 (20μM) and 6- methoxyflavone decreased the Papp to 3.03 (10μM) and 2.49 (20μM). Both 5- and 6- methoxyflavone showed no statistical significant differences from the control. The structure activity relationships with reference to P-gp inhibition clearly indicated that the C3 and C7 positioning of the methoxy-group on the A ring played a major role in the inhibition of Rho 123 transport. Conclusion: All the selected modulators showed inhibition of Rho 123 transport across the jejunum. This should affect the bioavailability of the substrates of P-gp and other active transporters. In summary, this study describe the inhibitory interaction of selected flavonoids with P-gp. Structure activity relationships were identified describing the inhibitory potency of the flavonoids based on methoxy groups positioning. The inhibitory potency results were 3-methoxyflavone > 7- methoxyflavone > 5-methoxyflavone> 6-methoxyflavone / Thesis (M.Sc. (Pharmaceutics))--North-West University, Potchefstroom Campus, 2005.

Over 5000 flavonoids have been identified so far and many of these are known to have antineoplastic properties. The relationships between the targeting activities by these compounds on cancer cells and the specific features that determine their molecular structures are not completely elucidated. Here we report the differential cytotoxic effects of two unsubstituted ring B flavonoids that differ solely in the presence of a C2, C3 double bond in ring C, on human cancer cells of the lung (A549), pancreas (MIA PaCa-2, Panc28), colon (HCT 116, CaCo-2), Liver (HepG2), and breast (SKBr3). These compounds were extracted from Chromolaena leivensis (Hieron) a plant belonging to the genus Chromolaena reputed to have antitumor activities. 3, 5 dihydroxy-7-methoxyflavone induce apoptosis in cancer cells of the lung A549, pancreas MIA PaCa-2 and Panc28, and colon HCT116, but not on Caco-2; whereas 3,5 dihydroxy-7-methoxyflavanone display proliferative effects in A549, Panc 28, MIA PaCa, and HCT116 cells at low concentrations, and slight cytotoxicity only on CaCo-2, a cancer cell line with a higher differentiation status than other cells tested. At the concentrations studied (5-80µM) neither compound demonstrated activity against cancer cells of the liver (HepG2) or breast (SKBr3) as indicated by MTT cell viability assays. When used in combination with 3,5 dihydroxy-7-methoxyflavone in pancreatic cancer cells, the targeting preference of 3, 5 dihydroxy-7-methoxyflavanone is altered, and a significant increase in inhibition of cell viability is observed 48 hours after dosing. The presence or absence of the C2, C3 double bond in ring C, accounts for electrochemical and structural changes that dictate differential specificity towards cancer cells. 3,5 dihydroxy-7-methoxyflavone has a more planar structure, whereas the absence of the double bond in C2, C3 causes ring B to adopt a perpendicular orientation to the plane formed by rings A and C and the OH group at C3.