Shuji Kitagawa

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We studied the delivery of retinoic acid to skin by using cationic liposomes consisting of double-chained cationic surfactant, phosphatidylcholine (PC) and retinoic acid in excised guinea pig dorsal skin. Egg yolk PC liposomes contaning retinoic acid at a molar ratio of 4 : 1 increased the delivery of retinoic acid about two-fold, compared with its addition as an isopropyl myristate solution. Cationic liposomes containing 1,2-dioleoyl-3-trimethylammonium propane (DOTAP) further enhanced the incorporation dependent on the DOTAP content. Liposomes consisting of DOTAP, egg yolk PC, and retinoic acid at a molar ratio of 2 : 2 : 1 induced a 3.7-fold increase in the skin incorporation compared with the egg yolk PC liposomes without DOTAP. Significant difference was not observed when either dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) was used instead of egg yolk PC as well as when dimethyldipalmitylammonium was used instead of DOTAP. These results suggest the potential of the use of the cationic liposomes for the intradermal delivery of lipophilic drugs like retinoic acid.

Overexpression of P-glycoprotein (P-gp), a plasma membrane transporter which extrudes chemotherapeutic agents out of cells, has been associated with the multidrug resistance (MDR) of cancer cells. It has been revealed that flavonoids and other polyphenols inhibit P-gp activity. Due to their inhibitory activities of polyphenols on P-gp function and their physiological safety, they are possible candidates for modulators of MDR. To determine suitable candidates, it is important to clarify the structure–activity relationships of their inhibitory activities on P-gp function. Determining the structure–activity relationships is also meaningful because the intake of dietary polyphenols may also alter drug pharmacokinetics and pharmacodynamics via inhibition of P-gp-mediated drug efflux in tissues such as the intestinal epithelium, blood-brain barrier, hepatocytes and renal tubular cells. This is a review of our recent investigations using multidrug-resistant P-gp overexpressing KB-C2 cells.

We studied the effects of flavonoids, naringenin (flavanone), baicalein (flavone), kaempferol, quercetin, myricetin, morin, and fisetin (flavonols) as well as two glycosides of quercetin on P-glycoprotein (P-gp) function in multidrug-resistant P-gp overexpressing KB-C2 cells. Flavonoids such as kaempferol and quercetin increased the accumulation of rhodamine-123 dependent on their chemical structure. Analysis by flow cytometry indicated that the increase in substrate accumulation was due to the inhibition of substrate efflux. Naringenin, which lacks the 2,3-double bond in the C ring, had no effect, although it was more hydrophobic than myricetin, fisetin and morin. Therefore, the planar structure of the flavonoids seemed to be important for their interaction with P-gp. The effects of other flavonoids on the accumulation of daunorubicin were in the order of kaempferol>quercetin, baicalein>myricetin>fisetin, morin. Quercetin-3-O-glucoside and rutin had no effect. The order of the effects corresponded with that of the partition coefficients. Difference in the number and position of hydroxyl groups in flavonoid molecules by themselves seemed to have little effect. These results suggested that hydrophobicity as well as planar structure is important for the inhibitory effects of flavonoids on P-gp-mediated transport.

We studied the effects of tea catechins, (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECG), and (-)-epigallocatechin gallate (EGCG) on the P-glycoprotein (P-gp) function in multidrug-resistant P-gp over-expressing KB-C2 cells. EC did not have any effects on cellular accumulation of P-gp substrates, rhodamine-123 and daunorubicin, but the other catechins increased the accumulation in the order of EGC<ECG<EGCG. The effects of EGCG were larger than those of verapamil and quercetin. Since these catechins inhibited the efflux of P-gp substrates, the elevation of substrate accumulation seemed to be induced by the inhibition of the efflux transporter. The results showed that the inhibitory effects of the catechins did not depend on their total hydrophobicity, but significantly depended on their chemical structure. The presence of the galloyl moiety on the C-ring markedly increased the n-octanol/PBS partition coefficients of the catechins and their activity on P-gp. On the other hand, the presence of the trihydric pyrogallol group as the B-ring decreased the partition coefficients but increased the activity on P-gp, compared with the action of the corresponding catechins with a dihydric catechol B-ring.

We studied the formation and stability of vesicles consisting of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP) and phosphatidylcholines by electron spin resonance (ESR) analysis and observation of their hemolytic activities. In contrast with previous findings on dimethyldialkylammoniums, DOTAP formed vesicles at 37 °C with phosphatidylcholines containing either saturated acyl chains such as dimyristoylphosphatidylcholine (DMPC) or unsaturated acyl chains such as dilinoleoylphosphatidylcholine (DLPC). Phosphatidylcholines made the bilayer more rigid and significantly reduced the hemolytic activity of DOTAP. In the presence of equimolar concentration of DOTAP and phosphatidylcholines, formation of tightly aggregated structures of several erythrocytes was observed, as previously reported for the vesicles containing dimethyldipalmitylammonium. These findings indicate that DOTAP vesicles were stabilized by phosphatidylcholines with either saturated acyl chains or unsaturated acyl chains, and the interaction with the lipid bilayer of biological membranes as cationic vesicles became prominent with minimal membrane damage by DOTAP monomers.

To investigate the effects of flavonoids on free radical-mediated biological membrane damage and the interaction of flavonoids with heme proteins, we studied the effects of quercetin, its glycosides (rutin and quercetin-3-O-glucoside), morin and (−)epicatechin on the hemolysis of the bovine erythrocytes induced by the hydrophilic free radical initiator, 2,2′-azobis(2-amidinopropane)dihydrochloride (AAPH), and their interaction with hemoglobin. These flavonoids retarded the onset of the hemolysis dose-dependently. The effects of quercetin and other flavonoids were much greater than those of dihydric-phenols studied previously. The inhibitory effects of quercetin and its glycosides were stronger than those of morin and (−)epicatechin. In the absence of AAPH, the relatively hydrophobic flavonoids quercetin and morin induced the oxidation of oxyhemoglobin to methemoglobin. Oxidation by quercetin was especially marked. However, this oxidation did not induce hemolysis. These findings indicate that relatively hydrophobic flavonoids penetrate the cytoplasm of erythrocytes, interact with hemoglobin, and oxidize the heme iron.

Using excised guinea pig dorsal skin, we examined the effects of three surfactants, anionic sodium dodecylsulfate (SDS), cationic n-dodecyltrimethylammonium bromide and non-ionic n-dodecyl-β-D-maltoside, all of which commonly have an n-dodecyl group, on in vitro skin permeation of the cationic drug ketotifen. All these three surfactants increased the skin permeation of ketotifen. Among the surfactants tested, anionic SDS had the largest enhancement effects, and significantly increased the permeation at concentrations over 1 mM. The enhancement effect of the same anionic surfactant on the permeation of anionic salicylate was smaller and similar to that of cationic n-dodecyltrimethylammonium. The enhancement effects of SDS on ketotifen permeation were more marked than those of the cationic surfactant but differed from previous findings of their effects on other drugs permeation. Analysis of the retention of ketotifen in the skin suggested that SDS-induced increase in the transfer of hydrophilic ketotifen to the skin is the main reason for the marked increase in skin permeation.

The membrane fluorescent probe N-((4-(6-phenyl-1,3,5-hexatrienyl)phenyl)propyl)trimethyl-ammonium (TMAP-DPH) has an additional three-carbon spacer between the fluorophore and the trimethylammonium substituent of 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). As a basic study to clarify the transport mechanism of amphiphilic quaternary ammoniums, we observed the characteristics of the transbilayer transport of TMAP-DPH in bovine blood platelets and bovine adrenal chromaffin cells using the albumin extraction method. We compared these inward transport rates with those of TMA-DPH. TMAP-DPH crossed into the cytoplasmic layers of the membranes more slowly than TMA-DPH after rapid binding to the outer halves of the plasma membranes. The transport rate markedly depended on temperature. Time to reach the half-maximal incorporated amount of TMAP-DPH increased threefold accompanied by an increase in the concentration from 0.2 to 1.5 μm. The transport was stimulated significantly by various types of membrane perturbations such as modification of sulfhydryl-groups by N-ethylmaleimide and benzyl alcohol-induced increase in the fluidity of the lipid bilayer. The saturation phenomenon suggested the presence of the regulatory process in the transbilayer transport of TMAP-DPH.