Toshiyuki Sakaeda

The monitoring of the blood concentration at 2 h (C₂) after the oral administration of a cyclosporine (CsA) microemulsion was reconfirmed to be useful for the prediction of systemic exposure, the area under the blood concentration-time curve from 0 to 4 h (AUC_0-4), in a group of Japanese patients, consisting of 33 children aged 5-15 years and 19 young adults aged 16-27 years, with a greater correlation for C₂ (r = 0.927) than the trough concentration (r = 0.488). The dose-normalized AUC_0-4 was independent of gender or indications for CsA, while it depended on body size, i.e., the age (P = 0.065) and total body weight (P = 0.026). MDR1 C3435T had a weak, but insignificant effect (P = 0.072); it was about 22-31% lower in the patients with TT³⁴³⁵. Co-administration of a steroid and further treatment with nifedipine had a more intensive effect (P = 0.018); co-administration resulted in a 51% increase in the dose-normalized AUC_0-4. A strong effect was also observed for the serum total cholesterol level (P = 0.001). Collectively, the discrepancies in the results on MDR1 C3435T among investigators might be due to variability in the age/total body weight, co-administration drugs or serum lipid level.

With the growing clinical usage of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins), the number of reports concerning serious drug–drug interaction has been increasing. Because recent studies have shown that conversion between acid and lactone forms occurs in the body, drug–drug interaction should be considered on both acid and lactone forms. Thus, we investigated the inhibitory effects of acid and lactone forms of eight statins, including one recently withdrawn, cerivastatin, and two recently developed, pitavastatin and rosuvastatin, on cytochrome P450 (CYP) 2C8, CYP2C9, and CYP3A4/5 metabolic activities and multidrug resistance protein 1 (MDR1) transporting activity. The inhibitory effects of statins on CYP metabolic activities and MDR1 transporting activity were investigated using human liver microsomes and MDR1-overexpressing LLC-GA5-COL150 cells, respectively. The acid forms had minimal inhibitory effects on all CYP activities tested, except for fluvastatin on CYP2C9-mediated tolbutamide 4-hydroxylation (IC₅₀ = 1.7 μM) and simvastatin on CYP3A4/5-mediated paclitaxel 3-hydroxylation (12.0 μM). Lactone forms showed no or minimal inhibitory effects on CYP2C8, CYP2C9, and CYP2C19 activities, except for rosuvastatin on the CYP2C9 activity (20.5 μM), whereas they showed stronger inhibitory effects on the CYP3A4/5 activity with the rank order of atorvastatin (5.6 μM), cerivastatin (8.1 μM), fluvastatin (14.9 μM), simvastatin (15.2 μM), rosuvastatin (20.7 μM), and lovastatin (24.1 μM). Pitavastatin and pravastatin had little inhibitory effect, and a similar order was found also for testosterone 6β-hydroxylation. MDR1-mediated transport of [³H]digoxin was inhibited only by lactone forms, and the rank order correlated with that of inhibitory effects on both CYP3A4/5 activities. Inhibitory effects on MDR1 activity, and on both CYP3A4/5 activities, could be explained by the lipophilicity; however, a significant correlation was found between the lipophilicity and inhibitory effects on CYP2C8-mediated paclitaxel 6α-hydroxylation. We showed the difference between the acid and lactone forms in terms of drug interaction. The lipophilicity could be one of the important factors for inhibitory effects. In the case of statins, it is important to examine the effects of both forms to understand the events found in clinical settings, including the pleiotropic effects.

The effects of micafungin on cytochrome P450 3A4 (CYP3A4) metabolic and multidrug resistance protein 1 (MDR1) transport activities were investigated and compared with those of amphotericin B and four azole antifungal drugs (ketoconazole, itraconazole, fluconazole and miconazole). The effects on the metabolic activity of CYP3A4 were examined by measuring nifedipine oxidase activity in human liver microsomes and the effects on MDR1 transport activity were evaluated using [3H]digoxin in MDR1-overexpressing LLC-GA5-COL150 cells. An inhibitory effect on CYP3A4 activity was found for ketoconazole, itraconazole and miconazole, with 50% inhibitory concentrations of 11.7, 32.6 and 74.2 nM, respectively. Fluconazole and micafungin had only slight inhibitory effects and amphotericin B had no effect. The MDR1-mediated transport of [3H]digoxin was inhibited by ketoconazole and itraconazole, and slightly by miconazole. It is suggested that micafungin and amphotericin B would be unlikely to cause drug-drug interactions by inhibition of CYP3A4 and MDR1. A positive correlation between the inhibitory effects on CYP3A4 and MDR1 activities was observed, and the physicochemical mechanisms involved and impact on clinical treatment should be studied further.

Multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. A number of various types of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics. The first investigation of the effects of MDR1 genotypes on pharmacotherapy was reported in 2000; a silent single nucleotide polymorphism (SNP), C3435T in exon 26, was found to be associated with the duodenal expression of MDR1, and thereby the plasma concentration of digoxin after oral administration. In the last 5 years, clinical studies have been conducted around the world on the association of MDR1 genotype with MDR1 expression and function in tissues, and with the pharmacokinetics and pharmacodynamics of drugs; however, there are still discrepancies in the results on C3435T. In 1995, a novel concept to predict in vivo oral pharmacokinetic performance from data on in vivo permeability and in vitro solubility has been proposed, and this Biopharmaceutical Classification System strongly suggested that the effects of intestinal MDR1 on the intestinal absorption of substrates is minimal in the case of commercially available oral drugs, and therefore MDR1 genotypes are little associated with the pharmacokinetics after oral administration. This review summarizes the latest reports for the future individualization of pharmacotherapy based on MDR1 genotyping, and attempts to explain discrepancies.

The reversing effects of carvedilol and other β‐adrenoceptor antagonists on multidrug resistance (MDR) were assessed in HeLa cells and the MDR1‐overexpressing derivative Hvr100–6 cells, established by stepwise increases of vinblastine concentration in the culture medium. The inhibitory effects on the transcellular transport and intracellular accumulation of [³H]vinblastine and [³H]daunorubicin were also assessed using LLC‐GA5‐COL150 cell monolayers, established by transfection of human MDR1 cDNA into porcine kidney epithelial LLC‐PK, cells. The cytotoxic effects of vinblastine, paclitaxel, doxorubicin and daunorubicin in Hvr100–6 were reversed 1.4‐ to 7.1‐fold by carvedilol at the realistic clinical concentration of 1 μM, whereas other β‐adrenoceptor antagonists had weaker or no such effects. Transport experiments using LLC‐GA5‐COL150 cell monolayers demonstrated that this effect of carvedilol was due to the inhibition of MDR1‐mediated transport of vinblastine, paclitaxel, doxorubicin and daunorubicin. These MDR1‐mediated reversing effects of carvedilol were similar to those of 1 μM verapamil, suggesting that carvedilol could be a candidate modulator of MDR in clinical use. Since other β‐adrenoceptor antagonists had no inhibitory effect on transport, the effects of carvedilol were not related to β‐adrenoceptors and might have been due to antioxidant activity. (Cancer Sci 2003; 94: 81–86)

Purpose. Recently, MDR1 (P-glycoprotein) and related transporters have been suggested to play a fundamental role in regulating apo- ptosis, but little information is available concerning the role of...

The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, pravastatin, was compared with simvastatin and lovastatin from the viewpoint of susceptibility to interaction with or via the multidrug transporter, MDR1 (P-glycoprotein). This was carried out using the MDR1-overexpressing cell line LLC-GA5-COL150, established by transfection of MDR1 cDNA into porcine kidney epithelial LLC-PK1 cells, and [3H]digoxin, which is a well-documented substrate for MDR1. Pravastatin, at 25–100 μM, had no effect on the transcellular transport of [3H]digoxin whereas simvastatin and lovastatin suppressed the basal-to-apical transport of [3H]digoxin and increased the apical-to-basal transport. It was suggested that recognition by MDR1 was due to the hydrophobicity. In conclusion, simvastatin and lovastatin are susceptible to interaction with or via MDR1, but pravastatin is not. This is important information when selecting the HMG-CoA reductase inhibitors for patients taking drugs that are MDR1 substrates.

This study was performed to determine the exclusion criteria that differentiate poorly absorbed drugs from good drug candidates, and to accelerate drug development by exclusion of unnecessary assessment. The molecular and pharmacokinetic properties of 222 commercially available oral drugs were tabulated and their correlations were analyzed. The exclusion criteria obtained were 1) a molecular weight of more than 500, and 2) a ClogP value of more than 5. Exceptions to molecular weight criteria were compounds with a sugar moiety, high atomic weight, and large cyclic structure. It was also suggested that being a substrate for MDRI (P-glycoprotein) does not always result in poor bioavailability, and that drug development by chemical modification of a seed or lead compound with quantitative structure activity relationship analysis can result in lower bioavailability, higher bound fraction and lower urinary excretion, which would hamper later development processes and might result in considerable drug-drug interaction. The criteria should be adjusted according to the pharmacological profiles of the agents in question and depending on the estimated profit, but ignoring these criteria may result in a significant waste of time and money during drug development.