In the near future, not only “systemic pharmacokinetics” but also “intracellular pharmacokinetics” seems to be important in Drug Delivery System (DDS) research for gene therapy. Beyond the basic philosophy of DDS of “delivering the optimal amounts of drugs to a target site”, it is now necessary to “express the gene (as a drug) efficiently in a target cell for a required period” in gene therapy. To achieve these objectives, vectors for introducing the gene into the target cell are being improved, and techniques to efficiently express the transgene and to regulate the transgene expression are being developed. DDS is expected to play a large part in achieving this goal. Here, we review a novel DDS technology to satisfy these criteria.

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We studied the intrahepatic disposition characteristics of galactosylated polyethylenimine (Gal-PEI)/plasmid DNA (pDNA) complexes using rat liver perfusion experiment. After intraportal administration, transfection activity in liver of Gal-PEI complexes was approximately 26-fold higher than that of native PEI complexes. To evaluate the relationship between hepatic gene expression and disposition profiles, hepatic disposition of Gal-PEI complexes were pharmacokinetically analyzed by use of perfused rat liver, which enables uptake characteristics intrinsic to the liver to be elucidated. Moment analysis revealed that both complexes exhibited very high single-pass extraction. To characterize each kinetic process in hepatic uptake of Gal-PEI complexes, their outflow profiles were analyzed based on a two-compartment dispersion model. Consequently, the tissue binding affinity of Gal-PEI complexes was 3.0-fold larger than that of native PEI complexes, suggesting the increasing of hepatic binding affinity much enhanced the hepatic gene transfection efficiency. In contrast, galactosylation of PEI did not affected internalization (and/or sequestration) rate.

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Phenolsulfonphthalein (PSP) has been selected as a model drug that is eliminated from both the kidney and liver in rats. Although the renal PSP transport system has been studied, few details of the biliary excretion of PSP have been reported. We investigated the biliary excretion system for PSP in rats. It has been reported that the biliary excretion of many organic anions from hepatocytes into bile is mediated by a primary active transporter, referred to as multidrug resistance-associated protein 2 (Mrp2/abcc2). The biliary excretion of PSP in SD rats was significantly decreased in the presence of Mrp2 inhibitors. The biliary excretion of PSP in Eisai hyperbilirubinemic rats (EHBR), hereditarily Mrp2-defective rats, was significantly lower than that in SD rats. Moreover, an efflux experiment using Caco-2 cells was carried out to confirm Mrp2-mediated PSP transport. Mrp2 inhibitors significantly decreased PSP efflux from Caco-2 cells. These results suggest that Mrp2 contributes to the biliary excretion of PSP in SD rats.

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To understand the mechanism underlying the highly liver-selective distribution of pitavastatin, uptake experiments were performed using rat hepatocytes.
The uptake of pitavastatin into rat hepatocytes is carrier-mediated and involved nonspecific diffusion in the presence of Na⁺. The michaelis constant (K_m) was 26.0 μmol/L, maximal uptake velocity (V_max) was 3124 pmol/min/mg protein, and non-specific uptake (P_dif) was 1.16 μL/min/mg protein. There were no remarkable differences in these kinetic parameters between the presence and absence of Na⁺.
Experiments using metabolic inhibitors revealed that energy-dependent systems contribute to the uptake of pitavastatin in the liver. Some organic anions reduced the uptake into rat hepatocytes in a concentration-dependent manner. The observed rates of inhibition of pitavastatin uptake by BSP, TCA and pravastatin were compared with the predicted rates. The predicted values were calculated, assuming that BSP, TCA and pravastatin inhibit the uptake of pitavastatin in a competitive manner. The observed inhibition by BSP and TCA was similar to that predicted, but the observed inhibition by pravastatin was considerably less than that predicted.
In conclusion, most of the pitavastatin taken up into the liver is transported by multiple carrier-mediated transporters such as Na⁺-independent multispecific anion transporters and energy-dependent transporters. In addition, these systems for pitavastatin may have features in common with the BSP and TCA transport system, and may partially involve the pravastatin transport system.

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To separately assess intestinal and hepatic first-pass effects with absorption ratio data, we have established an experimental model of rats double-cannulated into the portal and jugular veins. The model allows us to take blood samples simultaneously from conscious rats that have recovered from surgical damage. Double cannulation did not alter the physiological and hematological conditions. Moreover, the plasma concentration profiles of unchanged drug following oral and intravenous administration in the double-cannulated rats were not different from those of rats single-cannulated into the jugular vein. These results suggest that the model can be useful for separately assessing intestinal and hepatic first-pass effects.
We evaluated the first-pass effects in the intestine and the liver separately using this model. S-1452, as a model drug with 94% absorption ratio, was administered intravenously and orally to the double-cannulated rats, and the drug concentrations in the portal and systemic plasma were determined, and the rates of elimination from the intestine and liver were estimated. In the first pass, approximately 26% and 56% of the dose were extracted by the intestine and liver, respectively.
This method, in which the animal is not restricted nor under anesthesia, allows us to obtain reliable values of individual first pass effects in the intestine and liver. This method can also be an effective tool for assessing the site and extent of drug-drug interaction on the first-pass effects.

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Midazolam is a highly lipophilic drug that is widely used in preanesthetic medication. Recently, terpenes have been reported to show an enhancing effect on percutaneous absorption of drugs. The effect of terpenes (l-menthol, d-limonene, RS-(±)-β-citronellol, geraniol) on the in vitro percutaneous absorption of midazolam through rat skin was evaluated using unjacketed Franz diffusion cells. Since midazolam is a lipophilic drug, percutaneous penetration is low and a percutaneous penetration enhancer is necessary for its percutaneous absorption. The terpenes (5%, w/v) in combination with 30% ethanol, and 20% propylene glycol significantly increased the percutaneous absorption of midazolam in comparison to the control. In vitro data suggested that d-limonene is the most effective enhancer among terpenes and other penetration enhancers such as Azone^®. In in vivo percutaneous absorption assays, the midazolam formulation using d-limonene could penetrate through rat skin, but the other terpenes could not penetrate. In conclusion, d-limonene in combination with ethanol can be used to enhance the percutaneous absorption of the highly lipophilic drug midazolam.

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We sequenced all 13 exons of the CYP3A4 gene derived from 48 Japanese subjects. One subject possess the 20070 T>C mutation in the exon 10 (result in leu293Pro substitution, namely CYP3A4^*18), as heterozygote. Thus, we investigated the frequency of CYP3A4^*18 in 118 Japanese population using polymerase chain reaction-restriction fragment length polymorphism with Msp I and determined that the frequency of the CYP3A4^*18 allele was 1.3%.

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Two novel haplotypes of CYP2D6 were found in Japanese subjects. One haplotype of the human CYP2D6 gene, newly designated as CYP2D6^*44 allele, had both a novel single nucleotide polymorphism (SNP) of 2950G>C in intron 6 donor splice junction and a known SNP (82C<T). The newly detected mutation was as follows: SNP, 030418Tsubuko001; GENE NAME, CYP2D6; ACCESSION NUMBER, M33388; LENGTH, 25 bases; 5'-CGGATGTGCAGCG/CTGAGCCCATCTG-3'. In addition, we found the other haplotype, newly designated as CYP2D6^*21B allele, containing −1584C>G, −1235A>G, −740C>T, −678G>A, and a gene conversion with CYP2D7 gene in intron 1 associated with CYP2D6^*21. Both CYP2D6^*44 and CYP2D6^*21B alleles would cause a splicing error or a frameshift with impaired drug metabolizing function mediated by CYP2D6.

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