Current Drug Metabolism - Volume 2, Issue 4, 2001

In the last decade, a lot of amino acid transporters were identified by molecular cloning and assigned to the classically characterized amino acid transport systems. Among them, ones which belong to the heterodimeric amino acid transporter family are unique because of their broad substrate selectivity and their pathological implications as well as their structural features. The heterodimeric amino acid transporter family is a subfamily of SLC7 solute transporter family which includes 14-transmembrane cationic amino acid transporters as well as 12-transmembrane heterodimeric amino acid transporters. The members of heterodimeric amino acid transporter family are linked via a disulfide bond to single membrane spanning type II membrane glycoproteins such as 4F2hc (4F2 heavy chain) and rBAT (related to b0,+-amino acid transporter). Six members are associated with 4F2hc and one is linked to rBAT. The neutral amino acid transporter of this family seems to rely on the hydrophobic interactions for their substrate recognition which can explain their broad substrate selectivity. Because of this characteristic, they can permeate amino-acid-related drugs and contribute to the pharmacokinetics of these drugs. A neutral amino acid transporter LAT1 (L-type amino acid transporter 1) has actually been shown to be present at the blood-brain-barrier. Because the members of the heterodimeric amino acid transporter family exhibit variety of substrate selectivity, it is proposed that this family members have been diverged from the prototype neutral amino acid transporter such as LAT1 by acquiring the mechanisms for the recognition of electric charges on the substrate amino acid side chains. The dysfunction or hyperfunction of the members of the heterodimeric amino acid transporter family are involved in some diseases and pathologic conditions. The genetic defects of the renal and intestinal transporters BAT1 / b0,+AT (b0,+-type amino acid transporter 1 / b0,+-type amino acid transporter) and y+LAT1 (y+L-type amino acid transporter 1) result in the amino aciduria with sever clinical symptoms such as cystinuria and lysinuric protein intolerance, respectively. LAT1 is proposed to be involved in the progression of malignant tumor. xCT (x-C-type transporter) functions to protect cells against oxidative stress, while its over-function may be damaging neurons leading to the exacerbation of brain damage after brain ischemia. Therefore, these transporters would be candidates for therapeutic targets based on new strategies. Through the interaction with the associating proteins, the transporters of this family would be endowed with more possibility to be regulated via intracellular and extracellular signalling pathways, which is critical to tune the transporter functions to meet the metabolic requirements of cells.

Anthracycline antibiotics are very effective neoplastic agents widely used clinically. However, because of their many adverse effects (e.g. cardiotoxicity, leukopenia and alopecia), their clinical use has been limited. In order to minimize their adverse effects in clinical cancer chemotherapy, anthracyclines must be selectively transported into tumor cells. If there are differences in transport characteristics between tumor and normal cells, it should be possible to establish a strategy for selectively delivering anthracyclines to tumor cells on the basis of the differences. In human cultured leukemia HL60 cells, as tumor cells, and human fresh mononuclear cells, as normal cells, doxorubicin, pirarubicin, daunorubicin and idarubicin were incorporated via a common carrier-mediated system, but the carriers were different in the two cell types. In HL60 cells, it was indicated that a nucleoside transport system contributed, at least in part, to the transport of doxorubicin and pirarubicin, but not daunorubicin and idarubicin, and its contribution to pirarubicin transport was found in other tumor cells, i.e. mouse ovarian sarcoma M5076 and Ehrlich ascites carcinoma cells. On the other hand, in mononuclear cells, there was no involvement of a nucleoside transport system for the four anthracyclines examined. Therefore, we thought that with the modification of an anthracycline molecule as a substrate for the nucleoside transport system, the anthracycline could be delivered selectively to tumor cells.

Acquirement of drug resistance by tumor cells is a major chemotherapeutic problem. It is well known that typical multidrug resistance is caused by P-glycoprotein and multidrug resistance related protein (MRP1) which belong to the ATP binding cassette (ABC) transporter family. Ishikawa proposed that the ATP-dependent glutathione-S-conjugate export pump (GS-X pump) and phase III detoxification system are essential to drug metabolism, and this constituted a new concept in drug metabolism and the detoxification of xenobiotics. The GS-X pump has been revealed to belong to the ABC transporter family and suggested to the contribution to anticancer drug resistance. The GS-X pump actively effluxes the glutathione S-platinum (GS-Pt) complex. We cloned novel ABC transporter cDNA from the PC-14 / CDDP cell line, and the cloned cDNA was designated as a short-type MRP homologue, SMRP. Further investigation suggested that SMRP is a splicing variant of MRP5. The MRP5 mRNA levels in tumors from lung cancer patients treated with platinum regimen were significantly higher than in tumors from patients treated with non-platinum regimens, and the MRP5 expression levels were correlate with the GCS expression levels that is the rate-limiting step enzyme in glutathione biosynthesis. These results suggested that MRP5 take part in the function of GS-X pump. Recently many transporter molecules belong to the ABC transporter family such as MRP family have been identified, and appear to express in various human tissues. It can be presumed that their molecules are affected by the disposition and metabolism of drugs, but their substrates are still unclear. If the substrate specificity is revealed in the future, it is expected that the anticancer agents transporter, moreover anti cancer drug resistance mechanisms, can be clarified. This review is cited in the cisplatin resistance and the GS-X pump, and finally describes an overview of the MRPs substrates recently clarified, mainly about anticancer drugs.

Biochemical modulation, which is more effective with the use of antitumor agents, has recently played very important role in cancer chemotherapy. In this review, it was reported that some of the methylxanthine derivatives, e.g. caffeine, were useful for modulator and attempted to defined the relation between the effect of methylxanthine derivatives on the doxorubicin transport and antitumor activity. Caffeine and theobromine inhibited the doxorubicin efflux from tumor cells, increased the doxorubicin concentration in a tumor, and enhanced the antitumor effect of doxorubicin. However, the caffeine metabolites, which had no effect on the doxorubicin efflux, did not increase antitumor activity. Moreover, caffeine and theobromine did not enhance the side toxicity of doxorubicin on the lipid peroxide level, DNA biosynthesis and the doxorubicin concentrations in normal tissues.Moreover, we investigated the effect of the combination of doxorubicin with caffeine or theobromine on the change in cyclic adenosine 3',5'-monophosphate (cyclic AMP) in tissues in vivo, and the effect of cyclic AMP on doxorubicin efflux in vitro, and measured the distribution of caffeine and theobromine in normal and tumor tissues. In Ehrlich ascites carcinoma bearing mice, the level of cyclic AMP in a tumor was decreased by doxorubicin. With the combination of caffeine or theobromine and doxorubicin, the cyclic AMP level recovered to the control level. This tendency was not seen in normal tissues (heart and liver). Moreover, the doxorubicin efflux from the Ehrlich cells was inhibited on the addition of cyclic AMP in vitro. And the caffeine concentration in the tumors was the same as that in the heart, and was increased in combination with doxorubicin compared with that in the caffeine-only group during the 4 hr after caffeine treatment. Furthermore, the doxorubicin efflux was promoted by the supply of energy (addition of glucose), influx was decreased relatively, doxorubicin efflux needs the existence of glucose and the inhibition of energy related drug export pump by caffeine induced inhibition of doxorubicin efflux. The treatment of doxorubicin nor caffeine, and any treatment schedule did not change the amount and appearance of GLUT 1 as glucose transporter on Ehrlich ascites carcinoma cell. For the mentioned above, we thought as concerns the increase of antitumor activity of doxorubicin by caffeine which is xanthine derivatives as follows. Caffeine distributes, high level in tumor, keeps the cyclic AMP level, and effects glucose transport or doxorubicin transport depend on energy and inhibits doxorubicin efflux. And then DNA synthesis was increased with the maintenance the concentration of doxorubicin in tumor. These action did not show in normal tissues, caffeine did not influence the side toxicity of doxorubicin. These results suggested that caffeine which is one of xanthine derivatives will be useful for biochemical modulator.

A number of studies have appeared recently on the underlying mechanisms of liposome-cell interactions under in vitro conditions, in which isolated cell populations or cell lines were used. However, our knowledge of how liposomes interact with cells and the parameters that influence this in vivo is limited. We will summarize and discuss the relevant studies on this matter in this article. In addition, researchers in this field have long been aware of the interaction of liposomes with blood (or serum / plasma) proteins in vivo and their potential role in the process of the clearance of liposomes from the circulation. Some of the 'opsonizing' proteins, such as complement components, immunoglobulins, which enhance the interactions of liposomes with 'phagocytic cells' have been identified. However, the issue of which types of opsonins determine the fate of liposomes in vivo and how liposomal physicochemical properties such as size, charge and fluidity play an important role in the process of liposome clearance is not clear. Our own observations of one of opsonins, complement component are reviewed herein.As opposed to the fate of conventional liposomes, we briefly touch on the interaction of surface-modified liposomes, which are designed to avoid interactions with blood proteins and / or cells (sterically stabilized liposomes, long-circulating liposomes) and to actively target specific cells or tissues (targeted liposomes: immunoliposomes). Blood proteins such as opsonins are not usually thought to play an important role in the clearance of such liposomes.

There is a wide range of methods available for studying the transport of drugs across the blood-brain barrier (BBB) which is equipped with several systems to transport drugs as well as endogenous nutrients and waste products. The in vivo brain microdialysis technique, which allows direct sampling of the brain interstitial fluid (ISF), is a powerful means of characterizing influx and efflux transport across the BBB. In this paper, we review our results from the successful application of this technique to BBB drug transport studies. The drugs investigated include novel and CNS-active peptides, some agents that are actively removed from the brain ISF across the BBB, and a brain-directed prodrug.