Takeo Kawaguchi

Clinical Pharmacokinetics of Cytarabine Formulations

Cytarabine (cytosine arabinoside, Ara-C) is an effective chemotherapeutic agent for the treatment of acute myelogenous leukaemia and lymphocytic leukaemias. As cytarabine is an S-phase-specific drug, prolonged exposure of cells to cytotoxic concentrations is critical to achieve maximum cytotoxic activity. However, the activity of cytarabine is decreased by its rapid deamination to the biologically inactive metabolite uracil arabinoside. This rapid deamination is the reason for the ongoing search for effective formulations and derivatives of cytarabine that cannot be deaminated and exhibit better pharmacokinetic parameters.Protection of cytarabine from fast degradation and elimination has been investigated by encapsulating the drug into pharmaceutically acceptable carriers. Cytarabine derivatives have shown promise in vitro and in animal models. For example, ancitabine (cyclocytidine), enocitabine and cytarabine ocfosfate have been used clinically in Japan. Cytarabine encapsulated into multivesicular liposomes has been approved in several countries for the intrathecal treatment of lymphomatous meningitis.Although many compounds have been investigated, few cytarabine derivatives are currently available for clinical use. Further research is needed to improve the efficacy of cytarabine against haematological and solid tumours.

Enhanced delivery of zidovudine through rat and human skin via ester prodrugs

In an attempt to improve the skin delivery characteristics of Zidovudine (AZT, azidothymidine), five aliphatic esters (acetate, butyrate, hexanoate, octanoate, and decanoate) of AZT were synthesized and assessed as prodrugs of AZT. While the water solubility of the esters is lower than that of AZT, the solubilities in isopropylmyristate (IPM) and the partition coefficients (n-octanol: buffer) are higher. Susceptibility to enzymatic hydrolysis in the rat skin homogenate increases as the acyl chain of the ester is lengthened. Among the esters, acetate (C2-AZT) and hexanoate (C6-AZT) showed 2.4- and 4.8-fold enhanced permeation in human skin from an apolar vehicle (IPM) relative to application of AZT itself, respectively.

Sustained transdermal delivery of zidovudine via controlled release of penetration enhancer

Abstract In vitro permeation of zidovudine (azidothymidine, AZT), a potent antiviral agent acting on acquired immunodeficiency syndrome, through a rat skin from isopropyi myristate (IPM) solution was significantly enhanced by the addition of N -methyl-2-pyrrolidone (MP) as a penetration enhancer. When 50 μl of the IPM solution containing 12 mg/ml of AZT and 20% of MP was applied on rat abdominal skin, about 1 μM of the plasma concentration of AZT was observed at 1–2 h after the application, though the concentration decreased rapidly and was undetectable at 6 h. Though 34% of the applied AZT remained in the IPM solution on the rat skin at 10 h, MP was not detected. In order to maintain a constant level of MP in the solution, ethylene-vinyl acetate copolymer membrane was used for its controlled release in the transdermal delivery system of AZT. A considerable plasma concentration of AZT could be maintained for 10 h after application of this MP release-controlled transdermal system.

Intraarterial infusion of 5‐fluoro‐2‐deoxyuridine‐C8 dissolved in a lymphographic agent in malignant liver tumors. A preliminary report

The drug 5‐fluoro‐2‐deoxyuridine‐C8 (FUdR‐C8), one of the lipophilic prodrugs of FUdR, was dissolved in an oily lymphographic agent (Lipiodol Ultra Fluid, Andre Gelbe Laboratory, Paris, France; Ethiodol, Savage Laboratories, Melville, NY) and used for the intraarterial treatment of malignant liver tumors. From August 1985 to June 1988, 33 patients with hepatocellular carcinoma and 13 patients with metastatic liver tumors were treated with this agent at the Kumamoto University Hospital and its affiliated hospitals. The response rate (complete remission [CR] and partial remission [PR]) was 27.6% for hepatocellular carcinomas and 46.1% for metastatic liver tumors. The cumulative 1‐year survival rate was 55.1% for hepatocellular carcinomas and 70.0% for metastatic liver tumors. More than a 50% decrease in the tumor marker level was observed in ten of 21 patients with hepatocellular carcinoma and in two of eight patients with metastatic liver tumors. The side effects, which were transient and controlled with conservative treatment, included fever, abdominal pain, nausea, vomiting, and acute gastritis. Cancer 64:2437–2444, 1989.

Synthetic Nucleosides and Nucleotides. 40. Selective Inhibition of Eukaryotic DNA Polymerase α by 9-(β-D-Arabinofuranosyl)-2-(p-n-butylanilino)adenine 5′-Triphosphate (BuAaraATP) and Its 2′-Up Azido Analog: Synthesis and Enzymatic Evaluations†

Starting from 2,3,5-tri-O-acetyl-2-iodoadenosine, 9-(beta-D-arabinofuranosyl)-2-(p-n-butylanilino)adenine and its 2(S)-azido counterparts were synthesized in seven steps. These exhibited only moderate growth-inhibitory effects against mouse leukemic P388 cells (IC50 = 13-24 microM), although 5-triphosphate derivatives showed strong and selective inhibitory action on calf thymus DNA polymerase alpha, but not on beta- and epsilon-polymerases from eukaryotes.

Percutaneous absorption of azidothymidine in rats

Abstract Percutaneous absorption of azidothymidine (AZT) was examined in rats. When AZT was applied as a solution in 10% oleic acid/water on the abdominal skin which had been treated with 10% oleic acid/water for 24 h, considerable plasma concentrations of AZT were observed.

Bromocriptine Reverses P-Glycoprotein-mediated Multidrug Resistance in Tumor Cells

One of the most important causes of anticancer treatment failure is the development of multidrug resistance (MDR). The main characteristics of tumor cells displaying the MDR phenomena are cross‐resistance to structurally unrelated cytotoxic drugs having different mechanisms of action and the overexpression of the MDR1 gene, which encodes a transmembrane glycoprotein named P‐ glycoprotein (P‐gp). This study evaluated whether bromocriptine, a D2 dopaminergic receptor agonist, influenced anticancer drug cytotoxicity and P‐gp activity in a P‐gp‐expressing cell line compared to a non‐expressing subline. The Ki values for P‐gp of cyclosporine and verapamil were 1.09 and 540 μM, respectively, and that of bromocriptine was 6.52 μM in a calcein‐AM efflux assay using porcine kidney epithelial LLC‐PK1 and L‐MDR1 cells, overexpressing human P‐gp. Bromocriptine at 10 μM reduced the IC50 of doxorubicin (DXR) in K562‐DXR from 9000 to 270 ng/ml and that of vincristine (VCR) in K562‐VCR from 700 to 0.30 ng/ml, whereas the IC50 values of DXR and VCR in the K562 subline were only marginally affected by these drugs. Bromocriptine restored the anticancer effect of DXR, VCR, vinblastine, vinorelbine and etoposide on MDR‐tumor cells overexpressing P‐gp. These observations suggest that bromocriptine has the potential to reverse tumor MDR involving the efflux protein P‐gp in the clinical situation.

Antileukemic Activities and Mechanism of Action of 2′-Deoxy-4′-methylcytidine and Related Nucleosides

Abstract Antileukemic activity of several analogues containing 2′-deoxy-4′-methylcytidine and its araC counterpart were evaluated against murine leukemic P388 cells in vitro and in vivo. Both compounds showed significant cytostatic activity (both IC50=0.4 μM) in vitro and the former compound administered intraperitoneally at a dose of 3 mg/kg/day × 5 showed high activity (T/C=175%) in vivo. The mechanism of action of these 5′-triphosphates on DNA polymerases in detail will be also described.

Rectal absorption of Zidovudine

Abstract Rectal absorption of Zidovudine (3′-azido-3′-deoxythymidine, AZT) was investigated in order to determine the feasibility of developing AZT suppositories. Although the small intestine shows a higher absorption rate than the stomach and large intestine in rats, considerable absorption of AZT was observed from the lower large intestine (8 cm above the anus). A sustained-release suppository was prepared by direct compression of hydroxypropyl cellulose (HPC) with AZT. During in vivo experiments in rats, this suppository (10 mg or 37.5 μmol AZT/kg) maintained constant plasma levels above 1 μM for more than 6 h. The results suggest that suppositories can prove useful as an alternative dosage form of AZT administration.

Nonenzymatic and Enzymatic Hydrolysis of 5-Fluoro-2′-deoxyuridine (FUdR) Esters

The chemical and enzymatic reactivity of 5-fluoro-2′-deoxyuridine prodrugs esterified at the 3′ and 5′ positions with several acyl groups has been investigated. The enzymatic reactivity was affected by the acyl structure, the site of esterification, and the number of esters in the prodrug molecule.