Shinichi Ohashi

Embolic materials for endovascular treatment of cerebral lesions.

Recently developed soft microcatheters can be maneuvered endovascularly into the brain, permitting treatment of lesions without conventional neurosurgery. Progress in biomaterial science has contributed significantly to the development of this new therapeutic modality termed intravascular neurosurgery or interventional neuroradiology. Although embolic materials play an important role, ideal materials have yet to be devised. Various embolic materials in clinical use are reviewed, such as cyanoacrylates, ethylene-vinyl alcohol copolymer mixtures, Ethibloc, ethanol, estrogen, poly(vinyl acetate), cellulose acetate polymer, poly(vinyl alcohol), gelatin sponges, microfibrillar collagen, surgical silk sutures, detachable balloons, and coils. The materials are reviewed in the context of treatment application for various brain lesions, such as arteriovenous malformations, cerebral aneurysms, and head and neck tumors. Further developments in biomaterial polymer science can bring about progress against brain diseases.

Application of thermosensitive polymers as a new embolic material for intravascular neurosurgery

Application of thermosensitive polymers as an embolic material for intravascular neurosurgery was investigated. We intended to use thermosensitive polymers to occlude vessels by precipitation in response to body temperature. Copolymers of N-isopropylacrylamide (NIPAM) and N-n-propylacrylamide (NPAM) were selected as thermosensitive polymers. To determine the optimal lower critical soluble temperature (LCST) for the embolic material, we developed an in vitro flow model. In this study the copolymers with an LCST of 24-26 degrees C showed appropriate precipitation. To prove the occlusion of vessels in vivo, we injected the copolymers into a rabbit kidney through a microcatheter. The extent of embolization was judged by angiography and histological examination. An acute toxicity test of the copolymer of NIPAM and NPAM was performed in comparison with that of the NIPAM monomer. The copolymer used in this paper showed no acute toxicity in mice. Water solubility, non-adhesiveness, and non-toxicity are the advantages of the use of thermosensitive polymers as an embolic material. By changing the LCST, various embolic materials can be designed. Based on our results, we believe that the application of thermosensitive polymers as a new embolic material is very promising.

Synthesis of the conjugate of superoxide dismutase with the copolymer of divinyl ether and maleic anhydride retaining enzymatic activity

Superoxide dismutase (SOD) was covalently conjugated with the copolymer of divinyl ether and maleic anhydride (DIVEMA) with average molecular weight of 30000 and a narrow molecular weight distribution (Mw/Mn= 1.4). To avoid the crosslinkage and the decrease in enzymatic activity, a pHreversible amino group-protecting agent, 1,1-dimethylmaleic anhydride (DMMAn), was applied in the synthesis. The amino groups of SOD were firstly protected by DMMAn; and then, after the remaining amino groups were reacted with the acid anhydride groups of DIVEMA, the protective agent was removed by lowering of the pH to 6.0. By use of this protective agent the crosslinked product could be minimized. The DIVEMA-SOD conjugate could be purified by preparative GPC. The resulting DIVEMA-SOD conjugate, free from native SOD, retained almost the same enzymatic activity as that of native SOD as determined by the method of McCord.

Modification of physico-chemical and biopharmaceutical properties of superoxide dismutase by conjugation to the co-polymer of divinyl ether and maleic anhydride

Abstract The co-polymer of divinyl ether and maleic anhydride (MW 30 000) was covalently conjugated to superoxide dismutase in the presence of the protecting agent 2,3-dimethylmaleic anhydride as previously reported in this journal (T. Hirano et al., Synthesis of the conjugate of superoxide dismutase with the copolymer of divinyl ether and maleic anhydride retaining enzymatic activity, J. Control. Release 28 (1994) 203–209). The polymer-enzyme conjugate obtained has been now characterised with respect to its physico-chemical, structural, pharmacokinetic and biological properties. Polymer conjugation was found not to modify the protein structure as revealed by circular dichroism measurements but did enhance the thermal stability of the enzyme. On the other hand, covalent coupling of the polymer on the protein surface was found to decrease the reversible capability of exchanging the metal co-factor Cu 2+ of superoxide dismutase. The pharmacokinetic profile of the conjugate after intravenous administration in rats indicated that the conjugate presented a rapid distribution phase, while the elimination rate was significantly delayed with respect to the native form. Immunoprecipitation studies were performed by using anti-superoxide dismutase serum, which was enriched with polyclonal antibodies. Decreased antigenicity of the polymer conjugate with respect to the native form was observed.

Enhancement of macrophage and natural killer cell antitumor activity by DIVEMA-adriamycin conjugate.

Divinyl ether and maleic anhydride (DIVEMA) is a synthetic polyanion known to stimulate antitumor activity in macrophages (Mφ) and natural killer cells (NK). This compound is used as a carrier of anticancer drugs because of its efficient covalent binding and release of anti-cancer drugs. Recently, the conjugate of DIVEMA with adriamycin (ADM) was reported to have higher tumor control activity in mice-bearing intraperitoneal dissemination of fibrosarcoma (FSaI) cells than either DIVEMA or ADM alone. We here report on the effects of intraperitoneal (ip) injection of this conjugate on the cytotoxicity of peritoneal Mφ and splenic NK against FSaI cells. The conjugate clearly enhanced Mφ cyotoxicity, the maximum cytotoxicity being obsereved 7 days after ip. This cytotxicity enhancement was similar to the activation by DIVEMA, but it was stronger than that by ADM alone. On the other hand no activation of splenic NK by administration of either the conjugate or ADM was observed, despite remarkable NK activation by DIVEMA. These findings indicate that a part of the immunostimulatory activity of DIVEMA remains in the conjugate when DIVEMA is used as a carrier for ADM. Consequently the data also suggest that the antitumor effects of the conjugate resulted from combined effects of the host-mediated activity and the direct cytotoxicity of ADM itself.

Pharmacokinetical change of adriamycin by the conjugation with DIVEMA(a copolymer of divinylether and maleic anhydride).

Adriamycin (ADR) was covalently conjugated with a copolymer of divinyl ether and maleic anhydride (DIVEMA). Either sole ADR or DIVEMA-ADR conjugate was injected intraperitoneally into mice, and the concentration of ADR in the serum and peritoneal fluid (PF) was measured by HPLC (high-performance liquid chromatography). A reversed phase liquid chromatography column enabled to measure the concentration of ADR as low as 6 ng/ml. In sole ADR administration, the ADR concentration in the PF reached 179 ng/ml at one hour then gradually decreased to 13 ng/ml at 24 hours after i.p. injection. In contrast, the concentration in the serum fell rapidly from 16.8 ng/ml at one hour to 6.6 ng/ml at 2 hours after the injection. In DIVEMA-ADR conjugate, the ADR concentration in the PF was elevated to 4, 057 ng/ml at one hour and decreased more slowly than in sole ADR. Even at 24 hours after i.p. administration of the conjugate, ADR was kept as high as 90 ng/ml in the PF. On the other hand, the serum concentration remained under the limit of detection at all times measured. Pharmacokinetic parameters were obtained based on the time-concentration curves of either sole ADR or DIVEMA-ADR administration, using two-compartment model. The results showed that the conjugate increased the AUC(area under the curve) of ADR in the PF by 15.4 times higher than that of sole ADR. These remarkable ability of DIVEMA conjugate on pharmacokinetics of ADR would be greatly contributable for improvement of cancer chemotherapy, especially for the treatment of peritoneal dissemination from intraabdominal malignancies.