Paclitaxel (1, Taxol ® ), a diterpenoid originally isolated from the bark of the Pacific yew, Taxus brebiforia, is a powerful inhibitor of microtubule assembly in tumor cells and is currently used in the treatment of various kinds of cancers.
1)However, poor water solubility (0.4 mg/ml) compromises its use as an anticancer agent. Cremophor EL ® (polyoxyethylated castor oil) was therefore designed as a solubilizing agent to enhance water solubility (60 mg/ml), but causes hypersensitivity in patients. Conversely, docetaxel (2, Taxotere ® ), an analogue of paclitaxel exerting powerful antitumor activity, is more soluble (14 mg/ml) than paclitaxel, but solubility is not greatly improved.To date, much effort has been devoted to the synthesis of paclitaxel analogues with reduced side effects and improved solubility. For instance, a number of taxoids bound to hydrophilic functionalities such as polyethylene glycols, [2][3][4] amino acids, 5) phosphates, 6-8) malic acid 9) and sugars 10,11) have been identified as prodrugs of 1. Interestingly, a naturally occurring glycoside-bound paclitaxel (7-O-xylosylpaclitaxel, 3 12,13) ) reportedly shows potent activity in the disassembly of microtubules, which prompted us to export new sugar-bound taxoids.At an earlier stage of our project, efforts were made to synthesize sugar-bound taxoids enzymatically. However, the attempted glycosylation with a variety of enzymes failed entirely, probably due to both the intrinsically poor water-solubility and reactivity of 1. Accordingly, we turned our attention to the chemical synthesis of sugar-bound taxoids. Two problematic issues exist in the glycosylation of 7-OH of 1. One is the poor solubility of 1 in organic solvent, and the other is the inevitable use of a Lewis acid to activate anomeric functionalities of sugars. We were thus concerned that the baccatin nucleus would be damaged if conventional glycosylation protocols were adopted. In this context, we designed glycosyloxy acetic acid ( 4 14) ) and achieved esterification of 4 to 7-OH of 1.We have recently reported on the water-solubility and in vitro antitumor activity of taxoids 5a-d and 6a-e, which are esterified with 4 at either 7-OH of 1 or 10-OH of 2.15) We report herein in vivo antitumor activity of these new taxoids. (Fig. 3, 5a-d, 6a-e) was synthesized as described previously.
MATERIALS AND METHODS
Chemicals and Agents Each taxoid15) The quantity of those taxoids was analyzed by high-performance liquid chromatography (HPLC) on a C18 column (Taxil 5 mm, 250ϫ4.6 mm I.D.; MetaChem Technologies, Torrance, CA, U.S.A.) using methanol/waterϭ65/35 as eluent with a flow rate of 0.5 ml/min at 35°C. Taxoids were detected by absorption at 230 nm using an SPV-10A photodiode array detector (Shimadzu, Kyoto, Japan). Human serum AB type was purchased from Dainippon Pharmaceutical (Osaka, Japan). Rat serum was prepared from Sprague Dawley rats (Nippon Ensuiko Sugar Refining Co., Ltd.; Japan: b Kaken Pharmaceutical Co., Ltd.; Shinomiya, Japan: and c Department of Life Science, Kurashik...