**4.2 The formation and the anticancer mechanism of PG-PTX**

PG-PTX (paclitaxel poliglumex, CT-2103, Xyotax®, Opaxio®) is a water-soluble macromolecular conjugate that links PTX with PG (Li et al., 1996). PTX is conjugated by ester linkage to the γ-carboxylic acid side chains, leading to a relatively stable conjugate (Li et al., 1998a). Because the conjugation site is the 2 hydroxyl group of PTX, which is a crucial site for tubulin binding, the conjugate does not interact with β-tubulin and is inactive (Li, 2002; Rogers, 1993). The median MW of PG-PTX is 38.5 kDa, with a PTX content of approximately 36% on a w/w basis, equivalent to about one PTX ester linkage per 11 PG units of the polymer (Fig. 3) (Bonomi, 2007; Rogers, 1993).

Morphological analysis and biochemical characterizations demonstrate that both PTX and PG-PTX are able to induce apoptosis in cells expressing wild-type p53 or mutant p53, to arrest cells in the G2/M phase of the cell cycle, and to down-regulate HER-2/neu expression. Furthermore, when PG-PTX is compared with other water-soluble derivatives of PTX, including small-molecular-weight sodium pentetic acid-PTX and polyethylene glycol-PTX conjugate (MW 5 kDa), they all show the same effects on telomeric association, mitotic index chromatin condensation, and formation of apoptotic bodies (Multani et al., 1999). These results indicate that PG-PTX has the same mechanisms of action as PTX.

Fig. 2. Illustration of EPR effect and endocytosis. Different from blood vessels in normal tissue (A), those in tumor tissue (B) have porous openings, through which large-size conjugate leaks and is preferentially trapped and distributed to the tumor tissue. Once in the tumor tissue, the conjugate is taken up by the tumor cells through a cellular process called endocytosis (C). The conjugate releases active agent (D) via metabolism by lysosomal enzymes inside the lysosome of the tumor cell.
