Paclitaxel-Conjugated PAMAM Dendrimers Adversely Affect Microtubule Structure through Two Independent Modes of Action

摘要

Paclitaxel (Taxol^®) is an anti-cancer drug that induces mitotic arrest via microtubule hyperstabilization, but causes side effects due to its hydrophobicity and cellular promiscuity. The targeted cytotoxicity of hydrophilic paclitaxel-conjugated polyamidoamine (PAMAM) dendrimers has been demonstrated in cultured cancer cells. Mechanisms of action responsible for this cytotoxicity are unknown—i.e., whether the cytotoxicity is due to paclitaxel stabilization of microtubules — as is whether paclitaxel is released intracellularly from the dendrimer. To determine whether the conjugated paclitaxel can bind microtubules, we used a combination of ensemble and single microtubule imaging techniques in vitro. We demonstrate that these conjugates adversely affect microtubules by: (1) promoting the polymerization and stabilization of microtubules in a paclitaxel-dependent manner; and (2) bundling pre-formed microtubules in a paclitaxel-independent manner, potentially due to protonation of tertiary amines in the dendrimer interior. Our results provide mechanistic insights into the cytotoxicity of paclitaxel-conjugated PAMAM dendrimers and uncover unexpected risks of using such conjugates therapeutically.