Taxol-Conjugated Pamam Dendrimers Utilize Three Modes of Action on Microtubule Structure

Paclitaxel (Taxol) is a cancer drug that causes cell death by stabilizing microtubules and consequently arresting cell division. Previously, the cytotoxicity of taxol-conjugated PAMAM dendrimers was demonstrated in cancer cells. The exact mode(s) of action responsible for the potent cytotoxicity of these dendrimers were not examined but the literature provides uncertainty that the taxol could be released from the dendrimer carrier after cellular entry. Accordingly, we asked whether the taxol-dendrimer conjugate itself is able to bind microtubules. To address this question, we investigated the effect of these conjugates on microtubules in vitro using total internal reflection fluorescence microscopy (TIRFM) and transmission electron microscopy (TEM). We find that the taxol-dendrimer conjugate affects microtubule structure in two ways: (1) the conjugate can bind tubulin during tubulin polymerization and stabilize it into a tubular structure and (2) the conjugate can bundle microtubules in a manner that is not dependent on taxol, but dendrimer electrostatics. Both of these modes of action would arrest cell division and consequently kill the cell. This is the first time that the binding of taxol-conjugated dendrimers to microtubules has been demonstrated in vitro. Furthermore, our results provide both mechanistic insights into the cytotoxicity of the previously characterized taxol-conjugated PAMAM dendrimers and additional evidence for the potential of these and similar conjugates as cancer therapeutics.