Chemical modification of paclitaxel (Taxol) reduces P-glycoprotein interactions and increases permeation across the blood-brain barrier in vitro and in situ.

The purpose of this work was to introduce a chemical modification into the paclitaxel (Taxol) structure to reduce interactions with the product of the multidrug resistant type 1 (MDR1) gene, P-glycoprotein (Pgp), resulting in improved blood-brain barrier (BBB) permeability. Specifically, a taxane analogue, Tx-67, with a succinate group added at the C10 position of Taxol, was synthesized and identified as such a candidate. In comparison studies, Tx-67 had no apparent interactions with Pgp, as demonstrated by the lack of enhanced uptake of rhodamine 123 by brain microvessel endothelial cells (BMECs) in the presence of the agent. By contrast, Taxol exposure substantially enhanced rhodamine 123 uptake by BMECs through inhibition of Pgp. The transport across BMEC monolayers was polarized for both Tx-67 and Taxol with permeation in the apical to basolateral direction greater for Tx-67 and substantially reduced for Taxol relative to basolateral to apical permeation. Taxol and cyclosporin A treatments also did not enhance Tx-67 permeation across BMEC monolayers. In an in situ rat brain perfusion study, Tx-67 was demonstrated to permeate across the BBB at a greater rate than Taxol. These results demonstrate that the Taxol analogue Tx-67 had a reduced interaction with Pgp and, as a consequence, enhanced permeation across the blood-brain barrier in vitro and in situ.

Overcoming the blood-brain barrier to taxane delivery for neurodegenerative diseases and brain tumors.

The blood-brain barrier (BBB) effectively prevents microtubule (MT)-stabilizing drugs from readily entering the central nervous system (CNS). A major limiting factor for microtubule-stabilizing drug permeation across the BBB is the active efflux back into the circulation by the overexpression of the multidrug-resistant gene product 1 (MDR1) or P-glycoprotein (P-gp). This study has focused on strategies to overcome P-gp-mediated efflux of Taxol analogs, MT-stabilizing agents that could be used to treat brain tumors and, potentially, neurodegenerative diseases such as Alzheimer's disease. However, taxol is a strong P-gp substrate that limits its distribution across the BBB and therapeutic potential in the CNS. We have found that addition of a succinate group to the C-10 position of paclitaxel (Taxol) results in an agent, Tx-67, with reduced interactions with P-gp and enhanced permeation across the BBB in both in vitro and in situ models. Our studies demonstrate the feasibility of making small chemical modifications to Taxol to generate analogs with reduced affinity for the P-gp but retention of MT-stabilizing properties, i.e., a taxane that may reach and treat therapeutic targets in the CNS.

Amyloid peptide toxicity and microtubule-stabilizing drugs.

Based on microtubule (MT) disruption observed in primary neurons exposed to fibrillar amyloid peptides (A beta), we tested the potential protective effect of MT-stabilizing drugs such as Taxol against A beta-induced disruption of the cytoskeleton. Although Taxol was strongly protective, the fact that it does not cross the blood brain barrier (BBB) led us to synthesize and test other agents with MT-stabilizing properties and possible penetration into the brain. Our studies have thus far demonstrated that several MT-stabilizing agents, including some with structures quite different from that of Taxol, showed significant protective effects. However, not all agents that promoted MT-assembly were protective, suggesting additional mechanisms are involved in the actions of the drugs. A small number of neuroprotective compounds appear to have potential to enter the brain and thus might be tested to see if they slow progression of neurodegeneration in an appropriate animal model of Alzheimer's disease.

Single-site chemical modification at C10 of the baccatin III core of paclitaxel and Taxol C reduces P-glycoprotein interactions in bovine brain microvessel endothelial cells.

A single-site modification of paclitaxel analogs at the C10 position on the baccatin III core that reduces interaction with P-glycoprotein in bovine brain microvessel endothelial cells is described. Modification and derivatization of the C10 position were carried out using a substrate controlled hydride addition to a key C9 and C10 diketone intermediate. The analogs were tested for tubulin assembly and cytotoxicity, and were shown to retain potency similar to paclitaxel. P-glycoprotein interaction was examined using a rhodamine assay and it was found that simple hydrolysis or epimerization of the C10 acetate of paclitaxel and Taxol C can reduce interaction with the P-glycoprotein transporter that may allow for increased permeation of taxanes into the brain.