Photoaffinity labeling approach to map the Taxol-binding site on the microtubule

Tritiation of azido-labeled diiodo cabazitaxel (Jevtana) and docetaxel (Taxotere) derivatives to generate 3 H-photoaffinity probes

Radiolabelled azidophenyl analogues can make powerful photoaffinity probes for the identification of molecular targets. We describe our efforts to prepare tritiated azidophenyl analogues of the taxols cabazitaxel and docetaxel. Late-stage tritiation by isotope exchange with diiodo precursors resulted in reduction of the azide moiety, which could only be overcome by addition of high excess of a sacrificial azide. Iodine-deuterium exchange experiments on a model system established that deiodination with concomitant azide reduction is a general problem when performing such isotope-exchange reactions on azide-containing aryl iodides.

Cabazitaxel is more active than first-generation taxanes in ABCB1(+) cell lines due to its reduced affinity for P-glycoprotein

PURPOSE

The primary aim of this study was to determine cabazitaxel's affinity for the ABCB1/P-glycoprotein (P-gp) transporter compared to first-generation taxanes.

METHODS

We determined the kinetics of drug accumulation and retention using [¹⁴C]-labeled taxanes in multidrug-resistant (MDR) cells. In addition, membrane-enriched fractions isolated from doxorubicin-selected MES-SA/Dx5 cells were used to determine sodium orthovanadate-sensitive ATPase stimulation after exposure to taxanes. Custom [³H]-azido-taxane analogues were synthesized for the photoaffinity labeling of P-gp.

RESULTS

The maximum intracellular drug concentration was achieved faster with [¹⁴C]-cabazitaxel (5 min) than [¹⁴C]-docetaxel (15-30 min). MDR cells accumulated twice as much cabazitaxel than docetaxel, and these levels could be restored to parental levels in the presence of the P-gp inhibitor PSC-833 (valspodar). Efflux in drug-free medium confirmed that MDR cells retained twice as much cabazitaxel than docetaxel. There was a strong association (r² = 0.91) between the degree of taxane resistance conferred by P-gp expression and the accumulation differences observed with the two taxanes. One cell model expressing low levels of P-gp was not cross-resistant to cabazitaxel while demonstrating modest resistance to docetaxel. Furthermore, there was a 1.9 × reduction in sodium orthovanadate-sensitive ATPase stimulation resulting from treatment with cabazitaxel compared to docetaxel. We calculated a dissociation constant (Kd) value of 1.7 µM for [³H]-azido-docetaxel and ~ 7.5 µM for [³H]-azido-cabazitaxel resulting in a 4.4 × difference in P-gp labeling, and cold docetaxel was a more effective competitor than cabazitaxel.

CONCLUSION

Our studies confirm that cabazitaxel is more active in ABCB1(+) cell models due to its reduced affinity for P-gp compared to docetaxel.

Protein structure modeling in the proteomics era

Structural proteomics aims to understand the structural basis of protein interactions and functions. A prerequisite for this is the availability of 3D protein structures that mediate the biochemical interactions. The explosion in the number of available gene sequences set the stage for the next step in genome-scale projects -- to obtain 3D structures for each protein. To achieve this ambitious goal, the slow and costly structure determination experiments are supplemented with theoretical approaches. The current state and recent advances in structure modeling approaches are reviewed here, with special emphasis on comparative protein structure modeling techniques.

Mechanisms of Taxol resistance related to microtubules

Since its approval by the FDA in 1992 for the treatment of ovarian cancer, the use of Taxol has dramatically increased. Although treatment with Taxol has led to improvement in the duration and quality of life for some cancer patients, the majority eventually develop progressive disease after initially responding to Taxol treatment. Drug resistance represents a major obstacle to improving the overall response and survival of cancer patients. This review focuses on mechanisms of Taxol resistance that occur directly at the microtubule, such as mutations, tubulin isotype selection and post-translational modifications, and also at the level of regulatory proteins. A review of tubulin structure, microtubule dynamics, the mechanism of action of Taxol and its binding site on the microtubule are included, so that the reader can evaluate Taxol resistance in context.