In what ways does albumin influence paclitaxel's cellular uptake?

Albumin influences paclitaxel uptake mainly when paclitaxel is delivered as albumin-bound (nab-paclitaxel). Key ways:

- Improves solubility and pharmacokinetics
- Albumin keeps paclitaxel soluble without Cremophor EL, enabling longer circulation time and better tumor exposure.

- Endothelial transcytosis to the tumor
- Albumin binds to gp60 (albondin) on endothelial cells and triggers caveolae-mediated transcytosis.
- This transports albumin-bound paclitaxel across the blood vessel wall into the tumor interstitial space.

- Tumor accumulation and retention
- The tumor microenvironment often expresses SPARC (secreted protein acidic and rich in cysteine), which binds albumin.
- SPARC binding can concentrate and retain albumin-bound paclitaxel in the tumor vicinity, enhancing local exposure.

- Cellular uptake within the tumor
- After extravasation, tumor/stromal cells can internalize albumin-bound paclitaxel via albumin-binding pathways (potential receptors such as megalin/LRP1 and other albumin receptors).
- This uptake delivers paclitaxel closer to its intracellular target (tubulin).

- Drug release inside cells
- Once inside endosomes/lysosomes, paclitaxel dissociates from albumin and binds tubulin to exert its cytotoxic effect.

- Net effect
- The albumin carrier shifts delivery toward tumors, reduces solvent-related toxicity, and can enhance cellular uptake and intracellular drug delivery in tumors, though the degree of benefit can vary by tumor type and SPARC expression.

Note: While gp60-mediated transcytosis and SPARC interaction are well-described mechanisms in the Nab-paclitaxel paradigm, the extent to which SPARC or other albumin receptors predict response can be variable across cancers.