Carbon nanotubes (CNTs) have shown promise for in vivo cancer treatment by enabling efficient drug delivery. This study demonstrates the use of chemically functionalized single-walled carbon nanotubes (SWNTs) to deliver paclitaxel (PTX) to tumors in mice, achieving higher tumor suppression efficacy than the clinical drug Taxol®. The SWNTs were functionalized with branched polyethylene glycol (PEG) chains via a cleavable ester bond to form a water-soluble SWNT-PTX conjugate. This conjugate exhibits prolonged blood circulation and enhanced tumor uptake due to the enhanced permeability and retention (EPR) effect, leading to significantly higher tumor suppression efficacy. The SWNT-PTX conjugate was found to be non-toxic to normal organs, with drug molecules released from the nanotubes and excreted via the biliary pathway. The study also showed that SWNT-PTX has a higher tumor-to-normal organ PTX uptake ratio compared to Taxol® and PEGylated PTX, indicating its potential for effective cancer therapy with minimal side effects. The results suggest that SWNT-based drug delivery systems could be a promising platform for future cancer treatment. The study highlights the potential of carbon nanotubes as a versatile platform for drug delivery, with their unique properties enabling efficient drug targeting and delivery to tumors. The findings also emphasize the importance of optimizing the surface chemistry and size of nanotubes for desired pharmacokinetics and targeting efficiency. The study provides a foundation for further exploration of carbon nanotubes in therapeutic applications.Carbon nanotubes (CNTs) have shown promise for in vivo cancer treatment by enabling efficient drug delivery. This study demonstrates the use of chemically functionalized single-walled carbon nanotubes (SWNTs) to deliver paclitaxel (PTX) to tumors in mice, achieving higher tumor suppression efficacy than the clinical drug Taxol®. The SWNTs were functionalized with branched polyethylene glycol (PEG) chains via a cleavable ester bond to form a water-soluble SWNT-PTX conjugate. This conjugate exhibits prolonged blood circulation and enhanced tumor uptake due to the enhanced permeability and retention (EPR) effect, leading to significantly higher tumor suppression efficacy. The SWNT-PTX conjugate was found to be non-toxic to normal organs, with drug molecules released from the nanotubes and excreted via the biliary pathway. The study also showed that SWNT-PTX has a higher tumor-to-normal organ PTX uptake ratio compared to Taxol® and PEGylated PTX, indicating its potential for effective cancer therapy with minimal side effects. The results suggest that SWNT-based drug delivery systems could be a promising platform for future cancer treatment. The study highlights the potential of carbon nanotubes as a versatile platform for drug delivery, with their unique properties enabling efficient drug targeting and delivery to tumors. The findings also emphasize the importance of optimizing the surface chemistry and size of nanotubes for desired pharmacokinetics and targeting efficiency. The study provides a foundation for further exploration of carbon nanotubes in therapeutic applications.