The Sulfamate Small Molecule CAIX Inhibitor S4 Modulates Doxorubicin Efficacy

PLoS One. 2016 Aug 11;11(8):e0161040. doi: 10.1371/journal.pone.0161040. eCollection 2016.


Carbonic anhydrase IX (CAIX) is a tumor-specific protein that is upregulated during hypoxic conditions where it is involved in maintaining the pH balance. CAIX causes extracellular acidification, thereby limiting the uptake of weak basic chemotherapeutic agents, such as doxorubicin, and decreasing its efficacy. The aim of this study was to determine if doxorubicin efficacy can be increased when combined with the selective sulfamate CAIX inhibitor S4. The effect of S4 on doxorubicin efficacy was tested in vitro using cell viability assays with MDA-MB-231, FaDu, HT29 -CAIX high and HT29 -CAIX low cell lines. In addition, the efficacy of this combination therapy was investigated in tumor xenografts of the same cell lines. The addition of S4 in vitro increased the efficacy of doxorubicin in the MDA-MB-231 during hypoxic exposure (IC50 is 0.25 versus 0.14 µM, p = 0.0003). Similar results were observed for HT29-CAIX high with S4 during normoxia (IC50 is 0.20 versus 0.08 µM, p<0.0001) and in the HT29 -CAIX low cells (IC50 is 0.09 µM, p<0.0001). In vivo doxorubicin treatment was only effective in the MDA-MB-231 xenografts, but the efficacy of doxorubicin was decreased when combined with S4. In conclusion, the efficacy of doxorubicin treatment can be increased when combined with the selective sulfamate CAIX inhibitor S4 in vitro in certain cell lines. Nevertheless, in xenografts S4 did not enhance doxorubicin efficacy in the FaDu and HT29 tumor models and decreased doxorubicin efficacy in the MDA-MB-231 tumor model. These results stress the importance of better understanding the role of CAIX inhibitors in intratumoral pH regulation before combining them with standard treatment modalities, such as doxorubicin.

MeSH terms

  • Animals
  • Antibiotics, Antineoplastic / pharmacology
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / enzymology
  • Breast Neoplasms / pathology
  • Carbonic Anhydrase IX / antagonists & inhibitors
  • Carbonic Anhydrase Inhibitors / pharmacology*
  • Cell Hypoxia / drug effects
  • Cell Proliferation / drug effects
  • Doxorubicin / pharmacology*
  • Female
  • Gene Expression Regulation, Neoplastic / drug effects*
  • HT29 Cells
  • Humans
  • Mice
  • Mice, Nude
  • Phenylurea Compounds / pharmacology*
  • Sulfonamides / pharmacology*
  • Tumor Cells, Cultured
  • Xenograft Model Antitumor Assays


  • 4-(3'-(3',5'-dimethylphenyl)ureido)phenyl sulfamate
  • Antibiotics, Antineoplastic
  • Carbonic Anhydrase Inhibitors
  • Phenylurea Compounds
  • Sulfonamides
  • Doxorubicin
  • Carbonic Anhydrase IX

Grants and funding

The authors acknowledge financial support from METOXIA (Metastatic Tumors Facilitated by Hypoxic Micro-Environment; EU 7th Research Framework Programme – Theme HEALTH; Grant no.: 222741), NGI Pre-Seed grant (n° 93612005), Kankeronderzoekfonds Limburg from the Health Foundation Limburg and the Dutch Cancer Society (KWF UM 2011-5020, KWF UM 2009-4454, KWF MAC 2013-6425, KWF MAC 2013-6089, KWF UM 2015-7635).