Carbonic Anhydrase IX Expression in Clear Cell Renal Cell Carcinoma and Normal Tissues: Experiences From (Radio) Immunotherapy

published the results of a large cohort of patients (n ϭ 730) with sporadic clear cell renal cell carcinoma (ccRCC) that was analyzed for the possible relationship between expression of carbonic anhydrase IX (CAIX) and outcome.Although they used the cut point criteria for CAIX tumor expression identified previously by Bui et al, 2,3 they could not confirm the results of the University of California, Los Angeles group, who reported that in metastatic RCC low (Յ 85%) CAIX staining is an independent poor prognostic factor for survival. 2Although low CAIX expression was associated with increased risk of death as a result of RCC in an univariate analysis, after adjusting for nuclear grade or coagulative tumor necrosis, this was no longer the case. 1 Also in their study, Leibovich et al 1 determined CAIX expression in a wide variety of normal tissues and a few other tissues.They report, "CAIX expression was present in gastric mucosa, pancreatobiliary epithelium, and small intestine crypt base.Also, CAIX was seen in two specimens containing mesothelial cells, ovarian surface epithelium, and fetal rete testis."According to the authors, the CAIX expression levels in various normal tissues are high.Therefore, they raised the question of whether CAIX-directed therapeutic strategies will have a safe enough toxicity profile, while pointing also to the literature of anti-CAIX monoclonal antibody-based therapy that have not caused significant toxicities thus far. 1 We would like to comment on these two matters specifically.Regarding CAIX expression in normal tissues, it has been reported previously by other groups, in addition to the article by Leibovich et al 1 and the studies they refer to by Ivanov et al. [4][5][6] Furthermore, from the article, it does not become clear how the high expression in the various normal tissues is related to the high CAIX expression for which most ccRCC tumors are known.In our opinion, this is relevant information in light of a discussion on the toxicity profile.6][7] Furthermore, because CAIX expression is regulated by the transcription factor HIF-1␣ that in turn is regulated (among others) by the protein Von Hippel-Lindau (VHL), and because ccRCC is molecularly characterized by (functional) loss of the VHL gene, this explains the ubiquitous CAIX expression in ccRCC. 7,8n contrast, the CAIX expression in non-RCC tumors is the result of locoregional hypoxia, leading to locoregional CAIX expression. 8egarding the toxicity profile of anti-CAIX-directed therapy with monoclonal antibodies, we point out that we have ample experi-