carbonic anhydrase IX


monoclonal antibody


clear cell (conventional) RCC


von Hippel–Lindau


hypoxia-inducible transcription factors.


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The changing clinical landscape for the treatment of patients with metastatic RCC compels accurate prediction of prognosis, survival, and response to therapy. Secondly, the possibility of preoperatively identifying renal tumour type might have important implications in the choice of treatment for renal neoplasms: stratification of patients with renal masses might help determine management, particularly with current surgical advances focusing on less-invasive and nephron-sparing open surgery. Thirdly, new targeted therapies are still needed because of the relative promiscuity of kinase inhibitors. When considering molecular markers that might serve these goals, carbonic anhydrase IX (CAIX) has drawn considerable interest from different research areas. It appears to combine a range of characteristics varying from diagnostic and prognostic marker to predictor of survival and appropriate therapeutic target in patients with renal cancer, to prognostic marker in a wide variety of carcinomas. The versatility of this molecule is quite remarkable, because generally the mentioned characteristics are not combined in one molecule.

Before discussing the possible impact of this marker on clinical practice in RCC a brief discussion is warranted. CAIX is member of a large family of enzymes catalysing the hydration of carbon dioxide. At least 15 different α-carbonic anhydrase isoforms have been isolated in mammals, where these zinc enzymes play crucial physiological roles [1]. The active site of CAIX is situated extracellularly and contributes to acidification of the extracellular environment. The nomenclature CAIX/G250/MN is recognition of the different lines of research that have been ongoing. G250 refers to monoclonal antibody (mAb) G250 described in 1986 [2], an antibody selected based on the very restricted expression pattern of the target molecule in normal tissues, whereas almost ubiquitous expression in renal cancer specimens was observed [2]. MN/CAIX was described in 1994 as a human tumour-associated protein [3]. Research on these two seemingly unrelated molecules merged when the mAbG250 target molecule was molecularly characterized and shown to be identical to CAIX/MN [4], hence CAIX/G250/MN. The coming together of the two lines of research also clarified the leading mechanism of CAIX expression in RCC, more specifically clear cell (conventional) RCC (ccRCC). One of the most important molecular events in the carcinogenesis of ccRCC is loss of functional von Hippel–Lindau (VHL) protein expression [5], leading to stabilization of specific transcription factors, the so-called hypoxia-inducible transcription factors (HIFs). Studies on CAIX had shown that these factors are an absolute requirement for CAIX expression. Thus, the homogeneous expression of CAIX in ccRCC as seen with mAbG250 could readily be explained by VHL mutations in ccRCC.

Immunohistochemical analysis of tissue microarrays from patients treated by radical nephrectomy and subsequent survival-tree analysis has shown that a threshold of >85% CAIX-positive staining provided an accurate prediction of survival [6]. Low CAIX staining was an independent prognostic marker for poor survival [7]. Moreover, high (>85%) expression of CAIX in ccRCC has been associated with response to interleukin 2 [8]. Thus, CAIX expression appears to be linked to various aspects of tumour characteristics. It is possible that the correlation between CAIX and prognosis, survival, and response to interleukin 2 therapy is the consequence of HIFs and consequently ccRCC. Indeed, a strong association has been found between aberrant VHL expression and survival, suggesting that CAIX could be seen as a surrogate marker. However, because CAIX also contributes to the extracellular tumour cell environment, CAIX expression itself may be of importance in the maintenance and progression of RCC.

Selective tumour targeting with mAbG250 has been studied extensively. The first (biopsy-based) clinical trial showed several pivotal aspects: most notably virtually no uptake in other tissues resulting in excellent tumour visualization, and very high tumour uptake [9]. Despite the excellent visualization of tumour deposits, diagnostic imaging was not pursued because the need was non-existent. However, with the percentage of renal masses discovered incidentally increasing, and new therapeutic methods becoming available, imaging might become of importance to distinguish more potentially malignant tumours (ccRCC) from less aggressive variants. The first prospective clinical trial with 124I-labelled chimeric G250 showed a very high specificity and sensitivity for identifying ccRCC in patients with suspect renal masses, a clear indication of the potential clinical utility [10]. Whether this imaging method can be used to follow therapy effects remains to be determined.

Finally, can CAIX be seen as an appropriate therapeutic target for ccRCC? Various (nonrandomized) clinical trials have now been completed with therapeutic efforts focused on radioimmunotherapy and passive immunotherapy in patients with metastatic RCC. Thus far, mAbG250 treatment appears to lead to extended survival time. However, randomized trials are necessary to show this unequivocally. Alternatively, the development of specific CAIX inhibitors might be contemplated as an alternative, particularly because blockage of enzymatic activity might be detrimental to the tumour environment.

In conclusion, many issues need to be addressed to validate CAIX for these different applications, despite compelling initial evidence in diverse settings. One molecule for all applications is almost too good to be true. Secondly, the association of CAIX with response to interleukin 2 is intriguing, but the relation with targeted therapies has not been studied; this certainly requires study. Thirdly, application of CAIX imaging might significantly improve the clinical management of patients with kidney tumours. Finally, kinase inhibitors such as SU11248 (Sutent®, Pfizer Inc., La Jolla CA, USA) and BAY 43-9006 (Sorafenib®, Bayer Pharmaceuticals, West Haven, CT, USA) have greatly impacted on the clinical management of patients with renal cancer; results with these orally available small-molecule drugs have been very promising leading to major responses and a survival advantage. However, these kinase inhibitors are promiscuous and target much more than tumour cells alone. As there is a very strong case that the vast majority of ccRCC express CAIX, CAIX-guided therapies must be further explored to take advantage of the unique molecular switch that plays such a dominant role in this disease.


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