With the FDA approval of a new small-molecule drug to treat HIV infection by blocking the CCR5 chemokine receptor and with several other drugs of this class in development for this and other indications, there is an increased interest in determining the potential influence on tumor promotion or suppression that blocking this receptor may have. Large, long-term clinical studies would be the ideal method for evaluating the potential increase in cancer risk, and at least one such study is under way (see http://clinicaltrials.gov/show/NCT00665561?order=49).
In the meantime, the results of several registrational trials of these agents are in the public domain, although these data are shorter-term and may not reflect the true risk of increased malignancy given the protracted course of neoplastic development. Furthermore, many large epidemiological surveys of cancer rates in persons with the naturally occurring mutation of CCR5 delta-32, which results in the expression of nonfunctioning receptors, exist. This knowledge, combined with our understanding of the role of CCR5 in oncogenesis and tumor promotion, can provide some indication of the expected influence on cancer risk that the use of CCR5 antagonists may have.
OVERVIEW OF THE CHEMOKINE SYSTEM
Chemokines are chemical messengers, proteins secreted by immune cells that orchestrate immune responses. Chemokines bind to chemokine receptors on the surface of immune cells to effect this orchestration. Other proteins, like chemokines, that bind to receptors are referred to as the ligands of receptors. While more than 600 chemokine receptor proteins have been identified, only 18 have been cloned. These have been characterized as 2 major (CCR and CXCR) and 2 minor (CX3CR and XCR) groups. Receptors in each group only bind one group of chemokines, but the same chemokines may bind multiple receptors in the same group. For example, there have been 4 naturally occurring ligands that exhibit specificity to CCR5 identified thus far: MIP-1α, MIP-1β, MCP-1, and RANTES. However, RANTES also binds to CCR1 and CCR3 but does not bind to any of the receptors from the CXCR, CX3CR, or XCR group. Interactions between this network of chemokines and their receptors orchestrate the immune response.1
One of the best-understood chemokine receptors is CCR3. The CCR3 receptor is expressed primarily on eosinophils and appears to be responsible for their migration; blocking this receptor inhibits chemotaxis in vitro. Eotaxin, a ligand for the CCR3 receptor, seems to be overexpressed in patients with asthma, a disease characterized by eosinophilic hyperactivity.2 While the chemokine system regulates normal immune function and response to pathogens, it has also been exploited by various pathogens. The CCR5, CXCR4, and to a lesser extent the CCR3 receptors are utilized in conjunction with the CD4 cell surface receptor by HIV for entry into host T cells. While the exact function of CCR5 is somewhat less well understood, CCR5 seems to function in lymphocyte trafficking like other chemokine receptors. Natural killer (NK) cells and antigen-specific T cells when presented with foreign antigens secrete CCR5 ligands, such as RANTES, into the surrounding extracellular matrix. These ligands attract CCR5-positive cells, such as mature T cells, NK cells, and monocytes, to areas of infection and contribute to an immune and inflammatory response.3
ROLE OF THE CCR5 SYSTEM IN CANCER
Many studies have tried to elucidate the role of chemokines and their receptors in cancer. Indeed, many chemokines were first isolated from the supernatants of cell cultures of tumors. The fact that many chemokines were identified in this manner might, at first glance, make one think that each chemokine and its receptor is involved, if not in the induction of cancer, then in the progression of, angiogenesis in, and production of extracellular stroma for tumors. Not surprisingly, the picture is vastly more complicated. The same chemokine may have completely different effects on different types of malignant cells. The same chemokine that promotes growth in one tumor type may block growth in another and may do so by different mechanisms.
RANTES, the primary ligand of CCR5, has been shown to be overexpressed in breast cancer,4 and breast cancer cell lines have been shown to migrate toward RANTES.5 A study of CCR5 expression in a large sample of various subtypes of T-cell non-Hodgkin lymphomas (NHLs) shows the complex associations between the various lymphoma subtypes and expression of various chemokine receptors.6 Consistent expression of CCR5 was shown only in a small subset of NHLs, ie, anaplastic lymphoma kinase–positive anaplastic large-cell lymphoma. In a detailed analysis of 141 patients with T-cell–associated NHLs, CXCR3 expression was typical of smaller T–cell lymphomas (eg, angioimmunoblastic lymphoma) and CCR4 expression was typical of other lymphoma types. CCR5 expression was not consistently seen in any lymphoma types. Another preclinical study of the CCR5 antagonist TAK-779 showed that TAK-779 blocked RANTES-induced cell invasion and proliferation of prostate cancer cells in cell culture.7
EPIDEMIOLOGICAL STUDIES OF PERSONS WITH THE CCR5 DELTA-32 MUTATION
The delta-32 mutation of the CCR5 receptor gene is present in up to 20% of the white population worldwide.8 Persons who have 2 copies of this mutant gene have been shown to be highly resistant to HIV infection; those with a single copy can be infected but experience an attenuated course of disease.9 There have been many epidemiological studies that have compared the prevalence of the delta-32 mutation in persons with certain cancers with its prevalence in similar, nondiseased populations (Table 1). One particularly well-studied malignancy is NHL in HIV-positive persons. There are 2 retrospective studies that show a significantly reduced risk of NHL in persons with HIV/AIDS who have the CCR5 delta-32 mutation.10,11 The research by Dean and colleagues10 also examined RANTES in B-cell culture and found that it has a significant proliferative effect. This suggests that the role of the delta-32 mutation in NHL may be more than just ameliorating the effects of HIV infection; the presence of the mutant CCR5 may also attenuate chemokine-mediated B-cell proliferation in NHL.
In addition to these retrospective studies in HIV-positive persons with NHL, there are several studies of similar design that are looking at the prevalence of the delta-32 mutation in HIV-negative persons with various other malignancies and in matched controls. One study of HIV-negative patients with pancreatic cancer in San Francisco showed that the presence of the delta-32 mutation was not significantly different in those patients compared with a group of matched controls.12 A French study also failed to show any significant increase of the delta-32 mutation in patients with melanoma.13 A similarly designed study looked at Turkish patients with a variety of malignancies: breast, laryngeal, thyroid, and brain.14 In this study, although a trend toward greater prevalence of delta-32 mutation in breast carcinoma patients was noted, the association did not reach statistical significance. Although an opposite trend in laryngeal cancer patients was noted, the results were also not statistically significant. Of the 30 thyroid and 20 brain cancer patients in the Turkish study, no delta-32 mutations were identified, although the frequency of the allele (2.2%) was low in the general Turkish population that served as controls.
In addition to these retrospective analyses, there is one prospective study of patients with alcoholic cirrhosis and the likelihood of their developing hepatocellular carcinoma (HCC) or dying based on the presence of the delta-32 mutation.15 By the end of the study (mean follow-up of 62.9 months), HCC had developed in 12 of the 36 patients (33%) who had had at least 1 copy of the delta-32 mutation and 50% had died, compared with 35% and 34%, respectively, of those in the wild-type CCR5 homozygote group—again, not a statistically significant difference.
These studies of the prevalence of genetic mutations of CCR5 ligands in certain malignancies serve to further elucidate the potential role of CCR5 antagonism in cancer risk.9-13 Arguably, though they may not replicate completely the effects of blocking CCR5, because while it is tempting to assume that a mutated version of a CCR5 ligand is unable to bind CCR5 and accordingly unable to elicit the effects of the wild-type ligand, it is possible that mutant ligands might bind receptors more avidly and may therefore either block or agonize CCR5. In the San Francisco study of pancreatic cancer, the investigators looked at RANTES mutations as well as the CCR5 delta-32 mutation.12 The odds ratio of pancreatic cancer in patients with 1 mutant copy of the RANTES gene was shown to be 0.99; in patients homozygous for the RANTES mutation, the odds ratio was 0.82—not a statistically significant difference.
These epidemiological studies all examine the association of mutant CCR5 or mutated CCR5 ligands and their association with the risk of developing cancers.9-13 Another approach in evaluating the role of CCR5 in cancer is to study patients with established disease and see how the presence of mutated CCR5 affects disease progression. A Spanish study of cancer patients showed that the allelic frequency of delta-32 was no different in a cohort of 547 women with breast cancer than in the general population.16 The effect of the delta-32 mutation on disease-free survival was more complex. In patients whose breast cancer had the p53 mutation (p53 is a tumor-suppressor gene that is often mutated in a variety of cancers), the delta-32 was associated with a shortened disease-free survival. However, in those patients without the p53 mutation, the delta-32 had no effect on survival.