Topic Article
Sirolimus Reduces the Incidence and Progression of UVB-Induced Skin Cancer in SKH Mice even with Co-administration of Cyclosporine A
Brian C Wulff, Donna F Kusewitt, Anne M VanBuskirk, Jennifer M Thomas-Ahner, F Jason Duncan and Tatiana M Oberyszyn
Journal of Investigative Dermatology (2008), 128, 2467-2473; doi:10.1038/jid.2008.260

Sirolimus: A Potential Chemopreventive Agent

Anthony Fernandez ¹, Shasa Hu ¹ and Robert S. Kirsner ¹

Journal of Investigative Dermatology (2008) 128, 2352. doi:10.1038/jid.2008.260

Solid-organ transplant recipients requiring chronic immunosuppressive therapy experience a substantially increased incidence of aggressive malignancies, of which cutaneous malignancies are most common (DiGiovanna, 1998; Berg and Otley, 2002). In some studies, skin cancers have been estimated to occur at a greater than 65-fold increased frequency in transplant recipients than in the general population (Hartevelt et al., 1990; Jensen et al., 1999). This increased risk of skin cancer is likely due to a combination of previous and ongoing UV light exposure and pharmacologic immune suppression (Hojo et al., 1999).

Additionally, tumors in immunosuppressed transplant patients seem to be more aggressive than tumors found in the general population. The risk of metastasis from cutaneous squamous cell carcinoma in organ transplant patients is approximately 7%, significantly greater than that of the general population. Furthermore, in transplant recipients from Sydney, Australia, 27% of deaths (4 years after transplantation) were due to metastatic skin cancer (Ong et al., 1999; Otley and Maragh, 2005). Clearly, reducing the risk of aggressive skin cancers in immunosuppressed transplant recipients is a significant clinical challenge.

Wulff and colleagues (2008) studied two immunosuppressive medications commonly used after transplantation—cyclosporine A (CsA) and sirolimus (SRL)—to determine their individual and combined effects on tumor development. In murine models exposed to UV light, the authors reported enhanced tumor size and progression in CsA-treated mice. These clinical observations were associated with increased dermal mast cell numbers and higher levels of transforming growth factor (TGF)-β1 in the skin. However, UV light–exposed mice that received either SRL or CsA plus SRL exhibited fewer tumors, with diminished size and reduced progression compared with vehicle-treated mice. Dermal mast cells and TGF-β1 levels were not increased in these mice. These findings suggest the potential utility of SRL (whether or not CsA is used) in protecting transplant patients (and perhaps others) from developing skin cancer.

QUESTIONS
1. What is the clinical evidence that immunosuppression increases skin cancer risk?
2. Why is sirolimus potentially a chemopreventive agent?
3. How well did the experimental model replicate the clinical scenario depicted by the researchers?
4. What were the major findings of the study?
5. What are the proposed mechanisms by which sirolimus might work?
6. What may be the clinical implications of this article?
7. What further studies could be performed?

ANSWERS
1. It is well established that solid-organ transplant recipients requiring chronic immunosuppressive therapy have a substantially higher incidence of aggressive malignancies, with cutaneous malignancies being most common (DiGiovanna, 1998; Berg and Otley, 2002). In fact, some studies have estimated that skin cancer occurs at a greater than 65-fold increase in frequency in the transplant population compared with the general population (Hartevelt et al., 1990; Jensen et al., 1999). Whereas the ratio of basal cell carcinoma (BCC) to squamous cell carcinoma (SCC) is approximately 4–5:1 in the general population, it is reversed—to approximately 1:2—in the transplant population, such that SCC predominates (Buell et al, 2005). In addition to BCC and SCC, other cutaneous malignancies, such as Kaposi’s sarcoma, melanoma, and merkel cell carcinoma, occur with increased frequency in immunosuppressed patients (Otley and Maragh, 2005; Berg and Otley, 2002). Kaposi’s sarcoma has been estimated to occur at an 84-fold increased frequency in the transplant population compared with the general population (Berg and Otley, 2002).

Studies examining skin cancer incidence in transplant patients have found that incidence rises with survival time after transplantation. In Australia, malignant skin lesions were present in 7% of transplant recipients within 1 year after transplantation and in 70% after 20 years (Stockfleth et al., 2004). Numerous studies have documented a direct relationship between the intensity of immunosuppression and the incidence of skin cancer, suggesting that lower levels of immunosuppression would result in fewer skin cancers (Otley and Maragh, 2005). Reports of regression of such tumors after discontinuation of immunosuppressive medications further highlight the important role played by immunosuppression in the development of skin tumors (Otley and Maragh, 2005). The obvious increased risk of graft rejection after immunosuppressive discontinuation, however, makes this strategy undesirable.

In addition to a greater incidence of skin cancer, the tumors in immunosuppressed transplant patients seem to be more aggressive than those in the general population. The risk of metastasis from cutaneous SCC in organ transplant patients is approximately 7%, significantly greater than that seen in the general population. Furthermore, in transplant recipients from Sydney, Australia, 27% of deaths after the fourth year post-transplant were attributed to metastatic skin cancer, demonstrating that skin cancer in this population can significantly impact survival (Ong et al., 1999; Otley and Maragh, 2005).

Carcinogenesis in transplant patients is multifactorial, but use of immunosuppressive medication appears to be the single most important risk factor for cancer (Vasudev and Hariharan, 2007). Although the contribution of individual immunosuppressants to the development of skin cancers is difficult to ascertain, data suggest that cyclosporin A exposure leads to changes in keratinocyte gene expression that may in turn lead directly to pro-carcinogenic states (Tiu et al, 2006; Hojo et al., 1999). Specifically, cyclosporine-induced TGF-β production is implicated in cyclosporine-induced carcinogenesis, because anti-TGF-β monoclonal antibodies, but not control antibodies, prevented an increase in the number of metastases in one study (Hojo et al., 1999). Human papillomavirus (HPV) infection is also implicated in immunosuppression-related carcinogenesis. For SCC, HPV DNA has been detected more frequently in SCC tumors of transplant recipients (12/16, 75%) than in those of non-immunosuppressed patients (7/19, 37%) (Stockfleth et al., 2004). In contrast, the HPV detection rate was similar in BCC specimens. Overall, 22 HPV types were identified, with HPV types 5 and 8 being detected most commonly in SCCs from transplant recipients (Stockfleth et al., 2004). Furthermore, immunosuppressive therapy indirectly promotes carcinogenesis by altering immune surveillance and the body’s ability to destroy precancerous and cancerous cells (Berg and Otley, 2002).

2. Sirolimus, also known as rapamycin, is a structural analog of the macrolide antibiotic FK506 isolated from a strain of Streptomyces hygroscopicus collected from the soil of Rapa Nui (Easter Island) (Euvrard et al., 2004). First isolated as an antifungal agent, sirolimus was subsequently found to have potent immunosuppressive and antiproliferative properties, both of which are caused by inhibiting the mammalian target of rapamycin (mTOR). Sirolimus first binds to FK-binding protein-12 (FKBP-12) to form a cytoplasmic complex, which then subsequently binds to and inhibits mTOR. The immunosuppressive effects of sirolimus result from inhibiting mTOR propagation of IL-2-mediated signal transduction, thus hindering stimulation of lymphocyte division and antibody production. The antiproliferative properties of sirolimus are exerted largely via the dephosphorylation and inactivation of p70 ribosomal protein S6 kinase caused by mTOR inhibition, resulting in blockage of cell-cycle progression at the transition between G1 and S phases (Euvrard et al., 2004). Other mechanisms implicated in the antineoplastic properties of sirolimus include inhibition of IL-10 and cyclin D1 production, induction of apoptosis, increased production of E-cadherin and p27, and prevention of angiogenesis by impairing production of vascular endothelial growth factor (VEGF) (Euvrard et al., 2004; Vasudev and Hariharan, 2007; Khariwala et al., 2006). Of particular interest in skin cancer development, sirolimus has been shown to decrease expression of a variety of molecules known to play a role in UVB-mediated carcinogenesis, including matrix-degrading metalloproteinases, tumor necrosis factor- α (TNF- α), and p53 (Euvrard et al., 2004).

The antiproliferative properties of sirolimus have been demonstrated in both cell culture and murine studies. Several studies have shown that sirolimus can inhibit the growth of a variety of malignant cell types in culture, including rhabdomyosarcoma, neuroblastoma, small-cell lung cancer, osteosarcoma, pancreatic cancer, breast cancer, prostate cancer, colon cancer, brain tumors, and leukemia (Huang and Houghton, 2001; Huang and Houghton, 2002; Douros and Suffness, 1981). In mice, studies simulating metastasis of colon cancer have shown that sirolimus use is associated with decreased metastatic area and that this was associated with a decrease in neovascularization (Guba et al., 2002). In another study, examining a murine model of renal cell cancer pulmonary metastasis, sirolimus reduced—whereas cyclosporine increased—the number of pulmonary metastases (Luan et al. 2003). In both studies, the association between sirolimus use and decreased levels of VEGF were hypothesized to be important in the drug’s positive effects.

3. This study was designed to model the clinical situation in which a patient with a history of sun exposure, after receiving a transplant and beginning immunosuppressive treatment, avoids further sun exposure. In this scenario, cancer induction and promotion are presumed to have occurred before the onset of immunosuppression.

One important question that needs to be addressed is to what extent skin cancer development in SKH-1 hairless mice parallels skin cancer development in humans. Several characteristics have led to extensive use of the SKH-1 hairless mouse strain in studies examining the effects of UV radiation exposure on skin. Because the SKH-1 mouse is hairless, it does not need to be shaved before being exposed to UV radiation. It predictably develops SCC following UVB exposure, making it an attractive model for the study of photocarcinogenesis.

Despite these advantages, there is evidence that the response of SKH-1 mouse skin to UV radiation is significantly different from that of human skin. The human epidermis and stratum corneum are more than 20 times thicker than the SKH-1 mouse counterparts, implying that the type and amount of UV radiation penetrating to the basal layer will differ (Kligman et al., 1982; Sterenborg et al., 1986). Also, SKH-1 mice are albino and thus do not make pigment in response to UV exposure. This attribute alone is an indication that they are not an ideal model system with which to compare non-albino humans. Furthermore, whereas humans generate predominantly BCCs in response to UV radiation exposure, SKH-1 mice generate almost exclusively SCCs (even in the absence of immunosuppression), implying important intrinsic differences in their response to UV radiation ( De Gruijl and van der Leun, 1991). Another difference between SKH-1 mouse skin and human skin is that the former has little to no photoreactivation capacity to repair cyclobutane pyrimidine dimmers (Berton et al., 1997). Also, mouse skin responds to moderate UV radiation overexposure (3–5 minimal erythema doses (MED)) with mostly edema and very little erythema (Cole et al., 1983). All of these attributes make the SKH-1 mouse strain a dubious model for studying UV-induced skin damage in humans.

In addition to the intrinsic problems of using SKH-1 mice, important aspects of the methodology of these experiments may impair replication in an appropriate clinical scenario. First, the mice were exposed only to 1 MED of UVB light, whereas the natural sunlight to which humans are exposed contains a much broader spectrum of UV wavelengths. Also, the mice were given immunosuppressive therapy for only 9 weeks. Since studies suggest that malignant skin lesions are present in only 7% of transplant recipients within 1 year after transplantation, we cannot be sure of the role that immunosuppressive therapy played in tumor progression/inhibition after such a short period of time (Stockfleth et al., 2004). In fact, the median interval from transplant to malignancy in some studies was more than 4 years, and longest in those with isolated SCC lesions (Buell et al, 2005). Finally, we cannot be sure that the mice were truly immunosuppressed, because medication was administered intraperitoneally and no objective measures (such as serum trough levels of cyclosporine or sirolimus) were performed to ensure systemic levels comparable to those seen in transplant patients.

4. When only tumors at least 2 mm in diameter were included in their analyses, the vehicle- and cyclosporine-alone groups had significantly more tumors (mean 7.7 and 9.1 per mouse, respectively) than the sirolimus or sirolimus+cyclosporine groups (mean 1.5 and 1.7 per mouse, respectively). By week 9 of immunosuppression, the total tumor burden per mouse (Figure 1b in Wulff et al., 2008) and the mean tumor area per mouse (Figure 1c) were significantly larger in cyclosporine-treated mice than in the other treatment groups. The investigators state that mice treated with cyclosporine alone developed the most aggressive tumors, but it is unclear whether this finding was statistically significant. There was a trend toward increased dermal neutrophil infiltration in cyclosporine-treated mice, but this increase did not reach significance in comparison with vehicle-treated mice. A significant increase in the number of tryptase-positive mast cells was seen in paratumoral dermis of cyclosporin A–treated mice compared with the three other treatment groups. Finally, proTGF-β1 protein levels were significantly higher in cyclosporine-treated “non-tumoral” dorsal skin than in the other three treatment groups.

5. The investigators propose that sirolimus counteracts unwanted immune system alterations resulting directly or indirectly from cyclosporine exposure. In particular, they implicate differences in mast cell infiltration and TGF-β1 production. Although it is unclear why there is a difference in tumoral mast cell density between the cyclosporine group and the other treatment groups, several previous studies have shown that mast cells are significantly increased in some neoplasias, including oral, skin, breast, lip, and cervical cancer (Rojas et al., 2005; Iamaroon et al., 2003; Coussens et al., 1999; Kankkunen et al., 1997; Cabanillas-Saez et al., 2002). Furthermore, mast cell density has been associated with poor prognosis and increased metastasis (Kankkunen et al., 1997). Mast cells release chymase and tryptase, both of which can promote extracellular matrix degradation and stimulate angiogenesis (Rojas et al., 2005). Other factors released by mast cells, including heparin, histamine, VEGF, and fibroblast growth factor-2, are also implicated in angiogenesis. Increased mast cell density in melanomas has also been associated with increased microvessel density and poor prognosis (Ribatti et al., 2003).

In a similar manner, TGF-β1 levels were elevated in the cyclosporine-treated groups. TGF-β1 plays an important role in the maintenance of tissue homeostasis and has been shown to be overexpressed in a variety of human cancers, including skin cancer (Li et al., 2005 ). In addition, there is evidence that TGF-β1 signaling is important for tumor invasion and metastasis (Leivonen and Kahari, 2007). Transgenic mice that overexpress TGF-β1 using K5 and K14 promoters show many changes needed for tumor progression, including angiogenesis, inflammation, and basement membrane degradation (Li et al., 2005). Again, the mechanism underlying increased TGF-β1 expression in the cyclosporine-treated group is unclear. However, it is interesting that another group (Hojo et al., 1999) also found a link between cyclosporine treatment, adenocarcinoma growth, and increased TGF-β levels. Their study suggests that cyclosporine can promote cancer progression via a direct cellular effect that is independent of its effect on the host’s immune cells. In this study, anti-TGF-β antibodies, but not control antibodies, were able to prevent increased metastases (Hojo et al., 1999).

Of note, TGF-β1 is synthesized and secreted in a latent form in which the N-terminal latency–associated peptide (LAP) remains noncovalently bound to the C-terminal mature TGF-β1. The functionally active form of TGF-β1 is cleaved from the LAP (Li et al., 2005). Because only proTGF-β1 levels were determined in this study, it is not certain that levels of active TGF-β1 differed among the four treatment groups.

In addition, the theoretical mechanisms underlying the antiproliferative effects of sirolimus (as discussed in answer 2) are implicated in the results seen here.

6. The obvious clinical implication of this study is that perhaps the use of sirolimus in post-transplantation immunosuppressive regimens will decrease the incidence of skin cancer, as well as other cancers, in solid-organ transplant patients. There has long been interest in immunosuppressive agents other than cyclosporine in the renal transplant field due to the well-known nephrotoxic risks associated with its use. This has led to more than 10 years of experimentation using the non-nephrotoxic sirolimus in place of cyclosporine.

Preliminary data concerning the incidence of skin cancer in renal transplant patients taking sirolimus were drawn from five multicenter studies that assessed 1,886 patients receiving various immunosuppressive regimens at 2 years post-transplant (Mathew et al., 2004). The incidence of skin malignancy was lower in all groups of patients treated with sirolimus, especially in the 215 patients who had cyclosporine withdrawn compared with those who continued to receive cyclosporine (2.3% vs. 4.7%). Another study examined sirolimus use in 1,008 renal transplant patients over an 11-year period (Yakupoglu et al., 2006). The incidences of various malignancies common to transplant patients over the entire follow-up period were 2.4% for skin tumors, 0.4% for post-transplant lymphoproliferative disorders, and 0.2% for renal cell carcinoma. All of these incidences were significantly lower than for patients on other immunosuppressive regimens that did not include sirolimus. In addition, an earlier paper describing the same 1,008 patients reported only four deaths following diagnosis of malignancy, none of which was related to skin cancers (Kahan et al., 2003).

There is also evidence that sirolimus therapy may be beneficial to transplant patients who already have a history of developing multiple skin cancers. One study reported on 23 renal transplant recipients with multiple skin cancers who had their immunosuppressive regimens changed to sirolimus (Tessmer et al., 2006). Although their follow-up time of 22 months after onset of sirolimus administration was admittedly short, their data suggested that conversion to sirolimus may reduce the incidence of new skin cancers in such patients. Furthermore, sirolimus has been shown to reduce the frequency of other malignancies that may affect the skin. One such report described 15 renal transplant patients with biopsy-proven cutaneous Kaposi’s sarcoma who were switched from cyclosporine to sirolimus (Stallone et al., 2005). Three months after sirolimus therapy had begun, all cutaneous Kaposi’s sarcoma lesions had disappeared. Remission was confirmed histologically in all patients, and, importantly, there were no acute episodes of rejection or changes in kidney-graft function.

7. The anticarcinogenic properties of sirolimus make it a very attractive immunosuppressive agent, both within the field of transplantation medicine and in other areas of medicine. Several obvious questions raised by this article concern the role of sirolimus on levels of TGF-β1 and tumor-associated mast cells. It would be interesting to analyze tumor growth and use of cyclosporine and/or sirolimus in TGF-β1- or mast-cell-deficient mouse strains.

However, in light of the data suggesting that mice may not be ideal models in which to study UV-induced skin damage in humans, and because sirolimus use in humans has been progressively increasing for more than a decade, it seems that the most attractive future studies will involve humans and, ideally, will be randomized, double-blinded control trials.

With this in mind, there are numerous questions to investigate concerning immunosuppressed transplant patients: Does the addition of prophylactic retinoids to sirolimus-based immunosuppressive regimens further reduce the incidence of skin cancer (McKenna and Murphy, 1999)? Has HPV DNA been detected in skin cancers that develop in patients taking sirolimus? If so, which subtypes predominate? Is sirolimus use associated with decreased incidence of melanoma in this population (Karbowniczek et al., 2008)? Are higher doses of sirolimus associated with increased antineoplastic effects? Can topical sirolimus applied to sun-exposed areas decrease skin cancer incidence in patients unable to tolerate oral sirolimus (Rauktys et al., 2008)? Does the extent of post-transplant UV exposure play as significant a role in skin cancer development in sirolimus-treated patients as it does in patients on other regimens?

Just as exciting are the possibilities of using sirolimus as an immunosuppressant in other areas of medicine, including dermatology. This is highlighted by a recent report describing a patient with pemphigus vulgaris who experienced resolution of Kaposi’s sarcoma and maintained remission of pemphigus after switching from methotrexate to sirolimus (Saggar et al., 2008). There is a wealth of possible studies in dermatology, including questions such as: does sirolimus use decrease incidence of malignancies in patients with dermatologic inflammatory diseases requiring long-term immunosuppression? In immune-mediated diseases? Is sirolimus a safe and effective medication in patients with a history of malignancy who require immunosuppression? Does use of sirolimus result in better outcomes in patients with paraneoplastic inflammatory dermatoses? As the answers to these questions are slowly unraveled, it will be interesting to see where sirolimus eventually rests in the hierarchy of immunosuppressive medications.

REFERENCES
Berg D, Otley CC (2002) Skin cancer in organ transplant recipients: epidemiology, pathogenesis, and management. J Am Acad Dermatol 47:1–17

Berton TR, Mitchell DL, Fischer SM, Locniskar MF (1997) Epidermal proliferation but not quantity of DNA photodamage is correlated with UV-induced mouse skin carcinogenesis. J Invest Dermatol 109:340–7

Buell JF, Hanaway MJ, Thomas M, Alloway RR, Woodle ES (2005) Skin cancer following transplantation: the Israel Penn International Transplant Tumor Registry experience. Transplant Proc 37:962–3

Cabanillas-Saez A, Schalper JA, Nicovani SM, Rudolph MI (2002) Characterization of mast cells according to their content of tryptase and chymase in normal and neoplastic human uterine cervix. Int J Gynecol Cancer 12:92–8

Cole CA, Davies RE, Forbes PD, D’Aloisio L (1983) Comparison of action spectra for acute cutaneous responses to ultraviolet radiation: man vs albino mouse. Photochem Photobiol 37:623–31

Coussens LM, Raymond WW, Bergers G, Laig-Webster M, Behrendtsen O, Werb Z et al. (1999) Inflammatory mast cells up-regulate angiogenesis during squamous epithelial carcinogenesis. Genes Dev 13:1382–97

De Gruijl FR, van der Leun JC (1991) Development of skin tumors in hairless mice after discontinuation of ultraviolet radiation. Cancer Res 51:979–84

DiGiovanna JJ (1998) Posttransplantation skin cancer: scope of the problem, management, and role for systemic retinoid chemoprevention. Transplant Proc 30:2771–5, discussion 2776–8

Douros J, Suffness M (1981) New antitumor substances of natural origin. Cancer Treat Rev 8:63–87

Euvrard S, Ulrich C, Lefrancois N (2004) Immunosuppressants and skin cancer in transplant patients: focus on rapamycin. Dermatol Surg 30:628–33

Guba M, Von Breitenbuch P, Steinbauer M, Koehl G, Flegel S, Hornung M et al. (2002) Rapamycin inhibits primary and metastatic tumor growth by antiangiogenesis: involvement of vascular endothelial growth factor. Nat Med 8:128–35

Hartevelt MM, Bavinck JN, Kootte AM, Vermeer BJ, Vandenbroucke JP (1990) Incidence of skin cancer after renal transplantation in the Netherlands. Transplantation 49:506

Hojo M, Morimoto T, Maluccio M, Asano T, Morimoto K, Lagman M et al. (1999) Cyclosporine induces cancer progression by a cell-autonomous mechanism. Nature 397:530–4

Huang S, Houghton PJ (2001) Resistance to rapamycin: a novel anticancer drug. Cancer Metastasis Rev 20:69–78

Huang S, Houghton PJ (2002) Inhibitors of mammalian target of rapamycin as novel antitumor agents: from bench to clinic. Curr Opin Invest Drugs 3:295–304

Iamaroon A, Pongsiriwet S, Jittidecharaks S, Pattanaporn K, Prapayasatok S, Wanachantararak S (2003) Increase of mast cells and tumor angiogenesis in oral squamous cell carcinoma. J Oral Pathol Med 32:195–9

Jensen P, Hansen S, Møller B, Leivestad T, Pfeffer P, Geiran O et al. (1999) Skin cancer in kidney and heart transplant recipients and different long-term immunosuppressive therapy regimens. J Am Acad Dermatol 40 (2 pt 1):177–86

Kahan BD, Knight R, Schoenberg L, Pobielski J, Kerman RH, Mahalati K et al. (2003) Ten years of sirolimus therapy for human renal transplantation: the University of Texas at Houston experience. Transplant Proc 35(Suppl 3A):25S–34S

Kankkunen JP, Harvima IT, Naukkarinen A (1997) Quantitative analysis of tryptase and chymase containing mast cells in benign and malignant breast lesions. Int J Cancer 72:385–8

Karbowniczek M, Spittle CS, Morrison T, Wu H, Henske EP (2008) mTOR is activated in the majority of malignant melanomas. J Invest Dermatol 128:980–7

Khariwala SS, Kjaergaard J, Lorenz R, Van Lente F, Shu S, Strome M (2006) Everolimus (RAD) inhibits in vivo growth of murine squamous cell carcinoma (SCC VII). Laryngoscope 116:814–20

Kligman LH, Akin FJ, Kligman AM (1982) Prevention of ultraviolet damage to the dermis of hairless mice by sunscreens. J Invest Dermatol 78:181–9

Leivonen SK, Kahari VM (2007) Transforming growth factor-beta signaling in cancer invasion and metastasis. Int J Cancer 121:2119–24

Li AG, Lu SL, Han G, Kulesz-Martin M, Wang XJ (2005) Current view of the role of transforming growth factor beta 1 in skin carcinogenesis. J Investig Dermatol Symp Proc 10:110–7

Luan FL, Ding R, Sharma VK, Chon WJ, Lagman M, Suthanthiran M (2003) Rapamycin is an effective inhibitor of human renal cancer metastasis. Kidney Int 63:917–26

Mathew T, Kreis H, Friend P (2004) Two-year incidence of malignancy in sirolimus-treated renal transplant recipients: results from five multicenter studies. Clin Transplant 18:446–9

McKenna DB, Murphy GM (1999) Skin cancer chemoprophylaxis in renal transplant recipients: 5 years of experience using low dose acitretin. Br J Dermatol 140:656–60

Ong CS, Keogh AM, Kossard S, Macdonald PS, Spratt PM (1999) Skin cancer in Australian heart transplant recipients. J Am Acad Dermatol 40(1): 27–34

Otley CC, Maragh SL (2005) Reduction of immunosuppression for transplant associated skin cancer: rationale and evidence of efficacy. Dermatol Surg 31:163–8

Rauktys A, Lee N, Lee L, Dabora SL (2008) Topical rapamycin inhibits tuberous sclerosis tumor growth in a nude mouse model. BMC Dermatol 8:1

Ribatti D, Ennas MG, Vacca A, Ferreli F, Nico B, Orru S et al. (2003) Tumor vascularity and tryptase-positive mast cells correlate with a poor prognosis in melanoma. Eur J Clin Invest 33:420–5

Rojas IG, Spencer ML, Martinez A, Maurelia MA, Rudolph MI (2005) Characterization of mast cell subpopulations in lip cancer. J Oral Pathol Med 34:268–73

Saggar S, Zeichner JA, Brown TT, Phelps RG, Cohen SR (2008) Kaposi’s sarcoma resolves after sirolimus therapy in a patient with pemphigus vulgaris. Arch Dermatol 144:654–7

Stallone G, Schena A, Infante B, Di Paolo S, Loverre A, Maggio G et al. (2005) Sirolimus for Kaposi’s sarcoma in renal-transplant recipients. -N Engl J Med 352:1317–23

Sterenborg HJCM, de Gmijl FIR, van der Leun JC (1986) UV-induced epidermal hyperplasia in hairless mice. Photodermatology 3:206–14

Stockfleth E, Nindl I, Sterry W, Ulrich C, Schmook T, Meyer T (2004) Human papillomaviruses in transplant-associated skin cancers. Dermatol Surg 30:604–9

Tessmer CS, Magalhaes LV, Keitel E, Valar C, Gnatta D, Pra RL et al. (2006) Conversion to sirolimus in renal transplant recipients with skin cancer. Transplantation 82:1792–3

Tiu J, Li H, Rassekh C, van der Sloot P, Kovach R, Zhang P (2006) Molecular basis of posttransplant squamous cell carcinoma: the potential role of cyclosporine a in carcinogenesis. Laryngoscope 116:762–9

Vasudev B, Hariharan S (2007) Cancer after renal transplantation. Curr Opin Nephrol Hypertens 16:523–8

Wulff BC, Kusewitt DF, VanBuskirk AM, Thomas-Ahner JM, Duncan FJ, Oberyszyn TM (2008) Sirolimus reduces the incidence and progression of UVB-induced skin cancer in SKH mice even with co-administration of cyclosporine A. J Invest Dermatol 128:2467–73

Yakupoglu YK, Buell JF, Woodle S, Kahan BD (2006) Individualization of immunosuppressive therapy. III. Sirolimus associated with a reduced incidence of malignancy. Transplant Proc 38:358–61

¹ Department of Dermatology and Cutaneous Surgery, University of Miami Miller School of Medicine, Miami, Florida, USA