Azathioprine: Friend or Foe?

I.M. Leigh; C.M. Proby; G.J. Inman; C.A. Harwood


The British Journal of Dermatology. 2019;180(5):961-963. 

Advances in genomics technologies provide new opportunities to understand the mechanisms of skin carcinogenesis, with the promise of developing novel targeted therapies and personalized cancer treatments.[1,2] Recent expansion of national registration of skin cancer in the U.K. has demonstrated the huge number and increasing frequency of keratinocyte cancers (KC), highlighting the importance of better understanding their molecular pathogenesis in order to aid diagnostic, treatment and prevention strategies. This has been exemplified by improved knowledge of the role of Hedgehog signalling pathway in basal cell carcinoma, which has led to the development of targeted therapies for advanced and metastatic disease.

Although germline mutations in syndromes predisposing to cutaneous squamous cell carcinoma (cSCC) are well established in xeroderma pigmentosum, epidermodysplasia verruciformis and Ferguson Smith disease, cSCC have a very high mutational burden and distinguishing between 'driver' and 'passenger' mutations has proved a significant challenge. Indeed, identification of somatic driver mutations in sporadic cSCC has only recently been established with high throughput genomic and transcriptomic sequencing studies.

The NOTCH gene family plays a critical role in epidermal differentiation and loss of function mutations in NOTCH1 and NOTCH2 have been consistently identified early in squamous carcinogenesis.[3] Mutations in TP53 have been extensively reported in both animal models and human tumours. Further publications have also highlighted other potential drivers such as CARD11 mutations[2] and loss of function mutations in transforming growth factor beta receptors TGFBR1 and TGFBR2.[4]

In skin, the identification of driver mutations is not only complicated by the high mutational burden, but is confounded by the finding in normal sun-exposed skin of mutations in key tumour suppressor genes, particularly NOTCH1, NOTCH2, FAT1, TP53 and FGFR3. This provides a 'prepro cancer' carpet of mutations, from which the high rates of cancer gene mutations in actinic keratosis and both invasive and metastatic cSCC arise.[5] Somatic mutations occurring in cancers can be reflective of exogenous carcinogens in addition to underlying biological processes, which both leave unique mutational signatures. New bioinformatic techniques have identified more than 30 such signatures based on a simple classification of single base mutations combined with the base 5′ and 3′ to each mutation.[6]

The dominant ultraviolet radiation (UVR) signature in skin cancers (COSMIC signature 7) reflects the critical role of exogenous UVR in skin carcinogenesis, with characteristic C>T mutations in pyrimidine bases. Looking across the spectrum of SCCs from different origins including head and neck SCC, there are thought to be common genomic, pathway network and immunological features distinguishing squamous carcinomas from other cancers.[7]

It has been well established that patients with immunosuppression, notably organ transplant recipients, have a greatly increased risk of KC. cSCC, in particular, is increased in incidence 100–250-fold compared with the general population.[8] The pathogenesis of these tumours is likely to involve a complex interaction between UVR, reduced tumour surveillance resulting from immunosuppressive drug exposure, direct drug carcinogenesis and possibly oncogenic viruses such as human papillomaviruses.

A recently published study[1] of whole exome sequencing in 40 primary cSCC and 15 derived keratinocyte cell lines included samples from both immunosuppressed and immunocompetent patients. This not only confirmed the high level of recurrent mutations in TP53, NOTCH1/2 and CDKN2A with low levels of activating RAS mutations found in previous studies, but also applied mutational signature analysis. The majority of these samples showed signature 7 as expected, but in addition an entirely novel signature, designated signature 32, that showed a highly statistically significant association between azathioprine exposure and the novel signature 32 mutational pattern (P < 0·0001).

Azathioprine is a widely used immunosuppressive agent with known toxicities, which are carefully managed. Although no longer a drug of first choice following organ transplantation, azathioprine remains an extremely effective treatment, even as a monotherapy, for many diseases that fall within the immune-mediated inflammatory disease spectrum, including inflammatory bowel disease (IBD) and rheumatoid arthritis. Azathioprine prodrug is converted to mercaptopurine and metabolized to 6 thioguanine (6-TG) in the liver. The immunosuppressive effect is mediated by incorporation of 6-TG into replicating DNA (DNA-6TG) and inhibition of de novo purine synthesis.

Studies of the effect of DNA-6TG in keratinocytes have shown this to result in photosensitivity to ultraviolet A (UVA), which can be measured clinically. The dual effect of direct UVA-induced DNA damage and increased UVB mutagenesis through reduced repair of UVB-induced DNA damage is proposed to contribute to the increased skin cancer risk associated with azathioprine.[9] There is also strong support from epidemiological studies that azathioprine has a procarcinogenic role, both in organ transplant recipients[10] and in other immune-mediated inflammatory diseases, particularly IBD.[11] Exposure to thiopurines in patients with IBD has been shown in most case series and retrospective studies to be associated with a significantly increased cSCC risk, with hazard ratios of up to 5·9 and odds ratios of up to 5·26.[12] This risk appears to be related to duration of therapy, cumulative dose and age.

This new evidence for an azathioprine-specific mutational signature in cSCC reveals how directly mutagenic this drug can be in association with sunlight (Figure 1). It also emphasizes the importance of effective day-long and year-round sun protection for any patient on long-term azathioprine as UVA is highly prevalent at the earth's surface throughout the day, all year round, even through glass. As with all medications, the risks should be balanced against the benefits, particularly with the need to treat potentially life-threatening diseases with an effective drug. It is important that skin surveillance and early diagnosis and treatment are part of routine management for patients on azathioprine, including those with IBD or other autoimmune diseases, in whom it is currently less commonly instituted than in organ transplant recipients. Patients should be given advice on the skin cancer risk and the importance of photoprotection and self-skin examination and should have rapid access for diagnosis and treatment of suspicious skin lesions. If skin (pre)-malignancies develop, withdrawal of azathioprine is not inevitable but should be discussed, where possible, with an appropriate multidisciplinary team.

Figure 1.

Ultraviolet radiation (UVR) induces clonal mutations in normal skin and these driver gene mutations are found in actinic keratosis and invasive cutaneous squamous cell carcinoma (cSCC). The analysis of these mutations for signatures of endogenous or exogenous processes shows a typical ultraviolet signature with C>T mutations (signature 7). cSCC in patients who have received azathioprine also bear a distinctive signature (32) which appears to be related to the extent of drug exposure. It is not yet known whether actinic keratoses from patients treated with azathioprine bear signature 32 mutational signatures.

With this compelling new mutational signature evidence, we believe it is now even more important for physicians prescribing azathioprine to be aware of the increased risk of cutaneous malignancy and to routinely provide appropriate educational and surveillance support to minimize this risk. Further research to optimize such risk-reduction strategies is now urgently required.