**15.1 Pharmacogenomics of thiopurine therapy**

Azathioprine is a pro-drug which is metabolised to 6-mercaptopurine and then to a number of further metabolites including 6-thioguanine (6TG) and 6-methylmercaptopurine (6MMP) which are associated with myelo- and hepato-toxicity when they accumulate at high levels. Important enzymes in this pathway include thiopurine methyl transferase (TPMT) and hypoxanthine phosphoribosyltransferase (HPRT). It is recognised that polymorphisms of TPMT can influence TPMT activity and hence levels of 6TG and 6MMP. About 1 in 300 individuals have homozygote TPMT mutations and AZA or 6MP therapy results in very

fraught with difficulty. In the CESAME study, 16 of the 23 lymphomas were in patients with

Not all studies have attempted to evaluate the risk of lymphoma associated with immunomodulator therapy. The studies which have attempted to make this estimation are heterogenous and frequently there is a lack of distinction between lifetime exposure to these drugs, the cumulative doses received and whether cessation of a drug returns any lymphoma risk back to baseline. The CESAME study group attempted to answer some of these questions (Beaugerie et al., 2009a) by analysing patients who have *continuing*, *discontinued* or *never received* thiopurines. The SIRs for these three groups were 6.86 (95% CI 3.94 to 11.31), 1.44 (95% CI 0.17 to 5.20) and 1.43 (CI 95% 0.53 to 3.12) respectively. This may suggest that discontinuation of thiopurines returns risk to baseline but the results were not

It is not clear whether it is the disease itself, its treatment or a combination of the two which might put IBD patients at increased risk of lymphoma. It is possible that the use of drugs such as thiopurines and biologics are a marker of more aggressive disease and it is disease severity which disposes to lymphoma development. However, modern management of IBD has led to earlier use of these drugs, often in patients who do not have severe disease but possess risk factors for complicated disease, in an attempt to alter the natural history of the condition.

Severity of disease as a risk factor for lymphoma has not been analysed in any depth in IBD patients but there is some evidence available from other autoimmune diseases. A study of 378 RA patients diagnosed with lymphoma found no significant association with individual drugs but a marked increased risk with high disease activity which conferred a 70-fold increased risk (Baecklund et al., 2006). The authors concluded that it was the disease activity, not its treatment that was important in lymphomagenesis. In the CESAME study, some data regarding disease activity was collated but was not linked to lymphoma risk

It is important to recognise a number of other risk factors for lymphoma development which

Azathioprine is a pro-drug which is metabolised to 6-mercaptopurine and then to a number of further metabolites including 6-thioguanine (6TG) and 6-methylmercaptopurine (6MMP) which are associated with myelo- and hepato-toxicity when they accumulate at high levels. Important enzymes in this pathway include thiopurine methyl transferase (TPMT) and hypoxanthine phosphoribosyltransferase (HPRT). It is recognised that polymorphisms of TPMT can influence TPMT activity and hence levels of 6TG and 6MMP. About 1 in 300 individuals have homozygote TPMT mutations and AZA or 6MP therapy results in very

Crohn's disease (Beaugerie et al., 2009a).

**14.5 Exposure to immunomodulators** 

statistically significant.

**14.6 Disease severity** 

(Beaugerie et al., 2009a).

**15. Other risk factors for lymphoma development** 

**15.1 Pharmacogenomics of thiopurine therapy** 

are relevant to the IBD population but have not been well studied yet.

high levels of 6TG causing myelo-toxicity. Those with heterozygote mutations have intermediate TPMT activity and dose adjustment of AZA and 6MP may be required.

Thiopurines are also commonly used drugs for the management of acute lymphoblastic leukaemia (ALL) in paediatric patients. Following treatment, these patients have an increased risk of subsequently developing secondary myelodysplasia or acute myeloid leukaemia but it had been thought that this risk is associated with alkylating agents, epipodophyllotoxins or radiation therapy rather than due to AZA. However, Bo et al showed that paediatric patients with allelic variations of the TPMT gene (and lower TPMT levels) had a higher rate of secondary leukaemias in this setting (Bo et al., 1999). Thiopurines result in the incorporation of 6TG, a purine analogue, into lymphocyte DNA which can activate DNA repair mechanisms. This introduces the risk of point mutations and chromosomal abnormalities during repair processes. It follows that higher levels of 6TG may increase this risk further, possibly explaining the link with lymphomagenesis in IBD patients treated with thiopurines.

Disanti et al made an interesting observation in a cohort of IBD patients treated with 6MP over a 37 year period. The investigators divided the group of over 600 patients in to those who developed a sustained leukopenia of <4.0x109/l for 20 or more days and those who did not. They found that there was an increased risk of haematological malignancies in the group with sustained leukopenia (p=0.014) (Disanti et al., 2006).

Although, the TPMT levels were not known in these patients, further investigation in to the association between TPMT and lymphoma risk may be intriguing.

#### **15.2 Radiation exposure**

The use of imaging modalities such as computed tomography and fluoroscopy have been an important component for the diagnosis and assessment of IBD but there has been increasing concern regarding the cancer risk associated with diagnostic ionising radiation (Brenner and Hall, 2007). Much of the risk of medical radiation exposure is extrapolated from studies of populations near nuclear explosions and occupational exposure but there is some evidence in certain medical settings (Brenner et al., 2003). For example, young patients with scoliosis who have had repeated chest x-rays were at increased risk of breast malignancy (Doody et al., 2000).

Recent evidence suggests that CT imaging is being used more frequently in IBD patients, particularly those with Crohn's disease (Newnham et al., 2007, Kroeker et al., 2011). Leukaemia is well recognised as a long term consequence of radiation exposure and this risk is higher in children (Darby et al., 1992, Shimizu et al., 1990). However, there is little evidence to confirm an increased risk of lymphoma in patients exposed to diagnostic ionising radiation though the mechanisms of oncogenesis may be similar. There was no increased risk of lymphoma seen in patients receiving radiotherapy for uterine cancer nor amongst tuberculosis patients with repeated pneumothorax who had an average of 77 chest x-rays on follow up (Boice, 1992). A study including 318 NHL patients found no increased exposure to diagnostic radiation compared to controls if imaging from the 12 months immediately prior to lymphoma diagnosis was excluded. The authors felt that radiological procedures within this period were performed to investigate the lymphoma rather than a causative factor (Boice et al., 1991).

Evaluating Lymphoma Risk in Inflammatory Bowel Disease 341

for severe or complicated disease are now being identified which should allow selection of

There is ample evidence that immunomodulators and anti-TNF drugs are very effective for the treatment of IBD. In a 30 year review, Fraser et al found that there were 64% and 87% remission rates at 6 months for patients treated with AZA for Crohn's disease and ulcerative colitis respectively (Fraser et al., 2002). Feagan et al found 65% remission at 40 weeks with MTX for Crohn's disease (Feagan et al., 2000). In the SONIC study, there was 57% remission at 1 year for combined AZA and IXB therapy (Colombel et al., 2010). The CHARM study and its open label extension, ADHERE, for adalimumab in Crohn's disease found improved fistula healing rates, 57% decreased hospitalisation and improved Work Productivity Scores (Panaccione et al., 2010). A Markov model found that the benefits of azathioprine for the treatment of Crohn's disease outweighed the risk of lymphoma but such calculations are

Additionally, there is evidence that stopping these drugs may be harmful to patients. Azathioprine withdrawal leads to relapse within 18 months at 21% vs 8% (p=0.02, NNH=8) (Lemann et al., 2005). Methotrexate withdrawal leads to relapse within 40 weeks in 61% vs 35% (p=0.04, NNH=4) (Feagan et al., 2000). Infliximab withdrawal leads to hospitalisation within 1 year in 38% vs 23% (p=0.05, NNH=7) (Rutgeerts et al., 2004). However, the CESAME study did suggest that stopping immunomodulator therapy did return the risk of

No form of screening is able to predict lymphoma development. Although, the role of vitamin D status and TPMT expression on lymphoma risk is intriguing, there is insufficient evidence to recommend the routine testing of these parameters to guide patient management. Prophylactic use of antivirals in renal transplant recipients has been shown to reduce the risk of post-transplant lymphoproliferative disorders by as much as 83% and the use of this strategy in IBD patients on immunosuppression is warranted (Funch et al., 2005). The morbidity associated with IBD, the efficacy of these drugs and the risks of stopping them are important factors in making management decisions with patients. In many

Adams, A. E., Zwicker, J., Curiel, C., Kadin, M. E., Falchuk, K. R., Drews, R. & Kupper, T. S.

Altekruse, S., Kosary, C., Krapcho, M., Neyman, N., Aminou, R., Waldron, W., Ruhl, J.,

Altieri, A., Bermejo, J. L. & Hemminki, K. 2005. Familial risk for non-Hodgkin lymphoma

Aozasa, K., Takakuwa, T. & Nakatsuka, S. 2005. Pyothorax-associated lymphoma: a lymphoma developing in chronic inflammation. *Adv Anat Pathol,* 12, 324-31.

2004. Aggressive cutaneous T-cell lymphomas after TNFalpha blockade. *J Am Acad* 

Howlander, N., Tatalovich, Z., Cho, H., Mariotto, A., Eisner, M., Lewis, D., Cronin, K., Chen, H., Feuer, E., Stinchcomb, D. & Edwards, B. 2010. SEER Cancer Statistics

and other lymphoproliferative malignancies by histopathologic subtype: the

patients who are most likely to gain from aggressive treatment (Beaugerie et al., 2006).

inherently based on estimations and assumptions (Lewis et al., 2000).

lymphoma back to baseline.

**17. References** 

patients, the benefits will outweigh the risks.

Review, 1975-2007. National Cancer Institute.

Swedish Family-Cancer Database. *Blood,* 106, 668-72.

*Dermatol,* 51, 660-2.
