Sirolimus after kidney transplantation.

The risk of cancer is increased after kidney transplantation and is predominantly attributed to oncogenic immunosuppression. It is a leading cause of death for kidney allograft recipients, and analyses of transplant registry data suggest that mortality related to cancer is on the increase. Strategies to reduce the risk of cancer bymodifying immunosuppression are therefore desirable but must be balanced against other important adverse events such as allograft failure. An immunosuppressant that protects against cancer and avoids the nephrotoxicity of calcineurin inhibitors such as tacrolimus and ciclosporin would fulfil two important functions for kidney allograft recipients. Sirolimus, an inhibitor of mammalian target of rapamycin (mTOR) with perceived anti-neoplastic and non-nephrotoxic properties, could potentially challenge the dominance of calcineurin inhibition for immunosuppression after kidney transplantation. The linked article by Knoll and colleagues (doi:10.1136/bmj. g6679) examined the use of sirolimus after kidney transplantation. This commendable systematic review and meta-analysis has meticulous methods, with empirical data combined at an individual patient level for 5876 kidney allograft recipients from 21 randomised controlled trials of sirolimus compared with non-sirolimus immunosuppressive regimens. The authors included studies evaluating sirolimus as a de novo treatment and also after conversion from other immunosuppressive treatments. Sirolimus reduced the risk for overall and non-melanoma skin cancer by 40% and 56%, respectively. But this benefit was confined to patients switching to sirolimus from other treatments. The most striking finding was an increased risk of death (driven by increased cardiovascular deaths and deaths related to infection) among patients taking sirolimus in both de novo and conversion studies. This is a new finding and one that puts the future of sirolimus, and other mTOR inhibitors, at the centre of a difficult trade-off between risks and benefits in the setting of kidney transplantation. How should we interpret these data for the benefit of kidney allograft recipients? The use of de novo sirolimus is already limited in the immediate postoperative period after a transplant because of concerns about its use in the context of complications, including delayed graft function and poor wound healing. As sirolimus does not reduce the risk of cancer but does increase mortality in these patients, it is difficult to justify de novo use except in exceptional circumstances. Most experience with sirolimus is from patients converting from calcineurin inhibition after the early postoperative period, and this is where the benefits of cancer reduction are seen, albeit with an increased risk of mortality. Sirolimus conversion is currently undertaken in patients who need to discontinue their calcineurin inhibitor. The switch can be proactive as part of a calcineurin inhibitor withdrawal protocol (in anticipation of improved long term allograft survival that remains unproved) or reactive secondary to the development of cancer or side effects related to calcineurin inhibition (such as thrombotic microangiopathy or polyoma nephropathy that compromise allograft survival). Sirolimus, however, can be difficult to tolerate after conversion. A systematic review and meta-analysis by Lim and colleagues reported discontinuation rates of 21.6% after conversion to mTOR inhibitors after kidney transplantation (compared with 9.6% for patients who continued to take calcineurin inhibitors, P=0.020). Alternative options include conversion of calcineurin inhibitors to belatacept, a selective inhibitor of T cell activation. But belatacept has its own drawbacks that prohibit widespread use, and it remains unlicensed in some countries. Therefore conversion to sirolimus to prevent allograft loss in selected patients could be justified as kidney allograft failure itself is a strong independent risk factor for subsequent death. The most common indication for conversion to sirolimus after kidney transplantation is probably the development of skin cancer. While skin cancers might be the commonest cancer occurring after transplant, however, mortality from skin cancers is low at 3.2% of all cancer related mortality. Most of these deaths are caused by melanoma, which comprises only 2.7% of all skin cancers after transplantation. Sirolimus conversion could be justified in a narrow subset of kidney allograft recipients with melanoma but not for those with non-melanoma skin cancers. While non-melanoma skin cancers are unsightly and can affect a patient’s quality of life, prolonging life itself would probably be deemed of greater importance. The other pressing issue is whether increased mortality is a concern with sirolimus alone or a class effect. Everolimus or temsirolimus are two other examples of mTOR inhibitors, and