DISCUSSION BOARD PANEL DE DISCUSION

Chronic allograft nephropaty diagnosed by means of protocol biopsies

Daniel Serón 17664dsm@comb.es

Nephrology Department. Hospital de Bellvitge. L´Hospitalet. Barcelona. Spain.

Introduction

The first studies aimed to characterize renal allograft histology in stable grafts were done in the early eighties and challenged the assumption that renal histology was normal in patients apparently doing well. An interstitial infiltrate was noticed in a high proportion of stable grafts. In spite of this finding, patients evolved favourably during the first year of follow up. Infiltrating interstitial cells were also observed in patients with postransplant acute tubular necrosis and cyclosporine toxicity (1, 2). Despite, the interstitial infiltrate was more intense in patients with acute rejection, the number of infiltrating cells was an insufficient criteria to clearly separate rejecting from non-rejecting grafts (3, 4). These findings not only stimulated the phenotypic characterization of allograft biopsies as an attempt to define more precise criteria for the diagnosis of acute rejection, but also stimulate the study of interstitial infiltrates in stable grafts in order to determine whether the higher the degree of inflammation, the worse the long term outcome. However, a relationship between the intensity or phenotype of interstitial infiltrate and outcome has not been convincingly demonstrate (5). The main reason for the lack of correlation between the number of interstitial infiltrating cells and graft evolution in stable grafts, was the high variability in the number of interstitial infiltrating cells in different tissue samples obtained from the same patient as well as in different microscopic fields from the same section (6,7). Thus, it became evident that renal core biopsies were too small to estimate with a reasonable degree of precision the intensity of the interstitial infiltrate in the whole kidney. International standarization of the evaluation of renal allograft pathology according to the Banff (8) criteria or CCTT (9) classification, proposed the evaluation of the intensity of acute inflammatory changes in all renal compartments as an attempt to improve the reliability of the diagnosis of acute rejection. Banff criteria have been also applied to the study of the temporal evolution and prognostic significance of acute lesions in stable grafts. An association between the severity of acute lesions early after transplantation and outcome has been recently described (10).

The introduction of new immunosuppressants has allowed a very important reduction o in the incidence and severity of acute rejection. Accordingly, the contribution of acute rejection to graft failure has decreased, while the importance of CAN as the leading cause of late graft failure has become more evident (11). The histological diagnosis of CAN, according to Banff criteria, relies on the presence of tubulo-interstitial chronic renal damage either with or without arterial narrowing. In the early eighties, studies dealing with protocol biopsies showed that a proportion of stable renal allografts displayed interstitial fibrosis. The severity of interstitial fibrosis was related to cyclosporine treatment and correlated with serum creatinine at the time of biopsy. Thus, these first studies already suggested that interstitial chronic damage in stable grafts might be a marker of poor outcome (12).

Chronic allograft nephropathy in protocol biopsies

Studies of protocol biopsies have shown that approximatelly 40% of stable grafts display CAN during the first 6 months after transplantation (7, 10, 13, 14) . The incidence and severity of this lesion increases thereafter, and at 2 years, it is present in approximatelly two thirds of stable grafts (15). The presence of CAN not only implies a poor outcome (7, 10, 13, 14, 15, 16) but it has been shown that early appearance of CAN constitutes an independent predictor of renal allograft survival from other clinical parameters associated to late outcome such as acute rejection or serum creatinine (7). Furthermore, we have recently observed that at approximatelly 3 months of follow up, patients in whom CAN is associated with transplant vasculopathy show a more ominous prognosis than patients in whom CAN is not associated with vessel involvemet (17).

Advanced donor age, acute rejection before performing the protocol biopsy, prolongued cold ischemia time or increased cyclosporine exposure are all independent predictors of CAN in protocol biopsies (7, 13, 14). It is worth to notice that elevated serum cholesterol in the recipient constitutes a risk factor for the development of transplant vasculopathy but not for the development of chronic tubulointerstitial damage (17).

Taken together, the above mentioned findings suggest that protocol biopsies allow the diagnosis of CAN before this condition is clinically manifested.

           

Clinical trials to modify the natural history of chronic allograft nephropathy

Experimental studies suggest that the natural history of CAN can be modified by means of pharmacological intervention (19, 20). Furthermore, it has been recently shown that the appearance and progression of transplant vasculopathy in the transplanted heart can be efficiently prevented or treated (21, 22). However, information about the potential benefit of any preventive or therapeutical maneuvre to modify the natural history of CAN is rather scarce. This situation is mainly due to the methodological difficulties encountered in the design of trials aimed to prevent or treat this entity. In any primary or secondary intervention trial, the primary efficacy variable must necessarily rely on graft survival, since there are no surrogate mesasures of this parameter allowing to evaluate the efficacy of any treatment early after transplantation. Neither the presence of acute rejection, nor serum creatinine at a given point of follow up allow a precise estimation of graft survival (23).

It has been estimated that the minimum sample size for a trial aimed to prevent CAN is approximatelly 10 times higher than for a trial aimed to prevent acute rejection. Since the end point of such a a trial must necessarily rely on the study of long term allograft survival, time of follow up is also rather long .

The number of estimated patients to be enrolled in a treatment trial is lower than in a prevention trial (24). However, the advanced degree of chronic renal lesions at the time of diagnosis does not suggest that there are many chances for improvement (25) and accordingly, at this advanced stage of disease, a trial testing a drug that has the potential to modify the natural history of CAN could lead to the false conclusion that the drug is inefficient.

These considerations explain why at present, results from large, randomized and prospective trials evaluating any therapy to prevent or treat CAN are lacking.

The potential utility of protocol biopsies in trials aimed to prevent or treat chronic allograft nephropathy

In the transplanted heart, intracoronary angiographies and intracoronary ultrasounds allow to prospectively and precisely monitor the appearance and progression of transplant vasculopathy (26, 27). Such diagnostic tools have facilitated the design of trials aimed to prevent or treat heart transplant vasculopathy (21, 22). On the contrary, no prospective methods to monitor the appearance and progression of CAN are available in the kidney.

The study of renal allograft protocol biopsies has revealed that the presence of CAN early after transplantation constitutes a powerful independent predictor of long term. The predictive value on graft survival of incipient chronic tubulointerstital lesions in protocol biopsies evaluated according to the Banff criteria is superior to other clinical predictors of outcome. This observation suggests that protocol biopsies might be useful to monitor the appearance and progression of CAN in a similar way as intracoronary angiographies or intracoronary ultrasounds are employed to monitor transplant vasculopathy and accordingly they could become a useful tool in the design of trials aimed to modify the natural history of CAN.

We have evaluated the possible utility of protocol biopsies in the design of clinical trials aimed either to prevent or treat CAN in a large sample of patients in whom a protocol biopsy was performed at approximatelly 3 months after surgery (17). The incidence of CAN at 3 months was 40 %. Power calculations showed that in an hypothetical prevention trial in which the presence or absence of CAN at 3 months were considered the primary efficacy variable, the minimum sample size to detect a 50 % reduction of the incidence of CAN, aproximately 300 patients in the treatment and in the placebo group would be necessary. This figure is not different from the number of patients enrolled in trials aimed to prevent acute rejection. Thus our data suggest that protocol biopsies could be a useful primary efficacy variable in trials aimed to prevent CAN. Obviously, these calculation only constitue an approximate approach to the evaluation of the utility of protocol biopsies. Probably a later timing of the biopsy and the definition of quantitative parameters to estimate chronic renal lesions could further reduce the minimum sample size in such a trial.

On the other hand, we also evaluated the possible utility of early protocol biopsies in the design of a trial aimed to treat CAN. For this purpose, the minimum sample size to detect a 50% improvement in graft survival at 5 years was calculated using the presence of CAN in the protocol biopsy performed at 3 months as the selection criteria to include patients. In this case, approximately 220 patients in the treatment and in the placebo group would be necessary to detect a 50 % increase in graft survival at 5 years. An important advantage of this design is that it allows to avoid the exposure to the adverse events of the study drug in patients not displaying CAN, and who will probably not benefit from the study drug. However ir should be taken into consideration that about 1000 patients should be biopsies in order to enroll the 440 patients needed for such a trial.

In summary, the relevance of CAN as the major cause of late graft failure points out the necessity to evaluate in the clinical setting, the possible beneficial effect of the drugs that modify the natural history of CAN in the experimental setting. The contrast between the large number of trials aimed to prevent acute rejection in comparison to the lack of trials aimed to prevent CAN is mainly explained by the methodological difficulties encountered in the design of strategies allowing to reduce minimum sample size or time of follow up. Protocol biopsies allow the early detection of CAN. Since the presence of CAN in protocol biopsies constitutes an independent predictor of late outcome, it has been proposed that biopsies could become a primary efficacy variable in primary intervention trials. Power calculation suggest that protocol biopsies may allow an important reduction in the number of patients to be enrolled in a prevention trial as well as an important reduction in the time of follow up.

References

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