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Keywords:

  • HTK solution;
  • pancreas transplant;
  • pancreas transplantation alone;
  • simultaneous pancreas-kidney (SPK) transplantation;
  • UW preservation solution

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Prior single-center studies have reported that pancreas allograft survival is not affected by preservation in histidine-tryptophan-ketoglutarate (HTK) versus University of Wisconsin (UW) solution. To expand on these studies, we analyzed the United Network for Organ Sharing (UNOS) database of pancreas transplants from July 2004, through February 2008, to determine if preservation with HTK (N = 1081) versus UW (N = 3311) impacted graft survival. HTK preservation of pancreas allografts increased significantly in this time frame, from 15.4% in 2004 to 25.4% in 2008. After adjusting for other recipient, donor, graft and transplant center factors that impact graft survival, HTK preservation was independently associated with an increased risk of pancreas graft loss (hazard ratio [HR] 1.30, p = 0.014), especially in pancreas allografts with cold ischemia time (CIT) ≥12 h (HR 1.42, p = 0.017). This reduced survival with HTK preservation as compared to UW preservation was seen in both simultaneous pancreas-kidney (SPK) transplants and pancreas alone (PA) transplants. Furthermore, HTK preservation was also associated with a 1.54-fold higher odds of early (<30 days) pancreas graft loss as compared to UW (OR 1.54, p = 0.008). These results suggest that the increasing use of HTK for abdominal organ preservation should be re-examined.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Successful solid organ transplantation requires minimizing the damage from cold ischemia and reperfusion during organ recovery. For abdominal organs, the two most commonly used preservative solutions in the United States are University of Wisconsin (UW) solution and histidine-tryptophan-ketoglutarate (HTK) solution. UW solution has been the standard for abdominal organ preservation fluid since 1987 (1); however, HTK has replaced UW solution at many centers since it was approved for use in kidney and liver preservation. In addition to significantly lower cost (2), other potential advantages of HTK include reduced risk of reperfusion hyperkalemia, better tissue perfusion due to lower viscosity and better cell preservation over a wider range of temperatures (3).

The experience with HTK in pancreas allograft preservation has been limited to small series that have reported equivalent patient and graft survival for allografts preserved with UW and HTK solutions (2,4–10). However, these studies have been powered to detect only very large differences in graft survival, and have lacked sufficient sample size to appropriately adjust for confounders through multivariate regression models.

To expand on previous reports, we analyzed a large national registry of pancreas transplants from 2004 through 2008. The primary outcome measured was graft survival, which is very reliably measured through this registry. We then performed multivariate regression analyses to adjust for potential confounding recipient, donor, graft and transplant center characteristics. Finally, we analyzed subgroups of pancreas transplants by cold ischemia time (CIT), donor age and transplant type (simultaneous pancreas-kidney (SPK) or pancreas alone (PA)), as well as transplant center volume to determine if HTK versus UW preservation impacted graft survival differentially in these subgroups.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The study population consisted of 4392 patients undergoing pancreas transplantation between July 1, 2004, and February 28, 2008, as reported to the United Network for Organ Sharing (UNOS). Patients <18 years of age and patients in whom a solution other than HTK or UW was used were excluded from the study. Patients were divided into two groups: ‘UW’= organ recovery with UW solution (n = 3311) and ‘HTK’= organ recovery with HTK solution (n = 1081). The primary outcome was graft survival, defined as postoperative time alive with a functioning allograft, censored for loss to follow-up or administrative end-of-study. Patient death information was supplemented by linkage with the Social Security Master Death File.

Risk factors for graft loss were determined using Cox proportional hazards models, and risk factors for early graft loss (<30 days) were determined using logistic regression. Multivariate models were adjusted for the following confounders that were felt to be (or shown in prior studies to be) clinically relevant: recipient age, gender, ethnicity, body mass index (BMI), hypertension, hospitalized and prior transplant; donor age, gender, ethnicity, BMI and cause of death and transplant type (SPK or PA), donor type (donation after cardiac death or donation after brain death) and CIT.

To adjust for center-specific effects, models incorporating all the covariates listed above in addition to center volume were performed, with clustered variance estimates to account for correlation among patients and practice patterns by transplant center. We also evaluated the proportion of HTK use by transplant center volume divided into deciles.

Unless otherwise specified, all tests were 2-sided with statistical significance set at α= 0.05. Comparisons of donor and transplant characteristics between UW and HTK subgroups are reported using unpaired 2-tailed t-tests for continuous covariates and chi-squared tests of independence for categorical variables. For Cox models, proportional hazards assumptions were confirmed by inspection of complementary log-log plots. Data were missing for <5% of all covariates and handled by casewise deletion, with the exception of CIT that was imputed as previously described (11,12). All analyses were performed using Stata 10.0/MP for Linux (StataCorp, College Station, TX).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The proportion of pancreas allografts recovered with HTK preservation increased from 15.4% in 2004 to 25.4% in 2008. Of note, during this time frame, 50% of pancreas transplants were performed by 24 transplant centers (Figure 1A). However, HTK utilization was not associated with transplant center volume (Figure 1B). HTK-preserved allografts were less likely to be utilized in SPK transplants and had a statistically significantly shorter CIT (Table 1). Further, donors of HTK-preserved allografts were more likely to be Caucasian as compared to UW-preserved allograft (Table 1).

image

Figure 1. Distribution of pancreas transplants and HTK use by transplant center. (A) Cumulative proportion of pancreas transplants performed by transplant centers with decreasing volumes. Note that 50% of pancreas transplants were performed by 24 centers. (B) Average proportion of HTK use among centers by deciles of pancreas transplant volume.

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Table 1. Donor and transplant characteristics
 UW (n = 3311)HTK (n = 1081)p
  1. By 2-sided t-test for continuous variables and by chi-squared test for categorical variables. Bold values represent statistically significant differences between HTK and UW.

Age (y)25.525.80.5
Female (%)32.331.70.7
Ethnicity, Caucasian (%) 64.0 77.3 <0.001
BMI24.124.10.9
Cause of death (%)  0.1
Anoxia10.513.1 
CVA22.820.5 
Head trauma66.466.1 
Donor after cardiac death (%) 2.4 1.60.1
SPK (%) 68.0 63.6 0.009
CIT (h) 13.5 11.5 <0.001

Despite the fact that HTK preservation was used in recipients with shorter CIT, HTK preservation was associated with a significantly increased risk of graft loss on univariate analysis (hazard ratio [HR] 1.27, CI 1.08–1.48, p = 0.003). After adjusting for donor, recipient, graft and transplant center factors impacting graft survival, HTK preservation was still independently associated with a 30% increased risk of graft loss on multivariate analyses when compared with UW (HR 1.30, p = 0.014) (Table 2). In subgroup analyses, HTK-preserved SPK allografts had an increased risk of pancreas graft loss as compared to UW-preserved SPK allografts (HR 1.32, p = 0.023). The effect of HTK preservation on reduced graft survival of both SPK and PA allografts was observed immediately after transplant, consistent with the effect resulting from a difference in the efficacy of the preservative solution (Figure 2). Furthermore, HTK-preserved allografts with CIT ≥12 h (HR 1.42, p = 0.017) or a history of a prior transplant (HR 2.10, p = 0.016) had an even greater risk of graft loss when compared to UW-preserved allografts in adjusted models (Table 2). Finally, center volume did not seem to modify the effect of HTK preservation on graft survival, as higher risk was observed at both larger volume centers (≥50 pancreas transplants during the study period; adjusted HR 1.37, p = 0.008) and smaller volume centers (<50 pancreas transplants during the study period; adjusted HR 1.33, p = 0.03).

Table 2. Subgroup analysis of the effect of HTK preservation on pancreas allograft survival
 Multivariate patient level*Multivariate patient and center level**
HR (95% CI)p-ValueHR (95% CI)p-Value
  1. * Adjusted for recipient age, gender, ethnicity, BMI, hospitalized and prior transplant; donor age, gender, ethnicity, BMI and cause of death; transplant type (SPK or PA), donor type (donation after cardiac death or donation after brain death) and CIT.

  2. **Adjusted for same covariates as above and also adjusted for center volume, with clustered variance estimates to account for correlation among patients and practice patterns by transplant center.

All pancreas transplants1.27 (1.07–1.50)0.0061.30 (1.05–1.60)0.014
(n = 1081 HTK and 3311 UW)
SPK1.29 (1.03–1.63)0.0291.32 (1.04–1.68)0.023
(n = 688 HTK and 2250 UW)
CIT ≥12 h1.42 (1.10–1.85)0.0081.42 (1.06–1.90)0.017
(n = 391 HTK and 1499 UW)
Prior transplant2.10 (1.27–3.49)0.0042.10 (1.15–3.84)0.016
(n = 86 HTK and 266 UW)
Donors age > 40 years1.29 (0.88–1.88)0.20 1.34 (0.85–2.11)0.20 
(n = 151 HTK and 424 UW)
image

Figure 2. Kaplan–Meier graft survival curves for SPK and PA transplant based on preservative solution. (A) SPK graft survival by HTK versus UW. (B) PA graft survival by HTK versus UW.

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A closer examination of the allograft survival curves in Figure 2 suggested a dominant early effect of preservation solution; this was consistent with a recent single-center report (2). To differentiate between early and late effects in the national population, we developed a multivariate logistic regression model to examine the effect of HTK preservation on early pancreas graft loss within the first 30 postoperative days. Indeed, a dominant early effect was seen for most subgroups. For all pancreas transplants, HTK preservation resulted in a 1.54-fold higher odds of early graft loss (Odds Ratio, OR 1.54, p = 0.008) (Table 3). SPK transplants (OR 1.50, p = 0.021) and grafts with CIT ≥12 h (OR 1.58, p = 0.006) also had a higher odds of early graft loss with HTK preservation.

Table 3. Subgroup analysis of the effect of HTK preservation on early (< 30 days) pancreas allograft loss
 Multivariate patient level*Multivariate patient and center level**
OR (95% CI)p-ValueOR (95% CI)p-Value
  1. *Adjusted for recipient age, gender, ethnicity, BMI, hospitalized and prior transplant; donor age, gender, ethnicity, BMI and cause of death; transplant type (SPK or PA), donor type (donation after cardiac death or donation after brain death) and CIT.

  2. **Adjusted for same covariates as above and also adjusted for center volume, with clustered variance estimates to account for correlation among patients and practice patterns by transplant center.

All pancreas transplants1.44 (1.13–1.84)0.0031.54 (1.12–2.13)0.008
(n = 1081 HTK and 3311 UW)
SPK1.45 (1.06–1.98)0.0191.50 (1.06–2.12)0.021
(n = 688 HTK and 2250 UW)
CIT ≥12 h1.59 (1.08–2.33)0.0191.58 (0.99–2.53)0.006
(n = 391 HTK and 1499 UW)
Prior transplant1.84 (0.77–4.38)0.20 1.93 (0.72–5.19)0.20 
(n = 86 HTK and 266 UW)
Donors age > 40 years1.50 (0.88–2.55)0.10 1.61 (0.92–2.82)0.09 
(n = 151 HTK and 424 UW)

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

In this national study of pancreas transplants between 2004 and 2008, we found that the use of HTK solution is an independent risk factor for decreased graft survival and early graft loss. This effect was seen in both SPK and PA transplants. The increased risk of graft loss with HTK preservation was seen despite the fact that HTK was utilized in allografts with shorter CIT, which are less likely to fail.

One strength of this study is the large sample size identified in the national registry. Prior studies trying to assess the impact of HTK versus UW preservation on pancreas allograft survival have been severely underpowered to detect meaningful differences in graft survival (2,4–9). As a conservative estimate in a previous study (4), it would require 157 patients in each arm to have an 80% chance of showing a 10% difference in graft survival. Thus, it is not surprising that prior reports with series of 10 to 78 patients (matched to historical controls) have not been able to identify a statistically significant difference with HTK versus UW preservation, and have further lacked the ability to perform stable multivariate regression analyses to control for confounding recipient, donor and transplant variables.

Trends from previously reported series of HTK preservation of pancreas allografts are consistent with several of our statistically significant findings. A review of the Michigan Organ Procurement Organization experience with HTK in pancreas transplantation identified 36 HTK-preserved allografts and matched them to historical UW controls (2). While not achieving statistical significance, there was a clear trend toward poorer graft survival with HTK preservation at 30 days (HTK = 87% and UW = 94%) (2). This difference was mostly due to notably poorer PA graft survival with HTK preservation, as the 30-day PA graft lost was 14.3% for HTK as compared to 5.9% for UW (2). Agarwal et al. reported on 78 HTK-preserved pancreas allografts compared to historical UW controls and found shorter CIT for HTK-preserved allografts (9 h) versus UW-preserved allografts (11 h) with similar 1-year graft survival for HTK (93%) and UW (92%) (4). Certainly, one explanation for these results are the short CIT (<12 h) in this (4) and prior case series (2, 5–9) of HTK preserved pancreas allografts, as the UNOS data would suggest that HTK more strongly impacts graft survival when CIT ≥12 h.

While the precise mechanism by which HTK preservation may impact human pancreas allograft survival remains unclear, there have been extensive studies of HTK and UW preservation of porcine pancreas allografts that suggest porcine pancreas allografts stored in HTK are more edematous than those stored in UW (13–17). These gross findings also correlated with histological evidence of increased interstitial edema and cellular swelling (15–17). At least one group noted clinical correlation with HTK-preserved human pancreas allografts, as they reported that all HTK-flushed pancreas allografts appeared more edematous as compared to UW-flushed pancreas allografts (7). In vitro models of both human hepatocytes (18) and human liver endothelial cells (19) found that UW was significantly superior to HTK at reduction of cellular necrosis, mitochondrial dysfunction and intracellular ATP depletion during ischemia-reperfusion. Clearly, further work remains to elucidate the critical cellular signaling pathways to target during the ischemia-reperfusion that occurs during organ recovery and transplantation.

It is important to acknowledge several limitations to our study. As with any analysis utilizing the UNOS database, our conclusions rely on the assumption that there is no systematic bias generated by reporting error or missing data. However, the primary endpoint for this analysis is graft survival, which is reliably captured in the UNOS database. Residual or unmeasured confounders that could impact organ preservation and survival include: surgeon technique, volume of initial flush used at the time of organ recovery, differences in immunosuppression protocols, the fat content/quality of the allograft and center-specific practices. However, presumably these differences would only bias our results if there was a substantial interaction between preservative solution and these other variables, such that these variables not only impacted graft survival but also modified the impact of preservative solution on graft survival. Further, since causes of graft failure are poorly captured in the UNOS database (over 50% of the data are missing), a meaningful analysis of specific causes of graft loss in HTK versus UW-preserved pancreas allografts could not be performed.

In conclusion, our analysis of the national experience with HTK and UW pancreas allograft preservation demonstrates that HTK preservation is an independent risk factor for reduced graft survival for pancreas allografts and in particular those with prolonged CIT. These findings may be utilized to help guide clinical decision making in the selection of organ preservation solution and suggest that despite its lower cost, enthusiasm about HTK as a universal preservative solution should be re-examined.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The UNOS National Data Registry is supported in part by Health Resources and Services Administration contract 231–00-0115. The analyses described here are the responsibility of the authors alone and do not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products or organizations imply endorsement by the U.S. Government.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
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