Research Article: Hyperbranched Polyglycerol as a Colloid in Cold Organ Preservation Solutions

Date Published: February 23, 2015

Publisher: Public Library of Science

Author(s): Sihai Gao, Qiunong Guan, Irina Chafeeva, Donald E. Brooks, Christopher Y. C. Nguan, Jayachandran N. Kizhakkedathu, Caigan Du, Daqing Ma.


Hydroxyethyl starch (HES) is a common colloid in organ preservation solutions, such as in University of Wisconsin (UW) solution, for preventing graft interstitial edema and cell swelling during cold preservation of donor organs. However, HES has undesirable characteristics, such as high viscosity, causing kidney injury and aggregation of erythrocytes. Hyperbranched polyglycerol (HPG) is a branched compact polymer that has low intrinsic viscosity. This study investigated HPG (MW-0.5 to 119 kDa) as a potential alternative to HES for cold organ preservation. HPG was synthesized by ring-opening multibranching polymerization of glycidol. Both rat myocardiocytes and human endothelial cells were used as an in vitro model, and heart transplantation in mice as an in vivo model. Tissue damage or cell death was determined by both biochemical and histological analysis. HPG polymers were more compact with relatively low polydispersity index than HES in UW solution. Cold preservation of mouse hearts ex vivo in HPG solutions reduced organ damage in comparison to those in HES-based UW solution. Both size and concentration of HPGs contributed to the protection of the donor organs; 1 kDa HPG at 3 wt% solution was superior to HES-based UW solution and other HPGs. Heart transplants preserved with HPG solution (1 kDa, 3%) as compared with those with UW solution had a better functional recovery, less tissue injury and neutrophil infiltration in syngeneic recipients, and survived longer in allogeneic recipients. In cultured myocardiocytes or endothelial cells, significantly more cells survived after cold preservation with the HPG solution than those with the UW solution, which was positively correlated with the maintenance of intracellular adenosine triphosphate and cell membrane fluidity. In conclusion, HPG solution significantly enhanced the protection of hearts or cells during cold storage, suggesting that HPG is a promising colloid for the cold storage of donor organs and cells in transplantation.

Partial Text

Cold preservation of donor organs, tissues and cells with a specialized solution is a common strategy to minimize ischemic injury prior to transplantation, and so far many different preservation solutions have been developed for this purpose [1]. In heart transplantation, the University of Wisconsin (UW) solution has been demonstrated to be a safe and effective preservation solution for heart transplants [2–4], and is associated with improved short-term survival and less acute ischemic necrosis in the early post-transplant period [5–7]. One of the important components in UW solution, ET-Kyoto solution [8] and KPS-1 solution [1], is hydroxyethyl starch pentafraction (HES). Inclusion of HES is essential to counteract graft interstitial edema and endothelial cell swelling for improving viability and outcomes of donor organ preservation [9, 10], but it may have negative effects on its application as a cold organ preservation solution for transplantation. HES is commonly used for volume resuscitation of critically ill patients, but its safety has recently come into question. Fluid resuscitation with HES is associated with coagulopathy, pruritus and acute kidney injury [11–15], and HES impairs immediate donor kidney function after transplantation [16]. Two other important limitations associated with HES in UW solution have also been suggested: (i) it increases the viscosity of the solution, resulting in an increase in vascular resistance of donor organs to the initial flushing/perfusion that could shorten graft survival [17, 18]; and (ii) it causes the hyperaggregation of human red blood cells (RBCs) due to the adsorption of HES to RBCs [19–21], which may result in stasis of blood and incomplete washout from donor organs before transplantation. Moreover, HES is a highly heterogeneous polymer with high polydispersity index (PDI). Therefore, there is an unmet need for an alternative colloid that is superior to HES in terms its biocompatibility as well as for maximally limiting donor organ injury during organ procurement or preservation.

HES-based UW solution is commonly used for both aortic in situ flush for heart transplants and ex vivo cold storage of kidney, liver and pancreas. Recently, in comparison with HTK solution in preservation of deceased-donor organs, UW solution was demonstrated to reduce graft loss in kidney transplants even beyond 12 months [55–57], and prolong graft survival in both liver and pancreas transplants [58, 59]. However, there are a number of donor organs that do not function well upon transplantation after the preservation with the UW solution [10], suggesting that an improved method or a new solution for maximally limiting this injury is needed, and perhaps by which increasing the donor pool by using expanded criteria or marginal donors could be possible [60]. The present study demonstrates, for the first time, that the replacement of HES and raffinose with HPG, a compact biocompatible polymer, in the UW solution enhances the protection of donor hearts from cold ischemic injury and cultured cells (HUVECs and H9c2 cells) from cold-induced cell death. Importantly, the cytoprotection of low MW HPGs was found to be superior to that of the higher MW HPGs.

In the present study we investigated the potential of HPG as a colloidal substitute in an organ preservation solution. Using our initial screening studies, we determined that HPG 1 kDa at 3 wt% concentration is superior in preventing in organ injury compared to other molecular sizes and concentrations of HPG. There was clear MW dependence. HPG solution showed superior performance compared to UW solution in terms of functional recovery, graft function in both isografts and in allografts in mouse models. HPG solution produced better survival in cultured cells in cold temperature. The maintenance of the intercellular ATP and preservation of the membrane fluidity of the cells may be responsible for the enhanced protection offered by HPG solution. Our studies demonstrate that HPG based solutions are superior to HES based UW solution in many aspects and has the potential to replace current cold preservation solutions in the protection of donor organs, tissues or cells for transplantation.