Current status of the organ replacement approach for malignancies and an overture for organ bioengineering and regenerative medicine

Abstract

Significant achievements in the organ replacement approach for malignancies over the last 2 decades opened new horizons, and the age of “Transplant Oncology” has dawned. The indications of liver transplantation for malignancies have been carefully expanded by a strict patient selection to assure comparable outcomes with non-malignant diseases. Currently, the Milan criteria, gold standard for hepatocellular carcinoma, are being challenged by high-volume centers worldwide. Neoadjuvant chemoradiation therapy and liver transplantation for unresectable hilar cholangiocarcinoma has been successful in specialized institutions. For other primary and metastatic liver tumors, clinical evidence to establish standardized criteria is lacking. Intestinal and multivisceral transplantation is an option for low-grade neoplasms deemed unresectable by conventional surgery. However, the procedure itself is in the adolescent stage. Solid organ transplantation for malignancies inevitably suffers from “triple distress,” i.e., oncological, immunological, and technical. Organ bioengineering and regenerative medicine should serve as the “triple threat” therapy and revolutionize “Transplant Oncology.”

Introduction

Cancer is one of the leading causes of death worldwide and the number of cases increases steadily as the global society matures (http://www.who.int/topics/cancer/en/index.html). Three modalities, i.e., surgery, chemotherapy, and radiation therapy, are well-established cancer treatments. The organ replacement approach, i.e., solid organ transplantation, for malignant neoplasms in the abdomen is conducted only in rare situations where conventional means are not helpful. As we strive to save patients with devastating malignancies desperately seeking a cure, indications for transplantation have been carefully expanded over the last 2 decades with varying degrees of success. Overlap between transplantation and oncology continues to grow and now the age of “Transplant Oncology” has dawned. However, because transplantation is by nature totally dependent on donation and because its indications for cancer are largely heterogeneous, orthodox analytical approaches such as randomized control trials or meta-analyses are not easily undertaken. This review aims to provide a snapshot on the current status and summarize the controversial issues of solid organ transplantation for malignancies in the blooming stage. Finally, allotransplantation connotes fundamental limitations; forethoughts on organ bioengineering and regenerative medicine are mentioned.

Liver Transplantation (LT)

Primary tumors

Hepatocellular carcinoma (HCC)

The Milan criteria,¹ allowing LT for patients with up to 3 HCCs with none of them larger than 3 cm in diameter or a single HCC ≤ 5 cm without vascular invasion or extrahepatic metastases, was reported by Mazzaferro et al. in 1996. These criteria afford a 4-y survival of 75% and remain as the gold standard, yet they are too restrictive for selecting transplant candidates. A handful of transplant centers have made attempts to push the limit that was referred to as the “Metro Ticket.”² Well-known expanded criteria include the University of California San Francisco (≤6.5 cm if single lesion, ≤4.5 cm if ≤3 lesions with total tumor diameter ≤ 8 cm),^3,4 up-to-seven (≤7 as the sum of the size of the largest tumor [in cm] and the number of tumors),⁵ Tokyo (up to 5 lesions with a maximum diameter ≤ 5 cm),⁶ Asan (up to 6 lesions with a maximum diameter ≤ 5 cm),⁷ Hangzhou (total diameter ≤8 cm or histopathologic grade I or II and preoperative α fetoprotein level ≤ 400 ng/ml if total diameter > 8 cm),⁸ Scientific Registry of Transplant Recipients database (total tumor volume ≤ 115 cm³ or α fetoprotein ≤ 400 ng/ml),⁹ Kyoto (up to 10 lesions, maximum diameter ≤ 5 cm, and des-gamma-carboxy prothrombin ≤ 400 mAU/ml),^10,11 and Kyushu (≤5 cm or des-gamma-carboxy prothrombin ≤ 300 mAU/ml).^12,13 The last 4 are distinct from others because they have incorporated into the selection criteria surrogate markers of tumor biology in addition to morphological properties of HCC. The reported outcomes of these expanded criteria are all within the acceptable range achieving >70% 5-y survival, i.e., comparable to non-HCC LT. However, none of them have gained recognition to replace the original Milan criteria and an international consensus meeting is long awaited.

On the other hand, vigorous efforts have been employed to downstage HCC to meet criteria for LT and to select patients with favorable tumor biology. A wide variety of treatment modalities have been described, such as liver resection,^14,15 transarterial chemoembolization or chemoinfusion,^16-20 radiofrequency ablation,^14,15,20,21 ethanol injection,^14,15 and more recently, radioembolization with yttrium-90.^19,20 Each transplant center is using its own inclusion criteria, downstaging protocol, definition of successful downstaging, timing of LT, and immunosuppressive protocol.^22-24 Although a significant heterogeneity lies among studies, downstaging of HCC is arguably a promising alternative for those who would otherwise be out of options. Currently, patients with up to 5 HCCs (all lesions not exceeding 4 cm with total tumor diameter up to 12 cm) and single HCC up to 8 cm in diameter seem to be considered for downstaging (successful downstaging rate, 24–69%); their 5-y survival rates range from 55–94%.^22,23

Meanwhile, liver resection vs. LT for early HCC is also a matter of dispute.^25-27 If LT is being considered for a patient with advanced HCC, the “success” rate using either the expanded Milan criteria or after downstaging needs to be carefully balanced with the scarcity of organs and ensuring equity with non-HCC transplant candidates on the waiting list. Treatment decision-making in HCC is influenced by surgeon- and institution-related factors and the importance of a multidisciplinary team approach cannot be overstated.^28-31

Perihilar cholangiocarcinoma (CCA)

Surgical resection is the treatment of choice for resectable perihilar CCA. For unresectable disease, neoadjuvant chemoradiation therapy followed by LT has been acknowledged as a curative option in selected cases.^32-35 It has provided an excellent recurrence-free 5-y survival rate reaching 68% while maintaining equal to or even better quality of life than those who underwent LT for other liver disease.^36,37 Encouraged by the success of the Mayo protocol, other transplant centers in the United States have applied a similar approach to unresectable perihilar CCA and have achieved equivalent results.³⁸ However, the definition of “resectability” of the disease demands extra caution. The candidates for surgical resection appear to be different when the studies from Western countries are compared with those of Japan. For example, there is a great difference in the portion of patients with Bismuth type IV, ranging from 0% to >40%.^39-48 Thus, the reported survival data needs to be interpreted carefully. The Nagoya group has recently reported that 73 patients who underwent resection for Bismuth type IV tumor and/or combined vascular resection, therefore fulfilling the “unresectability criteria” defined by the Mayo group, had a 60.4% survival rate at 5 y, similar to the survival of LT recipients.⁴⁹ A new staging system for perihilar CCA that is expected to become the “common language” to provide a basis for discussing surgical treatment of this disease has raised several concerns and warrants further international collaboration.^50-52

Intrahepatic CCA

In contrast to perihilar CCA, LT for intrahepatic CCA fare worse. The disappointing results from the initial experiences with a 3-y survival in the 30% range are eloquently summarized in a recent review.⁵³ Most recently, a multicenter study from Spain demonstrated better survival rates in 27 patients reaching 51% at 5 y.⁵⁴ The University of California Los Angeles group described promising data of neoadjuvant and adjuvant therapy combined with LT in the treatment of intrahepatic and hilar CCA.⁵⁵ They further proposed a prognostic scoring system to identify patients who may benefit from LT.⁵⁶ Seven independent risk factors (multifocal tumor, perineural invasion, infiltrative subtype, lack of neoadjuvant and/or adjuvant therapies, primary sclerosing cholangitis, hilar CCA, and lymphovascular invasion) were assigned certain risk score points based on the corresponding parameter estimate using the Cox model. Patients in the low risk category (score 0–3) had a significantly higher 5-y survival of 78% compared with intermediate risk (score 4–7, 19%) and high risk (score 8–15, 0%) categories. In contrast to the Mayo group, they insist that a biopsy should be obtained before neoadjuvant therapy to evaluate tumor biology.^56,57 Although a marked progress has been made in understanding the pathogenesis of intrahepatic CCA,⁵⁸ LT as a treatment option remains highly controversial.

Others

For combined HCC-CCA, the data on the outcomes are scarce and the prognoses reported in the earlier series are not very promising, with the median overall survival ranging from 20 mo–3.6 y.^59-63 One study that analyzed patients in the Surveillance, Epidemiology, and End Results database (1973–2007) found that LT (19 patients) and resection (35 patients) for localized combined HCC-CCA had similar 3-y survival rates in the 50% range, which was inferior to LT for HCC (1447 patients, 78%).⁶³ At the moment, liver resection is preferred over LT for resectable combined HCC-CCA. Intriguingly, the Spanish study previously mentioned showed similar 5-y survival rates between combined HCC-CCA (15 patients, 78%) and the matched cohort of HCC (30 patients, 86%) after LT.⁵⁴ Precise imaging criteria for the pretransplant diagnosis of HCC are important to identify lesions suspicious for combined HCC-CCA. Needle biopsy of the tumor should ideally be performed to confirm histology, although it might be difficult in a clinical setting.^54,62,63

A recent systematic review described marginal outcomes of LT for fibrolamellar HCC with 5-y survival rates ranging from 29–55%.⁶⁴ However, most of the collected cases were from the 1980s and early 1990s before the proposal of the Milan criteria. In addition, perioperative patient management has made great progress since then and the disappointing results of the earlier series are not surprising. There are 2 publications to date reporting successful living donor LT (including 1 case of salvage transplantation after hepatectomy) as an effective alternative in highly selected cases.^65,66 The role of LT in the treatment of fibrolamellar HCC has yet to be defined.

Three large series (>50 patients) of LT for hepatic epithelioid hemangioendothelioma (HEHE) reported satisfactory 5-y survival rates ranging from 55–83%.^67-69 All of them have proposed an aggressive attitude toward unresectable HEHE even in patients with lymph node involvement or extrahepatic disease because both factors did not significantly affect survival when appropriate multimodal treatments were employed. Pretransplant medical condition and vascular invasion have been described as poor prognostic predictors.^68,69 The use of anti-vascular endothelial cell growth factor agents is expected to further improve outcomes.^68,70 Recently, Grotz et al. stated that patients with tumor size ≤ 10 cm, number up to 10, and extent of hepatic involvement up to 4 segments, are candidates for liver resection rather than LT and further investigations are warranted to clearly define the role of each surgical treatment.⁷¹ In the pediatric population, LT remains a good treatment modality in patients with unresectable HEHE, although the prognoses may not be as promising as other primary liver tumors, i.e., hepatoblastoma and HCC.⁷²

The data on LT for primary sarcoma are disappointing. Six patients with primary sarcomas showed recurrent disease at a median of 2 mo and all died early after LT with only 1 patient surviving the first year.⁷³ Likewise, Orlando et al. stated that hemangiosarcomas are an absolute contraindication to LT due to its diminished overall survival of 7.2 ± 2.6 mo.⁷⁴ However, for hepatic undifferentiated embryonal sarcoma, a rare malignancy in the pediatric population, 6 patients underwent LT combined with multimodal therapy and all remained disease-free with a median follow-up of 35 mo.⁷⁵

For unresectable hepatoblastoma, LT continues to be the only treatment option. In the US, the number of new cases of hepatoblastoma increased by almost 4-fold during the last 3 decades and LT by 20-fold.⁷⁶ The 5-y overall survival rate has gradually improved to over 75%, similar to that for patients with HCC.^76-78 Patients are considered unamenable to LT only if there are one or more extrahepatic lesions that are unresponsive to chemotherapy.⁷⁸ A Japanese study has proposed a new algorithm to allocate patients to either LT or liver resection depending on their response to chemotherapy.⁷⁹

Metastatic liver tumors

Neuroendocrine tumor (NET)

Several patient selection criteria and prognostic factors such as concurrent resection of other organs, primary tumor site, hepatomegaly, and recipient age have been proposed but there has been a lack of clinical evidence to clearly delineate candidates for transplantation.^80-84 Recently, a European registry study described outcomes of 213 cases that underwent LT for metastatic NET, the largest number of patients to date; 5-y overall and disease-free survival rates were 52% and 30%, respectively.⁸⁵ Hepatomegaly, poor tumor differentiation, and major resection concurrent with LT were revealed to be poor prognostic indicators and better survival rates of nearly 60% were achieved in cases performed after 2000 because of more favorable patient characteristics.⁸⁵ There is a growing consensus that LT for metastatic NET should be considered for unresectable disease, no extrahepatic disease, well-differentiated NET (NET G1/G2), and should not be associated with major extrahepatic resection.⁸⁵ Medical treatment continues to evolve and the timing of LT remains controversial.⁸⁵

Colorectal cancer

Once deemed an absolute contraindication, LT for unresectable colorectal liver metastasis may become a viable option again.^86,87 By using a mammalian target of rapamycin inhibitor for primary immunosuppression, a Norwegian pilot study of 21 patients achieved 60% overall survival at 5 y, far exceeding the reported outcomes of chemotherapy in this subset of patients.⁸⁷ Although 19 of 21 (90%) patients had recurrent disease, a significant proportion of them were accessible to further local treatments and 7 (33%) had no evidence of disease at the last follow-up.⁸⁷ However, this study was conducted because of the exceptional situation in Norway where a surplus of organs exists and an ethical dilemma needs to be solved.^87,88 Meanwhile, an Austrian group has suggested a novel technology to detect micrometastases in histologically negative lymph nodes to select patients who may benefit from LT.⁸⁹ Tremendous progress in LT over the last several decades as well as new tools developed for patient selection and the notably improved efficacy of chemotherapy may evoke a paradigm shift in the treatment of colorectal cancer.

Others

Only a handful of cases of LT for unresectable gastrointestinal stromal tumor (GIST) metastases have been reported to date. Husted et al. strongly argued against the use of LT for metastatic GIST because of its dismal outcome.⁷³ However, others described more promising survival with the combination of imatinib, allowing disease-free intervals over 24 mo with the longest survival reaching 10 y at the time of publication.^90-93 LT for unresectable metastatic GIST may afford prolonged survival in highly selected patients with time to liver metastases > 2 y, low or intermediate risk level of GIST, and positive KIT (CD117) or PDGFR gene mutation status.⁹³

Similarly, there are only 3 case reports of LT (2 living and 1 deceased donor) for unresectable liver metastases of solid pseudopapillary tumor of the pancreas.^94-96 The patients remain disease-free at 2 and 5 y after LT.^94-96 Selection criteria for LT have yet to be determined.

Intestinal and Multivisceral Transplantation

Multivisceral transplantation for extensive malignancies in the upper abdomen was first reported in 1989.⁹⁷ Since then, its short-term outcomes have improved significantly and indications of either intestinal, liver-intestinal, or multivisceral (full/modified) transplantations⁹⁸ have expanded for a wide variety of tumors such as desmoid (Gardner’s syndrome), neuroendocrine, adenocarcinoma, schwannoma, lymphoma, and sarcoma that would otherwise be unresectable.^99-112 Currently, there is no standardized indication of intestinal and multivisceral transplantation for neoplastic disease. However, for high-grade malignancies such as adenocarcinoma and lymphoma, the disappointing oncological results in the earlier series obviously warrant deliberate patient selection, if not contraindicated.¹⁰³ Because all patients are subject to life-long, intensive immunosuppression, the biological behavior of any neoplasm is essentially unpredictable and needs to be carefully weighed with the invasiveness of these procedures even if complete tumor removal is technically feasible. Novel immune monitoring techniques on the horizon^113-120 as well as accumulating evidence of the significance of donor specific antibodies^108,121-126 are critical to surmount the immunological dilemma of preventing intestinal rejection, the Achilles’ heel of transplanting small bowel contained allografts,¹²⁷ and tumor progression.

Application of transplantation techniques in cancer surgery

Autotransplantation

Ex vivo tumor resection and organ reimplantation (autotransplantation) for extensive hepatic and gastrointestinal malignancies involving mesenteric and/or celiac root deemed unresectable by conventional surgery is an offspring of solid organ transplantation. It has now evolved from single organ^128-135 to multivisceral surgery.^136,137 The major advantages of this approach are excellent exposure enabling complete tumor eradication in a bloodless field at the back table and eliminating the use of immunosuppression.^136,137 Nevertheless, it is associated with potential risks of vascular thrombosis and ischemia reperfusion injury as well as uncertain oncological outcomes, similar to allotransplantations.^136,137 These complex transplantations are still far from perfect and each case should undergo comprehensive, multidisciplinary discussion in a combined transplant and oncology program.

Others

Several other surgical approaches derived from solid organ transplantation have given rise to other complex operations for extensive malignancies. Hannoun et al. first reported ante-situm (transection of the suprahepatic inferior vena cava followed by ventral rotation of the liver) hepatic resection with total vascular exclusion, venovenous bypass, and hypothermic perfusion (which has also been used in in situ liver resection) in 1991.¹²⁹ Since then, liver tumors involving the confluence of hepatic veins and/or the retrohepatic vena cava were resected by this approach in more than 50 patients and every report underscores the paramount importance of deliberate patient selection.^{130,134,138-142} Ciancio et al. depicted the use of the mobilization technique in LT for resection of renal cell carcinoma with tumor thrombus in the inferior vena cava¹⁴³ and en bloc mobilization of the pancreas and spleen, which was derived from multivisceral transplantation as well as organ procurement, for resection of large tumors in the left upper abdomen.¹⁴⁴ Both procedures were successfully performed with minimum risk while maximizing the chance of complete resection.

Closing Remarks

Cancer treatment evolves constantly and multidisciplinary team discussion is imperative. Surgery is only one part of cancer patient care and all transplant surgeons and physicians should work closely with the surgical and medical oncologists. If the removal of a tumor appears to be beneficial from an oncological viewpoint, its resectability should be carefully determined by a team of surgeons experienced in hepatopancreatobiliary, gastrointestinal, and transplant surgery; however, this is not always possible. More importantly, it is vital to recognize the “triple distress,” fundamental limitations of solid organ transplantation for malignancies.

(1) Oncological distress: The so-called “selection criteria” for transplantation is totally dependent on 2 factors: (A) number of organs available, and (B) biological behavior of the disease. In the era of deteriorating organ availability, long-term outcomes after transplantation for malignancies cannot be inferior to other indications; otherwise, the principle of equity will be lost for other patients on the waiting list. On the contrary, in rare circumstances such as Norway where organ supply surpasses demand, clinical trials challenging the current consensus are permissive.⁸⁷

(2) Immunological distress: Induction of immune tolerance after transplantation and the crosstalk between transplant and tumor immunity,^145,146 both organ- and disease-specific, has yet to be elucidated. How to determine an adequate level of immunosuppression to prevent rejection and tumor progression at the same time remains the key question.

(3) Technical distress: The history of transplantation is one of a series of innovation. Novel techniques have served as life-saving options in complicated cases that were once considered absolute contraindications.^147,148 However, these complex procedures carry significant risks of predominantly vascular complications.

As we endeavor on our arduous odyssey for patients with malignancies that cannot be treated by standard cancer therapy, the aforementioned hardships will inevitably block our way as the undefeated enemy. Transplantation is an ever-unfinished enterprise and a victim of its own success. No matter how close we get, allotransplantation does not allow us to accomplish the ultimate goal to bring definitive cure for all patients worldwide. Organ bioengineering and regenerative medicine is the only hope; the “triple threat” cancer therapy that serves as an ultimate solution to the necessary evils of solid organ transplantation for malignancies.

The clinical application of organ bioengineering and regenerative medicine may proceed in a stepwise manner. For example, it is well known that the limit of liver resection is determined by the hepatic reserve and future liver remnant. Patients with a normal liver who require extensive hepatectomy beyond this limit because of tumor size and location are at risk for liver failure. If we successfully assemble and implant a small liver derived from the patient’s own cells by organ bioengineering, it can function as an auxiliary liver “autograft”¹⁴⁹ while the remnant liver regenerates rapidly. This autograft only needs to work until the patient’s own liver becomes large enough to meet the physiological demands. As the new technology improves, larger and durable liver autografts should be available that can completely replace the deceased liver, similar to LT for end-stage liver cirrhosis performed in everyday clinical practice. Likewise, for intestinal transplantation, a first step might be the implantation of a short-lasting autograft for temporary nutritional support while the patient undergoes intestinal rehabilitation for short-gut syndrome.¹⁵⁰

Organ bioengineering and regenerative medicine will revolutionize the field of “Transplant Oncology.”

Glossary

Abbreviations:

LT

liver transplantation

HCC

hepatocellular carcinoma

CCA

cholangiocarcinoma

HEHE

hepatic epithelioid hemangioendothelioma

NET

neuroendocrine tumor

GIST

gastrointestinal tumor

10.4161/org.29245