A Rabbit Model for Selective Portal Vein Embolization

A new animal model of atrophy-hypertrophy complex and liver damage following Yttrium-90 lobar selective internal radiation therapy in rabbits

Lobar selective internal radiation therapy (SIRT) is widely used to treat liver tumors inducing atrophy of the treated lobe and contralateral hypertrophy. The lack of animal model has precluded further investigations to improve this treatment. We developed an animal model of liver damage and atrophy–hypertrophy complex after SIRT. Three groups of 5–8 rabbits received transportal SIRT with Yttrium 90 resin microspheres of the cranial lobes with different activities (0.3, 0.6 and 1.2 GBq), corresponding to predicted absorbed radiation dose of 200, 400 and 800 Gy, respectively. Another group received non-loaded microspheres (sham group). Cranial and caudal lobes volumes were assessed using CT volumetry before, 15 and 30 days after SIRT. Liver biochemistry, histopathology and gene expression were evaluated. Four untreated rabbits were used as controls for gene expression studies. All animals receiving 1.2 GBq were euthanized due to clinical deterioration. Cranial SIRT with 0.6 GBq induced caudal lobe hypertrophy after 15 days (median increase 34% -ns-) but produced significant toxicity. Cranial SIRT with 0.3 GBq induced caudal lobe hypertrophy after 30 days (median increase 82%, p = 0.04). No volumetric changes were detected in sham group. Transient increase in serum transaminases was detected in all treated groups returning to normal values at 15 days. There was dose-dependent liver dysfunction with bilirubin elevation and albumin decrease. Histologically, 1.2 GBq group developed permanent severe liver damage with massive necrosis, 0.6 and 0.3 GBq groups developed moderate damage with inflammation and portal fibrosis at 15 days, partially recovering at 30 days. There was no difference in the expression of hepatocyte function and differentiation genes between 0.3 GBq and control groups. Cranial SIRT with 0.3 GBq of ⁹⁰Y resin microspheres in rabbits is a reliable animal model to analyse the atrophy–hypertrophy complex and liver damage without toxicity.

Yttrium-90 Portal Vein Radioembolization in Sprague-Dawley Rats: Dose-Dependent Imaging and Pathological Changes in Normal Liver

PURPOSE

Portal vein embolization (PVE) is an established neoadjuvant method to induce future liver remnant hypertrophy prior to surgical resection of hepatic tumors. The purpose of our study was to examine the feasibility of PVE with glass ⁹⁰Y microspheres (Y90 PVE) in Sprague-Dawley rats. We tested the hypothesis that increased doses of Y90 PVE would increase target lobe fibrosis and atrophy.

METHODS

Twenty-two rats were assigned to four groups for Y90 PVE to the right median lobe: very high- (273.8 MBq; n = 2), high- (99.9 MBq; n = 10), medium- (48.1 MBq; n = 5), and low-dose (14.8 MBq; n = 5). An untreated control group included seven rats. ⁹⁰Y PET/CT of ⁹⁰Y distributions confirmed lobar targeting. MRI volumes were measured at baseline, 2-, 4-, 8- and 12-weeks. Explanted hepatic lobes were weighed, sectioned, and stained for H&E and immunohistochemistry. Digitized slides allowed quantitative measurements of fibrosis (20 foci/slide).

RESULTS

Ex vivo measurements confirmed 91-97% activity was localized to the target lobe (n = 4). The percent growth of the target lobe relative to baseline was - 5.0% (95% CI - 17.0-6.9%) for high-, medium dose rats compared to + 18.6% (95% CI + 7.6-29.7%) in the low-dose group at 12-weeks (p = 0.0043). Radiation fibrosis increased in a dose-dependent fashion. Fibrotic area/microsphere was 22,893.5, 14,946.2 ± 2253.3, 15,304.5 ± 4716.6, and 5268.8 ± 2297.2 μm² for very high- (n = 1), high- (n = 4), medium- (n = 3), and low-dose groups (n = 5), respectively.

CONCLUSION

Y90 PVE was feasible in the rat model, resulted in target lobe atrophy, and dose-dependent increases in hepatic fibrosis at 12 weeks. The onset of imaging-based volumetric changes was 8-12 weeks.

Assessment of hepatic microvascular flow and density in patients undergoing preoperative portal vein embolization

BACKGROUND

The microvascular effects occurring after unilateral preoperative portal vein embolization (PVE) are poorly understood. The aim of this study was to assess the microvascular changes in the embolized and the non-embolized lobes after right PVE.

METHODS

Videos of the hepatic microcirculation in patients undergoing right hemihepatectomy following PVE were recorded using a handheld vital microscope (Cytocam) based on incident dark field imaging. Hepatic microcirculation was measured in the embolized and the non-embolized lobes at laparotomy, 3-6 weeks after PVE. The following microcirculatory parameters were assessed: total vessel density (TVD), microcirculatory flow index (MFI), proportion of perfused vessel (PPV), perfused vessel density (PVD), sinusoidal diameter (SinD) and the absolute red blood cell velocity (RBCv).

RESULTS

16 patients after major liver resection were included, 8 with and 8 without preoperative PVE. Microvascular density parameters were higher in the non-embolized lobes when compared to the embolized lobes (TVD: 40.3 ± 8.9 vs. 26.8 ± 4.6 mm/mm² (p < 0.003), PVD: 40.3 ± 8.8 vs. 26.7 ± 4.7 mm/mm² (p < 0.002), SinD: 9.2 ± 1.7 vs. 6.3 ± 0.8 μm (p < 0.040)). RBCv, PPV and the MFI were not significantly different.

CONCLUSION

The non-embolized lobe has a significantly higher microvascular density, however without differences in microvascular flow. These findings indicate increased angiogenesis in the hypertrophic lobe.

Evaluation of the rabbit liver by direct portography and contrast-enhanced computed tomography: anatomical variations of the portal system and hepatic volume quantification

Background

The study was aimed at: (1) describing the incidence of anatomic variations of the portal system in the rabbit using direct portography; and (2) estimating the liver volume and caudate lobe volume by using contrast-enhanced computed tomography (CECT) in the same animal model.

Methods

Forty-six New Zealand white rabbits were included. All of them underwent direct portography and unenhanced CECT. Conventional liver rabbit portal system anatomy (type 1) consisted of the bifurcation of the main portal vein (MPV) into the right portal vein (RPV) and left portal vein (LPV), which subsequently divided into medial left portal vein and lateral left portal vein. Trifurcation of the LPV was considered type 2. The LPV that divides into four smaller branches was classified as type 3. Other configurations of the portal system, including particular cases of MPV branching, were grouped as type 4. Liver lobes were manually segmented.

Results

The incidence of each type of portal system anatomy was: type 1, 67.4%; type 2, 15.2%; type 3, 13.0%); and type 4, 4.3%. The mean volume of the caudate lobe was 19.1 ml ± 5.7 ml and of the cranial lobes it was 66.7 ml ± 13.7 ml, and the total liver volume was 85.7 ml ± 16.7 ml.

Conclusions

In New Zealand white rabbits, type 1 is the prevalent type of portal system, liver volume is about 86 ml, and the caudate and cranial lobes are separated. This information could be important when planning experimental rabbit liver procedures.

Hepatic parenchymal transection increases liver volume but not function after portal vein embolization in rabbits

BACKGROUND

Associating liver partition with portal vein ligation for staged hepatectomy induces more extensive liver hypertrophy than ligation alone; however, the mechanisms underlying the accelerated liver regrowth and the functional quality of the hypertrophic liver are presently elusive. This study, therefore, investigated the effect of parenchymal transection on liver volume and function after portal vein embolization in a standardized rabbit model.

METHODS

Twelve rabbits were subjected to portal vein embolization of the cranial liver lobes and randomized between parenchymal transection of the left lateral liver lobe versus no transection (portal vein embolization only). Liver volume of the nonembolized liver lobe was assessed using computed tomography-volumetry, and liver uptake function was determined by ^99mTc-mebrofenin hepatobiliary scintigraphy before and 3 and 7 days after portal vein embolization.

RESULTS

The increase in nonembolized liver volume 3 days after portal vein embolization was 2.7-fold greater in the transected group compared with the portal vein embolization only group (56 ± 16% vs 21 ± 12%, respectively, P < .01) and 1.7-fold greater 7 days after portal vein embolization (113 ± 34% vs 68 ± 24%, P < .01). Liver uptake function did not differ between groups before portal vein embolization (8.4 ± 3.7%/min in the transection group vs 8.9 ± 1.6%/min) on day 3 (33.2 ± 4.7% after transection vs 30.3 ± 4.6%/min, respectively) and day 7 after portal vein embolization (42.6 ± 8.4% vs 39.1 ± 5.3%/min, respectively).

CONCLUSION

Parenchymal transection after portal vein embolization increases liver growth in terms of volume but not function. These results indicate that the rapid volume increase observed after associating liver partition with portal vein ligation for staged hepatectomy does not coincide with the clinically more relevant functional increase. Quantitative liver function tests might be essential in associating liver partition with portal vein ligation for staged hepatectomy to better assess the hypertrophy response and improve clinical decision-making.

Triggered liver regeneration: from experimental model to clinical implications

BACKGROUND

Major liver resection is the only therapeutic option for patients with malignant liver tumors. However, extended hepatectomy often leads to postoperative liver failure, mainly due to insufficient amounts of the remnant liver. Recently, selective portal vein occlusion (PVO) has been introduced to increase the remnant liver volume. This novel surgical technique initiated a progressive development in liver surgery, resulting in a significant increment in potential candidates for curative liver resection.

SUMMARY

The theoretical basis for this great advancement is formed by an understanding of the mechanisms of PVO-induced liver regeneration, mainly obtained from animal studies. The aim of this review is to give a comprehensive overview of the relevant animal models of PVO and to discuss the main characteristics of triggered liver regeneration, including the induced hemodynamic, morphological and functional alterations as well as the underlying molecular mechanisms, which might be of interest in both the laboratory and the clinic. Key Messages: Although basic research revealed the main characteristics of PVO-triggered liver regeneration within the last decades, several important issues regarding the regenerative process remain uncertain. To answer these open questions, additional well-designed animal experiments are needed in the future, which allow further refinement of this surgical technique.

A review of animal models for portal vein embolization

BACKGROUND

Portal vein embolization (PVE) is a preoperative intervention to increase the future remnant liver (FRL) through regeneration of the non-embolized liver lobes. This review assesses all the relevant animal models of PVE available, to guide researchers who intend to study PVE.

MATERIALS AND METHODS

We performed a systematic literature search in Medline and Pubmed, from 1993-June 2013, using search headings "PVE" and "portal vein ligation". Articles were included when meeting the selection criteria: experimental animal study on PVE or portal vein ligation and experiments described in 5 animals or more.

RESULTS

Sixty-one articles were selected, describing six different animal models. Most articles reported experiments with rats, rabbits, and pigs. In rats, the increase in wet-weight ratio of the non-occluded liver or total liver weight is greatest in the first 7 d with values ranging from 75%-80.5% on day 7. The volume increase of FRL in the rabbit model is greatest in the first 7 d with values ranging from 33.6%-80% on day 7. In pigs, the largest gain in volume of the FRL was seen in the first 2 wk.

CONCLUSIONS

The choice of the model depends on the specific aim of the study. Evaluating the increase in liver volume and liver function after PVE, larger animals as the pig, rabbit, or the dog is useful because of the possibility to apply computed tomography volumetry. To evaluate mechanisms of regeneration after PVE, the rat model is useful, because of the variety of antibodies commercially available.

Enhanced tumor growth after portal vein embolization in a rabbit tumor model

BACKGROUND

Preoperative portal vein embolization (PVE) is used to increase future remnant liver volume through induction of hepatocellular regeneration. This event, however, potentially enhances tumor growth. The aim of our study was to assess tumor growth and liver regeneration after PVE in a rabbit hepatic tumor model. The VX2 carcinoma is derived from a virus-induced papilloma tumor in rabbits. The tumor grows rapidly, and its blood supply is similar to that of human hepatocellular carcinoma.

MATERIALS AND METHODS

Two weeks after subcapsular implantation of a VX2 carcinoma in the cranial liver lobe, New Zealand White rabbits were allocated to a control or PVE group (n = 5 per group). In the PVE group, the portal vein branch to the cranial liver lobes (80%) was embolized using particles and coils, leaving the caudal liver lobe (20%) free. In the tumor control group, the liver was mobilized. Computed tomography volumetry was performed on days 3, 7, 10, and 14. Tumor growth rate (TGR), hepatocellular proliferation rate, and liver damage parameters were assessed before PVE and on days 1, 3, 7, 10, and 14.

RESULTS

Portography confirmed complete occlusion of the portal vein branch to the cranial liver lobes in all PVE rabbits. The hypertrophy response and proliferation rate in the nonembolized liver lobes were significantly higher in the PVE group, which was confirmed by liver-to-body weight index assessment. TGR was increased in both groups, with a significantly larger increase in the PVE group over time (day 14: mean, 34.4 ± 4.3 mL/d versus control: mean, 24.1 ± 7.2 mL/d; P < 0.05).

CONCLUSIONS

TGR was significantly increased after PVE in the rabbit tumor model. This finding supports the notion that PVE potentially enhances tumor growth, along with the regeneration of the nonembolized liver lobe.

Accurate quantitation of Ki67-positive proliferating hepatocytes in rabbit liver by a multicolor immunohistochemical (IHC) approach analyzed with automated tissue and cell segmentation software

Determination of hepatocyte proliferation activity is hampered by the presence of Ki67-positive non-parenchymal cells. We validated a multicolor immunohistochemical (IHC) approach using multispectral tissue and cell segmentation software. Portal vein branches to the cranial liver lobes of 10 rabbits were embolized, leading to atrophy of the cranial lobes and hyperplasia of the caudal lobes. Slides from cranial and caudal lobes (n=20) were double-stained (CK8+18 and Ki67) and triple-stained (CK8+18, Ki67, and CD31). The Ki67 proliferation index was calculated using automated tissue and cell segmentation software and compared with manual counting by two independent observers. A substantial variation was seen in the number of Ki67-positive hepatocytes in the different specimens in both double and triple staining (range, 0-50). Correlation coefficients between manual counting and the digital analysis were 0.76 for observer 1 (p<0.001) and 0.78 for observer 2 (p<0.001) with double staining and R(2) = 0.91 for observer 1 and R(2) = 0.89 for observer 2, p<0.001 with triple staining. In conclusion, in rabbit, the hepatocellular proliferation index can be reliably determined using automated tissue and cell segmentation software in combination with IHC multiple staining. Our findings may be useful in clinical practice when Ki67 proliferation index yields prognostic significance.

Bile salts predict liver regeneration in rabbit model of portal vein embolization

BACKGROUND

Portal vein embolization (PVE) is employed to increase future remnant liver (FRL) volume through induction of hepatocellular regeneration in the nonembolized liver lobe. The regenerative response is commonly determined by CT volumetry after PVE. The aim of the study was to examine plasma bile salts and triglycerides in the prediction of the regenerative response following PVE.

METHODS

PVE of the cranial liver lobe was performed in 15 rabbits, divided into three groups: NaCl (control), gelatin sponge (short-term occlusion), and polyvinyl alcohol particles with coils (PVAc, long-term occlusion). In all rabbits CT volumetry and blood sampling were performed prior to PVE and on days 3 and 7. Plasma bile salts and triglycerides were correlated with volume increase of the nonembolized liver lobe.

RESULTS

After 3 and 7 d, respectively, FRL volume was increased in both embolized groups, with the largest hypertrophy response observed in the PVAc group. Plasma bile salt levels were increased after PVE, especially in the PVAc group at day 3 (P < 0.01 compared to gelatin sponge). Plasma bile salts at day 3 predicted FRL volume increase at day 7 showing a positive correlation of 0.811 (P < 0.001). Levels of triglycerides were not significantly altered in either of the PVE procedures.

CONCLUSIONS

Plasma bile salt levels early after PVE strongly correlated with the regenerative response in a rabbit model of PVE, showing more pronounced elevation with larger volume increase of the nonembolized lobe. Therefore, plasma bile salts, but not triglycerides, can be used in the prediction of the regenerative response after PVE.

Short-term effects of combined hepatic vein embolization and portal vein embolization for the induction of liver regeneration in a rabbit model

PURPOSE

Alternative methods to optimize the hypertrophy response after portal vein embolization (PVE) are desired. This study assessed the effect of hepatic vein embolization (HVE) in addition to PVE on liver hypertrophy response in a standardized rabbit model.

MATERIALS AND METHODS

Thirty rabbits were allocated to groups according to intervention: PVE alone, HVE alone, and a combination of HVE and PVE. The liver regeneration response of the nonembolized, caudal liver was assessed by computed tomographic volumetry, liver-to-body weight index, and the amount of proliferating hepatocytes.

RESULTS

The caudal liver volume (CLV) increased significantly more in the PVE and combined PVE/HVE group than in the HVE group at 3 and 7 days after the procedure (P < .01). There were no significant differences in CLV increase or degree of hypertrophy between the PVE and combined embolization groups. The caudal liver-to-body weight index was significantly higher in the PVE and combined embolization groups than in the HVE group on day 7 (P < .01). The index was also significantly higher in the combined PVE/HVE group compared with the PVE group (P = .008). The caudal liver tissue of the PVE and combined groups contained a significantly higher number of proliferating hepatocytes compared with the HVE group on day 7 (P < .01).

CONCLUSIONS

Although histologic and additional regenerative changes are seen, HVE in addition to PVE has no additional short-term effect on hypertrophy response. The combination of HVE and PVE may therefore have little use in a clinical setting.

Liver regeneration after portal vein embolization using absorbable and permanent embolization materials in a rabbit model

OBJECTIVE

To compare the safety and hypertrophy response after portal vein embolization (PVE) using 2 absorbable and 3 permanent embolization materials.

BACKGROUND

Portal vein embolization is used to increase future remnant liver volume preoperatively. Application of temporary, absorbable embolization materials could be advantageous in some situations, provided sufficient hypertrophy is achieved from the nonembolized lobe.

METHODS

Six groups of rabbits (n = 5) underwent PVE of 80% of the total liver volume using saline (sham), gelatin sponge, fibrin glue, polyvinyl alcohol particles with coils, n-butyl cyanoacrylate, or polidocanol. The rabbits were killed after 7 days. Portography, computed tomographic volumetry, Doppler ultrasonography, laboratory liver function and damage parameters (nonembolized) liver-to-body weight ratio, immunohistochemistry, and cytokine and growth factor tissue levels were assessed to examine the differences in the liver regeneration response.

RESULTS

Polidocanol was discontinued because of toxic reactions in 3 rabbits. Gelatin sponge was the only material that was absorbed after 7 days and resulted in less hypertrophy of the nonembolized lobe than the other 3 materials. There were no significant differences in hypertrophy response between the other 3 embolization groups. Volumetric data obtained from computed tomography were supported by liver-to-body weight ratio and the amount of proliferating hepatocytes. The volume gain of the nonembolized lobe was proportional to the volume loss of the embolized liver lobes. The number of Kupffer cells in the embolized liver lobe was significantly higher in the fibrin glue, polyvinyl alcohol particles with coils, and n-butyl cyanoacrylate groups than in the sham and gelatin sponge groups. However, the levels of interleukin-6, tumor necrosis factor-α, hepatocyte growth factor, and transforming growth factor-β1 were significantly lower.

CONCLUSIONS

Temporary occlusion using gelatin sponge for PVE resulted in significantly less hypertrophy response than the use of permanent embolization materials. Except for polidocanol, none of the embolization materials exhibited evident hepatotoxicity.