Effects of Administration Dose, Species and Strain Variations on MicroCT Imaging
Characteristics of a Long-Acting Blood-Pool Agent in Normal Rodents
GN Ton1, C Burrascano2, WC Dow2, DA Bakan2, JP Weichert1
1University of Wisconsin-Madison,WI and 2Alerion Biomedical Inc., San Diego, CA

Results and Discussion

B48

Chylomicron

CE

E
C

CMR

Fatty
Acids

AC

B,E-receptor

LPL

HDL

Capillaryendothelium

Hepatocyte
Fatty acid-albumincomplexes

Fig 1. Hepatic uptake of Chylomicron Remnants

The macromolecular structure of theblood-pool contrast agent, Fenestra VC (BP),is shown in Fig. 2. Attachment of PEG chainsto the surface of the ITG vehicle prevents
Apo-E association and decreases its uptake byhepatocytes resulting in prolonged circulatory
retention. The goal of this study is to evaluate
the in vivo CT imaging characteristics of theblood-pool agent in various strains of miceand rats as a function of administration dose.

BPFig 2. Structure of BP and ITG Particles

The blood-pool contrast agent (Fenestra VC) was obtained from ART Advanced
Research Technologies Inc. (Montreal, Canada). The final iodine concentration and mean
particle size of this lipid emulsion were 50 mg I/mL and 180 nm, respectively. Fenestra VC
was administered as a single i.v. bolus dose via tail vein injection. Three doses (7.5, 10, and15 mL/kg) were given to female Sprague-Dawley rats (n=2-3/group) and two doses (10 and15 mL/kg) were injected to female C57Bl/6 (n=3) mice. Other strains of mice (Balb/c, FVB,Nude, and Scid) were administered with 15 mL/kg (n=3). Anesthetized animals werescanned using a GE eXplore Locus microCT system (80 kVp, 450 µA, 200 msec/view, 5frames/view, 400 views/scan and 93 µm resolution) prior to and at predetermined timeintervals following administration of the blood-pool agent.

Images were recontructed with the EVSBeam software (GE Medical System)and subsequently displayed and analyzed using both Amira 3-D visualization software(V3.1) and an open-source MicroView program (http://Microview.sf.net). CT valuesof volumetric ROIs in the inferior vena cava (IVC) and liver were normalized toHounsfield Unit (HU). Normalized signal intensity (HU/voxel) was obtained from thedifference of enhanced SI and the baseline value.
Fig 6. VC-Enhanced images of anesthetized C57B/6 mice
10 min following injection of Fenestra VC.
10mL/kg

15mL/kg

15mL/kg

CoronalviewSL
BV

Excellent vascular contrast enhancement and soft tissue
identification was achieved with 15 mL/kg VC.

S

15 mL/kg
t=10 min

KSPL
Renal Veins

CDSP

S

500(B)

600
SD Rat_SI in IVC_15 mL/kg VC
C57Bl/6_SI in IVC_15 mL/kg VC
SD Rat_SI in Liver_15 mL/kg VC
C57Bl/6_SI in LIver_15 mL/kg VC

Balb/c_15 mL/kg VC
FVB_15 mL/kg VC
C57Bl/6_15 mL/kg VC
Nude_15 mL/kg VC
Scid_15 mL/kg VC

500400400300300200200100

100001020304005010IVCS10 mL/kg
t=10 min

7.5 mL/kg
t=10 minFig 3. shows Fenestra VC-enhanced 3D microCT images of an anesthetized rat 10minutes following intravenous administration of a single 15 mL/kg dose. Combined 3D
surface rendering and an axial slice image (Fig. 3A) shows enhanced hepatic vascularnetwork. The renal cortex and renal medulla of the left kidney (K) are clearly visualized inFig 6B. Effect of administration dose on the in vivo imaging efficacy of Fenestra VC isshown in Fig 3(C-E). Coronal CT images (C-E) of anesthetized rats obtained 10 minutes
after injection of the blood pool at three different doses show excellent vascularenhancement was achieved at 15 mL/kg dose. At the lowest test dose (7.5 mL/kg), adequate
vascular intensity and soft tissue identification could be obtained at 10 minutes post
injection. Due to the relatively large size of the rats, only a portion of the body could be
imaged at a time.

2030

Fig 7. Time sequential VC-Enhanced images of anesthetized C57B/6 micefollowing injetion of Fenestra VC (15 mL/kg).
Baseline

24h6h10 min

GBLL

L

600
SI in IVC_15 mL/kg
SI in IVC_10 mL/kg
SI in IVC_7.5 mL/kg
SI in Liver_15 mL/kg
SI in Liver_10 mL/kg
SI in Liver_7.5 mL/kg

GI tract

B
SI in IVC_15 mL/kg VC
SI in IVC_10 mL/kg VC
SI in Liver_15 mL/kg VC
SI in Liver_10 mL/kg VC

Inherent poor soft tissue contrast was observed with the baseline scan.
Fenestra VC visibly remained in the vasculature for several hours, producing
excellent vascular contrast enhancement. Elimination via hepatobiliary system wasevident after 24 hours.

40

In vivo imaging results demonstrate that the vascular contrast agent provided sustained
vascular contrast enhancement for several hours in a variety of rodent models. Bothadministration dose and species variations had significant effects on the in vivo imaging
characteristics of Fenestra VC, however slight and manageble differences in the vascularsignal intensity were observed between various strains of mice. Fenestra VC with its
superior blood pool activity and acceptable injection tolerance profiles provides substantial

Materials and Methods

500
4005004003003002002001001000Conclusions

0102030

Post-Injection Time (Hours)

40

Coronalview

SFig 4. Normalized Signal Intensity-Time Profiles of Fenestra VC inSD Rats (A) and C57Bl/6 Mice (B)A50

The effect of species variation on the blood pool activity of Fenestra VC is clearlyshown in Fig 5A. Blood clearance, as measured by a decrease in vascular signal intensity,
was significantly faster in SD rats than in C57Bl/6 mice, affording a higher level of liver
localization in rats 24 hours post injection of Fenestra VC. Vascular contrast enhancement
profiles of the blood pool agent in five different strains of mice following intravenous
administration of 15 mL/kg dose is shown in Fig 5. Among the female mice, the FVB
group displayed a slower rate of vascular clearance, while the other strains all exhibitedcomparable rates albeit somewhat faster than that observed for the FVB. Results from theimaging study demonstrated that effects of species variation on the in vivo imaging
charactertistics of Fenestra VC were more significant than that of strain variation.
All the tested mice and rats remained active and displayed normal social behaviors
prior to their sacrifice within 1-4 weeks after injection. Intravenous administration ofFenestra VC at a dose of 15 mL/kg followed by a serial CT scanning according to thespecified protocol was well tolerated in both rats and mice.

S

600

40

Post-Injection Time (Hours)

Post-Injection Time (Hours)

SP15 mL/kg
t=10 min(A)6000ES0LAxialview

S
Hepatic vasculature

ITGMaterials and Methods

10mL/kgFig 3. VC-Enhanced MicroCT Vascular Images of Anesthetized Rats at T=10 minA
B

Normalized Signal Intensity (HU/Voxel).

AFenestra VC is a refined, surface modified
version of our hepatocyte-selective contrast
agent ITG (Fenestra LC). This agent mimics
chylomicron remnants and thereby selectively
delivers the contrast-producing moiety into thehepatic parenchyma following administration
Association of circulatory Apo-E with the ITG
particles is believed to be essential forhepatocyte recognition. Thus, interfering with
its Apo-E interaction is excpected to retard
hepatocyte recognition (Fig. 1).

Fig 5. Normalized Signal Intensity-Time Profiles of Fenestra VC(A) in SD Rats vs C57Bl/6 Mice and (B) in Various Strains of Mice (B)Normalized Signal Intensity (HU/Voxel).

Introduction
Recently introduced microCT scanners1,2 capable of sub 20-micron spatial resolution
afford the potential to more effectively monitor disease model conditions in small animals.
Despite being able to provide this exceedingly high spatial resolution, the widespread
adoption of this imaging method in living systems has been hindered by the relative lack ofsoft tissue contrast associated with taking such thin tomographic slices and comparably longacquisition times that preclude the use of conventional water-soluble contrast agents. In
order to achieve a comparatively prolonged and stable circulatory half-life, a novel longacting vascular CT contrast agent has been developed and evaluated for its ability tovisualize and noninvasivetly assess vascular characteristics in normal and disease modelconditions.3,4 The vascular contrast agent was recently commercialized and is now known as
FenestraTM VC.

50010203050

Post-Injection Time (Hours)

Normalized contrast enhancement profiles of the blood pool agent in the blood andliver tissues following various administration doses in female SD rats and C57Bl/6 mice areshown in Fig 4. Imaging data demonstrated that the degree of tissue contrast intensity inboth rats and mice was dose dependent. Vascular signal intensity in rats and mice decreased
to approximately 73% and 77%, respectively, after 6 hours following intravenous
administration of a single bolus dose. Unlike water-soluble contrast agents which rapidly
diffuse into extravascular space, the blood pool contrast agent remained in the circulation
for several hours, producingly susbtained vascular contrast enhancement.

flexibility in the type and scope of studies that can be performed.

References[1] Paulus MJ, et. al, Neoplasisa (2000) 2:62-70.
[2] Paulus MJ, et. al, Lab Animals (2001) 30:1-10.
[3] Weichert JP, et.al, Academic Radiol. (1998) 5:S16-19.
[4] Weichert JP, et.al, Radiology, (2000) 216:865-871.

Supported in part by the UWCCC, UW-Radiology, and Alerion Biomedical Inc.
gn.ton@hosp.wisc.edu, Poster #241