Magnitude of enhanced permeability and retention effect in tumors with different phenotypes: 89Zr-albumin as a model system

Abstract

Targeted nanoparticle-based technologies show increasing prevalence in radiotracer design. As a consequence, quantitative contribution of nonspecific accumulation in the target tissue, mainly governed by the enhanced permeability and retention (EPR) effect, becomes highly relevant for evaluating the specificity of these new agents. This study investigated the influence of different tumor phenotypes on the EPR effect, hypothesizing that a baseline level of uptake must be exceeded to visualize high and specific uptake of a targeted macromolecular radiotracer.

Methods: These preliminary studies use (89)Zr-labeled mouse serum albumin ((89)Zr-desferrioxamine-mAlb) as a model radiotracer to assess uptake and retention in 3 xenograft models of human prostate cancer (CWR22rv1, DU-145, and PC-3). Experiments include PET and contrast-enhanced ultrasound imaging to assess morphology, vascularization, and radiotracer uptake; temporal ex vivo biodistribution studies to quantify radiotracer uptake over time; and histologic and autoradiographic studies to evaluate the intra- and intertumoral distribution of (89)Zr-desferrioxamine-mAlb.

Results: Early uptake profiles show statistically significant but overall small differences in radiotracer uptake between different tumor phenotypes. By 20 h, nonspecific radiotracer uptake was found to be independent of tumor size and phenotype, reaching at least 5.0 percentage injected dose per gram in all 3 tumor models.

Conclusion: These studies suggest that minimal differences in tumor uptake exist at early time points, dependent on the tumor type. However, these differences equalize over time, reaching around 5.0 percentage injected dose per gram at 20 h after injection. These data provide strong support for the introduction of mandatory experimental controls of future macromolecular or nanoparticle-based drugs, particularly regarding the development of targeted radiotracers.

FIGURE 1

Representative coronal and transverse PET images showing ⁸⁹Zr-DFO-mAlb radiotracer uptake in mice bearing contralateral CWR22rv1, DU-145, or PC-3 human prostate xenografts on lower left and right flanks. For mouse bearing CWR22rv1 tumors, temporal PET images recorded at 1, 4, and 20 h after intravenous administration of radiotracer are presented to demonstrate change in radiotracer biodistribution and image contrast over time. White lines indicate approximate level of coronal or transverse slice. H = heart; Ki = kidney; L = Liver; T = tumor; Trans. = transverse.

FIGURE 2

(A) Bar chart showing change in ⁸⁹Zr-DFO-mAlb uptake in blood pool (representative data for CWR22rv1 models shown) and 3 tumor models over time (h). (B) Tumor-to-blood time–activity contrast curves.

FIGURE 3

B mode (left) and CEUS maximum-intensity-projection images (right) of representative CWR22rv1, DU-145, and PC-3 tumors. B mode images show heterogeneous echo patterns with hypoechogenic and echo-free areas consistent with necrotic zones. CEUS maximum-intensity-projection data depict differences in vascularization among the 3 tumor types; signal increase due to microbubble presence is coded as green. MIP = maximum intensity projection.

FIGURE 4

Microscopy images of adjacent histologic CWR22rv1, DU-145, and PC-3 tumor tissue slices stained with H&E (purple), albumin-bound Evans blue (fluorescence image; red), and DAR (green) showing intratumoral distribution of ⁸⁹Zr-DFO-mAlb. Tumors were harvested and fresh frozen at 20 h after intravenous administration of radiotracer. Evans blue dye was administered intravenously 10–15 min before sacrifice. Scale bar = 2 mm.