Pairwise comparison of 89Zr- and 124I-labeled cG250 based on positron emission tomography imaging and nonlinear immunokinetic modeling: in vivo carbonic anhydrase IX receptor binding and internalization in mouse xenografts of clear-cell renal cell carcinoma

Abstract

Purpose: The PET tracer, (124)I-cG250, directed against carbonic anhydrase IX (CAIX) shows promise for presurgical diagnosis of clear-cell renal cell carcinoma (ccRCC) (Divgi et al. in Lancet Oncol 8:304-310, 2007; Divgi et al. in J Clin Oncol 31:187-194, 2013). The radiometal (89)Zr, however, may offer advantages as a surrogate PET nuclide over (124)I in terms of greater tumor uptake and retention (Rice et al. in Semin Nucl Med 41:265-282, 2011). We have developed a nonlinear immunokinetic model to facilitate a quantitative comparison of absolute uptake and antibody turnover between (124)I-cG250 and (89)Zr-cG250 using a human ccRCC xenograft tumor model in mice. We believe that this unique model better relates quantitative imaging data to the salient biological features of tumor antibody-antigen binding and turnover.

Methods: We conducted experiments with (89)Zr-cG250 and (124)I-cG250 using a human ccRCC cell line (SK-RC-38) to characterize the binding affinity and internalization kinetics of the two tracers in vitro. Serial PET imaging was performed in mice bearing subcutaneous ccRCC tumors to simultaneously detect and quantify time-dependent tumor uptake in vivo. Using the known specific activities of the two tracers, the equilibrium rates of antibody internalization and turnover in the tumors were derived from the PET images using nonlinear compartmental modeling.

Results: The two tracers demonstrated virtually identical tumor cell binding and internalization but showed markedly different retentions in vitro. Superior PET images were obtained using (89)Zr-cG250, owing to the more prolonged trapping of the radiolabel in the tumor and simultaneous washout from normal tissues. Estimates of cG250/CAIX complex turnover were 1.35 - 5.51 × 10(12) molecules per hour per gram of tumor (20 % of receptors internalized per hour), and the ratio of (124)I/(89)Zr atoms released per unit time by tumor was 17.5.

Conclusion: Pairwise evaluation of (89)Zr-cG250 and (124)I-cG250 provided the basis for a nonlinear immunokinetic model which yielded quantitative information about the binding and internalization of radioantibody bound to CAIX on tumor cells in vivo. (89)Zr-cG250 is likely to provide high-quality PET images and may be a useful tool to quantify CAIX/cG250 receptor turnover and cG250-accessible antigen density noninvasively in humans.

FIGURE 1

Non-linear compartmental model of intravenously injected radiolabeled cG250 antibody in mice with the CAIX-expressing tumor xenografts. The parameter k(i,j) is the fractional rate (/h) of exchange of the radiolabel from compartment j to compartment i.

FIGURE 2

Analysis of internalization and cell-surface binding at 37°C up to 48 h (top graph (a)) and 4°C up to 4 h (bottom graph (b)) in vitro of ⁸⁹Zr-cG250 and ¹³¹I-cG250 by the RCC cell line SK-RC-38.

FIGURE 3

Noninvasive serial-PET imaging in tumor-bearing mice. Representative transverse (trans.) and coronal PET images of nude mice bearing subcutaneous SK-RC-38 xenografts in the lower hind limb imaged with ¹²⁴I- or ⁸⁹Zr-cG250 at various time points up to 11 d p.i.. Arrows indicate position of tumor. Images are parameterized as %ID/g.

FIGURE 4

Tumor time-activity ROI concentration curves for ¹²⁴I- or ⁸⁹Zr-cG250 uptake in SK-RC-38 xenografts (n = 5 for each cohort) up to 10 and 11 d p.i., respectively. Data are expressed as (mean ± standard error of the mean (SEM)).

FIGURE 5

Biodistribution data following serial PET imaging (11 d for ⁸⁹Zr-cG250 and 10 d for ¹²⁴I-cG250). Groups consisted of 4 or 5 mice each. Data for ¹²⁴I-cG250 is from group of animals with normal thyroid at time of tracer injection. The data are expressed as the mean %ID/g ± SEM. (*) p < 0.05, (**) p < 0.005.

FIGURE 6

Results of non-linear compartmental model of intravenously injected radiolabeled cG250 antibody in mice with the CAIX-expressing tumor xenografts. The data points represent the mean values among the mice injected with ⁸⁹Zr-cG250 ((a) and (b)) and ¹²⁴I-cG250 ((c) and (d)). Panels (a) and (b) show the observed and model-derived activity concentrations (%ID/g) in tumor and panels (c) and (d) the observed and model-derived activity (% of total tumor activity internalized in the tumor cells). In all cases, the model yields reasonably good fits to the data.