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Review
. 2012 Sep;14(3):389-99.
doi: 10.1208/s12248-012-9348-3. Epub 2012 Mar 31.

Tissue distribution studies of protein therapeutics using molecular probes: molecular imaging

Simon-Peter Williams  1
Affiliations
Review

Tissue distribution studies of protein therapeutics using molecular probes: molecular imaging

Simon-Peter Williams. AAPS J. 2012 Sep.

Abstract

Molecular imaging techniques for protein therapeutics rely on reporter labels, especially radionuclides or sometimes near-infrared fluorescent moieties, which must be introduced with minimal perturbation of the protein's function in vivo and are detected non-invasively during whole-body imaging. PET is the most sensitive whole-body imaging technique available, making it possible to perform biodistribution studies in humans with as little as 1 mg of injected antibody carrying 1 mCi (37 MBq) of zirconium-89 radiolabel. Different labeling chemistries facilitate a variety of optical and radionuclide methods that offer complementary information from microscopy and autoradiography and offer some trade-offs in whole-body imaging between cost and logistic difficulty and image quality and sensitivity (how much protein needs to be injected). Interpretation of tissue uptake requires consideration of label that has been catabolized and possibly residualized. Image contrast depends as much on background signal as it does on tissue uptake, and so the choice of injected dose and scan timing guides the selection of a suitable label and helps to optimize image quality. Although only recently developed, zirconium-89 PET techniques allow for the most quantitative tomographic imaging at millimeter resolution in small animals and they translate very well into clinical use as exemplified by studies of radiolabeled antibodies, including trastuzumab in breast cancer patients, in The Netherlands.

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Figures

Fig. 1
Fig. 1
Schematic of a dual-modality PET/optical construct used to explore the properties of IRDye800CW conjugates by Cohen at al. (4). Left Zr-89 chelated with desferrioxamine B. Right An IRDye800CW moiety. Both linked to lysine side groups on the antibody
Fig. 2
Fig. 2
Near-infrared fluorescence imaging with Zr89-bevacizumab-IRDye800CW from Cohen et al. [4]. Mouse with bilateral FaDu tumors (white arrows) imaged 24 h after injection with 40 μg of antibody (265 pmol, approximately 1.3 mg/kg) labeled 1:1 with IRDye800CW and 100 μCi of Zr-89. From the Zr-89 signal, it can be estimated that the signal arises from 6 to 12 pmol of dye. The image was acquired in 1 s. Despite this very sensitive detection in the superficial tumor, note the absence of signal from within the body
Fig. 3
Fig. 3
Time series of quantitative Zr-89 cetuximab image slices in mice, from Aerts et al. [39]. Note the blood pool progressively clears as the bilateral HT29 tumors (red arrows) show progressive antibody uptake over time. The mouse was injected with 100 μg of antibody carrying 200 μCi of Zr-89 (protein dose of approximately 3.3 mg/kg), which was sufficient to obtain images out to 5 days post-injection. The scan time was 25 min. Reprinted by permission of the Society of Nuclear Medicine from Aerts et al. [39] (Fig. 2)
Fig. 4
Fig. 4
Radiolabeling with Zr-89: covalent conjugation of a chelating group, followed later by chelation of Zr-89 at the time of use. The bifunctional (lysine-reactive, zirconium-chelating) reagent shown is the commercially available p-isothiocyanatobenzyl-desferrioxamine B
Fig. 5
Fig. 5
Time series of representative Zr-89 trastuzumab images (frontal view) in a patient receiving trastuzumab therapy as described in Dijkers et al. (71). Note the heart and great vessels with blood pool diminishing slowly over 5 days. Note the absence of antibody in the brain cavity, except for blood pool and a previously undetected metastatic brain lesion that is visible at the top of the skull and shows progressive uptake over the 5 days. The injected dose was 10 mg of Zr89-trastuzumab, 38 MBq (1 mCi) of Zr-89 illustrating the sensitivity of the imaging
Fig. 6
Fig. 6
Zr-89 trastuzumab uptake in various liver and bone lesions in three metastatic breast cancer patients as described in Dijkers et al. (71). The injected dose was 10 mg of Zr89-trastuzumab, 38 MBq (1 mCi) of Zr-89. Note the presence of multiple small bony and soft tissue lesions in these PET images illustrating the effective spatial resolution [from Fig. 3 in Dijkers et al. (71)]
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