ImmunoPET Imaging of Endogenous and Transfected Prolactin Receptor Tumor Xenografts

Periplasmic synthesis and purification of the human prolactin antagonist Δ1-11-G129R-hPRL

The human prolactin antagonist Δ_1-11-G129R-hPRL is a 21.9 kDa recombinant protein with 188 amino acids that downregulates the proliferation of a variety of cells expressing prolactin receptors. Periplasmic expression of recombinant proteins in E. coli has been considered an option for obtaining a soluble and correctly folded protein, as an alternative to cytoplasmic production. The aim of this work was, therefore, to synthesize for the first time, the Δ_1-11-G129R-hPRL antagonist, testing different activation temperatures and purifying it by classical chromatographic techniques. E. coli BL21(DE3) strain was transformed with a plasmid based on the pET25b( +) vector, DsbA signal sequence and the antagonist cDNA sequence. Different doses of IPTG were added, activating under different temperatures, and extracting the periplasmic fluid via osmotic shock. The best conditions were achieved by activating at 35 °C for 5 h using 0.4 mM IPTG, which gave a specific expression of 0.157 ± 0.015 μg/mL/A₆₀₀ at a final optical density of 3.43 ± 0.13 A₆₀₀. Purification was carried out by nickel-affinity chromatography followed by size-exclusion chromatography, quantification being performed via high-performance size-exclusion chromatography (HPSEC). The prolactin antagonist was characterized by SDS-PAGE, Western blotting, reversed-phase high-performance liquid chromatography (RP-HPLC) and MALDI-TOF–MS. The final product presented > 95% purity and its antagonistic effects were evaluated in vitro in view of potential clinical applications, including inhibition of the proliferation of cancer cells overexpressing the prolactin receptor and specific antidiabetic properties, taking also advantage of the fact that this antagonist was obtained in a soluble and correctly folded form and without an initial methionine.

Tumour-associated macrophages process drug and radio-conjugates of the dead tumour cell-targeting APOMAB® antibody

APOMAB (chDAB4) is a dead tumour cell-targeting antibody which has been used preclinically as a diagnostic agent and therapeutically as a radioimmunotherapy and antibody drug conjugate (ADC). However, little is known of the intra-tumour processing of chDAB4 when bound to dead tumour cells. In this study we examine the role of macrophages in the in vitro and in vivo processing of radiolabelled chDAB4 and a chDAB4 ADC. We found that chDAB4 binds to macrophages in vitro, resulting in the killing of macrophages when using the ADC, chDAB4-SG3249. Free drug released by the macrophage processing of chDAB4-SG3249 could result in killing of 'bystander' Lewis lung (LL2) carcinoma cells. Furthermore, macrophages phagocytosed chDAB4-bound dead LL2 cells and were killed when they phagocytosed chDAB4-SG3249-bound dead LL2 cells in vitro. In vivo, we found markedly different tumour retention of chDAB4 in the LL2 tumour model depending on whether it was radiolabelled with a residualising radionuclide (⁸⁹Zr), which is retained intracellularly, or a non-residualising radionuclide (¹²⁴I), which can diffuse out of the cell. This prolonged retention of ⁸⁹Zr vs¹²⁴I indicated intra-tumoral processing of chDAB4 in vivo. The tumour uptake of ⁸⁹Zr-chDAB4 was reduced after macrophage depletion, which also reduced the efficacy of the chDAB4 ADC in vivo. This study shows that macrophages can process chDAB4 and chDAB4 ADC in vitro and shows the importance of tumour-associated macrophages in the tumour retention of chDAB4 and the efficacy of chDAB4 ADC in vivo.

ImmunoPET: Concept, Design, and Applications

Immuno-positron emission tomography (immunoPET) is a paradigm-shifting molecular imaging modality combining the superior targeting specificity of monoclonal antibody (mAb) and the inherent sensitivity of PET technique. A variety of radionuclides and mAbs have been exploited to develop immunoPET probes, which has been driven by the development and optimization of radiochemistry and conjugation strategies. In addition, tumor-targeting vectors with a short circulation time (e.g., Nanobody) or with an enhanced binding affinity (e.g., bispecific antibody) are being used to design novel immunoPET probes. Accordingly, several immunoPET probes, such as ⁸⁹Zr-Df-pertuzumab and ⁸⁹Zr-atezolizumab, have been successfully translated for clinical use. By noninvasively and dynamically revealing the expression of heterogeneous tumor antigens, immunoPET imaging is gradually changing the theranostic landscape of several types of malignancies. ImmunoPET is the method of choice for imaging specific tumor markers, immune cells, immune checkpoints, and inflammatory processes. Furthermore, the integration of immunoPET imaging in antibody drug development is of substantial significance because it provides pivotal information regarding antibody targeting abilities and distribution profiles. Herein, we present the latest immunoPET imaging strategies and their preclinical and clinical applications. We also emphasize current conjugation strategies that can be leveraged to develop next-generation immunoPET probes. Lastly, we discuss practical considerations to tune the development and translation of immunoPET imaging strategies.