Purpose: Praluzatamab ravtansine (CX-2009) is a conditionally activated Probody drug conjugate (PDC) comprising an anti-CD166 mAb conjugated to DM4, with a protease-cleavable linker and a peptide mask that limits target engagement in normal tissue and circulation. The tumor microenvironment is enriched for proteases capable of cleaving the linker, thereby releasing the mask, allowing for localized binding of CX-2009 to CD166. CX-2009 was evaluated in a phase I/II clinical trial for patients with advanced solid tumors. Patients and Methods: Eligible patients had metastatic cancer receiving ≥2 prior treatments. CX-2009 was administered at escalating doses every 3 weeks (0.25–10 mg/kg) or every 2 weeks (4–6 mg/kg). Primary objective was to determine the safety profile and recommended phase II dose (RP2D). Results: Of 99 patients enrolled, the most prevalent subtype was breast cancer (n = 45). Median number of prior therapies was 5 (range, 1–19). Dose-limiting toxicities were observed at 8 mg/kg every 3 weeks and 6 mg/kg every 2 weeks. On the basis of tolerability, the RP2D was 7 mg/kg every 3 weeks. Tumor regressions were observed at doses ≥4 mg/kg. In the hormone receptor–positive/HER2-nonamplified breast cancer subset (n = 22), 2 patients (9%) had confirmed partial responses, and 10 patients (45%) had stable disease. Imaging with zirconium-labeled CX-2009 confirmed uptake in tumor lesions and shielding of major organs. Activated, unmasked CX-2009 was measurable in 18 of 22 posttreatment biopsies. Conclusions: CD166 is a novel, ubiquitously expressed target. CX-2009 is the first conditionally activated antibody–drug conjugate to CD166 to demonstrate both translational and clinical activity in a variety of tumor types.

This review highlights the added value of PET imaging in Central Nervous System (CNS) tumors, which is a tool that has rapidly evolved from a merely diagnostic setting to multimodal molecular diagnostics and the guidance of targeted therapy. PET is the method of choice for studying target expression and target binding behind the assumedly intact blood–brain barrier. Today, a variety of diagnostic PET tracers can be used for the primary staging of CNS tumors and to determine the effect of therapy. Additionally, theranostic PET tracers are increasingly used in the context of pharmaceutical and radiopharmaceutical drug development and application. In this approach, a single targeted drug is used for PET diagnosis, upon the coupling of a PET radionuclide, as well as for targeted (nuclide) therapy. Theranostic PET tracers have the potential to serve as a non-invasive whole body navigator in the selection of the most effective drug candidates and their most optimal dose and administration route, together with the potential to serve as a predictive biomarker in the selection of patients who are most likely to benefit from treatment. PET imaging supports the transition from trial and error medicine to predictive, preventive, and personalized medicine, hopefully leading to improved quality of life for patients and more cost-effective care.

The high affinity of an antibody can result in restricted tumor penetration and heterogenous tumor distribution, with preferential binding of the antibody to tumor cells localized around tumor vasculature. This so-called “binding site barrier” effect limits the efficacy of antibody-based therapies like antibody–drug conjugates (ADC). In this issue, Bordeau and colleagues introduce an original approach to overcome this barrier through transient competitive inhibition of antibody–antigen binding. By coadministration of an anti-idiotypic anti-trastuzumab domain antibody as a competitive inhibitor, increased tumor penetration of trastuzumab as well as enhanced efficacy of the ADC ado-trastuzumab emtansine were observed in tumor-bearing mice See related article by Bordeau et al., p. 4145

Introduction: Compared to the antibody and drug components of an ADC, the linker part has been somewhat neglected. However, its importance for the reduction of failures in ADC approvals is increasingly recognized. Next of being a stable glue between drug and antibody, an ideal linker should improve the manufacturability and widen the therapeutic window of ADCs. Areas covered: The biopharmaceutical company LinXis started an ADC development program in which platinum(II) is the key element of the first metal-organic linker. The cationic complex [ethylenediamineplatinum(II)]²⁺, herein called ‘Lx®’, is used successfully for conjugation of drugs to antibodies. Expert opinion: Based on lessons learned from ADC development, Lx linker technology fulfills most of the desirable linker characteristics. Lx allows large-scale cost-effective manufacturing of ADCs via a straightforward two-step ‘plug-and-play’ process. First clinical candidate trastuzumab-Lx-auristatin F shows favorable preclinical safety as well as outstanding in vivo tumor targeting performance and therapeutic efficacy.

Background: Approximately 20% of invasive ductal breast malignancies are human epidermal growth factor receptor 2 (HER2)-positive. These patients receive neoadjuvant systemic therapy (NAT) including HER2-targeting therapies. Up to 65% of patients achieve a pathological complete response (pCR). These patients might not have needed surgery. However, accurate preoperative identification of a pCR remains challenging. A radiologic complete response (rCR) on MRI corresponds to a pCR in only 73% of patients. The current feasibility study investigates if HER2-targeted PET/CT-imaging using Zirconium-89 (⁸⁹Zr)-radiolabeled trastuzumab can be used for more accurate NAT response evaluation. Methods: HER2-positive breast cancer patients scheduled to undergo NAT and subsequent surgery received a ⁸⁹Zr-trastuzumab PET/CT both before (PET/CT-1) and after (PET/CT-2) NAT. Qualitative and quantitative response evaluation was performed. Results: Six patients were enrolled. All primary tumors could be identified on PET/CT-1. Four patients had a pCR and two a pathological partial response (pPR) in the primary tumor. Qualitative assessment of PET/CT resulted in an accuracy of 66.7%, compared to 83.3% of the standard-of-care MRI. Quantitative assessment showed a difference between the SUV_R on PET/CT-1 and PET/CT-2 (ΔSUV_R) in patients with a pPR and pCR of −48% and −90% (p = 0.133), respectively. The difference in tumor-to-blood ratio on PET/CT-1 and PET/CT-2 (ΔTBR) in patients with pPR and pCR was −79% and −94% (p = 0.133), respectively. Three patients had metastatic lymph nodes at diagnosis that were all identified on PET/CT-1. All three patients achieved a nodal pCR. Qualitative assessment of the lymph nodes with PET/CT resulted in an accuracy of 66.7%, compared to 50% of the MRI. Conclusions: NAT response evaluation using ⁸⁹Zr-trastuzumab PET/CT is feasible. In the current study, qualitative assessment of the PET/CT images is not superior to standard-of-care MRI. Our results suggest that quantitative assessment of ⁸⁹Zr-trastuzumab PET/CT has potential for a more accurate response evaluation of the primary tumor after NAT in HER2-positive breast cancer.

Non-invasive imaging of atherosclerosis can help in the identification of vulnerable plaque lesions. CD40 is a co-stimulatory molecule present on various immune and non-immune cells in the plaques and is linked to inflammation and plaque instability. We hypothesize that a ⁸⁹Zr-labeled anti-CD40 monoclonal antibody (mAb) tracer has the potential to bind to cells present in atherosclerotic lesions and that CD40 Positron Emission Tomography (PET) can contribute to the detection of vulnerable atherosclerotic plaque lesions. To study this, wild-type (WT) and ApoE^−/− mice were fed a high cholesterol diet for 14 weeks to develop atherosclerosis. Mice were injected with [⁸⁹Zr]Zr-anti-CD40 mAb and the aortic uptake was evaluated and quantified using PET/Computed Tomography (CT) imaging. Ex vivo biodistribution was performed post-PET imaging and the uptake in the aorta was assessed with autoradiography and compared with Oil red O staining to determine the tracer potential to detect atherosclerotic plaques. On day 3 and 7 post injection, analysis of [⁸⁹Zr]Zr-anti-CD40 mAb PET/CT scans showed a more pronounced aortic signal in ApoE^−/− compared to WT mice with an increased aorta-to-blood uptake ratio. Autoradiography revealed [⁸⁹Zr]Zr-anti-CD40 mAb uptake in atherosclerotic plaque areas in ApoE^−/− mice, while no signal was found in WT mice. Clear overlap was observed between plaque areas as identified by Oil red O staining and autoradiography signal of [⁸⁹Zr]Zr-anti-CD40 mAb in ApoE^−/− mice. In this proof of concept study, we showed that PET/CT with [⁸⁹Zr]Zr-anti-CD40 mAb can detect atherosclerotic plaques. As CD40 is associated with plaque vulnerability, [⁸⁹Zr]Zr-anti-CD40 mAb has the potential to become a tracer to detect vulnerable atherosclerotic plaques.