Copper-64-Labeled Daratumumab As a PET Imaging Tracer for Multiple Myeloma: Potential Use for Minimal Residual Disease Detection

Introduction : Minimal residual disease (MRD) is known to be an independent predictor of progression free survival (PFS) in multiple myeloma (MM). Methods of MRD detection include flow cytometry, PCR based sequencing, and imaging. However, irrespective of the sensitivity of MRD detection, techniques based on bone marrow biopsies cannot account for a "patchy," non-homogenous bone marrow infiltration. Positron emission tomography (PET) coupled with computed tomography (CT) using ¹⁸fluoro-deoxyglucose as the radiotracer (¹⁸FDG-PET) is a commonly used imaging method that is much more sensitive than traditional radiographs, but it has several limitations; for one, lesions with a low metabolic rate may result in false negatives. Also, diffuse tracer uptake by normal marrow can obscure focal MM lesions. Daratumumab (Dara), a human anti-CD38 IgG₁ (ĸ subclass) antibody against the highly expressed plasma cell receptor CD38, has been recently approved by the FDA for the treatment of MM. Based on the idea that Dara can selectively target MM cells irrespective of their metabolism, our group labeled Dara with the positron-emitting isotope copper-64 (⁶⁴Cu-DOTA-Dara) in order to generate a novel imaging agent that can detect MM cells with high specificity. Our group already safely used ⁶⁴Cu-labeled trastuzumab to detect HER2-positive tissue in breast cancer patients (Mortimer JE et. al. 2014), and here we provide the first pre-clinical evidence that ⁶⁴Cu-labeled antibodies can be considered powerful imaging tools for MM detection.

Methods :Commercially availableDara was conjugated with the NHS active ester of DOTA (Macrocyclics). DOTA-conjugated antibody was incubated with ⁶⁴Cu, chased with 1 mM diethylenetriamine pentaacetic acid (DTPA), and purified on a preparative size-exclusion column. The immunoreactivity of ⁶⁴Cu-DOTA-Dara was also performed using purified CD38-Fc (Chimerigen Laboratories) in an HPLC size shift assay. To assess whether ⁶⁴Cu-DOTA-Dara can localize to MM cells in vivo, 5 x 10⁶ human MM1S GFP+/Luc+ cells were injected intravenously into 6 NOD-SCID mice. At 14 days, when the bioluminescence signal in the bones of the mice were detected, mice were injected with ⁶⁴Cu-DOTA-Dara (50 µCi, 10 µg), and PET imaging was performed at 21 and 44 hr after the injection (Figure 1A). Cancer free mice were used as controls (Figure 1B). ¹⁸F-FDG PET/CT scans in the same group of mice were also performed. Biodistribution studies and histological analysis were also conducted.

Results : ⁶⁴Cu-DOTA labeling did not affect Dara immunoreactivity against the antigen in the HPLC size shift assay. The biodistribution studies show that ⁶⁴Cu-DOTA-Dara was highly detectable in the femurs, tibias, sternum and skull, known sites of MM cell invasion, which were negative in cancer-free control mice. The radionuclide was also highly detected in the lung, liver, and spleen, relative to controls. In contrast, the majority of ⁶⁴Cu-DOTA-Dara remained in the blood of the control mice. Moreover, PET images show that ⁶⁴Cu-DOTA-Dara PET/CT displays a higher resolution and specificity in detecting MM cells in the bones not only compared to ¹⁸F-FDG PET/CT, but even compared to bioluminescence signals from the MM cell line. To assess the specificity of ⁶⁴Cu-Dara signal compared to ¹⁸F-FDG, studies using step sections in histological analysis are in process.

Conclusions : Our data support the use of ⁶⁴Cu-DOTA-Dara as a novel sensitive imaging agent to detect MM. The relatively short half life of ⁶⁴Cu (12.7 hours) limits potential marrow toxicity and is sufficiently long for PET/CT imaging up to 48 hours after administration of the radionuclide. A pilot study assessing the safety and feasibility of ⁶⁴Cu-DOTA-Dara imaging in human subjects is under way.

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