🧬 biology

Immuno-PET imaging of CD38 expression in multiple myeloma using a 64Cu-labeled nanobody

This study demonstrates that a 64Cu-labeled anti-CD38 nanobody (WF121) enables specific, high-contrast immuno-PET imaging of CD38 expression in multiple myeloma xenografts, validating its potential as a non-invasive companion diagnostic for guiding targeted therapies.

Original authors: Sébastien GOUARD, Tacien Petithomme, Patricia Le Saëc, Vincent Nguyen, Judith Fresquet, Séverine Marionneau-Lambot, Clément Bailly, Mickaël Bourgeois, Nicolas Boisgerault, Joëlle Gaschet, Michel Chére

Published 2026-06-28

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Sébastien GOUARD, Tacien Petithomme, Patricia Le Saëc, Vincent Nguyen, Judith Fresquet, Séverine Marionneau-Lambot, Clément Bailly, Mickaël Bourgeois, Nicolas Boisgerault, Joëlle Gaschet, Michel Chérel

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Picture: Finding the "ID Badge" of Cancer

Imagine Multiple Myeloma (a type of blood cancer) as a group of rogue cells hiding in the body. These bad cells wear a specific "ID badge" on their surface called CD38. Doctors use powerful drugs (like Daratumumab) to hunt down these badges, but sometimes the badges disappear, or the drugs don't work on everyone. Currently, doctors have to guess if a patient has enough badges to make the treatment worth the cost and side effects.

This study is about building a high-tech, glowing flashlight that can find these specific ID badges inside the body before treatment starts. This would help doctors decide exactly who needs the expensive drug and who doesn't.

The Tools: A Tiny Drone and a Glowing Battery

To build this flashlight, the researchers used two special ingredients:

The Drone (The Nanobody): Instead of using a giant, slow-moving antibody (which is like a heavy cargo ship), they used a nanobody. Think of a nanobody as a tiny, agile drone. Because it is so small (about 1/10th the size of a normal antibody), it can zip through the bloodstream, find its target, and clear out of the body very quickly.
- The specific drone used here: It was designed to lock onto the CD38 badge.
The Glowing Battery (Copper-64): To make the drone visible to a camera, they attached a radioactive "battery" called Copper-64.
- Why Copper-64? Other batteries (like Gallium-68) run out of power too fast (in about an hour). Copper-64 lasts much longer (about 12 hours). This is like giving the drone a long-lasting battery so it can be tracked for a full day, allowing doctors to take pictures at different times to get the clearest possible image.

The Experiment: A Test Drive in Mice

The researchers tested this "glowing drone" in mice that had been given two types of tumors:

Tumor A (MM1S): These cells wore the CD38 ID badge.
Tumor B (U266): These cells did not wear the badge.

What happened?

The Lock-On: When the researchers checked the cells under a microscope, the nanobody only stuck to the cells with the CD38 badge. It ignored the ones without it.
The Flight Path: When they injected the glowing drone into the mice, it moved incredibly fast. Within minutes, it cleared out of the blood and went straight to the kidneys (the body's filtration system), which is normal for small molecules.
The Target Hit: The drone didn't just float around; it found the CD38-positive tumors and stuck to them.
- The Result: The tumors with the badges lit up brightly on the PET scan (the camera). The tumors without the badges remained dark.
- The Contrast: The difference between the "lit up" tumor and the background was huge. It was like turning on a bright spotlight in a dark room; you could clearly see the target.

The "Drone" Behavior: Does it get stuck?

One concern with these tiny drones is whether they get stuck inside the cells or the kidneys.

Inside the Cells: The study found that the drone mostly stayed on the outside of the cell (the membrane), with only a tiny bit going inside. This is good because it means the "glow" stays right where the badge is, making the picture sharp.
Inside the Kidneys: The kidneys acted like a washing machine, filtering the drone out of the blood. The study showed that the kidneys cleared the drone efficiently over 24 hours, meaning it didn't get stuck there permanently.

The Conclusion: A Clearer Picture

The paper concludes that this new tool—a Copper-64 labeled nanobody—works exactly as hoped in a preclinical (mouse) setting.

It finds the cancer cells wearing the CD38 badge.
It ignores cells that don't have the badge.
It provides a clear, high-contrast image very quickly (within hours).

In simple terms: The researchers successfully built a tiny, long-lasting, glowing tracker that can spot the specific "ID badge" of multiple myeloma cancer cells in mice. This suggests that in the future, doctors could use a similar tool to take a "snapshot" of a patient's cancer to see if it has the right target for treatment, potentially saving patients from unnecessary side effects and costs.

Note: This study was performed entirely in mice and cell cultures. The paper does not claim this tool is ready for use in humans yet, but it provides the first proof that this specific combination (Copper-64 + Anti-CD38 Nanobody) works effectively in a living system.

Technical Summary: Immuno-PET Imaging of CD38 Expression in Multiple Myeloma Using a 64Cu-Labeled Nanobody

Problem Statement
Multiple Myeloma (MM) is a hematological malignancy where CD38 serves as a validated therapeutic target and biomarker, primarily targeted by the monoclonal antibody Daratumumab. However, clinical management is complicated by primary refractoriness, acquired resistance due to CD38 downregulation, and the high cost and toxicity of treatment. Currently, there is a lack of a companion diagnostic tool to accurately phenotype CD38 expression across all tumor lesions non-invasively. While full-length radiolabeled antibodies (e.g., Daratumumab) have been used for Immuno-PET, their slow pharmacokinetics necessitate delayed imaging (several days post-injection) and result in poor tumor-to-background ratios in the liver and spleen. Furthermore, standard FDG-PET lacks specificity for MM lesions. The study addresses the need for a rapid, high-contrast imaging agent capable of early detection and accurate phenotyping of CD38 expression to guide patient stratification.

Methodology
The study utilized a preclinical model to evaluate a copper-64 ( $^{64}$ Cu) labeled anti-CD38 single-domain antibody (nanobody), designated WF121.

Molecular Engineering: The WF121 nanobody was produced with a His-tag and conjugated to the NOTA chelator (1,4,7-Triazacyclononane-1,4,7-triacetic acid) via lysine residues. Mass spectrometry confirmed an average of 1.5 chelators per nanobody molecule.
Radiolabeling: The conjugate was radiolabeled with $^{64}$ Cu ( $T_{1/2} = 12.7$ h) produced via the $^{64}$ Ni(d,2n) $^{64}$ Cu reaction. Radiolabeling yields consistently exceeded 95%, and immunoreactivity was measured at 81.4%.
In Vitro Characterization: Binding specificity was assessed via flow cytometry on CD38-positive (MM1S) and CD38-negative (U266) cell lines. Internalization was evaluated using confocal microscopy.
In Vivo Models: Subcutaneous xenografts were established in C.B-17/IcrHanHsd-Prkdc-scid mice using MM1S (CD38+) and U266 (CD38-) cells.
Imaging and Analysis: Mice underwent dynamic PET/CT scans (0–2 hours) and sequential static acquisitions at 1, 2, and 24 hours post-injection. Quantitative uptake was measured as %ID/mL (tumor) and %ID/g (organs). Ex vivo biodistribution, autoradiography, and histological staining (HPS) were performed at 2 and 24 hours to validate imaging data and localize tracer distribution.

Key Results

Binding and Internalization: Flow cytometry confirmed selective binding of WF121 to CD38+ MM1S cells with an estimated $K_d$ of ~10 nM, while showing no binding to CD38- U266 cells. Confocal microscopy revealed that WF121 is predominantly membrane-associated, with partial internalization observed in ~70% of cells within 15–60 minutes.
Pharmacokinetics: Dynamic PET imaging demonstrated rapid blood clearance of the nanobody. Tumors were detectable as early as 5–15 minutes post-injection. Renal uptake was high initially (47.8% of total activity at 5–15 min), increasing to 63.5% by 115–125 min, indicating predominant renal clearance.
Tumor Contrast: In MM1S (CD38+) models, tumor uptake was significantly higher than in U266 (CD38-) models at all time points.
- At 24 hours, MM1S tumor uptake was $2.74 \pm 0.38$ %ID/mL, compared to $0.64 \pm 0.07$ %ID/mL in U266.
- The tumor-to-tumor ratio (MM1S/U266) improved over time, reaching 4.3 at 24 hours.
- Ex vivo biodistribution confirmed these findings, with MM1S uptake at 24 hours being 5.58 times higher than U266 uptake.
Clearance and Localization: Autoradiography confirmed homogeneous tracer distribution within MM1S tumors, with signal intensity slightly higher at the periphery. In the kidneys, the tracer was localized predominantly in the renal cortex. Biodistribution data showed efficient renal elimination, with kidney-associated activity decreasing from 68% to 30% (decay-corrected) between 2 and 24 hours, indicating no sustained retention.

Significance and Claims
The authors claim this study provides the first preclinical evidence that a $^{64}$ Cu-labeled anti-CD38 nanobody enables effective early and delayed immuno-PET imaging of CD38 expression in multiple myeloma.

Key claims regarding the significance of the work include:

Feasibility of $^{64}$ Cu-Nanobody Conjugates: The study demonstrates the successful combination of an anti-CD38 nanobody with copper-64, leveraging the isotope's longer half-life (12.7 h) to allow for delayed imaging, which may improve lesion detection near the kidneys after tracer clearance.
Diagnostic Potential: The high tumor-to-background contrast and rapid acquisition times support the potential of nanobody-based radiotracers as companion diagnostics for non-invasive phenotyping and patient stratification in CD38-targeted therapies.
Advantage Over Full-Length Antibodies: The results reinforce the advantage of nanobodies over full-length antibodies by overcoming limitations related to slow distribution, high background in the liver/spleen, and the need for delayed imaging.
Clinical Relevance: The findings suggest that CD38 immuno-PET could offer a more specific identification of myeloma lesions compared to FDG-PET, particularly in relapsed or refractory settings where patient selection is critical.

The paper maintains a modest tone, noting that while the results are promising, future clinical studies are required to define specific uptake thresholds for therapeutic response and to validate these findings in human patients.