Development of an Antibody-Targeted Nanoparticle for Early Detection of Minimal Residual Disease in Multiple Myeloma

Development of methods for the early detection of minimal residual disease (MRD) is a central challenge to the effective treatment of patients with multiple myeloma (MM). The presence of MRD is directly linked to both the durations of treatment response as well as to long-term survival outcomes. Current diagnostic methods utilize serologic biomarkers and/or repeated bone marrow samplings that do not take into account the spatial heterogeneity of the tumor microenvironment; as such, they often lead to false negative results. Available diagnostic imaging agents are not designed to detect plasma-cell populations; they rely on passive tumor accumulation that hinders both their detection specificity and sensitivity.

Here, we utilize ultra-small gadolinium-based nanoparticles (Gd-NPs) that are conjugated to monoclonal antibodies to enable magnetic resonance imaging of plasma cells in a murine model of MM. We demonstrate the first example of utilizing a non-invasive imaging agent to improve early detection of MRD after therapeutic administration.

To develop a plasma-cell targeted MRI contrast agent that would improve early detection of MRD, we fabricated ultra-small, sub-5 nm Gd-NPs that were conjugated to monoclonal antibodies directed against different receptors on the surfaces of MM cells. We postulated that these ultra-small Gd-NPs would minimally affect antibody targeting upon conjugation and could achieve improved in vivo imaging of plasma cell populations. For targeting of MM cells, we selected antibodies against the signaling lymphocytic activation molecule-F7 (SLAMF7) and the B-cell maturation antigen (BCMA). Both antigens are almost exclusively present on the surfaces of malignant B-cells. To generate our MM-targeted imaging construct, the surfaces of our Gd-NPs were decorated with free N-hydroxysuccinimide (NHS) groups and were conjugated to NHS groups on the surface of anti-SLAMF7 and BCMA antibodies via a bissulfosuccinimidyl suberate crosslinker.

An MRI study was undertaken to compare the efficiencies of the various nanoparticle constructs (Gd-NP, NP-SLAMF7, and NP-BCMA) to identify identical plasma cell burdens and as compared to the FDA-approved contrast agent Magnevist^TM. Gd uptake in the spine and femurs of animals were visualized using a 7T MRI machine with a T1-GRE sequence. The specificity of each of the administered contrast agents to target MM cells was confirmed by animal sacrifice immediately after MRI; the femurs and vertebral tissues of each animal were harvested for histologic assessment after staining by H&E and by Prussian blue, which showed sheets of marrow-infiltrating plasma cells and which labeled Gd deposits, respectively. This quantification demonstrated the enhanced sensitivity of NP-SLAMF7 and NP-BCMA, as compared to the passive targeting agents Gd-NP and Magnevist^TM, to detect plasma cell populations. As soon as 30 min after the intravenous injection of the nanoparticle-antibody complexes, animals that had been administered NP-SLAMF7 demonstrated an ~3.8-fold increase in the SNR for plasmacytomas in the spine while those that had received NP-BCMA exhibited a ~12-fold enhancement. Note that the NP-BCMA conjugate demonstrated better tumor uptake than NP-SLAMF7 (p= 0.0045, one-sided paired t-test), which was attributed to the greater numbers of surface BCMA antigens on MM cells as assessed on a per cellular basis.

We subsequently evaluated their targeting efficiencies both in vitro and in vivo prior to performing a comparative study to determine their detection capabilities when compared to other clinically-available diagnosis, such as computed tomography scans and methods levels of serum lambda light-chains. Comparisons of AUC for the SNR detected by each modality and over the entire duration of the experiment further supported the superiority of the NP-BCMA for both early diagnostic of MM and MRD diagnostic.

Selection of alternative cell surface targets may also enable detection of MRD in patients diagnosed with leukemia or other hematological malignancies. Ultra-small nanoparticle-antibody complexes are, thus, poised to impact both the diagnosis and treatment of MM. Their successful application may further afford insights into the fabrication of other targeted constructs that may help translate the full potential of nanomedicines to improve the care and survival of cancer patients.