Daratumumab, a Novel Therapeutic Human CD38 Monoclonal Antibody, Induces Killing of Refractory Patient-Derived Multiple Myeloma Cells, Growing in a Novel Humanized Mouse MM Model

Abstract 940

The evolution of multiple myeloma (MM) is a multi-step process during which mature B cells acquire genetic mutations in multiple genes, which typically takes place in the bone marrow (BM) microenvironment. This, together with the difficulty to culture MM in vitro or to grow MM in vivo in animal models has been the main reason during past decades for poor progress in preclinical research with patient-derived myeloma (pMM) cells. Recently, we developed a unique human-mouse hybrid model that allows engraftment and outgrowth of pMM cells by implementing a technology that is based on first generating a human bone environment in immune deficient mice (Groen et al. 2012) and that is subsequently capable of supporting growth of injected pMM cells. The model offers the opportunity (1) to study the pathobiology of myeloma, and (2) to evaluate, preclinically, new therapeutics for MM treatment, including antibody testing against pMM cells, obtained from patients who acquired resistance to conventional and novel drugs.

Daratumumab (DARA) is a human CD38 antibody with broad-spectrum killing activity. Daratumumab induces effective killing of MM tumor cells via complement dependent cytolysis (CDC), ADCC (antibody dependent cellular cytolysis) and ADCP (antibody-dependent phagocytosis). DARA represents a novel promising treatment for MM and other hematological malignancies and is currently tested in Phase I/II clinical trials. In these clinical studies the adverse events have been manageable and marked reductions in paraprotein and bone marrow plasma cells have been observed. In the current study, we asked whether DARA was able to inhibit growth of refractory tumor cells in our human-mouse hybrid model.

To this end, immune-deficient RAG2^−/−g_c^−/−-mice were implanted subcutaneously with biphasic calcium phosphate (BCP) particles (2–3 mm) loaded with culture expanded human mesenchymal stromal cells (MSCs). Eight weeks later, the humanized scaffolds in mice (n=45) were injected with 0.5–5×10⁶ pMM cells obtained from different refractory, MM patients. The pMM cells were gene-marked with a GFP-luciferase lentiviral construct for imaging of viable tumor cells. Bioluminescent imaging (BLI) was used to follow myeloma outgrowth in time and to visualize the effect of treatment. The pMM cells were obtained from patients at diagnosis (type 1); at end stage disease, after a history of MPT (melphalan, prednisone, thalidomide, type 2); or from a patient refractory to chemotherapy with bortezomib (BORT), adriamycine and dexamethasone (DEX) (type 3).

Mice carrying the pMM cells received similar treatment as the patients or were treated with DARA in a dose range of 1x 50 μg (low dose (LD)) or 2 to 3x 200 μg/mouse (high dose (HD)). BLI showed that the type 1 pMM-bearing mice responded well to all treatments, including DARA; type 2-bearing pMM mice showed no reduction in tumor growth after chemotherapy, but DARA treatment (LD) resulted in an almost complete elimination of circulating MM cells, as assessed with a CD138 antibody, in blood and BM. In a second experiment, type 2-pMM bearing mice were treated with a high DARA dose early (day 34, 50 and 72, 3 times HD, tumor size/BLI signal <200 cmp/cm²) or late (day 50 and 72, 2 times HD, tumor size/BLI signal >8000 cpm/cm²). A significant reduction of overall tumor load, as measured with BLI was observed, which interestingly did not differ between the high and low tumor load group. A reduction of circulating tumor cells (CD138+) was observed for both conditions, which was most obvious in the early treated mice and in agreement with the observations in the first experiment. Type 3 (resistant) pMM-bearing mice showed only a minor response to DEX and BORT, but were highly sensitive to melphalan. When DEX- and BORT-treated mice were treated with a single injection of DARA, this resulted in a complete remission in 3 out of 4 mice and a reduction of the tumor load by 50% in the fourth BORT-treated mouse.

In conclusion, our results demonstrate that DARA is effective against multiple myeloma cells derived from therapy- naïve or -refractory patients grafted in a humanized mouse model. In addition, this humanized MM model can be used to study the potential and mechanism of action of DARA in vivo. This novel MM model might be used to predict responsiveness of myeloma patients to particular treatments.